RHENIUM COMPLEXES WITH POTENTIALLY

RHENIUM COMPLEXES WITH POTENTIALLY MULTIDENTATE LIGANDS CONTAINING THE AMINO, IMINO, HYDROXY AND THIOL GROUPS

J. Mukiza

2016

RHENIUM COMPLEXES WITH POTENTIALLY MULTIDENTATE LIGANDS CONTAINING THE AMINO, IMINO, HYDROXY AND THIOL GROUPS

by

Janvier Mukiza

Submitted in fulfilment of the requirements for the degree of Philosophiae Doctor in the Faculty of Science at the Nelson Mandela Metropolitan University

2016

Promoter: Prof. T.I.A. Gerber

DECLARATION I, Janvier Mukiza (212438778) hereby declare that the thesis for Philosophiae Doctor is my own work and that it has not previously been submitted for assessment or completion of any postgraduate qualification to another University or for another qualification.

Janvier Mukiza

Official use: In accordance with Rule G4.6.3, 4.6.3 A treatise/dissertation/thesis must be accompanied by a written declaration on the part of the candidate to the effect that it is his/her own work and that it has not previously been submitted for assessment to another University or for another qualification. However, material from publications by the candidate may be embodied in a treatise/dissertation/thesis.

Table of contents

J.Mukiza

Table of Contents Acknowledgements

vii

Abstract

viii

List of Publications

xiv

Crystallographic Data

xvi

Chapter 1 Introduction 1.1

General Background

1

1.2

Aim and Motivation of Project

2

1.3

General Chemistry of Rhenium

3

1.3.1

Rhenium(-I)

3

1.3.2

Rhenium(0)

3

1.3.3

Rhenium(I)

3

1.3.3.1 Coordination chemistry of rhenium(I)

4

1.3.4

Rhenium(II)

9

1.3.5

Rhenium(III)

10

1.3.6

Rhenium(IV)

11

1.3.7 Rhenium(V) 1.3.7.1 Coordination chemistry of rhenium(V)

16

Rhenium(VI)

20

1.3.9 Rhenium(VII)

21

1.3.8

1.4

12

Rhenium Radiopharmaceuticals

22

1.4.1

Rhenium and technetium in nuclear medicine

22

1.4.2

Preparation of rhenium and technetium radiopharmaceuticals

22

1.4.3

Classification of rhenium and technetium radiopharmaceuticals

23

1.5 References

26

Chapter 2 Experimental 2.1 Handling of rhenium Nelson Mandela Metropolitan University

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J.Mukiza

2.2 Materials

31

2.2.1

Precursor compounds

31

2.2.2

General laboratory chemicals

34

2.3 Instrumentation

35

2.4 References

36

Chapter 3 Dimeric and Monomeric Rhenium Complexes of Orotic Acid and 2-Mercapto-orotic Acid 3.1 Introduction

37

3.2 Experimental

40

3.2.1

Synthesis of (μ-Br)(μ-O)(μ-oa)[Re2IVBr(OEt)2(PPh3)2] (1)

40

3.2.2

Synthesis of (μ-Br)(μ-O)(μ-oa)[Re2IVBr2(OiPr)(PPh3)2] (2)

41

3.2.3

Synthesis of [ReBr(dab)(oa)(PPh3)2] (3)

41

3.2.4

Synthesis of [ReO(py)2(OEt)(oa)] (4)

41

3.2.5

Synthesis of (Ph4P)[Re(CO)3(H2O)(oa)] (5) and (Et3N) [Re(CO)3(H2O)(oa)].2H2O (6)

42

3.2.6

Synthesis of (μ-Cl) (μ-O)(μ-moa)2[Re(PPh3)]2 (7)

42

3.2.7

Synthesis of (μ-Cl)(μ-O)(μ-oa)[Re2IVCl2(OiPr)(PPh3)2] (8)

43

3.2.8

X-ray crystallography

43

3.3 Results and discussion 3.3.1

43

(μ-Br)(μ-O)(μ-oa)[Re2IVBr(OEt)2(PPh3)2] (1) and (μ-Br)(μ-O)(μ-oa)[Re2IVBr2(OiPr)(PPh3)2] (2)

43

3.3.2

[ReBr(dab)(oa)(PPh3)2] (3)

49

3.3.3

[ReO(py)2(OEt)(oa)] (4)

53

3.3.4

(Ph4P)[Re(CO)3(H2O)(oa)] (5) and (Et3N) [Re(CO)3(H2O)(oa)].2H2O (6)

56

3.3.5

(μ-Cl) (μ-O)(μ-moa)2[Re(PPh3)]2 (7)

61

3.3.6

(μ-Cl)(μ-O)(μ-oa)[Re2IVCl2(OiPr)(PPh3)2] (8)

66

3.4 Conclusion

70

3.5 References

71

Nelson Mandela Metropolitan University

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Chapter 4 Reactivity of 5-Amino-orotic Acid and its Schiff Base and Carboxamide Derivatives to Rhenium(V) and (I) 4.1 Introduction

83

4.2 Experimental

88

4.2.1

Synthesis of salicylimine-orotic acid (H2soa)

88

4.2.2

Synthesis of (μ-Br)(μ-O)(μ-aoa)[Re2IVBr(OEt)2(PPh3)2] (1)

88

4.2.3

Synthesis of [ReBr(aoa)(apd)(PPh3)2] (2)

89

4.2.4

Synthesis of [ReI(coa)(PPh3)2]I (3)

89

4.2.5

Synthesis of [ReOCl(soa)(PPh3)2] (4)

89

4.2.6

Synthesis of (μ-Br)(μ-O)(μ-aoa)[Re2IVBr2(oiPr)(PPh3)2] (5)

90

4.2.7

Synthesis of [Re(CO3)(amef)(H2O)] (6)

90

4.2.8


90

4.3 Results and discussion

91

4.3.1

Salicylimine-orotic acid (H2soa)

91

4.3.2

(μ-Br)(μ-O)(μ-aoa)[Re2IVBr(OEt)2(PPh3)2] (1)

92

4.3.3

[ReBr(aoa)(apd)(PPh3)2] (2)

97

4.3.4

[ReI(coa)(PPh3)2]I (3)

102

4.3.5

[ReOCl(soa)(PPh3)2] (4)

106

4.3.6

(μ-Br)(μ-O)(μ-aoa)[Re2IVBr2(oiPr)(PPh3)2] (5)

109

4.3.7

[Re(CO3)(amef)(H2O)] (6)

114

4.4 Conclusion

118

4.5 References

119

Chapter 5 Dimeric Rhenium(IV) and Monomeric Rhenium(III) and (V) Complexes of 6-Hydroxypicolinic Acid 5.1 Introduction

128

5.2 Experimental

130

5.2.1

Synthesis (μ-O)(μ-hpa)2[ReIV2Br(OEt)(PPh3)2] (1)

5.2.2

Synthesis of (μ-Cl)(μ-O)(μ-hpa)[Re2IVCl2(OEt)(PPh3)2] (2) and


130

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[ReIIICl2(Hhpa)(PPh3)2] (3)

131

5.2.3

Synthesis of cis-[ReOX2(Hhpa)(PPh3)] (X = Br(4), Cl(5))

131

5.2.4

Synthesis of [ReIIIBr2(Hhpa)(PPh3)2] (6)

132

5.2.5

Synthesis of (μ-O)(μ-hpa)2[ReIV2I(OEt)(PPh3)2] (7)

132

5.2.6

Synthesis of (μ-Br)(μ-O)(μ-hpa)[Re2IVBr2(OEt)(PPh3)2] (8)

132

5.2.7


133

5.3 Results and discussion 5.3.1

133

(μ-O)(μ-hpa)2[ReIV2I(OEt)(PPh3)2] (1) and (μ-Cl)(μ-O)(μ-hpa)[Re2IVCl2(OEt)(PPh3)2] (2)

133

5.3.2

cis-[ReOX2(Hhpa)(PPh3)] (X = Br(4), Cl(5))

139

5.3.3

cis-[ReX2(Hhpa)(PPh3)2] (X = C(3), Br(6))

147

5.3.4

(μ-O)(μ-hpa)2[ReIV2I(OEt)(PPh3)2] (7)

152

5.3.5

(μ-Br)(μ-O)(μ-hpa)[Re2IVBr2(OEt)(PPh3)2] (8)

156

5.4 Conclusion

158

5.5 References

159

Chapter 6 Rhenium(I) and (V) Complexes with Monodentate S-donor and Bidentate S,N-donor Ligands 6.1 Introduction

170

6.2 Experimental

174

6.2.1

Synthesis of 4-phenyl-5-(pyridin-2-yl)-1-((pyridin-2-yl)methyl)-1H imidazole-2(3H)-thione (Hppit)

174

6.2.2

Synthesis of [Re(CO)3(ppit)]2 (1)

175

6.2.3

Synthesis of (n-Bu4N)[ReO(dtz)2] (2)

175

6.2.4

Synthesis of [Re(CO)3Cl(dptu)2] (3)

176

6.2.5

Synthesis of [Re(CO)3(Hdtz)(dttz)] (4)

176

6.2.6

Synthesis of (μ-Cl)2[Re(CO)3(ptz)]2 (5)

176

6.2.7

Synthesis of [ReO(dtz)(tmmb)] (6)

177

6.2.8


177



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6.3.1 4-phenyl-5-(pyridin-2-yl)-1-((pyridin-2-yl)methyl)-1H imidazole-2(3H)-thione (Hppit)

177

6.3.2

[Re(CO)3(ppit)]2 (1)

180

6.3.3

(n-Bu4N)[ReO(dtz)2] (2)

183

6.3.4

[Re(CO)3Cl(dptu)2] (3)

186

6.3.5

[Re(CO)3(Hdtz)(dttz)] (4)

189

6.3.6

(μ-Cl)2[Re(CO)3(ptz)]2 (5)

193

6.3.7

[ReO(dtz)(tmmb)] (6)

196

6.4 Conclusion

200

6.5 References

201

Chapter 7 Coordination Mode of Aroylhydrazone-Based Ligands to Rhenium(I) and (V) 7.1 Introduction

214

7.2 Experimental

216

7.2.1

Synthesis of [Re(CO)3Cl(Hdpa)] (1)

216

7.2.2

Synthesis cis-[ReOCl2(Hhtmb)(PPh3)] (2)

217

7.2.3

Synthesis of [Re(CO)3Br(H2hpmb)] (3)

217

7.2.4

Synthesis of [ReO(Hhtmb)(hieb)] (4)

218

7.2.5

Synthesis of cis-[ReOBr2(Hhpb)(PPh2)3] (5)

218

7.2.6

Synthesis of (μ-O)[ReO(pbz)2]2 (6)

218

7.2.7


219


219

7.3.1

[Re(CO)3Cl(Hdpa)] (1)

219

7.3.2

cis-[ReOCl2(Hhtmb)(PPh3)] (2)

224

7.3.3

[Re(CO)3Br(H2hpmb)] (3)

228

7.3.4

[ReO(Hhtmb)(hieb)] (4)

232

7.3.5

cis-[ReOBr2(Hhpb)(PPh2)3] (5)

237

7.3.6

(μ-O)[ReO(pbz)2]2 (6)

241

7.4 Conclusion Nelson Mandela Metropolitan University

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J.Mukiza

7.5 References

246

Chapter 8 Rhenium Complexes of (Pyridin-2-yl)methanol Derivatives, 2Aminopyrimidine and its Schiff Base Derivatives 8.1 Introduction

259

8.2 Experimental

262

8.2.1

Synthesis of cis-[ReOCl2(epm)(PPh3)] (1)

262

8.2.2

Synthesis of cis-[ReOCl2(mpm)(PPh3] (2)

263

8.2.3

Synthesis of cis-[ReOCl2(iapm)(PPh3)] (3)

263

8.2.4

Synthesis of [Re(CO)3(mpm)2] (4)

263

8.2.5

Synthesis of cis-[ReOBr2(ispm)(PPh3)] (5)

264

8.2.6

Synthesis of [Re(CO)3(hpbo)]2 (6)

264

8.2.7

Synthesis of [Re(CO)3Cl(pmpa)] (7)

264

8.2.8

Synthesis of [Re(CO)3Cl(amp)2] (8)

265

8.2.9

Synthesis of trans-[ReOI2(pppa)] (9)

265

8.2.10 X-ray crystallography 8.3 Results and discussion

266 266

8.3.1

cis-[ReOCl2(epm)(PPh3)] (1) and cis-[ReOCl2(mpm)(PPh3)] (2)

266

8.3.2

cis-[ReOCl2(iapm)(PPh3)] (3)

271

8.3.3

[Re(CO)3(mpm)2] (4)

275

8.3.4

cis-[ReOBr2(ispm)(PPh3)] (5)

278

8.3.5 [Re(CO)3(hpbo)]2 (6)

281

8.3.6

[Re(CO)3Cl(pmpa)] (7)

284

8.3.7

[Re(CO)3Cl(amp)2] (8)

287

8.3.8

trans-[ReOI2(pppa)] (9)

290

8.4 Conclusion

294

8.5 References

295

Chapter 9 Conclusion and Future work


309

vi

Acknowledgements

J.Mukiza

Acknowledgements I would like to express my sincere gratitude to my supervisor, Prof. T.I.A. Gerber, for giving me the opportunity to conduct my research under his supervision and for his time, encouragement, professional guidance and support throughout in this study. I am also grateful to Dr. E. Hosten and Dr. Richard Betz for assistance with the crystallographic analysis. Many thanks to Prof. Zenixole Tshentu for his helpful thoughts about Rhenium Chemistry. Many thanks to the Rhenium Chemistry Group at Nelson Mandela Metropolitan University: Dr. K.C. Potgieter, Dr. N.C. Yumata, G. Habarurema and X. Schoultz for their collaboration and sharing thoughts with me about Rhenium Chemistry. I would like to acknowledge H. Schalekemp, P. Gaika and V. Maqoko for technical assistance. I am grateful to the Lanthanide Chemistry Group at Nelson Mandela Metropolitan University: Dr. A. Abrahams, L.C. Coetzee, M. Kwakhanya, T. Madanhire and M. Cameron for their advice and collaboration during this study. Many thanks to the other colleagues in both the Organic and Inorganic Chemistry Laboratories at Nelson Mandela Metropolitan University for their assistance. I am also grateful towards the Nelson Mandela Metropolitan University, the South African National Research Foundation (NRF) and the Republic of Rwanda for funding. I would like to acknowledge the Rwandan Embassy in South Africa for foreign affairs assistance and valuable advice during our stay in the Republic of South Africa. I would like to express my appreciation towards my mother, brothers, sisters and other family members and all of my friends for their love, support, encouragement and inspiration. Many thanks to God for being my protector, guide and strength.


vii

Abstract

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Abstract The chemistry of rhenium has received considerable interest due to its versatility in various catalytic applications, fixation and especially the potential use of

186

Re and

188

Re radionuclides in nuclear medicine. This study investigates the synthesis and

characterisation of rhenium complexes with potentially multidentate ligands containing the amino, imino, hydroxyl and thiol groups. It reports new rhenium complexes in the +1, +3, +4 and +5 oxidation states, which display structural diversity, from monomers to ligand-bridged dimers as well as metal-metal multiply bonded dimers. The reaction of orotic acid (H2oa) and 2-mercapto-orotic acid (H2moa) with trans[ReOX3(PPh3)2] (X = Cl, Br) were studied and led to the formation of ligand-bridged dimers with metal-metal multiple bonds i.e. (µ-Br)(µ-O)(µ-oa)[Re2IVBr(OEt)2(PPh3)2], (μ-X)(µ-O)(µ-oa)[Re2IVX2(OiPr)(PPh3)2]

and

(µ-Cl)(µ-O)(µ-moa)2[Re(PPh3)]2. The

reaction of H2oa with [ReO2(py)4]Cl, [Re(dab)Br3(PPh3)2]

(H2dab = 1,2-

diaminobenzene) and [Re(CO)5Cl] were also studied and monomeric complexes [ReO(py)2(OEt)(oa)], [Re(dab)Br(oa)(PPh3)2] and (Ph4P)[Re(CO)3(H2O)(oa)] were isolated. The treatment of 5-amino-orotic acid (H2aoa) with [ReOBr3(PPh3)2] led to dimers with metal-metal triple bonds ReIV≡ReIV i.e. (µ-Br)(µ-O)(µ-oa)[Re2IVBr(OEt)2(PPh3)2], (μBr)(µ-O)(µ-oa)[Re2IVBr2(OiPr)(PPh3)2],

as

well

as

the

monomer

[ReV(apd)Br(aoa)(PPh3)2] (apd2− = 5-imidopyrimidine-2,4-dione). The chelating ligand 5-aminopyrimidine-2,4-dione (H2apd)

was formed by oxorhenium(V)-catalysed

decarboxylation of 5-amino-orotic acid (H2aoa) (see Scheme 1). The reaction of the Schiff base derivative of 5-amino-ortic acid, salicylimine-orotic acid (H2soa), with trans-[ReOI2(OEt)(PPh3)2] in ethanol was also investigated and led to the formation of

the

rhenium(III)

complex

salt

[Re(coa)I(PPh3)2]I

[Hcoa

=

5-(2-

hydroxybenzylideneamino)pyrimidine-2,4(1H,3H)-dione]. The chelating Hcoa is also formed from the oxorhenium(V)-catalysed decarboxylation of H2soa and coordinates to the rhenium(III) ion as a monoanionic tridentate N,O,O-donor chelate via the phenolate and ketonic oxygens, and the imino nitrogen atom.


viii

Abstract

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However, decarboxylation of H2soa was not observed in its reaction with [ReOCl3(PPh3)2], which led to the isolation of [ReOCl(soa)(PPh3)]. The reaction of the carboxamide derivative of 5-aminoorotic acid, 5-(5-aminopyrimidine-2,4(1H,3H)dioxamido)-1,2,3,6-tetrahedro-2,6-dioxopyrimidine-4-carboxylic acid (H2ampa) with [Re(CO)5Cl] in ethanol led to the formation of a zwitterionic rhenium(I) complex [Re(CO)3(H2O)(amef)] [amef = {5-(5-ammoniumpyrimidine-2,4(1H,3H)-dioxamido)1,2,3,6-tetrahedro-2,6-dioxopyrimidine-4-ethylformate}]. The chelating ion amef was formed from the combined tricarbonylrhenium(I)-catalysed esterification and aminoprotonation of H2ampa (see Scheme 1) and coordinates to the fac-[Re(CO)3]+ core as a dianionic bidentate N,N-donor chelate via the amido nitrgens. O

OH NH2

HN

NH2

HN trans-[ReOBr3(PPh3)2]

+ CO2 O

O

N H

O

N H

O

H2apd

H2aoa O

H N

HO

O

H N

O NH

trans-[ReOI2(OEt)(PPh3)2]

N

NH

+ CO2

O

N

OH

O

Hcoa

OH O O

OH

H2N

HN O N H

O

O NH

H N

O

O

+

H2soa

N H

O

Reflux, 24 h

NH

H N

[Re(CO)5Cl] / EtOH HN O

H3N

O O

N H

O

O

N H amef

H2ampa

Scheme 1: Rhenium-catalysed decarboxylation, esterification and amino-protonation of the 5-aminoorotic acid and its derivatives observed in this study


ix

Abstract

J.Mukiza

The doubly and singly ligand-bridged dimeric rhenium(IV) complexes with a metalmetal triple bond ReIV≡ReIV, i.e. (µ-O)(µ-hpa)2[ReIV2X(OEt)(PPh3)2] (X = Br; I) and (µX)(µ-O)(µ-hpa)[ReIV2X2(OEt)(PPh3)2] (X = Cl, Br), in which the H2hpa is

6-

hydroxypicolinic acid, were obtained from the reaction of H2hpa with [ReOX3(PPh3)2] and cis-[ReO2I(PPh3)2] in ethanol. For these complexes, the H2hpa acts as a dianionic bridging and tridentate chelate. The monomeric oxorhenium(V) complexes cis-[ReOX2(Hhpa)(PPh3)] were the only products formed by the reaction of [ReOX3(PPh3)2] with H2hpa in acetonitrile. The complex [ReIIIBr2(Hhpa)(PPh3)2] was obtained from the reaction of [ReOBr3(PPh3)2] with H2hpa in 2-propanol, while [ReIIICl2(Hhpa)(PPh3)2],

was

isolated

together

with

(µ-Cl)(µ-O)(µ-

hpa)[ReIV2Cl2(OEt)(PPh3)2], from the reaction of trans-[ReOCl3(PPh3)2] with H2hpa in ethanol. For both cis-[ReOX2(Hhpa)(PPh3)] and [ReIIIX2(Hhpa)(PPh3)2] the H2hpa acts as a monoanionic bidentate chelate and coordinate to rhenium via the carboxylate oxygen and pyridinic nitrogen atoms. The reactivity of dithizone H2dtz (see Figure 1) with (n-Bu4N)[ReOCl4] and [Re(CO)5Cl] in ethanol was investigated, and the complexes (n-Bu4N)[ReO(dtz)2] and [Re(CO)3(Hdtz)(dttz)] (dttz = 2,3-diphenyl-5-thione-tetrazolium zwitterions) (see Figure 1) were obtained. Dttz was derived from the double deprotonation of dithizone which led to the cyclisation in dttz. The reaction of dithizone with [ReOBr3(PPh3)2] in presence of

KSCN led to the isolation of

[ReO(dtz)(tmmb)] (Htmmb =

triphenylphosphazenomethinimino-N-mercaptobenzenamine) (see Figure 1). The Htmmb was derived from the decomposition of H2dtz in which one of its decomposition products reacted with KSCN and triphenylphosphine. S

N

N

S N

N H

H N

N

N N

NH N

H2dtz

H N

P S

dttz Htmmb

Figure 1: Structures of H2dtz, dttz and Htmmb


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Abstract

J.Mukiza

The H2dtz in [ReO(dtz)(tmmb)] acts as a dianionic bidentate N,S-donor chelate and coordinates to oxrhenium(V) via the thiolic sulfur and hydrazinic nitrogen. The dttz in [Re(CO)3(Hdtz)(dttz)] acts as a neutral monodentate ligand coordinating to the fac[Re(CO)3]+ core via the thionic sulfur atom, and H2dtz as a monoanionic bidentate chelate via the azoic nitrogen and thiolic sulfur atoms. Rhenium(I) complexes [Re(CO)3(ppit)]2

(Hppit

imidazole-2(3H)thione,

= see

4-phenyl-5-(pyridin-2-yl)-1-((pyridin-2-yl)methyl)-1HFigure

2),

[Re(CO)3Cl(dptu)2]

(dptu

=

1,3-

diphenylthiourea, see Figure 2) and (μ-Cl)2[Re(CO)3(ptz)]2 (ptz = phenothiazine, see Figure 2) were also synthesised from the reaction of [Re(CO)5X] (X = Cl; Br) with a two-fold molar excess of the ligand in toluene.

HN N

N H

N N

H N

S

S

N H S

dptu

Hppit

ptz

Figure 2: Structures of Hppit, dptu and ptz The coordination modes of aroylhydrazone-based ligands such as (propan-2ylidene)benzohydrazide (Hdpa),

(Hpbz),

di-2-pyridyketone-2-aminobenzoylhydrazone

2-hydroxy-N'-(propan-2-ylidene)benzohydrazide

(H2hpb),

2-hydroxy-N'-

((thiophen-3-yl)methylene)benzohydrazide (H2htmb) and 2-hydroxy N’((pyridin-2yl)methylene)benzohydrazide (Hhpmb) (see Figure 3) to the fac-[Re(CO)3]+, [ReO]3+ and [ReO3]4+ cores were studied. The reaction of Hhdpa and H2hpmb with [Re(CO)5X] in ethanol led to the isolation the complexes [Re(CO)3Cl(Hdpa)] and [Re(CO)3Br(H2hpmb)].

The

O)[ReO(pbz)2]2

isolated

were

complexes from

the

and

(μ-

[ReOBr3(PPh3)2]

with

cis-[ReOBr2(Hhpb)(PPh3)] reaction

of

salicylhydrazide (Hshz) and [ReO2(py)4]Cl with benzohydrazide (Hbhz) in acetone respectively. The reaction of [ReOI2(OEt)(PPh3)2] with H2htmb in acetonitrile led to the decomposition of the ligand, with one part reacting with acetonitrile leading to the new ligand 2-hydroxy-N’-(1-iminomethyl)benzohydrazide (H2hieb) in the complex


xi

Abstract

J.Mukiza

[ReO(Hhtmb)(hieb)]. However, the same reaction with trans-[ReOCl3(PPh3)2] as a starting material led to the complex cis-[ReOCl2(Hhtmb)(PPh3)].

O N N H X

R2

S X = OH, R1 =

,R =H 2

H2htmb

R1 N

X = NH2 , R1 / R2 =

Hdpa

X = H, R1 / R2 = CH3

Hpbz

N X = OH, R1 =

, R2 = H H2hpmb

: R / R = CH X = OH, 1 2 3

H2hpb

Figure 3: The various aroylhydrazone-based ligands used in this study The Schiff base N-((pyridin-2-yl)methylene)pyrimidin-2-amine (pmpa) have shown instability in alcohols and ketone. Depending on the type of alcohol and ketone used, the decomposition of pmpa results in various (pyridin-2-yl)methanol derivatives (see Figure 4) and their rhenium(V) and (I) complexes such as cis-[ReOCl2(epm)(PPh3)] (Hepm

=

ethoxy(pyridin-2-yl)methanol),

methoxy(pyridin-2-yl)methanol),

cis-[ReOCl2(mpm)(PPh3)]

cis-[ReOCl2(iapm)(PPh3)]

(Hmpm

(Hiapm

= =

(isopropylamino)(pyridin-2-yl)methanol), [Re(CO)3(mpm)]2, cis-[ReOBr2(ispm)(PPh3)] (Hispm = isopropyloxy(pyridin-2-yl)methanol) and [Re(CO)3(hpbo)]2 (Hhpbo = 4hydroxy-4-(pyridin-2-yl)butan-2-one) were isolated from the reaction solution. These ligands act as monoanionic bidentate N,O-donor chelates and coordinate to rhenium via the alcoholate oxygen and pyridinic nitrogen atoms. The reaction of 2-aminopyrimidine amp (see Figure 5) and its Schiff base N-(-3(pyrimidin-2-ylimino)propylidene)pyrimidin-2-amine) (pppa) (see Figure 5) with [Re(CO)5Cl] and trans-[ReOI2(OEt)(PPh3)2] were investigated. The octahedral complex [Re(CO)3Cl(amp)2] was obtained from the reaction of amp with [Re(CO)5Cl] in toluene. The seven-coordinated pentagonal bipyramidal oxorhenium complex [ReVOI2(pppa)] was obtained from the reaction of trans-[ReOI2(OEt)(PPh3)2] with amp in 2-propanol in presence of triethylamine. The pppa ligand was formed by a combined oxorhenium(V)-catalysed condensation and dehydroxylation of amp and 2-propanol. Nelson Mandela Metropolitan University

xii

Abstract

J.Mukiza N N

O

O N

pmpa

N

N OH Hmpm

N

OH

Hepm

O HN

O +

N Hhpbo

OH

N OH Hiapm

N OH Hispm

Figure 4: Structures of pmpa and (pyridin-2-yl)methanol derived from its decomposition in alcohol and ketone

N

NH2 N

N

N

N

N N

N pppa

amp

Figure 5: Structures of amp, and Schiff base ion pppa

Keywords: Rhenium, ligand-bridged dimers, aroylhydrazones, dithizone, orotic acid, 2-mercapto-orotic acid, 6-hydroxypicolinic acid, Schiff base, metalmetal triple bond.


xiii

List of Publications

J.Mukiza

List of Publications The following publications emanated from this work. [1]

J. Mukiza, E.C. Hosten, T.I.A. Gerber, Dimeric rhenium(IV) and monomeric rhenium(I) and (V) complexes of orotic acid, Polyhedron 98 (2015) 251–258.

[2]

J. Mukiza, T.I.A. Gerber, E. Hosten, F. Taherkhani , M. Nahali, A (μ-O)(μBr)ReIV2 metal–metal triple bond complex with a bridging tridentate ligand: Synthesis, structure and DFT study, Inorg. Chem. Commun. 49 (2014) 5–7.

[3]

J. Mukiza, T.I.A. Gerber, E. Hosten, Decarboxylation of 5-amino-orotic acid and a Schiff base derivative by rhenium(V), Inorg. Chem. Commun., 57 (2015) 54− 57.

[4]

J. Mukiza, T.I.A. Gerber, E. Hosten, The reaction of dithizone with the ReO3+ core. Formation of a mixed double complex salt containing a coordinated phosphazenohydrazide, Inorg. Chem. Commun.,47 (2014), 164–167.

[5]

J. Mukiza, T.I.A. Gerber, E. Hosten, Coordination of Bidentate AroylhydrazoneBased N,N-Donor Ligands to the fac-[Re(CO)3]+ Core, J. Chem. Crystallogr. 44 (2014), 368–375.

[6]

J. Mukiza, T.I.A. Gerber, E. Hosten, A.S. Ogunlaja, F. Taherkhani, M. Amini M. Nahali, Trans,trans,trans-[ReO2I2(PPh3)2], a rare rhenium(VI) complexSynthesis and DFT study, Inorg. Chem. Commun., 51 (2015) 83–86.

[7]

Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of ammonium 5-aminoorotate monohydrate, C5H10N4O5, Z. Kristallogr. NCS, 229 (2014), 359-360.

[8] Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of 4-phenyl-5-(pyridin-2-yl)-1-((pyridin-2-yl)methyl)-1H-imidazole-2(3H)thione, C20H16N4S, Z. Kristallogr. NCS, 229 (2014), 335-336. [9]

Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of bis(4,5-diaza-fluoren-9-one-К2N,N)dioxo-μ2oxodirhenium(V)-acetonitrile(1:2), C26H18Cl4N6O5Re2, Z. Kristallogr. NCS, 229 (2014) 341-342.


xiv

List of Publications [10]

J.Mukiza

Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of (μ2-acetato-К2O,O’)-(μ2-chlorido)-(μ2-oxido)bis(dichlorido-triphenylphosphane-rhenium(IV)), C38H33Cl5O3P2Re2, Z. Kristallogr.NCS, 230 (2015) 283-285.

[11]

Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of (μ2-acetato-К2O,O’)-(/2-bromido)-(μ2-oxido)tetrabromido-triphenylphosphane-P-К-triphenylphosphaneoxide-К-Odirhenium(IV)), C38H33Br5O4P2Re2, Z. Kristallogr. NCS 230 (2015) 177-179.

[12]

Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of trans-((2-aminophenolato-КO)-(2-hydroxyphenylimidoК2N,O)-bromidobis(triphenylphosphane)-rhenium(V))bromide-acetonitrilewater (1:2:1), C52H49Br2N4O3P2Re, Z. Kristallogr. NCS 230 (2015) 165-167.

[13]

Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of trans-dibromido-ethoxido-oxidobis(triphenylphosphane)-rhenium(V)–an orthorhombic poymorph, C38H35Br2O2P2Re, Z. Kristallogr. NCS 230 (2015) 141-144.

[14]

Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of tricarbonylchlorido-(N-(pyridine-2-yl)-N-((4-tertbutyl) benzoyl)pyridine-2-amine)rhenium(I) C24H21ClN3O4Re, Z. Kristallogr. NCS 230 (2015) 44-46.

[15]

Janvier Mukiza, Thomas I. A. Gerber, Eric C. Hosten and Richard Betz, Crystal structure of chlorido-tricarbonyl-bis(2-pyridylmethanone-N,N’)rhenium(I) C14H8ClN2O4Re, Z. Kristallogr. NCS, 230 (2015) 341-343.


xv

Crystallographic data Supplementary data for the crystal structures of the ligands and their corresponding rhenium complexes were determined and stored on the compact disk that is included in this thesis (attached to the inside back cover). These data include the: (i) Full crystal data and details of the structure determinations; (ii) Final coordinates and equivalent isotropic displacement parameters of the nonhydrogen atoms; (iii) Hydrogen atoms positions and isotropic displacement parameters; (iv) Isotropic displacement parameters; (v) All bond distances and bond angles; (vi) Torsion angles; (vii) Contact distances; (viii) Hydrogen-bonds.

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Chapter 1 Introduction 1.1. General Background Coordination chemistry is of interest and plays a considerable role in the development of new target-specific radiopharmaceuticals [1]. In a similar way, the coordination chemistry of rhenium has been recently well explored due to the potential therapeutic applications of the

186

Re and

188

Re radionuclides in nuclear

medicine [1, 2]. Rhenium ([Xe] 4f14 6s2 5d5) is one of the rarest elements and occur in low abundance (0.7 ppb) in the earth’s crust [3, 4]. It occurs naturally as a mixture of the two non-radioactive isotopes 185Re (37.4 %) and 187Re (62.6 %). The suitability of

186

Re and

188

Re in therapeutic nuclear medicine is related to their

favourable nuclear properties [5]. Both isotopes are radiochemically active and they are both γ and β-emitting agents [4]. Their relative half life times are 90 h and 17 h, β-emission energies are 1.07 MeV and 2.12 MeV, and their γ-emission energies are 137 keV and 155 keV respectively [1, 6]. Rhenium is located in Group VII on the Periodic Table and is probably the most versatile of all the transition metals [7]. Due to its location in the middle of the d-block of transition metals, it exhibits the properties of both early and later transition metals. Consequently, it displays a variety of coordination compounds in all oxidation states from -1 to +7 [7]. The low oxidation states of rhenium (as for other transition metals) are stabilised by π-acceptor ligands including CO, PR3 and RCN via π-back bonding, while the stabilisation of high oxidation states necessitates π-donor ligands including oxo, imido, sulfido and nitrido based ligands [8]. Rhenium complexes are thermodynamically and kinetically stable and show a variety of structural diversity in which the monomers, ligand-bridged dimers, metal-metal bonded species and clusters are known [7, 9]. The chemistry of rhenium and technetium are similar to each other. The 99mTc radionuclide is a γ-emitter and also finds application in nuclear medicine, specifically in the diagnostic imaging of various organs [4]. The similar physical properties of rhenium and technetium, particularly displaying the same photo-emission energy, are of interest in nuclear medicine. These enable the


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monitoring of the biodistribution of radiopharmaceuticals based on these metals using the same γ-ray camera [10]. Another advantage is that the analogous rhenium and technetium complexes are expected to share the same biodistribution pattern in the body [11].

1.2. Aim and Motivation of Project The coordination chemistry of rhenium has attracted interest due to the potential application of the

186/188

Re radionuclides in radiotherapy [12]. Ligands containing

hydroxyl, thiol and amino groups stabilize rhenium in various oxidation states [10]. Rhenium complexes, particularly in the +1 (d6) and +5 (d2) oxidation states, have shown greater stability and good biodistribution in therapeutic nuclear medicine [13]. Also, transition metals complexes with multidentate ligands containing the imino group (Schiff bases) are of interest due to their biological, antifertility, catalytical and enzymatic activities [14]. The multidentate Schiff base ligands have shown a chelating ability to rhenium, and the resulting complexes have been extensively explored for the development of therapeutic radiopharmaceuticals [15]. The aim of this project is the synthesis, spectroscopic, electrochemical and crystallographic characterisation of rhenium complexes in various oxidation states with potentially multidentate ligands containing the amino, imino, hydroxyl and thiol groups. These ligands, especially those containing the amino group, are advantageous in coordination chemistry since they can be easily functionalised and derivatised [16]. This project will lead to new rhenium complexes and contribute to the coordination chemistry of the metal, particularly in the +1, +3, +4 and +5 oxidation states. The metals technetium [Kr] 4d5 5s2) and rhenium have comparable atomic radii and consequently, their chemistry is very similar to each other and homologous complexes display the same coordination parameters [17]. Hence the coordination chemistry of rhenium is of interest since it can be successfully used for modelling the technetium coordination chemistry [17].


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1.3. General Chemistry of Rhenium As previously reported, rhenium compounds may exist in large number of oxidation states from -I to +VII, and each of them is represented by a considerable number of species available in the literature [18, 19]. 1.3.1. Rhenium(-I) The chemistry of rhenium(-I) is relatively undeveloped and is mainly represented by rhenium carbonyl clusters [20]. Rhenium(-I) has a d8 electronic configuration and its most well-known compound is the anion [Re(CO)5]−, obtained from the reduction of [Re2(CO)10] with Na/Hg [21]. The [Re(CO)5]− anion adopts a trigonal bipyramidal geometry, as proved by the crystal structure of the complex [Yb(THF) 6][Re(CO)5]2 [21]. 1.3.2. Rhenium(0) Rhenium(0) adopts a d7 electronic configuration and may exist either in monomeric or dimeric form; the latter mostly with a Re−Re single bond. Rhenium(0) complexes are rarest and limited to carbonyl derivatives [22]. The most well-known rhenium(0) compound is the dimeric [Re2(CO)10] in which the two pentacarbonylrhenium moieties are bonded together via a Re−Re single bond [22]. The complex [Re2(CO)10] can be obtained from carbonylation of [Re 2O7] or reduction of [ReCl5] or [K2ReCl6] under CO pressure [23]. The carbonyl ligands of [Re2(CO)10] are labile and under thermal or photolytic conditions, they can easily be replaced by π-acceptor ligands like phosphines, arsines, silanes, pyridines and isocyanides [22]. Cleavage of the Re−Re single bond in [Re2(CO)10] may occur photochemically leading to the pentacarbonyl radicals [Re(CO)5]∙ [24]. 1.3.3. Rhenium(I) Rhenium(I) is an electron-rich species and consequently its stabilization requires πacidic ligands such as carbonyls, isonitriles, phosphines, and nitrosyls [25]. It exhibits a d6 electronic configuration and forms a variety of low-spin octahedral complexes, and most of them are those based on the fac-[Re(CO)3]+ core. The fac-[Re(CO)3]+ core has a small size which allows flexibility and this makes it most promising for Nelson Mandela Metropolitan University

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labelling a variety of low molecular weight biomolecules [16, 26]. In addition, the radiopharmaceuticals based on this core are of interest due to its high stability in water compared to those based on the [ReO]3+ core, which are prone to oxidation to perrhenate [ReO4]− in water [27]. Rhenium(I) complexes are prepared in aqueous solution and the important precursors

are

[Re(CO)5X]

(X

=

Cl,

Br),

[Re(H2O)3(CO)3]+

and

(μ-X)2

[Re(CO)3(THF)]2 (X = Cl, Br) [25, 26]. Rhenium(I) complexes based on the fac[Re(CO)3]+ core exhibit thermodynamic and kinetic stability leading to a broad application of them in radioimaging, radiotherapy and luminescence [16, 26]. The precursor complex [Re(H2O)3(CO)3]+ can be synthesised by reduction of perrhenate [ReO4]- with sodium borohydride in aqueous solution in the presence of carbon monoxide [25]. Another method was developed for the synthesis of [Re(H2O)3(CO)3]+ which consists of using K2[H3BCO2] as both a reducing agent and source of carbon monoxide [28]. The aqua ligands of [Re(H2O)3(CO)3]+ are labile and can easily be replaced by a variety of multidentate ligands including amines, amides, thioethers, imines, thiols, carboxylate and phosphines [28]. The precursor [Re(CO)5X] (X = Cl, Br) is synthesised by oxidative cleavage of [Re2(CO)10] with X2 [29]. The dimeric precursor (μ-X)2[Re(CO)3(THF)]2 is easily obtained by the refluxing of [Re(CO)5X] in THF, and it has no intermetallic Re−Re bond, but two Re atoms bridged by halide ligands [26, 30]. 1.3.3.1 Coordination chemistry of rhenium(I) The coordination chemistry of rhenium(I) is dominated by the fac-[Re(CO)3]+ core. The fac-[Re(CO)3]+ core is well known and the most important rhenium(I) core in which the three CO ligands are facially arranged with three vacant coordination sites. It can coordinate to a variety of mono- and multidentate ligands, including amines, amides, thioethers, imines, thiols, carboxylates and phosphines resulting either in monomeric or dimeric complexes with a distorted octahedral geometry around the rhenium(I) center [16]. (a) N,O-donor ligands The derivatives of pyridine are an important class of ligands to the fac-[Re(CO)3]+ core [31]. The reaction of the bidentate N,O-donor chelate 2-acetylpyridine (acpy) Nelson Mandela Metropolitan University

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Chapter 1

J. Mukiza

with (Et4N)2[ReBr3(CO)3] in cold methanol leads to the complex [Re(CO)3Br(acpy)] (Figure 1.1) The acpy ligand is coordinated to the fac-[Re(CO)3]+ core via the neutral ketonic oxygen and pyridinic nitrogen [31].

Br N

OC Re OC

O CO

CH3

Figure 1.1: Structure of [Re(CO)3Br(acpy)] The glucosamine conjugates with N,N,O-donor atoms are known to be potential imaging agents utilizing the fac-[M(CO)3]+ core (M = Re and Tc) and their ability of binding tricarbonyl core was initially proved using the cold surrogate fac-[Re(CO)3]+ [32]. The reaction of 2-(2-(2-hydroxybenzyl)-2-(pyridine-2-ylmethyl)diaminoethyl)-2deoxy-β-D-glucopyranose (Hhddg) with an equivalent amount of [Re(H2O)3(CO)3]Br in boiling ethanol was investigated and led to the isolation of distorted octahedral complex [Re(CO)3(hddg)] (Figure 1.2) [32]. The deprotonated Hhddg ligand acts as a tridentate N,N,O-donor chelate coordinating to the fac-[Re(CO)3]+ core via the phenolic oxygen, and amino and pyridinic nitrogens. OH O

HO HO

OH

HN

N N

O Re CO

OC CO

Figure 1.2: Structure of [Re(CO)3(hddg)]


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J. Mukiza

(b) N,N-donor ligands The complexes based on the fac-[Re(CO)3]+ core bearing the bidentate N,N-donor chelate 2,4,7,9-tetraphenylphenanthroline are of interest in nuclear medicine, especially when the 2,9-phenyl groups have been either meta or para-CF3 substituted. They have been identified as screening agents against a selection of HeLa and A549 cancer cell lines [33]. The reaction of the meta substituted derivative 2,9-bis(3-(trifluoromethyl)phenyl)-4,6-diphenyl-1,10-phenanthroline

(tpdp)

with

[Re(CO)5Cl] in toluene has been investigated and led to the distorted octahedral complex [Re(CO)3Cl(tpdp)] (Figure 1.3) [33]. The tpdp ligand coordinated to the fac[Re(CO)3]+ core as N,N-donor chelate via both pyridinic nitrogen atoms.

Cl N

N Re

OC

CO CO

F

F F

F

F F

Figure 1.3: Structure of [Re(CO)3Cl(tpdp)] Most of the chelates containing three nitrogen donor atoms have shown the ability to chelate facially to the fac-[Re(CO)3]+ core. A typical example is N'-(2-(2(dimethylamino)ethylamino)ethyl)acetamidine (dae). The reaction of dae with [Re(CO)3(NCMe)3]BF4 in acetonitrile results in the cationic complex with a BF4− counterion

[Re(CO)3(dae)]BF4 (Figure 1.4) [34]. The dae ligand is facially

coordinated to rhenium and acts as a tridentate N,N,N-donor chelate.


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Chapter 1

J. Mukiza

NH2

NH H3C

N

H3C

N

BF4

Re

CH3

OC

CO CO

Figure 1.4: Structure of [Re(CO)3(dae)]BF4 (c) S,N-donor ligands Thiosemicarbazones are versatile S,N-donor ligands and coordinate to the fac[Re(CO)3]+ core either as neutral or in their deprotonated form, and the resulting complexes have been shown interesting biological activities [35]. The reaction of 4phenyl-1-(1-(pyridin-2-yl)ethylidene)thiosemicarbazide

(Hppet)

with

(Et4N)2[Re(CO)3Br3] in methanol leads to [Re(CO)3(ppet)] (Figure 1.5) [35]. The ligand is deprotonated and coordinated to the fac-[Re(CO)3]+ core as a tridentate N,N,S-donor chelate via imino and pyridinic nitrogens, and thiolic sulfur.

CH3 OC

N Re

OC CO

N N S HN

Figure 1.5: Structure of [Re(CO)3(ppet)] The coordination mode of tridentate ligands containing thiolic sulfur and amino and pyridyl nitrogen atoms to the fac-[Re(CO)3]+ core has been investigated [36]. These ligands are mostly deprotonated and act as tridentate N,N,S-donor chelates resulting in distorted octahedral complexes of the fac-[Re(CO)3]+ core. A typical example is the reaction of (Et4N)2[Re(CO)3Br3] with N,N-bis(mercaptoethyl)(aminomethyl)-2pyridine (Hmeap) in methanol which leads to the complex [Re(CO) 3(meap)] (Figure 1.6) [36].


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Chapter 1

J. Mukiza

CO CO

OC Re

N

S N HS

Figure 1.6: Structure of [Re(CO)3(meap)] (d) S,O and S-donor ligands Benzoylthiourea derivatives are known to form stable complexes with a large number of transition metals, including rhenium, and they exhibit various coordination modes [37, 38]. In the majority of their complexes with the fac-[Re(CO)3]+ core, they act either as bidentate S,O-donor or monodentate S-donor ligands [37]. The reaction of [Re(CO)5Br] with N-((diethyl)carbamothioyl)benzamide (Hdecb) in toluene leads to the isolation of the complex [Re(CO)3Br(Hdecb)] (Figure 1.7) [37]. Hdecb acts as neutral bidentate ligand and coordinates via the thionic sulfur and ketonic oxygen atoms.

CO O HN

Re

CO CO

S Br N

Figure 1.7: Structure of [Re(CO)3Br(Hdecb)] The similar reaction using 4-bromo-N-((diethyl)carbamothioyl)benzamide (Hbeb) leads to the isolation of [Re(CO)3Br(Hbeb)2] (Figure 1.8), in which Hbeb acts as a neutral monodentate ligand and coordinates to rhenium(I) via the thionic sulfur atom only [38].


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J. Mukiza

Br

Br O OC

CO

O

CO Re

HN

S

NH

S Br

N

N

Figure 1.8: Structure of [Re(CO)3Br(Hbeb)2] 1.3.4. Rhenium(II) Rhenium in formal oxidation state +II has a d5 electronic configuration and forms monomeric complexes, and often also dimeric complexes with a Re−Re triple bond [39]. Its compounds are relatively rare due to their instability and they often disproportionate to thermodynamically more stable rhenium(I) and rhenium(III) complexes [40]. The reduction of the electronic density on the metal centre is achieved through its coordination with π-acceptor ligands. Consequently, the coordination chemistry of rhenium(II) is governed by phosphines and other πacceptor ligands, including isocyanide derivatives and carbonyls [40]. Due to their paramagnetism and their open-shell electronic configuration, rhenium(II) complexes found application in magnetochemistry and catalysis [41, 42]. Rhenium(II) complexes based on the [Re2X4] (X = Cl, Br) core, in which the two rhenium atoms are triply bonded, have been reported in the literature [30]. A typical example is the complex (μ-dppm)2[Re2Cl4] (Figure 1.9) in which the bridging bis(diphenylphosphino)methane (dppm) is coordinated to the [Re 2Cl4] core [41, 43]. A few rhenium(II) carbonyl complexes have been described in the literature. Recently, Alberto and co-workers synthesised the rhenium(II) anion [ReIIBr4(CO)2]2− from

the

reduction

of

(Et4N)[ReIIIBr4(CO)2]

in

acetonitrile

[42].

The

salt

(Et4N)2[ReBr4(CO)2] is air stable and the bromide ligands are easily substituted by mono-, bi- and tridentate ligands resulting in the polynuclear complexes [Re(CO)2Br2(X)n], in which X is a neutral coordinated imidazole, pyridine or phenantroline [42]. These complexes are octahedral and show a relative stability Nelson Mandela Metropolitan University

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under aerobic conditions, both as solids and in solution [42]. Some of rhenium(II) complexes with aromatic and aliphatic amines are also described in the literature [42, 44].

P Cl Cl

Re P

P Re

Cl Cl

P

Figure 1.9: Structure of (μ-dppm)2[Re2Cl4] 1.3.5. Rhenium(III) Rhenium(III) has a d4 electronic configuration and the metal centre is effectively stabilised by σ-donor and π-acceptor ligands. A number of mononuclear derivatives in this oxidation state are reported in the literature [11, 12, 45, 46]. Most of these complexes are relatively stable, leading to an interest for their possible application as radiopharmaceuticals [45]. The chemistry of rhenium(III) is dominated by octahedral complexes of the general form [ReX2L4]+ and [ReX3L3] (L = phosphines, alkyl, aryl or arsines; X = Cl, Br). They are usually prepared by the reduction of the rhenium(V) precursors trans-[ReOX3L2] and trans-[ReOX2(OEt)L2] in the presence of reducing agents, to facilitate the removal of oxygen from the metal or by ligand substitution from trans-[ReX3(MeCN)(PPh3)2] [35]. The chemistry of trans-[ReX3(MeCN)(PPh3)2] and [ReCl3(benzil)(PPh3)] have been well explored as the main useful precursors of six-coordinated

rhenium(III)

[ReIIICl2(bat)(PPh3)2] and

complexes

[11,

45].

The

complexes

cis-

fac-[ReIIICl3(dpa)(PPh3)] were recently isolated by the

reaction of trans-[ReCl3(MeCN)(PPh3)2] with 1-phenyl-1,3-butadione (Hbat) and 2,2’dipyridylamine (dpa) under nitrogen in acetonitrile [45]. A variety of rhenium(III) polypyridyl complexes under different reaction conditions were isolated from [ReCl3(benzil)(PPh3)] [44].


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Five- and seven-coordinated rhenium(III) complexes are also known in the literature. The complexes [ReIII(terpy)2X]Y2 (X = OH, NCS, Cl; Y = PF6, SCN) are sevencoordinated and were isolated from the reduction of trans-[ReVO2(py)4)]+ by 2,2’:6,6’’terpyridine (terpy) [47]. The reaction of 2,6-bis(2-hydroxyphenyliminomethyl)pyridine (H2dhp) with cis-[ReO2I(PPh3)2] in ethanol also led to the seven-coordinated complex [ReIII(dhp)2]X (X = I, ReO4) [48]. The five-coordinated rhenium(III) complexes can be isolated

from

the

reduction/substitution

of

oxorhenium(V).

The

complex

III

[Re (mbm)3(PPh3)2] was isolated from the reaction of trans-[ReOCl3(PPh3)2] with pmethoxybenzyl-mercaptan (Hmbm) [48]. The few dinuclear rhenium(III) complexes with a Re−Re quadruple bond are also known. Cotton and co-workers isolated the complex (1,6-H3N(CH2)6NH3)[Re2Cl8] in which the two rhenium atoms in the [Re2Cl8]2− anion are quadruply bonded with a Re−Re bond distance of 2.2326(7) Å [49]. 1.3.6. Rhenium(IV) Rhenium in the +IV oxidation state adopts a d3 electronic configuration and is predominately found with σ-donor ligands, and with the resulting complexes both inert and stable [12]. The chemistry of rhenium(IV) is mostly represented by monomeric octahedral halides complexes of the type [ReX6]2−, [ReX4L2], [ReX4L] and [ReX5L]− [34], where L represents monodentate or bidentate neutral ligands with P (phosphines), O (ethers, ketones), N (isocyanides, bipyridine derivatives, isothiocyanate), S (thioethers) and As (arsines) as donor atoms [7, 19]. Rhenium(IV) can also form dimeric complexes in which Re−Re are triply bonded [7, 43]. In the presence of water, rhenium(IV) complexes tend to also form a Re═O bond, which is characteristic of the high oxidation states V, VI and VII [43]. Rhenium(IV) complexes are synthesized from the reduction of perrhenate in presence of HX (X = Cl, Br), leading to the formation of the [ReX6]2− anion [11]. [ReX6]2− is a useful precursor of other rhenium(IV) complexes in which the labile species (Et4N)[ReCl5] and [ReCl4(MeCN)2] are the intermediates in synthesis [19, 50]. Rhenium(IV) complexes can also be synthesised from the oxidation of mononuclear rhenium(III) compounds under more forcing conditions in presence of


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halocarbons [51]. A typical example is the complex [ReCl4(PPh3)2] which was isolated from the oxidation of [ReIIICl3(RCN)(PPh3)2] in carbon tetrachloride as shown by the reaction below [51]:

trans-[ReCl3(RCN)(PPh3)2] + CCl4

trans-[ReCl4(PPh3)2] + RCN +

1

2

C2Cl6

The [ReCl4(PPh3)2] may also be isolated as by-product from the reaction of trans[ReOX3(PPh3)2] with various carboxylic acid derivatives [51]. 1.3.7. Rhenium(V) One of the characteristic features of rhenium(V) is the existence of the d2 electronic configuration in a variety of diamagnetic and stable complexes. These complexes are based on various metal cores such as oxo [ReV=O]3+, dioxo [O═ReV═O]+, dinuclear oxo-briged [O═ReV−O−ReV═O]4+, nitrido [ReV≡N]2+, imido [ReV═NR]3+, sulfido [ReV═S]3+, amido [ReV−NHR]4+ and hydrazino [ReV(HxNNR)n]m+ and mostly display a distorted octahedral geometry around the rhenium centre [2, 52, 53]. These complexes are of the type [ReOX5]2−, [ReOX4L]−, [ReOX3L2], [ReO2X4]3−, [ReO2L4]+, [(ReOX2L2)2O], [ReNX2L3] and [Re(NR)X3L2] (X = Cl, Br, I, and L = MeCN, PPh3, py, DMSO [52, 53]. The suitability of these ligands (O2−, N3−, RN2− and S−2) for rhenium(V) is related to their π-donor capabilities, enabling the stabilisation of high oxidation states. (a) Rhenium(V) oxo cores, [ReO]3+ and [ReO2]+ Rhenium(V) complexes based on the oxo [ReV═O]3+ and dioxo [O═ReV═O]+ cores are prepared from the reduction of perrhenate with reducing ligands (mostly phosphines) in strongly acidic solution [19]. They can also be synthesised from ligand substitution reaction of [ReOX3(PPh3)2] and [ReOX2(OEt)(PPh3)2], initially prepared from the reduction of perrhenate [19]. The complexes containing the dinuclear oxo-briged [O═ReV−O−ReV═O]4+ core result from the dimerization of rhenium(V) complexes with the [ReV═O]3+ core in the presence of water. During this process, the ligand in trans position of the oxo ligand exchanges with water and thereafter two molecules of the monooxo species dimerize through the formation of a μ-oxo bond [54].


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Oxorhenium(V) complexes may undergo various reactions depending on the reaction conditions: With strong oxidants, they are oxidized to perrhenate [19]. As an example, the reaction of [ReOCl4(H2O)]− with nitrite ion in 10 M HCl surprisingly led to the formation of [ReO4]− according to the reaction shown below [11, 19]: 5 [ReOCl4(H2O)] + 2 NO2

3 [ReO4] + 2 [ReCl5(NO)]2 + 10 HCl

The monooxorhenium(V) species may be reduced by the removal of the terminal oxide, leading to mononuclear rhenium(III) complexes, and no rhenium(IV) intermediates were detected [19]. A typical example is the reaction of trans[ReOCl3(PPh3)2] with triphenylphosphine (PPh3) in acetonitrile, which leads to trans[ReCl3(MeCN)(PPh3)2] [11, 19]. A similar reduction occurs upon the reaction of bidentate 1,2-bis(diphenylphosphino)ethane (dppe) with [ReOCl3(dppe)], leading to the isolation of [ReCl2(dppe)2]Cl [55]. trans-[ReOCl3(PPh3)2] + PPh3 + MeCN 2 [ReOCl3(dppe)] + 3 dppe

trans-[ReCl3(MeCN)(PPh3)2] + OPPh3 2 [ReCl2(dppe)2]Cl + (dppe)O2

In certain reaction conditions, oxorhenium(V) may undergo a disproportionation reaction to rhenium(IV) and rhenium(VII). A typical example [ReOX4]− (X = Cl, Br) which disproprotionates in hot water according to the reaction [19, 56]:

3 [ReOX4] + 5 H2O

[ReO4] + 2 [ReO2] + 10 HX + 2 X

The disproportionation of oxorhenium(V) to rhenium(IV) and rhenium(VII) species also occurs by the reaction of [ReOCl5]2− with hot water [19]:

3 [ReOCl5]2 + 5 H2O

[ReO4] + 2 [ReCl6]2 + 2 HCl + Cl

Ligand substitution in oxorhenium(V) species may quantitatively occur either rapidly or by a prolonged reaction time. These reactions depend not only on the lability and the type of ligands to be substituted (leaving groups) by the desired ligands (incoming groups), but also on the stability of the product [19]. Some of ligand substutition reactions of the oxorhenium(V) species are shown below [19]:


13

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J. Mukiza

[ReOCl2(OEt)(PPh3)2] + Hacac

[ReOCl2(acac)(PPh3)2] + EtOH [ReO2(dppe)2]I + 2 PPh3

cis-[ReO2I(PPh3)2] + 2 dppe

trans-[ReOCl2(OEt)(PPh3)2] + HCl

trans-[ReOCl3(PPh3)2] + EtOH

The substitution by water or hydroxyl group is often followed by deprotonation leading to the cis or trans-dioxorhenium(V) and sometimes μ-oxo containing rhenium(V) species as shown in the reactions [19, 57]:

trans-[ReOCl3(PPh3)2] + 4 py + H2O 2 trans-[ReOCl3(PPh3)2] + 6 py + H2O

Acetone Reflux Benzene o

25 C Acetone

trans-[ReOI2(OEt)(PPh3)2] + H2O

Trans-dioxorhenium(V)

species

25oC

may

trans-[ReO2(py)4]Cl + 2 HCl + 2 PPh3 [(ReOCl2(py)2)2O] + 2 pyH+ + 2 Cl + 4 PPh3

cis-[ReO2I(PPh3)2] + HI + EtOH

undergo

protonation

oxohydroxorhenium(V) complexes, but the same reaction do not

to

give

occur for

monooxorhenium(V) species [19]. As an example, [ReO 2(en)2]+ produces the precipitate [ReO(OH)(en)2](ClO4)2 in HClO4 [14]. Similarly, the addition of an acid to [ReO2L4]+ (L = py, ½ dppe) yields the cations [ReO(OH)L4]2+ [55, 58] . (b) Rhenium(V) imido cores, [ReVNR]3+ The rhenium(V) organoimido core [Re═N−R]3+ is of radiopharmacological interest due to the stability of the nitrogen core, on which different organic substituents can be bonded. The biological activity of [Re═N−R]3+ can then be adjusted by varying the nature and properties of the substituent R, leading to the development of new derivatives with excellent stability [59]. The chemistry of rhenium(V) complexes based on the imido core [Re═N−R]3+ has been also extensively investigated due to their use as catalysts. They have been shown to be catalytically active in the hydroamination of alkynes, the synthesis of various nitrogen heterocycles, alkenes metathesis, and the activation of hydrocarbons, including methane and cycloaddition reactions with unsaturated organic substrates [60, 61].


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Depending on the steric and electronic environments, the [ReVNR]3+ core can adopt three different bonding geometries, shown in Figure 1.10 [62].

R N Re I

R

R

N

N+

Re II

Re III

Figure 1.10: Geometry representation in [ReVNR]3+ core In bent the geometry I, the sp2-hybridized nitrogen atom acts as a 2-electron donor to rhenium while in the linear geometry II, the sp2-hybridized nitrogen atom acts as 2electron donor in which the lone pair of electrons occupies an orbital that is N(2p) in character. In the linear geometry III, the sp-hybridized nitrogen atom acts as a 4electron donor with the lone pair on nitrogen being donated into a π-acceptor orbital of rhenium [62]. Contrary to the oxo moiety Re═O, the imido core Re═N−R is extremely stable, even in

fairly

drastic

reaction

conditions

[19].

Imidorhenium(V)

complexes

[Re(NPh)X3(PPh3)2] (X= Cl, Br) have been extensively synthesised from the condensation of trans-[ReOX3(PPh3)2] with primary aromatic amines in boiling ethanol or benzene. However the same reaction using primary aliphatic amines does not occur [19]. EtOH or benzene trans-[ReOX3(PPh3)2]+ PhNH2

Reflux

[ReNPhX3(PPh3)2] + H2O

The related rhenium(V) complexes [Re(NNCOPh)X2(PPh3)2] (X= Cl, Br) are synthesised by reacting trans-[ReOX3(PPh3)2] with aroylhydrazines which act as bidentate ligands in a mixture of benzene and ethanol [19]. N Ph

X

N

X

Re O

PPh3 PPh3

Figure 1.11: Structure of [Re(NNCOPh)X2(PPh3)2] Nelson Mandela Metropolitan University

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A versatile method of synthesis of imidorhenium(V) complexes is ligand substitution reactions with the [Re(NPh)X3(PPh3)2] and [Re(NMe)X3(PPh3)2] precursors [63]. Rhenium(V) imido complexes can also be synthesised from the alkylation or acylation of rhenium (V) complexes based on the nitrido core [Re≡N]2+ [64]. 1.3.7.1. Coordination chemistry of rhenium(V) (a) N,O-donor ligands Studies have shown that monooxorhenium(V) complexes containing bidentate N,Odonor chelates always have the phenolic oxygen coordinated in a position trans to the oxo ligand [65]. The reaction of 2-methylquinoline-8-ol (Hmqn) with trans[ReOCl3(PPh3)2] in toluene resulted in cis-[ReOCl2(mqn)(PPh3)] (Figure 1.12). The deprotonated mqn is coordinated as a bidentate chelate via quinolinic nitrogen and phenolic oxygen and both chlorides are in cis arrangement [65]. O

Cl

Cl CH3

Re Ph3P

N

O

Figure 1.12: Structure of cis-[ReOCl2(mqn)(PPh3)] The

reaction

of

(4Z)-4-(2-aminobenzylidenamino)-1,2-dihydro-2,3-dimethyl-1-

phenylpyrazol-5-one (H2nap) with trans-[ReOBr3(PPh3)2] in ethanol resulted in the imidorhenium(V) complex [Re(nap)Br2(PPh3)]Br (Figure 1.13) [66]. The H2nap is doubly deprotonated and acts as a tridentate N,N,O-donor chelate coordinated to rhenium through the imido and imino nitrogens, and a neutral ketonic oxygen [66].

Ph3 P

Br O Re

N

N

Br N

N

CH3

Br

CH3

Figure 1.13: Structure of [Re(nap)Br2(PPh3)]Br Nelson Mandela Metropolitan University

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When H2nap was reacted with cis-[ReO2I(PPh3)2], the reaction led to the monooxorhenium(V) octahedral complex [ReO(Hnap)(OEt)(PPh3)]I (Figure 1.14) [56]. The monoanionic Hnap coordinated to the [ReO]3+ core via the imino and amino nitrogens and ketonic oxygen, resulting in an octahedral cationic complex [56].

CH3 I

HN Ph3P

N

N

O Re

CH3

N

O

EtO

Figure 1.14: Structure of [ReO(Hnap)(OEt)(PPh3)]I (b) S,N-donor ligands Rhenium(V) complexes containing N,S-donor chelates constitute an important class in bioinorganic drugs due to their therapeutic application in nuclear medicine [67]. The reaction of the tetradentate S3N-donor ligand 2-(3-mercaptopropylthio)-N-(2mercaptoethyl)acetamide (H3mma) with trans-[ReOCl3(PPh3)2] in methanol with sodium acetate leads to the isolation of the distorted square-pyramidal complex [ReO(mma)] (Figure 1.15) [67]. The trianionic mma3− coordinates to rhenium via the thiolic sulfur, sulfido sulfur and amido nitrogen atoms. O

S

O

N

Re S

S

Figure 1.15: Structure of [ReO(mma)] The reaction of 3-((azepane-1-carbothioamido)(phenyl)methlene)-1,1-diethylthiourea (H2amdt) with (n-Bu4N)[ReOCl4] in methanol with NaN3 resulted in the dimeric rhenium(V) nitrido based complex (μ-O)[ReN(Hamdt)]2 (Figure 1.16) [68]. Nelson Mandela Metropolitan University

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The monoanionic Hamdt− acts as tridentate ligand and coordinates to the nitrido core [Re≡N]2+ through the hydrazino nitrogen and two thionic sulfur atoms. Cationic rhenium(V) complexes containing the (μ-O)2[Re2O2]2+ core without a metalmetal bond are rare in the literature [69]. The first was isolated from the reaction of the N2S-donor chelate bis(benzimidazol-2-ylethyl)sulfide (btn) with cis-[ReO2I(PPh3)2] in methanol. The reaction led to the dimeric cationic rhenium(V) complex (μO)2[ReO(btn)]2I2 (Figure 1.17). The btn chelate coordinates to the (μ-O)2[Re2O2]2+ core via the imino nitrogens of two imidazole rings and a sulfur atom [69].

H N N HN

N

N S Re

N O

S

N

S Re

N S

NH

N N H

N

Figure 1.16: Structure of (μ-O)[ReN(Hamdt)]2 H N S

HN

N

O Re

O

I

N

O

O

I

Re N

N S

NH

N H

Figure 1.17: Structure of (μ-O)2[ReO(btn)]2I2 Nelson Mandela Metropolitan University

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(c) S,O and N,S,O-donor ligands Derivatives of benzoylthiourea are the most well-known S,O-donor ligands and coordinate to different cores of rhenium(V) resulting either in monomeric or dimeric complexes, depending on the reaction conditions [37]. A typical example is the reaction of N-(morpholine-4-thioyl)benzamide (Hmtb) with [ReOCl3(PPh3)2] in acetone containing three drops of Et3N. The reaction leads to the isolation of the monomeric octahedral complex [ReOCl2(mtb)(PPh3)] (Figure 1.18), in which the monoanionic mtb− coordinates to rhenium via the thiolic sulfur and ketonic oxygen [37]. The same product is obtained upon stirring of Hmtb with (n-Bu4N)[ReOCl4] in cold methanol without adding Et3N [37].

O Ph3 P Cl

O

Re Cl

N

S N

O Figure 1.18: Structure of [ReOCl2(mtb)(PPh3)] The

N-(N’,N’-diethylaminothiocarbonyl)benzimidoyl

chloride

reacts

with

2-

aminophenol in dry acetone at 40o C in the presence of Et3N to yield the potentially tridentate

N,S,O-donor

chelate

N-(N’,N’-dietylaminothiocarbonyl)-N’-(2-

hydroxyphenyl)benzamidine (H2dhb) [70]. The reaction of H2dhb with (nBu4N)[ReOCl4] in cold methanol results in the distorted square-pyramidal complex [ReOCl(dhb)] (Figure 1.19) in which the ligand is doubly deprotonated and acts as tridentate N,S,O-donor [70]. [ReOCl(dhb)] reacts with 2-aminoacetic acid (Hama) in methanol/dichloromethane mixture giving the octahedral complex [ReO(dhb)(ama] (Figure 1.20). The deprotonated ama replaces the chloride and coordinates to rhenium as a bidentate via the neutral amino nitrogen and carboxylato oxygen [70].


19

Chapter 1

J. Mukiza

N

N N

O

S

Re O

Cl

Figure 1.19: Structure of [ReOCl(dhb)]

N

N N

O

S

Re O

NH2

O O

Figure 1.20: Structure of [ReO(dhb)(ama)] 1.3.8. Rhenium(VI) Rhenium(VI) is the least stable oxidation state due to its d 1 electron configuration, and its coordination compounds are rare. Rhenium(VI) center is stabilised by strong donor ligands such as terminal oxo and nitrido groups. Consequently, the coordination compounds of rhenium(VI) are mostly those based on the [ReO 2]2+ and [ReN]3+ cores [71, 72]. They are frequently isolated as intermediates in reduction of rhenium(VII) compounds or oxidation of rhenium(V) precursors and are often prone to undergo further redox reactions [19,71]. Rhenium(VI) compounds have been shown in coordination numbers from four to eight, but the octahedral geometry is the preferred one for most of its coordination compounds [71, 72]. The most representative rhenium(VI) complexes are those of the tetrahalide type [ReOX4] and [ReNX4]− (X = Cl, Br). They display a distorted square pyramidal geometry in which the multiple bonded oxo and nitrido are in the apical position, and they tend to adopt a six-coordinated octahedral geometry, either by intramolecular association in the solid state or by coordination with monodentate ligands [73]. Nelson Mandela Metropolitan University

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Both [ReOX4] and [ReNX4]− are used as precursors in the synthesis of nitride- and oxo-based rhenium(VI) complexes [72]. The most useful in synthesis is (nBu4N)[ReNCl4], which is obtained from the reduction of (n-Bu4N)[ReO4] with NaN3 in presence of HCl [73]. The [ReOCl4] is more reactive and acts as a precursor of rhenium compounds in oxidation state other than six. It is reduced to [ReOCl 5]2− by concentrated hydrochloric acid, to [ReOCl3(py)2] by pyridine, to [ReCl6]2− by thionyl chloride and it may disproportionate to [ReO2] and [ReO4]− in water [19]. Rhenium hexafluoride [ReF6] and metal trioxide [ReO3] are also known in the literature [19]. 1.3.9. Rhenium(VII) Rhenium(VII) exhibits a d0 electronic configuration and exhibits coordination numbers from four to nine. The chemistry of rhenium(VII) is dominated by oxides, and the most well-known are the perrhenate ion [ReO4]− and the high volatile polymeric dirhenium heptoxide [Re2O7] [19]. In presence of reducing agents such as phosphines in acidic solution, the [ReO4]− ion undergoes reduction to oxorhenium(V) complexes. The [Re2O7] is soluble in water to form a strongly acidic solution of perrhenic acid HReO4, from which various metal perrhenates with exceptional stability may be prepared [19]. Other important reaction of [Re2O7] is reductive carbonylation, which leads to [Re0(CO)5]2, and the absorption of water leading to [Re2O7(H2O)2] [19, 23]. The few halides and oxohalides of rhenium(VII) such as [ReF7], [ReF8]−, [ReOF6]−, [ReO2F4]−, [ReO2F3], [ReO3Cl2]− and [ReO3Cl] are known in the literature [19, 73]. The enneahydridorhenate ion [ReH9]2− is another type of interesting rhenium(VII) compound [69]. It can be prepared from the reaction of perrhenate with alkali metals in ethanol and the highest yield has been achieved from the reaction of Na[ReO4] and sodium leading to Na2[ReH9] [19, 75]. The coordination chemistry of rhenium(VII) is dominated by hard multiply-bonded πdonor ligands, particularly O2−, N3− and RN2− [76]. The labile complexes [ReO3Cl(dmso)2] and [ReO3Cl(dmf)2] in which the ligands act as monodentate Odonor were synthesised from the reaction of [ReO 3Cl] with appropriate ligand in dry CCl4 [19]. In a similar way, the less stable complexes [ReOCl 3L2] (L = py, ½ dipy, MeCONMe2, (Me2N)2(CO) and PO(NMe2)3) are formed [19]. The reaction of [ReH9]2− Nelson Mandela Metropolitan University

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with tertiary phosphines or arsines in isopropanol leads to the complexes [ReH8L]− (L = PPh3, PEt3, Pn-Bu3 and AsPh3) [19]. The neutral complexes [ReH7L] (L = PPh3, PEtPh2, PEt2Ph, AsEt2Ph and ½ dppe) have been also synthesised from the reaction of [ReOCl3L2] or [ReO(OR)Cl2L2] and [ReCl4L2] with LiAlH4 catalyst in tetrahydrofuran (THF) [19]. The cis-dioxorhenium(VII) complexes of catechol

and its derivatives

have been also synthesised and their ability to transfer oxygen atom to an olefin has been well explored [74].

1.4. Rhenium Radiopharmaceuticals 1.4.1. Rhenium and technetium in nuclear medicine Radiopharmaceuticals contain radionuclides and they are used in nuclear medicine for diagnostic imaging and radiotherapy for various diseases. In nuclear medicine, the potential application of radioisotopes is related to their nuclear characteristics such as chemical properties, mode of decay, half-life and tissue penetration ability [77]. Rhenium radiopharmaceuticals involve both

186

Re and

188

Re radionuclides, and

they are applied in radiotherapy due to their favourable nuclear properties. Rhenium and technetium display similar chemistry. The β-emitter 99mTc is applied in diagnostic imaging radiopharmaceuticals for imaging of various organs [4]. The favourable nuclear properties of

186

Re and

188

Re radionuclides are their tissue

penetration ability which is suitable for small and larger tumour treatment, their relative half-lives which are long enough to prolong the treatment period, and their βemission energies which are high enough to deliver the radiation dose [11]. The use of

99m

Tc radionuclide for diagnostic imaging is related to its nuclear characteristics

such as a short half-life (t1/2 = 6.02 h) and γ-ray emission of 142 keV [6]. This energy is efficient for penetrating the imaged organ since the defined optimum energy range for the medical imaging facilities currently present in various hospitals is 100-200 keV [6, 8]. 1.4.2. Preparation of rhenium and technetium radiopharmaceuticals The preparation of rhenium and technetium radiopharmaceuticals are very similar and starts from the permetallate ions [186/188ReO4]− and [99mTcO4]−, obtained from different radionuclide generators [6]. During the preparation, the metal ions have to be reduced by appropriate reducing agents (mostly a Sn(II) salt), and coordinated by Nelson Mandela Metropolitan University

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suitable chelating ligands which will stabilise the low oxidation state of the Lewis acidic metal center, and mainly determine the biodistribution patterns of the radiopharmaceutical in the body [1, 6]. In addition to that, the buffer solution for adjusting the pH to the labelling conditions, catalysts and stabilisers are added to the reaction system [6]. Such procedures commonly occur in so-called “instant kits” and the reaction must be optimized since the expected purity and yield of products should be reproducible at 95 % [1]. After performing their chromatographic quality control, the produced radiopharmaceuticals should be ready for injection without further purification [1]. 1.4.3. Classification of rhenium and technetium radiopharmaceuticals The radiopharmaceuticals based on rhenium and technetium are divided into three generations according to their biodistribution modes or respective synthetic approach [6]. (a) First generation radiopharmaceuticals The first generation radiopharmaceuticals are those with the biological distribution strictly determined by blood flow and perfusion. They can be called perfusion agents since they do not display any targeting functions in the body [6]. The nuclear medicine application of these radiopharmaceuticals mainly depends on their physiochemicals properties including hydrophilicity, charge and size of complex. These properties have been also proved to determine the biological distribution of radiopharmaceutical between the tissues [6]. Most of commercially available technetium-based imaging agents are first generation radiopharmaceuticals [1]. The first generation radiopharmaceuticals are often applied when the targeted systems are glomerular filtration, phagocytosis, hepatocytes and bones absorption [8]. For, example gluconic acid and glucoheptonic acid (Figure 1.21) [TcO] 3+-based complexes have been introduced for renal imaging, and they are today used for skeleton scintigraphy [78].


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Chapter 1

J. Mukiza OH

OH

OH

OH O

O HO

HO

OH OH

OH OH

OH

OH

B

A

Figure 1.21: Structure of gluconic acid (A) and glucoheptonic acid (B) Another example of first generation rhenium and technetium radiopharmaceuticals is 186/188

Re−HEDP and

(Figure 1.22). The

99m

Tc−HEDP (HEDP = 1-hydroxyethylidene-1,1-diphosphonate)

186/188

Re−HEDP has been successfully used for palliation of pain

relief from metastatic bone cancer [79]. The

99

Tc−HEDP is widely used for skeletal

and hepatobiliary system imaging [79]. O

CH3 O

P

C

P

OH

OH

OH

OH

OH

Figure 1.22: Structure of HEDP (b) Second generation radiopharmaceuticals The second generation radiopharmaceuticals target specific biological molecule receptors and bind to it. Hence, the radiopharmaceutical distribution to the specific site in the body is facilitated by such biological molecule receptors [1, 6]. The ligands coordinated to the second generation

186/188

Re and

99m

Tc radiopharmaceuticals must

be bifunctional, so that they can stabilise the metal ion in a particular oxidation state through coordination, and covalently link the radiopharmaceutical to the biovector [1]. The possible biological molecule receptors are peptides, antibodies, proteins or pharmacophores [6]. The connecting functionality of the radiopharmaceutical to the biovector commonly occurs through carboxylate or amine groups which can conveniently be activated with standard strategies from organic chemistry [1, 6]. Consequently, the carboxylate and/or amine functions of polyaminopolycarboxylic acids have been applied as chelating agents in preparation of second generation rhenium and technetium Nelson Mandela Metropolitan University

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radiopharmaceuticals [6]. Typical examples are 186/188

Re−DOTA and

99m

186/188

Re−DTPA and

99m

Tc−DTPA,

Tc−DOTA radiopharmaceuticals in which DTPA is

diethylenetriaminepentaacetic acid and DOTA is 1,4,7,10-tetraazacyclododecane1,4,7,10-tetraacetic acid (Figure 1.23) [6]. O O

OH N

OH

OH

O

OH

O N

HO

N

N N

O

O OH

HO

N OH DOTA

N

O

OH DTPA

O

O

Figure 1.23: Structure of DTPA and DOTA The

186/188

Re−DTPA is used as a potential radiopharmaceutical for intravascular

radiation therapy while scintigraphy [80]. The

99m

Tc−DTPA complex is used for kidney and brain

186/188

Re−DOTA is the potential agent for treatment of patients

with ovarian cancer while the

99m

Tc−DOTA radiopharmaceutical is used for kidney,

lung and liver imaging [81, 82]. (c) Third generation radiopharmaceuticals In this case, the essential parts of biological molecules (mostly hormones) are mimicked and the radiopharmaceutical complex is incorporated into the carbon skeleton [6]. The radionuclides of rhenium or technetium are incorporated into the carbon skeleton by coordination to carboxylate or aminethiol groups. The radioactive metal center and its ligands are insulated by metal-oxygen double bonds in order to mimic the electronic structure of the respective hormone [6]. Some of hormones such as progesterone and testosterone have been synthesised and tested for this application [82]. A typical example is the oxorhenium(V) radiopharmaceutical, synthesised from the radionuclide rhenium and tetradentate amino, amido, thioether and thiol groups incorporated into the ligand [6]. This oxorhenium(V) complex is Nelson Mandela Metropolitan University

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known to be incorporated in female hormone called estradiol and replaces C and D rings of that hormone as shown in Figure 1.24 [6]. O

OH

N C

D

O

N

A

B

HO

S

Re A

S

B

HO 1

2

Figure 1.24: Structure of estradiol hormone (1) and rhenium-estradiol template (2)

1.5. References [1]

U. Abram, R. Alberto, J. Braz. Chem. Soc., 17, 1486, 2006.

[2]

T.I.A. Gerber, D. Luzipo, P. Mayer, J. Coord. Chem., 57, 1345, 2004.

[3]

T.A. Patton, C.E. Strouse, C.B. Knobbler, J.A. Gladysz, J. Am. Chem. Soc., 105, 5804, 1983.

[4]

Z.M. Jalil, M.M. Qassim, H.M. Hamed, Magazine of Al-Kufa University for Biology, 5, No. 2, 273, 2013.

[5]

N. Iznaga-Escobar, Appl. Radiat. Isot., 54, 399, 2001.

[6]

J. Sophie, W.A. Herrmann, K.E. Fritz, J. Organomet. Chem., 751, 83, 2014.

[7]

J. Mukiza, T.I.A. Gerber, E. Hosten, F. Taherkhani, M. Nahali, Inorg. Chem. Commun., 49, 5, 2014.

[8]

E. Zangoni, PhD thesis, Universita Degli Studi di Padova, 2009.

[9]

F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Adv. Inorg. Chem., 6th Ed. J. Wiley Interscience, New York, 1999.

[10]

I. Booysen, PhD Thesis, Nelson Mandela Metropolitan University, 2009.

[11]

N.C. Yumata, MSc Dissertation, Nelson Mandela Metropolitan University, 2009.


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Chapter 1 [12]

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N.C. Yumata, G. Habarurema, J. Mukiza, T.I.A. Gerber, E. Hosten, F. Taherkhani, M. Nahali, Polyhedron, 62, 89, 2013.

[13]

A.L. Balch, R.H. Holm, J. Am. Chem. Soc., 58, 5201, 1966.

[14]

S. Kumar, D.N. Dhar, P.N. Saxena, J. Sci. Ind. Res., 68, 181, 2009.

[15]

H. Luo, S. Liu, S.G. Rettig, C. Orvig, Can. J. Chem., 73, 2272, 1995.

[16]

J. Mukiza, T.I.A. Gerber, E. Hosten, J. Chem. Crystallogr., 44, 368, 2014.

[17]

V.Y. Kukushki, Coord. Chem. Rev., 139, 375, 1995.

[18]

N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2end ed., Butterworth-Heinemann, Oxford, 1998.

[19]

G. Rouschias, Chem. Rev., 74, 531, 1974.

[20]

B. Royon, C.C. Romao, Compr. Organomet. Chem. III, 5, 855, 2007.

[21]

P.V. Poplaulokhin, X. Chen, E.A. Meyers, S.G. Shore, Inorg. Chem., 45, 10115, 2006.

[22]

M.R. Churchill, K.N. Amoh, H.J. Wasserman, Inorg. Chem., 20, 1609, 1981.

[23]

C.C. Romao, Encycl. Inorg. Chem., John Wiley & Sons, 2006.

[24]

K.M. Wilma, R.T. Walters, W.L. Waltz, W. Andrew, L.M. Dorfman, J. Am. Chem. Soc., 104, 1842, 1982.

[25]

J.C. Vites, M.M. Lynam, Coord. Chem. Rev., 1, 142, 1995.

[26]

F. Vitali, F. Calderazzo, Gazz. Chim. Ital., 102, 587, 1972.

[27]

R. Alberto, R. Schibli, A. Egli, R. Waibel, U. Abram, A.P. Schubiger, Coord. Chem. Rev., 190, 901, 1999.

[28]

R. Alberto, R. Schibli, A. Egli, A. P. Schubiger, J. Am. Chem. Soc., 120, 7987, 1998.

[29]

A.E. Leins, N.J. Coville, J. Organomet. Chem., 407, 359, 1991.

[30]

F. Calderazzo, I.P. Mavani, F. Vitali, I. Bernal, J.D. Korp, J.L. Atwood, J. Organomet. Chem., 160, 207, 1978.

[31]

W. Wang, B. Spingler, R. Alberto, Inorg. Chim. Acta, 355, 394, 2003.


27

Chapter 1 [32]

J. Mukiza

M.L. Bowen, N.C. Lim, C.B. Ewart, R. Misri, C.L. Ferreira, H. Urs, M.J. Adam, C. Orvig, Dalton Trans., 9227, 2009.

[33]

R. Carl, S. Watkins, S.M. Hamphrey, P.C.P. Bulman, S. Ashby, Y. Chao, C.J. Herbert, A. Mueller, RSC Adv., 3, 23963, 2013.

[34]

T. Perera, P.A. Marzilli, F.R. Fronczek, L.G. Marzilli, Inorg. Chem., 49, 2123, 2010.

[35]

I.S.G. Santos, U. Abram, R. Alberto, E.V. Lopez, A. Sanchez, Inorg. Chem., 43, 1834, 2004.

[36]

N. Lazarova, J. Babich, J. Valliant, P. Schaffer, S. James, J. Zubieta, Inorg. Chem., 44, 6763, 2005.

[37]

U. Abram, N. Hung Huy, Inorg. Chem., 46, 5310, 2007.

[38]

B. Schmitt, T.I.A. Gerber, R. Betz, E. Hosten, Inorg. Chem. Commun., 24, 136, 2012.

[39]

Y. Sasaki, J. Nucl. Radiochem. Sci., 6, 145, 2005.

[40]

L. Fadini, A. Togni, Chimia, 58, 2011, 2004.

[41]

J.E. Fergusson, Coord. Chem. Rev., 1, 459, 1966.

[42]

F. Zobi, L. Kromer, B. Spingler, R. Alberto, Inog. Chem., 48, 8965, 2009.

[43]

K.A. Conner, R.A. Walton, Compr. Coord. Chem.,4, 203, 1987.

[44]

L.E. Helberg, S.D. Orth, M. Sabat, W.D. Harman, Inorg. Chem., 35, 5584, 1996.

[45]

P. Mayer, N.C. Yumata, T.I.A. Gerber, A. Abrahams, S. Afr. J. Chem., 63, 180, 2010.

[46]

X.L. Luo, R.H. Crabtree, J. Am. Chem. Soc., 112, 4813, 1990.

[47]

K. Potgieter, P. Meyer, T.I.A. Gerber, I.N. Booysen, Polyhedron, 28, 2808, 2009.

[48]

M.S. Papadopoulos, P. Bouziotis, I.C. Pirmettis, C.P. Raptopoulou, A. Terzis, E. Chiotellis, Inorg. Chim. Acta, 290, 247, 1999.

[49]

F.A. Cotton, J.H. Matonic, D.O. Silva, Inog. Chim. Acta, 234, 115, 1995.


28

Chapter 1 [50]

G. Rouschias, G. Wilkinson, J. Chem. Soc., 489, 1968.

[51]

G. Rouschias, G. Wilkinson, J. Chem. Soc., 465, 1966; M. Freni, V.

J. Mukiza

Valenti, Gazzetta, 91, 1357, 1961. [52]

N. P. Johnson, F. I. M. Taha, G. Wilkinson, J. Chem. Soc., 2614, 1964.

[53]

I. Booysen, T.I.A. Gerber, E. Hosten, P. Mayer, J. Coord. Chem., 60, 867, 2007.

[54]

K.J.C. Van Bommel, W. Verboom, H. Kooijman, A. L. Spek, D. N. Reinhoudt., Inorg. Chem., 37, 4197, 1998.

[55]

M. Freni, D. Giusto, P. Romiti, Gazz. Chim. Ital., 97, 833, 1967.

[56]

H.S. Prop, PhD Thesis, Massachusetts Institute of Technology, 1979.

[57]

M.D. Al-Sabti, K. M. Tawfiq, M. J. Al-Jeboori, Um-Sal. Sc. J., 6, 32, 2009.

[58]

D.E. Grove, G. Wilkinson, J. Chem. Soc., 1224, 1966.

[59]

M.T. Ahmet, B. Coutinho, J.R. Dilworth, J.R. Miller, S.J. Parrot, Y. Zheng, Polyhedron, 15, 2041, 1996.

[60]

W.A. Nugent, B.L. Haymore, Coord. Chem. Rev., 31, 123, 1980.

[61]

T.I.A. Gerber, J. Perils, J.G.H du Preez, Inorg. Chim. Acta, 278, 96, 1998.

[62]

C.F. Eduards, W.P. Griffith, A.J.P. White, D.J. Williams, J. Chem. Soc. Dalton Trans., 957, 1992.

[63]

R.A. Eikey, M.M. Abu-Omar, Coordi. Chem. Rev., 243, 83, 2003.

[64]

A. Voigt, U. Abraham, R. Kirmse, Inorg. Chem. Commun.,1, 203, 1998.

[65]

Y. Miyashita, T. Ohashi, A. Imai, N. Amir, K. Fujisawa, O. Ken-ichi, Sci. Technol. Adv. Mater., 6, 660, 2005.

[66]

K.C. Potgieter, T.I.A. Gerber, P. Mayer, S. Afr. J. Chem., 64, 184, 2011.

[67]

M.C. Archer, J.R. Dilworth, D.V. Griffiths, M.J. Al-Jaboori, J.D. Kelly, L. Canzhong, M.J. Rosser, Y. Zheng, J. Chem. Soc., Dalton Trans., 1403, 1997; B. Johannsen, H. Spies, Top. Curr. Chem., 77, 176, 1996.

[68]

I.S.M. Pedro, H.H. Nguyen, A. Hagenbach, S. Bergemann, G. Ronald,


29

Chapter 1

J. Mukiza

V.M. Deflon, U. Abram, Dalton Trans., 42, 5111, 2013. [69]

T.I.A. Gerber, K.C. Potgieter, P. Mayer, Inorg. Chem. Commun., 14, 118, 2011.

[70]

U. Abram, N. Hung Huy, J. Grewe, S. Jenniffer, B. Kuhn, Inorg. Chem., 47, 5136, 2008.

[71]

J. Mukiza, T.I.A. Gerber, E. Hosten, A.S. Ogunlaja, F. Taherkhani, M. Amini, M. Nahali, Inorg. Chem. Commun., 51, 83, 2015.

[72]

O.E.M. Vargas, PhD Thesis, University of Zürich, 2011.

[73]

U. Abram, M. Braun, S. Abram, R. Kirmse, A. Voigt, J. Chem. Soc., Dalton, Trans., 231, 1998; W. Noh, G.S. Girolami, Dalton Trans., 674, 2007.

[74]

J.V. Goldstone, MSc Dissertation, Massachusetts Institute of Technology 1993.

[75]

A.P. Ginsberg, C.R. Sprinkle, Inorg. Chem., 8, 2212, 1969.

[76]

J.G. Malm, H. Selig, J. Inorg. Nucl. Chem., 20, 189, 1961.

[77]

M. Neves, A. Kling, R.M. Lambrecht, Appl. Radiat . Isot., 57, 657, 2002.

[78]

M.J. Abrams, S.K. Larsen, J. Zubieta, Inorg. Chem., 30, 2031, 1991.

[79]

W. Brenner, W.U. Kampen, C.V. Forstner, B. Corrina, M. Zuhayra, C. Muhle, N. Czech, E. Henze, J. Nucl. Med., 42, 1545, 2001.

[80]

B.T. Hsieh, J.F. Hsieh, S.C. Tsai, W.Y. Lin, H.T. Huang, G. Ting, S.J. Wang, Nucl. Med. Biol., 26, 967, 1999.

[81]

R. Varshney, P.P. Hazari, J.K. Uppal, P. Sunil, R. Stromberg, M. Allard, A.K. Mishra, Cancer Bio. Ther., 11, 893, 2011.

[82]

W.A. Volkert, T.J. Hoffman, Chem. Rev., 99, 2269, 1999; J. Chen, Z. Cheng, T.J. Hoffman, S.S. Jurisson, T.P. Quinn, J. Cancer Res.,60, 5649, 2000.


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Chapter 2 Experimental 2.1. Handling of rhenium The metal rhenium may exist in the four isotopes

185

Re,

187

Re,

186

Re and

188

Re of which the last two are radioactive. The two non-radioactive isotopes

occur naturally as

185

Re and

187

Re with a natural abundance of 37.4% and

62.6% respectively. No special precautions were taken in handling rhenium chemicals during this work since they only contain non-radioactive rhenium.

2.2. Materials 2.2.1. Precursor compounds (a) Ammonium perrhenate Ammonium perrhenate (NH4)[ReO4] was obtained from Sigma-Aldrich in +99% purity and required no further purification. (b) Rhenium pentacarbonylhalide Halogenated pentacarbonylrhenium [Re(CO)5X] (X = Cl or Br) were used as starting materials for rhenium(I) complexes synthesis. These reagents were obtained from Sigma-Aldrich in 98% purity and were used without any further purification. (c) trans-[ReOCl3(PPh3)2] To a mixture of 0.9 g of ammonium perrhenate (NH 4)[ReO4] in 3 cm3 of hydrochloric acid was added 5.0 g of triphenylphosphine in 50 cm 3 of glacial acetic acid under nitrogen. A bright green precipitate of trans-[ReOCl3(PPh3)2] was formed and it was purified by filtration, washing with glacial acetic acid and diethyl ether and drying under vacuum [1]. Yield = 95%. Anal. Calc. for C36H30P2OCl3Re (mol. wt. = 833.09 g/mol): C, 51.9; H, 3.6; Cl, 12.9. Found: C, 51.9; H, 3.6; Cl, 12.9 %.


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(d) trans-[ReOBr3(PPh3)2] To a solution of 1.0 g of (NH4)[ReO4] in 3 cm3 of concentrated hydrobromic acid was added to 5.0 g of triphenylphosphine in 50 cm 3 of glacial acetic acid under nitrogen. A yellow precipitate formed was filtered off, washed with glacial acetic acid and diethyl ether, and dried under vacuum [1]. Yield = 90%. Anal. Calc. for C36H30P2OBr3Re (mol. wt. = 966.46 g/mol): C, 44.7; H, 3.1; Br, 24.8 Found: C, 44.4; H, 3.1; Br, 24.2 %. (e) trans-[ReOI2(OEt)(PPh3)2] A mass of 5.0 g of triphenylphosphine in 30 cm3 of ethanol was added to 1.0 g of (NH4)[ReO4] in 5 cm3 of hydroiodic acid (56%) and the mixture was heated under reflux for 15 minutes. The solution was allowed to cool to room temperature and the resultant dark green precipitate was filtered off and washed with ethanol and diethyl ether, and dried under vacuum [2]. Yield 86%. Anal. Calc. for C38H35I2O2P2Re (mol. wt. = 1142.8 g/mol): C, 44.5; H, 3.4; I, 24.8. Found: C, 44.7; H, 3.9; I, 25.2 %. (f) cis-[ReO2I(PPh3)2] A mixture of 1.0 g of trans-[ReOI2(OEt)(PPh3)2] in 50 cm3 of acetone and 2 cm3 of water was stirred at room temperature for an hour. The purple crystalline product which formed was filtered off, washed with acetone and diethyl ether and dried under vacuum [2]. Yield 81%. Anal. Calc. for C36H30IO2P2Re (mol. wt. = 869 g/mol): C, 49.7; H, 3.5; I, 14.6. Found: C, 49.7; H, 3.5; I, 14.4 %. (g) trans-[ReO2(py)4]Cl A mixture of 3 cm3 of pyridine and 0.5 cm3 of water was added to a solution of 0.5 g of trans-[ReOCl3(PPh3)2] in 10 cm3 of acetone. The resultant mixture was refluxed for 90 minutes and generated an orange precipitate which was washed with toluene (2 x 3 cm3) and diethyl ether (3 x 2 cm3), and dried under vacuum [3]. Yield = 89%. Anal. Calc. for C20H20ClN4O2Re (mol. wt. = 570.06 g/mol): C, 42.1; H, 3.5; Cl, 14.2; N, 9.8. Found: C, 42.7; H, 3.5; Cl, 14.4; N, 9.9 %.


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(h) (n-Bu4N)[ReOCl4] A mass of 1.02 g of (NH4)[ReO4] was dissolved in 25 cm3 of water. To this, 5 cm3 of a 75% aqueous (n-Bu4N)Cl solution was added with stirring.

The

resultant white precipitate of (n-Bu4N)[ReO4] was removed by filtration, washed with ethanol and dried under vacuum. A suspension of 0.7 g of this salt in about 100 cm3 of chloroform was chlorinated by HCl gas (dried with H2SO4) for 4 hours. During this time the solution turned deep red. The volume was reduced, with the solution turning yellow. Upon addition of 50 cm 3 of a nheptane/diethyl ether mixture a yellow-green precipitate formed, which was removed by vacuum filtration and washed with ethanol. It was dried under vacuum [4]. Yield = 82 %. Anal. Calcd. for C16H36NOCl4Re (mol. wt. = 586.5 g/mol): C, 32.8; H, 6.2; N, 2.4; Cl, 24.2. Found: C, 32.7; H, 6.2; N, 2.4; Cl, 24.2 %. (i) trans-[ReCl3(MeCN)(PPh3)2] A mixture of trans-[ReOCl3(PPh3)2] (5.0 g, 6.0 mmol), triphenylphosphine (5.0 g,19.0 mmol), acetonitrile (35 cm3) and toluene (40 cm3) was stirred for 2 h at reflux under nitrogen. After an hour an orange solid was formed upon cooling to room temperature, and was collected through vacuum filtration and washed with 100 cm3 of ethanol [5]. Yield = 87 %, m.p. 289-295 °C. Anal. Calcd. for C38H32P2CI3Re: C, 53.2; H, 3.9; N, 1.6. Found: C, 53.1; H, 3.8; N, 1.6 %. (j) trans-[ReBr3(dab)(PPh3)2] (H2dab = 1,2-diaminobenzene) A mixture of trans-[ReOBr3(PPh3)2] (179 mg, 185 μmol) and H2dab (20 mg, 185 μmol) in ethanol (20 cm3) was heated under reflux for 1 h. The resulting green solution was allowed to cool to room temperature, and the dark red product was filtered off, washed with ethanol and diethyl ether, and dried under vacuum [6]. Yield = 85%, m.p. 232 °C. Anal. Calcd. for C42H36Br3N2P2Re: C, 56.64; H, 3.93; N, 3.03. Found: C, 54.72; H, 3.79; N, 2.89 %.


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2.2.2. General laboratory chemicals All common laboratory chemicals were of analytical grade and were used without any further purification. The following chemicals were commercially obtained and used as received: Benzoyl chloride

Aldrich (≥98%)

Orotic acid

Aldrich (≥98%)

5-Aminoorotic acid

Aldrich (99%)

6-Hydroxypicolinic acid

Aldrich (98%)

Triphenylphosphine

Aldrich (99%)

Dithizone

Merck

Phenothiazine

Aldrich (98%)

Potassium thiocyanate

Merck

Di-2-pyridyl-ketone

Aldrich (99%)

1,3-Diphenylthiourea

Aldrich (98%)

Salicylaldehyde

Aldrich (≥ 99%)

2-Aminopyrimidine

Merck (> 98)

Pyridine-2-carbaldehyde

Aldrich (99%)

Benzohydrazide

Aldrich (99%)

2-Aminobenzohydrazide

Aldrich (99%)

Thiophene-2-carbaldehyde

Merck (98%)

2-Hydroxybenzohydrazide

PubChem

Tetraphenylphosphonium chloride

Fluka (97%)

Dipicolylamine

ChemSpider


34

Chapter 2

J. Mukiza

2-Mercapto-orotic acid

Aldrich

Triethylamine

Aldrich (≥99.5%)

Triphenyphosphite

Aldrich (97%)

Pyridine

Aldrich (99.8%)

1,2-Diaminobenzene

PubChem

Tetrabuthylammonium chloride

Aldrich (≥97%)

2.3. Instrumentation The 1H NMR spectra were obtained at 300 K by using a Bruker Advance III 400 MHz spectrometer. Deuterated chloroform and dimethyl sulfoxide were used as solvent and the peak positions were obtained relatively to tetramethylsilane (SiMe4). The infrared spectra were recorded on a Bruker Platinum Tensor 27 ATR-IR spectrophotometer in the 4000-200 cm-1 range. Optical

spectra

were

obtained

by

using

a

Perkin-Elmer

330

spectrophotometer. The concentrations are given in 10-4 mol dm-3 and extinction coefficients (ε) in dm3 mol-1cm-1. Melting points were determined using a Stuart SMP 30 Electrothermal 1A9100 and Stuart Scientific SMP 3 melting point instruments. The elemental analyses for carbon, hydrogen, nitrogen and sulfur were performed on a Vario EL (ElementarAnalysensystem GmbH) instrument. For single crystal X-ray crystallography analysis (done at 200 K), a Bruker Kappa Apex II diffractometer in the conventional ω-2θ scan mode and monochromatic Mo-Kα radiation (λ = 0.71073 Å) was used. The cyclic voltammetry studies were carried out with a Bas Epsilon Version 1.30.64 system which consists of a platinum working electrode, a platinum auxiliary electrode and a pseudo silver/silver chloride Re-5 reference Nelson Mandela Metropolitan University

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Chapter 2

J. Mukiza

electrode. The supporting electrolyte tetrabutylammonium perchlorate (TBAP) had a concentration of 0.10 M and the respective complex concentrations were 0.001 M. The sample solutions were deoxygenated by nitrogen before runs. Conductivity measurements were carried out at 293 K using a HI 2300 EC/TDS/ conductometer.

2.4. References [1]

N.P. Johnson, C.J.L. Lock, G. Wilkinson, Inorg. Synth., 9, 145, 1967.

[2]

J. Wiley & Sons, Inc. Inorganic Syntheses, 29, 149, 1992.

[3]

M.D. Al-Sabti, K. M. Tawfiq, M. J. Al-Jeboori, Um-Sal. Sc. J., 6, 32, 2009.

[4] (a) T. Lis, B. Jezowska-Trzebiatowska, Acta Crystallogr., B33, 1248, 1977; R. Alberto, R. Schibli, A. Egli, P.A. Schubiger, W.A. Herrmann, G. Artus, U. Abram, T.A. Kaden, J. Organomet. Chem., 492, 217, 1995. [5] N.C. Yumata, MSc Dissertation, Nelson Mandela Metropolitan, University, 2010. [6] I. Booysen, PhD Thesis, Nelson Mandela Metropolitan University, 2009.


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Chapter 3

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Chapter 3 Dimeric and Monomeric Rhenium Complexes of Orotic Acid and 2-Mercapto-orotic Acid 3.1. Introduction Considerable attention has been paid to heterocyclic compounds, specifically pyrimidine (Figure 3.1) and derivatives, as ligands for transition metals [1]. Pyrimidine is advantageous in coordination chemistry due to its ability of being functionalised, which results in multifunctional chelates with various coordination modes in the synthesis of mononuclear and polynuclear transition metal complexes [2]. Some of these complexes exhibit antibacterial, antitumour and antifungal activities and play an important role in processes such as the catalysis of drug interaction with biological molecules [3]. N N

Figure 3.1: Line structure of pyrimidine There is a renewed interest in the coordination chemistry of rhenium due, not only to the application of its

186/188

Re isotopes in therapeutic nuclear medicine, but also to its

versatility in various catalytic applications [4]. This versatility is enhanced by the large number of stable oxidation states (-I to +VII) and the types of structures that the metal can form (from monomers to ligand-bridged multimers, and even from metalmetal multiply bonded complexes to clusters) [5]. The reaction therefore of rhenium with versatile ligands may lead to interesting and unexpected coordination compounds, and to this end the focus of this study is on the multifunctionalised pyrimidine orotic acid (6-uracil carboxylic acid or 1,2,3,6-tetrahydro-2,6-dioxo-4pyrimidine carboxylic acid or vitamin B13, H2ao) and its derivative 2-mercapto-orotic acid (6-hydroxy-2-mercaptopyrimidine-4-carboxylic acid, H2moa) as possible ligands for rhenium(I), (IV) and (V). Orotic acid (Figure 3.2) is a functionalised pyrimidine and is well-known due to its great biological importance. It is the only key precursor in the biosynthesis of Nelson Mandela Metropolitan University

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Chapter 3

J. Mukiza

pyrimidine nucleotides of nucleic acids in living organisms [6, 7]. Orotic acid and its derivatives are used in medicine as a carrier for certain metal ions [8]. Recently, metal orotates have been used in curing syndromes related to metal ions deficiency es and also as promising therapeutic agents for cancer and heart diseases [7, 8]. They also exhibit bacteriostatic and cytostatic properties [9, 10]. Metal orotate complexes can also be a good building block for the construction of supramolecular systems [10]. O

O

OH

N

HN O

OH

N H

O

HS

N

OH

(b)

(a)

Figure 3.2: Line structures of (a) orotic acid (H2oa), and (b) 2-mercapto-orotic acid (H2moa) Multidentate functionality and stronger hydrogen bonding of orotic acid and its derivatives make them the versatile ligands to chelate, as well as to bridge metals ions, leading to mononuclear and polynuclear metal complexes with exceptional stability [11a]. Orotic acid is known to coordinate to metal ions through the four oxygen atoms of the carboxyl or carbonyl groups and two nitrogen atoms, forming monodentate, bidentate, bis(bidentate), tridentate bridging, tetradentate bridging and mixed chelating-bridging coordination modes [11a]. 2-Mercapto-orotic acid as well as the other pyrimidine and pyridine derivatives containing hydroxyl or thiol groups adjacent to imino nitrogen atom exhibit the enolketo tautomerisation (Scheme 3.1), both in the solid state and in solution, which can model their coordination mode to the metal ions. This tautomerisation is explained by the mobility of the unstable hydrogen atom of the OH and SH groups which is close to the basic nitrogen atom and consequently could easily be transferred to it [11b]. 2Mercapto-orotic acid may coordinate to metal through the two oxygen atoms of the carboxyl or hydroxyl groups, two nitrogen atoms and sulfur atom resulting in the mononuclear and ligand-bridged multimers, and even metal-metal multiply bonded complexes. Nelson Mandela Metropolitan University

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Chapter 3

J. Mukiza O

O

OH

N

N HS

OH

N

OH

HS

N H

O

Scheme 3.1: Enol-keto tautomerism of 2-mercaptoorotic acid (H2moa) In this study, the reaction of trans-[ReOBr3(PPh3)2] with a two-fold molar excess of orotic acid (H2oa) in ethanol led to the isolation of the ligand-bridged complex (µBr)(µ-O)(µ-oa)[Re2IVBr(OEt)2(PPh3)2] (1). The orotate anion oa2- acts as a bridging ligand with the coordination of a neutral ketonic oxygen to one rhenium ion, and the coordination of a carboxylic oxygen and amidic nitrogen to the other rhenium ion. Both ethoxides are coordinated to the rhenium bonded to the ketonic oxygen. Both rhenium ions are therefore in the +IV (d3) oxidation state, intimating the existence of a metal-metal triple bond. By changing the solvent to propan-2-ol (iPrOH), the very similar compound (µ-Br)(µ-O)(µ-oa)[Re2IVBr2(iOPr)(PPh3)2] (2) was isolated, with the difference being the replacement of the two ethoxides in 1 by a bromide and 2propanoxide in 2. By using trans-[ReOCl3(PPh3)2] as precursor in propan-2-ol containing triethylamine, the chloro homologous product of 2 was isolated from the reaction solution, i.e. (µ-Cl)(µ-O)(µ-oa)[Re2IVCl2(iOPr)(PPh3)2] (8). The reduction to Re(IV) in the bridged dimers is surprising, and it was intimated that it is the result of disproportionation of Re(V) to Re(IV) and Re(VII), as was observed earlier in the formation of (µ-O)2ReIV complexes [12]. Since the oxo group acts as a bridging ligand in complexes 1 and 2, we attempted to eliminate the oxo oxygen from the reaction matrix by using the oxo-free starting material [Re(dab)Br3(PPh3)2]

(H2dab = 1,2-diaminobenzene) as a source of

rhenium(V). The reaction of the latter with a twofold molar excess of H2oa in ethanol led to the isolation of the rhenium(V) monomer [Re(dab)Br(oa)(PPh3)2] (3), in which dab2- is co-ordinated monodentately via an imido nitrogen, and oa2- bidentately through the carboxylate oxygen and amidic nitrogen. In order to establish the effect of the removal of PPh3 as a possible ligand, the trans-dioxo complex [ReVO2(py)4]Cl was used as starting material in the reaction with H2oa, leading to the rhenium(V) monomer [ReO(py)2(OEt)(oa)] (4). The coordination mode of oa2- is the same as in


39

Chapter 3

J. Mukiza

complex 3. Complex 4 is a rare example of a rhenium(V) complex containing two pyridine ligands in cis positions to each other. For example, in the complexes [ReO(OR)Cl2(py)2] a trans,trans arrangement is adopted [13]. Orotic acid was also reacted with [ReI(CO)5Cl] in the presence of triethylamine (Et3N) and

tetraphenylphosphonium

chloride

[Ph4PCl]

and

the

complex

salt

(Ph4P)[Re(CO)3(H2O)(oa)] (5) was isolated as product. Repeating the reaction without

(Ph4P)Cl

led

to

the

isolation

of

the

similar

salt

(Et3NH)[Re(CO)3(H2O)(oa)].2H2O (6). Reported rhenium(I) complexes are mostly neutral, and a monomeric complex anion containing the tricarbonylrhenium(I) moiety is extremely rare. Neutral rhenium(I) complexes, containing water as a ligand, of the type fac-[Re(CO)3(H2O)(nc)], were synthesized from (NEt4)2[Re(CO)3Br3] and Hnc (4-imidazolecarboxylic acid; pyridine-2,4-dicarboxylic acid) in water, with nccoordinated as a monoanionic N,O-donor chelate [14]. The reaction of trans-[ReCl3(MeCN)(PPh3)2] with a two-fold molar excess of 2mercaptoorotic acid (H2moa) in ethanol under aerobic conditions led to the isolation of the doubly ligand-bridged complex (µ-Cl)(µ-O)(µ-moa)2[Re(PPh3)]2 (7). Each moa2- anion acts as a bridging ligand with the coordination of the charged thiolate sulfur to one rhenium ion, and the carboxylic oxygen and neutral pyrimidinic nitrogen to the other rhenium ion. The ligands in complex 7 contain seven negative charges which would reflect the +3 charges on one rhenium ion and +4 on the other. Both rhenium ions are bonded together and the distance Re(1)−Re(1i) = 2.5426(7) Å would reflect a bond order of 3.5. Complex 7 was also isolated from the reaction of trans-[ReOCl3(PPh3)2] with a two-fold molar excess of H2moa under reflux in ethanol.

3.2. Experimental 3.2.1. Synthesis of (μ-Br)(μ-O)(μ-oa)[Re2IVBr(OEt)2(PPh3)2] (1) To a solution of H2oa (32 mg, 0.206 mmol) in 10 cm3 of ethanol was added trans[ReOBr3(PPh3)2] (100 mg, 0.103 mmol) in 10 cm3 of ethanol. The yellow mixture was heated under reflux for 24 hours, with the colour changing to green. The solution was allowed to cool to room temperature, and a black-grey precipitate was filtered off. A small volume of dichloromethane was added to the filtrate, which was left to evaporate slowly at room temperature. Black crystals were collected after a month. Nelson Mandela Metropolitan University

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Yield: 69 %; m.p. 217 °C. IR (νmax/cm-1): ν(N−H) 3115; νs(CO2) 1717; νa(CO2) 1325; ν(C═O) 1666, 1623; ν(Re−O−Re) 691; ν(Re−N) 501; ν(Re−O) 442. 1H NMR (d6DMSO, ppm): 0.94 (t, 6H, OCH2CH3), 3.97 (q, 4H, OCH2), 7.24-7.78 (m, 31H). UVvis (DMF, λmax(ε, M-1cm-1)): 345 (34200), 361sh (28100), 457 nm (4300). 3.2.2. Synthesis of (μ-Br)(μ-O)(μ-oa)[Re2IVBr2(OiPr)(PPh3)2] (2) A mass of 32 mg of H2oa (0.206 mmol), dissolved in 10 cm3 of propan-2-ol (iPrOH), was added to trans-[ReOBr3(PPh3)2] (100 mg, 0.103 mmol) in 10 cm3 of 2-propanol. The yellow mixture was heated under reflux for 24 hours, and after cooling to room temperature a green precipitate was collected by filtration. The filtrate was left to evaporate at room temperature, and after two weeks dark green crystals were collected. Yield = 68 %; m.p. = 209 °C. IR (νmax/cm-1): ν(N−H) 3055; νs(CO2) 1710; νa(CO2) 1338; ν(C═O) 1664, 1628; ν(Re−O−Re) 690; ν(Re−N) 484; ν(Re−O) 442. 1H NMR (d6-DMSO, ppm): 1.48 (d, 6H, 2 x CH3), 4.22 (m, 1H, CH), 7.17-7.82 (m, 31H). UV-vis (DMF, λmax(ε, M-1cm-1)): 342 (13300), 497 nm (2020). 3.2.3. Synthesis of [ReBr(dab)(oa)(PPh3)2] (3) To a mixture of H2oa (32 mg, 0.206 mmol) and [Re(dab)Br3(PPh3)2] (109 mg, 0.103 mmol) in 20 cm3 of ethanol was added 3 drops of triethylamine. The resulting brownred mixture was heated under reflux for 24 hours, and after cooling to room temperature, the solution was filtered with no precipitate having been formed. A small volume of dichloromethane was added to the filtrate, which was left to evaporate at room temperature. After three weeks black crystals were harvested. Yield = 60 %; m.p. = 151 °C. IR (νmax/cm-1): ν(NH2) 2993, 3002; ν(N−H) 3226; νs(CO2) 1674; νa(CO2) 1365; ν(Re═N) 1030; ν(C═O) 1635, 1629; ν(Re−N) 498; ν(Re−O) 432. 1H NMR (d6-DMSO, ppm): 5.82 (s, 1H, NH), 6.92 (s, 1H, CH), 7.14 (dd, 2H), 7.47 (dd, 2H), 7.55-7.69 (m, 30H). UV-vis (MeOH, λmax(ε, M-1cm-1)): 273 (43800), 285sh (32200), 437 nm (5310). 3.2.4. Synthesis of [ReO(py)2(OEt)(oa)] (4) A mixture of H2oa (53 mg, 0.35 mmol) and trans-[ReO2(py)4]Cl (100 mg, 0.175 mmol) in 20 cm3 of ethanol was heated under reflux for 24 hours. Thereafter, the solution was cooled to room temperature and filtered, with no precipitate been formed. The filtrate was left to evaporate at room temperature, and orange crystals Nelson Mandela Metropolitan University

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were collected after a week. Yield = 62 %; m.p. = 202°C. IR (ν max/cm-1): ν(N−H) 3121; νs(CO2) 1663; νa(CO2) 1342; ν(C═O) 1622, 1645; ν(Re═O) 947; ν(Re−N) 528; ν(Re−O) 438. 1H NMR (d6-DMSO, ppm): 7.94 (d, 2H), 7.63 (dd, 1H), 7.55 (dd, 1H), 7.48 (t, 2H), 6.99 (d, 2H), 6.99 (t, 2H), 6.93 (s, 1H, CH), 3.96 (q, 2H, OCH2), 0.97 (t, 3H, OCH2CH3). UV-vis (MeOH, λmax(ε, M-1cm-1)): 310 (2200), 272 nm (1745). 3.2.5. Synthesis of (Ph4P)[Re(CO)3(H2O)(oa)] (5) and (Et3NH)[Re(CO)3(H2O)(oa)].2H2O (6) [Re(CO)5Cl] (100 mg, 0.276 mmol) and H2oa (43 mg, 0.276 mmol) were added to 15 cm3 of ethanol containing 4 drops of triethylamine (Et3N). To this mixture was also added a solution of tetraphenylphosphonium chloride (Ph4PCl) (103 mg, 0.276 mmol) (for 5) in 5 cm3 of water. The resulting colourless mixture was refluxed for 24 hours, giving a yellow solution, which was filtered after being cooled to room temperature. No precipitate was formed. Colourless crystals, suitable for X-ray crystallography, were grown in one month by the slow evaporation of the filtrate at room temperature. 5: Yield = 63 %; m.p. = 283 °C. IR (νmax/cm-1): ν(N−H) 3288; ν(C≡O) 2010, 1875br; νs(CO2) 1666; νa(CO2) 1363; ν(C═O) 1631, 1619; ν(Re−N) 526; ν(Re−O) 476. 1H NMR (d6-DMSO, ppm): 6.73 (s, 1H, CH), 7.66-8.05 (m, 20H, PPh4). UV-vis (MeOH, λmax(ε, M-1cm-1)): 274 (3080), 331 nm (1910). 6: Yield = 75 %; m.p. = 278 °C. IR (νmax/cm-1): ν(N−H) 3205, 3092; ν(C≡O) 2010, 1875br; νs(CO2) 1666; νa(CO2) 1363; ν(C═O) 1622; ν(Re−N) 526; ν(Re−O) 476. 1H NMR (d6-DMSO, ppm): 1.00 (t, 9H, 3CH3), 2.00 (s, 1H, NH), 2.59 (q, 6H, 3CH2), 6.73 (s, 1H, CH). UVvis (MeOH, λmax(ε, M-1cm-1)): 333 nm (1280). 3.2.6. Synthesis of (µ-Cl)(µ-O)(µ-moa)2[Re(PPh3)]2 (7) To a solution of H2moa (50 mg, 0.290 mmol) in 10 cm3 of ethanol was added trans[ReCl3(MeCN)(PPh3)2] (125 mg, 0.145 mmol) in 10 cm3 of ethanol. The orange mixture was heated under reflux for 24 hours, with the colour changing to dark brown. The solution was allowed to cool to room temperature, and no precipitate was observed. A small volume of dichloromethane was added to the filtrate, which was left to evaporate slowly at room temperature. Black crystals were collected after a three weeks. Yield: 70 %; m.p. 198 °C. IR (νmax/cm-1): ν(N−H) 3055; ν(Re−O−Re) 690; ν(Re−N) 509; ν(Re−O) 540; ν(Re−S) 455, 428. 1H NMR (d6-DMSO, ppm): 7.34-


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J. Mukiza

7.82 (m, 34H). UV-vis (MeOH, λmax(ε, M-1cm-1)): 304 (9700), 343 (6120), 456 nm (2000). 3.2.7. Synthesis of (µ-Cl)(µ-O)(µ-oa)[Re2IVCl2(OiPr)(PPh3)2] (8) A mass of 32 mg of H2oa (0.206 mmol), dissolved in 10 cm3 of propan-2-ol (iPrOH), was added to trans-[ReOCl3(PPh3)2] (86 mg, 0.103 mmol) in 10 cm3 of 2-propanol containing 3 drops of triethylamine (Et3N). The bright-green mixture was heated under reflux for 7 days, and after cooling to room temperature a dark green precipitate was collected by filtration. The recrystallization of the solid from an ethanol/dichloromethane mixture produced dark green crystals of 8.OPPh3.iPrOH after two months. Yield = 43 %; m.p. = 217 °C. IR (νmax/cm-1): ν(N−H) 3057; νs(CO2) 1701; νa(CO2) 1317; ν(C═O) 1671, 1659; ν(Re−O−Re) 691; ν(Re−N) 491; ν(Re−O) 442. 1H NMR (d6-DMSO, ppm): 1.46 (d, 6H, 2 x CH3), 4.21 (m, 1H, CH), 7.15-7.81 (m, 31H). UV-vis (Methanol, λmax(ε, M-1cm-1)): 380 (4800), 482 nm (1940). 3.2.8. X-ray crystallography Single crystal X-ray crystallography studies were performed at 200 K using a Bruker Kappa Apex II diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71069 Å). For data collection, APEX-II was used while for cell refinement and data reduction, SAINT was used [15]. The structures were solved by direct methods applying SHELXS-97 [16], or SIR97 [17] and refined by least-squares procedures using SHELXL-97 [15], with SHELXLE [17] as a graphical interface. All nonhydrogen atoms were anisotropically refined and hydrogen atoms were calculated in idealised geometrical positions. Data were corrected by absorption effects using the numerical method using SADABS [15].

3.3. Results and discussion 3.3.1. (μ-Br)(μ-O)(μ-oa)[Re2IVBr(OEt)2(PPh3)2] (1) and (μ-Br)(μ-O)(μ-oa)[Re2IVBr2(OiPr)(PPh3)2] (2) The complex 1 and 2 were prepared by the heating under reflux of trans[ReOBr3(PPh3)2] with a twofold molar excess of H2aoa in ethanol (for 1) and propan2-ol (for 2).


43

Chapter 3

J. Mukiza EtOH Reflux, 24 h

-Br)(-O)(-oa)[Re2Br(OEt)2(PPh3)2] (1)

trans-[ReOBr3(PPh3)2] + H2oa i

PrOH

Reflux, 24 h

-Br)(-O)(-oa)[Re2Br2(iPrO)(PPh3)2] (2)

The complex 1 has black grey crystals while the crystals of 2 are dark green. Both 1 and 2 are soluble in dimethylformamide, dimethylsulfoxide and acetonitrile to give dark green solution. They are weakly soluble in non-polar organic solvents like dichloromethane and chloroform. In the IR spectra of 1 and 2 the v(Re−O−Re) is displayed as a strong peak around 690 cm-1, and the coordination of the oa2- ligand is indicated by bands for v(Re−N) around 490 cm-1, v(Re−O) at 442 cm-1, and the symmetric and asymmetric CO2 stretching vibrations around 1710 and 1330 cm-1 respectively [18]. The UV-visible absorption spectra in DMF are characterized by an intense absorption around 345 nm, and a less intense band at 457 nm ( 4300) for 1 and at 497 (ε 2020) for 2. The former peak appears at 279 nm in the spectrum of the free ligand H2oa in methanol, and is ascribed to an intraligand

* transition. The

latter peak is indicative of a d-d transition. For triply-bonded metal-metal complexes of the type (µ-O)2Re2 a strong absorption band was observed at about 490 nm with extinction coefficients around 104 dm3mol-1cm-1 for Re(IV) compounds [12, 19]. This band has been assigned to the The

* transition within the Re−Re triple bond.

1

H NMR signals in solution were observed in the region expected for

diamagnetic complexes, indicating that there are no unpaired electrons in the dimers. Sharp peaks for the proton signals of the coordinated alcoxide ligands are observed, while the aromatic region is clouded by multiplets due to the presence of PPh3 protons [18]. The

31

P NMR signals in solution were also recorded and only one peak was

observed for each complex [at 29.50 ppm for 1 and at 30.20 ppm 2] [18].


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J. Mukiza

92

% Transmittance

82 72 62

Complex 1

52 42

Complex 2

32 22 12

3300

2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 3.3: Overlay IR spectra of complexes 1 and 2 The cyclic voltammetry of complexes 1 and 2 in acetonitrile exhibits simple electrochemistry during a reductive potential sweep (Figure 3.4). For complexes 1 and 2, two one-electron quasi-reversible oxidations in acetonitrile were observed at 1.01 V and 0.603 V for 1, and at 0.918 V and 0.601 V for 2. Subsequent cathodic scans did not show the reduction processes. The signal around +1 V is ascribed to the redox process Re(IV)Re(IV) to Re(V)Re(IV), with the lower value to the Re(V)Re(IV) to Re(V)Re(V) process [18].

Figure 3.4: Overlay cyclic voltammograms of complexes 1 and 2 in MeCN Nelson Mandela Metropolitan University

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Figure 3.5 shows the ORTEP drawing of the structure of complex 1. The coordination environment of each rhenium ion in the complex is different in the edgeshared bi-octahedral structure, with a bridging oxide ion O(5), a bridging bromide ion Br(2) and a bridging chelate ligand oa2-. The rhenium ion Re(2) is coordinated in a bidentate manner by N(1) and the carboxylate oxygen O(3) of oa2-, the bromide Br(1), phosphorus P(1), and the bridging oxide O(5) and bromide Br(2) ions. The rhenium ion Re(1) is coordinated by the phosphorus atom P(2), the ethoxides O(7) and O(8), the carbonylic oxygen atom O(1) of oa2-, and the bridging oxide O(5) and bromide Br(2) ions. The Re(1)−O(7) [1.891(2) Å] and Re(2)−O(8) [1.890(3) Å] bond lengths show that both ethanol molecules are coordinated in the deprotonated ethoxide form [20]. The two coordinated ethoxides are, to a small extent (< 4%), disordered with the bromides Br(3) and Br(4). The Re−Re distance is 2.5664(4) Å, which is slightly longer than in the bridged compounds

[(Ph3P)Cl2ReIV(µ-O)(µ-Cl)(µ-EtCO2)ReIVCl2(PPh3)]

(2.52

Å)

and

[(Ph3P)ClRe(µ-O)(µ-Cl)(µ-EtCO2)2ReCl(PPh3)] (2.514 Å) [21], and considerably longer than in the (µ-O)2ReIV2 complexes [Re2(µ-O)2L2](PF6)4 (2.364(1) Å, L = tris(2pyridylmethyl)amine; 2.368(1) Å, L = ((6-methyl-2-pyridyl)methyl)bis(2-pyridylmethyl)amine) [14]. The observed range for (µ-O)2Re2 complexes is 2.362– 2.381 Å [14,19, 20]. Since the rhenium ions are formally in the +4 oxidation state with a d 3-d3 electron configuration, it would intimate a Re≡Re triple bond. expected” Re≡Re bond has been explained in terms of a σ2π2

A “longer than *2

, rather than the

conventional σ2π2 2, electronic configuration [22]. The former configuration was supported by molecular orbital calculations, which intimated that the interaction of the orbitals of bridging ligands would place the energy of the

orbital higher than

that of the * orbital, thus providing some antibonding character to the Re≡Re bond [14, 22]. This may be due to the strong interaction of the p orbitals of the bridging oxo and bromide ions with the bonding molecular

orbital. The short Re(1)−O(5)

and Re(2)−O(5) bond lengths of 1.913(2) and 1.879(2) Å respectively support this destabilization of the than 3 (σ2π2 large

2

orbital, which would lead to a formal bond order of 1, rather

), and consequently to a longer Re≡Re triple bond which leads to a

Re−O(5)−Re

bond

angle

of

85.19(7)˚.

The

O(5)−Re(1)−Br(2)

and

O(5)−Re(2)−Br(2) bond angles are 106.37(5)˚ and 108.85(5)˚ respectively.


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Figure 3.5: ORTEP view of complex 1 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity In the bridging chelate oa2- the C(1)−O(1) bond is double [1.259(3) Å], and consequently the Re(1)−O(1) length of 2.168(2) Å is considerably longer than the Re(2)−O(3) bond [2.048(2) Å]. Nitrogen N(1) is coordinated as an amide, with the Re(2)−N(1) bond length equal to 2.099(2) Å [23]. The C(3)−C(4) [1.343(3)Å] and O(2)−C(2) [1.233(3) Å] bonds are double. The coordination of the two phosphines are symmetrical in the structure, with the P(1)−Re(2)−Re(1)−P(2) torsion angle equal to 4.94(4)°. The O(3)−Re(2)−Re(1)−O(8) axis deviates considerably from linearity, with the O(3)−Re(2)−Re(1) and Re(2)−Re(1)−O(8) angles equal to 148.10(5) and 146.33(9)° respectively. There is nothing unusual about the other bond lengths and angles in the molecule. The proton on nitrogen atom N(2) is involved in intramolecular hydrogen-bonds with oxygen atom O(3). The hydrogen-bond parameters are summarised in Table 3.1.


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Table 3.1: Hydrogen-bond distance (Å) and angle (o) in complex 1 D−H∙∙∙A N(2)−H(2)∙∙∙O(3)

D−H 0.8800

H∙∙∙A 1.8900

D∙∙∙A 2.769(3)

D−H∙∙∙A 179.00

Figure 3.6: Crystal packing diagram of complex 1 showing intramolecular hydrogenbonds The structure of complex 2 is shown in Figure 3.7. The coordination milieu around Re(2) is exactly the same as around Re(2) in complex 1. However, around Re(1) two sites are occupied by a bromide Br(1) and a 2-propanoxide O(7), compared to two ethoxide oxygens in 1. This difference is probably steric in nature due to the larger iso-propyl group. The Re−Re distance of 2.5639(7) Å is practically the same as in 1. The bond lengths of the oa2- donor atoms to the rhenium ions differ little from those observed in 1 [Re(2)−N(1) = 2.11(1) Å, Re(2)−O(3) = 2.058(9) Å, Re(1)−O(1) = 2.154(8) Å]. The N(1)−C(1)−O(1) angle equals 122(1)°, and the bite angle of oa2[N(1)−Re(2)−O(3) = 72.7(4)°] is smaller than in complex 1 [74.36(7)°]. The O(3)−Re(2)−Re(1)−O(7) torsion angle of 3.9(7)° differs little from the corresponding angle [4.6(2)°] in 1. Nelson Mandela Metropolitan University

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Figure 3.7: Molecular structure of complex 2 showing atom labelling. Hydrogen atoms have been omitted for clarity The proton on nitrogen atom N(2) is involved in an intramolecular hydrogen-bond with oxygen atom O(2). The hydrogen-bond parameters are summarised in Table 3.2. Table 3.2: Hydrogen-bond distance (Å) and angle (o) in complex 2 D−H∙∙∙A N(2)−H(2)∙∙∙O(2)

D−H 0.8800

H∙∙∙A 1.8700

D∙∙∙A 2.753(15)

D−H∙∙∙A 177.00

3.3.2. [ReBr(dab)(oa)(PPh3)2] (3) The monomeric complex [ReBr(dab)(oa)(PPh3)2] (3) was synthesised by reacting two equivalents of orotic acid (H2oa) with the imido-based complex [ReBr3(dab)(PPh3)2] (H2dab = 1,2-diaminobenzene) in ethanol with triethylamine (Et3N).

[ReBr3(dab)(PPh3)2] + H2oa

EtOH / Et3N

[ReBr(dab)(oa)(PPh3)2] (3) + 2 HBr

Reflux, 24 h


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The dark-green complex is air-stable, and it is soluble in the polar organic solvents dimethylformamide, dimethylsulfoxide, ethanol and methanol, and it is weakly soluble in non-polar organic solvents. In the IR spectrum the Re═N stretch appears as a medium-intensity band at 1030 cm-1 (Figure 3.8). The bands due to νsym(CO2) and νasym(CO2) appear at 1674 and 1365 cm-1. The difference of ~310 cm-1 confirms the monodentate coordination mode of the carboxylate group. The characteristic bands for ν(Re−N) and ν(Re−O) appear at 498 and 432 cm-1 respectively [18]. The 1H NMR spectrum clearly shows the presence of six phenyl rings of both PPh 3, with multiplet signal in the aromatic region [7.55-7.69 ppm]. The four protons of phenylimido moiety occur as two doublets of doublet signals appearing at 7.14 ppm and 7.47 ppm. There is a singlet at 5.82 ppm which is assigned to the NH protons. The CH proton in oa2− occurs as a singlet at 5.82 ppm [18]. The electronic spectrum in MeOH exhibits a high energy band at 273 nm, which appears at 279 nm in the spectrum of the free ligand H 2oa, and is assigned to an intra-ligand

* transition. The shoulder at 285 nm is likely to be a metal-to-ligand

charge transfer [dxy(Re) → *(oa2-) MLCT], with the lower-energy absorption at 437 nm due to a (dxy)2 → (dxy)1(dxz)1 transition (taking the N═Re−O as the z axis) [18]. 98

% Transmittance

88 78 68 58 48 38 28 18 3300

2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 3.8: IR spectrum of complex 3 Nelson Mandela Metropolitan University

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Chapter 3 The

cyclic

J. Mukiza voltammogram

of

complex

3

in

acetonitrile

exhibits

simple

electrochemistry during a reductive potential sweep. Complex 3 produces two redox waves at 1.159 V and -0.566 V. The anodic signal is assigned to the redox process Re(V) to Re(VI), whereas the signal in the cathodic region is ascribed to the Re(V) to Re(IV) reduction [18]. Figure 3.9 illustrates the molecular structure of complex 3. The rhenium Re(V) ion lies at the centre of a distorted octahedron. The basal plane is defined by the deprotonated nitrogen N(11), two phosphorus atoms P(1) and P(2), and the bromide Br(1). The deprotonated carboxylate oxygen O(11) and imido nitrogen N(21) lie in trans axial positions [O(11)−Re−N(21) = 176.2(1)°], and N(22) is not coordinated. The ligand oa2- acts as a bidentate chelate, with a bite angle O(11)−Re−N(11) of 74.0(1)°, which contributes considerably to the distortion in the complex. The N(21)−Re−Br(1), N(21)−Re−P(1), N(21)−Re−P(2) and N(21)−Re−N(11) angles are all larger than 90° [93.2(1), 96.1(1), 93.4(1) and 108.6(1)° respectively]. The Re−N(21) bond length of 1.735(3) Å falls in the range observed for rhenium(V)imido nitrogen bonds [24], and the Re−N(21)−C(21) angle of 166.1(3)° illustrates the sp hybridization of N(21). The Re−N(11) [2.152(3) Å] bond is longer than those observed for Re(V)-amide bonds, and the Re−O(11) [2.090(3) Å] bond is normal [18, 25]. Bond distances in the orotate anion clearly show the double bonds C(13)−O(13) [1.241(7) Å], C(14)−O(14) [1.229(5) Å] and C(11)−C(12) [1.351(6) Å]. Surprisingly, the O(11)−C(15)−C(11)−N(11) torsion angle is 0.0(5)°. The propensity of imidorhenium(V) complexes to form neutral complexes is illustrated by complex 3, which contains the dianionic bidentate N-,O--donor ligand oa2-, two neutral phosphines and a chloride. It is also illustrated by the reaction of [Re(p-NC6H4CH3)Cl3(PPh3)2] with quinolone-2-carboxylic acid (Hcxa) to form [Re(pNC6H4CH3)Cl2(cxa)(PPh3)], in which electroneutrality is maintained by the bidentate monoanionic chelate cxa-, one phosphine and two chlorides [14]. The protons on nitrogen atom N(22) and N(12) are involved in intramolecular hydrogen-bonds. All hydrogen-bond parameters are summarised in Table 3.3.


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Figure 3.9: ORTEP view of complex 3 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and the dichloromethane solvent of crystallisation have been omitted for clarity Table 3.3: Hydrogen-bond distances (Å) and angles (o) in complex 3 D−H∙∙∙A N(12)−H(12)∙∙∙O(13)

D−H 0.8800

H∙∙∙A 1.9300

D∙∙∙A 2.788(6)

D−H∙∙∙A 165.00

N(22)−H(22A)∙∙∙O(14)

0.8800

1.8700

2.726(6)

164.00

N(22)−H(22A)∙∙∙N(21)

0.8800

2.5600

2.857(6)

101.00

N(22)−H(22B)∙∙∙O(12)

0.8800

2.1200

2.966(6)

160.00


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Figure 3.10: Crystal packing diagram of complex 3 displaying intramolecular hydrogen-bonds 3.3.3. [ReO(py)2(OEt)(oa)] (4) The monomeric complex [ReO(py)2(OEt)(oa)] (4) was synthesised by reacting two equivalents of orotic acid (H2oa) with trans-[ReO2(py)4]Cl under reflux for 24 hours in ethanol. EtOH [ReO2(py)4]Cl + H2oa + EtOH

Reflux, 24 h

[ReO(py)2(OEt)(oa)] (4) + H2O + HCl + 2 py

The orange-coloured complex 4 is air-stable and it is soluble in polar organic solvents like dimethylformamide, dimethylsulfoxide, ethanol and methanol and it is weakly soluble in non-polar organic solvents like dichloromethane and chloroform. In the IR spectrum the ν(Re═O) appears as a medium-intensity band at 947 cm-1, with δ(OEt) as a strong band at 914 cm -1. The difference in the νsym(CO2) and νasym(CO2) is 320 cm-1. The 1H NMR spectrum clearly shows the presence and magnetic inequivalence of the two pyridine rings, with six signals in the aromatic region [6.9-8.0 ppm] [18], and they are a doublet at 7.94 ppm, a doublet of a doublet at 7.63 ppm, a doublet of a Nelson Mandela Metropolitan University

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doublet at 7.55 ppm, a triplet at 7.48 ppm, a doublet at 6.99 ppm and a triplet at 6.99 ppm.

Figure 3.11: IR spectrum of complex 4 In the electronic spectrum in MeOH the intra-ligand

* transition occurs at 272

nm, with a ligand-to-metal charge transfer (MLCT) band at 310 nm. The former intraligand

* transition peak appears at 279 nm in the spectrum of the free ligand

H2oa in methanol [18]. The

cyclic

voltammogram

of

complex

4

in

acetonitrile

exhibits

simple

electrochemistry during a reductive potential sweep. Complex 4 shows two redox waves at 1.207 V and -0.510 V. The formal signal is assigned to the oxidation process Re(V) to Re(VI), whereas the signal in the cathodic region is ascribed to the Re(V) to Re(IV) reduction [18]. In the structure of complex 4 (Figure 3.12) the octahedron around Re(1) is formed by the two pyridyl N(3) and N(4) atoms, the amido N(11) and carboxylate O(13) donor atoms of oa2- in the basal plane, and the oxo O(5) and ethoxide O(4) in trans axial positions [O(4)−Re(1)−O(5) = 175.4(2)°]. The bite angle of oa2- [N(11)−Re−O(13) = 79.7(2)°] is considerably larger than in complexes 2 and 3, with the result that the basal trans angles are relatively close to linearity [N(2)−Re−N(11) = 171.3(2)°, N(3)−Re−O(13) = 176.7(2)°]. As expected, the repulsion between the oxo O(5) and the charged basal donor atoms [O(5)−Re−N(11) Nelson Mandela Metropolitan University

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= 95.1(2)°, O(5)−Re−O(13) = 94.0(2)°] are larger than with the neutral basal donor atoms [O(5)−Re−N(2) = 89.4(2)°, O(5)−Re−N(3) = 89.1(2)°].

Figure 3.12: ORTEP view of complex 4 showing 50% probability displacement ellipsoids and atom labelling The Re═O(5) distance of 1.694(4) Å is the same as for similar trans-oxoethoxorhenium(V) complexes (average of 1.691(2) Å) [26], and it compares well with those reported for the [ReO(OEt)X2(PPh3)2] series [X = I, 1.699(4); Br, 1.715(9); Cl, 1.678(6) Å] [26]. The Re−N(pyridyl) distances are significantly unequal [Re−N(2) = 2.171(4); Re−N(3) = 2.143(4) Å], due to their trans coordination to the amido nitrogen N(11) [Re−N(11) = 2.071 (4) Å]

and carboxylate O(13) [Re−O(13) =

2.053(4) Å] respectively. The Re−O(4)(ethoxo) bond length [1.854(4) Å] is similar to those observed in the literature [18, 26, 27], but it is substantially less than 2.04 Å, which is considered to be representative of a Re(V)−O single bond [19]. Partial multiple bonding in Re−O(4) is consistent with the large Re−O(4)−C(41) angle of 166.1(4)°. The proton on nitrogen atom N(12) is involved in a hydrogen-bond with the oxygen atom O(13). Nelson Mandela Metropolitan University

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Table 3.4: Hydrogen-bond distance (Å) and angle (o) in complex 4 D−H∙∙∙A N(12)−H(12)∙∙∙O(13)

D−H 0.85(4)

H∙∙∙A 2.11(4)

D∙∙∙A 2.943(6)

D−H∙∙∙A 165(4)

Figure 3.13: Crystal packing diagram of complex 4 displaying intramolecular hydrogen-bonds 3.3.4. (Ph4P)[Re(CO)3(H2O)(oa)] (5) and (Et3NH)[Re(CO)3(H2O)(oa)].2H2O (6) The

two

rhenium(I)

complex

salts

(Ph4P)[Re(CO)3(H2O)(oa)]

(5)

and

(Et3NH)[Re(CO)3(H2O)(oa)].2H2O (6) were prepared by the heating under reflux of [Re(CO)5Cl] with a two-fold molar excess of H2ao in ethanol with triethylamine Et3N and tetraphenylphosphonium chloride Ph4PCl for 24 hours (for 5), and without Ph4PCl for 24 h (for 6), as summarised below. EtOH / Et3N Ph4PCl / H2O, Reflux 24 h

(Ph4Cl)[Re(CO)3(H2O)(oa)](5) + 2 HCl + 2 CO

[Re(CO)5Cl] + H2oa EtOH / Et3N, H2O Reflux, 24 h

(Et3NH)[Re(CO)3(H2O)(oa)].2 H2O (6) + HCl + 2 CO

Complexes 5 and 6 are both yellow in colour and they are soluble in polar organic solvents, but weakly soluble in non-polar organic solvents like dichloromethane, chloroform and toluene.


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The IR spectra of 5 and 6 (Figure 3.14) display a strong sharp band at 2010 cm-1 and a broad band at 1875 cm -1. These patterns are typical in a facial isomer arrangement around the rhenium(I) [5]. The difference in the νsym(CO2) and νasym(CO2) is 300 cm-1, and the bidentate coordination of oa2- is shown by ν(Re−N) at 526 and ν(Re−O) at 476 cm-1.

% Transmittance

90 80 70

Complex 5

60 50

Complex 6 40 30 3300

2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 3.14: Overlay IR spectra of complexes 5 and 6 In the UV-visible spectrum the intra-ligand

* transition is observed at 274 nm,

and the only other band in the spectrum at 331 nm (for complex 6) and 333 nm (for complex 7) are assigned to metal-to-ligand charge transfer (MLCT), d(Re) → *(oa2-) [18, 28]. In the 1H NMR spectra of complexes 5 and 6 the two one-proton singlets at 5.68 and 5.77 ppm are ascribed to the CH and NH protons on the oa2- ring respectively [18]. The structures of 5 and 6 are shown in Figures 3.15 and 3.17. The rhenium(I) atom lies in a distorted octahedral environment, with the water O(5) and two donor atoms N(1) and O(3) of oa2- in a facial arrangement, imposed by the fac-[Re(CO)3]+ core. The distortion is mainly the result of the trans angles N(1)−Re−C(7) = 172.12(8)°, O(3)−Re−C(6) = 174.29(7)° and O(5)−Re−C(8) = 174.38(7)° for complex 5 and N(11)−Re−C(3) = 171.39(11)°, O(13)−Re−C(2) = 174.40(11)° and O(5)−Re−C(4) = 172.74(13)° for complex 6 (Table 3.12). Again, the chelate oa2- is dianionic with the Nelson Mandela Metropolitan University

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charged amido N(1) and O(3) (for complex 5) and N(11) and O(13) (for complex 6) donor atoms forming a bite angle of 75.54(6)° and 75.16(8)° for 5 and 6 respectively. In complex 5, the N(1)−C(1)−C(5)−O(3) torsion angle is 9.2(2)° and the corresponding torsion angle in 6 [N(11)−C(11)−C(15)−O(13)] equals 1.8(4)o.

Figure 3.15: ORTEP view of the molecular structure of complex 5 showing 50% probability displacement ellipsoids and atom labelling

The Re−N(1) [2.164(2) Å] and Re−O(3) [2.126(2) Å] bond lengths for 5 and their corresponding Re−N(11) [2.168(2) Å] and Re−O(13) [2.137(2) Å] bond lengths for complex 6 are, as expected, longer than those observed in the rhenium(IV) and (V) complexes 1-4. The Re−O(5)H2 bond distance of 2.205(2) Å for complex 5 and Re−O(5)H2 bond distance of 2.186(2) Å for complex 6 are the same as that found [2.198(5) Å] in fac-[Re(CO)3(H2O)(nc)], referred to earlier [14]. The three Re−C bond lengths [average 1.900(2) Å for 5 and 1.902(3) Å for 6] fall at the lower end of the range observed [1.900(2)−1.928(2) Å] for similar complexes [29]. Nelson Mandela Metropolitan University

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The protons on nitrogen atom N(2) and oxygen atom of the coordinated water molecule O(5) are involved in intramolecular hydrogen-bonds. All hydrogen-bond parameters are summarised in Table 3.5. Table 3.5: Hydrogen-bond distances (Å) and angles (o) in complex 5 D−H∙∙∙A N(2)−H(2)∙∙∙O(2)

D−H 0.8400

H∙∙∙A 2.0100

D∙∙∙A 2.886(2)

D−H∙∙∙A 176.00

O(5)−H(5A)∙∙∙O(2)

0.831(11)

1.941(14)

2.7149(19)

155(2)

O(5)−H(5B)∙∙∙O(1)

0.840(13)

1.774(13)

2.611(2)

174(2)

Figure 3.16: Crystal packing diagram of complex 5 displaying intramolecular hydrogen-bonds In complex 6 the protons on the water of crystallisation and the nitrogen atom of the cation Et3NH+ are involved in intermolecular hydrogen-bonds with themselves and the anion [Re(CO)3(H2O)(oa)]− (Figure 3.18). The anion [Re(CO)3(H2O)(oa)]− itself also displays intramolecular hydrogen-bonds. All hydrogen-bonds parameters are summarised in Table 3.6 Nelson Mandela Metropolitan University

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. Figure 3.17: ORTEP view of complex 6 showing 50% probability displacement ellipsoids and atom labelling

Figure 3.18: Crystal packing diagram of complex 6 displaying hydrogenbonds Nelson Mandela Metropolitan University

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Table 3.6: Hydrogen-bonds distances (Å) and angles (o) in complex 6 D−H∙∙∙A O(5)−H(5A)∙∙∙O6)

D−H 0.85(4)

H∙∙∙A 1.83(4)

D∙∙∙A 2.655(4)

D−H∙∙∙A 166(3)

O(5)−H(5B)∙∙∙O(11)

1.791(14) 2.629(3)

2.271(9)

173(5)

O(6)−H(6A)∙∙∙O(14)

0.83(3)

1.9(3)

2.78(1)

174(4)

O(6)−H(6B)∙∙∙O(12)

0.84(4)

1.94(4)

2.734(4)

157(4)

O(7)−H(7A)∙∙∙O13)

0.84(4)

1.93(4)

2.746(4)

165(4)

O(7)−H(7B)∙∙∙O(14)

0.84(5)

2.21(5)

2.940(4)

146(3)

N(8)−H(8)∙∙∙O(7)

0.87(5)

1.83(5)

2.698(4)

177(7)

N(12)−H(12)∙∙∙Br(1)

0.81(4)

2.05(5)

2.863(3)

175(5)

3.3.5. (µ-Cl)(µ-O)(µ-moa)2[Re(PPh3)]2 (7) The dimeric complex (µ-Cl)(µ-O)(µ-moa)2[Re(PPh3)]2 (7) was prepared by the heating under reflux of trans-[ReCl3(MeCN)(PPh3)2] with a twofold molar excess of 2mercapto-orotic acid (H2moa) in ethanol under aerobic conditions. Complex 7 was also isolated in high yield from the reaction of trans-[ReOCl3(PPh3)2] with a twofold molar excess of H2moa under reflux in ethanol. It is soluble in alcohol, dimethylformamide, chloroform, dichloromethane, dimethylsulfoxide and acetonitrile to give brown solutions. Figure 3.21 shows the ORTEP drawing of the molecular structure of complex 7. The coordination environment of each rhenium ion in the complex is similar in the edgeshared bi-octahedral structure, with a bridging chloride Cl(1), a bridging oxide ion O(4) and two bridging chelate ligands moa2-. Each rhenium ion is coordinated in a bidentate manner by the neutral pyrimidinic nitrogen atom N(1) and the carboxylate oxygen O(1) of one moa2-, and a thiolate sulfur S(1) of the second moa2−, in addition to a phosphorus atom P(1). The oxidation state of the rhenium atoms in 7 is different from that in 1 and 2. The intramolecular bond lengths in the tridentate moa 2− chelates indicate that they are both dianionic. Both the C(1)−O(1) [1.30(1) Å] and S(1)−C(14) [1.74(1) Å] bonds are single. In the pyrimidine ring the C(11)−C(12) Nelson Mandela Metropolitan University

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[1.34(2) Å] and N(1)−C(14) [1.33(1) Å] bonds are double. Each moa2− ligand therefore coordinates in the form:

Re

Re

N

S

O O NH O Figure 3.19: Coordination mode of moa2− anion to rhenium in complex 7 Together with the negative charges provided by the bridging Cl(1) and oxo O(4), the rhenium metal atoms are therefore in the +4 and +3 oxidation states, intimating that the rhenium-rhenium metal-metal bond has a bond order of 3.5. Complex 7 therefore has a unique metal-metal (d3−d4) bond with 3.5 formal bonds between the two rhenium atoms in the face-sharing bi-octahedral structure. The five d orbitals of each Re center split into t2g- and eg-like orbitals, with the latter unoccupied in a formal sense since they are of much higher energy than the formal orbitals. The t2g-type orbitals combine to form a σ(a’1), a degenerate pair of (e’), a pair of

*

(e’’) and a

σ*(a’’2) molecular orbital [30].

 

t2g

t2g



 Figure 3.20: Molecular orbitals energy diagram for the ReIV−ReIII unit in complex 7


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The electronic structure of Re−Re bond in 7 is therefore described by the configuration σ2

4 *1

. However, molecular orbital calculations have shown that the

natural orbital populations of the

and

*

orbitals in a similar d3−d4 dimer are 3.47

and 1.53 respectively, quite different from the formal values of 4.0 and 1.0 for *

and

orbitals respectively [30]. This scenario will lower the bond order and lengthen the

Re−Re bond.

Figure 3.21: Molecular structure of complex 7 showing 40% probability displacement ellipsoids and atom labelling. Hydrogen atoms, disorded PPh 3 and water of crystallisation have been omitted for clarity The bond distance Re(1)−O(4) of 1.916(8) Å, in which O(4) is the bridging oxide, is in good agreement with the corresponding value in complex 1. The Re−O(4)−Re bond angle of 83.1(4)˚ is also close to the 85.19(7)˚ found in 1. The bond angle Re−Cl−Re [63.14(8)˚] in 7 is larger than the Re−Br−Re [59.50(1)˚] angle in 1. The bond distances Re(1)−S(1i), Re(1)−N(1), Re(1)−Cl(1) and Re(1)−O(1) are equal to 2.392(2) Å, 2.154(7) Å, 2.428(3) Å and 2.071(7) Å respectively, with the trans angles Cl(1)−Re(1)−P(1) [171.40(7)˚], O(1)−Re(1)−O(4) [162.2(3)˚] and S(1i)−Re(1)−N(1) Nelson Mandela Metropolitan University

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[169.9(2)˚] showing a significant deviation from linearity. The non-linearity of these trans angles affects the bond angles S(1i)−Re(1)−O(1) [97.7(2)˚], O(1)−Re(1)−N(1) [75.7(3)˚], Cl(1)−Re(1)−S(1i) [93.83(6)˚] and Cl(1)−Re(1)−O(4) [106.9(2)˚], which significantly deviate from orthogonality. The proton on nitrogen atom N(2) is involved in an intermolecular hydrogen-bond with oxygen atom O(6) of crystallisation water. The proton on the water of crystallisation is also involved in an intermolecular hydrogen-bond with oxygen atom O(2) of the one of moa2− chelate. The hydrogen-bond parameters are summarised in Table 3.7. Table 3.7: Hydrogen-bond distances (Å) and angles (o) in complex 7 D−H∙∙∙A N(2)−H(2)∙∙∙O(6)

D−H 0.8800

H∙∙∙A 1.9400

D∙∙∙A 2.810(14)

D−H∙∙∙A 170.00

O(6)−H(6A)∙∙∙O(2)

0.8700

2.5600

2.881(15)

103.00

Figure 3.22: Crystal packing diagram of complex 7 showing intermolecular hydrogen-bonds (blue-dashed) Nelson Mandela Metropolitan University

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The infrared spectrum of 7 displays an intense peak at 690 cm-1, which is ascribed to v(Re−O−Re). The coordination of moa2− to rhenium is supported by the weak intensity peaks at 455 and 428 cm-1 which are assigned to v(Re−S), an intense peak at 509 cm-1 assigned to v(Re−O)CO2- and the medium intensity peak at 540 cm-1 is assigned to v(Re−N). The enol-keto tautomerisation in moa2− is supported by the weak intensity peak at 3055 cm-1 which is ascribed to v(N−H). The

1


diamagnetic complexes, indicating that there are no unpaired electrons in the dimer. The aromatic region is dominated by multiplet signals due to the presence of the PPh3 protons. The UV-visible absorption spectrum of 7 in methanol is characterized by an absorption around 304 nm (ε ~ 9700), assigned to the intraligand

* transition in

moa2−, and a less intense at 343 nm (ε 6120) due to a ligand-to-metal charge transfer transition (LMCT). The peak at 456 nm (ε 2000) is indicative of a d-d transition. For triply-bonded metal-metal complexes of the type (µ-O)2Re2 a strong absorption bond was observed at about 490 nm with extinction coefficients around 104 dm3mol-1cm-1 for Re(IV) compounds. This band has therefore been assigned to * transition within the Re−Re metal-metal bond [13]

the

0.96 0.86

Absorbance

0.76 0.66 0.56 0.46 0.36 0.26 0.16 0.06 290

340

390

440

490

540

Wavelength (nm) Figure 3.23: UV-Vis spectrum of complex 7 in MeOH


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The cyclic voltammogram in acetonitrile shows simple electrochemistry for both reductive and oxidative waves. The sweep in positive potential shows non-reversible processes at 1.147 and 0.545 V. The former peak is ascribed to the oxidation from Re(IV)Re(III) (d3−d4) to Re(IV)Re(IV) (d3−d3), with the lower value due to the oxidation from Re(IV)Re(IV) (d3−d3) to Re(V)Re(V) (d2−d2). The sweep in negative potential shows two non-reversible peaks at -1.672 and -1.166 V, which are assigned to the reduction from Re(IV)Re(IV) (d3−d3) to Re(III)Re(IV) (d4−d3) and Re(IV)Re(III) ((d3−d4) to Re(III)Re(III) (d4−d4) respectively. 3.3.6. (µ-Cl)(µ-O)(µ-oa)[Re2IVCl2(OiPr)(PPh3)2] (8) The attempts to prepare the chloro analogues of complexes 1 and 2 using trans[ReOCl3(PPh3)2] as starting complex with H2oa under similar reaction conditions were unsuccessful, and only led to the recovery of the starting material. However, the reaction of trans-[ReOCl3(PPh3)2] with H2oa in propan-2-ol containing 3 drops of triethylamine (Et3N), with the increase of the reaction time from 24 hours to 7 days was successful, and led to the isolation of the chloro analogue of complex 2, i.e. (µCl)(µ-O)(µ-oa)[Re2IVCl2(OiPr)(PPh3)2] (8). i

trans-[ReOCl3(PPh3)2] + H2oa

PrOH / Et3N

-Cl)-O)(-oa)[Re2IVCl2(OiPr)(PPh3)2] (8)

Reflux, 7 days

The complex 8 is dark green in the solid state and in solution, and it is soluble in alcohols, dimethylformamide, dimethylsulfoxide and acetonitrile, and weakly soluble in solvents such as dichloromethane and chloroform. The peak at 380 nm (ε 4800) in the UV-visible absorption spectrum of 8 in methanol is assigned to a ligand-to-metal charge transfer transition (LMCT). The peak at 482 nm (ε 1940) reflects a d-d transition. For triply-bonded metal-metal complexes of the type (µ-O)2Re2 a strong absorption bond was observed at about 490 nm with extinction coefficients around 104 dm3mol-1cm-1 for Re(IV) compounds. This band has therefore been assigned to the

* transition within the Re−Re metal-metal

bond [13].


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0.94 0.84

Absorbance

0.74 0.64 0.54 0.44 0.34 0.24 0.14 330

380

430

480

530

Wavelength (nm) Figure 3.24: UV-Vis spectrum of complex 8 in MeOH The strong peak at 691 cm-1 in the IR spectrum of 8 is assigned to v(Re−O−Re). The coordination of the oa2- chelate is indicated by bands for v(Re−N) at 491 cm-1, v(Re−O) at 442 cm-1, and the symmetric and asymmetric CO2 stretching vibrations at 1701 and 1317 cm-1 respectively. The low intensity band at 3057 cm-1 is ascribed to v(N−H). In the cyclic voltammogram of 8 in acetonitrile the sweep at positive potentials shows non-reversible processes at 1.237 and 0.508 V, which are due to the oxidation processes from Re(IV)Re(IV) (d3−d3) to Re(IV)Re(V) (d3−d2) and Re(IV)Re(V) (d3−d2) to Re(V)Re(V) (d2−d2) respectively. The sweep in negative potential shows two non-reversible peaks at -1.01 and -0.707 V, which are assigned to the reduction processes from Re(IV)Re(IV) (d3−d3) to Re(III)Re(IV) (d4−d3) and Re(III)Re(IV) ((d4−d3) to Re(III)Re(III) (d4−d4) respectively. The 1H NMR signal of complex 8 in solution were observed in the region expected for diamagnetic complexes, indicating that there are no unpaired electrons in the dimmer, and it is practically similar to that of complex 2.


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91

% Transmittance

81 71 61 51 41 31 2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 3.25: IR spectrum of complex 8 The crystal structure of complex 8 is shown in Figure 3.26. The coordination milieu around the rhenium(IV) ions is different in the edge-shared bi-octahedral structure. Only one oa2- ligand bridges the two metal centres, in addition to the oxide O(5) and chloride Cl(2) bridges. The donor atoms N(1) and carboxylate O(3) are coordinated to Re(1), together with P(1) and Cl(1). The neutral ketonic oxygen O(1) is bonded to Re(2), which is also coordinated to the 2-propanoxide O(8), chloride Cl(3) and P(2). The Re−Re distance of 2.5341(4) Å is considerably shorter than 2.5664(4) Å in complex 1 and 2.5639(7) Å in 2, but is longer compared to the compounds with only chloride and oxo bridges, i.e. [(Ph3P)Cl2ReIV(µ-O)(µ-Cl)(µ-EtCO2)ReIVCl2(PPh3)] [2.52(1) Å], [(Ph3P)ClRe(µ-O)(µ-Cl)(µ-EtCO2)2ReCl(PPh3)] [2.514 Å] [13] and [(Ph3P)Cl2ReIV(µ-O)(µ-Cl)(µ-MeCO2)ReIVCl2(PPh3)]

[2.5124(6)

Å]

[31].

The

Re−O(5)−Re bond angle of 83.2(2)° is smaller than in complex 1 [85.19(7)°] and [83.7(3)°] in 2, mainly due to the larger Re−Re distance in both complexes 1 and 2 compared to complex 8. The bond lengths of the oa2- donor atoms to the rhenium are close to those observed in 1 and 2 [Re(1)−N(1) = 2.098(4) Å, Re(1)−O(3) = 2.043(4) Å, Re(2)−O(1) = 2.152(4) Å]. The bond angle O(1)−C(1)−N(1) and the bite angle O(3)−Re(1)−N(1) equal to 121.0(5)° and 74.2(2)° respectively, and they are in good agreement with the similar bond angles in complexes 1 and 2.


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Figure 3.26: Crystal structure of complex 8 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms, the triphenylphosphine oxide and the 2-propanol of crystallisation have been omitted for clarity There is one intermolecular hydrogen-bond between the proton on the nitrogen atom N(2) and the oxygen atom O(7) of co-crystallised triphenylphosphine oxide OPPh3 [N(2)−H(2)∙∙∙O(7)], with D−H = 0.8800 Å, H…A = 1.8200 Å, D…A = 1.695(7) Å and D−H…A = 172.00 o.


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Figure 3.27: Crystal packing diagram of complex 8 showing intermolecular hydrogen-bonds (blue-dashed)

3.4. Conclusion The reaction of orotic acid (H2oa) with different rhenium(V) and (I) starting materials were studied. The complex (µ-Br)(µ-O)(µ-oa)[Re2IVBr(OEt)2(PPh3)2] (1) was obtained from the reaction of H2oa with [ReOBr3(PPh3)2] in ethanol. The complexes (µ-X)(µO)(µ-oa)[Re2IVX2(iOPr)(PPh3)2] (X = Cl(8),

Br(2)) were also obtained from the

reaction of H2oa with [ReOX3(PPh3)2] in propan-2-ol. The anion oa2- in 1, 2 and 8 is coordinated as a tridentate bridging ligand with the coordination of a neutral ketonic oxygen to one rhenium ion, and the coordination of a carboxylic oxygen and amidic nitrogen to the other rhenium ion. Rhenium(V) monomers [Re(dab)Br(oa)(PPh3)2] (3) and [ReO(py)2(OEt)(oa)] (4) were obtained from the reaction of H2oa with [Re(dab)Br3(PPh3)2] and [ReO2(py)4]Cl respectively in ethanol. The anionic rhenium(I) monomer [Re(CO)3(H2O)(oa)]− (5) was isolated from the reaction of H2oa with [Re(CO)5Cl] in ethanol. The anion oa2- in 3, 4 and 5 is bidentately coordinated through the carboxylate oxygen and amidic nitrogen.


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The reaction of 2-mercapto-orotic acid (H2moa) with [ReCl3(MeCN)(PPh3)2] in ethanol under aerobic conditions led to the doubly ligand-bridged dimeric complex (µ-Cl)(µ-O)(µ-moa)2[Re(PPh3)]2 (7), with seven negative charges, which reflects the existence of Re(III) and Re(IV) ions. Each moa2- anion acts as a bridging ligand with the coordination of the charged thiolate sulfur to one rhenium ion, and the carboxylic oxygen and neutral pyrimidinic nitrogen to the other rhenium ion. The two Re(III) and Re(IV) ions are bonded together with a bond order of 3.5. The results in this study show that the dimeric ReIV≡ReIV complex formation only occurred when all of triphenylphosphine, a halide and an oxo group in alcohol are present in the rhenium(V) starting material and reaction solution. When any of these components are absent, monomeric complexes containing orotate were isolated.

3.5. References [1]

A.N. Srivastava, N.P. Singh, C.K. Chriwastaw, J. Serb. Chem. Soc., 79, 421, 2014; M.L. Tong, L. Zhuo-Jia, W. Li, Z. Shao-Liang, C. Xiao-Ming, Cryst. Growth Des. , 2, 443, 2002.

[2]

P.O. Lumme, H. Knuuttila, Polyhedron, 14, 1553, 1995; M.O.Q. Susana, I.S.H. Nogueira, V. Felix, G.B.M. Drew, Polyhedron, 21, 2783, 2002.

[3]

M. Sonmez, M. Celebi, I. Berber, Eur. J. Med. Chem., 45, 1935, 2010; M. Hazra, T. Dolai, A. Pandey, S.K. Dey, A. Patra, Bioinorg. Chem. Appl., 2, 2014, 2014.

[4]

G.S. Owens, J. Arias, M.M. Abu-Omar, Catal. Today, 55, 317, 2000.

[5]

F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic Chemistry, 6th ed., J. Wiley Interscience, New York, 1999.

[6]

J. Lieberman, A. Kornerg, E. S. Simms, J. Biol. Chem., 215, 403, 1995.

[7]

D. Michalska, K. Hernik, R. Wysokiński, B. Morzyk-Ociepa, A. Piertraszko, Polyhedron, 26, 4303, 2007.

[8]

O.Z. Yaşilel, M. S. Soylu, H. Ölmez, O. Büyükgüngör, Polyhedron, 25, 2985, 2006.

[9]

O.Z. Yaşilel, H.Paşaoğlu, K. Akdağ, O. Büyükgüngör, Polyhedron, 26,


71

Chapter 3

J. Mukiza

2761, 2007. [10]

P. Castan, S. Wimmer, E. Colacio-Rodriguez, A.L. Beauchamp, S. Cros, J. Inorg. Biochem., 38, 225, 1990.

[11] (a) O.Z. Yaşilel, O. Büyükgüngör, H. Erer, A. Mutlu, Polyhedron, 28, 156, 2009. (b) M.O.Q. Susana, I.S.H. Nogueira, V. Felix, G.B.M. Drew, Polyhedron, 21, 2783, 2002; R.A. Coxall, S.G. Harris, D.K. Henderson, S. Persons, P.A. Tasker, R.E.P. Winpenny, J. Chem. Soc. Dalton Trans., 2349, 2000. [12]

G. Bohm, K. Wieghardt, B. Nuber, J. Weiss, Inorg. Chem., 30, 3464, 1991; H. Sugimoto, M. Kamei, K. Umakoshi, Y. Sasaki, M. Suzuki, Inorg. Chem., 35, 7082, 1996.

[13]

R. Graziani, U. Casellato, R. Rossi, A. Marchi, J. Cryst. Spectrosc. Res. 15, 573, 1985; S. Fortin, A.L. Beauchamp, Inorg. Chim. Acta, 279, 159, 1998.

[14]

S. Mundwiler, M. Kündig, K. Ortner, R. Alberto, Dalton Trans., 1320, 2004.

[15]

APEX2, SADABS, SAINT, 2010, Bruker AXS Inc., Madison, Wisconsin, USA.

[16]

A. Altomare, M.C. Burla, G. Cascarano, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polodori, J. Appl. Cryst., 28, 842, 1995.

[17]

C.B. Hubschle, G.M. Sheldrick, B. Dittrich, J. Appl. Cryst., 44, 1281, 2011.

[18]

J. Mukiza, T.I.A. Gerber, E. Hosten, Polyhedron, 98, 251, 2015.

[19]

S. Ikari, T. Ito, W. McFarlane, M. Nasreldin, B.-L. Ooi, Y. Sasaki, A.G. Sykes, J. Chem. Soc., Dalton Trans., 2621, 1993.

[20]

T. Lis, Acta Crystallogr., B31, 1594, 1975.

[21]

F.A. Cotton, B.M. Foxman, Inorg. Chem., 7, 1784, 1968; F.A. Cotton, R. Eiss, B. Foxman, Inorg. Chem., 8, 950, 1969.

[22]

H.B. Bürgi, G. Anderegg, P. Bläuenstein, Inorg. Chem., 20, 3829, 1981.

[23]

L. Wei, J.W. Babich, Inorg. Chem., 43, 6445, 2004; F. Refosco, F. Tisato, C. Bolzati, G. Bandoli, J. Chem. Soc., Dalton Trans., 605, 1993.

[24] B. Machura, M. Wolff, I. Gryca, Inorg. Chim. Acta, 370, 7, 2011. [25] S.M.O. Quintal, H.I.S. Nogueira, V. Felix, M.G.B. Drew, New J. Chem., 24, Nelson Mandela Metropolitan University

72

Chapter 3

J. Mukiza

511, 2000; T.I.A. Gerber, D. Luzipo, P. Mayer, J. Coord. Chem., 57, 1399, 2004. [26]

A.M. Lebuis, C. Roux, A.L. Beauchamp, Acta Crystallogr., C49, 33, 1993.

[27]

J.M. Mayer, Inorg. Chem., 27, 3899, 1988.

[28]

J. Bossert, C. Daniel, Chemistry, 12, 4835, 2006.

[29]


[30]

K. Sacto, Y. Nakao, H. Sato, S. Sakaki J. Phys. Chem. A, 110, 9710, 2006.

[31]

J. Mukiza, T.I.A. Gerber, E. Hosten, R. Betz, Z. Kristallogr. NCS, 230, 283, 2015.


73

Chapter 3

J. Mukiza

Table 3.8: Crystal and structure refinement data for complexes 1 and 2 1

2

Formula

C45H41Br2N2O7P2Re2

C44H39Br3N2O6P2Re2

Formula Weight

1319.13

1365.83

Crystal System

Triclinic

Triclinic

Space group

P-1

P-1

a (Å)

9.8794(3)

11.0670(5)

b (Å)

13.2646(4)

14.6585(8)

c (Å)

18.8904(6)

17.6100(9)

α (deg.)

82.434(1)

112.273(2)

β (deg.)

76.571(1)

92.356(2)

γ (deg.)

75.837(1)

95.340(2)

Volume (Å3)

2327.28(12)

2621.9(2)

Z

2

2

Density (g/cm3)

1.882

1.730

Absorption coefficient (mm-1)

7.085

7.001

F(000)

1265

1300

θ range (deg.)

2.0-28.4

1.9-27.9

Index ranges h

-13/13

-13/14

k

-17/17

-19/19

l

-23/25

-23/22

Reflection measured

41792

42784

Independent/observed reflections

11551/10161

12234/7433

Data/parameters

11551/562

12234/534

Goodness-of-fit on F2

1.03

1.05

Final R indices [I>2σ(I)]

0.0173

0.0603

(wR2 = 0.0372)

(wR2 = 0.1991)

0.83/-0.97

4.35/-2.66

-3

Largest diff. peak/hole (eÅ )


74

Chapter 3

J. Mukiza


4

Formula

C47H38BrN4O4P2Re.CH2Cl2 C17H17N4O6Re

Formula Weight

1135.79

559.56

Crystal System

Triclinic

Tetragonal

Space group

P-1

P41

a (Å)

10.5785(4)

8.0850(2)

b (Å)

13. 5929(5)

8.0850(2)

c (Å)

17.6877(6)

28.7348(7)

α (deg.)

80.479(2)

β (deg.)

77.652(2)

γ (deg.)

77.422(2)

Volume (Å3)

2405.98(15)

1878.31(10)

Z

2

4

Density (g/cm3)

1.568

1.979


3.580

6.512

F(000)

1124

1080

θ range (deg.)

2.0-28.4

2.5-28.3

Index ranges h

-14/14

-10/10

k

-18/18

-10/10

l

-23/23

-38/38

Reflection measured

42252

18699


11960/10583

4665/4397

Data/parameters

11960/563

4665/258


1.11

1.08


0.0344

0.0186

(wR2 = 0.0904)

(wR2 = 0.0384)

2.66/-2.18

0.79/-0.85

-3



75

Chapter 3

J. Mukiza

Table 3.10: Crystal and structure refinement data for complexes 5 and 6 5 Formula

C24H20P.C8H4N2O8Re

6 C8H4N2O8Re. C6H16N.2H2O

Formula Weight

781.71

580.57

Crystal System

Monoclinic

Triclinic

Space group

P21/n

P-1

a (Å)

8.8737(4)

9.5669(6)

b (Å)

15.7579(7)

11. 0393(6)

c (Å)

21.5553(10)

11.5896(6)

α (deg.)

80.479(2)

63.386(2)

β (deg.)

100.051(2)

81.205(2)

γ (deg.)

77.422(2)

66.437(2)

Volume (Å3)

2967.8(2)

1002.56(10)

Z

4

2

1.568

1.923

Absorption coefficient (mm )

4.204

6.116

F(000)

1536

568

θ range (deg.)

1.9-28.3

2.0-28.3

Index ranges h

-11/11

-12/12

k

-21/118

-14/14

l

-28/27

-15/15

Density (g/cm3) -1

Reflection measured

41717

18283


7386/6589

4992/4801

Data/parameters

7386/405

4992/277


1.03

1.08


0.0180

0.0182

(wR2 = 0.0374)

(wR2 = 0.0472)

1.62/-0.66

1.82/-1.15

Largest diff. peak/hole (eÅ-3)


76

Chapter 3

J. Mukiza


8

C46H34ClN4O7P2Re2S2.

C44H39Cl3N2O6P2Re2.

2H2O

C18H15O.C3H8O

Formula Weight

1 324.73

1570.85

Crystal System

Monoclinic

Triclinic

Space group

C2/c

P-1

a (Å)

23.3695(10)

12.3941(6)

b (Å)

11.2188(5)

12.9600(6)

c (Å)

19.1409(9)

21.096(1)

α (deg.)

90.701(2)

β (deg.)

106.005(2)

γ (deg.)

95.057(2) 113.204(2)

Volume (Å3)

4823.4(8)

Z

3098.3(3)

4

2

1.824

1.684


5.281

4.167

F(000)

5272

1552

θ range (deg.)

2.0−28.4

1.7−28.4

Index ranges h

-31/31

-16/16

k

-14/14

-17/17

l

-25/25

-28/28

Reflection measured

63457

55093


6037/5293

15300/12718

Data/parameters

6037/300

15300/738


1.45

1.05


0.0523

0.0393

(wR2 = 0.14131)

(wR2 = 0.1124)

3.15/-2.74

1.96/-3.56

Density (g/cm3) -1



77

Chapter 3

J. Mukiza

Table 3.12: Selected bond lengths (Å) and angles (°) for complexes 1 and 2 Bond length 1 Re(1)−Re(2)

2.5664(4)

2 Re(1)−O(5)

1.942(7)

R(1)−O(5)

1.913(2)

Re(1)−Re(2)

2.5639(7)

Re(2)−O(5)

1.879(2)

Re(1)−Br(2)

2.572(2)

Re(1)−Br(2)

1.6033(3)

Re(1)−O(1)

2.154(8)

Re(2)−Br(2)

1.5682(4)

Re(1)−O(7)

1.864(8)

Re(2)−N(1)

2.099(2)

Re(2)−O(5)

1.902(8)

Re(1)−O(1)

2.168(2)

Re(2)−Br(2)

2.565(2)

Re(1)−O(7)

1.891(2)

Re(2)−Br(3)

2.442(1)

Re(2)−O(3)

2.048(2)

Re(2)−O(3)

2.058(9)

C(2)−O(2)

2.233(3)

Re(2)−N(1)

2.11(1)

C(1)−O(1)

1.259(3)

C(2)−O(2)

1.22(2)

C(3)−C(4)

1.343(3)

C(3)−C(4)

1.33(2)

Bond angles 1 O(3)−Re(2)−Re(1)

148.10(5)

2 Re(1)−O(5)−Re(2)

83.7(3)

Re(2)−Re(1)−O(8)

146.33(9)

N(1)−Re(2)−O(3)

72.7(4)

N(1)−Re(2)−O(3)

72.7(4)

O(5)−Re(1)−O(7)

157.1(3)

Re(1)−O(5)−Re(2)

85.19(7)

Re(1)−Br(2)−Re(2)

59.88(4)

O(3)−Re(2)−O(5)

156.36(7)

O(1)−Re(1)−Br(1)

176.0(2)

Re(1)−Br(2)−Re(2)

59.50(1)

O(3)−Re(2)−Re(1)

145.3(2)

N(1)−Re(2)−Br(1)

167.87(6)

N(1)−C(1)−O(1)

122(5)

O(1)−Re(1)−O(7)

170.21(8)

Br(3)−Re(2)−N(1)

160.2(3)

N(1)−C(1)−O(1)

122(1)

Br(1)−Re(1)−O(1)

176.0(2)


78

Chapter 3

J. Mukiza

Table 3.13: Selected bond lengths (Å) and angles (°) for complexes 3 and 4 Bond length 3 Re(1)−N(21)

1.735(3)

4 Re(1)−O(5)

1.694(4)

Re(1)−O(11)

2.090(3)

Re(1)−N(11)

2.071(4)

Re(1)−N(11)

2.152(3)

Re(1)−N(2)

2.171(4)

Re(1)−P(1)

1.497(1)

Re(1)−O(4)

1.851(4)

Re(1)−P(2)

2.487(1)

Re(1)−O(13)

2.053(4)

Re(1)−Br(1)

2.5315(5)

Re(1)−N(3)

2.143(4)

Bond angles 3 N(21)−Re(1)−O(11)

176.2(1)

4 O(5)−Re(1)−O(4)

175.4(2)

P(1)−Re(1)−P(2)

170.41(4)

O(13)−Re(1)−N(3)

176.7(2)

Br(1)−Re(1)−N(11)

158.14(9)

N(2)−Re(1)−N(11)

171.3(2)

N(11)−Re(1)−O(11)

74.0(4)

O(5)−Re(1)−N(11)

95.1(2)

N(21)−Re(1)−N(11)

108.6(1)

N(2)−Re(1)−N(3)

86.3(2)

Br(1)−Re(1)−O(11)

84.27(7)

N(11)−Re(1)−O(13)

79.7(2)

Br(1)−Re(1)−N(21)

93.2(1)

O(5)−Re(1)−O(13)

94.0(2)

P(1)−Re(1)−N(11)

89.16(8)

O(4)−C(4)−C(42)

111.8(6)


79

Chapter 3

J. Mukiza

Table 3.14: Selected bond lengths (Å) and angles (°) for complexes 5 and 6 Bond lengths 5 Re(1)−N(1)

2.164(2)

6 Re(1)−O(5)

2.186(2)

Re(1)−O(3)

2.126(1)

Re(1)−O(13)

2.137(2)

Re(1)−O(5)

2.205(2)

Re(1)−N(11)

2.168(2)

Re(1)−C(8)

1.892(2)

Re(1)−C(2)

1.896(3)

Re(1)−C(6)

1.906(2)

Re(1)−C(3)

1.913(3)

Re(1)−C(7)

1.903(2)

Re(1)−C(4)

1.897(3)

Bond angles 5 O(3)−Re(1)−C(6)

174.29(7)

6 O(13)−Re(1)−C(2)

174.4(1)

O(5)−Re(1)−C(5)

174.38(7)

O(5)−Re(1)−C(4)

172.7(1)

N(1)−Re(1)−C(7)

172.12(8)

N(11)−Re(1)−C(3)

171.4(1)

N(1)−Re(1)−O(3)

75.54(6)

O(13)−Re(1)−N(11)

75.16(8)

O(3)−Re(1)−O(5)

78.94(5)

O(5)−Re(1)−O(3)

77.95(8)

O(5)−Re(1)−N(1)

81.23(6)

O(5)−Re(1)−N(11)

79.35(8)

N(1)−Re(1)−C(6)

99.32(7)

N(11)−Re(1)−C(2)

100.4(1)

C(7)−Re(1)−C(8)

90.63(9)

C(2)−Re(1)−C(4)

88.0(14)


80

Chapter 3

J. Mukiza

Table 3.15: Selected bond lengths (Å) and angles (°) for complex 7 Re(1)−Re(1i)

Bond lengths 2.5426(7) Re(1)−Cl(1)

2.428(3)

Re(1)−P(1)

2.434(2)

Re(1)−O(1)

2.071(7)

Re(1)−O(4)

1.916(8)

Re(1)−N(1)

2.154(7)

Re(1)−S(1i)

2.392(2)

S(1)−C(14)

1.74(1)

O(3)−C(13)

1.22(1)

N(1)−C(11)

1.38(1)

N(1)−C(14)

1.33(1)

N(2)−C(13)

1.40(2)

N(2)−C(14)

1.35(1)

C(12)−C(13)

1.43(2)

C(11)−C(12)

1.34(2)

O(2)−C(1)

1.21(1)

O(1)−C(1)

1.30(1)

P(1)−C(31)

1.82(1)

Cl(1)−Re(1)−P(1)

Bond angles 171.40(7) O(1)−Re(1)−O(4)

162.2(3)

S(1i)−Re(1)−N(1)

169.9(4)

S(1i)−Re(1)−O(1)

97.7(2)

O(1)−Re(1)−N(1)

75.7(3)

Cl(1)−Re(1)−S(1i)

93.83(6)

Cl(1)−Re(1)−O(4)

106.9(2)

Re(1)−O(4)−Re(1i)

83.1(1)

Re(1)−Cl(1)−Re(1i)

63.14(8)

Re(1i)−Re(1)−Cl(1)

58.43(4)

Cl(1)−Re(1)−N(1)

78.6(2)

Re(1i)−Re(1)−P(1)

129.39(6)

Re(1i)−Re(1)−O(1)

146.2(2)

P(1)−Re(1)−N(1)

97.2(2)

S(1i)−Re(1)−P(1)

89.47(8)

P(1)−Re(1)−O(1)

82.9(2)


81

Chapter 3

J. Mukiza

Table 3.16: Selected bond lengths (Å) and angles (°) for complex 8

Re(1)−Re(2)


2.336(2)

Re(1)−Cl(2)

2.429(2)

Re(1)−P(1)

2.458(2)

Re(1)−O(3)

2.0434(4)

Re(1)−O(5)

1.889(4)

Re(1)−N(1)

2.098(4)

Re(2)−Cl(2)

2.459(2)

Re(2)−Cl(3)

2.307(2)

Re(2)−P(2)

2.454(2)

Re(2)−O(1)

2.152(4)

Re(2)−O(5)

1.930(4)

Re(2)−O(6)

1.865(5)

O(1)−C(1)

1.265(7)

N(1)−C(1)

1.330(7)

N(2)−C(2)

1.340(8)

Re(1)−O(5)−Re(2)

Bond angles 83.2(2) Cl(1)−Re(1)−N(1)

164.2(2)

Cl(2)−Re(1)−P(1)

172.92(6)

O(3)−Re(1)−O(5)

157.7(2)

Cl(2)−Re(2)−P(2)

174.45(5)

Cl(3)−Re(2)−O(1)

175.3(1)

O(5)−Re(2)−O(6)

157.5(2)

O(3)−Re(1)−N(1)

74.2(2)

Re(1)−Cl(2)−Re(2)

62.44(4)

Cl(2)−Re(2)−O(5)

105.2(1)

Cl(2)−Re(1)−O(5)

108.4(1)

Re(2)−Re(1)−O(3)

144.9(1)

O(1)−Re(2)−O(6)

80.4(2)

Cl(1)−Re(1)−Cl(2)

91.23(6)

Cl(2)−Re(2)−Cl(3)

89.02(6)

O(1)−Re(2)−O(5)

84.6(2)

O(1)−C(1)−N(2)

121.0(5)

C(1)−N(1)−C(4)

118.4(5)


82

Chapter 4

J. Mukiza

Chapter 4 Reactivity of 5-Amino-orotic Acid and its Schiff Base and Carboxamide Derivatives to Rhenium(V) and (I) 4.1. Introduction The metal rhenium, a member of Group 7 on the Periodic Table, is probably the most versatile of all the transition metals. From its position in the d-block it displays a large variation in its coordination chemistry, with properties of both the early and late transition metals, giving stable coordination compounds in all the oxidation states from 0 to +7 [1, 2]. The metal also displays structural diversity, with monomers, ligand-bridged dimers, metal-metal multiple bonded species and clusters are quite common [1, 2]. This chapter reports the reactivity of 5-amino-orotic acid (or 2,6-dioxo-5-amino1,2,3,6-tetrahydropyrimidine-4-carboxylic acid; H2aoa) and its Schiff base derivative salicylimine-orotic

acid

{5-(2-hydroxybenzylideneamino)-1,2,3,6-tetrahydro-2,6-

dioxopyrimidine-4-carboxylic acid; H2soa} to oxorhenium(V) and its carboxamide derivative

{5-(5-aminopyrimidine-2,4(1H,3H)-dioxamido)-1,2,3,6-tetrahedro-2,6-

dioxopyrimidine-4-carboxylic acid; H2ampa} to tricarbonylrhenium(I).

O O

OH HO

H N

NH2

HN O

O

NH N

O

O

OH

H2N

HN

O N H H2aoa

O

OH

NH

H N O O

N H

H2soa

N H

O H2ampa

Figure 4.1: Line structures of 5-amino-orotic acid (H2aoa), Schiff base (H2soa) and carboxamide (H2ampa) derivatives used in this study 5-Amino-orotic acid and its derivatives are of interest due to their biological activity. They exhibit antimetabolity activity, in which it inhibits certain enzymes catalysing the synthesis, degradation and interconversion of pyrimidine bases [3]. In monomeric Nelson Mandela Metropolitan University

83

O

Chapter 4

J. Mukiza

complexes, 5-amino-orotic acid (H2aoa) can potentially act as a bidentate N,O-donor chelate, either via the carboxylate oxygen and the amino nitrogen (giving a sixmembered chelate ring) or via the carboxylate oxygen and pyrimidine nitrogen giving a five-membered metallocycle [4]. In dimeric complexes, it can act as bridging ligand and coordinates bidentately through a carboxylate oxygen and amido nitrogen atom to one metal ion, and in a monodentate manner to the second metal ion via a neutral carbonyl oxygen [1]. In

this

study,

the

Schiff

base

salicylimine-orotic

acid

{or

5-(2-

hydroxybenzylideneimino)-1,2,3,6-tetrahydro-2,6-dioxopyrimidine-4-carboxylic acid} was synthesised from the condensation reaction of 5-amino-orotic acid with an excess of salicylaldehyde in methanol. The carboxamide 5-(5-aminopyrimidine2,4(1H,3H)-dioxamido)-1,2,3,6-tetrahedro-2,6-dioxopyrimidine-4-carboxylic

acid

(H2ampa) was obtained from the self-condensation of two molecules of 5-aminoorotic acid in pyridinic solution containing triphenylphosphite (PhO)3P as catalyst [5]. O O

OH

HO NH2

HN

+

O

N O

MeOH OH

HO

O

N H

H2soa O

H2aoa O

O

OH (PhO)3P / Pyridine

HN

Reflux, 4 h O

N H

OH

H2N

NH

H N

NH2 2

O NH + H2O

Reflux, 48 h

O

H N

O

HN O O

N H

O

N H

+ H2O O

H2ampa

H2aoa

Scheme 4.1: Synthesis of Schiff base (H2soa) and carboxamide (H2ampa) derivatives of 5-amino-orotic acid (H2aoa) The reaction of trans-[ReOBr3(PPh3)2] with 5-amino-orotic acid (H2aoa) in ethanol led to the isolation of the triply-bridged dimer (µ-O)(µ-Br)(µ-aoa)[Re2IVBr(OEt)2(PPh3)2] Nelson Mandela Metropolitan University

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(1). The 5-amino-orotate anion aoa2- acts as a bridging ligand with the coordination of a neutral ketonic oxygen to one rhenium ion, and the coordination of a carboxylic oxygen and amidic nitrogen to the other rhenium ion, leaving the amino group uncoordinated. Both ethoxides are coordinated to the rhenium bonded to the neutral ketonic oxygen. Both rhenium ions are therefore in the +IV (d3) oxidation state, intimating the existence of a metal-metal triple bond. By changing the solvent to propan-2-ol (iPrOH) with addition of triethylamine (Et3N), the very similar compound (µ-Br)(µ-O)(µ-aoa)[Re2IVBr2(iOPr)(PPh3)2] (5) was isolated, with the difference being the replacement of the two ethoxides in 1 by a bromide and 2-propanoxide in 5. Surprisingly, the reaction of 5-amino-orotic acid with trans-[ReOBr3(PPh3)2] in propan-2-ol produced the complex [ReVBr(apd)(aoa)(PPh3)2] (2) (apd2− = 5imidopyrimidine-2,4-dione).

The

chelating

ligand

5-aminopyrimidine-2,4-dione

(H2apd; Figure 4.2) was formed by oxorhenium(V)-catalysed decarboxylation of 5amino-orotic acid H2aoa, and it is coordinated via the dinegative imido nitrogen only. The 5-aminoorotate anion aoa2- in complex 2 acts as a bidentate N,O-donor chelate and coordinates via the carboxylate oxygen and pyrimidine nitrogen giving a fivemembered metallocycle. NH2

HN O

N H

O

Figure 4.2: Line structure of 5-aminopyrimidine-2,4-dione (H2apd) The reaction of the Schiff base derivative of 5-amino-orotic acid, salicylimine-orotic acid (H2soa) with trans-[ReOI2(OEt)(PPh3)2] in ethanol led to the rhenium(III)

complex

[Re(coa)I(PPh3)2]I

(3)

[Hcoa

formation of =

5-(2-

hydroxybenzylideneamino)pyrimidine-2,4(1H,3H)-dione; Figure 4.3]. The chelating Hcoa is also formed from the oxorhenium(V)-catalysed decarboxylation of H2soa and coordinates to the rhenium(III) ion as a monoanionic tridentate N,O,O-donor chelate via the phenolate and ketonic oxygens and imino nitrogen atom. Decarboxylation of salicylimine-orotic acid was not observed in its reaction with trans-[ReOCl3(PPh3)2], which led to the isolation of [ReOCl(soa)(PPh3)] (4), in which soa2- acts as a


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tridentate N,O,O-donor ligand and coordinates to oxorhenium(V) via the carboxylate and phenolate oxygens and imino nitrogen, as expected.

H N

O NH

N O OH Figure 4.3: Line structure of Hcoa The research interest in the decarboxylation of organic acids has arisen mainly because of OMP decarboxylase, which is a key enzyme in the biosynthesis of nucleic acids, with the decarboxylation of orotidine 5’-monophosphate to form uridine 5’-monophosphate [6]. The photochemical decarboxylation of orotic acid to uracil was reported previously [7], and yields of uracil increased if photocatalysis occurs in the presence of Fe(II) or Cu(II) ions and 2-propanol [8]. Metal-ion catalysed decarboxylation of various organic acids, such as dimethyloxaloacetic acid [9] and aminomalonic acid [10] has been reported. O

OH NH2

HN

trans-[ReOBr3(PPh3)2]

+ CO2 O

O

O HO

NH2

HN

N H

O

N H H2apd

H N

H2aoa H N

O

O


NH N O

O NH

N O OH

+ CO2

Hcoa

OH H soa 2

Scheme 4.2: Oxorhenium(V)-catalysed decarboxylation of 5-amino-orotic acid (H2aoa) and salicylimine-orotic acid (H2soa) observed in this study


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Although the mechanism of the oxorhenium(V)-catalysed decarboxylation of 5amino-orotic acid and its Schiff base derivative salicylimine-orotic acid (Scheme 4.2) observed in this study is still under investigation, the results reported here indicate that decarboxylation does not occur if the carboxylate group is coordinated to the metal, as is the case for the ligands aoa2- and soa2- in complexes 2 and 4 respectively. In the Re(III) complex 3 the coordination of the monoanionic tridentate Schiff base occurs via the charged phenolate oxygen, the neutral imino N(3) and neutral O(1), leaving the carboxylate group uncoordinated, again leading to subsequent

decarboxylation.

Computational

studies

on

the

mechanism of

decarboxylation of orotic acid analogues indicated that mechanisms involving carboxylic proton transfer to the 2- or 4-oxygen on the orotic ring of these derivatives are energetically more favourable than direct decarboxylation without proton transfer [9]. Since both these oxygens are uncoordinated in the imido-coordinated apd2- in 2 and since the 2-oxygen is not coordinated in 3, decarboxylation is possible. The reaction of H2ampa with [Re(CO)5Cl] in ethanol led to the

formation of a

zwitterionic

[amef

rhenium(I)

complex

[Re(CO)3(H2O)(amef)]

(6)

=

(5-

ammoniumpyrimidine-2,4(1H,3H)-dioxamido)-1,2,3,6-tetrahedro-2,6-dioxopyrimidine4-ethylformate] ion (Scheme 4.3). The chelating ion amef is formed from the combined tricarbonylrhenium(I)-catalysed esterification, and amino-protonation of H2ampa and coordinates to the fac-[Re(CO)3]+ core as a dianionic bidentate N,Ndonor chelate via the amido nitrgens.

+

O O

OH

H2N

NH

H N HN O O

N H

O

O

N H

H3N

O

O

Reflux, 24 h O

NH

H N

[Re(CO)5Cl] / EtOH HN O

O

N H

O

O

N H amef

H2ampa

Scheme 4.3: Tricarbonylrhenium(I)-catalysed esterification and amino-protonation of H2ampa leading to the amef ion observed in this study


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Transition metals complexes including rhenium have received the considerable interest due to their ability to catalyse many reactions in organic transformations, and it is due to the presence of an incomplete d-shell of these metals in their ionised form. The ability of rhenium complexes [Re(CO)5X] (X = Cl, Br) to catalyse esterification reaction was previously reported in the literature [11], and it has proved that it is due to the Lewis acidity character of these complexes [11]. The Lewis acidity character also enable them to catalyse the reactions such as the FriedelCrafts alkylation of benzene and its derivatives and activation of C(sp3)−H and C(sp2)−H bonds, which allows the introduction of several function groups or substituents [11, 12]. The low-valent character of [Re(CO)5X] (X = Cl, Br) complexes have been also explored in organic chemistry for promoting oxidative cyclisation reactions [11].

4.2. Experimental 4.2.1. Synthesis of salicylimine-orotic acid (H2soa) To a solution of 5-amino-orotic acid (H2aoa) (400 mg, 23.4 mmol) in 25 cm3 of methanol was added an excess of salicylaldehyde (7.5 cm 3, 70.2 mmol). The mixture was heated under reflux for 48 hours, and then the solution was cooled to room temperature. The resulting yellow precipitate was filtered off, and the excess of salicylaldehyde was removed by washing with ethanol, before drying under vacuum. Yield = 86 %, m.p. = 315°C. IR (νmax/cm-1): ν(N−H) 3337, 3457; ν(O−H) 3156; ν(CO2H) 1717; ν(C═O) 1671, 1667; ν(C═N) 1609. 1H NMR (d6-DMSO, 295 K, ppm): 11.54 (s, 1H, CH═N), 10.86, 10.32, 9.44 (s, 1H, 3 x NH), 7.73 (t, 1H), 7.60 (t, 1H), 7.08 (d, 1H), 7.03 (t, 1H). 4.2.2. Synthesis of (μ-Br)(μ-O)(μ-aoa)[Re2IVBr(OEt)2(PPh3)2] (1) Two equivalents of H2aoa (34 mg) and 100 mg (0.101 mmol) of trans[ReOBr3(PPh3)2] were heated under reflux in 20 cm 3 of ethanol for 24h. After the heating was stopped, the solution was cooled to room temperature, and a black precipitate was removed by filtration. A volume of 5 cm3 of dichloromethane was added to the mother liquor, and the mixture was stored at -5 ˚C. After 3 weeks black crystals, suitable for X-ray diffraction studies, were collected by filtration. crystals were washed with acetone

and

dried

under

vacuum.

The

(µ-O)(µ-Br)(µ-

aoa)[Re2Br2.16(OEt)1.84(PPh3)2]. 2CH2Cl2: Black. Yield: 64%, m.p. 198˚C. Anal. Calcd Nelson Mandela Metropolitan University

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for C44.68H42.20N3Br2.16O6.84P2Re2.2CH2Cl2: C, 35.6; H, 3.1; N, 2.8. Found: C, 35.4; H, 3.3; N, 2.9 %. IR (cm-1): ν(N−H) 3330w, 3458w; ν(Re−O−Re) 694s; ν(C═O) 1602s, 1664s; ν(Re−N) 472m; ν(Re−O) 440m. UV-vis (MeOH, λmax (ε, M-1cm-1)): 328 (19500), 435 nm (360). 4.2.3. Synthesis of [ReBr(aoa)(apd)(PPh3)2] (2) To a solution of H2aoa (35 mg, 0.206 mmol) in 10 cm3 of propan-2-ol was added trans-[ReOBr3(PPh3)2] (100 mg, 0.103 mmol) in 15 cm3 of propan-2-ol. The mixture was heated under reflux for 24 hours, with the colour changing to green. The reaction solution was cooled to room temperature, and a dark green precipitate was filtered off. The filtrate was left to evaporate slowly at room temperature, and after a month dark green crystals, suitable for X-ray crystallographic study, were harvested. Yield = 85 mg (68 % based on Re), m.p. 343 °C. IR (νmax/cm-1): ν(CO2) 1678; ν(C═O) 1604, 1585; ν(Re═N) 1057; ν(Re−N) 534; ν(Re−O) 434. 1H NMR (d6-DMSO, 295 K, ppm): 7.08 (s, 1H, N(123)H), 7.21 (s, 1H, N(122)H), 7.34 (s, 1H, N(112)H), 7.40-7.70 (m, 31 H). UV-vis (MeOH, λmax (ε, M-1cm-1)): 329 (32400), 393 (13400), 442 (8550) nm. 4.2.4. Synthesis of [ReI(coa)(PPh3)2]I (3) To a solution of H2soa (48 mg, 0.175 mmol) in 10 cm3 of ethanol was added trans [ReOI2(OEt)(PPh3)2] (100 mg, 0.088 mmol) in 10 cm3 of ethanol. The dark-green mixture was heated under reflux for 24 hours, after which the reaction solution was cooled to room temperature. A brown precipitate was collected, and a volume of 5 cm3 of dichloromethane was added to the mother liquor. The mixture was left to evaporate slowly at room temperature, and after 3 weeks black crystals were collected by filtration. Yield = 44 mg (38 % based on Re), m.p. 295 °C. IR (ν max/cm1

): ν(C=O); ν(Re-N) 496; ν(Re-O(3)) 459; ν(Re-O(1)) 441. UV-vis (DMSO, λmax (ε, M-

1

cm-1)): 274 (1390), 356 (1120) nm.

4.2.5. Synthesis of [ReOCl(soa)(PPh3)] (4) A mixture of H2sor (66 mg, 0.24 mmol) and trans-[ReOCl3(PPh3)2] (100 mg, 0.12 mmol) in 25 cm3 of ethanol was heated under reflux for 48 hours, with the colour of the solution changing to black. After cooling to room temperature, a dark green precipitate

was

filtered

off.

Recrystallization


of

the

precipitate

in

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dichloromethane/ethanol mixture resulted in dark green crystals of X-ray quality. Yield = 84 mg ( 77 % based on Re), m.p. 326 °C. IR (νmax/cm-1): ν(CO2) 1705; ν(Re═O) 947; ν(Re−N) 492; ν(Re−O) 457, 438. 1H NMR (d6-DMSO, 295 K, ppm): 9.52 (s, 1H, CH=N), 8.08 (t, 1H), 8.46 (t, 1H), 7.64-7.82 (m, 17H). UV-vis (MeOH, λmax (ε, M-1cm-1)): 345 (47000), 446 (8530), 577 (3250) nm. 4.2.6. Synthesis of (μ-Br)(μ-O)(μ-aoa)[Re2IVBr2(OiPr)(PPh3)2] (5) To a mixture of H2aoa (50 mg, 0.291 mmol) and trans-[ReOBr3(PPh3)2] (140 mg, 0.146 mmol) in 25 cm3 of propan-2-ol (HOiPr) was added 0.7 cm3 of triethylamine (Et3N). The yellow mixture was heated under reflux for 15 hours, and after cooling to room temperature, the solution was filtered with no precipitate having been formed. A small volume of methanol and ethanol was added to the filtrate, which was left to evaporate at room temperature and after two months, dark green crystals suitable for X-ray diffraction studies were collected by filtration. Yield: 60%, m.p. 185˚C. IR (cm1

): ν(N−H) 3055, 3380; ν(Re−O−Re) 690; ν(C═O) 1652, 1694; ν(Re−N) 536;

ν(Re−O)CO2- 492; ν(Re−O(8)) 438; ν(Re−O(13)) 435. UV-vis (MeOH, λmax (ε, M-1cm1

)): 286 (8560), 408 (4450), 485 (1230).

4.2.7. Synthesis of [Re(CO)3(H2O)(amef)] (6) [Re(CO)5Cl] (100 mg, 0.276 mmol) and H2ampa (90 mg, 0.276 mmol) were added to 15 cm3 of ethanol. The resulting orange mixture was refluxed for 24 hours, giving a dark yellow solution, which was filtered after being cooled to room temperature. No precipitate was formed. Yellow crystals, suitable for X-ray crystallography, were grown in two months by the slow evaporation of the filtrate at room temperature. Yield = 50 %; m.p. = 278 °C. IR (νmax/cm-1): ν(C≡O) 2019, 1883; ν(C═O) 1607, 1637; ν(Re−N) 526, 511; ν(Re−O)H2O 489. 1H NMR (d6-DMSO, ppm): 2.04 (s, 3H, NH3), 7.02 (s, 1H, N(31)H), 9.06 (s, 2H, N(32)H, N(12)H ), 1.32 (t, 3H, CH3), 4.21 (q, 2H, CH2). UV-vis (MeOH , λmax(ε, M-1cm-1)): 269 (22070), 342 (2990), 374 (2330) . 4.2.8. X-ray crystallography Single crystal X-ray crystallography studies were performed at 200 K using a Bruker Kappa Apex II diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71069 Å). For data collection, APEX-II was used while for cell refinement and data reduction, SAINT was used [13]. The structures were solved by direct methods Nelson Mandela Metropolitan University

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applying SHELXS-97 [14], or SIR97 [15] and refined by least-squares procedures using SHELXL-97 [13], with SHELXLE [15] as a graphical interface. All non-hydrogen atoms were anisotropically refined and hydrogen atoms were calculated in idealised geometrical positions. Data were corrected by absorption effects using the numerical method using SADABS [13].

4.3. Results and discussion 4.3.1. Salicylimine-orotic acid (H2soa) The compound H2soa was obtained from the condensation reaction of 5-aminoorotic acid with an excess of salicylaldehyde in methanol (Scheme 4.1). The excess of salicylaldehyde was removed by washing the precipitate with ethanol, before drying under vacuum. H2soa is a yellow solid, weakly soluble in polar organic solvents, insoluble in non-polar organic solvents and it melts at 315°C. The infrared spectrum of H2soa displays medium-intensity peaks at 3337 and 3457 cm-1, which are assigned to v(N−H). The frequencies of the C═O stretches are assigned to the absorptions at 1671 and 1667 cm-1. The frequency of the CO2H stretch is assigned to the absorption at 1717 cm-1. The frequencies of the C═N and O−H stretches are assigned to the absorptions at 1609 and 3156 cm-1 respectively.

88

% Transmittance

78 68 58 48 38 28 3360

2860

2360

1860

1360

860

360

Wavenumber (cm-1) Figure 4.4: IR spectrum of H2soa Nelson Mandela Metropolitan University

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In the 1H NMR spectrum of H2soa three one-proton singlets at 9.44, 10.32 and 10.86 ppm are ascribed to the NH protons on the H2soa. The singlet signal at 11.54 ppm is assigned to the HC═N proton. The four phenyl ring protons occur as a triplet at 7.03 ppm, a doublet at 7.08 ppm, a triplet at 7.60 ppm and a doublet 7.73 ppm [4].

Figure 4.5: 1H NMR spectrum of H2soa in d6-DMSO 4.3.2. (μ-Br)(μ-O)(μ-aoa)[Re2IVBr(OEt)2(PPh3)2] (1) The complex 1 was prepared by the heating under reflux of [ReOBr3(PPh3)2] with a twofold molar excess of H2aoa in ethanol.

2 [ReOBr3(PPh3)2] + H2aoa + 2 EtOH

EtOH

Reflux, 24 h

1 + 2 PPh3 + 2 HBr + H2O + 2 Br -

Complex 1 is black in colour, not soluble in water, but soluble in dichloromethane, DMF, ethanol and methanol, to give dark green solutions. The infrared spectrum of 1 (Figure 4.6) displays an intense peak at 694 cm-1, which is ascribed to v(Re−O−Re).

Medium-intensity peaks at 443 and 497 cm -1 are

assigned to v(Re−O)CO2- and v(Re−N) respectively. The frequency of the C═O stretch of the coordinated neutral oxygen is assigned to the absorption at 1602 cm -1, with the uncoordinated C═O carbonyl stretching frequency occurring at 1664 cm -1. The frequencies of the C−H stretches of the coordinated ethoxide are assigned to the absorption at 2982 cm-1. The frequencies of the N−H stretches of the free amino Nelson Mandela Metropolitan University

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group and N−H stretch of the free amido group are assigned to the absorptions at 3331 and 3456 cm-1 respectively. The UV-visible absorption spectrum in methanol consists of an intense absorption at 328 nm ( 19500 dm3mol-1cm-1), and a less intense band at 435 nm ( 360). The former peak appears at 335 nm ( 6980) in the spectrum of the free ligand H2aoa in methanol, and is ascribed to an intraligand

* transition. The latter peak is

indicative of a d-d transition. For metal-metal complexes of the type (µ-O)2Re2 a strong absorption band was observed at about 490 nm with extinction coefficients around 104 dm3mol-1cm-1 for Re(IV) compounds [1]. This band has been assigned to * transition within the Re−Re triple bond. Weaker absorption bands at

the

longer wavelengths have been ascribed to transitions involving the

and * orbitals.

The cyclic voltammogram (CV) of 1 in acetonitrile shows three reversible redox waves at +0.108, +1.106 and -1.154 V. They are one-electron steps as confirmed by coulometric measurements, and are assigned to ReIV2/ReIVReV, ReIVReV/ReV2 and ReIV2/ReIIIReIV processes respectively [1].

% Transmittance

90

80

70

60

50

40 3380

2880

2380

1880

1380

880

380

Wavenumber (cm-1) Figure 4.6: IR spectrum of complex 1


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1.8063 1.6063

Absorbance

1.4063

H2aoa ligand

1.2063 1.0063

Complex 1

0.8063 0.6063 0.4063 0.2063 0.0063 293

343

393

443

493

Wavelength (nm) Figure 4.7: Overlay UV-Vis spectra of complex 1 and the H2aoa ligand in MeOH Figure 4.8 shows the ORTEP drawing of the molecular structure complex 1. The coordination environment of each rhenium ion in the complex is different in the edgeshared bi-octahedral structure, with a bridging oxide ion, a bridging bromide ion and a bridging chelate ligand aoa2-. The rhenium ion Re(1) is coordinated in a bidentate manner by N(1) and the carboxylate oxygen O(4) of aoa 2-, the bromide Br(2), phosphorus P(1), and the bridging oxide O(5) and bromide Br(1) ions. The rhenium ion Re(2) is coordinated by the phosphorus atom P(1), the ethoxide O(6), the ethoxide O(7)(84%)/Br(3)(16%), the carbonylic oxygen atom O(1) of aoa 2-, and the bridging oxide O(5) and bromide Br(1) ions. It should be noted that ethoxide and the carboxylic oxygens are coordinated trans to the bridging oxo O(5) and carbonylic oxygen O(1). The Re−Re distance is 2.5526(4) Å, which is slightly longer than in the bridged compounds [Ph3PCl2ReIV(µ-O)(µ-Cl)(µ-EtCO2)ReIVCl2PPh3] (2.52 Å) and [Ph3PCl(µO)(µ-Cl)(µ-EtCO2)2ReCl(PPh3)] (2.514 Å) [1, 2], and considerably longer than in the (µ-O)2Re2IV complexes [Re2(µ-O)2L2](PF6)4 (2.364(1) Å, L = tris(2-pyridylmethyl)amine; 2.368(1) Å, L = ((6-methyl-2-pyridyl)methyl)bis(2-pyridylmethyl)-amine) [16]. The range normally found for (µ-O)2Re2 complexes is 2.362–2.381 Å [16−19]. Since the rhenium ions in the diamagnetic complex 1 are formally in the +4 oxidation state with a d3-d3 electron configuration, it would indicate a Re≡Re triple bond. A “longer Nelson Mandela Metropolitan University

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than expected” Re≡Re bond has been explained in terms of a σ 2π2

*2

, rather than

the conventional σ2π2 2, electronic configuration [20]. The former configuration was supported by molecular orbital calculations, which intimated that the interaction of the orbitals of bridging ligands would place the energy of the

orbital higher than

that of the * orbital, thus providing some antibonding character to the Re≡Re bond [16, 20]. This may be due to the strong interaction of the p orbitals of the bridging oxo and bromide ions with the bonding molecular

orbital. The short Re(1)−O(5)

and Re(2)−O(5) bond lengths of 1.890(3) and 1.913(3) Å respectively support this destabilization of the than one of 3 (σ2π2

2

orbital, which would led to a formal bond order of 1, rather ), and consequently a longer Re≡Re triple bond which leads to

a large Re−O(5)−Re bond angle of 84.3(1)˚. The O(5)−Re(1)−Br(1) and O(5)−Re(2)−Br(1) bond angles are 109.65(9)˚ and 106.74(9)˚ respectively.

Figure 4.8: ORTEP view of complex 1 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and dichloromethane crystallisation solvent have been omitted for clarity


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In the bridging chelate aoa2- the C(11)−O(1) bond length is 1.267(5) Å, which makes it a double bond. The Re(2)−O(1) length [2.175(3) Å] is therefore considerably longer than the Re(1)−O(4) bond [2.045(4) Å]. The nitrogen N(1) is deprotonated and is coordinated as an amide, with the Re(1)−N(1) bond length equal to 2.090(4) Å. The C(13)−C(14) [1.360(6) Å] and O(2)−C(12) [1.231(5) Å] bonds are double. There is nothing unusual about the other bond lengths and angles in the molecule. The hydrogens of the uncoordinated N(3)H2 amino group are involved in intramolecular hydrogen bonds to O(2) and O(3). Table 4.1: Hydrogen-bond distances (Å) and angles (o) in complex 1 D−H∙∙∙A N(2)−H(2)∙∙∙O(2)

D−H 0.88(3)

H∙∙∙A 1.96(3)

D∙∙∙A 2.826(5)

D−H∙∙∙A 172(5)

N(3)−H(3A)∙∙∙O(3)

0.88(3)

2.27(6)

2.874(6)

126(4)

N(3)−H(3A)∙∙∙O(3)

0.88(3)

2.32(5)

3.059(6)

141(5)

N(3)−H(3B)···O(2)

0.88(4)

2.43(6)

2.736(7)

101(4)

N(3)−H(3B)···Cl(91)

0.88(4)

2.41(4)

3.132(9)

140(6)

Figure 4.9: Crystal packing and hydrogen-bonding for complex 1 showing intramolecular hydrogen-bonds (blue-dashed) Nelson Mandela Metropolitan University

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4.3.3. [ReBr(aoa)(apd)(PPh3)2] (2) The monomeric imidorhenium(V) complex [ReBr(apd) (aoa)(PPh3)2] (2) (apd2− = 5imidopyrimidine-2,4-dione) was isolated from the reaction of a two-fold molar excess of 5-amino-orotic (H2aoa) with trans-[ReOBr3(PPh3)2] in propan-2-ol (iPrOH) under reflux. The chelating ligand 5-aminopyrimidine-2,4-dione (H2apd) was formed by the oxorhenium(V)-catalysed decarboxylation of 5-aminoorotic acid (Scheme 4.2), and apd2− is coordinated via the dinegative imido nitrogen only. i

trans-[ReOBr3(PPh3)2] + 2 H2aoa

PrOH

Reflux, 24 h

[ReBr(aoa)(apd)(PPh3)2] (2) + CO2 + H2O + 2 HBr

The mechanism of decarboxylation of orotic acid analogues indicated that it involves carboxylic proton transfer to the 2- or 4-oxygen on the orotic ring of these derivatives, since it is energetically more favourable than direct decarboxylation without proton transfer [4, 21]. Since both these oxygens are uncoordinated in the imido-coordinated apd2- in complex 2, decarboxylation is possible. Complex 2 has dark-green coloured crystals and it is soluble in polar organic solvents like ethanol, methanol acetonitrile, dimethylformamide and dimethyl sulfoxide to give dark green solution. However, it is weakly soluble in non-polar organic solvents like dichloromethane, chloroform and hexane. A band of medium intensity in the infra-red spectrum (Figure 4.10) at 1059 cm-1 is ascribed to ν(Re═N) of the coordinated imido-uracil group. The bands at 1678 and 1604 cm-1 are assigned to ν(CO2) and ν(C═O) respectively. The bands displayed at 534 cm-1 and 434 cm-1 are assigned to v(Re−N) and v(Re−O) respectively [4]. The 1H NMR spectrum (Figure 4.11) is characterized by three singlets in the range 7.08–7.34 ppm and are assigned to the hydrogen atoms on N(123), N(122) and N(112) respectively. The protons of phenyl rings and the C-H proton of 5imidopyrimidine-2,4-dione are represented by the signals in the rage 7.40−7.70 ppm [4]. The UV-Vis spectrum of 2 (Figure 4.12) in methanol is characterised by an absorption at 329 nm which is assigned to an intraligand π→π* transition in the


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coordinated ligands. Absorption at 393 nm is assigned to a ligand-to-metal charge transfer (LMCT) and the band at 442 nm is due to a metal (dxy)2→(dxy)1(dxz)1 transition [4].

88

% Transmittance

78 68 58 48 38 28 18 8 3300

2800

2300

1800

1300

800

300


Figure 4.11: 1H NMR spectrum of complex 2 in d6-DMSO The cyclic voltammogram (CV) of complex 2 in acetonitrile (Figure 4.14) is characterised by reductive and oxidative waves. The sweep in positive potential Nelson Mandela Metropolitan University

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shows the quasi-reversible one-electron oxidation peak at E1/2 = +1.417 V which is ascribed to the oxidation from ReV (d2) to ReVI (d1). The sweep in negative potential shows the two non-reversible peaks at E1/2 = -2.136 and -0.975 V. These peaks are assigned to the reduction from ReV (d2) to ReIV (d3) and ReIV (d3) to ReIII (d4) respectively.

0.23

Absorbance

0.18

0.13

0.08

0.03 280

330

380

430

480

530

Wavelength (nm) Figure 4.12: UV-vis spectrum of complex 2 in MeOH

Figure 4.13: Cyclic voltammogram of complex 2 in MeCN The X-ray crystal structure (Figure 4.14) shows two complexes in the asymmetric unit, in which the bidentate ligand aoa2- is coordinated via the carboxylate oxygen and deprotonated pyrimidine nitrogen, with a free amino group NH2. However, the Nelson Mandela Metropolitan University

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monodentate ligand apd2- was formed by the decarboxylation of H2aoa, and is coordinated through the imido nitrogen only. The geometry around rhenium center is a distorted octahedral and this is mainly due to the deviation from linearity of the trans angles [P(11)−Re(1)−P(12) = 167.11(4)˚, P(21)−Re(2)−P(22) = 166.97(4)˚, Br(1)−Re(1)−N(111)

=

161.56(8)˚,

O(113)−Re(1)−N(121)

=

171.8(1)˚,

O(213)−Re(2)−N(221) = 172.2(1)˚, Br(2)−Re(2)−N(211) = 161.70(8)˚]. The distortion from an ideal octahedral geometry is also reflected by the bite angles [N(111)−Re(1)−N(121)

=

98.96(3)˚,

N(211)−Re(2)−N(221)

=

99.6(1)˚,

O(213)−Re(2)−N(121) = 74.63(10)˚, N(111)−Re(1)−O(113) = 75.0(1)˚], which show significant deviations from orthogonality.

Figure 4.14: ORTEP view of complex 2 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and propan-2-ol crystallisation solvent have been omitted for clarity The bond lengths Re(1)−N(121) [1.733(3) Å] and Re(2)−N(221) [1.719(3) Å] are typical of Re(V)-imido distances [2, 4], while the Re(1)−N(111) [2.155(3) Å] and Re(2)−N(211) [2.149(3) Å] bond lengths are typical of Re(V)-amido distances [4, 22]. Nelson Mandela Metropolitan University

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The carboxylate O(113), trans to the imido N(121), forms a bond length of 2.049(3) Å to the metal while carboxylate O(213), trans to the imido N(221) in the other complex,

forms

a

bond

length

of

2.045(2)

Å

to

the

metal.

The

two

V

Re −O(carboxylate) distances are in good agreement with similar bonds distances reported in the literature [2, 4]. The two phosphine ligands are coordinated in trans positions to each other, with a P(11)−Re−P(12) bond angle of 167.11(4)˚ in one complex and a P(21)−Re−P(22) bond angle of 166.97(4)˚ in the other. The Re(1)−Br(1) and Re(2)−Br(2) bond distances are equal to 2.5229(6) Å and 2.5179(6) Å respectively. The protons on the 2-propanol solvent of crystallisation are involved in intermolecular hydrogen-bonds (Table 4.2). The protons on the nitrogen atoms of the two asymmetric structures also contribute to both intra- and intermolecular hydrogenbonding. All hydrogen-bond parameters are summarised in Table 4.2. Table 4.2: Hydrogen-bond distances (Å) and angles (o) in complex 2 D−H∙∙∙A O(3)−H(3)∙∙∙O(111)

D−H 0.8400

H∙∙∙A 1.7900

D∙∙∙A 2.588(6)

D−H∙∙∙A 159.00

O(4)−H(4)∙∙∙O(211)

0.8400

1.9300

2.652(6)

143.00

N(113)−H(11A)∙∙∙O(114)

0.8800

2.1400

2.757(6)

127.00

N(113)−H(11B)∙∙∙O(121)

0.8800

2.1300

2.973(5)

161.00

N(213)−H(21A)∙∙∙O(214)

0.8800

2.1100

2.729(5)

127.00

N(213)−H(21B)∙∙∙O(212)

0.8800

2.4800

2.781(5)

101.00

N(213)−H(21B)∙∙∙O(221)

0.8800

2.1200

2.954(5)

157.00

N(112)−H(112)∙∙∙O(122)

0.8800

2.1600

3.026(5)

170.00

N(122)−H(122)∙∙∙O(112)

0.8800

1.8500

2.720(5)

172.00

N(123)−H(123)∙∙∙O(3)

0.8800

1.8100

2.688(7)

172.00

N(212)−H(212)∙∙∙O(222)

0.8800

2.0900

2.966(5)

170.00

N(222)−H(222)∙∙∙O(212)

0.8800

1.8700

2.744(5)

173.00

N(223)−H(223)...O(4)

0.8800

1.8200

2.698(6)

176.00


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Figure 4.15: Crystal packing diagram of complex 2 displaying hydrogen-bonds 4.3.4. [ReI(coa)(PPh3)2]I (3) The monomeric rhenium(III) complex salt [ReI(coa)(PPh3)2]I (3) (Hcoa = 5-(2hydroxybenzylideneamino)pyrimidine-2,4(1H,3H)-dione)

was

isolated

from

the

reaction of a two-fold molar excess of the Schiff base salicylimine-orotic acid or {5(2-hydroxybenzylideneamino)-1,2,3,6-tetrahydro-2,6-dioxopyrimidine-4-carboxylic acid, H2soa} with trans-[ReOI2(OEt)(PPh3)2] in ethanol under reflux for 24 hours. The tridentate N,O,O-donor chelating ligand Hcoa was formed by the oxorhenium(V)catalysed decarboxylation of the Schiff base H2soa (Scheme 4.2).

trans-[ReOI2(OEt)(PPh3)2] + H2soa

EtOH Reflux, 24 h

[ReI(coa)(PPh3)2]I (3) + CO2 + H2O

In complex 3, the coordination of the monoanionic tridentate Schiff base occurs via the charged phenolate oxygen, the neutral imino N(3) and neutral O(1), leaving the carboxylate group uncoordinated, again leading to subsequent decarboxylation. Complex 3 has dark green crystals and it is soluble in polar organic solvents, specifically alcohols, and it is a good electrolyte in methanol. However, the complex Nelson Mandela Metropolitan University

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3 shows a weak solubility in non-polar organic solvents like dichloromethane, chloroform and hexane. In the IR spectrum of 3, the typical Re−N, Re−O(3) and Re−O(1) stretching frequencies occur at 496, 459 and 441 cm-1 respectively. The vibration frequencies at 1662 and 3055 cm-1 are assigned to ν(C═O) and ν(N−H) respectively [4].

90 85

% Transmittance

80 75 70 65 60 55 50 45 2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 4.16: IR spectrum of complex 9 The 1H NMR spectrum consists of broad peaks with poor resolution, due to the paramagnetism of the d4 complex cation [4]. In the UV-visible spectrum of complex 3 in dimethylsulfoxide, the intra-ligand

*

transition is observed at 274 nm, and the only other band in the spectrum at (356 nm) is likely due to a combination of the ligand-to-metal charge transitions pπ(O−)→d*π(Re), pπ(O)→d*π(Re), pπ(N)→d*π(Re) and pπ(I−)→d*π(Re). Figure 4.17 shows the ORTEP drawing of the structure of complex 3. The tridentate ligand coa−, formed by the decarboxylation of H2soa, is coordinated to rhenium(III) ion via the deprotonated phenolate oxygen O(3), the neutral imino N(3) and the neutral oxygen O(1). The geometry around the rhenium center is distorted Nelson Mandela Metropolitan University

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octahedral. The two triphenylphosphine ligands are coordinated in trans positions to each other, with the trans angle P(11)−Re(1)−P(12) = 177.32(3)˚ close to linearity. The two additional trans angles [I(1)−Re(1)−N(3) = 176.05(7)˚, O(1)−Re(1)−O(3) = 168.88(8)˚] also show a small deviation from the linearity. The deviation from linearity of the three trans angles is the main cause of the distortion in complex 3. The constraints imposed by the coordination requirements of the tridentate coa− lead to the

deviation

from

N(3)−Re(1)−O(1) [94.11(6)˚],

the

orthogonality

[79.11(9)˚],

of

the

N(3)−Re(1)−O(3)

I(1)−Re(1)−P(2)

[91.92(2)˚],

rhenium-centered

[89.78(9)˚],

P(1)−Re(1)−O(1)

angles

I(1)−Re(1)−O(3) [89.61(7)˚]

and

P(1)−Re(1)−N(3) [88.84(8)˚]. The coordination of a neutral oxygen atom to Re(III) is unusual and rare, and the difference between the Re−O(1) [2.102(2) Å] and Re−O(3) [1.953(2) Å] bond lengths is significant. The bond distance ReIII−N(3) [2.071(2) Å] is in good agreement with the values 2.074(3) Å and 2.062(2) Å reported in the literature for ReIII−N(imine) distances for the similar complexes [22]. The O(1)−C(2) and N(3)−C(5) bond lengths are 1.274(4) Å and 1.306(4) Å respectively, and are typical of double bonds [4]. This is supported by the bond angles around N(3) [C(1)−N(3)−C(5) = 121.4(3)˚] and around C(2) [O(1)−C(2)−N(1) = 120.1(3)˚, N(1)−C(2)−N(2) = 119.2(3)˚], which are close to 120o as expected for sp2-hybridised nitrogen and carbon atoms. There

are

three

intermolecular

+

hydrogen-bonds

between

the

cation

−

[ReI(coa)(PPh3)2] , counterion I and crystallisation ethanol solvent. All hydrogenbond parameters are summarised in Table 4.3. Table 4.3: Hydrogen-bond distances (Å) and angles (o) in complex 3 D−H∙∙∙A N(1)−H(1A)∙∙∙O(4)

D−H 0.91(5)

H∙∙∙A 1.82(5)

D∙∙∙A 2.730(4)

D−H∙∙∙A 173(5)

N(2)−H(2A)∙∙∙I(2)

0.82(5)

2.65(5)

3.453(3)

168(8)

O(4)−H(4A)∙∙∙I(2)

0.8400

2.8600

3.679(4)

167.00


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Figure 4.17: ORTEP view of complex 3 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and dichloromethane and ethanol crystallisation solvents have been omitted for clarity

Figure 4.18: Crystal packing diagram displaying intermolecular hydrogen-bonds in complex 3 Nelson Mandela Metropolitan University

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4.3.5. [ReOCl(soa)(PPh3)] (4) The monomeric oxorhenium(V) complex [ReOCl(soa)(PPh3)]I (4) (H2soa = salicylimine-orotic acid or {5-(2-hydroxybenzylideneamino)-1,2,3,6-tetrahydro-2,6dioxopyrimidine-4-carboxylic acid) was synthesised from the reaction of a two-fold molar excess of H2soa with trans-[ReOCl3(PPh3)2] in ethanol under reflux for 48 hours.

trans-[ReOCl3(PPh3)2] + H2soa

EtOH Reflux, 48 h

[ReOCl(soa)(PPh3)] (4) + PPh3 + 2 HCl

In contrast to the reaction of H2soa with trans-[ReOl2(OEt)PPh3)2] in ethanol, decarboxylation was not observed in the reaction of H2soa with trans[ReOCl3(PPh3)2] in ethanol. The dianionic tridentate soa2- is coordinated to oxorhenium(V) via the phenolate O(1), the imino N(1) and carboxylate O(4). Complex 4 was also isolated from the reaction of trans-[ReCl3(MeCN)(PPh3)2] in air. It is soluble in dichloromethane, acetonitrile and chloroform, but weakly soluble in methanol and ethanol. It has a green colour in the solid state and in solution, and it is a 1:1 electrolyte in both acetonitrile and dichloromethane solutions. In the IR spectrum (Figure 4.19) the typical Re═O stretching frequency occurs at 947 cm-1, and two medium intensity peaks could be assigned for the two Re−O vibrations at 438 and 457 cm-1, while the peak appearing at 492cm-1 could be assigned to ν(Re−N). The bands occurring at 1705 and 1668 cm-1 are ascribed to ν(CO2) and ν(C═O) respectively. The typical N═C and N−H stretching frequencies occur at 1628 and 3057 cm-1 respectively. In the UV-visible spectrum of complex 4 (Figure 4.20) in methanol, the intra-ligand * transition of the coordinated soa2− anion occurs at 345 nm, and which appears at 343 nm in the free H2soa. Absorption at 446 nm and 577 nm are assigned to a ligand-to-metal charge transfer (LMCT) and a d-d transition respectively. The 1H NMR spectrum is dominated by multiplets of proton signals of the PPh 3 group and phenoxyl ring of soa2−, occurring as a triplet at 8.08 ppm, a triplet at 8.46 ppm


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and a multiplet in the range 7.64−7.82 ppm. The singlet at 9.23 ppm is assigned to HC═N. The two NH protons appear as singlets at 9.56 and 7.31 ppm.

100

% Transmittance

90

80

70

60

50

40 2800

2300

1800

1300

800

300


0.45 0.4

Absorbance

0.35

Complex 4

0.3 0.25

H2soa ligand

0.2 0.15 0.1 0.05 0 325

375

425

475

525

575

625

675

Wavelength (nm) Figure 4.20: Overlay UV-Vis spectra of complex 4 and H2soa in MeOH The single crystal X-ray crystallography analysis of complex 4 (Figure 4.21) shows that the dianionic tridentate soa2- is coordinated to oxorhenium(V) via the phenolate Nelson Mandela Metropolitan University

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oxygen O(1), the imino nitrogen N(1) and carboxylate oxygen O(4), with N(1), O(1) and O(4) in the coordination sites trans to the chloride Cl(1), oxo O(6) and P(1) respectively. The geometry around the rhenium(V) center is distorted octahedral. The distortion from an ideal octahedral geometry could be attributed to the significant deviation from the linearity of the trans angles P(1)−Re(1)−O(4) [166.6(1)˚], O(1)−Re(1)−O(6) [166.6(1)˚] and Cl(1)−Re(1)−N(1) [166.02(7)˚]. This distortion leads to the non-orthogonal angles O(1)−Re(1)−O(4) [96.0(1)˚], O(6)−Re(1)−N(1) [89.6(1)˚] and N(1)−Re(1)−O(1) [80.9(1)˚]. The Re−O(6) distance of 1.695(2) Å is typical for an oxo bond to Re(V) [4, 23, 24], and the difference between the Re−O(1) [1.949(2) Å] and Re−O(4) [2.094(2) Å] lengths is significant. The N(1)−C(1) bond is double [1.301(5) Å] [4], and the Re−N(1) [2.115(3) Å] is typical for Re(V)−imine bonds [4]. The ethanol of crystallisation solvent contributes to intermolecular hydrogen-bond, [Table 4.4].

Figure 4.21: ORTEP view of complex 10 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and ethanol crystallisation solvent have been omitted for clarity


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Table 4.4: Hydrogen-bond distances (Å) and angles (o) in complex 4 D−H∙∙∙A O(7)−H(7)∙∙∙O(8)

D−H 0.8400

H∙∙∙A 2.1000

D∙∙∙A 2.879(16)

D−H∙∙∙A 154.00

O(8)−H(8)∙∙∙O(5)

0.8400

2.1200

2.953(16)

170.00

N(21)−H(21)∙∙∙O(5)

0.89(3)

2.40(5)

2.702(5)

100(3)

N(21)−H(21)∙∙∙O(2)

0.89(3)

2.01(3)

2.879(4)

166(4)

N(22)−H(22)∙∙∙O(7)

0.88(5)

1.87(5)

2.754(6)

176(6)

Figure 4.22: Crystal packing diagram displaying intermolecular hydrogen-bonds in complex 4 (blue-dashed) 4.3.6. (μ-Br)(μ-O)(μ-aoa)[Re2IVBr2(OiPr)(PPh3)2] (5) The reaction of trans-[ReOBr3(PPh3)2] with a two-fold molar excess of 5-amino-orotic acid (H2aoa) in propan-2-ol with addition of triethylamine (Et3N) led to the isolation of the dimeric ligand-bridged complex (μ-Br)(μ-O)(μ-aoa)[Re2IVBr2(OiPr)(PPh3)2] (5). Complex 5 is quite different from 1, with the difference being the replacement of the Nelson Mandela Metropolitan University

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two ethoxides in 1 by a bromide and propan-2-oxide in 5. As for 1, it is in the +IV (d3) oxidation state, intimating the existence of a metal-metal triple bond ReIV≡ReIV. i

trans-[ReOBr3(PPh3)2] + H2aoa

PrOH / Et3N

Reflux, 15 h

-O)-Br)(-aoa)[Re2IVBr2(OiPr)(PPh3)2] (5)

In this study, the reaction of trans-[ReOBr3(PPh3)2] with 5-amino-orotic acid (H2aoa) in propan-2-ol without addition of triethylamine (Et3N) was investigated (paragraph 4.3.3), and produced the monomeric complex [ReVBr(aoa)(apd)(PPh3)2] (2) (apd2- = 5-imidopyrimidine-2,4-dione). For that reaction, the ligand apd2- was formed by the decarboxylation of H2aoa, and it is coordinated via the dinegative imido nitrogen only. The mechanism of decarboxylation of orotic acid analogues indicated that it involves the acidic carboxylic proton transfer to the 2- or 4-oxygen on the orotic ring of these derivatives. During the synthesis of complex 5, the triethylamine (Et3N) was added in the reaction mixture before heating under reflux. Since Et3N is base, it was interacted with the acidic carboxylic proton and consequently the decarboxylation of H2aoa was not possible, and therefore the reaction led to the dimeric complex (μBr)(μ-O)(μ-aoa)[Re2IVBr2(OiPr)(PPh3)2] (5) instead of the monomeric complex [ReVBr (aoa)(apd)(PPh3)2] (2). Complex 5 is dark green in colour and it is soluble in DMSO, dichloromethane, DMF, ethanol and methanol, to give light green solutions. The infrared spectrum of 5 (Figure 4.23) displays an intense peak at 690 cm-1, which is ascribed to v(Re−O−Re). A medium-intensity peak at 536 cm-1 is assigned to v(Re−N). Weak intensity peaks at 492 and 438 and 435 cm-1 are ascribed to v(Re−O)CO2-, v(Re−O(8)) and v(Re−O(13)) respectively. The frequency of the C═O stretch of the coordinated neutral oxygen is assigned to the absorption at 1652 cm-1, with the uncoordinated C═O carbonyl stretching frequency occurring at 1694 cm-1. The frequencies of the C−H stretches of the coordinated 2-propanoxide are assigned to the absorptions at 2922 and 2850 cm-1. The frequencies of the N−H stretches of the free amino group and N−H stretch of the free amido group are assigned to the absorptions at 3055 and 3380 cm-1 respectively. The cyclic voltammograms of complex 5 in acetonitrile shows two one-electron irreversible oxidations at 1.149 and 0.302 V. The former signal is ascribed to the redox process from Re(IV)Re(IV) to Re(V)Re(IV) with the latter assigned to the redox Nelson Mandela Metropolitan University

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process from Re(V)Re(IV) to Re(V)Re(V). The sweep in negative potential shows a non-reversible peaks at -0.455 and -0.236 V. These peaks are assigned to the oneelectron reduction processes from Re(IV)Re(IV) to Re(III)Re(IV) and Re(III)Re(IV) to Re(III)Re(III), respectively.

87

% Transmittance

77

67

57

47

37 3340

2840

2340

1840

1340

840

340

Wavenumber (cm-1) Figure 4.23: IR spectrum of complex 2 The UV-vis spectrum of complex 5 in methanol displays absorptions at 286 nm ( 8560) and 408 nm ( 4450), which are assigned to an intraligand

* transition

in the coordinated aoa2− and a combination of the ligand-to-metal charge transitions LMCT pπ(O−)→d*π(Re), pπ(N−)→d*π(Re) and pπ(O)→d*π(Re) respectively. The absorption at 485 nm ( 1230) reflects a d-d transition. For triply-bonded metal-metal complexes of the type (µ-O)2Re2 a strong absorption bond was observed at about 490 nm with extinction coefficients around 104 dm3mol-1cm-1 for Re(IV) compounds. This band has been assigned to the

* transition within the Re−Re triple bond

[1].


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0.95 0.85

Absorbance

0.75 0.65 0.55 0.45 0.35 0.25 0.15 0.05 277

327

377

427

477

527

Wavelength (nm) Figure 4.24: UV-Vis spectrum of complex 5 in MeOH The structure of complex 5 is shown in Figure 4.25. The coordination milieu around Re(1) is exactly the same as around Re(1) in complex 1. However, around Re(2) two sites are occupied by a bromide Br(3) and a 2-propanoxide O(8), compared to two ethoxide oxygens in 1. This difference could be steric in nature due to the larger isopropyl group. The Re−Re distance of 2.5673(4) Å is practically the same as in 1. The bond lengths of the aoa2- donor atoms to the rhenium ions differ little from those observed in 1 [Re(1)−N(11) = 2.099(5) Å, Re(1)−O(11) = 2.044(4) Å, Re(2)−O(13) = 2.150(4) Å]. The bond distances Re(1)−O(1) [1.782(4) Å] and Re(2)−O(1) [1.924(4) Å] in which O(1) is a bridging oxide are close to their corresponding lengths in 1. The bond distance Re(2)−O(8) [1.860(4) Å] for 2-propanoxide is shorter than Re−O(6) [1.935(4) Å] for ethoxide in 1. There is nothing unusual about the other bond lengths and angles in the bridging chelate aoa2- comparatively to those in complex 1. The proton on the methanol of crystallisation O(9) is involved in an intermolecular hydrogen-bond with the oxygen atom O(12) of the aoa2− chelate. The protons on the free amino nitrogen N(13) of the aoa2− chelate are involved in the intramolecular hydrogen-bonds with the oxygen atom O(12) and intermolecular hydrogen-bond with oxygen atom O(9) of the methanol crystallisation solvent. The proton on the free amidic nitrogen N(12) of the aoa2− chelate is also involved in an intramolecular Nelson Mandela Metropolitan University

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hydrogen-bond with oxygen atom O(14). The hydrogen-bond parameters are summarised in Table 4.5.

Figure 4.25: Molecular structure of complex 5 showing 40% probability displacement ellipsoids and atom labelling. Hydrogen atoms, methanol and ethanol crystallisation solvents have been omitted for clarity

Table 4.5: Hydrogen-bond distances (Å) and angles (o) in complex 5 D−H∙∙∙A O(9)−H(9)∙∙∙O(12)

D−H 0.8400

H∙∙∙A 2.1200

D∙∙∙A 2.95(1)

D−H∙∙∙A 167.00

N(12)−H(12)∙∙∙O(14)

0.91(4)

1.95(4)

2.849(8)

172(4)

N(13)−H(13A)∙∙∙O(12)

0.9900

2.2500

2.864(9)

120.00

N(13)−H(13A)∙∙∙O(12)

0.9900

2.3300

3.238(8)

153.00

N(13)−H(13B)∙∙∙O(9)

1.1600

1.8200

1.81(1)

140.00


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Figure 4.26: Crystal packing diagram of complex 5 displaying hydrogen-bonds (blue-dashed) 4.3.7. [Re(CO)3(H2O)(amef)] (6) The zwitterionic rhenium(I) complex [Re(CO)3(H2O)(amef)] (6) (amef = -(5ammoniumpyrimidine-2,4(1H,3H)-dioxamido)-1,2,3,6-tetrahedro-2,6-dioxopyrimidine4-ethylformate ion) was prepared by the heating under reflux of [Re(CO)5Cl] with a two-fold molar excess of H2ampa in ethanol. The chelating ion amef was formed by a combined tricarbonylrhenium(I)-catalysed esterification and amino-protonation of H2ampa (Scheme 4.3), and the amef− anion coordinates to the fac-[Re(CO)3]+ core as a bidentate N,N-donor chelate via the two negative amido nitrogens.

[Re(CO)5Cl] + H2ampa

EtOH Reflux, 24 h

[Re(CO)3(H2O)(amef)] (6) + 2 CO + HCl

Complex 6 is yellow in colour and it is soluble in polar and non-polar organic solvents to give yellow solutions.


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The IR spectrum of 6 (Figure 4.27) displays a strong sharp band at 2019 and a broad band at 1883 cm-1 (which is a combination of two bands) which are assigned to the three ν(C≡O) of the fac-[Re(CO)3]+ unit. The coordination of amef− to rhenium is supported by the weak intensity peaks at 511 and 526 cm-1 which are assigned to v(Re−N). The weak intensity peak at 489 cm-1 assigned to v(Re−O)H2O. The bands at 1607 and 1637 cm-1 are assigned to v(C═O). In the 1H NMR spectrum of 6, the −NH3+ protons produce a broad singlet peak at 2.04 ppm. The broad singlet peaks 7.02 ppm and 9.06 ppm are assigned to NH protons. The protons of the CH3 and CH2 occur as a triplet at 1.32 ppm and a quartet at 4.21 ppm respectively. In the UV-visible spectrum of complex 6 (Figure 4.27) in methanol, the intra-ligand * transition of the coordinated amef− anion occurs at 267 nm. The bands at 342 and 371 nm are assigned to a ligand-to-metal charge transfer (LMCT), with no d-d transitions as expected for a spin-paired d6 system.

Absorbance

2.03

1.53

1.03

0.53

0.03 250

300

350

400

450

500

Wavelength (nm) Figure 4.27: UV-Vis spectrum of complex 6 in MeOH The structure of 6 is shown in Figure 4.28. The rhenium(I) atom lies in a distorted octahedral environment, with the water O(8) and two deprotonated donor amido nitrogen atoms N(1) and N(11) of amef- in a facial arrangement, imposed by the fac[Re(CO)3]+ core. The amino nitrogen N(13) is protonated and this group is present as Nelson Mandela Metropolitan University

115

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J. Mukiza

a free –NH3+ appendix. The coordinated amef chelate is therefore a zwitter-ion, with two negative charges on N(1) and N(11), and a positive charge on N(13) giving an overall mono-anion. The rhenium(I) complex is therefore neutral. The distortion in the complex is mainly the result of the trans angles O(8)−Re−C(5) = 176.5(3)°, N(1)−Re−C(6) = 172.8(3)° and N(11)−Re−C(7) = 171.2(4)° (Table 4.10). The bidentate chelate has a bite angle of N(1)−Re−N(11) = 74.2(3)°, close to the values observed for other five-membered metallocycles found in this study. The Re−N(1) [2.155(5) Å] and Re−N(11) [2.15(1) Å] bond lengths agree well with the ReI−N(amido) distances reported in the literature for similar rhenium(I) complexes [2]. The Re−O(8)H2 bond distance of 2.206(7) Å is similar to that found [2.198(5) Å] in

fac-[Re(CO)3(H2O)(nc)]

(Hnc

=

4-imidazolecarboxylic

acid;

pyridine-2,4-

dicarboxylic acid) [26] and 2.205(2) Å found in fac-[Re(CO)3(H2O)(oa)]+ (H2oa = orotic acid) [2]. The Re−C(6) bond length of 1.917(8) Å fall in the the range observed [1.900(2)−1.928(2) Å] for similar complexes [24]. However, the two other Re−C distances of 1.878(9) Å and 1.89(1) Å fall at the lower end of that range and this was also observed in the complex fac-[Re(CO)3(H2O)(oa)]+ [2]. The coordination of amef− results in a five-membered metallocylic ring and the constraints imposed by this ring affect

the

bond

angles

O(8)−Re−N(1)

[79.4(2)°],

O(8)−Re−C(6) [95.7(3)°],

N(11)−Re−C(6) [99.8(4)°], N(1)−Re−C(5) [100.3(3)°] and N(11)−Re−C(5) [98.4(4)°], which are markedly deviated from orthogonality. In the chelate amef- the O(11)−C(12) [1.19(2) Å], O(12)−C(13) [1.29(2) Å] O(1)−C(1) [1.244(8) Å], O(31)−C(33) [1.230(7) Å] bond lengths are double. The C(11)−C(14) [1.39(1) Å] and C(31)−C(32) [1.346(8) Å] bonds are also double. The N(12)−C(12) [1.39(1) Å], N(11)−C(11) [1.39(1) Å], N(1)−C(31) [1.412(8) Å], N(32)−C(34) [1.370(9) Å] and N(31)−C(32) [1.395(8) Å] bond lengths are single. The N(1)−C(1) [1.308(9) Å] is a single bond, somehow shortened due to the coordination. There is nothing unusual about the other bond lengths and angles in the molecule. The protons on nitrogen atoms N(12), N(13), N(31) and N(31) are involved in the intramolecular hydrogen-bonds with oxygen atom O(11), O(1), O(6), O(12), O(2) and O(31) (Figure 4.29). The hydrogen-bond parameters are summarised in Table 4.6.


116

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J. Mukiza

Figure 4.28: Molecular structure of complex 6 showing 50% probability displacement ellipsoids and atom labelling. The disorded part of the amef ligand have been omitted for clarity Table 4.6: Hydrogen-bond distances (Å) and angles (o) in complex 6 D−H∙∙∙A N(12)−H(12)∙∙∙O(11)

D−H 0.8800

H∙∙∙A 1.8800

D∙∙∙A 2.73(2)

D−H∙∙∙A 161.00

N(13)−H(13A)∙∙∙O(1)

0.9100

1.8500

2.62(1)

140.00

N(13)−H(13A)∙∙∙O(6)

0.9100

2.3500

2.94(1)

122.00

N(13)−H(13C)∙∙∙O(12)

0.9100

1.7400

2.25(2)

113.00

N(31)−H(31)∙∙∙O(2)

0.8800

2.2700

2.645(8)

105.00

N(31)−H(31)∙∙∙O(1)

0.8800

2.0700

2.842(7)

145.00

N(32)−H(32)∙∙∙O(31)

0.8800

1.9900

2.859(7)

167.00


117

Chapter 4

J. Mukiza

Figure 4.29: Crystal packing diagram of complex 6, displaying hydrogen-bonds (blue-dashed)

4.4. Conclusion The reactivity of 5-amino-orotic acid (H2aoa), its Schiff base salicylimine-orotic acid (H2soa) and carboxamide {5-(5-aminopyrimidine-2,4(1H,3H)-dioxamido)-1,2,3,6tetrahydro-2,6-dioxopyrimidine-4-carboxylic acid; H2ampa} to the [ReO]3+ and fac[Re(CO)3]+ cores was investigated. The reaction of H2aoa with [ReOBr3(PPh3)2] in ethanol produced a dimer with

a metal-metal triple

bond (µ-O)(µ-Br)(µ-

aoa)[Re2IVBr(OEt)2(PPh3)2] (1). The same reaction in propan-2-ol however, by addition of triethylamine, gave a complex quite different from 1, i.e. (µ-Br)(µ-O)(µaoa)[Re2IVBr2(iOPr)(PPh3)2] (5). The anion aoa2- in 1 and 5 coordinated as a tridentate bridging ligand, with the coordination of a neutral ketonic oxygen to one rhenium ion, and a carboxylic oxygen and amidic nitrogen to the other rhenium ion. The complex [ReVBr(apd)(aoa)(PPh3)2] (2) (apd2− = 5-imidopyrimidine-2,4-dione) was formed from the reaction of H2aoa with [ReOBr3(PPh3)2] in propan-2-ol without triethylamine. The 5-aminopyrimidine-2,4-dione (H2apd) was formed by the oxorhenium(V)-catalysed decarboxylation of 5-amino-orotic acid, and coordinated via the dinegative imido nitrogen only. The anion aoa2- in 2 coordinated as a bidentate via the carboxylate oxygen and pyrimidine nitrogen. The rhenium(III) complex [Re(coa)I(PPh3)2]I

(3)

[Hcoa


=

5-(2-hydroxybenzylideneamino)pyrimidine-

118

Chapter 4

J. Mukiza

2,4(1H,3H)-dione] was isolated from the reaction of H2soa with [ReOI2(OEt)(PPh3)2] in

ethanol.

The

Hcoa

is also

formed

from

the

oxorhenium(V)-catalysed

decarboxylation of H2soa and coordinated as a monoanionic N,O,O-donor via the phenolate and ketonic oxygens and imino nitrogen atom. However, decarboxylation of H2soa was not observed by its reaction with [ReOCl3(PPh2)3] and the complex [ReOCl(soa)(PPh3)] (4) was isolated. The soa2- in 4 coordinated as expected via the carboxylate and phenolate oxygens and imino nitrogen. The reaction of H2ampa with [Re(CO)5Cl] in ethanol led to the complex [Re(CO)3(H2O)(amef)] (6) [amef = (5-ammoniumpyrimidine-2,4(1H,3H)-dioxamido)1,2,3,6-tetrahedro-2,6-dioxopyrimidine-4-ethylformate] ion. The ion amef was derived from the combined tricarbonylrhenium(I)-catalysed esterification and aminoprotonation of H2ampa and coordinates to the fac-[Re(CO)3]+ core as a dianionic N,N-donor via the amido nitrgens.

4.5. References [1]


[2]


[3]

B.Ž. Jovanović, F.H. Assaleh, A.D. Marninković, J. Serb. Chem. Soc., 69 949, 2004.

[4]

J. Mukiza, T.I.A. Gerber, E. Hosten, Inorg. Chem. Commun., 57, 54, 2015.

[5]

T. Mukherjee, B. Sen, E. Zangrando, G. Hundal, B. Chattopadhyay, Inorg. Chim. Acta, 406, 176, 2013.

[6]

R.W. McClard, M.J. Black, L.R. Livingstone, M.E. Jones, Biochemistry, 19 4699, 1980; A. Radzicka, R. Wolfenden, Science, 267, 90, 1995.

[7]

J.P. Ferris, P.C. Joshi, J. Org. Chem., 44, 2133, 1979.

[8]

T. Kimura, J. Kamimura, K. Takada, A. Sugimori, Chem. Lett., 237, 1976.

[9]

R. Steinberger, F.H. Westheimer, J. Am. Chem. Soc., 73, 429, 1951.

[10]

J.H. Fitzpatrick, D. Hopgood, Inorg. Chem., 13, 568, 1974.


119

Chapter 4

J. Mukiza

[11]

Y. Kuninobu, K. Takai, Chem. Rev., 111, 1938, 2009.

[12]

H. Chen, J.F. Hartwig, Angew. Chem. Int. Ed., 38, 3391, 1999.

[13]


[14]


[15]


[16]

P.I. Girginova, F.A.A. Paz, H.I.S. Nogueira, N.J.O. Silva, V.S. Amaral, J. Klinowski, T. Trindade, Polyhedron, 24, 563, 2005.

[17]

S. Ikari, T. Ito, W. McFarlane, M. Nasreldin, B.-L. Ooi, Y. Sasaki, A.G. Sykes, J. Chem. Soc., Dalton Trans., 2621, 1993.

[18]

T. Lis, Acta Crystallogr. B31, 1594, 1975.

[19]

G. Bohm, K. Wieghardt, B Nuber, J. Weiss, Angew. Chem. Int. Ed. Engl., 29, 787, 1990.

[20]

H.B. Bürgi, G. Anderegg, P. Bläuenstein, Inorg. Chem., 20, 3829, 1981.

[21]

L.M. Phillips, J.K. Lee, J. Am. Chem. Soc., 123, 12067, 2001.

[22]

N.P. Johnson, C.J.L. Lock, G. Wilkinson, Inorg. Synth., 9, 145, 1967.

[23]


[24] J. Mukiza, T.I.A. Gerber, E. Hosten, J. Chem. Crystallogr., 44, 368, 2014. [25] B. Machura, M. Wolff, R. Kruszynski, J. Kusz, Polyhedron, 28, 1211, 2009. [26] S. Mundwiler, M. Kündig, K. Ortner, R. Alberto, Dalton Trans., 1320, 2004.


120

Chapter 4

J. Mukiza


2

C45H42Br2N3O7P2Re2. 4C45H36BrN6O6P2Re. 2CH2Cl2

C18H18P.2C3H8O.2H2O

Formula Weight

1507.41

4874.05

Crystal System

Triclinic

Triclinic

Space group

P-1

P-1

a (Å)

10.6861(4)

12.2251(5)

b (Å)

13.3343(5)

18.6799(7)

c (Å)

20.0831(7)

24.8383(10)

α (deg.)

94.795(2)

76.135(2)

β (deg.)

103.987(3)

84.084(2)

γ (deg.)

110.030(3)

83.914(2)

Volume (Å3)

2565.05(17)

5458.1(8)

Z

2

1

1.952

1.483


6.719

3.079

F(000)

1451

2434

θ range (deg.)

1.1-28.3

1.7-28.4

Index ranges h

-14/14

-16/16

k

-17/17

-24/24

l

-26/26

-33/33

Reflection measured

44698

99184


12593/10590

27255/21767

Data/parameters

12593/638

27255/1226


1.07

1.03


0.0320

0.0350

(wR2 = 0.0754)

(wR2 = 0.0982)

2.16/-1.69

1.60/-1.16

Density (g/cm3) -1



121

Chapter 4

J. Mukiza


4

C47H38IN3O3P2Re.

C30H22ClN3O6PRe

CH2Cl2.C2H6O.I

3C2H6O

Formula Weight

1325.75

911.34

Crystal System

Triclinic

Triclinic

Space group

P-1

P-1

a (Å)

12.5109(4)

9.7306(4)

b (Å)

12. 5727(4)

12.0567(7)

c (Å)

17.2977(6)

17.5581(7)

α (deg.)

81.005(1)

100.094(2)

β (deg.)

78.210(1)

101.296(3)

68.639(1)

104.981(2)

Volume (Å )

2470.5(1)

1895.0(1)

Z

2

2

Density (g/cm3)

1.782

1.597


3.929

3.375

F(000)

1288

912

θ range (deg.)

2.0-28.4

1.8-28.3

Index ranges h

-16/16

-12/12

k

-16/14

-16/15

l

-21/23

-23/23

γ (deg.) 3

Reflection measured

43514

49342


12173/10612

9426/8632

Data/parameters

12173/614

9426/439

Goodness-of-fit on F

1.08

1.09


0.0563

0.0277

(wR2 = 0.0259)

(wR2 = 0.0749)

1.90/-1.91

1.68/-0.94

2



122

Chapter 4

J. Mukiza


2C44H40Br3N3O7P2Re2.

6 C15H11N6O11Re

C2H6O).2CH4O Formula Weight

2871.86

637.51

Crystal System

Triclinic

Triclinic

Space group

P-1

P-1

a (Å)

11.2227(7)

9.2352(3)

b (Å)

13.3707(8)

9.9988(4)

c (Å)

17.9362(1)

13.6460(5)

α (deg.)

92.734(3)

81.635(2)

β (deg.)

91.277(3)

83.279(2)

γ (deg.)

113.713(2)

75.235(2)

Volume (Å3)

2458.9(3)

1201.31(8)

Z

1

2

1.939

1.672


7.473

5.120

F(000)

1378

612

θ range (deg.)

2.0-28.4

2.1-28.3

Index ranges h

-14/15

-12/12

k

-17/16

-13/13

l

-23/23

-17/18

Reflection measured

119711

21857


12224/10501

5982/4836

Data/parameters

12224/580

5982/339


1.07

1.06


0.0386

0.0451

(wR2 = 0.0936)

(wR2 = 0.1200)

2.72/-3.25

4.49/-0.79

Density (g/cm3) -1



123

Chapter 4

J. Mukiza

Table 4.10: Selected bond lengths (Å) and angles (°) for complexes 1 and 2 Bond length 1 Re(1)−Re(2)

2.5526(4)

2 Re(1)−Br(1)

2.5229(2)

Re(1)−O(5)

1.890(3)

Re(1)−P(11)

2.509(1)

Re(2)−O(5)

1.913(3)

C(113)−N(113)

1.359(6)

R(1)−Br(1)

1.5592(6)

Re(1)−O(113)

2.049(3)

Re(2)−Br(1)

1.6141(6)

Re(1)−N(111)

1.55(3)

Re(1)−N(1)

2.090(4)

Re(2)−O(213)

2.099(2)

Re(1)−O(4)

2.045(4)

Re(2)−N(211)

2.149(3)

Re(2)−O(1)

1.906(7)

Re(1)−N(121)

1.733(3)

Re(2)−O(6)

1.935(4)

Re(2)−Br(2)

2.5179(6)

C(11)−O(1)

1.267(5)

Re(2)−Br(3)

2.442(1)

C(13)−C(14)

1.360(6)

Re(2)−N(221)

1.719(3)

O(2)−C(12)

1.231(5)

Re(2)−P(21)

2.473(3)

Bond angles 1 O(5)−Re(1)−Br(1)

109.65(9)

2 O(113)−Re(1)−N(121)

161.56(8)

O(5)−Re(2)−Br(1)

106.74(9)

P(11)−Re(1)−P(12)

167.11(4)

Re(1)−O(5)−Re(2)

84.3(1)

O(213)−Re(2)−N(221)

172.2(1)

Re(1)−Br(1)−Re(2)

59.12(2)

P(21)−Re(2)−P(22)

166.97(4)

O(4)−Re(1)−N(1)

74.6(2)

N(111)−Re(1)−N(121)

98.96(3)

O(1)−Re(2)−O(6)

171.0(1)

N(211)−Re(2)−N(221)

99.6(1)

Br(2)−Re(1)−N(1)

164.9(1)

O(213)−Re(2)−N(121)

74.6(1)

Br(1)−Re(2)−P(2)

170.65(3)

N(111)−Re(1)−O(113)

75.0(1)

O(2)−C(12)−N(2)

122(5)

Br(1)−Re(1)−N(111)

161.56(8)


124

Chapter 4

J. Mukiza

Table 4.11: Selected bond lengths (Å) and angles (°) for complexes 3 and 4 Bond length 3 Re(1)−O(3)

1.953(2)

4 Re(1)−O(6)

1.695(2)

Re(1)−O(1)

2.102(2)

Re(1)−O(4)

2.094(2)

Re(1)−P(2)

2.485(1)

Re(1)−O(1)

1.949(2)

Re(1)−N(3)

2.071(2)

Re(1)−N(1)

2.115(3)

Re(1)−I(1)

2.7139(4)

Re(1)−P(1)

2.4567(9)

Re(1)−P(1)

2.482(1)

Re(1)−Cl(1)

2.3517(9)

N(3)−C(5)

1.306(4)

N(1)−C(1)

1.301(5)

Bond angles 3 O(1)−Re(1)−O(3)

168.88(8)

4 O(1)−Re(1)−O(6)

166.6(1)

P(1)−Re(1)−P(2)

177.32(3)

Cl(1)−Re(1)−N(1)

166.02(7)

N(3)−Re(1)−I(1)

176.05(7)

P(1)−Re(1)−O(4)

176.39(7)

N(3)−Re(1)−O(1)

79.11(9)

O(6)−Re(1)−O(4)

96.0(1)

N(3)−Re(1)−O(3)

89.78(9)

O(6)−Re(1)−N(1)

86.6(1)

P(2)−Re(1)−N(3)

88.84(8)

O(4)−Re(1)−O(6)

96.0(1)

P(2)−Re(1)−O(1)

90.45(7)

Re(1)−O(1)−C(12)

134.7(2)

C(1)−N(3)−C(5)

121.4(3)

C(1)−N(1)−C(2)

117.9(3)


125

Chapter 4

J. Mukiza


Re(1)−Re(2)

Bond lengths 2.5673(4) Re(1)−Br(1)

Re(1)−Br(2)

2.5594(8)

Re(1)−P(1)

2.472(2)

Re(1)−O(1)

1.872(4)

Re(1)−O(11)

2.044(4)

Re(1)−N(11)

2.099(5)

Re(2)−Br(2)

2.5950(8)

Re(2)−Br(3)

2.4806(8)

Re(2)−P(2)

2.466(2)

Re(2)−O(1)

1.924(4)

Re(2)−O(8)

1.860(4)

Re(2)−O(13)

2.150(4)

O(13)−C(12)

1.263(6)

O(14)−C(13)

2.23(1)

C(11)−C(14)

1.349(9)

2.4667(8)

Re(1)−Br(2)−Re(2)

Bond angles 59.74(2) Re(1)−O(1)−Re(2)

O(1)−Re(1)−O(11)

156.8(2)

Br(1)−Re(1)−N(11)

159.2(2)

Br(2)−Re(1)−P(1)

172.50(4)

Br(3)−Re(2)−O(13)

174.9(1)

Br(2)−Re(2)−P(2)

175.81(4)

Br(1)−Re(1)−Br(2)

90.71(2)

Br(1)−Re(1)−O(11)

86.3(1)

Br(1)−Re(1)−O(1)

107.1(1)

Br(2)−Re(2)−O(1)

106.0(1)

Br(3)−Re(2)−P(2)

90.33(4)

Br(2)−Re(2)−O(13)

92.7(1)

Br(3)−Re(2)−O(1)

99.1(1)

Br(2)−Re(2)−O(8)

91.4(1)

Re(2)−Re(1)−N(11)

85.6(1)

Re(2)−Re(1)−O(11)

145.7(1)

Re(1)−Re(2)−O(8)

147.4(1)


85.1(2)

126

Chapter 4

J. Mukiza

Table 4.13: Selected bond lengths (Å) and angles (°) for complex 6 Re(1)−O(8)

Bond lengths 2.206(7) Re(1)−N(1)

2.155(5)

Re(1)−N(11)

2.15(1)

Re(1)−C(6)

1.917(8)

Re(1)−C(7)

1.878(9)

Re(1)−C(5)

1.89(1)

O(12)−C(8)

1.19(2)

O(12)−C(13)

1.29(2)

O(1)−C(11)

1.244(8)

O(31)−C(33)

1.230(7)

O(2)−C(2)

1.227(9)

N(12)−C(12)

1.39(1)

N(11)−C(11)

1.39(1)

N(1)−C(1)

1.308(9)

N(32)−C(34)

1.370(9)

N(1)−C(31)

1.412(8)

N(31)−C(32)

1.395(9)

C(11)−C(14)

1.39(1)

C(31)−C(32)

1.346(8)

N(11)−C(12)

1.39(2)

N(12)−C(13)

1.39(1)

N(32)−C(33)

1.358(8)

O(8)−Re(1)−C(5)

Bond angles 176.5(3) N(1)−Re(1)−C(6)

N(1)−Re(1)−C(7)

171.2(4)

N(1)−Re(1)−N(1)

74.2(3)

O(8)−Re(1)−N(1)

79.4(2)

O(8)−Re(1)−C(6)

95.7(3)

N(1)−Re(1)−C(5)

100.3(3)

N(11)−Re(1)−C(6)

99.8(4)

N(11)−Re(1)−C(5)

98.4(4)

C(5)−Re(1)−C(6)

84.3(4)

C(5)−Re(1)−C(7)

86.1(4)

C(6)−Re(1)−C(7)

88.1(4)


172.8(3)

127

Chapter 5

J. Mukiza

Chapter 5 Dimeric Rhenium(IV) and Monomeric Rhenium(III) and (V) Complexes of 6-Hydroxypicolinic Acid 5.1. Introduction Due to their inherent tendency to form chelating rings, picolinate ligands have shown the ability to form either mononuclear or polynuclear complexes with transition metals [1]. They can also act as bridging ligands for up to four metals ions, leading to their use as building blocks to assemble multi-dimensional frameworks [1]. Transition metal complexes of picolinic acid (or pyridine-2-carboxylic acid; Figure 5.1), as well as its derivatives with electron-withdrawing groups like hydroxyl and halogens, have been widely studied due to their coordination flexibility and a variety of physiological properties, particularly their insulinomimetic activity [2].

OH O

N

Figure 5.1: Line structure of picolinic acid Hydroxypicolinic acids as ligands have attracted attention since they display a variety of bonding modes [3]. Also, with pyridinecarboxylate ligands (HON), the complexes [ReOCl2(ON)(PPh3)] have shown catalytic activity for the conversion of ethane to propionic/acetic acids [2], and [ReO(CH3)(ON)2] is active in olefin oxidation [4, 5]. The coordination modes of hydroxyl derivatives of picolinic acid to other transition metals have been investigated, and their complexes with copper, platinum, palladium, molbdenum and tungsten [1] have been reported in the literature. This chapter will report and discuss the coordination mode of a hydroxyl derivative of picolinic acid, 6-hydroxypicolinic acid (H2hpa; Figure 5.2), to rhenium in the +III, +IV and +V oxidation states.


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OH N

OH

O

Figure 5.2: Line structure of 6-hydroxypicolinic acid (H2hpa) 6-Hydroxypicolinic acid exhibits enol-keto tautomerization (Scheme 5.1) which can model its coordination mode to transition metals. The tautomerisation is due to the mobility of the unstable hydrogen atom of the OH group which is close to the basic nitrogen atom and consequently could easily be transferred to it [1, 6].

OH

OH N

OH

O

O

H N

O

Scheme 5.1: Enol-keto tautomerism of 6-hydroxypicolinic acid (H2hpa) The reaction of trans-[ReOBr3(PPh3)2] with a two-fold molar excess of 6hydroxypicolinic acid (H2hpa) in ethanol led to the isolation of the doubly ligandbridged complex (µ-O)(µ-hpa)2[ReIV2Br(OEt)(PPh3)2] (1). Each hpa2- anion acts as a bridging ligand with the coordination of charged pyridolate oxygen to one rhenium ion, and the coordination of a carboxylic oxygen and neutral pyridinic nitrogen to the other rhenium ion. X-ray analysis showed that the ethoxide is, to a small extent (20%), disordered with a bromide. Both rhenium ions in 1 are therefore in the +IV (d3) oxidation state, intimating the existence of a metal-metal triple bond ReIV≡ReIV. The similar reaction however, by reducing the reaction time from 24 hours to 6 hours and using excess of trans-[ReOBr3(PPh3)2], led to the singly ligand-bridged dimer complex (µ-Br)(µ-O)(µ-hpa)[ReIV2Br2(OEt)(PPh3)2] (8). By using cis-[ReO2I(PPh3)2] as precursor in ethanol, the homologous product of 1 with iodide in place of bromide was isolated from the reaction solution, i.e. (µ-O)(µ-hpa)2[ReIV2I(OEt)(PPh3)2] (7). By using trans-[ReOCl3(PPh3)2] as precursor in ethanol, two products were isolated from the

reaction

solution,

i.e.

(µ-Cl)(µ-O)(µ-hpa)[ReIV2Cl2(OEt)(PPh3)2]

(2)

and

[ReIIICl2(Hhpa)(PPh3)2] (3). Complex 2 is also a bridged dimer, quite different from 1, only one hpa2- chelate bridges the two metal centres, and the coordination Nelson Mandela Metropolitan University

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environment of each rhenium is different. The neutral complex 3 contains rhenium in the oxidation state +III, with coordination of Hhpa - only occurring through the carboxylic oxygen and neutral pyridyl nitrogen. As observed from the reaction of orotic acid (H2oa) and 5-aminoorotic acid (H2aoa) with trans-[ReOBr3(PPh3)2] in ethanol, the reduction to Re(IV) in the bridged dimers is unusual, and it was intimated that it is the result of disproportionation of Re(V) to Re(IV) and Re(VII), as was observed earlier in the formation of ReIV(µ-O)2 complexes [7]. It is our belief that the formation of 1, 2, 7 and 8 also occurs by the disproportionation rather than the reduction by PPh3, which normally lead to rhenium(III) products. We could find no evidence of OPPh3 formation in any of the reactions. The d3 [Re(IV)] system would promote the formation of a metal-metal triple bond, as explained in the literature [8]. The complexes cis-[ReOX2(Hhpa)(PPh3)] (X = Br (4); Cl (5)) were the only products formed by the reaction of [ReOX3(PPh3)2] with H2hpa in acetonitrile, which we found surprising. Normally rhenium(III) products are formed in acetonitrile via the intermediate [ReX3(MeCN)(PPh3)2] [9]. Again, Hhpa- is coordinated in the same manner as in 3. The bromide equivalent of 3, i.e. [ReIIIBr2(Hhpa)(PPh3)2] (6), was obtained as a precipitate from the reaction of [ReOBr3(PPh3)2] with H2hpa in propan2-ol. It was the only product isolated, and the slow evaporation of the reaction mother liquor also produced 6.

5.2. Experimental 5.2.1. Synthesis of (µ-O)(µ-hpa)2[ReIV2Br(OEt)(PPh3)2] (1) To a solution of H2hpa (50 mg, 0.36 mmol) in 10 cm3 of ethanol was added trans[ReOBr3(PPh3)2] (150 mg, 155 µmol) in 10 cm3 of ethanol. The yellow mixture was heated under reflux for 24 hours, resulting in a black solution, which was filtered after being cooled to room temperature. No precipitate was obtained. The filtrate was left to evaporate slowly at room temperature, and after three weeks very dark green crystals were harvested by filtration. Yield: 41 %; m.p. 150 °C. IR (ν max/cm-1): νs(CO2) 1695; νa(CO2) 1316; ν(Re−O−Re) 722; ν(Re−N) 509, 519; ν(Re−O) 453br. 1H NMR (d6-DMSO, ppm): 1.02 (t, 3H, OCH2CH3), 4.01 (q, 2H, OCH2), 7.26-7.34 (m, 30H), 7.36 (dd, 2H), 7.76 (d, 2H), 8.20 (d, 2H). UV-vis (acetonitrile, λmax nm (ε, M-1cm-1)): 274 (22200), 335 (4360), 376 (2700), 483 (2750).


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5.2.2. Synthesis of (µ-Cl)(µ-O)(µ-hpa)[ReIV2Cl2(OEt)(PPh3)2] (2) and [ReIIICl2(Hhpa)(PPh3)2] (3) To trans-[ReOCl3(PPh3)2] (180 mg, 216 µmol) in 10 cm3 of ethanol was added 0.432 µmol of H2hpa (60 mg) in 10 cm3 of ethanol. The resulting yellow-green mixture was heated at reflux for 24 hours, resulting in a dark green solution. After cooling to room temperature a red-orange precipitate was removed by filtration. The recrystallisation of the precipitate in an ethanol/dichloromethane mixture gave red-orange crystals of complex 3. A volume of 2 cm3 of acetonitrile was added to the filtrate, and the mixture was left to evaporate slowly at room temperature. After a month dark green crystals of 2 were collected, and they were washed with ethanol and acetone. 2.3MeCN: Yield = 23 %, m.p. = 152 °C. IR (νmax/cm-1): νs(CO2) 1699; νa(CO2) 1312; ν(Re−O−Re) 721; ν(Re−N) 522; ν(Re−O) 457.1H NMR (d6-DMSO, ppm): 1.04 (t, 3H, OCH2CH3), 4.02 (q, 2H, OCH2), 7.26-7.34 (m, 30H), 7.38 (dd, 1H), 7.79 (d, 1H), 8.24 (d, 1H). Electronic spectrum (methanol, λ nm (ε, M-1cm-1)): 284 (18000), 329 (4200), 408 (3760). 3: Yield = 29 %, m.p. = 286 °C. IR (νmax/cm-1): ν(O−H) 3177; νs(CO2) 1678; νa(CO2) 1321; ν(C═N) 1621; ν(Re−N) 496; ν(Re−O) 451. Electronic spectrum (acetonitrile, λmax nm (ε, M-1cm-1)): 339 (8200), 447 nm (4000), 522 (2900). 5.2.3. Synthesis of cis-[ReOX2(Hhpa)(PPh3)] (X = Br (4); Cl (5)) To trans-[ReOX3(PPh3)2] (155 µmol) in 10 cm3 of acetonitrile was added 380 µmol of H2hpca (53 mg) in 10 cm3 of acetonitrile. The resulting mixture was heated at reflux for 4 hours, resulting in a dark green solution, which was filtered after being cooled to room temperature. No precipitate was obtained. The dark green crystals were grown in one week from the slow evaporation of mother liquor at room temperature. 4: Yield = 60 %, m.p. = 191 °C. IR (νmax/cm-1): ν(Re═O) 974; νs(CO2) 1693; νa(CO2) 1322; ν(Re−N) 496; ν(C═N) 1620; ν(Re−O) 442. 1H NMR (d6-DMSO, ppm): 5.08 (s, OH), 7.38-7.60 (m, 15H), 7.30 (dd, 1H), 7.43 (d, 1H), 7.65 (d, 1H). Electronic spectrum (acetonitrile, λ nm (ε, M-1cm-1)): 339 (6200), 358 nm (5460), 391 (2420). 5.OPPh3.H2O: Yield = 66 %, m.p. = 183 °C. IR (νmax/cm-1): ν(Re═O) 974; νs(CO2) 1693; νa(CO2) 1354; ν(Re−N) 497; ν(Re−O) 435. 1H NMR (d6-DMSO, ppm): 5.08 (s, OH), 7.38-7.60 (m, 15H), 7.30 (dd, 1H), 7.43 (d, 1H), 7.65 (d, 1H). Electronic spectrum (acetonitrile, λ nm (ε, M-1cm-1)): 308 (6200), 375 (5460).


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5.2.4. Synthesis of [ReIIIBr2(Hhpa)(PPh3)2] (6) To trans-[ReOBr3(PPh3)2] (213 mg, 220 µmol) in 10 cm3 of propan-2-ol was added 432 µmol of H2hpa (60 mg) in 10 cm3 of propan-2-ol. The resulting mixture was heated at reflux for 24 hours, resulting in a dark orange solution. After cooling to room temperature an orange precipitate was removed by filtration. The recrystallization of the solid from an ethanol/dichloromethane mixture produced orange crystals. Yield = 39 %, m.p. = 225 °C. IR (νmax/cm-1): ν(O−H) 3194; νs(CO2) 1677; νa(CO2) 1325; ν(Re−N) 499; ν(Re−O) 451. Electronic spectrum (acetonitrile, λmax nm (ε, M-1cm-1)): 311 (8200), 361 (4000). 5.2.5. Synthesis of (µ-O)(µ-hpa)2[ReIV2I(OEt)(PPh3)2] (7) To a solution of H2hpa (80 mg, 575 μmol) in 10 cm3 of ethanol was added cis[ReO2I(PPh3)2] (250 mg, 288 µmol) in 10 cm3 of ethanol. The purple mixture was heated under reflux for 24 hours, resulting in a dark green solution, which was filtered after being cooled to room temperature. No precipitate was obtained. The filtrate was left to evaporate slowly at room temperature, and after three months the dark green crystals were harvested by filtration. Yield: 65 %; m.p. 147 °C. IR (νmax/cm-1): νs(CO2) 1648; νa(CO2) 1369; ν(Re−O−Re) 691; ν(Re−N) 529, 537; ν(Re−O) 440, 453. 1H NMR (d6-DMSO, ppm): 1.04 (t, 3H, OCH2CH3), 4.03 (q, 2H, OCH2), 7.28-7.36 (m, 30H), 7.38 (dd, 2H), 7.78 (d, 2H), 8.24 (d, 2H). UV-vis (Methanol, λmax nm (ε, M-1cm-1)): 350 (17300), 438 (2400). 5.2.6. Synthesis of (µ-Br)(µ-O)(µ-hpa)[ReIV2Br2(OEt)(PPh3)2] (8) The mixture of trans-[ReOBr3(PPh3)2] (213 mg, 220 µmol) and H2hpa (15 mg, 110 µmol) in 15 cm3 of ethanol was heated under reflux for 6 hours, resulting in a dark green solution, which was filtered after being cooled to room temperature. No precipitate was obtained. The mother liquor was left to evaporate slowly at room temperature and dark green crystals suitable for X-ray crystallography were grown in two months. Yield = 57 %, m.p. = 148 °C. IR (νmax/cm-1): νs(CO2) 1642; νa(CO2) 1372; ν(Re−O−Re) 692; ν(Re−N) 536; ν(Re−O) 441.1H NMR (d6-DMSO, ppm): 1.05 (t, 3H, OCH2CH3), 4.03 (q, 2H, OCH2), 7.29-7.37 (m, 30H), 7.41 (dd, 1H), 7.82 (d, 1H), 8.27 (d, 1H). Electronic spectrum (acetonitrile, λ nm (ε, M-1cm-1)): 345 (1800), 450 nm (105).


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5.2.7. X-ray crystallography Single crystal X-ray crystallography studies were performed at 200 K using a Bruker Kappa Apex II diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71069 Å). For data collection, APEX-II was used while for cell refinement and data reduction, SAINT was used [10]. The structures were solved by direct methods applying SHELXS-97 [11], or SIR97 [12] and refined by least-squares procedures using SHELXL-97 [10], with SHELXLE [12] as a graphical interface. All non-hydrogen atoms were anisotropically refined and hydrogen atoms were calculated in idealised geometrical positions. Data were corrected by absorption effects using the numerical method using SADABS [10].

5.3. Results and discussion 5.3.1. (μ-O)(μ-hpa)2[Re2Br(OEt)(PPh3)2] (1) and (μ-O)(μ-Cl)(μ-hpa)[Re2Cl2(OEt)(PPh3)2] (2) The reaction of trans-[ReOX3(PPh3)2] with a two-fold molar excess of 6-hydroxy picolinic acid (H2hpa) in ethanol led to the isolation of the dimeric ligand-bridged complexes (μ-O)(μ-hpa)2[Re2Br(OEt)(PPh3)2] (1) (for X = Br) and (μ-O)(μ-Cl)(μhpa)[Re2Cl2(OEt)(PPh3)2] (2) (X = Cl). However, complex 2 was isolated together with the rhenium(III) complex [ReCl2(Hhpa)(PPh3)2] (3). Complexes 1 and 2 are ligand-bridged dimers and they are quite different each other. Both complexes 1 and 2 are therefore in the +IV (d3) oxidation state, intimating the existence of a metalmetal triple bond ReIV≡ReIV. [ReOCl3(PPh3)2] / EtOH Reflux, 24 h H2hpa [ReOBr3(PPh3)2] / EtOH Reflux, 24 h

-O)-Cl)(-hpa)[Re2Cl2(OEt)(PPh3)2] (2) + [ReCl2(Hhpa)(PPh3)2] (3)

-O)(-hpa)2[Re2Br(OEt)(PPh3)2] (1)

Complexes 1 and 2 are air stable and they are both dark green in the solid state. They are reasonably soluble in both polar and non-polar organic solvents, giving dark green solutions for both complexes. Nelson Mandela Metropolitan University

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In the infrared spectra of 1 and 2 the ν(Re−O−Re) is displayed as a strong peak around 720 cm-1, and the coordination of the hpa2- ligand is indicated by bands for ν(Re−N) around 520 cm-1, ν(Re−O) around 450 cm-1, and the symmetric and asymmetric CO2 stretching vibrations at about 1695 and 1315 cm -1 respectively. The difference of ~380 cm-1 between the latter two vibrations confirms the monodentate coordination mode of the carboxylate group. The

1


diamagnetic complexes, indicating that there are no unpaired electrons in the dimers. Sharp peaks for the proton signals of the coordinated ethoxide occur as triplet and quartet at 1.02 and 4.01 ppm respectively for 1 and 1.04 and 4.02 ppm for 2. The aromatic regions for 1 and 2 display the multiplet peaks in the range 7.267.34 ppm due to the PPh3. For 1, the pyridyl protons occur as a doublet of a doublet at 7.36 ppm, a doublet at 7.76 ppm and a doublet 8.20 ppm, and the similar peaks

% Transmittance

for 2 occur at 7.38, 7.79 and 8.24 ppm.

87

Complex 1

77

Complex 2

67 57 47 37 27 17 7 1750

1550

1350

1150

950

750

550

350

Wavenumber (cm-1) Figure 5.3: Overlay IR spectra of complex 1 and 2 The UV-visible absorption spectra of complexes 1 and 2 in methanol are characterized by an intense absorption around 284 nm (ε ~ 20000), assigned to the intraligand

* transition in hpa2− and a less intense band around 330 nm (ε

4300) due to a ligand-to-metal charge transfer (LMCT). The former peak appears at 279 nm in the spectrum of the free ligand H2hpa in methanol. The peak at 480 nm (ε 3000) for complex 1 is indicative of a d-d transition and it is found at 408 nm (ε 3760) Nelson Mandela Metropolitan University

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for complex 2. For triply-bonded metal-metal complexes of the type (µ-O)2Re2 a strong absorption bond was observed at about 490 nm with extinction coefficients around 104 dm3mol-1cm-1 for Re(IV) compounds. This band has been assigned to the transition within the Re−Re triple bond [13]. The cyclic voltammograms of complexes 1 and 2 (Figure 5.5) in methanol show both reductive and oxidative waves. For 1, two one-electron irreversible oxidations were observed at 0.917 and 0.289 V. The former signal is ascribed to the redox process from Re(IV)Re(IV) to Re(V)Re(IV) with the latter assigned to the redox process from Re(V)Re(IV) to Re(V)Re(V). For 2, similar signals are found at 0.761 and 0.516 V and the former signal is quasi-reversible. For 1, the sweep in negative potential shows a non-reversible peaks at -1.30 and -0.693 V. These peaks are assigned to the one-electron reduction processes from Re(IV)Re(IV) to Re(III)Re(IV) and Re(III)Re(IV) to Re(III)Re(III), respectively. For 2, the similar peaks are found at -1.19 and -0.432 V.

Absorbance

2.01

Complex 1

1.51

Complex 2

1.01

0.51

0.01 255

305

355

405

455

505

555

Wavelength (nm) Figure 5.4: Overlay UV-Vis spectra of complexes 1 and 2 in MeOH


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Figure 5.5: Overlay cyclic voltammograms of complexes 1 and 2 in MeOH Figure 5.6 shows the ORTEP drawing of the molecular structure of complex 1. The coordination environment of each rhenium ion in the complex is similar in the edgeshared bi-octahedral structure, with a bridging oxide ion O(1) and two bridging chelate ligands hpa2-. Each rhenium ion is coordinated in a bidentate manner by the pyridinic nitrogen atom and the carboxylate oxygen of one hpa2-, and pyridolate oxygen of the second hpa2-. In addition, a bromide, a phosphorus atom, and a bridging oxide O(1) are coordinated to the Re(1) ion, while Re(2) is bonded to a phosphorus, the bridging oxide O(1) and an ethoxide, which is disordered to the extent of 20% with a bromide. The Re−Re distance is 2.6268(4) Å, which is slightly longer than in the bridged compounds

[(Ph3P)Cl2ReIV(µ-O)(µ-Cl)(µ-EtCO2)ReIVCl2(PPh3)]

(2.52

Å)

and

[(Ph3P)ClRe(µ-O)(µ-Cl)(µ-EtCO2)2ReCl(PPh3)] (2.514 Å) [13], and considerably longer than in the (µ-O)2ReIV2 complexes [Re2(µ-O)2L2](PF6)4 (2.364(1) Å, L = tris(2pyridylmethyl)amine; 2.368(1) Å, L = ((6-methyl-2-pyridyl)methyl)bis(2-pyridylmethyl)amine) [16].The observed range for (µ-O)2Re2 complexes is 2.362–2.381 Å [7, 13−15]. Since the rhenium ions are formally in the +4 oxidation state with a d 3−d3 electron configuration, it would intimate a Re≡Re triple bond. A “longer than expected” Re≡Re bond has been explained in terms of a σ2π2

*2

, rather than the

conventional σ2π2 2, electronic configuration [8]. The former configuration was supported by molecular orbital calculations, which intimated that the interaction of Nelson Mandela Metropolitan University

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the orbitals of bridging ligands would place the energy of the that of the

*

orbital higher than

orbital, thus providing some antibonding character to the Re≡Re bond

[14, 15]. This may be due to the strong interaction of the p orbitals of the bridging oxo and bromide ions with the bonding molecular orbitals. The short average Re−O(1) bond length of 1.880(6) Å supports this destabilization of the which would lead to a formal bond order of 1, rather than 3 (σ2π2

orbital, 2

), and

consequently to a longer Re≡Re triple bond which leads to a large Re−O(1)−Re bond angle of 88.6(2)°.

Figure 5.6: ORTEP view of complex 1 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms, the disordered bromide and water of crystallisation have been omitted for clarity For the bridging chelate hpa2-, the Re−O(carboxylate) bond length [average = 2.029(6) Å] is shorter than the average Re−O(pyridolate) bond distance [2.073(6) Å]. The O(161)−Re(1)−N(11) bite angle is 75.7(3), and the O(161)−Re(1)−Re(2)−O(261) Nelson Mandela Metropolitan University

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axis is linear [torsion angle equals 3.1(5)°]. There is nothing unusual about the other bond lengths and angles in the molecule. The molecular structure of complex 2 is shown in Figure 5.7, and the crystal and structure refinement data are given in Table 5.4. The bond lengths and angles are given in Table 5.8. The coordination milieu around the rhenium(IV) ions is different in the edge-shared bi-octahedral structure. Only one hpa2- ligand bridges the two metal centres, in addition to the oxide O(1) and chloride Cl(2) bridges. The donor atoms N(1) and carboxylate O(12) are coordinated to Re(1), together with P(1) and Cl(1). The pyridolate O(11) is bonded to Re(2), which is also coordinated to the ethoxide O(8), chloride Cl(3) and P(2).

Figure 5.7: ORTEP view of complex 2 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and acetonitrile crystallisation solvent have been omitted for clarity The Re−Re distance of 2.5101(4) Å is considerably shorter than in complex 1, but is similar to the compounds with only chloride and oxo bridges, i.e. [(Ph 3P)Cl2ReIV(µO)(µ-Cl)(µ-EtCO2)ReIVCl2(PPh3)]

[2.52(1)


Å]

and

[(Ph3P)ClRe(µ-O)(µ-Cl)(µ138

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EtCO2)2ReCl(PPh3)] [2.514 Å] [13]. The Re−O(1)−Re bond angle of 82.7(1)° is considerably smaller than in complex 1 [88.6(2)°], mainly due to the larger Re−Re distance in the latter complex. The bond lengths of the hpa 2- donor atoms to the rhenium ions [Re(1)−N(1)

= 2.110(3), Re(1)−O(12) = 2.026(3), Re(2)−O(11) =

2.093(3) Å] differ little from those observed in 1 [respective average values are 2.131(7), 2.029(6) and 2.073(6) Å], and the bite angle of hpa2- [N(1)−Re(1)−O(12) = 75.2(1)°] is nearly identical to that in complex 1 [average of 75.5(3)°]. 5.3.2. Cis-[ReOX2(Hhpa)(PPh3)] (X = Br (4); Cl (5)) The oxorhenium(V) complexes cis-[ReOX2(Hhpa)(PPh3) (X = Br (4), Cl (5)) were obtained from the reaction of two equivalents of H 2hpa with [ReOX3(PPh3)2] (X = Br (4), Cl (5)) under reflux conditions for 4 hours in acetonitrile. [ReOBr3(PPh3)2] / CH3CN Reflux, 4 h

cis-[ReOBr2(Hhpa)(PPh3)] (4)

H2hpa [ReOCl3(PPh3)2] / CH3CN Reflux, 4 h

cis-[ReOCl2(Hhpa)(PPh3)] (5)

Monomeric oxorhenium(V) complexes containing bidentate monoanionic N,O-donor chelates are common in the literature [17, 18, 19]. There is no evidence of the reduction of rhenium(V) as was found in ethanol in paragraph 5.3.1. Complexes 4 and 5 are air-stable with dark green-coloured crystals and they are soluble in both polar and non-polar organic solvents to give light green solutions. In the IR spectra (Figure 5.8) the ν(Re═O) absorptions appear as medium-intensity bands at about 970 cm-1. The difference in the νsym(CO2) and νasym(CO2) is 370 cm-1, and ν(C═N) appears around 1620 cm-1. For complex 4, there two bands of medium intensity at 442 and 496 cm-1, assigned to ν(Re−O) and ν(Re−N) respectively. For complex 5, these bands (ν(Re−O) and ν(Re−N))

occur at 435 and 497 cm-1

respectively.


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95

% transmittance

85

75

65

Complex 4

55

Complex 5

45

35 1780

1580

1380

1180

980

780

580

380

Wavenumber (cm-1) Figure 5.8: Overlay IR spectra of complexes 4 and 5 In the 1H NMR spectra of 4 and 5 the three pyridyl protons occur as a doublet of doublet at 7.30, a doublet at 7.65 and a doublet at 7.43 ppm. The protons of the phenyl rings integrated for 15 protons for 4 and 5 occur as multiplet signals in the range 7.38−7.60 ppm. The UV-vis spectra of the complexes in acetonitrile (Figure 5.9) are quite different from each other. For complex 4, the three absorption peaks occur at 339, 358 and 391 nm, and they are assigned to intraligand

* transitions in the coordinated

Hhpa−, and a combination of the ligand-to-metal charge transfer transitions pπ(O−)→d*π(Re), pπ(N)→d*π(Re) and pπ(Br−)→d*π(Re), and a (dxy)2→(dxy)1(dxz)1 transition respectively. For complex 5, the two absorption peaks occur at 308 and 375 nm, and they are assigned to intraligand

* transitions in the coordinated

Hhpa− and a combination of the ligand-to-metal charge transitions pπ(O−)→d*π(Re), pπ(N)→d*π(Re) and pπ(Cl−)→d*π(Re) respectively. The cyclic voltammograms in methanol (Figure 5.10) show both reductive and oxidative waves. For 4, the sweep in positive potential shows non-reversible processes at 0.478 and 0.258 V. The former peak is ascribed to the oxidation from ReV to ReVI and the latter one is due to the oxidation from ReVI to ReVII. For 5, the Nelson Mandela Metropolitan University

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similar peaks occur at 0.557 and 0.256 V. For 4, the sweep in negative potential shows two non-reversible peaks at -0.983 and -0.535 V, which are assigned to the reduction from ReV to ReIV and ReIV to ReIII respectively. For 5, the similar peaks occur at -1.19 and -0.473 V.

1.45 1.25

Absorbance

Complex 4 1.05 0.85

Complex 5

0.65 0.45 0.25 0.05 300

320

340

360

380

400

420

440

460

Wavelength (nm) Figure 5.9: Overlay UV-vis spectra of complexes 4 and 5 in MeCN

Figure 5.10: Overlay cyclic voltammograms of complexes 4 and 5 in MeOH Nelson Mandela Metropolitan University

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An ORTEP view of the molecular structure of complex 4 (Figure 5.11) shows that the ligand Hhpa− acts as a monoanionic bidentate N,O-donor chelate and coordinates to the metal center through the neutral pyridinic nitrogen atom N(11) and anionic carboxylate oxygen atom O(11), resulting in a five-membered ring metallocycle. The crystal structure refinement data are given in Table 5.9 and the bond lengths and angles are given in Table 5.5. The geometry around rhenium center is a distorted octahedral in which the basal plane is defined by the phosphorous atom P(1), the bromides Br(1) and Br(2) and nitrogen N(11). The phenolic oxygen O(15) is uncoordinated and its proton forms a hydrogen-bond to O(12). The two bromides Br(1) and Br(2) are in a cis arrangement with the Br(1)−Re(1)−Br(2) angle of 88.08(1)o. The oxo ligand O(1) lies in a trans axial position to the coordinated carboxylate oxygen O(11). The deviation from an ideal octahedral geometry around the rhenium center is mainly due to the non-linear axis Br(1)−Re(1)−P(1) = 163.15(2)o, Br(2)−Re(1)−N(11) = 164.53(5)o and O(1)−Re(1)−O(11) = 162.57(7)o. The constraints imposed by the coordination mode of the Hhpa− anion to a five-membered ring metallocycle and the steric repulsion of the oxo ligand O(1) displaces the rhenium atom out of the mean equatorial plane, resulting in the deviation from the expected orthogonality of the rhenium-centered bond angles Br(1)−Re(1)−O(1) = 107.03(7)o, O(1)−Re(1)−N(11) = 95.05(7)o,

O(11)−Re(1)−N(11)

=

74.50(6)o,

P(1)−Re(1)−N(11)

=

92.56(5)o,

Br(1)−Re(1)−N(11) = 83.30(5)o and Br(2)−Re(1)−O(11) = 92.14(4)o. The Re−O(1) length of 1.667(2) Å falls in the range observed for Re═O distances in distorted octahedral monooxo-rhenium(V) complexes [2]. The Re(1)−O(11) bond distance [2.095(2) Å] agrees well with ReV−O(carboxylate) distances in similar oxorhenium(V) complexes [20, 21]. The bond length Re(1)−Br(1) [2.5131(3) Å] is significantly longer than the Re(1)−Br(2) [2.4741(1) Å], reflecting the larger trans effect of P(1) compared to the pyridinic nitrogen N(11). The Re(1)−N(11) bond length [2.156(2) Å] is typical for ReV−N(pyridyl) bond distances [18, 20]. The Re(1)−P(1) bond length [2.4671(6) Å] is in good agreement with Re−P bond distances reported in similar complexes [22]. The O(15)−C(15) bond distance of 1.319(2) Å is between a single and a double bond reflecting the tendency to enol-keto tautomerisation in the Hhpa− ligand. The other bond distances and angles in Hhpa− are normal.


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Figure 5.11: ORTEP view of complex 4 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity The proton on oxygen O(15) is involved in an intramolecular hydrogen-bond with the oxygen atom O(12) [O(15)−H(15)∙∙∙O(12)], and its parameters are summarised in Table 5.1. Table 5.1: Hydrogen-bond distance (Å) and angle (o) in complex 4 D−H∙∙∙A O(15)−H(1)∙∙∙O(12)

D−H 0.8400


H∙∙∙A 1.7900

D∙∙∙A 2.626(2)

D−H∙∙∙A 175.00

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Figure 5.12: Crystal packing diagram of complex 4, displaying hydrogen-bonds Crystals of complex 5 were obtained as the adduct 5.OPPh3.H2O from the acetonitrile preparative solution (Figure 5.11) and the Hhpa− anion is coordinated similarly as in complex 4. The crystal structure refinement data are given in Table 5.5 and the bond lengths and angles are given in Table 5.9. The rhenium(V) ion lies at the centre of a distorted octahedron. The basal plane is defined by the pyridyl nitrogen N(1), two chlorides and the phosphorus atom of PPh3. The carboxylic oxygen O(11) and the oxo O(2) lie in trans axial positions. The two chlorides are coordinated to the metal cis to each other, with a Cl−Re−Cl angle of 88.39(2)°. The metal is lifted out of the mean equatorial plane towards O(2), which results in the O(2)−Re−Cl(1), O(2)−Re−Cl(2) and O(2)−Re−N(1) angles being larger than 90° [105.58(6), 101.51(5) and 95.15(7)°, respectively]. The O(2)−Re−P(1) angle is close to orthogonality [89.38(5)°], and the O(2)−Re−O(11) angle deviates considerably from linearity [162.59(7)°]. The small bite angle of Hhpa− [74.39(5)°] contributes considerably to the distortion of the complex. Nelson Mandela Metropolitan University

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Figure 5.13: ORTEP view of complex 5 showing 50% probability displacement ellipsoids and atom labelling The Re−Cl(1) bond, trans to P(1), is longer [2.3730(6) Å] than the bond trans to the pyridyl N(1) [2.3462(6) Å], illustrating the larger trans influence of the phosphorus atom. The Re−O(2) length of 1.667(2) Å is close to that in complex 4, and falls in the range observed for distorted octahedral monooxo-rhenium(V) complexes [23, 24]. The bond distances Re(1)−N(11) [2.154(2) Å] and Re(1)−O(11) [2.054(1) Å] are typical for ReV−pyridyl and ReV−O(carboxylate) lengths [20, 23] respectively, and they are in good agreement with the similar distances in complex 4. Complexes of the formula [ReOCl2(Rsal)(PPh3)] (RsalH = bidentate N,O-donor Schiff base) were previously prepared from the reaction of [ReOCl3(PPh3)2] with RsalH. In all these compounds the oxygen-donor atom of the Rsal- ligands is coordinated trans to the Re═O bond [24]. The complex [ReOCl2(Mesal)(PPh2)] (Mesal = Nmethylsalicylideneimino) was isolated in both the cis and trans forms, which differ in the arrangement of the two chlorine atoms in the equatorial plane.


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The protons on the oxygen atom of the water of crystallisation are involved in intramolecular and intermolecular hydrogen-bonds with the oxygen atom O(3) of OPPh3. There is also an intermolecular hydrogen-bond between the proton of the uncoordinated oxygen atom O(13) and the oxygen atom of crystallisation water O(4). All hydrogen bonds parameters are summarised in Table 5.2.

Figure 5.14: Crystal packing diagram of complex 5, displaying hydrogen-bonds (blue-dashed) Table 5.2: Hydrogen-bond distances (Å) and angles (o) in complex 5 D−H∙∙∙A O(4)−H(4A)∙∙∙O(4)

D−H 0.84(2)

H∙∙∙A 1.85(2)

D∙∙∙A 2.677(2)

D−H∙∙∙A 174(3)

O(4)−H(4B)∙∙∙O(3)

0.83(2)

1.87(2)

2.683(2)

166(2)

O(13)−H(13A)∙∙∙O(4)

0.8400

1.6900

2.526(4)

176.00


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5.3.3. [ReX2(Hhpa)(PPh3)2] (X = Cl(3); Br (6)) The rhenium(III) complexes [ReX2(Hhpa)(PPh3)] (X = Cl (3), Br (6)) were obtained from the reaction of two equivalents of H2hpa with [ReOBr3(PPh3)2] in propan-2-ol for (6), while complex 3 was isolated (together with complex 2) from the reaction of two equivalents of H2hpa with [ReOCl3(PPh3)2] in ethanol. For both complexes 3 and 6, the two halides are in cis positions while both triphenylphosphines are in trans positions each other. [ReOCl3(PPh3)2] / EtOH Reflux, 24 h H2hpa [ReOBr3(PPh3)2] / iPrOH Reflux, 24 h

-O)-Cl)(-hpa)[Re2Cl2(OEt)(PPh3)2] (2) + [ReCl2(Hhpa)(PPh3)2] (3)

[ReBr2(Hhpa)(PPh3)2] (6)

Dihalogeno-rhenium(III) complexes with two triphenylphosphines in trans positions and halogens in cis positions as in complexes 3 and 6 were previously reported in the literature. Typical examples are the complexes [ReCl2(bat)(PPh3)2] (Hbat = benzoylacetone) [25] and [ReBr2(bp)PPh3)2] (Hbp = 2-hydroxybenzophenone) [18], and they were obtained from the reaction of trans-[ReCl3(MeCN)(PPh3)2 with Hbat and trans-[ReOBr3(PPh3)2] with Hbp respectively. Complexes 3 and 6 have orange crystals and they are weakly soluble in non-polar organic solvents, but very soluble in polar organic solvents, particularly dichloromethane and chloroform, to give orange-coloured solutions. In the IR spectra of [ReX2(Hhpa)(PPh3)2] (X = Cl(3), Br(6)) the bands due to νsym(CO2) and νasym(CO2) appear around 1675 and 1320 cm-1 respectively, again confirming the monodentate coordination mode of the carboxylate group (Figure 9.15). The typical vibrational mode of substituted pyridines, ν(C═N), appears at 1606 cm-1 for uncoordinated H2hap, and it is shifted by about 15 cm -1 to lower wave numbers, which is in agreement with the N,O-chelation involving the pyridinic nitrogen atom. The characteristic bands for ν(Re−N) and ν(Re−O) appear at about 500 and 450 cm-1 respectively. The single deprotonation of H2hpa is supported by the presence of the bands at 3177 cm-1 for complex 3 and 3194 cm-1 for complex 6 which could be assigned to v(O−H). Nelson Mandela Metropolitan University

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% Transmittance

88

78

68

Complex 3

58

Complex 6

48

38 2850

2350

1850

1350

850

350

Wavenumber (cm-1) Figure 5.15: Overlay IR spectra of complexes 3 and 6 The UV-vis spectrum of complex 3 in acetonitrile displays absorptions at 339 and 447 nm which are assigned to an intraligand

* transition in the coordinated

Hhpa− and a combination of the ligand-to-metal charge transitions LMCT [pπ(O−)→d*π(Re), pπ(N)→d*π(Re) and pπ(Cl−)→d*π(Re) respectively]. The absorption at 522 nm reflects a (dxy)2→(dxy)1(dxz)1 transition. For the similar rhenium(III) complex [ReCl2(bat)PPh3)2] (Hbat = benzoylacetone) a ligand-to-metal charge transfer (MLCT) absorption was found at 442 nm, and an absorption due to a (dxy)2→(dxy)1(dxz)1 transition was observed at 545 nm [24]. For complex 6, the UV-vis spectrum in acetonitrile displays only two absorption bands at 311 and 361 nm, which are assigned to an intraligand

* transition in the coordinated Hhpa− and a

combination of the ligand-to-metal charge transitions LMCT [pπ(O−)→d*π(Re), pπ(N)→d*π(Re) and pπ(Br−)→d*π(Re) respectively]. The cyclic voltammograms of complexes 3 and 6 in acetonitrile show both reductive and oxidative waves. For 3, the sweep in positive potential shows non-reversible processes at 1.871 and 0.034 V. The former peak is ascribed to the oxidation from ReIII to ReIV and ReIII to ReV respectively. The sweep in negative potential shows a non-reversible process at -1.105 V and it is assigned to the reduction from ReV to ReIV. For complex 6, the sweep in positive potential shows non-reversible processes at 1.034 and 0.27 V which are assigned to the oxidation from Re III to ReIV and ReIII to Nelson Mandela Metropolitan University

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ReV respectively. The sweep in negative potential also shows non-reversible processes at -1.552 and -0.693 V. These processes are ascribed to the reduction from ReV to ReIV and ReV to ReIII respectively. Figures 5.16 and 5.17 illustrate the molecular structures of complexes 3 and 6 respectively. The crystal structure refinement data are given in Table 5.6 and the bond lengths and angles are given in Table 5.10. The rhenium(III) ions lie at the centre of a distorted octahedron. In both complexes the two donor atoms of the Hhpa− chelate lie in trans positions to the two halides, and the trans angles in the two complexes are nearly identical [3: N(1)−Re−Cl(1) = 163.90(4), O(2)−Re−Cl(2) = 175.29(4),

P(1)−Re−P(2)

=

174.16(2);

6:

N(1)−Re−Br(2)

=

162.19(4),

O(12)−Re−Br(1) = 175.99(4), P(1)−Re−P(2) = 174.41(2)°], as are the bite angles of Hhpa− [76.99(6)° in 3, 76.69(6)° in 6]. The deviation from the linearity of the trans angles contribute more to the distortion. The Re−N and Re−O distances are not influenced by the halides [Re−N(1) = 2.165(2) in 3, 2.170(2) Å in 6; Re−O(2) = 2.022(1) in 3, Re−O(12) = 2.025(1) Å in 6].

Figure 5.16: ORTEP view of complex 3 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity Nelson Mandela Metropolitan University

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The proton on the uncoordinated oxygen O(1) in complex 3 is involved in an intramolecular hydrogen-bond with the chloride atom Cl(2) (Figure 5.18). In similar way, the proton on the uncoordinated oxygen O(11) for complex 6 is involved in an intramolecular hydrogen-bond with the bromide atom Br(2) (Figure 5.19). The hydrogen-bond parameters are summarised in Table 5.3.

Figure 5.17: ORTEP view of complex 6 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity Table 5.3: Hydrogen-bond distances (Å) and angles (o) in complexes 3 and 6 D−H∙∙∙A 3: O(1)−H(1)∙∙∙Cl(2)

D−H 0.8400

H∙∙∙A 2.1500

D∙∙∙A 3.1028(15)

D−H∙∙∙A 176.00

6: O(11)−H(11)∙∙∙Br(2)

0.8400

2.2600

3.1028(15)

176.00


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Figure 5.18: Crystal packing diagram of complex 3, displaying the intramolecular hydrogen-bond

Figure 5.19: Crystal packing diagram of complex 6, displaying the intramolecular hydrogen-bond Nelson Mandela Metropolitan University

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5.3.4. (μ-O)(μ-hpa)2[ReIV2I(OEt)(PPh3)2] (7) The reaction of cis-[ReO2I(PPh3)2] with a two-fold molar excess of 6-hydroxy picolinic acid (H2hpa) in ethanol led to the isolation of the dimeric ligand-bridged complex (μO)(μ-hpa)2[Re2I(OEt)(PPh3)2] (7). Complex 7 is homologous to 1 with the only difference the presence of the iodide ligand in place of bromide ligand in 1. It is also in the +IV (d3) oxidation state, intimating the existence of a metal-metal triple bond ReIV≡ReIV.

H2hpa + cis-[ReO2I(PPh3)2]

EtOH

-O)(-hpa)2[Re2I(OEt)(PPh3)2] (7)

Reflux, 24 h

Complex 7 is air stable and it is dark green in both liquid and solid state. It is reasonably soluble in alcohol, dichloromethane, chloroform and acetonitrile. In the the infrared spectrum of 7 the ν(Re−O−Re) is displayed as a strong peak at 691 cm-1, and the coordination of the hpa2- ligands is indicated by bands for ν(Re−N) at 529 and 537 cm-1, ν(Re−O) at 453 cm-1, and the symmetric and asymmetric CO2 stretching vibrations at about 1648 and 1369 cm-1 respectively. The peak at 440 cm-1 is assigned to ν(Re−O(2)) of the ethoxide group. The UV-visible absorption spectrum of complexes 7 in methanol is characterized by an absorption band around 350 nm (ε 17300) due to a combined ligand-to-metal charge transfer (LMCT) [pπ(O−)→d*π(Re) and pπ(N)→d*π(Re)] (Figure 5.20). The peak at 438 nm (ε 2400) is indicative of a d-d transition. For triply-bonded metalmetal complexes of the type (µ-O)2Re2 a strong absorption bond was observed at about 490 nm with extinction coefficients around 10 4 dm3mol-1cm-1 for Re(IV) compounds. This band has been assigned to the transition within the Re−Re triple bond [13]. The 1H NMR signal in solution was observed in the region expected for diamagnetic complexes, indicating that there are no unpaired electrons in the dimer. The aromatic region displays the multiplet peaks in the range 7.28-7.36 ppm due to the PPh3. The pyridyl protons occur as doublet of doublet at 7.38 ppm, doublet at 7.78 ppm and doublet 8.24 ppm. Sharp peaks for the proton signals of the coordinated ethoxide occur as triplet and quartet at 1.04 and 4.03 ppm respectively. Nelson Mandela Metropolitan University

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In the cyclic voltammogram of complex 7 (Figure 5.21) in methanol, both reductive and oxidative waves have observed. The two one-electron irreversible oxidations were observed at 0.268 and 0.197 V and they are assigned to the redox processes from Re(IV)Re(IV) to Re(V)Re(IV) and Re(V)Re(IV) to Re(V)Re(V). The sweep in negative potential shows a non-reversible peaks at -1.074 and -0.621 V. These peaks are assigned to the one-electron reduction processes from Re(IV)Re(IV) to Re(III)Re(IV) and Re(III)Re(IV) to Re(III)Re(III), respectively.

2.03

Absorbance

1.53

1.03

0.53

0.03 335

385

435

485

535

Wavelength (nm) Figure 5.20: UV-vis spectrum of complex 7 in MeOH

Figure 5.21: Cyclic voltammogram of complex 7 in MeOH Nelson Mandela Metropolitan University

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Figure 5.22 shows the ORTEP drawing of the molecular structure of complex 7, the crystal structure refinement data are given in Table 5.7 and the bond lengths and angles are given in Table 5.11. The coordination environment of each rhenium ion in the complex is similar in the edge-shared bi-octahedral structure, with a bridging oxide ion O(1) and two bridging chelate ligands hpa2- as in 1. As in 1, each rhenium ion is coordinated in a bidentate manner by the pyridinic nitrogen atom and the carboxylate oxygen of one hpa2-, and pyridolate oxygen of the second hpa2-. In addition, an iodide, a phosphorus atom, and a bridging oxide O(1) are coordinated to the Re(2) ion, while Re(1) is bonded to a phosphorus, the bridging oxide O(1) and an ethoxide.

Figure 5.22: ORTEP view of complex 7 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and ethanol of crystallisation have been omitted for clarity The Re−Re distance of 2.6298(5) Å is longer than 2.5101(4) Å in complex 2 and it is nearly identical to the 2.6268(5) Å in complex 1. The Re−O(1)−Re bond angle of 88.5(3)° is practically the same as in complex 1 [88.6(2)°], and it is mainly due to the Nelson Mandela Metropolitan University

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similar Re−Re distance in both complexes 1 and 7 , however, it is considerably larger than in complex 2 [82.7(1)°], mainly due to the larger Re−Re distance in complex 7 than in complex 2. The bond distances of the hpa2- donor atoms to both rhenium ions [Re(1)−N(1) = 2.138(9) Å, Re(1)−O(12) = 2.038(7) Å, Re(1)−O(21) = 2.089(6) Å, Re(2)−N(2) = 2.133(8) Å, Re(2)−O(22) = 2.029(7) Å, Re(2)−O(11) = 2.091(8) Å] differ little from those observed in 1 (Table 5.7), and the bite angles of hpa2- O(12)−Re(1)−N(1) and O(22)−Re(2)−N(2) equal to 75.4(3)°], and are nearly identical to that in complex 1 [average of 75.5(3)°]. There is one intermolecular hydrogen-bond between proton on the oxygen atom O(3)

of

ethanol

crystallisation

solvent

and

the

oxygen

atom

O(11)

[O(3)−H(3)∙∙∙O(11)], with D−H = 0.8400 Å, H…A = 2.4500 Å, D…A = 3.17(3) Å and D−H…A = 144.00 o (see Figure 5.23).

Figure 5.23: Crystal packing diagram of complex 7, displaying the intermolecular hydrogen-bond (blue-dashed) Nelson Mandela Metropolitan University

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5.3.5. Synthesis of (µ-Br)(µ-O)(µ-hpa)[ReIV2Br2(OEt)(PPh3)2] (8) The reaction of trans-[ReOBr3(PPh3)2] with a two-fold molar excess of 6-hydroxypicolinic acid (H2hpa) in ethanol under reflux for 24 hours led to the isolation of the doubly ligand-bridged complex (μ-O)(μ-hpa)2[Re2Br(OEt)(PPh3)2] (1) (paragraph 5.3.1). However, the reaction of H2hpa with a two-fold molar excess of trans[ReOBr3(PPh3)2] in ethanol, with the reduction of the reaction time from 24 hours to 6 hours did not yield complex 1 but the singly ligand-bridged complex (µ-Br)(µ-O)(µhpa)[Re2IVBr2(OEt)(PPh3)2] (8). Reflux 2 [ReOBr3(PPh3)2] + H2hpa + EtOH

6h

8 + OPPh3 + HBr + H2O

Complex 7 is air stable and it is dark green in solid state. It is soluble in chloroform, dichloromethane, acetonitrile and alcohols. The peak at 692 cm-1 in the infrared spectrum of complex 8 is assigned to ν(Re−O−Re). The coordination of the hpa2- chelate to rhenium is indicated by bands at 536 and 441 cm-1 for ν(Re−N) and ν(Re−O) respectively, and the symmetric and asymmetric CO2 stretching vibrations are at 1642 and 1371 cm-1 respectively (Figure 5.24). In the UV-visible absorption spectrum of 8 in acetonitrile, an absorption band at 345 nm (ε 1800) is due to a combined ligand-to-metal charge transfer (LMCT) [pπ(O−)→d*π(Re) and pπ(N)→d*π(Re)]. The peak at 450 nm (ε 105) is indicative of a dd transition. For triply-bonded metal-metal complexes of the type (µ-O)2Re2 a strong absorption bond was observed at about 490 nm with extinction coefficients around 104 dm3mol-1cm-1 for Re(IV) compounds. This band has been assigned to the transition within the Re−Re triple bond [13]. The cyclic voltammogram of complex 8 in methanol shows two non-reversible oneelectron oxidation peaks at 1.139 and 0.269 V, and the sweep in negative potential shows non-reversible peaks at -1.153 and -0.913 V. These peaks are similarly assigned to the corresponding signals in complex 7.


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75

% Transmittance

65 55 45 35 25 15 5 1600

1400

1200

1000

800

600

400

Wavenumber (cm-1) Figure 5.24: IR spectra of complex 8 The

1

H NMR spectrum in solution was observed in the region expected for

diamagnetic complexes, indicating that there are no unpaired electrons in the dimer, and it is practically the same to that of complex 2. The ORTEP diagram of the crystal structure of complex 8 is shown in Figure 5.35, and the crystal structure refinement data are given in Table 5.7. The bond lengths and angles are given in Table 5.12. The coordination milieu around the rhenium(IV) ions is different in the edge-shared bi-octahedral structure. Only one hpa2- ligand bridges the two metal centres, in addition to the oxide O(1) and bromide Br(2) bridges. The donor atoms N(1) and carboxylate O(12) are coordinated to Re(1), together with P(1) and Br(1). The pyridolate O(11) is bonded to Re(2), which is also coordinated to the ethoxide O(8), bromide Br(3) and P(2). The Re−Re distance of 2.5591(1) Å is shorter than in complexes 1 [2.6268(4) Å] and 7 [2.6298(5) Å], and it is also shorter than in the compounds (µ-Br)(µ-O)(µoa)[Re2IVBr(OEt)2(PPh3)2] [2.5664 (4) Å] and (µ-Br)(µ-O)(µ-oa)[Re2IVBr2(iOPr)(PPh3)2] [2.5639 (7) Å] (H2oa = orotic acid) [20]. However, it is longer than in the compound (μ-Br)(μ-O)(μ-MeCO2)[Re2IVBr4(OPPh3)(PPh3)2] [2.529(6) Å] [26] and in complex 2 [2.5101(4) Å], and practically similar to that reported in the complex (μ-Br)(μ-O)(μNelson Mandela Metropolitan University

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aoa)[Re2IVBr(OEt)2(PPh3)2] (H2aoa = 5-amino-orotic acid) [2.5526(4) Å] [27]. Due to the smaller Re−Re distance in complex 2 than in 8, the Re−O(1)−Re bond angle in 8 [84.9(3)°] is considerably larger than in complex 2 [82.7(1)°], and it is considerably smaller than in complex 1 [88.6(2)°] and 7 [88.5(3)°] mainly due to the larger Re−Re distance in 1 and 7 than in 8. The bond lengths of the hpa2- donor atoms to the rhenium ions [Re(1)−N(1) = 2.101(9), Re(1)−O(12) = 2.067(8) and Re(2)−O(11) = 2.142(9) Å] differ little from those observed in 7. The bite angle of hpa2[N(1)−Re(1)−O(12) = 75.3(4)°] is nearly identical to that in complex 7 [75.4(3)°].

Figure 5.25: ORTEP view of complex 8 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and ethanol of crystallisation have been omitted for clarity

5.4. Conclusion The reaction of 6-hydroxypicolinic acid (H2hpa) and with different oxorhenium(V) starting materials led to different products, depending on solvent, molar ratios and reaction time. With trans-[ReOX3(PPh3)2] (X = Cl, Br) in ethanol and a reaction time of 24 hours, and two-fold molar excess of H2hpa, the ReIV≡ReIV dimers (µ-O)(µhpa)2[Re2Br(OEt)(PPh3)2] (1) and (µ-Cl)(µ-O)(µ-hpa)[Re2Cl2(OEt)(PPh3)2] (2) were Nelson Mandela Metropolitan University

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formed. The bromo analogue of 2, complex 8, was isolated when the reaction time was shortened to six hours. The iodo analogue of 1, (µ-O)(µ-hpa)2[Re2I(OEt)(PPh3)2] (7), was obtained from cis-[ReO2I(PPh3)2]. The rhenium(III) monomer [ReCl2(Hhpa)(PPh3)2] (3) was obtained as a precipitate from the synthetic solution of complex 2, and its bromo analogue was isolated from the same reaction as for 1, but in propan-2-ol as solvent. The rhenium(V) monomers [ReOX2(Hhpa)(PPh3)] (X = Cl(4), Br(5)) were isolated from the reaction of [ReOX3(PPh3)2] and H2hpa (1:2 ratio) in acetonitrile.

5.5. References [1]

R.A. Coxall, S.G. Harris, D.K. Henderson, S. Persons, P.A. Tasker, R.E.P. Winpenny, J. Chem. Soc. Dalton Trans., 2349, 2000.

[2]

K. Boris-Marko, K. Matija, P. Zora, Croat. Chem. Acta, 85, 479, 2012.

[3]

P.I. Girginova, F.A.A. Paz, H.I.S. Nogueira, N.J.O. Silva, V.S. Amaral, J. Klinowski, T. Trindade, Polyhedron, 24, 563, 2005.

[4]

A.M. Kirillov, M. Haukka, M.V. Kirilova, A.J.L. Pombeiro, Adv. Synth. Catal., 347, 1435, 2005.

[5]

A. Deloffre, S. Halut, L. Salles, J.-M. Bregault, J.R. Gregorio, B. Denise, H. Rudler, J. Chem. Soc., Dalton Trans., 2897, 1999.

[6]

M.O.Q. Susana, I.S.H. Nogueira, V. Felix, G.B.M. Drew, Polyhedron, 21, 2783, 2002.

[7]

G. Bohm, K. Wieghardt, B. Nuber, J. Weiss, Inorg. Chem., 30, 3464, 1991; H. Sugimoto, M. Kamei, K. Umakoshi, Y. Sasaki, M. Suzuki, Inorg. Chem., 35, 7082, 1996.

[8]

K. Saito, Y. Nakao, H. Sato, S. Sakaki, J. Phys. Chem. A 110, 9710, 2006.

[9]

E. Marchesi, A. Marchi, L. Marvelli, M. Peruzzini, M. Brugnati, V. Bertolasi, Inorg. Chim. Acta, 358, 352, 2005.

[10]

APEX2, SADABS, SAINT, 2010, Bruker AXS Inc., Madison, Wisconsin,


159

Chapter 5

J. Mukiza

USA. [11]


[12]


[13]

F.A. Cotton, B.M. Foxman, Inorg. Chem., 7, 1784, 1968; F.A. Cotton, R. Eiss, B. Foxman, Inorg. Chem., 8, 950, 1969.

[14]

T. Lis, Acta Crystallogr. B31, 1594, 1975.

[15]

G. Bohm, K. Wieghardt, B Nuber, J. Weiss, Angew. Chem. Int. Ed. Engl., 29, 787, 1990.

[16]

F.A. Cotton, S.J. Lippard, Inorg. Chem., 4, 1621, 1965; A.M. Lebuis, C. Roux, A.L. Beauchamp, Acta Crystallogr., C49, 33, 1993.

[17]

A. Abrahams, I. Booysen, T.I.A. Gerber, P. Mayer, Bull. Chem. Soc. Ethiop., 22, 247, 2008.

[18]


[19]

T.I.A. Gerber, N.C. Yumata, R. Betz, Inorg. Chem. Commun., 15, 69, 2012.

[20]


[21]

Y. Kim, J.C. Hackett, R.W. Brueggemeier, J. Med. Chem., 47, 4032, 2004.

[22]


[23]


[24]

X. Chen, F.J. Femia, J.W. Babich, J. Zubieta, Inorg. Chim. Acta, 308, 80, 2000.

[25]

N.C. Yumata, MSc Dissertation, Nelson Mandela Metropolitan, University, 2010.

[26]


[27]

J. Mukiza, T.I.A. Gerber, E. Hosten, F. Taherkhani, M. Nahali, Inorg. Chem.


160

Chapter 5

J. Mukiza

Commun., 49, 5, 2014. Table 5.4: Crystal and structure refinement data for complexes 1 and 2 1 Formula

2

C50H41N2O8Re2 P2Br2.(H2O) C44H38Cl3NO5P2Re2. 3CH3CN

Formula weight

2654.71

1324.63

Crystal system

Monoclinic

Triclinic

Space group

C2/c

P-1

a (Å)

17.8560(6)

11.2459(5)

b (Å)

14.3110(2)

14.1196(7)

c (Å)

36.2420(12)

18.0377(9)

α (deg.)

104.323(2)

β (deg.)

96.032(1)

γ (deg.)

103.954(2) 108.216(2)

Volume (Å3) Z

9209.9(5)

2473.2(22)

4

2

3

Density (g/cm )

1.915

1.779


6.428

5.167

F(000)

5121

1292

θ range (deg.)

2.0-28.3

2.0-28.3

Index ranges h

-23/23

-15/15

k

-19/19

-18/18

l

-48/47

-21/23

Reflection measured

43529

43799


11446/10494

12237/11024

Data/parameters

11446/601

12237/599


1.40

1.17


0.0565

0.0254

(wR2 = 0.1186)

(wR2 = 0.0553)

2.23/-4.21

2.37/-1.30



161

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J. Mukiza


C24H19Br2NO4P

5 C24H19Cl2NO4PRe.C18H15OP. H2O

Formula weight

762.38

969.77

Crystal system

Monoclinic

Monoclinic

Space group

P21/c

P21/c

a (Å)

10.3450(3)

21.540(1)

b (Å)

16.3150(5)

11.5939(7)

c (Å)

14.9240(5)

16.2981(9)

β (deg.)

105.886(1)

96.341(2)

Volume (Å3)

2422.7(1)

4045.2(4)

Z

4

4

Density (g/cm3)

2.090

1.592


8.412

3.263

F(000)

1448

1928

θ range (deg.)

1.9-28.4

2.0-28.4

Index ranges h

-13/13

-28/28

k

-21/18

-15/15

l

-17/18

-21/21

Reflection measured

22154

110707


6022/5196

10078/9186

Data/parameters

6022/299

10078/496


1.03

1.13


0.0162

0.0176

(wR2 = 0.0334)

(wR2 = 0.0407)

0.69/-0.46

0.98/-0.79



162

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J. Mukiza


6

Formula

C42H34Cl2NO3P2Re

C42H34Br2NO3P2Re

Formula weight

919.75

1008.65

Crystal system

Triclinic

Triclinic

Space group

P-1

P-1

a (Å)

9.2159(5)

9.2814(5)

b (Å)

12.6597(7)

12.6715(7)

c (Å)

16.3801(9)

16.4346(9)

α (deg.)

83.305(3)

83.851(2)

β (deg.)

84.438(3)

84.809(2)

γ (deg.)

74.052(3)

73.960(2)

Volume (Å3)

1820.5(2)

1842.8(2)

Z

2

2

Density (g/cm3)

1.678

1.818


3.614

5.594

F(000)

912

984

θ range (deg.)

1.7-28.4

1.7-28.4

Index ranges h

-12/12

-12/12

k

-16/16

-16/16

l

-21/21

-21/21

Reflection measured

55889

50381


9057/8414

9203/8737

Data/parameters

9057/461

9203/461


1.06

1.08


0.0165

0.0152

(wR2 = 0.0378)

(wR2 = 0.0375)

0.89/-0.95

1.09/-0.94

-3



163

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J. Mukiza

Table 5.7: Crystal and structure refinement data for complex 7 and 8 7

8

Formula

C50H41lN2O8P2Re2.C2H6O

C44H38Br3NO5P2Re2

Formula weight

1405.18

1334.82

Crystal system

Triclinic

Monoclinic

Space group

P-1

P21/n

a (Å)

11.602(1)

9.8614(4)

b (Å)

14.468(1)

22.2612(9)

c (Å)

15.547(2)

19.7923(7)

α (deg.)

98.338(4)

β (deg.)

103.306(3)

γ (deg.)

99.397(4)

Volume (Å3)

2459.7(4)

4304.5(3)

Z

2

4

Density (g/cm3)

1.897

2.06


5.668

8.524

F(000)

1356

2536

θ range

1.8−28.4

2.1−28.3

Index ranges h

-15/15

-13/11

k

-19/19

-29/29

l

-20/20

-26/26

Reflection measured

42556

41987


12147/9849

10703/8673

Data/parameters

12147/602

10703/467


1.08

1.12

Final R indices [I>2σ>(I)]

0.0570

0.0601

(wR2 = 0.1453)

(wR2 = 0.1507)

3.07/-4.96

2.37/-4.18

-3



97.828(1)

164

Chapter 5

J. Mukiza

Table 5.8: Selected bond lengths (Å) and angles (°) for complex 1 and 2 Bond lengths 1 Re(1)−Re(2)

2.6268(4)

2 Re(1)−Re(2)

2.5101(4)

Re(1)−O(25)

2.078(6)

Re(1)−Cl(2)

2.407(1)

Re(1)−N(11)

2.119(7)

Re(1)−O(1)

1.895(2)

Re(2)−O(1)

1.877(6)

Re(1)−N(1)

2.110(3)

Re(2)−O(100)

1.88(1)

Re(2)−Cl(3)

2.382(1)

Re(2)−N(21)

2.141(7)

Re(2)−O(1)

1.906(2)

Re(1)−Br(1)

2.4996(6)

Re(2)−O(11)

2.093(3)

Re(1)−O(1)

1.883(6)

Re(1)−Cl(1)

2.353(1)

Re(1)−O(161)

2.041(6)

Re(1)−O(12)

2.026(3)

Re(2)−O(15)

2.067(6)

Re(2)−Cl(2)

2.440(1)

Re(2)−O(261)

2.016(6)

Re(2)−O(8)

1.931(3)

Bond Angles 1 Re(2)−Re(1)−O(161)

152.08(18)

2 Cl(1)−Re(1)−N(1)

167.69(9)

Br(1)−Re(1)−N(11)

168.3(2)

O(1)−Re(1)−O(12)

157.5(1)

O(1)−Re(2)−O(261)

153.8(2)

Cl(2)−Re(2)−P(2)

177.01(4)

O(1)−Re(1)−O(161)

153.1(3)

O(8)−Re(2)−O(11)

167.5(1)

O(1)−Re(1)−O(25)

129.8(2)

Re(1)−O(1)−Re(2)

82.7(1)

P(2)−Re(2)−O(100)

87.6(3)

O(1)−Re(2)−O(11)

86.09(1)

O(161)−Re(1)−N(11)

75.7(3)

N(1)−Re(1)−O(12)

75.2(1)

P(2)−Re(2)−N(21)

158.0(2)

Cl(2)−Re(1)−P(1)

169.97(4)

P(1)−Re(1)−O(25)

152.1(2)

Cl(3)−Re(2)−O(1)

162.33(8)

O(15)−Re(2)−O(100)

168.3(3)

Re(1)−Cl(2)−Re(2)

62.37(3)

Re(1)−O(1)−Re(2)

88.6(2)

Cl(1)−Re(1)−Cl(2)

92.95(4)


165

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J. Mukiza

Table 5.9: Selected bond lengths (Å) and angles (°) for complexes 4 and 5 Bond length 4 Re(1)−Br(1)

2.5131(3)

5 Re(1)−Cl(1)

2.3730(6)

Re(1)−P(1)

2.4671(6)

Re(1)−Cl(2)

2.3462(6)

Re(1)−O(11)

2.095(2)

Re(1)−O(2)

1.667(2)

Re(1)−Br(2)

2.4741(3)

Re(1)−O(11)

2.054(1)

Re(1)−O(1)

1.662(2)

Re(1)−P(1)

2.4764(6)

Re(1)−N(11)

2.156(2)

Re(1)−N(1)

2.154(2)

O(15)−C(15)

1.319(2)

N(13)−C(15)

1.317(3)

Bond angles 4 Br(1)−Re(1)−P(1)

163.15(2)

5 Cl(1)−Re(1)−P(1)

165.05(2)

O(1)−Re(1)−O(11)

162.57(7)

O(2)−Re(1)−O(11)

162.59(7)

P(1)−Re(1)−O(11)

77.25(5)

Cl(2)−Re(1)−N(1)

164.04(4)

P(1)−Re(1)−N(11)

92.56(5)

Cl(1)−Re(1)−Cl(2)

88.39(2)

Br(1)−Re(1)−O(1)

107.03(6)

O(2)−Re(1)−Cl(1)

105.58(6)

O(1)−Re(1)−N(11)

95.05(7)

O(2)−Re(1)−Cl(2)

101.51(5)

Br(2)−Re(1)−N(11)

164.53(5)

O(2)−Re(1)−N(1)

95.15(7)

Br(1)−Re(1)−Br(2)

88.08(1)

O(2)−Re(1)−P(1)

89.38(5)

P(1)−Re(1)−O(1)

89.57(6)

Cl(1)−Re(1)−O(11)

90.17(4)

Br(2)−Re(1)−O(11)

92.14(4)

O(11)−Re(1)−N(1)

74.39(5)

Br(1)−Re(1)−N(11)

83.30(5)

P(1)−Re(1)−O(11)

77.91(4)

O(11)−Re(1)−N(11)

74.50(6)

Cl(2)−Re(1)−P(1)

88.65(2)


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J. Mukiza

Table 5.10: Selected bond lengths (Å) and angles (°) for complexes 3 and 6 Bond length 3 Re(1)−Cl(1)

2.3690(5)

6 Re(1)−Br(1)

2.5174(5)

Re(1)−P(1)

2.4779(6)

Re(1)−Br(2)

2.5055(3)

Re(1)−O(2)

2.022(1)

Re(1)−P(1)

2.4770(6)

Re(1)−Cl(2)

2.3759(6)

Re(1)−P(2)

2.4665(6)

Re(1)−P(2)

2.4678(6)

Re(1)−O(12)

2.025(1)

Re(1)−N(1)

2.165(2)

Re(1)−N(1)

2.170(2)

O(2)−C(1)

1.302(2)

O(11)−C(15)

1.319(2)

Bond angles 3 Cl(1)−Re(1)−N(1)

163.90(4)

6 Br(1)−Re(1)−O(12)

175.99(4)

P(1)−Re(1)−P(2)

174.16(2)

Br(2)−Re(1)−N(1)

162.19(4)

P(1)−Re(1)−O(2)

93.24(4)

P(1)−Re(1)−P(2)

174.41(2)

Cl(2)−Re(1)−N(1)

98.49(4)

O(12)−Re(1)−N(1)

76.69(6)

P(2)−Re(1)−Cl(1)

89.53(2)

Br(1)−Re(1)−Br(2)

97.94(1)

Cl(1)−Re(1)−P(1)

90.61(2)

Br(2)−Re(1)−O(12)

85.61(4)

Cl(2)−Re(1)−O(2)

175.29(4)

Br(1)−Re(1)−N(1)

99.81(4)

Cl(1)−Re(1)−O(2)

86.98(4)

P(1)−Re(1)−N(1)

88.54(4)

P(2)−Re(1)−O(2)

92.59(4)

P(1)−Re(1)−O(12)

93.21(4)

N(1)−Re(1)−P(1)

88.82(4)

Br(1)−Re(1)−P(2)

85.80(2)

Cl(1)−Re(1)−Cl(2)

97.57(2)

Br(2)−Re(1)−P(1)

90.48(1)


167

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J. Mukiza


Re(1)−Re(2)

Bond lengths 2.6298(5) Re(2)−I(1)

Re(1)−P(1)

2.470(2)

Re(1)−O(1)

1.884(7)

Re(1)−O(2)

1.9157(4)

Re(1)−O(12)

2.038(7)

Re(1)−O(21)

2.089(6)

Re(1)−N(1)

2.138(9)

Re(2)−P(2)

2.496(2)

Re(2)−O(1)

1.886(7)

Re(2)−O(11)

2.091(8)

Re(2)−O(22)

2.029(7)

Re(2)−N(2)

2.133(8)

O(21)−C(21)

1.3(1)

O(11)−C(11)

1.2(1)

N(1)−C(11)

1.35(1)

2.694(1)

Re(1)−O(1)−Re(2)

Bond angles 88.5(3) O(1)−Re(1)−O(21)

O(1)−Re(1)−O(12)

152.3(3)

P(1)−Re(1)−N(1)

157.9(2)

I(1)−Re(2)−N(2)

166.9(2)

P(1)−Re(2)−O(11)

153.2(2)

O(1)−Re(2)−O(22)

154.2(3)

O(1)−Re(2)−O(11)

127.9(3)

O(1)−Re(1)−N(1)

126.8(3)

O(12)−Re(1)−O(21)

77.7(3)

P(2)−Re(2)−O(22)

76.9(2)

I(2)−Re(2)−O(1)

95.1(2)

O(12)−Re(1)−N(1)

75.4(3)

O(22)−Re(2)−N(2)

75.4(3)

O(1)−Re(1)−O(2)

104.5(3)

Re(1)−Re(2)−O(22)

150.8(2)

Re(2)−Re(1)−O(2)

105.8(2)

Re(2)−Re(1)−O(12)

151.9(2)


167.7(3)

168

Chapter 5

J. Mukiza


Re(1)−Re(2)

Bond lengths 2.5591(5) Re(1)−Br(1)

Re(1)−Br(2)

2.576(1)

Re(1)−P(1)

2.464(2)

Re(1)−O(1)

1.890(6)

Re(1)−O(12)

2.067(8)

Re(1)−N(1)

2.101(9)

Re(2)−Br(2)

2.583(1)

Re(2)−Br(3)

2.477(1)

Re(2)−P(2)

2.473(2)

Re(2)−O(1)

1.900(2)

Re(2)−O(8)

1.908(9)

Re(2)−O(11)

2.142(9)

O(11)−C(11)

1.25(1)

O(12)−C(16)

1.25(2)

N(1)−C(11)

1.32(1)

2.497(2)

Re(1)−O(1)−Re(2)

Bond angles 84.9(3) Br(1)−Re(1)−N(1)

Br(2)−Re(1)−P(1)

174.9(6)

O(1)−Re(1)−O(12)

159.6(3)

Br(2)−Re(2)−P(2)

173.95(7)

Br(3)−Re(2)−O(11)

173.9(2)

O(1)−Re(2)−O(8)

157.1(3)

Re(1)−Br(2)−Re(2)

59.48(3)

O(1)−Re(1)−N(1)

73.3(4)

Re(1)−Re(2)−O(8)

147.0(2)

Br(2)−Re(2)−Br(3)

89.50(4)

O(1)−Re(2)−O(11)

85.5(3)

O(8)−Re(2)−O(11)

80.6(4)

Br(2)−Re(1)−N(1)

83.6(3)

Br(2)−Re(1)−O(12)

88.6(2)

Br(2)−Re(1)−O(1)

108.0(2)

P(1)−Re(1)−O(1)

75.4(2)

P(2)−Re(2)−O(1)

78.6(2)


161.4(3)

169

Chapter 6

J.Mukiza

Chapter 6 Rhenium(I) and (V) Complexes with Monodentate S-donor and Bidentate S,N-donor Ligands 6.1. Introduction The coordination chemistry of rhenium and technetium with bidentate N,S-donor chelates and monodentate S-donor ligands has been extensively studied for radiopharmaceutical purposes [1]. Thiourea (Figure 6.1) has played a considerable role in the development of coordination chemistry and usually coordinates to transition metals as a monodentate S-donor ligand [1]. Thiourea and its derivatives are widely applied in a variety of fields, including medical, agricultural, qualitative and quantitative analytical chemistry [2, 3]. They have also shown in vitro activity against a large range of tumours [2, 4], and exhibit a variety of biological activities, including antiviral [2, 5], antibacterial [2, 6], antifungal [2, 6], antitubercular, herbicidal [2, 7], insecticidal, and as chelating agents, in catalysis, and in anion identification [6].

S H2N

NH2

Figure 6.1: Line structure of thiourea Rhenium complexes based on the fac-[Re(CO)3]+ core have received considerable interest due to their favourable radiopharmaceutical properties [8]. The potential radiopharmaceuticals based on the fac-[Re(CO)3]+ core show a higher stability compared to those based on the [ReO]3+ core [8]. The easy synthesis of [Re(CO)3(H2O)3]+ and [Re(CO)3X3]2− (X = Cl−, Br−) provides efficient synthons for the production of a variety of complexes containing the fac-[Re(CO)3]+ core since the aqua and halide ligands are labile and are easily substituted by a variety of ligands such as phosphines, imines, amines, thiols, thiones and others [9]. This chapter will report and discuss the complexes of fac-[Re(CO)3]+ core with the thiourea derivatives 1,3-diphenylthiourea (dptu) and 4-phenyl-5-(pyridin-2-yl)-1-((pyridin-2-yl)methyl)-1Himidazole-2(3H)thione (Hppit) (Figure 6.2).


170

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J.Mukiza

HN

S

S N

N H

N N

N H dptu

Hppit

Figure 6.2: Structures of thiourea derivatives used in this study The chemistry of dithizone (H2dtz) or 1,5-diphenyl-3-mercapto-formazan (Figure 6.3) has attracted attention due to its application in analytical chemistry and medicine. It is used as analytical reagent in coulometric titration of metals such as lead, mercury, silver copper and zinc [10, 11]. The chelating ability of dithizone makes it useful in the environmental quality control, specifically for environmental heavy metal assessment [3]. It is well-known for its medical applications [13-15] particularly for purity assessment of human pancreatic islet preparations used for transplantation in patients with Type 1 diabetes [13-15]. It is also efficient in the treatment of prostate cancer and pain due to bony metastases. The azoic group of dithizone and its homologous ligands have also been investigated for their essentially anti-HIV and anti-viral activities [13-15].

S

H N

N N

N H

Figure 6.3: Line structure of the dithizone ligand (H2dtz) Dithizone may undergo a double deprotonation leading to the cyclisation to dehydrodithizone or the 2,3-diphenyl-5-thione-tetrazolium zwitterion (dttz) (Figure 6.4). The formation of dttz from dithizone has been previously reported in literature [16]. Dttz was synthesised by Ogilvie and Corwin from the refluxing of dithizone for 30 min in acetic, acid followed by recrystallisation and washing with water [16]. In this work, the reaction of dithizone with [Re(CO)5Cl] surprisingly led to the cyclisation


171

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J.Mukiza

in dttz, and both dttz and the singly deprotonated dithizone anion Hdtz− coordinate to the fac-[Re(CO)3]+ core, giving the rhenium(I) complex [Re(CO)3(Hdtz)(dttz)]. S N N

N N

Figure 6.4: Structure of the 2,3-diphenyl-5-thione-tetrazolium zwitterion (dttz) Both dithizone (H2dtz) and its derivative, the 2,3-diphenyl-5-thione-tetrazolium zwitterion (dttz), may undergo thione-thiol tautomerisation (Scheme 6.1), leading to the diversity of their coordination compounds with transition metals. Dithizone coordinates to metal centres mostly as a bidentate ligand, binding through sulfur and nitrogen atoms to form a five-membered ring [15, 17]. It can also form polynuclear complexes bridging through the sulfur atom [17]. S N N

N H

SH

H N

N N

N

H N

S

H2dtz

S

2H+ + N

N

N

N

N

N

N N

dttz Scheme 6.1: Isomerisation of dithizone (H2dtz) to 2,3-diphenyl-5-thione-tetrazolium zwitterion (dttz) and their thione-thiol tautomerisation The reaction of dithizone with rhenium(V) precursors containing triphenylphosphine such as trans-[ReOX3(PPh3)2] (X = Cl, Br), cis-[ReO2I(PPh3)2] and trans[ReOI2(OEt)(PPh3)2] in ethanol was investigated and yields similar products [15]. The ligand

was

decomposed

and

the


decomposition

product

reacted

with

172

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triphenylphosphine from the rhenium(V) precursors to yield the new N,S-donor, triphenylphosphazeno-N-phenylmethanethiohydrazide (H2L; Figure 6.5) in which the Ph-N group on the azoic side of the dithizone molecule was replaced by triphenylphosphine [15]. Both H2dtz and H2L coordinate to rhenium as N,S-donors, resulting in the complex salt [ReO(dtz)2][ReO(HL)2] which contains the ionic oxorhenium(V) complexes [ReO(dtz)2]− and [ReO(HL)2]+ [15]. The use of rhenium(V) precursors which do not contain the triphenylphosphine, such as (n-Bu4N)[ReOCl4], was also investigated and led to the isolation of complex salt (n-Bu4N)[ReO(dtz)2] [15].

S

H N

P N

N H

Figure 6.5: Structure of triphenylphosphazeno-N-phenylmethanethiohydrazide (H2L) The reaction of dithizone with trans-[ReOBr3(PPh3)2] and KSCN in ethanol was also investigated. The ligand was decomposed and the decomposition product reacted with KSCN and triphenylphosphine from trans-[ReOBr3(PPh3)2] to yield the new N,Ndonor ligand triphenylphosphazenomethinimino-N-mercaptobenzenamine Htmmb (Figure 6.6). Both dtz−2 and tmmb− anions coordinate to oxorhenium(V) resulting in neutral complex [ReO(dtz)(tmmb)].

NH H N

P N

S

Figure 6.6: Structure of triphenylphosphazenomethinimino-N-mercaptobenzenamine (Htmmb)


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Dinuclear di-bridged complexes of general formula (μ-X)2[M2L8] (Figure 6.7), in which X is halogen or another mononegative bridging ligand, are known as Walton compounds [18] and may form with transition metals with d0−d0 to d6−d6 electron configurations [18].

Le

La

Lb

M

Le

M

Le

La

La

Lb

Le La

Figure 6.7: Geometrical representation in Walton compounds (μ-X)2[M2L8] Dinuclear hexacarbonylrhenium complexes (μ-X)2[Re(CO)3L]2 (X = Cl, Br; L = neutral and monodentate ligand) are good examples of rhenium(I) Walton compounds. These complexes are of interest since they may undergo substitution reactions in a similar way as the corresponding mononuclear pentacarbonyl halides [Re(CO)5X] [19]. Recently, the rhenium(I) Walton complex (μ-Cl)2[Re(CO)3(MeCN)]2 was reported in the literature [20]. In this work, the rhenium(I) Walton complex (μCl)2[Re(CO)3(ptz)]2 (ptz = phenothizine) will be reported and discussed.

H N

S Figure 6.8: Line structure of phenothiazine (ptz)

6.2. Experimental 6.2.1. Synthesis of 4-phenyl-5-(pyridin-2-yl)-1-((pyridin-2-yl)methyl)-1H imidazole-2(3H)-thione (Hppit) KSCN (1.23 g, 12.7 mmol) was dissolved in 50 cm 3 of dry acetone, and to this was added dropwise 12.7 mmol of benzoylchloride. A white precipitate of KCl separated out from the solution and was filtered off. The reaction mixture was heated under reflux under nitrogen for an hour and then cooled to room temperature. An amount of 12.7 mmol of bis((pyridin-2-yl)methyl)amine in 5 cm3 of dry acetone was then added dropwise and the reaction mixture was heated further under reflux for an hour, and


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then poured into ethanol, from which the product crystallized after three days. The crystals of Hppit were collected by filtration and washed with ethanol and dried under vacuum. Yield = 67%, m.p. = 232 °C. 1H NMR (295K, ppm): 4.55 (s, 1H, CH2), 5.20 (s, 1H, NH), 7.23 (t, 2H), 7.48 (m, 5H, phenyl protons), 7.50 (d, 1H), 7.70 (t, 2H) 8.01 (d, 1H), 8.50 (d, 2H). IR (νmax/cm-1): ν(C═S) 1243, ν(C═N) 1597, 1685, ν(N−H) 3014; ν(C−H) 2939. 6.2.2. Synthesis of [Re(CO)3(ppit)]2 (1) [Re(CO)5Br] (100 mg, 250 μmol) in 10 cm3 of toluene was added to 500 μmol (172 mg) of Hppit in 5 cm3 of toluene. The mixture was heated under reflux for 6 hours under nitrogen, resulting in a clear yellow solution, which was filtered after being cooled to room temperature. A yellow precipitate was obtained on standing at room temperature, and was removed by filtration. Crystals for X-ray crystallography were grown in two weeks by the slow evaporation of the filtrate at room temperature. Yield = 60 %, m.p. = 273° C. IR (νmax/cm-1): ν(C═O, fac) 2017, 1884; ν(C═N) 1581, 1603, 1711; v(Re−S) 420; ν(Re−N) 476. 1H NMR (295K, ppm): 5.04 (s, 4H, 2CH2), 6.84 (t), 7.31 (m, 14H), 7.32 (d, 2H), 7.63 (t, 4H), 7.86 (t, 2H), 8.67 (d, 4H). Electronic spectrum (acetonitrile, λ (ε, M-1cm-1)): 275 (2400), 297 (2000), 319 (3860), 345 nm (1060). 6.2.3. Synthesis of (n-Bu4N)[ReO(dtz)2] (2) To (n-Bu4N)[ReOCl4] (100 mg, 170 μmol) in 10 cm3 of ethanol was added 88 mg of H2dtz (342 μmol) in 10 cm3 of ethanol. The mixture was refluxed for 3 hours, resulting in a black solution, which was filtered after being cooled to room temperature, to give a black residue. Brown crystals were grown after 5 days from the slow evaporation of the mother liquor at room temperature. Yield = 68 %, m.p. = 302 °C. Conductivity (nitromethane) 86 ohm-1cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 953; ν(C═N) 1589; ν(N═N) 1549; ν(Re−S) 384; ν(Re−N) 459. 1H NMR (295K, ppm): 7.98 (d, 4H), 7.75 (d, 4H), 7.52 (t, 4H), 7.47 (t, 4H), 7.30−7.39 (m, 4H), 3.51 (dd, 8H), 1.45−1.82 (m, 16H), 0.97 (t, 12H). Anal. Calcd: C, 52.9; H, 5.8; N, 13.2. Found: C, 52.5; H, 5.4; N, 13.3 %. Electronic spectrum (acetonitrile, λ (ε, M-1cm-1)): 305 (5550), 448 nm (7460).


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6.2.4. Synthesis of [Re(CO)3Cl(dptu)2] (3) [Re(CO)5Cl] (105 mg, 290 μmol) in 10 cm3 of toluene was added to 580 μmol (132 mg) of dptu in 5 cm3 of toluene. This mixture was refluxed for 6 hours under nitrogen, resulting in a clear yellow solution, which was filtered after being cooled to room temperature. No precipitate was formed. Yellow crystals suitable for X-ray crystallography were obtained after two months by the slow evaporation of the mother liquor at room temperature. Yield = 69%, m.p. = 183 °C. IR (νmax/cm-1): ν(C═O, fac) 2012, 1878, 1907; v(Re−S) 472; ν(N−H) 3050−3470. 1H NMR (295K, ppm): 7.05 (d, 8H), 7.13 (s, 4H NH), 7.25 (t, 4H), 7.35 (d, 4H), 7.87 (d, 4H). Electronic spectrum (toluene, λ (ε, M-1cm-1)): 320 (5480). 6.2.5. Synthesis of [Re(CO)3(Hdtz)(dttz)] (4) To [Re(CO)5Cl] (105 mg, 290 μmol) in 10 cm3 of ethanol was added 580 μmol of H2dtz (153 mg) in 10 cm3 of ethanol. The resulting green mixture was refluxed for 6 hours, resulting in a purple solution, which was filtered after being cooled to room temperature, giving a purple solid. Dichloromethane was added to the mother liquor and the mixture was left at room temperature to produce the purple crystals after one week. Yield = 71 %, m.p. = 398 °C. Conductivity (methanol) 38 ohm-1cm2 mol-1. IR (νmax/cm-1): ν(C═O, fac) 2002, 1877, 1896; ν(N−H) 3182; ν(C═N) 1593, 1694; ν(Re−S) 405, 440; ν(Re−N) 474 . 1H NMR (295K, ppm): 7.02 (d, 2H), 7.15 (s, 1H, NH), 7.22 (t, 2H), 7.38 (q, 4H), 7.45 (d, 4H), 7.50−7.70 (m, 2H), 7.75 (d, 4H). Electronic spectrum (methanol, λ (ε, M-1cm-1)): 408 nm (3450), 561 nm (4800). 6.2.6. Synthesis of (μ-Cl)2[Re(CO)3(ptz)]2 (5) A mixture of [Re(CO)5Cl] (100 mg, 280 μmol) and 560 μmol of ptz (110 mg) in 20 cm3 of toluene was heated under reflux for 4 hours, resulting in a yellow solution. After being cooled to room temperature, the solution was filtered (no precipitate was formed) and the mother liquor was left to evaporate slowly at room temperature. Dark green crystals were obtained after two months. Yield = 66%, m.p. = 195 °C. IR (νmax/cm-1): ν(C═O, fac) 2021, 1896; ν(N−H) 3340; ν(Re−S) 428. 1H NMR (295K, ppm): 7.42 (m, 4H), 7.51 (t, 4H), 7.77 (d, 8H), 8.03 (s, 2H, 2NH). Electronic spectrum (methanol, λ (ε, M-1cm-1)): 316 (2990), 344 nm (1010).


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6.2.7. Synthesis of [ReO(dtz)(tmmb)] (6) [ReOBr3(PPh3)2] (110 mg, 114 μmol) was added to 228 μmol of H2dtz (58 mg) in 20 cm3 of ethanol. A mass of 11 mg of KSCN (0.114 mmol) was dissolved in 5 cm3 and added to the stirred mixture. The mixture was heated under reflux for 6 hours, resulting in a dark red solution, which was filtered after being cooled to room temperature, giving a red solid. Recrystallisation from an ethanol/dichloromethane mixture resulted in red crystals after 4 days from the slow evaporation at room temperature. Yield = 61%, m.p. = 145 °C. Conductivity (acetonitrile, 10-3 M) 73 ohm1

cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 981; ν(C═N) 1589, 1576; ν(P═N) 1399; ν(N═N)

1569; ν(Re−S) 415; ν(Re−N) 456, 477, 493. 1H NMR (295K, ppm): 6.42 (m, 15H), 7.29 (m, 3H), 7.89 (d, 2H), 7.96 (d, 2H), 6.67 (d, 3H), 7.75 (t, 2H), 7.65 (t, 4H), 7.78 (d, 2H). Electronic spectrum (acetonitrile, λ (ε, M-1cm-1)): 300 (3824), 445 nm (3745). 6.2.8. X-ray crystallography Single crystal X-ray crystallography studies were performed at 200 K using a Bruker Kappa Apex II diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71069 Å). For data collection, APEX-II was used while for cell refinement and data reduction, SAINT was used [21]. The structures were solved by direct methods applying SHELXS-97 [22], or SIR97 [23] and refined by least-squares procedures using SHELXL-97 [17], with SHELXLE [23] as a graphical interface. All non-hydrogen atoms were anisotropically refined and hydrogen atoms were calculated in idealised geometrical positions. Data were corrected by absorption effects using the numerical method using SADABS [21].

6.3. Results and discussion 6.3.1. 4-Phenyl-5-(pyridin-2-yl)-1-((pyridin-2-yl)methyl)-1H imidazole-2(3H)-thione (Hppit) The thiourea derivative Hppit was synthesized by reacting equimolar ratios of bis((pyridin-2-yl)methyl)amine, potassium thiocyanate and benzoylchloride in dry acetone. Pure Hppit was obtained as yellow crystals from ethanol. Hppit is soluble in most organic solvents giving yellow solutions. It is insoluble in water.


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The IR spectrum of Hppit is shown in Figure 6.9. It is characterized by an absorptions at 3014 and 2939 cm-1, which are assigned to the N−H and C−H stretching frequencies. The peaks at 1685 and 1597 cm-1 are assigned to ν(C═N). The absorption peak of medium intensity at 1243 cm-1 is assigned to ν(C═S).

O Cl

+

N

N H

Dry acetone N

+ KSCN

HN + H2O + KCl

S

Reflux, 4 h

N N N

Hppit

Scheme 6.2: Synthesis of Hppit

% Transmittance

91

81

71

61

51

41 2880

2380

1880

Wavenumber

1380

880

380

(cm-1)

Figure 6.9: IR spectrum of Hppit In the 1H NMR spectrum (Figure 6.10) of Hppit two one-proton singlets at 4.55 and 5.25 ppm are ascribed to the CH2 and NH2 protons respectively. The five phenyl ring protons occur as a multiplet peak 7.48 ppm. The protons of two pyridyl rings occur as a triplet at 7.23 ppm, a doublet at 7.50 ppm, a triplet 7.70 ppm, a doublet 8.01 ppm and a doublet at 8.50 ppm.


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Figure 6.10: 1H NMR spectrum of complex 1 in d6-DMSO The ORTEP diagram of Hppit (Figure 6.11) shows a thiono-tautomer of imidazole bonded to three aromatic systems, two of them being pyridine or a derivative thereof. The central heterocyclic moiety is essentially flat with one of the alkene-type carbon atoms deviating most by 0.0127(5) Å. The largest angle is created by methyl-pyridyl substituent. The dihedral angles that the imidazole ring make with the least-square planes of the aromatic substituents are 50.11(7)o (ring A), 52.21(7)o (ring B) and 77.68(7)o (ring C). The C═S double bond length of 1.689(1) Å is slightly shorter than the values reported for molecular structures featuring comparable N−(C═S)−N moieties [24, 25]. The bond distances N(3)−C(2) [1.404(2) Å] and N(4)−C(1) [1.455(2) Å] are single bonds [26]. The N(3)−C(4) [1.363(2) Å] and N(4)−C(4) [1.349(2) Å] bond lengths are

single [26]. The C(2)−C(3) bond length is double, with a length of

1.364(2) Å. The five bond angles inside the imidazole ring are N(4)–C(4)–N(3) [105.5(1)o], C(3)–N(4)–C(4) [111.37(9)o], C(2)–N(3)–C(4) [109.93(9)o], N(4)–C(3)– C(2) [106.8(1)o] and N(3)–C(2)–C(3) [106.4(1)o] respectively. The evidence of πelectrons delocalisation inside the pyridyl rings is supported by the bond lengths N(2)−C(12) [1.338(2) Å], N(1)−C(21) [1.339(2) Å], N(1)−C(22) [1.339(2) Å] and N(2)−C(11) [1.343(2) Å] which are between the single and double bonds [26].


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The C(2)−C(3) [1.364(2) Å] bond length is a double bond while C(2)−C(11) [1.467(2) Å] is a single bond [26].

Figure 6.11: ORTEP view of Hppit showing 50% probability displacement ellipsoids and atom labelling 6.3.2. [Re(CO)3(ppit)]2 (1) The complex [Re(CO)3(ppit)]2 (1) was isolated from the reaction of two equivalents of Hppit with [Re(CO)5Br] in toluene under reflux conditions.

2 [Re(CO)5Br] + 2 Hppit

Toluene Reflux, 6 h

[Re(CO)3(ppit)]2 + 4 CO + 2 HBr

Complex 1 is soluble in polar organic solvents like methanol, ethanol, dimethylsulfoxide and acetonitrile, but insoluble in non-polar organic solvents. The infrared spectrum of 1 (Figure 6.12) is dominated by the intense broad bands at 2017 and 1884 cm-1, assigned to v(C≡O) of the fac-[Re(CO)3]+ unit. The absorption peaks at 1581, with shoulder at 1603 cm-1, are ascribed to v(C═N) of the pyridyl rings. The coordinated imine [v(C═N)] is represented by the peak of high intensity at 1681 cm-1. The medium intensity peaks at 420 and 476 cm-1 are due to the Re−S and Re−N stretching vibrations respectively. Nelson Mandela Metropolitan University

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The UV-Vis spectrum of 1 (Figure 6.13) in acetonitrile consists of absorption at 319 nm which is ascribed to an intraligand π→π* transition of the coordinated ligand. The absorption peak appeared at 345 nm is ascribed to a metal-to-ligand charge transfer (MLCT). The 1H NMR spectrum of 1 in the aromatic region displays the peaks of the protons of the phenyl and pyridyl rings, and they integrate for 26 protons, appearing as a multiplet at 7.31 ppm, a doublet at 7.32 ppm, a triplet at 7.63 ppm, a triplet at 7.86 ppm and a doublet at 8.67 ppm.

% Transmittance

88 78 68 58 48 38 28 1850

1350

850

350


0.35

Absorbance

0.3 0.25 0.2 0.15 0.1 0.05 0 310

320

330

340

350

360

Wavelength (nm) Figure 6.13: UV-Vis spectrum of complex 1 in MeCN Nelson Mandela Metropolitan University

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The ORTEP diagram of the molecular structure of 1 (Figure 6.14) shows that it is a centrosymmetric dinuclear rhenium(I) complex with a rhombic (µ-S)2Re2 unit at the centre of the molecule. The dimer is produced by inversion of which the centre is positioned in the centre of the nearly square Re2S2 ring. Each rhenium(I) atom geometrically adopts a distorted octahedral arrangement with three carbonyl ligands coordinated in a facial arrangement, one imino nitrogen and two bridging thiolate sulfur atoms. The distortion from an ideal octahedron around Re(1) and Re(2) may be attributed to the deviation from linearity of the trans angles S(1)−Re(2)−C(5) = 168.4(1)o, N(4)−Re(1)−C(6) = 168.2(1)o and S(1i)−Re(1)−C(7) = 175.7(1)o. These distortions may also be due to the constraints imposed by the coordination mode of the deprotonated Hppit ligand, which forms a four-membered chelate ring with the bond angles

S(1)−Re(1)−S(1i)

=

82.25(2)°,

S(1i)−Re(2)−N(4)

=

88.34(7)o

and

S(1)−Re(1)−N(4) = 65.70(7)o, all deviating from orthogonality.

Figure 6.14: Crystal structure of complex 1 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms and toluene crystallisation solvent have been omitted for clarity Nelson Mandela Metropolitan University

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Both sulfur-bridge atoms are symmetrically coordinated to the rhenium(I) centers with bond lengths Re(1)−S(1) = 2.598(1) Å and Re(1)−S(1i) = 2.5466(9) Å. In comparison with other compounds featuring the same rhenium-sulfur motif, these Re−S distances are markedly longer [27, 28] with the sulfur-centered angle Re(1)−S(1)−Re(1i) [95.66(3)o] deviating from orthogonality. This may due to the constraints imposed by the imidazole ring and the coordination requirements of ppit−. The Re(1)−N(4) bond distance of 2.179(4) Å agrees well to those previously reported for rhenium(I)−N(imines) bonds [29]. The bonds lengths Re(1)−C(6) [1.921(3) Å], Re(1)−C(5) [1.902(3) Å] and Re(1)−C(7) [1.910(3) Å] agree well with those previously reported in similar rhenium(I) complexes [27, 28]. The bond distances N(3)−C(4) [1.343(3) Å] and N(4)−C(4) [1.327(4) Å] are shorter than the same bonds reported for the free ligand and are between single and double bonds [26] showing the π-delocalisation over N(3)−C(4)−N(4). This confirms the coordination mode of the sulfur atom as being an anionic thiolate moiety. The bond distances N(3)−C(2) = 1.396(4) Å and N(4)−C(3) = 1.382(4) Å and N(3)−C(1) = 1.464(4) Å are single bonds [26]. The π-electron delocalisation inside the pyridyl rings is supported by the bond lengths N(1)−C(12) [1.334(4) Å], N(2)−C(22) [1.338(4) Å], N(1)−C(22) [1.339(2) Å], which are between the single and double bonds [26]. 6.3.3. (n-Bu4N)[ReO(dtz)2] (2) The complex salt (n-Bu4N)[ReO(dtz)2] (2) was synthesised from the reaction of two equivalents of H2dtz with (n-Bu4N)[ReOCl4] in ethanol under reflux.

(n-Bu4N)[ReOCl4] + 2 H2dtz

EtOH Reflux, 3 h

(n-Bu4N)[ReO(dtz)2] + 4 HCl

It is a 1:1 electrolyte in nitromethane and is soluble in polar solvents such as water, ethanol, methanol, dimethylformide, dimethylsulfoxide and acetonitrile. In the infrared spectrum (Figure 6.15) the Re═O stretching frequency appears as a strong peak at 953 cm-1. The peaks at 1589 and 1549 cm-1 are assigned to v(C═N) and v(N═N) respectively. The medium intensity bands at 459 and 348 cm -1 are ascribed to v(Re−N) and v(Re−S) respectively.


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The UV-Vis spectrum of 1 (Figure 6.16) in acetonitrile consists of two absorptions at 305 and 448 nm which are ascribed to an intraligand π→π* and a (dxy)2→(dxy)1(dxz)1 transitions respectively. In the aromatic region of the 1H NMR spectrum, there are four proton signals at 7.98 ppm (d), 7.75 ppm (d), 7.52 ppm (t), 7.47 ppm (t) and a

% Transimittance

multiplet in the range 7.30−7.39 ppm. 87 82 77 72 67 62 57 52 47 42 37 1520

1320

1120

920

Wavenumber

720

520

320

(cm-1)

Figure 6.15: IR spectrum of complex 2

Absorbance

0.72 0.62

0.52 0.42 0.32 0.22 0.12 0.02 291

341

391

441

491

541

591

Wavelength (nm) Figure 6.16: UV-Vis spectrum of complex 2 in MeCN Single-crystal X-ray crystallography of 2 (Figure 6.17) shows that the dithizone ligands are doubly deprotonated and coordinated to the rhenium centre as bidentate chelates to yield the anionic complex [ReO(dtz)2]− with a n-Bu4N+ counterion. Coordination occurs through the anionic thiolate sulfur and deprotonated hydrazinic nitrogen atoms to form a five-membered ring metallocycle. The coordination geometry of [ReO(dtz)2]− is typically of oxorhenium(V) complexes with N2S2 ligands, Nelson Mandela Metropolitan University

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with the rhenium center five-coordinated with a N2S2 basal donor set and an apical oxo ligand, resulting in the distorted square-based pyramidal geometry. The distortion of square-pyramidal ideality can be attributed to the steric repulsion of the oxo ligand O(1) and the coordination requirements of the dtz2− ligand to rhenium(V) resulting in five-membered ring metallocycles. The bond length Re(1)−O(1) of 1.697(3) Å is typical for the Re═O bond distances reported for similar square pyramidal monooxorhenium(V) complexes [30]. The bond distance Re(1)−N(11) [2.025(3) Å] agrees well with those previously observed for rhenium(V)−hydrazinic nitrogen distances [15]. The bond distances Re(1)−S(1) and Re(1)−S(2) are equal to [2.302(3) Å] and fall in the range observed for similar complexes [15]. The angles O(1)−Re(1)−S(1) = 113.65(9)o, O(1)−Re(1)−N(11) = 105.1(1)o, O(1)−Re(1)−N(21) = 106.3(1)o contribute considerably to the distortion. This is mainly due to the steric repulsion of the oxo ligand O(1) and the constraints imposed by the coordination mode of dtz2−, resulting in five-membered ring metallocycles. Also, the S−Re−S and N−Re−N bond angles are profoundly nonlinear [S(1)−Re(2)−S(2) = 131.02(4)o, N(11)−Re(1)−N(21) = 148.9(1)o]. The bonds C(1)−N(12) [1.296(5) Å] and C(2)−N(22) [1.292(5) Å] are double, while C(1)−S(1) [1.762(4) Å] is a single bond [26]. These prove the high degree of electrons delocalisation along the S(1)−C(1)−N(12) and S(2)−C(2)−N(22) moieties in both dithizone ligands coordinated to rhenium. The bond C(2)−N(23) [1.400(5) Å] is single [26]. The bond lengths N(13)−N(14) = 1.270(4) Å and N(23)−N(24) = 1.268(5) Å agree well with azoic N═N bonds previously reported in similar complexes, with an average value of 1.268(5) Å [15]. The bond distances N(21)−N(22) = 1.371(4) Å and N(11)−N(12) = 1.354(4) Å show double bond character [15, 26] and these reflect further evidence of a high degree of electron delocalisation along the doubly deprotonated dithizone backbone.


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Figure 6.17: ORTEP view of complex 2 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity 6.3.4. [Re(CO)3Cl(dptu)2] (3) The complex [Re(CO)3Cl(dptu)] (3) was synthesised from the reaction of two equivalents of dptu with [Re(CO)5Cl] in toluene under reflux conditions.

[Re(CO)5Cl] + 2 dptu

Toluene Reflux, 6 h

[Re(CO)3Cl(dptu)2] + 2 CO

Complex 3 is air-stable, yellow coloured, not soluble in water and non-polar organic solvents. It is soluble in variety of polar organic solvents such as methanol, ethanol, dimethylsulfoxide, dimethylformamide and acetonitrile. The infrared spectrum of 3 (Figure 6.18) is dominated by intense absorption bands at 2012 and 1878 cm-1 with a shoulder at 1907 cm-1. These bands are ascribed to v(C≡O) of the fac-[Re(CO)3]+ unit. The band of medium intensity at 472 cm-1 is Nelson Mandela Metropolitan University

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ascribed to v(Re−S). The bands in the range 3050−3470 cm-1are assigned to the v(N−H), confirming that the dptu ligand is not deprotonated.

% Transmittance

95 85 75 65 55 45 35 3300

2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 6.18: IR spectrum of complex 3 The UV-vis spectrum of 3 in toluene consists of only one absorption peak at 320 nm which is ascribed to an intraligand π→π* transition of the coordinated ligand dptu. The 1H NMR spectrum shows that hydrogens of the phenyl rings integrate for 20 protons, appearing as a doublet at 7.05 ppm, a triplet at 7.25 ppm, a doublet at 7.35 ppm and a doublet at 7.87 ppm. The NH protons occur as singlet and broad peak at 7.13 ppm. The ORTEP view for the crystal structure of [Re(CO)3Cl(dptu)] is shown in Figure 6.19. The complex has a distorted octahedral geometry, with the chloride and two sulfur atoms in a facial arrangement, as imposed by the fac-[Re(CO)3]+core. The dptu coordinates as a neutral monodentate ligand with coordination via the thiocarbonyl sulfur atom only. The distortion from the octahedral ideality is mainly the result of the trans angles Cl(1)−Re(1)−C(5) = 176.2(1)°, S(1)−Re(1)−C(4) = 174.8(2)° and S(2)−Re(1)−C(4) = 173.4(1)°. The two bond distances Re(1)−C(4) and Re(1)−C(3) are the same and equal 1.917(5) Å, and fall in the range observed [1.900(2)−1.928(2) Å] for similar complexes [28]. However, the Re(1)−C(5) [1.897(4) Å] distance is somehow shorter than the other two Re−C bond distances and it may be due to the higher trans effect Nelson Mandela Metropolitan University

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of chloride. The rhenium atom is bonded to two of the ligands, through the sulfur atoms, with the bond lengths of Re(1)−S(1) and Re(1)−S(2) being 2.523(1) Å and 2.517(1) Å respectively. These bond distances correspond to similar bond lengths in the literature between the metal and a neutral sulfur donor atom [28, 31]. The bond distances C(1)−S(1) and C(2)−S(2) [at 1.708(3) Å and 1.710(3) Å respectively] are indicative of double bonds, lengthened somewhat by coordination. The bond angles around C(1) [S(1)-C(1)-N(12) = 120.4(2)°] and C(2) [N(21)-C(2)-N(22) = 119.0(3)°] indicate that these carbons are sp2-hybridized.

Figure 6.19: ORTEP view of crystal structure of complex 3 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity The oxygen atom O(5) is involved in a hydrogen-bond with proton on N(12). The chloride atom Cl(1) is

also involved in intramolecular hydrogen-bonds with the

protons on both nitrogen atoms N(11) and N(21). The hydrogen-bond parameters are summarised in Table 6.1. The packing in the unit cell (Figure 6.20) displays the intramolecular hydrogen-bonds (blue-dashed) together with intramolecular contacts (red-dashed).


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Table 6.1: Hydrogen-bond distances (Å) and angles (o) in complex 3 D−H∙∙∙A N(11)−H(11)∙∙∙Cl(1)

D−H 0.80(5)

H∙∙∙A 2.55(5)

D∙∙∙A 3.318(3)

D−H∙∙∙A 163(4)

N(12)−H(12)∙∙∙O(5)

0.82(5)

2.36(5)

3.034(4)

140(4)

N(21)−H(21)∙∙∙Cl(1)

0.89(5)

2.28(5)

3.170(3)

178(5)

Figure 6.20: Crystal packing of complex 3, displaying hydrogen-bonds (bluedashed) 5.3.5. [Re(CO)3(Hdtz)(dttz)] (4) The monomeric complex [Re(CO)3(Hdtz)(dttz)] (4) was synthesised from the reaction of two equivalent of H2dtz with [Re(CO)3Cl] in ethanol mixture under reflux.

EtOH [Re(CO)5Cl] + 2 H2dtz

Reflux, 6 h

[Re(CO)3(Hdtz)(dttz)] + 2 CO + HCl + 2 H+

Complex 4 is air-stable, light purple-coloured, not soluble in water but soluble in both polar and non-polar organic solvents. The cyclisation of dithizone to the 2,3-diphenyl5-thione-tetrazolium zwitterion (dttz) and the coordination mode of dttz

to

ruthenium(II) have been reported in the literature [32]. The intense red-coloured complex [Ru(trpy)(bpy)(dttz)](ClO4) (trpy = 2,2’:6’,2’’-terpyridine, bpy = bipyridine) Nelson Mandela Metropolitan University

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was synthesised from the reaction of dithizone (H2dtz) with [Ru(trpy)(bpy)Cl]Cl in aqueous ethanol solution [32]. The infrared spectrum of 4 (Figure 6.21) is dominated by intense bands at 2002 and 1877 cm-1 with a shoulder at 1896 cm-1. The three bands are ascribed to v(C≡O) of the fac-[Re(CO)3]+ unity. The band of medium intensity at 1593 with shoulder at 1634 cm-1 are ascribed to v(C═N). The weak intensity bands at 405 and 440 cm-1 are ascribed to v(Re−S), with the former value assigned to the sulfur atom of dttz. The v(Re−N) appears as a medium intensity peak at 474 cm-1. The single deprotonation of H2dtz is supported by the weak intensity and broad band appearing at 3182 cm-1, which is assigned to the v(N(8)−H).

Figure 6.21: IR spectrum of complex 4 In the UV-visible of complex 4, the two absorptions peaks at 561 and 408 nm are assigned to n→π* transition in the coordinated dttz and Hdtz− ligands and a metal-toligand-charge transfer (MLCT) respectively. In the 1H NMR spectrum of 4 the NH proton occurs as a singlet at 7.15 ppm. The phenyl rings protons integrate for 20 protons and are represented by six peaks appearing as a triplet at 7.02 ppm, a triplet at 7.22 ppm, a quartet at 7.38 ppm, a doublet at 7.45 ppm, a multiplet at 7.50−7.70 ppm and a doublet at 7.75 ppm. Nelson Mandela Metropolitan University

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The ORTEP view for the crystal structure of [Re(CO)3(Hdtz)(dttz)] is shown in Figure 6.22. The complex has a distorted octahedral geometry, with the single deprotonated Hdtz− acting as a bidentate N,S-donor chelate, coordinating to the rhenium(I) center via the neutral azoic nitrogen N(5) and anionic thiolate sulfur S(2). The zwitterion dttz acts as a monodentate S-donor chelate, coordinating via the sulfur S(1). The S(1), S(2) and N(5) are coordinated in a facial arrangement, as imposed by the fac[Re(CO)3]+core. The distortion from the octahedral ideality is mainly the result of the trans angles [S(1)−Re(1)−C(92) = 178.72(5)°, S(2)−Re(1)−C(91) = 177.79(6)° and N(5)−Re(1)−C(90) = 170.53(7)°] deviating from linearity. The bond distance Re(1)−N(5)

=

2.184(2)

Å,

with

the

angles

N(5)−Re(1)−C(91)

[99.90(7)°],

S(2)−Re(1)−N(5) [77.91(4)°] deviating from the orthogonality. Interestingly, the four angles

C(91)−Re(1)−C(92),

C(90)−Re(1)−C(92),

S(2)−Re(1)−C(92)

and

S(1)−Re(1)−S(2) are 90.94(8)°, 90.67(8)°, 90.1(2)° and 90.92(2)° respectively, and are remarkably close to orthogonality. The three bond distances Re(1)−C(91) [1.919(2) Å], Re(1)−C(90) [1.915(2) Å] and Re(1)−C(90) [1.9077(18) Å] fall in the observed range [1.900(2)−1.928(2) Å] for similar complexes [28]. The rhenium(I) atom is bonded to the two sulfur atoms S(1) (from dttz) and S(2) (from Hdtz−) with unequal bond lengths of 2.5552(5) and 2.4488(4) Å respectively. Both Re−S distances are close to the observed bond distances in the literature [31]. In the coordinated Hdtz− ligand the S(2)−C(52) bond length of 1.732(2) Å corresponds to a single bond, indicating that the negative charge is located on S(2). The donor atom N(5) is a neutral, with a N(5)−N(6) double bond of 1.281(2) Å. The N(7)−C(52) bond is double [1.319(2) Å], with N(7)−N(8) being single [1.330(2) Å]. The bonding parameters in the neutral dttz ligand are more complex. Due to electron delocalisation the bonds cannot be classified as absolutely single or double. For example, the double bond S(1)−C(1) has a length of 1.723(2) Å, close to that of S(2)−C(52), and since N(3)−N(4) [1.309(2) Å] is shorter than N(1)−N(2) bond length [1.317(2) Å], it is tentatively classified as being a double. The C(1)−N(1) and C(1)−N(4) bonds, at 1.356(3) and 1.354(2) Å respectively, are undoubtedly single, with the effect that the zwitterion dttz has a positive charge on N(4), with a negative charge on N(1), as shown earlier in Figure 6.4. Nelson Mandela Metropolitan University

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Figure 6.22: ORTEP view of complex 4 showing 40% probability displacement ellipsoids and atom labelling The packing in the unit cell (Figure 6.23) shows discrete monomeric and neutral units, with an intramolecular hydrogen-bond between S(1) and proton on nitrogen N(8). Table 6.2: Hydrogen-bond distance (Å) and angle (o) in complex 4 D−H∙∙∙A N(8)−H(78)∙∙∙S(1)

D−H 0.85(2)


H∙∙∙A 2.40(2)

D∙∙∙A 2.863(2)

D−H∙∙∙A 115.0(2)

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Figure 6.23: Crystal packing of complex 4, displaying the hydrogen-bond in light blue 6.3.6. (μ-Cl)2[Re(CO)3(ptz)]2 (5) The dimeric complex (μ-Cl)2[Re(CO)3(ptz)]2 (5) was synthesised from the reaction of two equivalent of phenothiazine (ptz) with [Re(CO)3Cl] in toluene under reflux conditions (Scheme 6.3). Complex 5 is a Walton compound, it is air-stable, not soluble in water, but soluble in both polar and non-polar organic solvents. The infrared spectrum of 5 (Figure 6.24) is dominated by intense bands at 2021 and 1896 (broad) cm-1. The two bands are ascribed to the three v(C≡O) of the fac[Re(CO)3]+ moiety. The medium intensity band appearing at 3340 cm-1 is assigned to v(N−H) in the coordinated phenothiazine ligand. The medium intensity band at 428 cm-1 is ascribed to v(Re−S). In the electronic spectrum, the π→π* transition in the free ptz ligand occurs at 316 nm and it is the same as in the coordinated ligand. The absorption peak at 344 nm is assigned to the metal-to-ligand charge transfer transition (MLCT) dπ(Re)→pπ*(Cl−).


193

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J.Mukiza H N H N

2 [Re(CO)5Cl] + 2

S

S Toluene

OC

Reflux, 4 h

CO Cl

Re OC

CO

+ 4 CO

Re Cl

CO

CO

S N H

Scheme 6.3: Synthesis of (μ-Cl)2[Re(CO)3(ptz)]2 (5)

Figure 6.24: IR spectrum of complex 5 In the 1H NMR spectrum the NH protons occur as a broad singlet at 8.03 ppm. The protons of the phenyl ring integrate for 16 protons, and are represented by three signals: a doublet at 7.77 ppm, a triplet at 7.51 ppm and a multiplet at 7.42 ppm. The ORTEP view of complex 5 (Figure 6.25) is characterised by the rhombic unit (µCl)2Re2 at the centre of (μ-Cl)2[Re(CO)3(ptz)]2. Each rhenium(I) atom has a distorted octahedral geometry and the molecule consists of two fac-[Re(CO)3]+ fragments bridged by two chloride atoms. The molecule has two phenothiazine ligands coordinated to rhenium which reside above and below the Re2Cl2 plane.


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The distortion from an ideal octahedral geometry around the rhenium in both halves of the dimer is supported by the non-linear bond angles Cl(1i)−Re(1)−C(51) = 175.74(6)o, Cl(1)−Re(1)−C(52) = 174.44(7)o and S(1)−Re(1)−C(50) = 174.75(6)o. These distortions lead to the deviation from the expected orthogonal angles Cl(1)−Re(1)−C(50) = 94.19(6)o, C(50)−Re(1)−C(52) = 90.07(9)o, S(1)−Re(1)−C(52) = 93.77(7)o, Cl(1)−Re(1)−S(1) = 81.77(2)o, Re(1)−Cl(1)−Re(1i) = 98.61(2)o and Cl(1i)−Re(1)−S(1) = 82.01(2)o.

Figure 6.25: Overview strucure of complex 5 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity The phenothiazine ligands are symmetrically coordinated to rhenium(I) with Re(1)−S(1) = 2.5220(5) Å (see Table 6.12). The two chloride ligands are symmetrically coordinated to rhenium(I) with very similar Re−Cl distances [Re(1)−Cl(1) = 2.5148(5) Å, Re(1)−Cl(1i) = 2.5127(5) Å], and are in good agreement Nelson Mandela Metropolitan University

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with Re−Cl distances reported in similar complexes [20]. The three Re−C(carbonyl) bond distances in one half of the dimer [Re(1)−C(50) = 1.926(6) Å, Re(1)−C(51) = 1.891(2) Å and Re(1)−C(52) = 1.908(2) Å] are similar to their corresponding lengths in the other half. These distances fall in the range 1.900(2)−1.928(2) Å previously reported for Re−C(carbonyl) distances observed in rhenium(I) complexes based on the fac-[Re(CO)3]+ core [28]. The bond distances S(1)−C(11) [1.771(2) Å]

and

S(1)−C(21) [1.7784(19) Å] are practically similar and single bonds [26]. The bond distances N(1)−C(12) = 1.398(3) Å, and N(1)−C(22) = 1.392(3) Å are single bonds [26]. There is one intramolecular hydrogen-bond [N(1)−H(71)∙∙∙O(51)], with D−H = 0.90(3) Å, H…A = 2.37(2) Å, D…A = 3.245(2) Å and D−H…A = 164(3) o. 6.3.7. [ReO(dtz)(tmmb)] (6) The oxorhenium(V) complex [ReO(dtz)(tmmb)] (6) was synthesised from the reaction of two equivalent of H2dtz and KSCN with trans-[ReOBr3(PPh3)2] in ethanol under reflux. EtOH [ReOBr3(PPh3)2] + 2 H2dtz + KSCN

Reflux, 6 h

[ReO(dtz)(tmmb)] + 3 HBr

Complex 6 is red and air-stable, not soluble in water but soluble in both polar and non-polar

organic

solvents.

Although

phosphoraneimine

[R3P═NR]

and

phosphoraneiminato [R3P═N−] complexes are known for a variety of transition metals [15, 19], the in situ formation of phosphoraneimines by nucleophilic attack is unusual. The reaction of [TiCl2(NPN)] (NPN = Ph(CH2SiMe2NPh)2) with KC8 under nitrogen in tetrahydrofuran produced a complex containing the (PhNSiMe2CH2)2P(═N)Ph trianion [34]. The two oxorhenium(V) complexes featuring the phosphoraneimine bond,

[ReO{o-(Ph3P═N)C6H4(NCH2CH2N(CH2CO2)2)}]

and

[ReOCl{o-

(Ph3P═N)C6H4(NCH2CH2N(CH2CO2H)2 CH2CO2)}] were isolated from the reaction of trans-[ReOCl3(PPh2)3] with N-(2-nitrophenyl)ethylenediamine-N’,N-diacetic acid in ethanol [35]. The in situ formation of phosphoraneimine as a ligand for transition metals is often observed by the trapping of nitrene species as an intermediate in a reaction involving C═N bond cleavage by phosphine [36]. The formation of the Htmmb ligand from dithizone and triphenylphosphine in ethanol occurs in three steps (Scheme 6.4). The first is a rhenium-catalysed step and it Nelson Mandela Metropolitan University

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consists of the decomposition of the hydrazinic bond (HN−NH) of dithizone in the presence of KSCN and PPh3 (from trans-[ReOBr3(PPh3)2]), which led to the formation of the 1-(phenylimino)-3-triphenylphosphonium)thiourea cation (a) together with S-(benzeneamino)methanethioimidate anion (b), and potassium ethoxide (EtOK). The second step consists of proton transfer from cation (a) to anion (b), leading to the neutral compounds, N-(phenylimino)triphenylphosphazenothioamide (c) together with S-(benzeneamino)methanethioamidic acid (d). The latter is also more likely a rhenium-catalysed step, and it consists of the substitution of a proton on the imine carbon atom on (d) by a triphenylphosphazeno group (-N=PPh3) on (c), leading to Htmmb ligand together with N-(phenylimino)methanethioamide (e).

S N

N H

N

H N

KSCN / EtOH

N

N

Htmmb + N

S N

H N

S

+ EtOK (b)

(a)

S

N

+

N H

PPh3, Reflux NH

P

HN

+ P

S

H N

Proton transfer Sub stitu tion Prot on tr ansf er

NH N

H

(e)

S N

P N

(c)

+H

S

H N

(d)

Scheme 6.4: The proposed decomposition of dithizone in the presence of KSCN and PPh3 to give the Htmmb chelate In the infrared spectrum of 6 (Figure 6.26) the Re═O stretching frequency is represented by sharp and medium intensity peak appearing at 981 cm-1. The bands at 1589 and 1576 cm-1 are assigned to the coordinated and free imine v(C═N) respectively. The bands at 1569 and 1399 cm-1 are assigned free v(N═N) and v(P═N). The band of medium intensity at 415 cm-1 is due to v(Re−S). The v(Re−N) is represented by the weak intensity bands at 456, 477 and 493 cm-1 respectively.


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Figure 6.26: IR spectrum of complex 6 The UV-Vis spectrum of 6 in acetonitrile consists of two absorptions at 300 and 445 nm. The first is ascribed to an intraligand π→π* transition of the coordinated ligand and the latter to the a (dxy)2→(dxy)1(dxz)1 transition. In the 1H NMR spectrum of 6, the NH protons produce a broad singlet peak at 8.11 ppm. The protons of the phenyl rings are integrated as 30 protons appearing as a doublet at 7.96 ppm, a doublet at 7.89 ppm, a triplet at 7.75 ppm, a triplet at 7.65 ppm, a multiplet at 7.29 ppm, a triplet at 6.78 ppm, a doublet at 6.67 ppm and a multiplet at 6.42 ppm. Single-crystal X-ray crystallography of complex 6 (Figure 6.27) proves that the dithizone ligand was doubly deprotonated and coordinated to the rhenium centre as a bidentate N,S-donor chelate, while Htmmb is singly deprotonated coordinating as a bidentate N,N-donor chelate to yield the neutral complex [ReO(dtz)(tmmb)]. The dtz2- anion coordinates to rhenium center through the anionic thiolate sulfur S(1) and deprotonated hydrazinic nitrogen N(1) to form a five-membered metallocyclic ring, while tmmb− coordinates via the anionic amino nitrogen N(5) and neutral imino nitrogen N(6) to form a five-membered ring metallocycle. The coordination number of rhenium(V) is typically five with N3S basal donor set and an apical oxo ligand, and geometry around metal center is distorted square-based pyramidal. The distortion is reflected by trans angles N(1)−Re(1)−N(5) = 147.66(7)o and S(1)−Re(1)−N(6) = Nelson Mandela Metropolitan University

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135.59(6)o, markedly deviating from linearity.

The distortion of square-pyramidal

ideality can be attributed to the steric repulsion of the oxo group O(1) and the coordination requirements of the dtz2− and tmmb− anions to rhenium(V), resulting in five-membered ring metallocycles which may impose considerable constraints on the bond lengths and angles.

Figure 6.27: Molecular structure of complex 6 showing 40% probability displacement ellipsoids and atom labelling The Re(1)−O(1)

bond length [1.695(2) Å] falls in the expected range for the

ReV−oxo bond, as was observed in related complexes [30]. Focussing on the dtz2− chelate, both the Re(1)−S(1) [2.2914(7) Å] and Re(1)−N(1) [2.030(2) Å] bonds are single. In the ligand backbone, both the N(1)−N(2) [1.367(3) Å] and N(1)−C(11) [1.434(3) Å] bonds are single, implying that N(1) is negatively charged and is coordinated as an amide. The N(2)−N(1)−C(11) bond angle is 110.5(2)o, supporting sp3-hybridization of N(1). The C(1)−N(2) [1.293(3) Å] and N(3)−N(4) [1.265(3) Å] bonds are double, making S(1) an anionic thiolic sulfur. In the tmmb − chelate, both P(1)−N(7)

and C(2)−N(6) are double bonds [1.615(2) Å and 1.346(3) Å

respectively], and S(2)−N(5) [1.736(2) Å] and N(5)−C(31) [1.426(4) Å] bonds are Nelson Mandela Metropolitan University

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single, implying a negative charge on N(5) and that N(6) is coordinated as a neutral imino nitrogen atom. The bond angle of 120.7(2)o for N(6)−C(2)−N(7) and P(1)−N(7)−C(2) [127.4(2) o] imply that both C(2) and N(7) are sp2-hybridized. All these bond lengths compare well with similar corresponding bonds reported in the literature [15, 26, 30].

6.4. Conclusion In this chapter the complexes of the [Re(CO)3]+ and [ReO]3+ cores were studied with derivatives of thiourea as ligands. In the complex fac-[Re(CO)3Cl(dptu)2] (dptu = 1,3-diphenylthiourea) the dptu ligands are monodentately coordinated through the neutral thione sulfur atom. The reaction of [Re(CO)5Cl] with dithizone (H2dtz) in ethanol produced fac-[Re(CO)3(Hdtz)(dttz)] as a product, with the anionic Hdtz− coordinated bidentately via the thiolate sulfur and neutral nitrogen atoms. The dttz ligand was formed by the cyclisation of dithizone to the 2,3-diphenyl-5-thione-tetrazolium zwitterion, and is coordinated through the neutral thione sulfur atom only. With the thiourea derivative Hppit, which also contains two pyridyl nitrogen atoms as possible donor atoms, the dimer [Re(CO)3(ppit)]2 was isolated, in which the anionic thiolate sulfur acts as a bridging donor atom to the two rhenium(I) centers, in addition to coordination also by an imine nitrogen atom, leaving free pyridyl rings. With phenothiazine (ptz), coordination occurs via the neutral sulfur atom only in the dimer (μ-Cl)2[Re(CO)3(ptz)]2. The reaction of H2dtz with (n-Bu4N)[ReOCl4] in ethanol produced the squarepyramidal complex [ReO(dtz)2]−, with N,S-coordination of dtz2−. Repeating the reaction with [ReOBr3(PPh3)2], the neutral square-pyramidal [ReO(dtz)(tmmb)] was isolated, with tmmb− formed by the substitution of one of the phenylimine groups of H2dtz by a triphenylphosphazene group.


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6.5. References [1]

R. D. Admas, M. Huang, J. H. Yamamoto, L. Zhang, Chem. Ber., 129, 137, 1996.

[2]

P.S. Hlabela, MSc Dissertation, Rhodes University, 2000.

[3]

R. Rossi, A. Marchi, A. Duatti, L. Magon, U. Casellato, R. Graziani, J. Organomet. Chem., 31, 75, 1971.

[4]

C. Sacht, M.S. Datt, S. Otto, A. Roodt, J. Chem. Soc., Dalton Trans., 4579, 2000.

[5]

C. Sun, H. Huang, M. Feng, X. Shi, X. Zhang, P. Zhou, Bioorg. Chem. Lett., 16, 162, 2006.

[6]

S. Saeed, N. Rashid, M. Ali, R. Hussain, P.G. Jones, Eur. J. Chem., 1, 221, 2010.

[7]

S.Y. Ke, G.J. Xue, Tetrahedron, 10, 63, 2006.

[8]

R. Alberto, R. Schibli, A. Egli, R. Waibel, U. Abram, A.P. Schubiger, Coord. Chem. Rev., 190, 901, 1999.

[9]

R. Alberto, R. Schibli, A. Egli, A.P. Schubiger, U. Abram, T.A. Kaden, J. Am. Chem. Soc., 190, 901, 1999.

[10]

J. L. Walsh, R. McCrackin, A. T. McPhail, Polyhedron, 17, 3221, 1998.

[11]

J. McB. Harrowfield, C. Pakawatchai, A.H. White, J. Chem. Soc., Dalton Trans., 36, 825, 1983.

[12]

M. C. Lo, Can. Med. Assoc. J., 82, 463, 1960.

[13]

C. Ricordi, Acta Diabetol. Lat., 27, 185, 1990.

[14]

M. Vandevelde, M. Wirtvrouw, AIDS Retroviruses, 12, 567, 1996.

[15]


[16]

J.W. Ogilvie, A.H. Corwin, J. Am. Chem. Soc., 83, 5023, 1961.

[17]

J. McB. Harrowfield, C. Pakawatchai, A.H. White, J. Chem. Soc., Dalton Trans., 825, 1983.

[18] S. Shalk, R. Hoffmann, C. Richard, F. Richard, H. Summerville, J. Am. Chem. Nelson Mandela Metropolitan University

201

Chapter 6

J.Mukiza

Soc., 102, 4556, 1980. [19]

F. Vitali, D. Calderazzo, Gazz. Chim. Ital., 102, 587, 1972.

[20]

J. Mukiza, T.I.A. Gerber, E.C. Hosten, R. Betz, Z. Kristallogr. NCS, 229, 355, 2014.

[21]


[22]


[23]


[24] J. Mukiza, T.I.A. Gerber, E.C. Hosten, R. Betz, Z. Kristallogr. NCS, 229, 335, 2014. [25]

F.H. Allen, Acta Crystallogr., B58, 380, 2002.

[26]

H.F. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G. Orpin, R. Taylor, J. Chem. Soc., Perkin Trans. II, 3, 1987.

[27]


[28] B. Schmitt, T.I.A. Gerber, E.C. Hosten, R. Betz, Inorg. Chem. Commun., 24, 136, 2012. [29] J. Mukiza, T.I.A. Gerber, E. Hosten, J. Chem. Crystallogr., 44, 368, 2014. [30]

K.G. Von Eschwege, J.C. Swarts, M.A.S. Aguino, T.S. Cameron, Acta Crystallographica, E68, m1518, 2012.

[31]


[32]

J.L. Walsh, R. McCraken, A.T. McPhail, Polyhedron, 182, 19, 1999.

[33]

K. Dehnicke, M. Krieger, W. Massa, Coord. Chem Rev., 17, 3221, 1998.

[34]

M. Morello, P. Yu, C.D. Carmichael, B.O. Patric, M.D. Fryuzuk, J. Am. Chem. Soc., 127, 12796, 2005.


202

Chapter 6 [35]

J.Mukiza

M. Allali, E. Benoist, M. Gressier, J. Jaudo, N. Habbadi, A. Souiz, M. Dartiguenave, Dalton Trans., 3178, 2004.

[36]

A. Saravanamuthu, D.M. Ho, M.E. Kerr, C. Fitzgerald, R.M.R. Bruce, A.E. Bruce, Krieger, Inorg. Chem. 32, 2202, 1993.

Table 6.3: Crystal and structure refinement data for complex Hppit Formula

C20H16N4S

Formula Weight

344.44

Crystal System

Monoclinic

Space group

P21/c

a (Å)

15.6504(6)

b (Å)

5.7055(2)

c (Å)

19.4320(7)

β (deg.)

95.371(2)

Volume (Å3)

1727.5(1)

Z

4

Density (g/cm3)

1.324


0.197

F(000)

720

θ range (deg.)

2.1-28.3

Index ranges h

-20/20

k

-7/7

l

-25/25

Reflection measured

23827


4290/3736

Data/parameters

4290/230


1.06


0.0332 (wR2 = 0.0873)



0.26/-0.22

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Chapter 6

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2

C46H30N8O6S2Re2.

C26H20N8OReS2.

2(C7H8)

C14H36N

Formula Weight

1411.61

953.31

Crystal System

Monoclinic

Monoclinic

Space group

I2/a

P21/n

a (Å)

22.07(2)

9.0987(8)

b (Å)

11.2696(8)

18.083(2)

c (Å)

23.765(2)

29.009(2)

β (deg.)

112.147(6)

98.555(4)

Volume (Å3)

5476.0(7)

4719.9(6)

4

4

1.712

1.342


4.553

2.703

F(000)

2768

1944

θ range (deg.)

1.9−28.3

2.3−28.3

Index ranges h

-28/29

-12/12

k

-13/15

- 23/24

l

-31/31

-38/38

Z 3

Density (g/cm ) -1

Reflection measured

25821

82475

Independent/observed reflections 6809/5939

11736/9673

Data/parameters

6809/314

11736/500


1.08

1.12


0.0216

0.0386

(wR2 = 0.0533)

(wR2 = 0.0826)

1.44/ 0.90

1.00/-2.86

2

-3



204

Chapter 6

J.Mukiza


4

Formula

C29H24ClN4O3ReS2

C29H21N8O3ReS2

Formula Weight

762.32

789.89

Crystal System

Monoclinic

Triclinic

Space group

P21/c

P-1

a (Å)

11.0329(5)

10.2290(4)

b (Å)

16.0515(7)

12.3260(5)

c (Å)

16.8993(8)

12.7340(5)

α (deg.)

86.939(2)

β (deg.)

93.314(2)

γ (deg.)

69.565(2) 88.052(2)

Volume (Å3)

2987.8(2)

1502.2(1)

Z

4

2

Density (g/cm3)

1.695

1.724


4.333

4.229

F(000)

1496

764

θ range (deg.)

1.8-28.3

2.1-28.4

Index ranges h

-14/14

-13/13

k

-21/21

-16/11

l

-19/22

-17/16

Reflection measured

39770

26814


7402/6088

7501/7281

Data/parameters

7402/377

7501/387


1.25

1.08


0.0298

0.0146

(wR2 = 0.0644)

(wR2 = 0.0359)

3.20/-1.38

1.04/-0.79

2



205

Chapter 6

J.Mukiza


6

Formula

C30H18Cl2N2O6Re2S2 C38H31N7OPReS2

Formula Weight

1009.90

833.00

Crystal System

Triclinic

Triclinic

Space group

P-1

a (Å)

7.7360(3)

9.9973(4)

b (Å)

9.5180(4)

11.8427(4)

c (Å)

11.0747(4)

15.8331(6)

α (deg.)

74.664(2)

96.909(1)

β (deg.)

72.493(2)

104.476(1)

γ (deg.)

87.767(2)

96.295(1)

Volume (Å3)

749.25(5)

1782.7(1)

Z

1

2

Density (g/cm3)

2.238

1.645


8.436

3.612

F(000)

476

876

θ range (deg.)

2.0-28.4

2.1-28.3

Index ranges h

-10/10

-13/13

k

-12/12

-15/15

l

-14/14

-20/21

P-1

Reflection measured

29785

32445


3740/3571

8860/7830

Data/parameters

3740/203

8860/451


1.06

1.02


0.0115

0.0233

(wR2 = 0.0271)

(wR2 = 0.0487)

0.55/-0.54

0.96/-0.47

2



206

Chapter 6

J.Mukiza

Table 6.7: Selected bond lengths (Å) and angles (°) for Hppit S(1)−C(4)

1.689(1)

Bond lengths N(1)−C(22)

1.339(2)

N(1)−C(21)

1.339(2)

N(2)−C(11)

1.343(2)

N(2)−C(12)

1.338(2)

N(3)−C(1)

1.455(2)

N(3)−C(2)

1.404(2)

N(3)−C(4)

1.363(2)

N(4)−C(3)

1.380(2)

N(4)−C(4)

1.349(2)

C(2)−C(3)

1.364(2)

C(2)−C(11)

1.470(2)

S(1)−C(4)−N(3)

126.71(9)

Bond angles S(1)−C(4)−N(4)

127.78(9)

C(3)−N(4)−C(4)

111.37(9)

C(2)−N(3)−C(4)

109.93(9)

N(4)−C(3)−C(31)

121.7(1)

N(3)−C(2)−C(11)

124.3(1)

N(4)−C(4)−N(3)

105.5(1)

N(3)−C(1)−C(21)

113.35(9)

N(4)−C(3)−C(2)

106.8(1)

N(3)−C(2)−C(3)

106.4(1)


207

Chapter 6

J.Mukiza

Table 6.8: Selected bond lengths (Å) and angles (o) for complex 1 Re(1)−S(1)

2.598(1)

Bond lengths Re(1)−S(1i)

2.5466(9)

Re(1)−N(4)

2.179(4)

Re(1)−C(5)

1.902(3)

Re(1)−C(6)

1.921(3)

Re(1)−C(7)

1.910(3)

S(1)−C(4)

1.752(3)

N(1)−C(11)

1.329(5)

N(1)−C(12)

1.334(5)

N(2)−C(21)

1.341(4)

N(2)−C(22)

1.338(4)

N(3)−C(1)

1.464(4)

N(3)−C(2)

1.396(4)

N(3)−C(4)

1.343(3)

N(4)−C(3)

1.382(3)

N(4)−C(4)

1.327(4)

C(2)−C(3)

1.379(4)

O(7)−C(7)

1.152(4)

S(1)−Re(1)−C(5)

Bond angles 168.5(1) N(4)−Re(1)−C(6)

168.2(1)

S(1i)−Re(1)−C(7)

175.7(1)

Re(1)−N(4)−C(3)

150.7(2)

S(1)−Re(1)−S(1i)

82.25(2)

S(1i)−Re(1)−N(4)

88.34(7)

N(4)−Re(1)−C(7)

97.98(7)

S(1i)−Re(1)−C(6)

89.8(1)

C(6)−Re(1)−O(7)

88.1(2)

C(5)−Re(1)−C(7)

89.2(2)

R(1)−C(7)−O(7)

178.2(3)

Re(1)−C(5)−O(5)

178.6(3)

Re(1)−C(6)−O(6)

178.0(3)

N(3)−C(4)−N(4)

111.4(2)

C(1)−N(3)−C(4)

124.5(2)

C(3)−N(4)−C(4)

107.0(2)

S(1)−Re(1)−N(4)

65.70(7)

S(1)−Re(1)−C(6)

102.5(1)

Re(1)−S(1)−Re(1i)

95.66(3)

Re(1i)−S(1)−C(4)

108.3(1)


208

Chapter 6

J.Mukiza

Table 6.9: Selected bond lengths (Å) and angles (°) for complex 2 Bond lengths Re(1)−S(1)

Re(1)−O(1)

1.697(3)

Re(1)−S(2)

2.302(1)

Re(1)−N(11)

2.305(3)

N(13)−N(14)

1.270(1)

C(1)−N(12)

1.296(3)

C(1)−S(1)

1.762(4)

N(11)−N(12)

1.354(4)

N(23)−N(2)

1.268(4)

C(2)−N(23)

1.400(5)

C(2)−N(22)

1.297(5)

N(21)−N(22)

1.371(4)

2.302(1)

O(1)−Re(1)−S(1)

Bond angles 113.65(9) O(1)−Re(1)−N(11)

105.1(1)

O(1)−Re(1)−N(21)

106.3(9)

S(1)−Re(1)−N(11)

80.42(8)

N(1)−N(12)−C(1)

114.9(3)

S(1)−Re(1)−S(2)

131.02(4)

N(11)−Re(1)−N(12)

148.85(3)

S(2)−Re(1)−O(1)

115.1(1)


209

Chapter 6

J.Mukiza

Table 6.10: Selected bond lengths (Å) and angles (°) for complex 3 Re(1)−S(1)

2.523(1)

Bond lengths Re(1)−S(2)

2.517(1)

Re(1)−Cl(1)

2.525(1)

Re(1)−C(3)

1.917(5)

Re(1)−C(4)

1.917(5)

Re(1)−C(5)

1.897(4)

S(1)−C(1)

1.708(3)

S(2)−C(2)

1.710(3)

N(11)−C(1)

1.340(5)

N(12)−C(1)

1.334(5)

N(21)−C(2)

1.335(5)

N(22)−C(2)

1.335(5)

Cl(1)−Re(1)−C(5)

Bond angles 176.2(1) S(1)−Re(1)−C(4)

174.8(2)

S(2)−Re(1)−C(4)

173.4(1)

S(1)−Re(1)−S(2)

81.45(3)

C(3)−Re(1)−C(4)

87.3(2)

Cl(1)−Re(1)−S(1)

88.48(3)

Cl(1)−Re(1)−S(2)

97.25(4)

C(3)−Re(1)−C(5)

90.24(17)

C(4)−Re(1)−C(5)

90.1(2)

Cl(1)−Re(1)−C(3)

89.25(14)

S(1)−C(1)−N(12)

120.4(3)

N(11)−C(1)−N(12)

118.3(3)

S(2)−C(2)−N(22)

118.1(3)

N(21)−C(2)−N(22)

119.0(3)

Re(1)−C(4)−O(4)

177.1(4)

Re(1)−C(5)−O(5)

177.5(3)

Re(1)−C(3)−O(3)

176.5(4)

S(2)−C(2)−N(21)

122.9(3)


210

Chapter 6

J.Mukiza

Table 6.11: Selected bond lengths (Å) and angles (°) for complex 4 Bond lengths Re(1)−S(2)

Re(1)−S(1)

2.5552(5)

Re(1)−N(5)

2.184(2)

Re(1)−C(90)

1.915(2)

Re(1)−C(91)

1.919(2)

Re(1)−C(92)

1.908(2)

S(1)−C(1)

1.723(2)

S(2)−C(52)

1.732(2)

N(1)−N(2)

1.317(2)

N(2)−N(3)

1.337(2)

N(3)−N(4)

1.309(2)

N(5)−N(6)

1.281(2)

N(7)−N(8)

1.330(2)

N(7)−C(52)

1.319(2)

N(6)−C(52)

1.377(2)

N(1)−C(1)

1.356(3)

N(4)−C(1)

1.354(2)

N(3)−C(11)

1.441(2)

N(3)−C(21)

1.441(3)

N(5)−C(41)

1.443(2)

2.4488(4)

S(1)−Re(1)−C(92)

Bond angles 178.72(5) S(2)−Re(1)−C(91)

N(5)−Re(1)−C(90)

170.53(7)

S(1)−Re(1)−S(2)

90.92(2)

S(1)−Re(1)−N(5)

86.58(4)

S(2)−Re(1)−N(5)

77.91(4)

S(2)−Re(1)−C(92)

90.13(6)

S(2)−Re(1)−C(90)

94.08(6)

C(90)−Re(1)−C(91)

88.06(9)

C(90)−Re(1)−C(92) 90.67(8)

C(91)−Re(1)−C(92)

90.40(8)

N(5)−Re(1)−C(91)

99.90(7)

S(2)−C(52)−N(7)

124.3(1)

S(1)−C(1)−N(4)

124.4(2)

N(8)−N(7)−C(52)

114.8(2)

N(3)−N(4)−C(1)

104.7(2)

N(1)−N(2)−N(3)

109.9(2)

N(2)−C(1)−N(4)

111.0(2)

N(2)−N(3)−N(4)

110.0(1)

N(2)−N(1)−C(1)

104.4(2)

Re(1)−C(90)−O(90)

178.4(2)

Re(1)−C(91)−O(91) 177.2(2)

Re(1)−C(92)−O(92)

178.7(2)

S(2)−C(52)−N(7)


177.79(6)

124.3(1)

211

Chapter 6

J.Mukiza

Table 6.12: Selected bond lengths (Å) and angles (°) for complex 5 Bond lengths Re(1)−Cl(1)

Re(1)−S(1)

2.5220(5)

Re(1)−C(50)

1.926(6)

Re(1)−C(51)

1.891(2)

Re(1)−Cl(1i)

2.5127(5)

Re(1)−C(52)

1.908(2)

S(1)−C(11)

1.771(2)

S(1)−C(21)

1.778(2)

N(1)−C(12)

1.398(3)

N(1)−C(22)

1.392(3)

2.5148(5)

Cl(1)−Re(1)−C(52)

Bond angles 174.44(7) S(1)−Re(1)−C(50)

Cl(1i)−Re(1)−C(51)

175.74(6)

Re(1)−Cl(1)−Re(1i)

Cl(1)−Re(1)−C(50)

94.19(6)

C(50)−Re(1)−C(51) 89.38(9)

C(50)−Re(1)−C(52)

90.07(9)

Cl(1)−Re(1)−S(1)

81.77(2)

S(1)−Re(1)−C(52)

93.77(7)

Cl(1i)−Re(1)−S(1)

82.01(2)


174.75(6) 98.61(2)

212

Chapter 6

J.Mukiza

Table 6.13: Selected bond lengths (Å) and angles (°) for complex 6 Bond lengths Re(1)−O(1)

1.695(2)

2.030(2)

Re(1)−N(5)

1.995(2)

Re(1)−N(6)

2.022(2)

S(1)−C(1)

1.754(2)

S(2)−N(5)

1.736(2)

S(2)−C(2)

1.729(2)

P(1)−N(7)

1.615(2)

N(2)−C(1)

1.293(3)

N(3)−C(1)

1.393(3)

N(6)−C(2)

1.346(3)

N(7)−C(2)

1.319(3)

N(3)−N(4)

1.265(3)

N(1)−N(2)

1.367(3)

N(4)−C(21)

1.430(3)

N(1)−C(11)

1.434(3)

N(5)−C(31)

1.426(4)

Re(1)−S(1)

2.2914(7)

Re(1)−N(1)

N(1)−Re(1)−N(5)

Bond angles 147.66(7) N(1)−Re(1)−N(6)

86.23(8)

S(1)−Re(1)−O(1)

109.56(7)

S(1)−Re(1)−N(1)

80.61(6)

S(1)−Re(1)−N(5)

89.21(7)

S(1)−Re(1)−N(6)

135.59(6)

O(1)−Re(1)−N(5)

107.20(9)

O(1)−Re(1)−N(1)

105.13(9)

O(1)−Re(1)−N(5)

114.81(9)

S(1)−C(1)−N(2)

121.5(2)

N(6)−C(2)−N(7)

120.7(2)

P(1)−N(7)−C(2)

127.4(2)

N(1)−N(2)−C(1)

115.0(2)

N(5)−S(2)−C(2)

97.5(1)

N(2)−N(1)−C(11)

110.5(2)

S(2)−N(5)−C(31)

110.1(2)


213

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J.Mukiza

Chapter 7 Coordination Mode of Aroylhydrazone-Based Ligands to Rhenium(I) and (V) 7.1. Introduction The coordination chemistry of aroylhydrazone-based ligands (Figure 7.1) is of interest due to the combination of potential donor atoms such as protonated or deprotonated amido nitrogen, oxygen, imino nitrogen and additional donor sites from aldehydes or ketones forming Schiff bases. There is a considerable interest in the hydrazides and their corresponding aroylhydrazone due to their variety of biological, biochemical and chemical activities [1]. Aroylhdrazone-based ligands exhibit physiological and biological activities in the treatment of several diseases such as tuberculosis [2], iron over-load [3, 4], and also as inhibitors for many enzymes [5]. They are important pharmacophoric cores of several anti-inflammatory and antiplatelet drugs [6]. Transition metal complexes of aroylhydrazones-based ligands have found considerable applications in various physical and chemical processes such as non-linear optics, sensors and medicine and they can effectively act as catalysts towards alkene epoxidation [7].

O N N H

R2 R1

Figure 7.1: General structure of aroylhydrazone-based ligands

The coordination mode of aroylhydrazone-based ligands to transition metals depends on the nature and oxidation state of the metal atom, the type of ligand as well as the presence of other species capable of competing for the coordination sites [8]. Aroylhydrazones are characterised by the trigonal N- and O-donor atoms that can coordinate to transitional metal ions [9], acting as bidentate [10], tridentate, tetradentate [11] or even pentadentate [12] chelates, depending on the heterocyclic ring-substituents attached to the hydrazone moiety. They may exhibit keto-enol tautomerization (Scheme 7.1) which can model the coordination to transition metals Nelson Mandela Metropolitan University

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J.Mukiza

as neutral or mononegative in the keto or enol form respectively [1]. Previously, the coordination mode of aroylhydrazones-based ligands to transition metals like copper(II), zinc(II) [8, 13, 14], nickel(II), cobalt(II) [15] and oxomolybdenum(VI) [6] have been investigated and the keto-enol tautomerisation of the ligands has occurred in most cases.

OH

O N N H

N

R2

R2

N R1

R1

Scheme 7.1: Keto-enol tautomerisation of aroylhydrazone-based ligands In this chapter, the aroylhydrazone-based ligands in Figure 7.2 (propan-2ylidene)benzohydrazide (Hdpa),

(Hpbz),

di-2-pyridyketone-2-aminobenzoylhydrazone

2-hydroxy-N'-(propan-2-ylidene)benzohydrazide

(H2hpb),

2-hydroxy-N'-

((thiophen-3-yl)methylene)benzohydrazide (H2htmb) and 2-hydroxy N’((pyridin-2yl)methylene)benzohydrazide

(Hhpmb)

have

been

synthesised

and

their

coordination mode to rhenium(I) and rhenium(V) are reported and discussed.

O N N H X X = NH2 , R1 / R2 =

R2

S X = OH, R1 =

,R =H 2

H2htmb

R1 N Hdpa

X = H, R1 / R2 = CH3

Hpbz

N X = OH, R1 =

, R2 = H H2hpmb

: R / R = CH X = OH, 1 2 3

H2hpb

Figure 7.2: The various aroylhydrazone-based ligands used in this study The chelating ligands Hdpa, H2hpmb and H2htmb have been successfully synthesised from condensation of the appropriate aroylhydrazine with an appropriate ketone or aldehyde in methanol according to the procedure reported in the literature [7]. The Hpbz and H2hpb ligands were obtained from the condensation of acetone with benzohydrazide (Hbhz) and salicylhydrazide (Hshz) respectively.


215

Chapter 7

J.Mukiza O

O N H

NH2 +

X

O R1

N

Reflux, 6 h MeOH

R2

N H

R1

O O

+ H3C

X X = OH, Hshz

+ H2O

X

O NH2

R2

N H

N

Reflux

N H

CH3

CH3

X

X = H, Hbhz

CH3 + H 2O

X = OH, H2hpb

X = H, Hpbz

Scheme 7.2: Synthesis of various aroylhydrazone-based ligands used in this study Surprisingly, the reaction of trans-[ReOI2(OEt)(PPh3)2] with 2-hydroxy-N'-((thiophen3-yl)methylene)benzohydrazide (H2htmb) in acetonitrile at room temperature led to the decomposition of the ligand, with one part reacting with acetonitrile leading to the new ligand 2-hydroxy-N’-(1-iminomethyl)benzohydrazide H2hieb (Scheme 7.3). Both H2htmb and H2hieb coordinated to oxorhenium(V) with H2htmb acting as a bidentate and H2hieb as a tridentate chelate. O

O N N H OH

MeCN

N H

o

25 C

S

H N

NH

OH

H2hieb

H2htmb

Scheme 7.3: Decomposition of H2htmb in MeCN to give H2hieb

7.2. Experimental 7.2.1. Synthesis of [Re(CO)3Cl(Hdpa)] (1) [Re(CO)5Cl] (100 mg, 0.277 mmol) in 10 cm3 of ethanol was added to 0.553 mmol (175 mg) of Hdpa in 5 cm3 of ethanol. The resulting yellow mixture was heated under reflux for 6 hours, resulting in an orange solution, which was filtered after being cooled to

room temperature. No

precipitate

was isolated

from filtration.

Dichloromethane (3 cm3) was added to the filtrate and orange crystals suitable for Xray measurement were grown in three weeks by the slow evaporation of the solvents Nelson Mandela Metropolitan University

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at room temperature. Yield = 67 %, m.p. = 247 °C. IR (νmax/cm-1): ν(C≡O, fac) 1863 br, 2015; ν(C═O) 1740; ν(C═N) 1668, 1628; ν(Re−N) 432; ν(N−H) 3365. 1H NMR (295K, ppm): ): 10.11 (s, NH), 8.55 (t, 2H), 7.05 (m, 1H), 7.40 (t, 1H), 7.65 (t, 2H), 8.10 (m, 1H), 8.35 (m, 2H), 8.85 (d, 1H), 9.15 (d, 2H), 10.11 (s, 1H), 7.45 (s, 1H). Electronic spectrum (acetonitrile, λ, (ε, M-1cm-1)): 310 nm (8655), 451 nm (1430). 7.2.2. Synthesis of cis-[ReOCl2(Hhtmb)(PPh3)] (2) To trans-[ReOCl3(PPh3)2] (150 mg, 0.18 mmol) in 10 cm3 of acetonitrile was added 0.36 mmol of H2htmb (88.5 mg) in 10 cm3 of acetonitrile. The resulting yellow-green mixture was stirred at room temperature for 24 hours, resulting in a black-brown solution, which was filtered giving a green precipitate. The mother liquor was left to room temperature and yellow-green crystals were grown in 4 days by slow evaporation. Yield = 72 %, m.p. = 169 °C. Conductivity (methanol, 10-3 M) 50 ohm1

cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 974; ν(N═C) 1662; ν(Re−N) 472; ν(Re−O) 445;

ν(O−H) 3429; ν(C−H) 2692. 1H NMR (295K, ppm): 6.58 (t, 6H), 6.69 (t, 1H), 6.71 (d, 6H), 6.80 (d, 3H), 7.10 (s, 1H, CH), 7.19 (s, 1H, OH), 7.22 (d, 1H), 7.31 (m, 3H), 7.48 (m, 1H), 7.60 (t, 1H). Electronic spectrum (dimethylformamide, λ (ε, M-1cm-1)): 336 nm (9100), 380 nm (6430), 478 nm (5450). . 7.2.3. Synthesis of [Re(CO)3Br(H2hpmb)] (3) [Re(CO)5Br] (110 mg, 0.27 mmol) in 10 cm3 of ethanol was added to 0.54 mmol (130 mg) of H2hpmb in 5 cm3 of ethanol. The resulting yellow mixture was heated under reflux for 6 hours, resulting in an orange solution, which was filtered after being cooled to room temperature. No precipitate was obtained from filtration. Dichloromethane (3 cm3) was added to the filtrate and orange crystals suitable for Xray crystallography were grown in one week by the slow evaporation of the solvents at room temperature. Yield = 61 %, m.p. = 258 °C. IR (νmax/cm-1): ν(C≡O, fac) 1894, 1923, 2027; ν(C═O) 1682; ν(C═N) 1605, 1643; ν(Re−N) 474; ν(N−H) 3246; ν(O−H) 3436. 1H NMR (295K, ppm): 8.68 (s, 1H, H(1)), 7.81 (t, 1H, H(3)), 7.62 (d, 1H, H(15)), 7.58 (s, 1H, N(3)H), 7.42 (d, 1H, H(23)), 7.38 (m, 1H, H(12), H(14), H(25)), 6.94 (d, 1H, H(26)), 6.88 (t, 1H, H(24)). Electronic spectrum (acetonitrile, λ, (ε, M1

cm-1)): 271 nm (9700), 322 nm (4080), 334 nm (3690), 408 nm (990).


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7.2.4. Synthesis of [ReO(Hhtmb)(hieb)] (4) To trans-[ReOI2(OEt)(PPh3)2] (180 mg, 0.125 mmol) in 10 cm3 of acetonitrile was added 0.25 mmol of H2htmb (61 mg) in 10 cm3 of acetonitrile. The resulting darkgreen mixture was stirred at room temperature for 24 hours, giving a brown solution, which was filtered giving a brown precipitate. The mother liquor was left to room temperature and black crystals were grown in 5 days by slow evaporation at room temperature. Yield = 79 %, m.p. = 182 °C. Conductivity (methanol, 10-3 M) 56 ohm1

cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 912; ν(N−H) 3069; ν(C═N) 1583, 1606, 1620;

ν(C═O) 1651; ν(Re−O) 440, 424; ν(Re−N) 476, 443; ν(C−H) 2584−2941; ν(N−H) 3069; ν(O−H) 3250. 1H NMR (295K, ppm): 1.11 (s, 3H, C(3)H3), 6.60 (t, 1H, H(13)), 6.70 (t, 1H), 6.78 (d, 1H, H(12)), 6.81 (d, 1H, H(14)), 7.18 (d, 1H), 7.29 (d, 1H, H(36)), 7.40 (m, 2H), 7.60 (m, 2H), 7.66 (d, 1H, H(33)), 8.00 (s, 1H, N(4)H), 8.10 (s, 1H, N(5)H), 8.20 (s, 1H, H(1)). Electronic spectrum (methanol, λ (ε, M-1cm-1)): 345 (16800), 460 nm (2180). 7.2.5. Synthesis of cis-[ReOBr2(Hhpb)(PPh3)] (5) To trans-[ReOBr3(PPh3)2] (150 mg, 0.155 mmol) in 10 cm3 of dry acetone was added 0.31 mmol of salicylhydrazide Hshz (47 mg) in 10 cm3 of dry acetone. The resulting yellow mixture was heated under reflux for 4 hours, for being filtered after cooling to room temperature. No precipitate was obtained from filtration. The filtrate was left at room temperature and green crystals were grown in 2 weeks by slow evaporation. Yield = 63 %, m.p. = 217 °C. Conductivity (methanol, 10-3 M) 35 ohm-1cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 982; ν(C═N) 1593, 1653; ν(Re−O) 415; ν(Re−N) 459; ν(C−H) 2980, 3053; ν(O−H) 3392. 1H NMR (295K, ppm): 1.00 (s, 6H, 2CH3), 6.58 (t, 1H, H(15)), 6.73 (d, 1H, H(13)), 6.90−7.55 (m, 15H, PPh3), 7.60 (t, 1H, H(14)), 7.86 (d, 1H, H(16)), 8.32 (br s, 1H, OH). Electronic spectrum (methanol, λ (ε, M-1cm-1)): 298 (12500), 430 nm (2000). 7.2.6. Synthesis of (μ-O)[ReO(pbz)2]2 (6) To trans-[ReO2(py)4]Cl (150 mg, 0.263 mmol) in 10 cm3 of dry acetone was added 0.526 mmol of benzohydrazide Hbhz (71 mg) in 10 cm3 of dry acetone. The resulting orange mixture was heated under reflux for 6 hours. No precipitate was obtained from the filtration of the reaction solution at room temperature. The mother liquor was


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left to room temperature and dark green crystals were grown in one week by slow evaporation. Yield = 62%, m.p. = 191 °C. Conductivity (acetonitrile, 10-3 M) 113 ohm1

cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 908; ν(Re−O−Re) 677; ν(C═N) 1664, 1669,

1578, 1591; ν(Re−O) 442, ν(Re−N) 467. 1H NMR (295K, ppm): 8.20 (d, 1H), 8.03 (d, 1H), 7.76-7.82 (m, 3H), 7.32-7.56 (m, 5H), 1.00 (s, 6H). Electronic spectrum (dimethylformamide, λ (ε, M-1cm-1)): 315 (2160), 428 (700). 7.2.7. X-ray crystallography Single crystal X-ray crystallography studies were performed at 200 K using a Bruker Kappa Apex II diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71069 Å). For data collection, APEX-II was used while for cell refinement and data reduction, SAINT was used [16]. The structures were solved by direct methods applying SHELXS-97 [17], or SIR97 [18] and refined by least-squares procedures using SHELXL-97 [17], with SHELXLE [18] as a graphical interface. All non-hydrogen atoms were anisotropically refined and hydrogen atoms were calculated in idealised geometrical positions. Data were corrected by absorption effects using the numerical method using SADABS [16].

7.3. Results and discussion 7.3.1. [Re(CO)3Cl(Hdpa)] (1) The complex [Re(CO)3Cl(Hdpa)] (1) was synthesised by the reaction of Hdpa with [Re(CO)5Cl] in ethanol according to the reaction:

[Re(CO)5Cl] + Hdpa

EtOH Reflux, 6 h

[Re(CO)3Cl(Hdpa)] + 2 CO

The best yield was obtained with a 2:1 molar ratio with respect to the ligand. Complex 1 is air-stable, orange-yellow in colour, not soluble in water but soluble in variety of organic solvents such as methanol, ethanol, dichloromethane, acetone, dimethylformamide, dimethylsulfoxide and chloroform. The infrared spectrum of 1 (Figure 7.3) is characterised by an intense broad band at 1863 and an intense band 2015 cm-1. These bands are ascribed to v(C≡O) of the fac-[Re(CO)3]+ unit [7, 19]. The band of medium intensity at 1740 cm-1 is assigned to Nelson Mandela Metropolitan University

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the non-coordinated v(C═O) of the ligand. The bands at 1668 cm-1 with a shoulder at 1628 cm-1 are ascribed to v(C═N) of the coordinated ligand. The bands of medium intensity at 432 cm-1 are ascribed to v(Re−N). The broad band at 3365 cm-1 is assigned to v(N−H), confirming that the Hdpa ligand is not deprotonated.

% Transmittance

87.5

77.5

67.5

57.5

47.5

37.5 3300

2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 7.3: IR spectrum of complex 1 The UV-Vis spectrum of 1 (Figure 7.4) in acetonitrile is characterised by an intense absorption at 310 nm and a less intense absorption at 451 nm. The former peak is ascribed to an intraligand π→π* electronic transition of Hdpa and the latter is assigned to a metal-to-ligand charge transfer transition (MLCT). In the 1H NMR spectrum of 1 (Figure 7.5) the singlet the furthest downfield (which integrates for one proton) at 10.11 ppm is assigned to the proton on N(2). The protons of the phenyl and pyridyl rings integrate for 12 protons, grouped in eight signals, showing the magnetic equivalence of the protons of the two pyridyl rings. These signals are represented as a multiplet at 7.05 ppm, a triplet at 7.40 ppm, a triplet at 7.65 ppm, a multiplet at 8.10 ppm, a multiplet at 8.35 ppm, a triplet at 8.55ppm, a doublet at 8.85 ppm and a doublet at 9.15 ppm.


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0.826 0.726

Absorbance

0.626 0.526 0.426 0.326 0.226 0.126 0.026 290

340

390

440

490

540

Wavelength (nm) Figure 7.4: UV-Vis spectrum of complex 1 in MeCN

Figure 7.5: 1H NMR spectrum of complex 1 in CDCl3 The ORTEP diagram of complex 1 (Figure 7.6) shows that the rhenium(I) complex contains the robust fac-[Re(CO)3]+ core in a distorted octahedral geometry. The rhenium atom is coordinated to three carbonyls in a facial orientation, two pyridyl nitrogens N(11) and N(21) and the chloride Cl(1). The coordination of the chelate forms a six-membered ring metallocycle with the metal. The Re(1)−N(11) and Re(1)−N(21) bond distances of 2.190(3) Å and 2.201(3) Å respectively agree well to those previously reported for rhenium(I)−N(imines) bond which typically fall in the range 2.15−2.22 Å [7, 19−21]. The bond distance Re(1)−Cl(1) = 2.478(1) Å is close to the average Re−Cl distance of 2.479(1) Å Nelson Mandela Metropolitan University

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observed in similar complexes [7]. The three Re−CO bond distances are 1.920(4) Å, 1.917(4) Å and 1.908(4) Å and they fall in the range observed [1.900(2)−1.928(2) Å] for similar complexes [7, 19]. The distortion from an ideal octahedron is the result of the trans angles Cl(1)−Re(1)−C(52)

[177.1(1)o]

N(21)−Re(1)−C(51)

[178.5(1)o]

and

N(11)−Re(1)−C(50) [176.6(2)o] being non-linear. These distortions may also be the result of the constraints imposed by the bidentate coordination mode of the Hdpa ligand, which forms a six-membered ring metallocycle with the bite angle N(11)−Re(1)−N(21) = 84.2(1)o, showing a deviation to orthogonality. These affect the bond angles Cl(1)−Re(1)−N(21) = 84.22(8)o, C(50)−Re(1)−C(51) = 88.9(2)o, Cl(1)−Re(1)−N(11) = 83.42(8)o which are significantly deviated from orthogonality.

Figure 7.6: ORTEP view of complex 1 showing 50% probability displacement ellipsoids and atom labelling. The ethanol solvent of crystallisation and water have been omitted for clarity The bond distance O(1)−C(3) [1.219(5) Å] is a double bond and agree well with the value observed for the non-coordinated Hdpa ligand [22].

The bond distance

N(2)−C(3) [1.376(5) Å] is a single bond [23] and this shows that there is no keto-enol Nelson Mandela Metropolitan University

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tautomerisation in the Hdpa ligand. The evidence of pi-electron delocalisation in the pyridyl ring is confirmed by the bond distance N(11)−C(11) [1.345(5) Å] which is between a single and double bond [23]. The bond distance N(1)−C(1) [1.287(5) Å] is a double bond while N(1)−N(2) [1.355(5) Å] is a single bond [23]. The crystal packing diagram of complex 1 (Figure 7.7) displays two intermolecular hydrogen-bonds [N(2)−H(721)∙∙∙O(90) and O(90)−H(91)∙∙∙O(91)]. It also shows two intramolecular hydrogen-bonds [N(3)−H(732)∙∙∙Cl(1) and N(3)−H(731)∙∙∙O(1)]. All hydrogen-bonds parameters are summarised in Table 7.1. Table 7.1: Hydrogen-bond distances (Å) and angles (o) in complex 1 D−H∙∙∙A O(90)−H(91)∙∙∙O(91)

D−H 0.8400

H∙∙∙A 1.9200

D∙∙∙A 2.70(1)

D−H∙∙∙A 155.00

N(2)−H(721)∙∙∙O(90)

0.85(5)

2.05(5)

2.836(6)

153(4)

N(3)−H(731)∙∙∙O(1)

0.84(5)

2.34(5)

2.872(6)

122(5)

N(3)−H(732)∙∙∙Cl(1)

0.91(7)

2.58(7)

3.466(5)

164(6)

Figure 7.7: Crystal packing of complex 1, displaying hydrogen-bonds (blue-dashed) Nelson Mandela Metropolitan University

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7.3.2. cis-[ReOCl2(Hhtmb)(PPh3)] (2) The complex cis-[ReOCl2(Hhtmb)(PPh3)] (2) was synthesised by the reaction of H2htmb with trans-[ReOCl3(PPh3)2].

trans-[ReOCl3(PPh3)2] + H2htmb

MeCN

cis-[ReOCl2(Hhtmb)(PPh3)] + PPh3 + HCl

o

25 C, 24 h

The complex is air-stable, green in colour and not soluble in water. It is soluble in polar organic solvents like methanol, ethanol and dimethylsulfoxide, but weakly soluble in solvents like dichloromethane, hexane, benzene and chloroform. The IR spectrum of 2 (Figure 7.8) shows an intense peak at 974 cm-1, which corresponds to ν(Re═O). The peak of medium intensity at 1622 cm-1 is ascribed to ν(C═N). The peaks appearing at 472 and 445 cm-1 are ascribed to ν(Re−N) and ν (Re−O) respectively. The wide peak at 3429 cm-1 is assigned to ν(O−H).

% Transmittance

82 72 62 52 42 32 22

3300

2800

2300

1800

1300

800

300

Wavember (cm-1) Figure 7.8: IR spectrum of complex 2 The UV-Vis spectrum of 2 (Figure 6.9) in dimethylformamide is characterised by the absorption bands at 336 and 380 nm, which are assigned to an intraligand π→π* electronic transition and a ligand-to-metal charge transfer transitions (LMCT)


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[pπ(O−)→d*π(Re) and pπ(N)→d*π(Re)] respectively. The high intensity absorption at 478 nm is due to a d-d transition [(dxy)2→(dxy)1(dxz)1].

0.88

Absorbance

0.78 0.68 0.58 0.48 0.38 0.28 0.18 0.08 320

370

420

470

520

570

Wavelength (nm) Figure 7.9: UV-Vis spectrum of complex 2 in DMF The 1H NMR spectrum (Figure 7.10) shows sharp peaks between 6.50 and 7.70 ppm. The spectrum is complicated by the protons of PPh 3, which overlap with some signals of the Hhtmb− chelate, so that the assignment of the signals to specific protons was impossible to make.

Figure 7.10: 1H NMR spectrum of complex 2 in CDCl3 Nelson Mandela Metropolitan University

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The ORTEP diagram of crystal structure complex 2 (Figure 7.11) shows that the ligand is coordinated to rhenium(V) atom as monoanionic, bidentate N,O-donor chelate through the neutral imino nitrogen N(2) and enolic oxygen O(1) resulting in a five-membered ring metallocyle. The geometry around rhenium atom is a distorted octahedral in which the equatorial plane is defined by the phosphorous atom P(1) of triphenylphosphine, the two chlorides Cl(1) and Cl(2), and the neutral imino nitrogen N(2). The oxo ligand O(2) and enolate oxygen O(1) are in trans axial positions. The two chlorides are in a cis arrangement with the bond angle Cl(1)−Re(1)−Cl(2) [88.13(5)]o closed to orthogonality. The deviation from an ideal rhenium-centered octahedron geometry manifests in the non-linear trans angles O(1)−Re(1)−O(2) = 158.9(2)o, Cl(1)−Re(1)−P(1) = 170.98(5)o, and Cl(2)−Re(1)−N(2) = 165.0(1)o. The angles

Cl(2)−Re(1)−P(1)

=

88.67(5)o,

O(1)−Re(1)−N(2)

=

72.76(15)o,

P(1)−Re(1)−O(1) = 85.00(10)o, Cl(1)−Re(1)−O(1) = 86.74o, Cl(1)−Re(1)−Cl(2) = 88.13(5)o show considerable deviation from orthogonality mainly due to the constraints imposed by the five-membered ring metallocycle. The steric repulsion of the oxo ligand O(2) is reflected by the rhenium-centered

bond angles

P(1)−Re(1)−O(2) = 87.85(14)o, O(2)−Re(1)−N(2) = 88.08(18)o, Cl(2)−Re(1)−O(2) = 106.76(15)o which significantly deviate from orthogonality. The Re(1)−O(2) bond length of 1.680(4) Å falls in the range previously reported for Re═O bonds in similar complexes [24]. The Re(1)−O(1) bond length [2.018(3) Å] is typical for a single bond of an anionic oxygen coordinated to oxorhenium(V) [24, 25]. The Re(1)−Cl(1) [2.3726(14) Å] bond is significantly longer than the Re(1)−Cl(2) [2.319(14) Å] one due to the larger trans effect of P(1) compared to the imino nitrogen N(2). The Re(1)−N(2) [2.132(4) Å] is consistence with neutral Re−N(imino) bonds [26]. In the backbone of the chelate, the N(1)−C(1) [1.3259(6) Å] and N(2)−C(2) [1.292(7) Å] bonds are double, with the N(1)−N(2) = 1.392(6) Å single [23]. The evidence of keto-enol tautomerisation in the ligand is supported by the bond distance O(1)−C(1) [1.292(7) Å] which is slightly lengthened comparatively to the value known for a ketonic double bond [23] and the bond angle O(1)−C(1)−N(1) [120.4(4) o] showing the sp2-hybridisation character on C(1).


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Figure 7.11: ORTEP view of complex 2 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity The crystal packing diagram of complex 2 (Figure 7.12) displays two intramolecular hydrogen-bonds [O(12)−H(12)∙∙S(1)] and [O(12)−H(12)∙∙∙N(1)]. The two hydrogenbond parameters are summarised in Table 7.2. Table 7.2: Hydrogen-bond distances (Å) and angles (o) in complex 2 D−H∙∙∙A O(12)−H(12)∙∙∙S(1)

D−H 0.8400

H∙∙∙A 2.5600

D∙∙∙A 3.174(5)

D−H∙∙∙A 131.00

O(12)−H(12)∙∙∙N(1)

0.8400

1.9800

2.664(6)

138.00


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Figure 7.12: Crystal packing of complex 2, displaying hydrogen-bonds (blue-dashed)

7.3.3. [Re(CO)3Br(H2hpmb)] (3) The rhenium(I) complex [Re(CO)3Br(H2hpmb)] (3) was isolated from the reaction of H2hpmb with [Re(CO)5Br] in ethanol as described by the following equation:

[Re(CO)5Br] + H2hpmb

EtOH Reflux, 6 h

[Re(CO)3Br(H2hpmb)] + 2 CO

The complex is reasonably soluble in polar organic solvents like methanol, ethanol and dimethylsulfoxide and acetonitrile. It also shows a great solubility in chlorinated solvents like dichloromethane and chloroform. The infrared spectrum of 3 (Figure 7.13) is dominated by the intense board bands at 2027 and 1894 cm-1 with shoulder at 1923 cm-1. These bands can be assigned to the v(C≡O) of the fac-[Re(CO)3]+ unit. The non-coordinated C═O of the ligand is described by the band of medium intensity at 1682 cm-1. The bands at 1643 with shoulder at and 1605 cm-1 are ascribed to the free and coordinated v(C═N) Nelson Mandela Metropolitan University

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respectively. The Re−N bonds are represented by an intense band at 474 cm -1. The broad band at 3246 cm-1 is ascribed to v(N−H), confirming that the H2hpmb ligand is not deprotonated. This is also supported by the peak of weak intensity at 3436 cm-1, which is ascribed to v(O−H).

90.5

% Transmittance

85.5 80.5 75.5 70.5 65.5 60.5 55.5 50.5 45.5 3300

2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 7.13: IR spectrum of complex 3 The UV-Vis spectrum of 3 (Figure 7.14) in acetonitrile is characterised by absorption peaks at 271, 322 and 334 nm. The first two bands are attributable to intraligand (IL) π→π* and n→π* transitions. The shoulder at 334 nm is assigned as a metal-to-ligand charge transfer (MLCT) [dπ(Re)→ pπ*(Br−)]. In the 1H NMR spectrum in CDCl3 the singlet the furthest downfield at 8.68 ppm is assigned to the methine proton C(1)H (see Figure 7.16 for labelling). There is also a one-proton singlet at 7.58 ppm, which is assigned to N(3)H. The one-proton triplet at 7.81 ppm and doublet at 7.62 ppm are assigned to the protons H(3) and H(15) respectively on the coordinated pyridyl ring.


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0.919 0.819

Absorbance

0.719 0.619 0.519 0.419 0.319 0.219 0.119 0.019 245

295

345

395

445

Wavelength (nm) Figure 7.14: UV-Vis spectrum of complex 3 in MeCN

Figure 7.15: 1H NMR spectrum of complex 3 in CDCl3 The crystal structure of complex 3 (Figure 7.16) shows that the H2hpmb ligand coordinates to the fac-[Re(CO)3]+ core as a bidentate N,N-donor chelate through pyridinic nitrogen N(1) and imino nitrogen N(2), forming a five-membered ring metallocycle. The bromide ligand lies in the trans position to carbon atom C(5). The distortion from an ideal octahedral geometry is due to the non-linear trans angles N(1)−Re(1)−C(3) = 172.5(1)o, N(2)−Re(1)−C(4) = 178.5(1)o and Br(1)−Re(1)−C(5) = 177.8(4)o. This distortion is also the result of the constraints imposed by the H2hpmb ligand, which forms a six-membered ring metallocycle and forms a bite angle of N(1)−Re(1)−N(2) = 74.2(1)o. Nelson Mandela Metropolitan University

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Figure 7.16: Molecular structure of complex 3 showing 50% probability displacement ellipsoids and atom labelling. Disorded bromide and carbonyl have been omitted for clarity The bond lengths Re(1)−C(3) [1.929(4) Å], Re(1)−C(5) [1.90(1) Å] and Re(1)−C(4) [1.912(4) Å] fall in the range [1.900(2)−1.928(2) Å] previously reported for similar complexes [7, 19, 27]. The Re(1)−N(1) and Re(1)−N(2) bond distances are identical at 2.179(3) Å and 2.180(3) Å respectively, and fall in the range 2.15−2.22 Å previously reported for rhenium(I)−N(imine) bonds [7, 19−21]. The bond length Re(1)−Br(1) [2.594(2) Å] is closed to the values reported for rhenium(I)−Br bond in similar complexes [27]. The bond O(1)−C(2) [1.213(5) Å] is a double bond [22] proving that there is no ketoenol tautomerisation in the ligand. The bond lengths N(3)−C(2) [1.378(5) Å] and N(2)−N(3) [1.367(4) Å] are typically single bonds of this kind and

N(2)−C(1)

[1.294(5) Å] is a double bond [22]. The crystal packing diagram of complex 3 (Figure 7.17) displays two intramolecular hydrogen-bonds and their parameters are summarised in Table 7.3.


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Table 7.3: Hydrogen-bond distances (Å) and angles (o) in complex 3 D−H∙∙∙A O(2)−H(2)∙∙∙Br(1)

D−H 0.8400

H∙∙∙A 2.4200

D∙∙∙A 3.260(3)

D−H∙∙∙A 176.00

N(3)−H(3)∙∙∙O(2)

0.8400

1.9100

2.614(4)

141.00

Figure 7.17: Crystal packing of complex 3, displaying hydrogen-bonds (blue-dashd) 7.3.4. [ReO(Hhtmb)(hieb)] (4) The complex [ReO(Hhtmb)(hieb)] (4) was synthesised by the reaction of two equivalents of H2htmb with trans-[ReOI2(OEt)(PPh3)2] in acetonitrile. Surprisingly, one equivalent of the H2htmb reacted with acetonitrile, resulting in the new ligand hieb2−. The Complex is of interest since oxorhenium(V) complexes containing ReV−N(amide) bonds are rare in the literature. It is soluble in polar organic solvents such as methanol, ethanol, acetonitrile and dimethylsulfoxide. It is not soluble in water and weakly soluble in solvents like chloroform and dichloromethane. The IR spectrum of 4 (Figure 7.18) shows a medium intensity absorption at 912 cm-1, which is assigned to ν(Re═O). This value is slightly lower than the expected range of 920−950 cm-1 found in octahedral oxorhenium(V) complexes with a phenolate oxygen trans to the oxo group [19]. There are three peaks in the region where Nelson Mandela Metropolitan University

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ν(C═N) normally appears: at 1620, 1606 and 1583 cm-1, and they are assigned to the stretching frequencies of C(2)═N(2), C(1)═N(1) and C(4)═N(5) respectively (see Figure 7.20 for atom labelling). The free C═O is represented by absorption band at 1651 cm-1. There are two bands of medium intensity at 476 and 493 cm-1, and they are assigned to ν(Re−N). The band at 440 cm-1, with shoulder at 424 cm-1, are assigned to the two ν(Re−O). The bands at 3069 and 3250 cm-1 are assigned to ν(N(5)−H) and ν(O(22)−H) respectively.

85

% Transmittance

75

65

55

45

35 3300

2800

2300

1800

1300

800

300

Wavenumber (cm-1) Figure 7.18: IR spectrum of complex 4 The UV-Vis spectrum of 4 (Figure 7.19) in methanol is characterised by the two absorption peaks at 345 and 460 nm. The former peak is assigned to the combination of a ligand-to-metal charge transfer transitions LMCT [pπ(O−)→d*π(Re), pπ(N−)→d*π(Re) and pπ(N)→d*π(Re)] and the latter is assigned to a (dxy)2→(dxy)1(dxz)1 transition. The 1H NMR spectrum of 4 shows two one-proton singlets the furthest downfield at 8.20 and 8.10 ppm, and these signals are assigned to the protons H(1) and N(5)H (see Figure 7.20 for labelling). The three proton-singlet at 1.11 ppm is assigned to C(5)H3. The aromatic region from 6.60−8.00 ppm contains 10 signals which integrate for the remaining 12 protons (excluding O(22)H, which could not be assigned) in the complex. Nelson Mandela Metropolitan University

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1.7 1.5 1.3

Absorbance

1.1

0.9 0.7 0.5 0.3 0.1 320

370

420

470

520

Wavelength (nm) Figure 7.19: UV-Vis spectrum of complex 4 in MeCN The single-crystal X-ray crystal structure determination of complex 4 (Figure 7.20) shows two anions Hhtmb− and hieb2− coordinated to the [ReO]3+ core. The Hhtmb− acts as bidentate N,O-donor chelate and coordinates to the metal center through the neutral imino nitrogen N(1) and charged enolic oxygen O(1) resulting, in a fivemembered ring metallocycle. The hieb2− acts as dianionic tridentate N,N,O-donor chelate and coordinates through the neutral imino nitrogen N(5), the anionic amido nitrogen N(3) and phenolic oxygen O(32). The deviation from ideal octahedral geometry around the rhenium is shown by the trans angles O(32)−Re(1)−N(5) = 157.2(1)o, O(2)−Re(1)−O(50) = 157.7(1)o and N(1)−Re(1)−N(3) = 168.3(1)o, showing a significant deviation from linearity. The bite angle of Hhtmb− N(1)−Re(1)−O(1) equals to 71.9(1)o, and the two bite angles of hieb2−

N(3)−Re(1)−N(5)

and

N(3)−Re(1)−O(32)

are

77.2(1)o and 90.4(1)o

respectively. The steric repulsion of oxo ligand O(50) is reflected by the rheniumcentered bond angles O(32)−Re(1)−O(50) [102.0(1)o], O(50)−Re(1)−N(5) [99.7(1)o] and O(50)−Re(1)−N(1) [85.9(1)o] showing significant deviation from orthogonality.


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The bond length Re(1)−O(50) [1.685(3) Å] falls within the range expected for Re═O bond distance [28]. The Re(1)−O(1) [2.074(2) Å] is a typical for a single bond of this kind [24, 25]. The phenolic oxygen O(1) is in trans position to the oxo oxygen O(50) and the bond length Re(1)−O(32) [1.99(3) Å] is slightly longer than Re−O(phenolate) distances observed when phenolic oxygen is in trans positions to the oxo ligand [26, 28]. The bond lengths Re(1)−N(5) [2.185(4) Å] and Re(1)−N(1) [2.024(4) Å] are consistence with Re−N(imino) bonds [26]. The nitrogen atom N(3) is amidic and it is supported by bond length Re(1)−N(3) [1.976(4) Å] which is close to the average value of 2.00(3) reported for Re−N(amide) bonds [29].

Figure 7.20: Overview of crystal structure complex 4 showing 50% probability displacement ellipsoids and atom labelling The bond distance O(2)−C(3) [1.230(4) Å] is a double bond and N(3)−C(3) [1.375(4) Å] is a single bond [22] supporting the non keto-enol tautomerisation in the hieb2− anion. This is also supported by the bond angle N(4)−N(3)−C(3) [113.8(3)o] which implies that N(3) is sp3 hybridized. The bond length N(3)−N(4) [1.406(4) Å] is single Nelson Mandela Metropolitan University

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[22]. The bond lengths N(4)−C(4) [1.321(5) Å] and N(5)−C(4) [1.313(5) Å] are between single and double bonds [22] showing the charge delocalisation over the N(4)−C(4)−N(5) part of the ligand. The bond distances O(1)−C(2) [1.302(4) Å] and N(2)−C(2) [1.319(4) Å] are between single and double bonds [22] supporting the keto-enol tautomerisation in Hhtmb− anion. The bond length N(1)−N(2) [1.398(4) Å] is single while N(1)−C(1) [1.290(4) Å] is a typical double bond [22]. The crystal packing diagram of complex 4 (Figure 7.21) displays five intramolecular hydrogen-bonds, and their parameters are summarised in Table 7.4. Table 7.4: Hydrogen-bond distances (Å) and angles (o) in complex 4 D−H∙∙∙A O(22)−H(22)∙∙∙S(1)

D−H 0.8400

H∙∙∙A 2.5800

D∙∙∙A 3.169(3)

D−H∙∙∙A 129.00

O(22)−H(22)∙∙∙N(2)

0.8400

1.8600

2.602(4)

146.00

N(4)−H(74)∙∙∙O(2)

0.74(4)

2.29(4)

2.578(4)

105 (4)

N(4)−H(74)∙∙∙O(2)

0.74(4)

2.19(4)

2.878(4)

156(5)

N(5)−H(75)∙∙∙O(22)

0.72(5)

2.17(5)

2.864(5)

163(6)

Figure 7.21: Crystal packing of complex 4, displaying hydrogen-bonds Nelson Mandela Metropolitan University

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7.3.5. cis-[ReOBr2(Hhpb)(PPh3)] (5) The synthesis of 5 was affected by reacting two equivalents of salicylhydrazide (Hshz) with trans-[ReOBr3(PPh3)2] in acetone. The coordinated singly deprotonated Schiff base ligand 2-hydroxy-N'-(propan-2-ylidene)-benzohydrazide (Hhpb−) was formed by the condensation of acetone with salicylhydrazide.

trans-[ReOBr3(PPh3)2] + Hshz + Me2CO

Reflux 4h

cis-[ReOBr2(Hhpb)(PPh3)] + PPh3 + HBr

The reaction of Hshz with trans-[ReOX3(PPh3)2] (X = Cl (a), Br (b) in acetonitrile and acetone has been previously investigated [24]. With a in acetonitrile the rhenium(III) complex [ReCl2(shz)(MeCN)(PPh3)2] was isolated, and with b in acetonitrile the dimeric shd4− bridged complex (μ-shd)[ReOBr(OPPh3)]2 was isolated in which the bridging shd4− resulted in the condensation of two Hshz ligands with the elimination of hydrazine [24]. In acetone, the reaction of Hshz with a led to the isolation of [ReOCl2(Hhpb)(PPh3)] [24]. The complex 5 is air-stable, light green in colour and soluble in both polar and non-polar organic solvents. The IR spectrum of 5 (Figure 6.22) displays an absorption band of medium intensity at 982 cm-1, which assigned to ν(Re═O). Absorption bands at 1593 and 1653 cm-1 are ascribed to the coordinated imine N(1)═C(1) and free imine N(2)═C(2) respectively. There are two bands of weak intensity at 415 and 459 cm-1 which could be assigned to ν(Re−O) and ν(Re−N) respectively. The broad band at 3392 cm-1 is assigned to ν(O−H). Two peaks are observed in the UV-Vis spectrum of 5 (Figure 7.23) in methanol: one at 300 nm (ε 12500), which is ascribed to an intraligand π→π* electronic transition of the coordinated Hhpb− chelate, and the other at 430 nm (ε 2000), which is assigned to a (dxy)2→(dxy)1(dxz)1 transition. The 1H NMR spectrum of 5 (Figure 7.24) in CDCl3 shows a broad singlet peak at 8.32 ppm, ascribed to the OH proton. The spectrum is dominated by multiplets in the range 6.90−7.55 ppm, due to the protons of PPh3. The signals of the four protons of the phenolic ring are distinct and appear as a doublet (H(16)) at 7.86 ppm, a triplet


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(H(14)) at 7.60 ppm, a doublet for H(13) at 6.73 ppm, and a triplet for H(15) at 6.58 ppm.

82

% Transmittance

72 62 52 42 32 22 3280

2780

2280

1780

1280

780

280


1.2

Absorbance

1

0.8

0.6

0.4

0.2

0 294

344

394

444

494

544

Wavelength (nm) Figure 7.23: UV-Vis spectrum of complex 5 in MeOH


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Figure 7.24: Aromatic region of 1HNMR spectrum of complex 5 in CDCl3 The molecular crystal structure of complex 5 (Figure 7.25) shows that the Hhpb− anion acts as monoanionic bidentate N,O-donor chelate and coordinates to the metal via the neutral imino nitrogen N(2) and enolic oxygen O(1), resulting in a fivemembered ring metallocycle. The geometry around rhenium center is distorted octahedral in which the basal plane is defined by the phosphorous atom P(1), the bromides Br(1) and Br(2), and nitrogen N(2). The phenolic oxygen O(2) is uncoordinated and forms hydrogen bond with the imino nitrogen N(1). The two bromides Br(1) and Br(2) lie in cis positions with a Br(1)−Re(1)−Br(2) angle of 88.36(1)o. The oxo ligand O(3) lies in a trans axial position to the monoanionic oxygen O(2). The deviation from an ideal octahedral geometry around the rhenium center is shown by the non-linear axis Br(1)−Re(1)−P(1) = 173.92(3)o, Br(2)−Re(1)−N(2) = 161.23(6)o and O(2)−Re(1)−O(3) = 162.62(9)o. The constraints imposed by the single deprotonated Hhpb− and the steric repulsion of the oxo ligand O(3) lead to the deviation from the orthogonality of rhenium-centered angles Br(1)−Re(1)−O(3) = 101.56(7)o

O(2)−Re(1)−N(2)

=

74.10(8)o,

P(1)−Re(1)−N(2)

=

92.73(8)o,

N(2)−Re(1)−O(3) 94.09(9)o Br(2)−Re(1)−O(3) = 104.38(7)o, P(1)−Re(1)−O(3) = 84.06(7)o. Nelson Mandela Metropolitan University

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Figure 7.25: Molecular structure of complex 5 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity The Re═O bond length of 1.680(2) Å is within the expected range and Re(1)−O(2) [2.046(2) Å] is typical for a single bond of this kind [24]. The bond length Re(1)−Br(1) [2.5135(4) Å] is significantly longer than the Re(1)−Br(2) [2.4747(5) Å] one due to the larger trans effect of P(1) compared to imino nitrogen N(2). The bond length Re(1)−N(2) [2.150(2) Å] is consistent with Re−N(imino) bonds [26]. The bond length Re(1)−P(1) [2.4793(7) Å] is in consistence with Re−P bond distances reported in similar complexes

[28]. The evidence of

charge

delocalisation

over

the

C(2)−N(2)−N(1)−C(1)−O(2) part of singly deprotonated ligand is indicated by the bond lengths N(1)−C(1) [1.306(3) Å], O(2)−C(1) [1.304(3) Å] and N(2)−C(2) [1.304(3) Å] which are intermediate between single and double bonds of this nature [22, 24]. The crystal packing diagram of complex 5 (Figure 7.27) displays one intramolecular hydrogen-bond [O(1)−H(1)∙∙∙N(1)], and its parameters are summarised in Table 7.5.


240

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Table 7.5: Hydrogen-bond distance (Å) and angle (o) in complex 5 D−H∙∙∙A O(1)−H(1)∙∙∙N(1)

D−H 0.8400

H∙∙∙A 1.8300

D∙∙∙A 2.569(3)

D−H∙∙∙A 146.00

Figure 7.26: Crystal packing of complex 5, displaying hydrogen-bonds 7.3.6. (μ-O)[ReO(pbz)2]2 (6) The dimeric complex (μ-O)[ReO(pbz)2]2 (6) was synthesised by reacting two equivalents of benzohydrazide (Hbhz) with trans-[ReO2(py)4]Cl in acetone. The coordinated and deprotonated Schiff base (propan-2-ylidene)benzohydrazide (pbz−) was formed by the condensation of Hbhz with acetone. Reflux trans-[ReO2(py)4]Cl + Hbhz + Me2CO

6h

(-O)[ReO(pbz)2]2 (6) + 4 py + HCl + H2O

The complex is air-stable, and soluble in polar organic solvents. The dimerisation reaction occurs in three steps (Scheme 6.2): the first step involves the formation of the cationic and monomeric complex [ReO(pbz)2]+ followed by hydrolysis to the Nelson Mandela Metropolitan University

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neutral hydroxy containing complex [ReO(pbz)2(OH)]. The third step is dimerization of [ReO(pbz)2(OH)] accompanied with dehydration to form (μ-O)[ReO(pbz)2]2 (6).

2 [ReO(pbz)2]+ + 2 H2O + 2 Cl

2 trans-[ReO2(py)4]Cl + 4 Hpbz

2 H2O

[(pbz)2ORe-O-ReO(pbz)2]

-H2O

2 [ReO(pbz)2(OH)] + 2 HCl

Scheme 7.4: Proposed reaction scheme for synthesis of (μ-O)[ReO(pbz)2]2 (6) The 1H NMR spectrum of 6 (Figure 7.27) shows that the protons of the pbz− chelates around each metal are magnetically equivalent, but that the sets around the different metals are not. There are two one-proton doublets at 8.20 and 8.03 ppm respectively, a three-proton multiplet in the range 7.76-7.82 ppm, and a five-proton multiplet in the range 7.32-7.56 ppm. The methyl protons produce a singlet at 1.00 ppm.

Figure 7.27: Aromatic region of 1HNMR spectrum of complex 6 in CD6SO The absorption band at 908 cm-1 in the IR spectrum of 6 (Figure 7.28) is ascribed to v(Re═O), and the one at 677 cm-1 to v(Re−O−Re). Absorption bands at 1664 and 1669 cm-1are ascribed to the uncoordinated imines, and the low intensity bands at 1578 and 1591 cm-1 to the coordinated imines. The bands at 442 and 467 cm-1 could be assigned to ν(Re−O) and ν(Re−N) respectively. Nelson Mandela Metropolitan University

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90.5

% Transmittance

80.5 70.5 60.5 50.5 40.5 30.5 20.5 1700

1500

1300

1100

900

700

500

300

Wavenumber (cm-1) Figure 7.28: IR spectrum of complex 6 in the range 400−1800 cm-1 The UV-Vis spectrum of 6 (Figure 7.29) in dimethylformamide (DMF) is characterised by the two absorption peaks at 315 and 428 nm. The former peak is ascribed to an intraligand π→π* electronic transition of the coordinated pbz− chelates and the latter is due to a d-d transition, as assigned before for complexes 4 and 5.

0.201

Absorbance

0.151

0.101

0.051

0.001 309

329

349

369

389

409

429

449

469

489

Wavelength (nm) Figure 7.29: UV-Vis spectrum of complex 6 in DMF Nelson Mandela Metropolitan University

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The ORTEP diagram of complex 6 (Figure 7.30) consists of a dimer based on the O═Re−O−Re═O backbone. The pbz− chelate coordinates to oxorhenium(V) in its deprotonated form, and therefore acts as a monoanionic bidentate N,O-donor chelate and coordinates via the neutral imino nitrogen and enolic oxygen, resulting in a five-membered ring metallocycle. There are two chelates similarly coordinated to each rhenium, and the dimer is centered at the bridging oxo ligand O(6). The rhenium atoms in both halves of the dimer are six-coordinated and adopt a distorted octahedral geometry in which the oxo ligand and the bridging oxygen in each half of the dimer occupy the axial positions trans to each other with the bond angle Re(1)−O(6)−Re(2) = 176.5(2)o. This distortion within the octahedron is mainly caused by the trans angles O(1)−Re(1)−O(2) = 174.2(2)o, 176.5(2)o,

O(6)−Re(2)−O(7)

=

178.5(2)o,

O(5)−Re(1)−O(6) =

N(12)−Re(1)−N(22)

=

174.2(2)o,

N(32)−Re(2)−N(42) = 172.5(2)o and O(3)−Re(2)−O(4) = 173.09(2)o all deviating from the linearity. The bond lengths Re(1)═O(5) [1.700(3) Å], Re(2)═O(7) [1.694(4) Å], Re(1)−O(6) [1.900(3) Å] and Re(2)−O(6) [1.917(3) Å] are typical of those found in similar complexes appearing in the literature [27, 30]. The four bond lengths Re(1)−O(1) [2.045(4) Å], Re(1)−O(2) [2.038(4) Å], Re(2)−O(4) [2.050(4) Å] and

Re(2)−O(3)

[2.049(4) Å] are similar to Re−O single bonds reported for similar complexes in the literature [24]. The bond lengths Re(1)−N(12) [2.119(5) Å], Re(1)−N(22) [2.125(5) Å], Re(2)−N(32) [2.125(5) Å] and Re(2)−N(42) [2.145(5) Å] are consistence with neutral ReV−N(imino) bonds which normally occur in the range 2.03 (1) Å −2.15(1) Å [26, 27]. The steric repulsion of the oxo ligands and the constraints imposed by the deprotonated Hpbz ligand in coordinating to rhenium, to form the five-membered ring, could contribute to the deviation from normal orthogonality of the rheniumcentered

angles

O(5)−Re(1)−N(22)

[90.2(2)o],

O(4)−Re(2)−O(6)

[88.0(2)o],

O(5)−Re(1)−N(12) [95.3(2)o], O(6)−Re(2)−N(42) [104.5(2)o], O(7)−Re(2)−N(32) [93.9(2)o], and O(2)−Re(1)−N(22) [75.9(2)o]. The bond distances O(1)−C(16) [1.295(7) Å], O(3)−C(36) [1.289(7) Å], O(2)−C(26) [1.287(7) Å] and O(4)−C(46) [1.287(7) Å] are between single and double bonds [22]. The bond lengths N(41)−C(46) [1.300(8) Å], N(31)−C(36) [1.296(8) Å], N(11)−C(16) [1.292(7) Å] and N(21)−C(26) [1.291(7) Å] show double bond character [22, 26] Nelson Mandela Metropolitan University

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supporting the keto-enol tautomerisation in Hpbz ligand. The bond lengths N(11)−N(12) [1.441(7) Å], N(31)−N(32) [1.406(7) Å], N(21)−N(22) [1.421(7) Å] and N(41)−N(42) [1.415(7) Å] are single bonds of this kind [22]. The bond lengths N(22)−C(27) [1.299(8) Å], N(37)−C(32) [1.288(8) Å] and N(12)−C(17) [1.285(8) Å] are double bonds [22], lengthened slightly due to the coordination.

Figure 7.30: ORTEP view of complex 6 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity

7.4. Conclusion The coordination behaviour of aroylhydrazone derivatives with the [ReO]3+ and fac[Re(CO)3]+ cores was investigated. The phenol derivative H2htmb produced the sixcoordinated complex [ReOCl2(Hhtmb)(PPh3)], where Hhtmb− coordinates as a monoanionic bidentate chelate via the charged enolate oxygen and imino nitrogen. The complex [ReO(Hhtmb)(hieb)] was synthesised by the reaction of H2htmb and [ReOI2(OEt)(PPh3)2] in acetonitrile. The dianionic ligand hieb2− was surprisingly Nelson Mandela Metropolitan University

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formed by the substitution of the thiophene group of H2htmb by acetonitrile, and it is coordinated as N,N,O-donor. The

reaction

of

salicylhydrazide

(Hshz)

with

[ReOBr3(PPh3)2]

produced

[ReOBr2(Hhpb)(PPh3)] in acetone. The monoanionic Hhpb− N,O-donor was formed by the condensation of Hshz and acetone. The condensation of acetone with benzohydrazide (Hbhz) to form the N,O-donor ligand pbz− was also observed in the reaction of [ReO2(py)4]Cl with Hbhz in acetone to form the oxo-bridged dimer (μO)[ReO(pbz)2]2. The reaction of [Re(CO)5X] (X = Cl, Br) with the pyridyl derivatives Hhdpa and H2hpmb produced [Re(CO)3Cl(Hdpa)] and [Re(CO)3Br(H2hpmb)] respectively, in which the bidentate chelates act as N,N-donor.

7.5. References [1]

H. Hosseini-Monfared, R. Bikas, J. Sanchiz, T. Lis, M. Siczek, J. Tucek, R. Zboril, P. Mayer, Polyhedron, 61, 45, 2013.

[2]

M. Mohan, M. P. Gupta, L. Chandra, N. K. Jha, Inorg. Chim. Acta, 61, 151, 1988.

[3 ] D.R. Richardson, P. Ponka, Am. J. Hermatol., 58, 299, 1988. [4]

D.S. Kalinowski, P. C. Sharpe, P.V. Bernhardt, D.R. Richardison, J. Med. Chem., 51, 331, 2008.

[5]

G. Tamasi, L. Chiasserini, L. Savini, A. Sega, R. Cini, J. Inorg. Biochem., 99, 1347, 2005.

[6]

S. Pasayat, S.P. Dash, Saswati, P.K. Majhi, Y.P. Patil, M. Nethaji, H.R. Dash, S.Das, R. Dinda, Polyhedron, 38, 198, 2012.

[7]


[8]

C. Gokce, R. Gup, Chemical Papers, 67, 1293, 2013.

[9]

A. Ray, C. Rizzoli, G. Pilet, C. Desplanches, E. Garriba, E. Rentschler, S. Mitra, Eur. J. Inorg. Chem., 2915, 2009.


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[10] M. B. Hursthouse, S. A. A. Jayaweera, A. Quick, J. Chem. Soc., Dalton Trans., 279, 1979. [11] H. Hossein-Monfared, S. Alavi, A. Farrokhi, M. Vahedpour, P. Mayer, Polyhedron, 30, 1842, 2011. [12]

S. Lin, S.X. Liu, J.Q. Huanga, C.C. Lin, Dalton Trans., 1595, 2002.

[13] S. Mondal, S. Naskar, A.K. Dey, E. Sinn, C. Eribal, S.R. Herron, S.K. Chattopdhyay, Inorg. Chim. Acta, 398, 98, 2013. [14]

A. Jamadar, PhD Thesis, University of New York, 2012.

[15]

S. Behera, MSc. Dissertation, National Institute of Technology, Rourkela, 2009.

[16]


[17]


[18]


[19]


[20]

T.I.A. Gerber, Z.R. Tshentu, P. Mayer, J. Coord. Chem., 58, 1271, 2004.

[21]

T.I.A. Gerber, A. Abrahams, P. Mayer, J. Coord. Chem., 58, 1387, 2005.

[22]

T.I.A. Gerber, J. Mukiza, E. Hosten, R. Betz, Z. Kristallogr. NCS, 229, 327, 2014.

[23]

F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G. Orpin, R. Taylor J. Chem. Soc., Perkin Trans., II, 3, 1987.

[24]

T.I.A. Gerber, N.C. Yumata, R. Betz, Inorg. Chem. Commun., 15, 69, 2012.

[25]

F.A. Cotton, S.J. Lippard, Inorg. Chem., 4, 1621, 1965.

[26]

K.C. Potgieter, MSc Dissertation, Nelson Mandela Metropolitan University, 2009.


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[27]

I.N. Booysen, PhD Thesis, Nelson Mandela Metropolitan University, 2009.

[28]

A. Abraham, I.N. Booysen, T.I.A. Gerber, P. Mayer, Bull. Chem. Soc. Ethiop., 22, 247, 2008.

[29]

U. Mazzi, E. Roncari, R. Rossi, V. Bertolasi, O. Traverso, L. Magon, Transition Met. Chem., 5, 289, 1980.

[30] J. Mukiza, T.I.A. Gerber, E. Hosten, R. Betz, Z. Kristallogr. NCS, 229, 341, 2014.


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C21H15ClN5O4Re.

2 C30H24Cl2N2O3PReS

C2H6OO Formula Weight

685.11

780.66

Crystal System

Monoclinic

Triclinic

Space group

P21/n

P-1

a (Å)

9.0000(3)

10.0240(5)

b (Å)

23.4860(7)

10.2800(5)

c (Å)

12.3160(3)

14.2150(7)

α (deg.)

89.587(2)

β (deg.)

104.954(1)

γ (deg.)

83.317(2) 83.170(2)

Volume (Å3)

2515.1(1)

1444.5(1)

Z

4

2

Density (g/cm3)

1.809

1.795


4.986

4.554

F(000)

1336

764

θ range (deg.)

1.9-28.3

2.0-28.0

Index ranges h

-12/2

-13/13

k

-31/29

-13/13

l

13/21

-18/18

Reflection measured

20502

23998


6268/5554

6874/5727

Data/parameters

6268/339

6874/362


1.23

1.05


0.0319

0.0378

(wR2 = 0.0568)

(wR2 = 0.1044)

1.49/-1.15

3.13/-1.86



-

249

Chapter 7

J.Mukiza


4

Formula

C16H11BrN3O5Re

C21H18N5O5ReS

Formula Weight

591.39

638.68

Crystal System

Triclinic

Triclinic

Space group

P-1

P-1

a (Å)

7.7191(3)

8.0620(2)

b (Å)

9.4438(4)

10.6870(3)

c (Å)

12.8575(6)

13.5190(4)

α (deg.)

77.585(2)

97.187(2)

β (deg.)

87.039(2)

99.701(1)

γ (deg.)

69.137(2)

93.166(1)

Volume (Å3)

855.01(6)

1135.63(5)

Z

2

2

Density (g/cm3)

2.297

1.868


9.476

5.485

F(000)

556

620

θ range (deg.)

1.6-28.4

1.9-28.5

Index ranges h

-10/10

-10/10

k

-12/12

-14/14

l

-17/17

-18/18

Reflection measured

14950

20210


4264/3495

5683/5347

Data/parameters

4264/247

5683/308


1.08

1.10


0.0240

0.0261

(wR2 = 0.0422)

(wR2 = 0.0659)

1.20/-0.82

4.78/-0.65

-3



250

Chapter 7

J.Mukiza


6

Formula

C28H26Br2N2O3PRe

C40H44N8O7R2

Formula Weight

815.49

1121.25

Crystal System

Monoclinic

Monoclinic

Space group

P21/c

P21/c

a (Å)

9.7768(3)

12.4069(3)

b (Å)

18.8593(6)

11.7414(2)

c (Å)

15.2556(5)

28.2535(6)

β (deg.)

96.666(1)

98.218(1)

Volume (Å3)

2793.9(2)

4073.6(2)

4

4

Density (g/cm )

1.939

1.828


7.300

5.997

F(000)

1568

2184

θ range (deg.)

1.7−28.3

1.5−28.3

Index ranges h

-13/11

-16/13

k

-22/25

- 14/15

l

-20/20

-37/37

Reflection measured

26892

55835


6937/6082

10117/8780

Data/parameters

5228/284

10117/522


1.10

1.25


0.0226

0.0389

(wR2 = 0.0485)

(wR2 = 0.0713)

0.75/-1.93

1.65/-1.97

Z 3

2



251

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J.Mukiza

Table 7.9: Selected bond lengths (Å) and angles (o) for complex 1 Re(1)−N(11)

2.190(3)

Bond lengths Re(1)−N(21)

Re(1)−Cl(1)

2.478(1)

Re(1)−C(50)

1.917(4)

Re(1)−C(51)

1.920(4)

Re(1)−C(52)

1.908(4)

O(1)−C(3)

1.219(5)

O(50)−C(50)

1.151(5)

O(51)−C(51)

1.138(5)

O(52)−C(52)

1.138(5)

N(1)−N(2)

1.355(5)

N(1)−C(1)

1.287(5)

N(2)−C(3)

1.376(5)

N(21)−C(21)

1.349(5)

N(11)−C(11)

1.345(5)

N(3)−C(32)

1.382(7)

Cl(1)−Re(1)−C(52)

Bond angles 177.1(1) N(11)−Re(1)−C(50)

2.201(3)

175.9(1)

N(21)−Re(1)−C(51)

178.5(1)

C(50)−Re(1)−C(51)

88.9(2)

N(11)−Re(1)−N(21)

84.2(1)

Cl(1)−Re(1)−N(21)

84.22(8)

Cl(1)−Re(1)−N(11)

83.42(8)

N(1)−N(2)−C(1)

117.6(3)

N(1)−N(2)−C(3)

119.9(3)

O(1)−C(3)−N(2)

122.9(4)

O(1)−C(3)−C(31)

124.2(4)

C(11)−C(1)−O(21)

118.9(3)

Re(1)−C(50)−C(50)

179.5(3)

Re(1)−C(51)−C(51)

177.7(4)

R(1)−C(5)−C(52)

177.4(4)

N(21)−R(1)−C(52)

94.0(1)


252

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J.Mukiza

Table 7.10: Selected bond lengths (Å) and angles (o) for complex 2 Re(1)−Cl(1)


2.319(1)

Re(1)−P(1)

2.475(1)

Re(1)−O(1)

2.018(3)

Re(1)−O(2)

1.680(4)

Re(1)−N(2)

2.132(4)

O(1)−C(1)

1.297(6)

N(1)−C(1)

1.325(6)

N(1)−N(2)

1.392(6)

N(2)−C(2)

1.292(7)

S(1)−C(21)

1.724(6)

S(1)−C(24)

1.701(7)

C(1)−C(11)

1.460(7)

C(2)−C(21)

1.431(8)

P(1)−C(51)

1.810(5)

P(3)−C(31)

1.813(5)

O(1)−Re(1)−O(2)

Bond angles 158.9(2) C(1)−Re(1)−P(1)

Cl(1)−Re(1)−Cl(2)

88.13(5)

Cl(1)−Re(1)−O(1)

86.7(1)

Cl(2)−Re(1)−P(1)

88.67(5)

P(1)−Re(1)−O(2)

87.9(1)

O(1)−Re(1)−N(2)

72.8(2)

O(2)−Re(1)−N(2)

88.1(2)

P(1)−Re(1)−O(1)

85.0(1)

Cl(2)−Re(1)−O(2)

106.8(2)

O(1)−C(1)−N(1)

120.4(4)

N(2)−N(1)−C(1)

110.0(4)

N(1)−N(2)−C(2)

117.3(4)

P(1)−Re(1)−N(2)

94.3(1)

Cl(2)−Re(5)−N(2)

165.0(1)

Re(1)−N(2)−N(1)

117.1(3)


170.98(5)

253

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J.Mukiza

Table 7.11: Selected bond lengths (Å) and angles (°) for complex 3 Re(1)−Br(1)

Bond lengths 2.5939(5) Re(1)−N(1)

2.179(3)

Re(1)−N(2)

2.180(3)

Re(1)−C(3)

1.929(4)

Re(1)−C(4)

1.912(4)

Re(1)−C(5)

1.90(1)

O(1)−C(2)

1.213(5)

N(3)−C(2)

1.378(5)

N(2)−N(3)

1.367(4)

N(2)−C(1)

1.294(5)

N(1)−Re(1)−C(3)

Bond angles 172.5(1) N(2)−Re(1)−C(4)

172.5(1)

Br(1)−Re(1)−C(5)

177.8(4)

Re(1)−C(4)−O(4)

179.4(4)

Re(1)−C(3)−O(3)

178.9(3)

Re(1)−C(5)−O(5)

178.0(1)

C(4)−Re(1)−C(5)

89.2(5)

Br(1)−Re(1)−N(2)

84.99(8)

N(1)−Re(1)−C(4)

98.9(1)

N(1)−Re(1)−C(5)

93.4(4)

N(3)−C(2)−O(1)

115.3(3)

N(2)−N(3)−C(2)

127.0(3)

N(3)−N(2)−C(1)

121.5(3)

N(1)−Re(1)−N(2)

74.2(1)


254

Chapter 7

J.Mukiza


Bond lengths 2.074(2) Re(1)−O(32)

Re(1)−O(50)

1.685(3)

Re(1)−N(1)

2.185(3)

Re(1)−N(3)

1.976(3)

Re(1)−N(5)

2.024(3)

O(1)−C(2)

1.302(4)

O(2)−C(3)

1.230(4)

N(3)−C(3)

1.375(4)

N(3)−N(4)

1.406(4)

N(4)−C(4)

1.321(5)

N(5)−C(4)

1.313(5)

N(1)−C(1)

1.290(4)

N(1)−N(2)

1.398(4)

N(2)−C(2)

1.319(5)

S(1)−C(11)

1.722(4)

1.999(3)

O(1)−Re(1)−O(32)

Bond angles 80.4(1) O(2)−Re(1)−O(50)

157.7(1)

N(1)−Re(1)−N(3)

168.3(1)

O(32)−Re(1)−N(5)

157.2(1)

O(32)−Re(1)−N(3)

90.4(1)

N(1)−Re(1)−N(5)

96.9(1)

O(1)−Re(1)−N(3)

97.1(1)

N(3)−Re(1)−N(5)

77.2(1)

O(50)−Re(1)−N(1)

85.9(1)

O(50)−Re(1)−N(5)

99.7(1)

O(1)−Re(1)−N(1)

71.9(1)

O(2)−C(2)−N(3)

120.6(3)

N(4)−C(4)−N(5)

123.8(3)

N(4)−N(3)−C(3)

113.8(3)

N(3)−N(4)−C(4)

115.2(3)

N(1)−C(1)−C(11)

132.0(4)

N(2)−N(1)−C(1)

118.1(3)

N(1)−N(2)−C(2)

110.4(3)

N(2)−C(2)−C(21)

119.7(3)

O(1)−C(2)−N(2)

121.8(3)

O(32)−Re(1)−O(50)

102.0(1)

Re(1)-O(1)-C(2)

119(2)


255

Chapter 7

J.Mukiza

Table 7.13: Selected bond lengths (Å) and angles (°) for complex 5 Bond lengths Re(1)−Br(2)

Re(1)−Br(1)

2.5135(4)

Re(1)−P(1)

2.4793(7)

Re(1)−O(2)

2.046(2)

Re(1)−O(3)

1.680(2)

Re(1)−N(2)

2.150(2)

O(2)−C(1)

1.304(3)

N(1)−N(2)

1.402(3)

N(1)−C(1)

1.306(3)

N(2)−C(2)

1.304(3)

2.4747(5)

Br(1)−Re(1)−P(1)

Bond angles 173.92(3) Br(2)−Re(1)−N(2)

O(2)−Re(1)−O(3)

162.62(9)

O(2)−Re(1)−N(2)

74.10(8)

Br(1)−Re(1)−O(3)

101.56(7)

P(1)−Re(1)−N(2)

92.73(6)

Br(1)−Re(1)−Br(2)

88.36(1)

Br(2)−Re(1)−O(3)

104.38(7)

N(2)−Re(1)−O(3)

94.09(9)

Br(2)−Re(1)−P(1)

92.52(2)

O(2)−C(1)−N(1)

122.1(2)

N(1)−N(2)−C(1)

111.9(2)

N(1)−N(2)−C(2)

114.4(2)

P(1)−Re(2)−O(3)

84.06(7)


161.23(6)

256

Chapter 7

J.Mukiza


2.045(4)

Bond lengths Re(1)−O(5)

Re(1)−O(2)

2.038(4)

Re(1)−O(6)

1.900(3)

Re(1)−N(12)

2.119(5)

Re(1)−N(22)

2.125(5)

Re(2)−O(3)

2.049(4)

Re(2)−O(4)

2.050(4)

Re(2)−O(6)

1.917(3)

Re(2)−O(7)

1.694(4)

Re(2)−N(32)

2.125(5)

Re(2)−N(42)

2.145(5)

O(1)−C(16)

1.295(7)

O(2)−C(26)

1.282(7)

O(3)−C(36)

1.289(7)

O(4)−C(46)

1.283(7)

N(11)−N(12)

1.441(7)

N(11)−C(16)

1.292(7)

N(12)−C(17)

1.285(8)

N(11)−C(16)

1.285(8)

N(21)−N(22)

1.421(7)

N(21)−C(26)

1.291(7)

N(22)−C(27)

1.299(8)

N(31)−N(32)

1.406(7)

N(31)−C(36)

1.296(8)

N(32)−C(37)

1.288(8)

N(41)−N(42)

1.415(7)

N(41)−C(46)

1.300(8)

1.700(3)

O(1)−Re(1)−O(2)

Bond angles 174.2(2) O(5)−Re(1)−O(6)

176.5(2)

Re(1)−O(6)−Re(2)

176.5(2)

N(12)−Re(1)−N(22)

174.2(2)

O(6)−Re(2)−O(7)

178.5(2)

N(32)−Re(2)−N(42)

172.5(2)

O(3)−Re(2)−O(4)

173.9(2)

O(2)−Re(1)−N(12)

102.1(2)

O(2)−Re(1)−N(22)

75.8(2)

O(5)−Re(1)−N(12)

95.3(2)

O(5)−Re(1)−N(22)

90.2(2)

O(6)−Re(1)−N(12)

88.2(2)

O(6)−Re(1)−N(22)

86.3(2)

O(3)−Re(2)−N(32)

76.0(2)

O(6)−Re(2)−N(42)

104.5(2)

O(4)−Re(2)−N(32)

103.4(2)

O(4)−Re(2)−N(42)

75.4(2)

O(6)−Re(2)−N(32)

85.7(2)


257

Chapter 7

J.Mukiza

O(6)−Re(2)−N(42)

86.9(2)

O(7)−Re(2)−N(32)

93.9(2)

O(3)−Re(2)−O(7)

92.6(2)

O(3)−Re(2)−N(42)

104.5(2)

O(7)−Re(2)−N(32)

93.9(2)

O(1)−Re(1)−N(12)

75.7(2)

O(1)−Re(1)−O(5)

93.4(2)

O(2)−Re(1)−O(6)

87.5(2)

O(4)−Re(2)−O(6)

88.0(2)

N(42)−N(41)−C(46)

109.8(5)

N(41)−N(42)−C(47)

116.0(5)

N(31)−N(32)−C(37)

115.0(5)

N(31)−N(32)−C(36)

110.6(5)

N(12)−N(11)−C(16)

110.4(4)

N(11)−N(12)−C(17)

112.9(5)

N(22)−N(21)−C(26)

109.7(4)

N(21)−N(22)−C(27)

115.9(5)

O(1)−C(16)−N(11)

125.0(5)

O(2)−C(26)−N(21)

126.8(5)

O(3)−C(36)−N(31)

126.3(6)

O(4)−C(46)−N(41)

126.8(6)

N(42)−C(47)−N(48)

120.9(5)

N(32)−C(37)−C(39)

121.0(5)

N(42)−C(47)−N(48)

121.1(6)


258

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Chapter 8 Rhenium Complexes of (Pyridin-2-yl)methanol Derivatives, 2-Aminopyrimidine and its Schiff Base Derivatives 8.1. Introduction The current interest in the coordination chemistry of rhenium mainly focusses on the synthetic aspects, structural and physiochemical properties for the development of radiopharmaceutical agents, and for nitrogen fixation and applications in catalysis [1]. It is for these reasons that the focus is also on pyridine and pyrimidine derivatives due to the considerable role that they have played in the development of coordination chemistry of other transition metals, and their biological activities [2]. The chelating ligands resulting in functionalisation and derivatisation of both pyridine and pyrimidine have been used for synthesis of mononuclear and polynuclear transition metal complexes (including rhenium) with exceptional stability [1−9]. Pyridine derivatives are vital heterocyclic compounds and have been found the applications in organic synthesis, particularly as agrochemical and synthetic intermediates [10]. A typical example is alkoxypyridines and amidopyridines which have already been widely applied in the field of agrochemical products [11]. A variety of pyridine derivatives have been proved to possess multiple biological and pharmacological activities as well as low toxicity toward mammals [12]. They exhibit a variety of medical activities including anti-mycobacterial [13], analgesic, antiparkinsonian, anti-convulsant [14], anti-timoral [15], cytotoxic [16], anti-malarial [17], anti-diabetic [18], inhibitory [19], fungicidal [20] and as receptor antagonists [21]. Pyrimidine is a member of heterocyclic diazine family compounds and it is a component of nucleic acids, nucleotides and corresponding nucleosides [22]. Pyrimidine derivatives display a broad range of pharmacological properties and intensive research has been focused on their anticancer activity [23]. They have been explored for their uses as histamine and adenosine receptor antagonists and other biological receptors and modulators [22]. A series of diaminopyrimidine derivatives exhibit the affinity to the 5-HT7 receptor [22]. Some metal complexes of


259

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J.Mukiza

pyrimidine derivatives are biologically important, and their anti-bacterial, anti-tumour and anti-fungal activities have been reported [24]. This chapter reports on tricarbonylrhenium(I) and oxorhenium(V) complexes of (pyridin-2-yl)methanol

and

2-aminopyrimidine

(Figure

8.1)

derivatives.

Oxorhenium(V) complexes are of interest due to their potential oxygen atom transfer ability [1, 25] and their utility as intermediates in the synthesis of a large number of neutral and ionic complexes, and their catalytic activity [1, 25]. Rhenium(I) complexes containing the fac-[Re(CO)3]+ core are of interest since they exhibit a high kinetic and thermodynamic stability making the radiopharmaceuticals based on the fac-[Re(CO)3]+ core more advantageous than those based on the [ReO]3+ core [26].

N

N NH2 OH

1

N 2

Figure 8.1: Structures of (pyridin-2-yl)methanol (1) and 2-aminopyrimidine (2) In this study, the (pyridin-2-yl)methanol derivatives used are those containing alkoxy groups such as ethoxy(pyridin-2-yl)methanol (Hepm), methoxy(pyridin-2-yl)methanol (Hmpm) and isopropyloxy(pyridin-2-yl)methanol (Hispm) as well as the amino- and keto-functionalised derivatives (isopropylamino)(pyridin-2-yl)methanol (Hiapm) and 4-hydroxy-4-(pyridin-2-yl)butan-2-one (Hhpbo) (see Scheme 8.1). These ligands are derived from the instability of the Schiff base derivative of 2-aminopyrimidine, N((pyridin-2-yl)methylene)pyrimidin-2-amine (pmpa), which decomposes in alcohol and ketone.

N N

N N

Figure 8.2: Line structure of N-((pyridin-2-yl)methylene)pyrimidin-2-amine (pmpa) Nelson Mandela Metropolitan University

260

Chapter 8

J.Mukiza

Depending on the type of alcohol and ketone used, the decomposition of N-((pyridin2-yl)methylene)pyrimidin-2-amine (Scheme 8.1) results in various (pyridin-2yl)methanol

derivatives

and

their

complexes,

cis-[ReOCl2(epm)(PPh3)]

oxorhenium(V) (1),

and

tricarbonylrhenium(I)

cis-[ReOCl2(mpm)(PPh3)]

(2),

cis-

[ReOCl2(iapm)(PPh3)] (3), [Re(CO)3(mpm)]2 (4), cis-[ReOBr2(ispm)(PPh3)] (5) and [Re(CO)3(hpbo)]2 (6). N-((Pyridin-2-yl)methylene)pyrimidin-2-amine (pmpa) was synthesised from the condensation of 2-aminopyrimidine (amp) and picolinaldehyde in methanol (Scheme 8.1). The pmpa was reacted with [Re(CO)5Cl] in toluene and the complex [Re(CO)3Cl(pmpa)] (7) was isolated as product. O O Reflux N Hhpbo

OH N

N Reflux, 4 h +

O N

N amp

MeOH NH2

O

N

OH Reflux

N

N OH Hmpm

N pmpa

Reflux HO O

HO HN

N

OH

Reflux

Hepm

O +

N

OH

Hiapm

N

OH

Hispm

Scheme 8.1: Synthesis of pmpa and its decomposition in alcohol and ketone leading to the (pyridin-2-yl)methanol derivatives The reaction of 2-aminopyrimidine (amp) with [Re(CO)5Cl] in toluene was also investigated and the complex [Re(CO)3Cl(amp)2] (8) was isolated. Surprisingly, the reaction of 2-aminopyrimidine with trans-[ReOI2(OEt)(PPh3)2] in propan-2-ol in the presence of triethylamine (Et3N) produced the seven-coordinated complex trans[ReVOI2(pppa)] (9) (pppa = N-(-3-(pyrimidin-2-ylimino)propylidene)pyrimidin-2-amine ion) (Figure 8.3).


261

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J.Mukiza

N

N

N

N

N N

Figure 8.3: Line structure of N-(-3-(pyrimidin-2-ylimino)propylidene)pyrimidin-2amine) ion (pppa) The chelating ligand N-(-3-(pyrimidin-2-ylimino)propylidene)pyrimidin-2-amine) ion (pppa) was formed by a combined oxorhenium(V)-catalysed condensation and dehydroxylation of 2-aminopyrimidine and 2-propanol (Scheme 8.2), and it is coordinated as a tetradentate N,N,N,N-donor chelate via the neutral imino and pyrimidinic nitrogen atoms.

OH N

NH2

2 N

trans-[ReOI2(OEt)(PPh3)2] / Et3N

N

N

N

N + H2O

N

N pppa

Scheme 8.2: Oxorhenium(V)-catalysed condensation and dehydroxylation of 2aminopyrimidine and 2-propanol leading to the pppa chelating ion

8.2. Experimental 8.2.1. Synthesis of cis-[ReOCl2(epm)(PPh3)] (1) To trans-[ReOCl3(PPh3)2] (103 mg, 0.12 mmol) in 10 cm3 of ethanol was added 0.24 mmol of pmpa (45 mg) in 10 cm3 of ethanol. The resulting yellow green mixture was refluxed for 4 hours, resulting in a dark green solution, which was filtered after being cooled to room temperature. No precipitate was obtained. Purple crystals were grown after 3 days from the slow evaporation of the mother liquor at room temperature. Yield = 67 %, m.p. = 146 °C. Conductivity (methanol, 10-3 M): 35 ohm1

cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 997; ν(Re−O) 442; ν(Re−N) 465; ν(C═N) 1672.

1

H NMR (295K, ppm): 1.11 (t, 3H, CH3), 3.41 (q, 2H, CH2), 5.58 (s, 1H, H(1)), 6.85 (t,


262

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1H, H(13)), 7.41 (d, 1H, H(15)), 7.48 (t, 1H, H(14)), 7.52-7.70 (m, 15H, PPh3), 8.50 (d, 1H, H(12)). Electronic spectrum (dichloromethane, λ (ε, M-1cm-1)): 338 (3600). 8.2.2. Synthesis of cis-[ReOCl2(mpm)(PPh3)] (2) A mixture of 45 mg (0.24 mmol) of pmpa and 103 mg (0.12 mmol) of trans[ReOCl3(PPh3)2] in 20 cm3 of methanol was heated under reflux for 4 hours, resulting in a dark-green solution, which was filtered after being cooled to room temperature. No precipitate was obtained. Purple crystals were grown in 4 days from the slow evaporation of the mother liquor at room temperature. Yield = 62 %, m.p. = 138 °C. Conductivity (methanol 10-3 M): 31 ohm-1cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 997; ν(Re−O) 442; ν(Re−N) 465; ν(C═N) 1672. 1H NMR (295K, ppm): 3.24 (s, 3H, CH3), 5.55 (s, 1H, H(1)), 6.88 (t, 1H, H(13)), 7.35-7.50 (m, 2H, H(14), H(15)), 7.52−7.68 (m, 15H, PPh3), 8.54 (d, 2H, H(12)). Electronic spectrum (methanol, λ (ε, M-1cm-1)): 360 (11500), 300 nm (36900). 8.2.3. Synthesis of cis-[ReOCl2(iapm)(PPh3)] (3) The addition of 45 mg (0.24 mmol) of pmpa in 10 cm3 of propan-2-ol to trans[ReOCl3(PPh3)2] (103 mg, 0.12 mmol) in 10 cm3 of propan-2-ol give a yellow-green solution which was refluxed for 5 hours, resulting in a dark green solution, which was filtered after being cooled to room temperature. No precipitate was formed. Green crystals were grown overnight from the slow evaporation of the mother liquor at room temperature. Yield = 78 %, m.p. = 151 °C. Conductivity (methanol, 10-3 M): 32 ohm1

cm2 mol-1. IR (νmax./cm-1): ν(Re═O) 996; ν(Re−O) 441; ν(Re−N) 464; ν(C═N) 1673.

1

H NMR (295K, ppm): 1.08 (d, 6H, C(3)H3, C(4)H3), 2.99 (m, 1H, C(2)H), 3.70 (s, 1H,

NH), 5.50 (s, 1H, CH), 7.00 (t, 1H), 7.50 (d, 1H), 7.59 (t, 1H), 7.65−7.80 (m, 15H), 8.62 (d, 1H). Electronic spectrum (dimethylformamide, λ (ε, M-1cm-1)): 347 (11470), 522 (3070), 633 nm (1230). 8.2.4. Synthesis of [Re(CO)3(mpm)]2 (4) [Re(CO)5Cl] (150 mg, 0.41 mmol) in 10 cm3 of methanol was added to 0.82 mmol of pmpa (151 mg) in 5 cm3 of methanol. The resulting yellow mixture was refluxed for 5 hours under nitrogen, resulting in an orange solution, which was filtered after being cooled to room temperature. No precipitate was observed. Orange crystals were grown in 4 days from the slow evaporation of the mother liquor at room temperature. Nelson Mandela Metropolitan University

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Yield = 65 %, m.p. = 172 °C. Conductivity (methanol, 10-3 M): 52 ohm-1cm2 mol-1. IR (νmax/cm-1): ν(C≡O, fac) 1863, 2013; ν(Re−O) 449; ν(Re−N) 488; ν(C═N) 1608. 1H NMR (295K, ppm): 3.22 (s, 6H, 2CH3), 5.57 (s, 2H, 2CH), 7.53 (t, 2H), 7.72 (d, 2H), 7.78 (t, 2H), 8.87 (d, 2H). Electronic spectrum (methanol, λ (ε, M-1cm-1)): 306 (850), 260 (2300), 375 nm (250). 8.2.5. Synthesis of cis-[ReOBr2(ispm)(PPh3)] (5) To trans-[ReOBr3(PPh3)2] (105 mg, 0.11 mmol) in 10 cm3 of propan-2-ol was added 0.22 mmol of pmpa (40 mg) in 5 cm3 of propan-2-ol. The resulting yellow mixture was refluxed for 6 hours, resulting in dark green solution, which was filtered after being cooled to room temperature. No precipitate was observed. Green crystals were grown in one week from the slow evaporation of the mother liquor at room temperature. Yield = 78 %, m.p. = 148 °C. Conductivity (acetonitrile, 10-3 M): 80 ohm1

cm2 mol-1. IR (νmax/cm-1): ν(Re═O) 967; ν(Re−O) 455; ν(Re−N) 488; ν(C═N) 1634.

1

H NMR (295K, ppm): 1.17 (d, 6H, 2CH3), 3.21 (s, 1H, C(2)H), 5.62 (s, 1H, C(1)H),

7.15 (t, 1H, H(14)), 7.22 (d, 1H, H(12)), 7.38 (t, 1H, H(13)), 7.45−7.80 (m, 15H, PPh3), 8.04 (d, 1H, H(15)). Electronic spectrum (acetonitrile, λ (ε, M-1cm-1)): 315 (1260), 450 (450), 502 nm (370). 8.2.6. Synthesis of [Re(CO)3(hpbo)]2 (6) A mixture of [Re(CO)5Cl] (100 mg, 0.28 mmol) and pmpa (103 mg, 0.56 mmol) in 15 cm3 of dry acetone was refluxed for 6 hours under nitrogen, resulting in a yellow solution, which was filtered after being cooled to room temperature. No precipitate was obtained. Yellow crystals were grown in 5 weeks from the slow evaporation of the mother liquor at room temperature. Yield = 55 %, m.p. = 167 °C. Conductivity (methanol, 10-3 M): 48 ohm-1cm2 mol-1. IR (νmax/cm-1): ν(C≡O, fac) 1869, 1890, 2012; ν(Re−O) 403; ν(Re−N) 487; ν(C═N) 1627; ν(C═O) 1712. 1H NMR (295K, ppm): 2.09 (s, 6H, 2CH3), 4.73 (d, 4H, 2C(17)H2), 4.82 (t, 2H, 2C(16)H), 7.78 (d, 1H, H(12)), 7.41 (t, 1H, H(14)), 8.06 (t, 1H, H(13)), 8.69 (d, 1H, H(15)). Electronic spectrum (dimethylformamide, λ (ε, M-1cm-1)): 272 (1360), 378 (1640), 430 (1600). 8.2.7. Synthesis of [Re(CO)3Cl(pmpa)] (7) [Re(CO)5Cl] (150 mg, 0.41 mmol) in 10 cm3 of toluene was added to 0.82 mmol (151 mg) of pmpa in 10 cm3 of toluene. The colourless mixture was heated under reflux Nelson Mandela Metropolitan University

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for 4 hours, resulting in a red solution. After being cooled to room temperature, the mixture was filtered off giving a red precipitate. Recrystallisation from an ethanol/dichloromethane mixture resulted in red crystals suitable for X-ray measurement after three days by the slow evaporation of the mother liquor at room temperature. Yield = 83 %, m.p. = 186 °C. IR (νmax/cm-1): ν(C≡O, fac) 1863, 1909, 2017; ν(Re−N) 434, 472; ν(C═N) 1608, 1583, 1568. 1H NMR (295K, ppm): 8.84 (s, 1H, C(1)H), 8.62 (d, 1H, H(15)), 8.04 (t, 1H, H(13)), 7.96 (d, 1H, H(12)), 7.64 (t, 1H, H(14)), 7.51 (d, 2H, H(22), H(24)), 7.41 (t, 1H, H(23)). Electronic spectrum (methanol, λ (ε, M-1cm-1)): 296 (5855), 482 nm (820). 8.2.8. Synthesis of [Re(CO)3Cl(amp)2] (8) [Re(CO)5Cl] (170 mg, 0.47 mmol) in 10 cm3 of ethanol was added to 0.94 mmol (89 mg) of amp in 5 cm3 of ethanol. The mixture was refluxed for 6 hours under nitrogen, resulting in a yellow solution, which was filtered after being cooled to room temperature. No precipitate was obtained. Yellow crystals suitable for X-ray crystallography were grown in three months by the slow evaporation of the mother liquor at room temperature. Yield = 64 %, m.p. = 193 °C. IR (νmax/cm-1): ν(C≡O, fac) 1886, 1913, 2023; ν(Re−N) 486; ν(C═N) 1633, 1585; ν(N−H) 2849, 2916. 1H NMR (295K, ppm): 4.02 (s, 4H, 2NH2), 7.45 (t, 2H, H(13)), 8.86 (d, 4H, H(12), H(14)). Electronic spectrum (dimethylformamide, λ (ε, M-1cm-1)): 321 (5500), 360 nm (3600). 8.2.9. Synthesis of trans-[ReOI2(pppa)] (9) To trans-[ReOI2(OEt)(PPh3)2] (150 mg, 131 μmol) in 10 cm3 of propan-2-ol was added 393 μmmol (37 mg) of amp in 10 cm3 of propan-2-ol with 5 drops of Et3N. The dark green mixture was heated under refluxed for 8 hours, resulting in a dark brown solution, which was filtered after being cooled to room temperature. No precipitate was removed. Green crystals suitable for X-ray crystallography were grown in two months by the slow evaporation of the mother liquor at room temperature. Yield = 45 %, m.p. = 248 °C. IR (νmax/cm-1): ν(Re═O) 909; ν(Re−N) 464, 506; ν(C═N) 1627, 1589, 1561. 1H NMR (295K, ppm): 8.23 (d, 2H, H(12), H(22)), 7.62 (dd, 2H, H(1), H(3)), 7.48 (t, 1H, H(2)), 7.42 (t, 2H, H(13), H(23)), 6.56 (t, 2H, H(14), H(24)). Electronic spectrum (dimethylsulfoxide, λ (ε, M-1cm-1)): 333 (4600), 425 (2490), 470 nm (2000).


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8.2.10. X-ray crystallography Single crystal X-ray crystallography studies were performed at 200 K using a Bruker Kappa Apex II diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71069 Å). For data collection, APEX-II was used while for cell refinement and data reduction, SAINT was used [27]. The structures were solved by direct methods applying SHELXS-97 [28], or SIR97 [29] and refined by least-squares procedures using SHELXL-97 [26], with SHELXLE [29] as a graphical interface. All non-hydrogen atoms were anisotropically refined and hydrogen atoms were calculated in idealised geometrical positions. Data were corrected by absorption effects using the numerical method using SADABS [27].

8.3. Results and discussion 8.3.1. cis-[ReOCl2(epm)(PPh3)] (1) and cis-[ReOCl2(mpm)(PPh3)] (2) The complexes 1 and 2 were synthesised from the reaction of two equivalents of the Schiff base pmpa with trans-[ReOCl3(PPh3)2] in ethanol and methanol respectively. The Schiff base pmpa has decomposed and one of its decomposition products reacted with ethanol (for complex 1), giving the bidentate N,O-donor ethoxy(pyridin2-yl)ethanolate (epm−) and methanol (for complex 2), giving the bidentate N,O-donor methoxy(pyridine-2-yl)methanolate (mpm−). EtOH trans-[ReOCl3(PPh3)2] + 2 pmpa

Reflux, 4 h

cis-[ReOCl2(epm)(PPh3)] (1) + PPh3 + HCl

MeOH Reflux, 4 h

cis-[ReOCl2(mpm)(PPh3)] (2) + PPh3 + HCl

Complexes 1 and 2 are air-stable, not soluble in water, but soluble in variety of organic

solvents

such

as

methanol,

ethanol,

dichloromethane,

acetone,

dimethylformamide, dimethylsulfoxide and chloroform to give dark green solutions. They are non-electrolytes in methanol, with a molar conductivity value around 30 ohm-1cm2mol-1. Figure 8.4 displays the overlay IR spectra of complexes 1 and 2 showing that the two spectra are practically the same. The absorption peak of medium intensity at 997 cmNelson Mandela Metropolitan University

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(for both 1 and 2) is assigned to ν(Re═O). The absorption peak of medium intensity

at 1672 cm-1 is ascribed to ν(C═N) in the pyridyl ring. The absorption peaks appearing at 442 and 465 cm-1 (for both complex 1 and 2) are ascribed to ν(Re−O)

% Transmittance

and ν (Re−N) respectively.

82

Complex 1

72

Complex 2

62 52 42 32 22 12 1600

1400

1200

1000

800

600

400

Wavenumber (cm-1) Figure 8.4: Overlay IR spectra of complex 1 and 2 Figure 8.5 displays the overlay UV visible spectra of complex 1 in dichloromethane and 2 in methanol. The absorption intensity at 360 nm for complex 2 is due to a ligand-to-metal charge transfer (LMCT). However, the similar ligand-to-metal charge transfer (LMCT) absorption intensity in complex 1 has been shifted at 338 nm. Complex 2 displays another absorption intensity at 300 nm which can be ascribed to an intraligand π→π* electrons transition of the coordinated ligand. The 1H NMR spectrum in the aromatic region is shown in Figure 8.6. It is dominated by signals of the protons of the phenyl rings of PPh 3, with multiplets in the 7.52-7.70 ppm region that integrate for 15 protons. The protons of the pyridyl ring integrate for 4 protons and occur as a doublet at 8.50 ppm (H(12)), a doublet at 7.41 ppm (H(15)), a triplet at 7.48 ppm (H(14)) and a triplet at 6.85 ppm (H(13)). The ethyl protons lead to a quartet at 3.41 ppm (C(4)H2) and a triplet at 1.11 ppm (C(5)H3). The 1H NMR spectrum of complex 2 in the aromatic region (Figure 8.7) is nearly identical to that of complex 1. Nelson Mandela Metropolitan University

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0.46

Complex 1

0.41

Absorbance

0.36

Complex 2

0.31 0.26 0.21 0.16 0.11 0.06 0.01 286

306

326

346

366

386

406

426

Wavelength (nm) Figure 8.5: Overlay UV-Vis spectrum of complex 1 in CH2Cl2 and complex 2 in MeOH

Figure 8.6: 1H NMR spectrum of complex 1 in d6-DMSO


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Figure 8.7: 1H NMR spectrum of complex 2 in d6-DMSO The ORTEP diagrams of complex 1 (Figure 8.8) and 2 (Figure 8.9) show that the bidentate ligands are coordinated to oxorhenium(V) as monoanionic, bidentate N,Odonor chelates and coordinate to the metal center through the neutral pyridinic nitrogen

and

anionic

alcoholate

oxygen,

resulting

in

five-membered

ring

metallocyles. The geometry around rhenium atom is distorted octahedral in which the equatorial plane is defined by the phosphorous atom P(1) of triphenylphosphine, two chlorides Cl(1) and Cl(2), and the neutral pyridinic nitrogen. The oxo ligand and alcoholate oxygen are in trans axial positions. The two chlorides are in cis positions with

the

bond

angle

Cl(1)−Re(1)−Cl(2)

[89.31(3)o]

for

complex

1

and

Cl(1)−Re(1)−Cl(2) [89.29(2)o] for 2 being close to the orthogonality. The deviation from an ideal rhenium-centered octahedron results in the non-linear trans angles O(1)−Re(1)−O(4) = 161.1(1)o, Cl(2)−Re(1)−P(1) = 174.44(3)o, Cl(1)−Re(1)−N(1)

=

169.70(8)o

for

complex

1

and

the

similar

angles

O(1)−Re(1)−O(3) = 161.21(7)o, Cl(2)−Re(1)−P(1) = 174.11(2)o, Cl(1)−Re(1)−N(1) = 169.91(5)o for complex 2. The constraints imposed by the five-membered ring metallocycle are reflected by bite angles O(1)−Re(1)−N(1) = 73.3(1)o (for 1) and 75.31(6)o (for 2). The steric repulsion of oxo ligand O(4) in complex 1 and O(3) for 2 is reflected by the rhenium-centered bond angles O(4)−Re(1)−N(1) = 86.77(10)o, Nelson Mandela Metropolitan University

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Cl(2)−Re(1)−O(4) = 96.46(8)o, Cl(2)−Re(1)−O(3) = 96.71(5)o, P(1)−Re(1)−O(3) = 88.50(5)o and O(3)−Re(1)−N(1) = 87.09(7)o which significantly deviate from orthogonality. The Re═O bond lengths of 1.696(2) Å and 1.69(2) Å in complex 1 and 2 respectively are within the range expected with alcoholate oxygens trans to the oxo group [29, 30]. The Re(1)−O(1) = 1.915(2) Å in 1 and Re(1)−O(1) = 1.921(2) Å in 2 are approximately similar and are shorter than the usual Re−O single bond [2.04(2) Å], which may be reflecting the delocalisation of π-electron density from the oxo bond to the trans Re−O bond [30, 31].

For complex 1, the bond distance Re(1)−Cl(2)

[2.4169(9) Å] is longer than the Re(1)−Cl(1) [2.359(1) Å], similar to complex 2 in which the bond distance Re(1)−Cl(2) [2.4196(6) Å] is longer than Re(1)−Cl(1) [2.3581(7) Å]. This reflects the larger trans effect of P(1) compared to the pyridinic nitrogen N(1) in both complexes. The Re−N bond distance in complexes 1 and 2 are 2.150(3) Å and 2.145(3) Å respectively and are typical for Re(V)−pyridyl bonds [32, 33]. The bond distances O(1)−C(1) = 1.394(5) Å (1) and O(1)−C(1) = 1.401(3) Å (2) show that they are single bonds.

Figure 8.8: Molecular structure of complex 1 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity Nelson Mandela Metropolitan University

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Figure 8.9: ORTEP view of complex 2 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atom have been omitted for clarity 8.3.2. Cis-[ReOCl2(iapm)(PPh3)] (3) The reaction of pmpa with trans-[ReOCl3(PPh3)2] in propan-2-ol led to ligand decomposition, and its decomposition product reacted with the solvent propan-2-ol giving the bidentate N,O-donor (isopropylamino)(pyridin-2-yl)methanol (Hiapm), which coordinated to rhenium as monoanionic chelate, resulting in complex 3. trans-[ReOCl3(PPh3)2] + pmpa + iPrOH

Reflux 5h

cis-[ReOCl2(iapm)(PPh3)] + PPh3 + HCl

Complex 3 is air-stable, not soluble in water but sparingly soluble in a variety of organic

solvents

such

as

methanol,

ethanol,

dichloromethane,

acetone,

dimethylformamide, dimethylsulfoxide and chloroform, to give dark green solutions. The iapm is similarly coordinated to oxorhenium(V) as the epm and mpm ligands in complexes 1 and 2. Consequently, the complex 3 displays the same geometry as complexes 1 and 2, particularly the two chlorides in cis potions and alcoholate oxygen coordinated in the trans position to the oxo ligand.


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The IR spectrum of complex 3 (Figure 8.10) shows an absorption of medium intensity peak at 996 cm-1, which corresponds to ν(Re═O). The peak of medium intensity at 1673 cm-1 is ascribed to ν(C═N) of the pyridyl ring. The peaks at 464 and 441 cm-1 are ascribed to ν(Re−N) and ν (Re−O) respectively. The UV-Vis spectrum of 3 (Figure 8.11) in dimethylformamide is characterised by the absorption peak at 347 nm, which is assigned to a ligand-to-metal charge transfer transition (LMCT) respectively. The two absorption peaks at 522 and 633 nm reflect a d-d and n→π* electronic transitions respectively. The 1H NMR spectrum of 3 (Figure 8.12) shows sharp and well resolved peaks. The multiplets in the region 7.65-7.80 ppm integrate for 15 protons and are assigned to the phenyl protons of PPh3. The four protons of the pyridyl ring are represented by a triplet at 7.00 ppm, a doublet at 7.50 ppm, a triplet at 7.59 and a doublet at 8.62 ppm. The methyl protons lead to a doublet at 1.08 ppm (C(3)H3 and C(4)H3) and the methine proton occurs as a multiplet at 2.99 ppm for C(2)H, and a singlet at 5.50 ppm is assigned to C(1)H). The N(2)H proton is represented by a singlet and broad peak at 3.70 ppm.

Figure 8.10: IR spectrum of complex 3 in the range 350-1800 cm-1


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1.25

Absorbance

1.05

0.85

0.65

0.45

0.25

0.05 327

377

427

477

527

577

627

677

Wavelength (nm) Figure 8.11: UV-Vis spectrum of complex 3 in DMF

Figure 8.12: 1H NMR spectrum of complex 3 in d6-DMSO The ORTEP diagram of complex 3 (Figure 8.13) shows that the ligand is coordinated to oxorhenium(V) as a monoanionic bidentate chelate similar to the chelates in complexes 1 and 2. The geometry around the rhenium is distorted octahedral in which the equatorial plane is defined by the phosphorus atom P(1) of triphenylphosphine, two chlorides Cl(1) and Cl(2), and the neutral pyridinic nitrogen Nelson Mandela Metropolitan University

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N(1). The oxo ligand and alcoholate oxygen are in trans axial positions. The two chlorides are in cis arrangement as supported by the bond angle Cl(1)−Re(1)−Cl(2) [90.38(3)o]. The Re═O bond length of 1.702(2) Å is slightly longer than the corresponding bond lengths in complexes 1 and 2, and also longer than the similar bond observed in oxorhenium(V) complex featuring an alcoholate oxygen trans to the oxo group [30, 31]. The Re(1)−O(1) = 1.914(2) Å is shorter than the usual single bond Re−O [2.04(2) Å], which may be reflecting the delocalisation of π-electron density from the oxo bond to the trans Re−O bond [29, 34]. The bond distance Re(1)−Cl(1) [2.425(1) Å] is longer than the Re(1)−Cl(2) [2.349(1) Å] due to the larger trans effect of P(1) compared to the pyridinic nitrogen N(1). The Re−N bond distance of 2.145(4) Å is typical for Re(V)−pyridyl bonds [32, 33]. The bond distances N(2)−C(2) [1.455(6) Å], N(2)−C(1) [1.381(6) Å] and O(1)−C(1) [1.399(5) Å] are single bonds of the type [33].

Figure 8.13: Crystal structure of complex 3 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity Nelson Mandela Metropolitan University

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The deviation from an normal rhenium-centered octahedron geometry is reflected by the non-linear trans angles O(1)−Re(1)−O(2) = 161.7(1)o, Cl(2)−Re(1)−N(1) = 168.7(1)o and Cl(1)−Re(1)−P(1) = 177.05(3)o. The coordination requirements of the iapm− anion forming the five-membered ring metallocycle and the steric repulsion of oxo ligand O(2) also result in the deviation from an ideal orthogonality, with the angles O(2)−Re(1)−N(1) = 86.5(1)o, P(1)−Re(1)−N(1) = 91.7(1)o, Cl(2)−Re(1)−O(2) = 104.6(1)o, Cl(1)−Re(1)−N(1) = 87.2(1)o and O(1)−Re(1)−N(1) = 75.7(2)o. 8.3.3. [Re(CO)3(mpm)]2 (4) Complex 4 was obtained from the reaction of two equivalents of the Schiff base pmpa with [Re(CO)5Cl] in methanol. The Schiff base pmpa has been decomposed and one of its decomposition products reacted with methanol, giving the bidentate N,O-donor ligand methoxy(pyridin-2-yl)methanol (Hmpm) which coordinated to rhenium giving complex 4.

Reflux 2 [Re(CO)5Cl] + 2 pmpa + MeOH

5h

[Re(CO)3(mpm)]2 (4) + 2 HCl + 4 CO

Complex 3 is air-stable with orange-coloured crystals, not soluble in water but sparingly soluble in a variety of organic polar and non-polar solvents. The infrared spectrum of 4 (Figure 8.14) is characterised by intense broad band at 1863 and an intense sharp band 2013 cm-1. These bands are ascribed to v(C≡O) of the fac-[Re(CO)3]+ unit with the former broad peak the result of the symmetric stretching vibrations of the two CO ligands in the equatorial plane. The band of medium intensity at 1608 cm-1 is assigned to v(C═N). The bands of weak intensity at 449 and 488 cm-1 are ascribed to v(Re−O) and v(Re−N) respectively. The aromatic region of the 1H NMR spectrum of 4 integrate for the 8 protons of the two pyridyl rings and are represented by a triplet at 7.53 ppm, a doublet at 7.72 ppm, a triplet at 7.78 ppm and a doublet at 8.87 ppm. These reflect that each proton of one pyridyl ring is magnetically equivalent to its homologous on the other ring. The two methyl groups in each chelate give a singlet at 3.22 ppm, with another singlet at 5.57 ppm [C(1)H].


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Figure 8.14: IR spectrum of complex 4 The UV-Vis spectrum of 4 (Figure 8.15) in methanol is characterised by absorption at 306 and 260 nm which are assigned to an intraligand π→π* transition in the coordinated mpm− ligand. The absorption at 376 nm is assigned to a metal-to-ligand charge transfer (MLCT).

Absorbance

0.202

0.152

0.102

0.052

0.002 257

277

297

317

337

357

377

397

417

437

Wavelength (nm) Figure 8.15: UV-Vis spectrum of complex 4 in MeOH Nelson Mandela Metropolitan University

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The ORTEP diagram of complex 4 (Figure 8.16) is characterized by a rhombic (µO)2Re2 unit at the centre of the molecule. The ligand coordinates to the rhenium(I) in a bidentate fashion via a neutral pyridinic nitrogen and an anionic bridging alcoholate oxygen atom. The dimer is produced by inversion of which the centre is positioned in the centre of the nearly square Re2O2 ring. Each rhenium(I) atom geometrically adopts a distorted octahedral arrangement with a pyridinic nitrogen and two bridging alcoholate oxygens coordinated in a facial arrangement as imposed by fac[Re(CO)3]+ core. The mpm− is symmetrically coordinated to both rhenium atoms and consequently both halves of the dimer are geometrically symmetric. The distortion from an ideal octahedron around Re(1) and Re(2) is the result of the deviation from linearity of the trans angles O(1)−Re(2)−C(4) = 169.5(2)o, N(1)−Re(1)−C(5) = 172.2(7)o, O(1)−Re(1)−C(3) = 174.3(2)o, and their homologous other half of the dimer. These distortions can also be attributed to the constraints imposed by the coordination mode of the mpm − anion, which forms a five-membered chelate ring with a bite angle O(1)−Re(1)−N(1) of 74.0(2)°.


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The Re(1)−N(1) bond distance of 2.190(3) Å agree well to those previously reported for neutral rhenium(I)−N(pyridinic) bonds [35−36]. The bond distances Re(1)−O(1) = 2.144(4) Å is shorter than Re(1)−O(1i) = 2.175(3) Å, but are close to the ReI−O(alcoholate) single bonds reported in similar dimeric complexes [35]. The two Re−CO bond distances of 1.904(6) Å and 1.915(5) Å fall in the range observed [1.900(2)−1.928(2) Å] for similar complexes [35−37], but the third Re−C(4)O bond distance of 1.898(6) Å is slightly below the range. 8.3.4. cis-[ReOBr2(ispm)(PPh3)] (5) The Schiff base pmpa was reacted with trans-[ReOBr3(PPh3)2] in propan-2-ol and the

complex

cis-[ReOBr2(ispm)(PPh3)]

(5)

(ispm−

=

isopropyloxy(pyridin-2-

yl)methanolate) was isolated as product. The chelating ligand (isopropyloxy)(pyridin2-yl)methanol was formed by the decomposition of pmpa in which one of its decomposition products reacted with propan-2-ol giving Hispm, instead of (isopropylamino)(pyridin-2-yl)methanol (Hiapm) as observed for the similar reaction with trans-[ReOCl3(PPh3)2]. The ispm− anion coordinates to oxorhenium(V) as a bidentate N,O-donor chelate.

trans-[ReOBr3(PPh3)2] + pmpa + iPrOH

Reflux 6h

cis-[ReOBr2(iapm)(PPh3)] + PPh3 + HBr

Complex 5 is air-stable, insoluble in water but weakly soluble in polar solvents to give dark green solutions. It is non-electrolyte in acetonitrile. The IR spectrum of complex 5 shows an absorption peak at 976 cm-1, which corresponds to ν(Re═O). The peak of medium intensity at 1634 cm-1 is ascribed to ν(C═N) of the pyridyl ring. The peaks at 455 and 488 cm-1 are ascribed to ν(Re−O) and ν (Re−N) respectively. The UV-Vis spectrum of 5 (Figure 8.17) in acetonitrile is characterised by an absorption at 315 nm which is assigned to an intraligand

* transition in the

coordinated iapm−. The two absorptions at 450 and 513 nm are assigned to a combination of

the ligand-to-metal charge transfer transitions [pπ(O−)→d*π(Re),

pπ(N)→d*π(Re) and pπ(Br−)→d*π(Re)] and a (dxy)2→(dxy)1(dxz)1 transition respectively.


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The 1H NMR spectrum of complex 5 in the aromatic region shows multiplet in the region 7.45−7.80 ppm which integrate for the 15 protons of the PPh3 phenyl rings, in addition to a doublet at 8.04 ppm (H(15)), a triplet at 7.38 ppm (H(13)), a doublet at 7.22 ppm (H(12)), and a triplet at 7.15 ppm (H(14)). The six methyl protons produce a singlet at 1.17 ppm, and C(1)H and C(2)H give one-proton singlets at 5.62 and 3.21 ppm respectively.

0.123

Absorbance

0.103

0.083

0.063

0.043

0.023 307

357

407

457

507

557

Wavelength (nm) Figure 8.17: UV-Vis spectrum of complex 5 in MeCN The ORTEP diagram of complex 5 (Figure 8.18) shows that the ligand is coordinated to oxorhenium(V) as a monoanion, similar to the chelates in complexes 1, 2 and 3. The geometry around rhenium atom is a distorted octahedron in which the equatorial plane is defined by the phosphorus atom P(1), two bromides Br(1) and Br(2) and the neutral pyridinic nitrogen N(1). The oxo ligand and alcoholate oxygen are in trans axial positions. The two bromide ligands are in a cis arrangement, with the bond angle Br(1)−Re(1)−Br(2) equal to 91.01(2)o. The deviation from an ideal rhenium-centered octahedron geometry is reflected in the non-linear trans angles O(1)−Re(1)−O(3) = 161.9(1)o, Br(2)−Re(1)−N(1) = 169.50(7)o and Br(2)−Re(1)−P(1) = 176.62(3)o. The bond angles Br(2)−Re(1)−N(1) [91.63(7)°],

P(1)−Re(1)−O(1)

[88.00(6)°],

P(1)−Re(1)−O(3)

[89.06(7)o]

and

Br(1)−Re(1)−O(3) [103.83(7)o] deviate markedly from orthogonality due to the coordination requirements of ispm− anion and the steric repulsion of the oxo ligand O(3). Nelson Mandela Metropolitan University

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The Re(1)═O(3) bond length of 1.699(2) Å is slightly shorter than the similar bond length in complex 3 and also longer than the reported Re═O bond lengths for oxorhenium(V) complexes featuring alcoholate oxygen trans to the oxo group [29, 30]. The Re(1)−O(1) = 1.918(2) is comparable to the similar distance in complex 3 and it is also shorter than the usual single bond Re−O [2.04(2) Å], which may be reflecting the delocalisation of π-electron density from the oxo bond to trans Re−O bond [30, 31]. The bond distance Re(1)−Br(1) [2.4935(6) Å] is shorter than the Re(1)−Br(2) [2.5796(4) Å], again reflecting the larger trans effect of P(1) compared to the pyridinic nitrogen N(1). The Re−N bond distance of 2.154(3) Å is slightly longer than the similar distance in complex 3 but it is typical for Re(V)−pyridyl bonds [3, 33]. The bond distances O(1)−C(1) [1.404(4) Å] and O(2)−C(1) [1.404(4) Å] are single bonds of this type [33]. The π-delocalisation inside pyridyl ring is supported by bond distances N(1)−C(15) [1.347(4) Å] and N(1)−C(11) [1.355(4) Å] being between single and double bonds [33].

Figure 8.18: ORTEP view of complex 5 showing 50% probability displacement ellipsoids and atom labelling. Hydrogen atom have been omitted for clarity Nelson Mandela Metropolitan University

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8.3.5. [Re(CO)3(hpbo)]2 (6) The dimeric complex 6 was synthesised from the reaction of two equivalents of the Schiff base pmpa with [Re(CO)5Cl] in acetone. The pmpa reacted with acetone giving the bidentate N,O-donor derivative 4-hydroxy-4-(pyridin-2-yl)butan-2-one (Hhpbo), which coordinated to rhenium(I), resulting in complex 6.

Reflux [Re(CO)3(hpbo)]2 (6) + 2 HCl + 4 CO

2 [Re(CO)5Cl] + 2 pmpa + Me2CO 6h

The synthesis of complex 6 in acetone is an unusual procedure since most of the reported rhenium(I) complexes based on the fac-[Re(CO)3]+ core have been isolated from the solvents such as toluene, tetrahydrofuran, acetonitrile and alcohols. Complex 6 is air-stable with yellow coloured crystals. It is not soluble in water but soluble in variety of organic polar and non-polar solvents. In the infrared spectrum of 6 (Figure 8.19) absorption bands at 2012 and 1869, with shoulder at 1890 cm-1 are ascribed to v(C≡O) of the fac-[Re(CO)3]+ unit. The band of medium intensity at 1712 cm-1 is assigned to the free ketonic (C═O) vibration frequency. The bands of weak intensity at 403 and 487 cm-1 are ascribed to v(Re−O) and v(Re−N) respectively. The C═N in pyridyl rings are represented by absorption at 1627 cm-1. The UV-Vis spectrum of 6 (Figure 8.20) in dimethylformamide shows the absorption at 272 nm which is assigned to an intra-ligand

* electronic transition in the

coordinated hpbo−. The absorption at 378 nm is assigned to a metal-to-ligand charge transfer (MLCT) transition as it is for the absorption at 430 nm. The 1H NMR spectrum of 6 in the aromatic region is similar to that of complex 4. The protons of both pyridyl rings are magnetically equivalent to the corresponding one on the other ring. They appear as a triplet at 7.41 ppm (H(14)), a doublet at 7.78 ppm (H(12)), a triplet at 8.06 ppm (H(13)) and a doublet at 8.69 ppm (H(15)). The assignment of the other signals in the spectrum is given in the experimental section in paragraph 8.2.6.


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1.278

Absorbance

1.078

0.878

0.678

0.478

0.278

0.078 268

318

368

418

468

Wavelength (nm) Figure 8.20: UV-Vis spectrum of complex 6 in DMF The crystal structure of complex 6 (Figure 8.21) is characterized by a rhombic (µO)2Re2 unit at the centre of the molecule as in complex 4. Similarly to complex 4, the hpbo− anion coordinates to the rhenium(I) centre as a bidentate via a neutral pyridinic nitrogen and an anionic bridging alcoholate oxygen atom. The dimer is produced by an inversion of which the centre is positioned in the centre of the nearly Nelson Mandela Metropolitan University

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square Re2O2 ring. Each rhenium(I) atom geometrically adopts a distorted octahedral arrangement with a pyridinic nitrogen and two bridging alcoholate oxygen atoms coordinated in a facial arrangement as imposed by the fac-[Re(CO)3]+ core. The distortion from an ideal octahedron around Re(1) and Re(2) is the result of the deviation from linearity of the trans angles O(11)−Re(1)−C(50) = 169.66(9)o, N(11)−Re(1)−C(51) = 175.1(1)o, O(21)−Re(1)−C(52) = 173.7(1)o in one half of dimer and their homologous angles O(11)−Re(2)−C(61) = 176.15(9)o, N(21)−Re(2)−C(60) = 174.83(9)o, O(21)−Re(1)−C(62) = 167.41(9)o in the other half. The distortions are also the result of the constraints imposed by the coordination mode of the hpbo− anion, which forms a five-membered chelate ring, and results in the deviation from the expected orthogonality of the angles O(21)−Re(1)−C(51) = 92.03(9)°, O(21)−Re(1)−N(11) = 86.03(6)o, O(11)−Re(1)−O(21) = 75.01(6)o in one half of dimer and

O(21)−Re(2)−C(61)

=

91.6(1)°,

O(11)−Re(2)−N(21)

=

88.36(6)o

and

C(60)−Re(2)−O(61) = 86.3(1)o in the other half.


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The coordination milieu around Re(1) and Re(2) is exactly the same as around Re(1) and Re(2) in complex 4. The Re(1)−N(11) and Re(21)−N(21) bond distances of 2.182(2) Å and 2.179(2) Å are somehow shorter than the similar bonds in complex 4. The bond distances Re(1)−O(11) and Re(2)−O(21) of 2.1430(4) Å and 2.137(2) Å are practically similar to Re−O in complex 4 (Table 7.2). There is nothing unusual in the Re−CO bonds distance for 6 (Table 8.2). The bond distances O(22)−C(28) and O(12)−C(18) of 1.196(4) Å and 1.207(4) Å respectively are indicative of double bonds [33], with the bond angles around C(28) [O(22)−C(28)−C(27) = 120.2(3)o] and C(18) [O(12)−C(18)−C(17) = 120.9(3)o] close to the expected 120o for sp2hybridization. The bonds O(11)−C(16) [1.423(3) Å] and O(21)−C(26) [1.420(3) Å] are single bonds [33]. 8.3.6. [Re(CO)3Cl(pmpa)] (7) The rhenium(I) complex [Re(CO)3Cl(pmpa)] (7) was isolated from the reaction of pmpa with [Re(CO)5Cl] in toluene under reflux.

[Re(CO)5Cl] + pmpa

Toluene

[Re(CO)3Cl(pmpa)] (7)+ 2 CO

Reflux, 4 h

The red complex 7 is air-stable and it is soluble in both polar and non-polar organic solvents like dimethylformamide, ethanol, methanol, propan-2-ol, dichlorormethane and chloroform. The infrared spectrum of 7 (Figure 8.22) displays absorption bands at 1863, 1909 and 2017 cm-1, which are assigned to v(C≡O) of the robust fac-[Re(CO)3]+ unit. Absorption bands at 1608, 1583 and 1568 cm-1 are assigned to the various C═N stretching vibration frequencies in the chelate pmpa. Peaks at 434 and 472 cm-1 are ascribed to v(Re−N). In the electronic spectrum (Figure 8.23) in MeOH the intra-ligand

* transition

occurs at 296 nm, with a ligand-to-metal charge transfer (MLCT, d(Re)→π*(pmpa)) band at 482 nm.


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Absorbance

0.505 0.405 0.305 0.205 0.105 0.005 280

330

380

430

480

530

580

Wavelength (nm) Figure 8.23: UV-vis spectrum of complex 7 in MeOH In the 1H NMR spectrum the one-proton singlet at 8.84 ppm is assigned to C(1)H. The pyridyl protons produce signals at 8.62 ppm (d, C(15)), 8.04 ppm (t, C(13)), 7.96 ppm (d, C(12)) and 7.64 ppm (t, C(14)). The three protons of the pyrimidine ring led to a two-proton doublet at 7.51 ppm (H(22), H(24)) and a triplet at 7.41 ppm (H(23)).


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The molecular structure of 7 is shown in Figure 8.24. The rhenium(I) atom lies in a distorted octahedral environment, with the chloride Cl(1) and two donor atoms N(1) and N(3) of pmpa in a facial arrangement, imposed by the fac-[Re(CO)3]+ core. The distortion is mainly the result of the trans angles Cl(1)−Re−C(2) = 175.75(8)°, N(1)−Re−C(3) = 173.76(9)° and N(3)−Re−C(4) = 172.20(7)° (Table 3.12) which deviate from linearity. The chelate pmpa is neutral and coordinates to rhenium via the imino nitrogen N(3) and pyridinic nitrogen atoms N(1), forming a bite angle of 74.33(7)°. The distortion from an ideal octahedral geometry is also reflected by the angles C(3)−Re−N(3) = 100.61(9)˚, N(1)−Re−C(4) = 97.91(9)˚, Cl(1)−Re−C(4) = 92.17(8)˚ and Cl(1)−Re−N(1) = 83.66(5)˚ which show significant deviation from orthogonality.

Figure 8.24: Molecular structure of complex 7 showing 40% probability displacement ellipsoids and atom labelling. Hydrogen atoms have been omitted for clarity


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The Re−N(1) bond distance of 2.173(2) Å is shorter than the Re−N(3) bond length of 2.198(2) Å, which is slightly longer than previously reported for rhenium(I)−N(imines) bonds, which typically fall in the range 2.15−2.17 Å [26]. The bond distance Re−Cl(1) [2.4708(7) Å] is in good agreement with ReI−Cl distances reported in similar complexes [7, 26]. The bond C(1)−N(3) [1.286(3) Å] is a double bond [26] and it is confirmed by the bond angle around C(1) [N(3)−C(1)−C(11) = 118.6(2)°], which is close to the expected 120o for a sp2-hybridized carbon atom. 8.3.7. [Re(CO)3Cl(amp)2] (8) Complex [Re(CO)3Cl(amp)2] (8) was prepared by the heating under reflux of two equivalents of 2-aminopyrimidine (amp) with [Re(CO)5Cl] in toluene.

[Re(CO)5Cl] + 2 amp

Toluene / N2

[Re(CO)3Cl(amp)2] (8) + 2 CO

Reflux, 6 h

Transition metal complexes of 2-aminopyrimidine are common in the literature. A typical example is the reaction of two equivalents of amp with MX2 (X = Cl, Br; M = Co, Ni) in presence of ten-fold excess of HX, which led to the complexes [MX4(ampH)2], (ampH = 2-aminopyrimidinium) [38]. Complex 8 is air stable, yellow coloured in both liquid and solid states and it is weakly soluble in both polar and nonpolar organic solvents. The bands at 1886, 1913 and 2023 cm-1 in the infrared spectrum of 8 (Figure 8.25) are assigned to the v(C≡O) of the fac-[Re(CO)3]+ unit. The bands at 1633 and 1585 cm-1 are ascribed to v(C═N) of the pyrimidine rings. The absorption frequency at 486 cm-1 is assigned to v(Re−N). The bands of medium intensity at 2849 and 2916 cm-1 are assigned to v(N−H) of the free amino groups. In the UV-Vis spectrum in dimethylformamide (Figure 7.26) the intra-ligand

*

transition occurs at 321 nm, with a ligand-to-metal charge transfer transition (MLCT), d(Re)→π*(amp) band at 360 nm. In the 1H NMR spectrum there is a singlet at 4.02 ppm which is assigned to the NH2 protons. The two pyrimidine ring protons produce two doublets at 7.45 ppm and 8.86 ppm. Nelson Mandela Metropolitan University

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Figure 8.25: IR spectrum of complex 8 0.555

Absorbance

0.455

0.355

0.255

0.155

0.055

317

367

417

467

Wavelength (nm) Figure 8.26: UV-vis spectrum of complex 8 in DMF The crystal molecular structure of 8 (Figure 8.27) shows that the rhenium atom is coordinated to three carbonyls in a facial orientation, two neutral amp chelates, which act as monodentate ligands and coordinate to rhenium via the pyrimidyl nitrogen atoms N(11) and N(11i), and the chloride Cl(1). The angle N(11)−Re−N(11i) equals 85.58(7)˚. Nelson Mandela Metropolitan University

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There is nothing unusual about the Re−C bond lengths. The Re−N(11)/Re−N(11i) bond distance of 2.230(2) Å are in good agreement with the range of 2.15−2.22 Å reported in the literature for rhenium(I)−N(imines) distances [26]. The bond distance Re−Cl(1) [2.476(2) Å] is close to that in complex 7.

Figure 8.27: Molecular structure of complex 8 showing 40% probability displacement ellipsoids and atom labelling The distortion from an ideal octahedron results in the trans angles [Cl(1)−Re−C(2) = 178.6(2)o, N(11)−Re−C(1i) = 178.14(8)o, N(11i)−Re−C(1) = 178.14(8)o] being nonlinear, and to the deviation from orthogonality of the bond angles Cl(1)−Re−N(11) = 92.25(6)˚, Cl(1)−Re−C(1) = 94.25(9)˚, C(1)−Re−C(2) = 87.1(2)˚, N(11)−Re−C(1) = 92.56(8)˚, N(11i)−Re−C(2) = 92.73(19)˚, C(1i)−Re−C(2) = 92.5(2)˚. The protons on nitrogen atom N(13) are involved in intramolecular hydrogen-bonds with the nitrogen atom N(12) and chloride atom Cl(1) (see Table 8.1).


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Table 8.1: Hydrogen-bond distance (Å) and angle (o) in complex 8 D−H∙∙∙A N(13)−H(13A)∙∙∙N(12)

D−H 0.88(2)

H∙∙∙A 2.13(2)

D∙∙∙A 2.999(3)

D−H∙∙∙A 171(2)

N(13)−H(13B)∙∙∙Cl(1)

0.86(2)

2.45(2)

3.192(3)

144.6(2)

Figure 8.28: Crystal packing diagram showing intramolecular hydrogen-bonds in complex 8 (blue-dashed) 8.3.8. trans-[ReOI2(pppa)] (9) The rhenium(V) complex trans-[ReOI2(pppa)] (9) (pppa = N-(-3-(pyrimidin-2ylimino)propylidene)pyrimidin-2-amine) ion) was prepared by the heating under reflux

of


with

a

two-fold

molar

excess

of

2-

aminopyrimidine (amp) in propan-2-ol with 5 drops of triethylamine (Et3N). i

trans-[ReOI2(OEt)(PPh3)2] + 2 amp

PrOH / Et3N Reflux, 8 h


trans-[ReOI2(pppa)] + 2 PPh3 + H2O +EtOH

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The chelating ion pppa was formed by a combined oxorhenium(V)-catalysed condensation and dehydroxylation of 2-aminopyrimidine and propane-2-ol (Scheme 8.2), and the pppa coordinates to the [ReO]3+ core as a monoanionic tetradentate N,N,N,N-donor chelate via the two neutral imino nitrogens and two neutral pyrimidinic nitrogens. Complex 9 is sparingly soluble in polar organic solvents such as dichloromethane, alcohol, chloroform and acetonitrile, to give a yellow solutions. The IR spectrum (Figure 8.29) shows an absorption peak at 909 cm-1, which corresponds to ν(Re═O). The vibration frequencies at 464 and 506 cm-1 are ascribed to Re−N. The peaks at 1589 and 1627 cm-1 are assigned to ν(C═N) of the coordinated pyrimidinic rings, and the one at 1561 cm-1 to ν(C═N) in the ligand backbone.

76

% Transmittance

66 56 46 36 26 16 6 1600

1400

1200

1000

800

600

400

Wavenumber (cm-1) Figure 8.29: IR spectrum of complex 9 In the electronic spectrum (Figure 8.30) in dimethylsulfoxide there are two absorption bands at 333 and 425 nm, which are due to a

* transition in the coordinated

ligand and a ligand-to-metal charge transfer transition [pπ(N)→d*π(Re)]. The absorption band at 470 nm reflects a (dxy)2→(dxy)1(dxz)1 transition.


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In the 1H NMR spectrum, a two-proton doublet the furthest down field at 8.23 ppm is assigned to the magnetically equivalent protons H(12) and H(22). The one-proton triplet is assigned to H(2). The assignment of the other signals is given in paragraph 8.2.9. 0.48 0.43

Absprbance

0.38 0.33 0.28 0.23 0.18 0.13 0.08 323

373

423

473

523

573

Wavelength (nm) Figure 8.30: UV-vis spectrum of complex 9 in DMSO

Figure 8.31: 1H NMR spectrum of complex 9 in CDCl3 Nelson Mandela Metropolitan University

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The structure of 9 is shown in Figure 8.32. The rhenium(V) atom is sevencoordinated and lies in a distorted pentagonal bipyramidal environment. The chelating ligand pppa coordinates to the [ReO]3+ core as a tetradentate N,N,N,Ndonor chelate and binds to the metal-center via the neutral imino nitrogens N(1) and N(2) and pyrimidinic nitrogens N(11) and N(12). There are also two anionic iodide ligands and a dianionic oxo ligand O(1). The two iodide ligands I(1) and I(2) lie in trans axial positions each other [I(1)−Re−I(2) = 168.45(1)°]. The carbon atom C(2) is catalytically dehydroxylated and a negative charge is delocalised over the N(1)C(1)C(2)C(3)N(2) ligand backbone. The overall effect is that the complex is neutral.

Figure 8.32: Molecular structure of complex 9 showing 40% probability displacement ellipsoids and atom labelling The N(11)−Re−N(21) bond angle [168.9(1)°] is remarkably linear, and the bite angles of

pppa

are

N(11)−Re−N(1)

=

58.8(1)°,

N(2)−Re−N(21)

=

59.1(1)°

and

N(1)−Re−N(2) = 73.1(1)°. The former two angles are small due to the formation of a four-membered metallocycle, which allows space for the oxo group O(1) to Nelson Mandela Metropolitan University

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the

pentagonal

plane.

The

O(1)−Re−N(11)

[84.6(1)°]

and

O(1)−Re−N(21) [84.4(1)°] are practically identical. All these latter five angles deviate from the ideal of 72o for a perfect pentagonal geometry. The bond lengths Re−N(1) and Re−N(2) are identical [2.189(2) Å] and are at the higher end of the range observed [2.05−2.12 Å] for ReV−N(imine) bonds [39]. The Re−N(11) and Re−N(21) bond lengths of 2.149(2) Å and 2.171(2) Å are typical for ReV−N(pyridyl or pyrimidyl) bond distances [3, 8]. The Re═O(1) bond length of 1.694(3) Å falls in the expected range for octahedral complexes. The Re−I(1) and Re−I(2) bond lengths equal to 2.7332(3) Å and 2.7338(3) Å respectively. The bond lengths N(1)−C(1) [1.323(6) Å] and N(2)−C(3) [1.324(5) Å] are double bonds lengthened somewhat due to coordination. The bond lengths C(1)−C(2) [1.387(6) Å] and C(2)−C(3) [1.381(6) Å] are single bonds. Pentagonal bipyramidal (pbp) complexes are not known for oxorhenium(V), and complex 9 is the first such example. They are known for rhenium(III) however. The reaction of [ReCl3(MeCN)(PPh3)2] with the planar pentadentate diacetylpyridine-bis(benzoylhydrazone) ligand H2DAPB and Et3N in propan-2-ol gave the sevencoordinated complex [ReIIICl(DAPB)(PPh3)], with the chloride and PPh3 in trans axial positions [40]. The pbp complex [ReIIIH2(hq)(PPh3)3] (hq = 2-hydroxyquinoline monoanion) was synthesised from [ReVH4(hq)(PPh3)2] with PPh3 [41].

8.4. Conclusion Complexes of the [ReO]3+ and fac-[Re(CO)3)]+ cores were studied with (pyridin-2yl)methanol derivatives, 2-aminopyrimidine (amp) and its Schiff base derives N((pyridin-2-yl)methylene)pyrimidin-2-amine

(pmpa)

and

N-(-3-(pyrimidin-2-

ylimino)propylidene)pyrimidin-2-amine) ion (pppa). The reaction of pmpa with different rhenium(V) and (I) starting materials in alcohols and acetone was investigated, and the ligand has decomposed giving a variety of bidentate N,O-donor (pyridin-2-yl)methanol derivatives: ethoxy(pyridin-2-yl)methanol (Hepm), methoxy(pyridin-2-yl)methanol (Hmpm), isopropyloxy(pyridin-2-yl)methanol (Hispm), (isopropylamino)(pyridin-2-yl)methanol (Hiapm) and 4-hydroxy-4-(pyridin-2yl)butan-2-one

(Hhpbo).

The

complexes

[ReOCl2(epm)(PPh3)]

(1),

[ReOCl2(mpm)(PPh3)] (2) and [ReOCl2(iapm)(PPh3)] (3) were isolated from the Nelson Mandela Metropolitan University

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reaction of pmpa with [ReOCl3(PPh3)2] in ethanol, methanol and propan-2-ol respectively. The complex [ReOBr2(ispm)(PPh3)] (5) was obtained from the reaction of pmpa with [ReOBr3(PPh3)2] in propan-2-ol. The ligands in 1, 2, 3 and 5 coordinated as a monoanionic via the alcoholate oxygen and pyridinic nitrogen. The dimeric rhenium(I) complexes [Re(CO)3(mpm)]2 (4) and [Re(CO)3(hpbo)]2 (6) were obtained from the reaction of pmpa with [Re(CO)5Cl] in methanol and acetone respectively. The hpbo− and mpm− anions in 4 and 6 coordinated via the pyridinic nitrogen and the bridging alcoholate oxygen. The reaction of pmpa with [Re(CO)5Cl] in toluene was also investigated and led to the complex [Re(CO)3Cl(pmpa)] (7). The pmpa in 7 acted as a neutral bidentate N,Ndonor and coordinated via the pyridinic and imino nitrogen atoms. The amp chelate was reacted with [Re(CO)5Cl] in toluene and the complex [Re(CO)3Cl(amp)2] (8) was isolated. The amp in 8 was monodentately coordinated to rhenium via pyrimidinic nitrogen atom. The reaction of amp with [ReOI 2(OEt)(PPh3)2] in propan-2-ol in the presence of triethylamine

produced the seven-coordinated

V

pentagonal bipyramidal complex [Re OI2(pppa)] (9). The pppa was formed by a combined oxorhenium(V)-catalysed condensation and dehydroxylation of 2aminopyrimidine and propan-2-ol, and coordinated as a neutral tetradentate via the two pyrimidinic and two imino nitrogens.

8.5. References [1]

B. Machura, M. Wolff, A. Switlicka, J. Palion, R. Kruszynski, J. Mol. Struct., 994, 256, 2011.

[2]

A.N. Srivastava, N.P. Singh, C.K. Chriwastaw, J. Serb. Chem. Soc., 79, 421, 2014; M.L. Tong, L. Zhuo-Jia, W. Li, Z. Shao-Liang, C. Xiao-Ming, Cryst. Growth Des. , 2, 443, 2002.

[3]


[4]


[5]



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[6]


[7]


[8]


[9]

B. Machura, M. Wolff, A. Switlicka, Inorg. Chem. Commun., 14, 17, 2011.

[10]

M. Hazra, T. Dolai, A. Pandey, S.K. Dey, A. Patra, Bioinorg. Chem. Appl., 2014, 2, 2014.

[11]

J. Garcia-Tojal, A. Garcia-Orad, A.A. Diaz, J. Inorg. Biochem., 84, 271, 2001.

[12]

E.A. Bakhite, A.A. Abd-Ella, M.E.A. El-Sayed, S.A.A. Abdel-Raheem, J. Agric. Food Chem., 62, 9982, 2014.

[13]

M.H. Geziginci, A.R. Martin, S.G. Franzablau, J. Med. Chem., 44, 1560, 2001.

[14]

A.E. Amr, H.H. Sayeda, M.M. Abdulla, Arch. Pharm. Chem. Life Sci., 338, 433, 2005.

[15]

M.T. Cocco, C. Congiu, V. Lilliu, V. Onnis, Bioorg. Med. Chem, 15, 1859, 2007.

[16]

C. Willman, R. Grunert, P.J. Bednraski, R. Troschutz, Bioorg. Med. Chem, 17, 4406, 2009.

[17]

B.N. Acharya, D. Thavaselvam, M.P. Kaushik, Med. Chem. Res., 17, 487, 2008.

[18]

R.H. Bahekar, M.R. Jain, P.A. Jadava, V.M. Prajapati, D.N. Patel, A.A. Gupta, A.Charma, R. Tom, D. Bandyopadhya, H. Modi, P.R. Patel, Bioorg. Med. Chem, 15, 6789, 2007.

[19]

Y. Kim, J.C. Hackett, R.W. Brueggemeier, J. Med. Chem., 47, 4032, 2004.

[20]

G.M. Badger, The Chemistry of Heterocyclic Compounds, Academic Press, New York and London, 223, 1961.

[21]

B. Butelmann, A. Alanine, A. Bourson, R. Gill, M. Heitz, V. Mutel, E. Pinard, G. Trube, R. Wyler, Bioorg. Med. Chem. Lett., 13, 829, 2003.


296

Chapter 8

J.Mukiza

[22]

A.L. Blake, MSc Dissertation, Georgia State University, 2010.

[23]

R. Dudhe, P.K. Sharma, P.Verma, A. Chaudhary, J. Adv. Sci. Res.,2, 10, 2011.

[24]

M. Sonmez, M. Celebi, I. Berber, Eur. J. Med. Chem., 45, 1935, 2010; M. Hazra, T. Dolai, A. Pandey, S.K. Dey, A. Patra, Bioinorg. Chem. Appl., 2, 2014, 2014.

[25]

B. Machura, M. Wolff, J. Mrozinski, R. Kruszynski, J. Kusz, Polyhedron, 28, 2 377, 2009.

[26]


[27]


[28]


[29]

N.C. Yumata, MSc Dissertation, Nelson Mandela Metropolitan, University, 2010.

[30]

A. Abrahams, I. Booysen, T.A.I. Gerber, P. Mayer, Bull. Chem. Soc. Ethiop., 22, 247, 2008.

[31]

F.A. Cotton, S.J. Lippard, Inorg. Chem., 4, 1621, 1965.

[32] A. Abrahms, PhD Thesis, Nelson Mandela Metropolitan University, 2009. [33]

F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G. Orpin, R. Taylor J. Chem. Soc., Perkin Trans., II, 3, 1987.

[34] C.B. Hubschle, G.M. Sheldrick, B. Dittrich, J. Appl. Cryst., 44, 1281, 2011. [35] W. Wang, B. Spingler, R. Alberto, Inorg. Chim. Acta, 355, 386, 2003. [36]


[37]

A.S. Mohamed Goher, Polyhedron, 12, 1751, 1993.

[38]

M.E. Masaki, B.J. Prince, M.M. Turnbull, J. Coord. Chem., 55, 1337, 2002.

[39]

K.C. Potgieter, MSc Dissertation, Nelson Mandela Metropolitan University,


297

Chapter 8

J.Mukiza

2009. [40]

A.S.M. Al-Shihri, J.R. Dilworth, S.D. Howe, J. Silver, R.M. Thompson, J. Davies, D.C. Povey, Polyhedron, 12, 2297, 1993.

[41]

T.M. McKinney, P.E. Fanwick, R.A. Walton, Inorg. Chem., 38, 1548, 1999.


2

Formula

C26H25Cl2NO3ReP

C25H23Cl2NO3ReP

Formula Weight

687.55

673.52

Crystal System

Monoclinic

Monoclinic

Space group

P21/n

P21/n

a (Å)

10.3511(3)

10.2032(4)

b (Å)

14.9427(5)

14.6636(5)

c (Å)

17.0443(6)

16.9408(6)

β (deg.)

105.206(2)

105.720(2)

Volume (Å3)

2544.00(15)

2439.81(16)

Z

4

4

Density (g/cm3)

1.795

1.834


5.078

5.293

F(000)

1344

1312

θ range (deg.)

2.1-28.3

2.1-28.3

Index ranges h

-13/13

-13/13

k

-18/19

-18/19

l

-22/22

-22/22

Reflection measured

23724

29655


6328/5554

6081/5401

Data/parameters

6238/338

6081/299


1.06

1.03


0.0246

0.0156

(wR2 = 0.0577)

(wR2 = 0.0360)

2.19/-1.19

0.69/-0.36



298

Chapter 8

J.Mukiza


4

Formula

C27H27Cl2N2O2ReP

C20H16N2O10Re2

Formula Weight

699.59

816.77

Crystal System

Monoclinic

Triclinic

Space group

P21/c

P-1

a (Å)

11.8234(3)

7.6661(4)

b (Å)

11.7104(3)

9.1921(4)

c (Å)

19.6971(4)

9.5159(4)

α (deg.)

, 63.135(2)

β (deg.)

97.698(1)

γ (deg.)

89.313(2) 74.620(2)

Volume (Å3)

2702.62(11)

572.22(5)

4

1

Density (g/cm )

1.719

2.370


4.780

10.626

F(000)

1372

380

θ range (deg.)

1.7-28.3

2.4-27.9

Index ranges h

-15/15

-10/10

k

-15/15

-12/12

l

-22/26

-11/12

Reflection measured

50471

9438


6731/5623

2683/2541

Data/parameters

6731/318

2683/155


1.09

1.15


0.0293

0.0230

(wR2 = 0.0744)

(wR2 = 0.0627)

2.55/-0.74

2.06/-1.84

Z 3



299

Chapter 8

J.Mukiza


6

Formula

C27H27Br2NO2ReP

C24H20N2O10Re2

Formula Weight

740.48

868.84

Crystal System

Monoclinic

Triclinic

Space group

P21/c

P-1

a (Å)

12.0096(2)

9.0620(4)

b (Å)

11.9254(2)

9.1856(4)

c (Å)

19.6960(3)

17.3177(8)

α (deg.)

75.375(2)

β (deg.)

97.525(1)

γ (deg.)

78.857(1) 70.693(1)

Volume (Å3)

2796.55(8)

1306.8(1)

Z

4

2

Density (g/cm3)

1.878

2.208


7.288

9.313

F(000)

1520

816

θ range (deg.)

2.0-28.4

1.2-28.3

Index ranges h

-15/12

-12/11

k

-15/14

-12/12

l

-26/26

-22/23

Reflection measured

40478

23633


6962/5568

6471/5898

Data/parameters

6962/318

6471/345


1.00

1.16


0.0240

0.0131

(wR2 = 0.0509)

(wR2 = 0.0292)

1.29/-0.52

0.83/-0.61

-3



300

Chapter 8

J.Mukiza


8

Formula

C13H8ClN4O3Re

C11H10ClN6O3Re

Formula Weight

489.89

495.91

Crystal System

Monoclinic

Orthorhombic

Space group

P12/n

Fddd

a (Å)

12.2165(6)

8.0431(3)

b (Å)

8.9879(4)

30.197(1)

c (Å)

13.3487(6)

30.826(1)

β (deg.)

100.657(2)

Volume (Å3)

1440.4(1)

7486.5(2)

Z

4

2

Density (g/cm3)

2.259

1.760


8.659

6.652

F(000)

920

3744

θ range (deg.)

2.1-28.3

1.9-28.3

Index ranges h

-16/16

-10/10

k

-11/11

-39/40

l

-17/13

-40/33

Reflection measured

26421

18094


3581/3237

2343/2094

Data/parameters

3581/199

2343/122


1.08

1.08


0.0151

0.0126

(wR2 = 0.0318)

(wR2 = 0.0317)

0.74/-0.46

0.65/-0.50

-3



301

Chapter 8

J.Mukiza

Table 8.6: Crystal and structure refinement data for complex 9 Crystal Data Formula

C11H9l2N6ORe

Formula Weight

681.25

Crystal System

Triclinic

Space group

P-1

a (Å)

7.7201(4)

b (Å)

9.1988(5)

c (Å)

11.1480(6)

α (deg.)

93.219(2)

β (deg.)

95.029(2)

γ (deg.)

102.760(2)

Volume (Å3)

766.83(7)

Z

2

Density (g/cm3)

2.951


11.956

F(000)

612

θ range

1.8−28.4

Index ranges h

-10/10

k

-11/11

l

-14/14

Reflection measured

13312


3841/3778

Data/parameters

3841/191


1.09

Final R indices [I>2σ>(I)]

0.0247 (wR2 = 0.0669)



2.69/-3.11

302

Chapter 8

J.Mukiza

Table 8.7: Selected bond lengths (Å) and angles (°) for complex 1 and 2 Bond lengths 1 Re(1)−Cl(1)

2.3594(9)

2 Re(1)−Cl(1)

2.3581(7)

Re(1)−O(4)

1.696(2)

Re(1)−O(3)

1.69(2)

Re(1)−P(1)

2.4615(9)

Re(1)−N(1)

2.145(2)

O(1)−C(1)

1.394(5)

O(2)−C(1)

1.395(3)

N(1)−C(11)

1.341(5)

N(1)−C(12)

1.351(3)

Re(1)−Cl(2)

2.4169(9)

Re(1)−Cl(2)

2.4196(6)

Re(1)−O(1)

1.915(2)

Re(1)−O(1)

1.9212(2)

Re(1)−N(1)

2.150(3)

Re(1)−N(1)

2.145(2)

O(3)−C(1)

1.35(1)

O(2)−C(1)

1.395(3)

N(1)−C(12)

1.347(5)

N(1)−C(12)

1.351(3)


169.70(8)

2 Cl(1)−Re(1)−N(1)

174.11(2)

O(1)−Re(1)−O(4)

161.1(1)

O(1)−Re(1)−O(3)

87.09(7)

O(4)−Re(1)−N(1)

86.8(1)

O(1)−Re(1)−N(1)

75.31(6)

P(1)−Re(1)−O(1)

85.92(7)

P(1)−Re(1)−O(3)

89.29(2)

Cl(2)−Re(1)−N(1)

86.84(8)

Cl(2)−Re(1)−N(1)

95.42(4)

Cl(2)−Re(1)−O(4)

96.46(8)

Cl(2)−Re(1)−O(3)

107.9(2)

Cl(2)−Re(1)−P(1)

174.44(3)

Cl(2)−Re(1)−P(1)

174.11(2)

O(1)−Re(1)−N(1)

73.3(1)

O(3)−Re(1)−N(1)

87.09(7)

P(1)−Re(1)−N(1)

91.33(8)

P(1)−Re(1)−N(1)

90.97(5)

Cl(1)−Re(1)−Cl(2)

89.31(3)

Cl(1)−Re(1)−Cl(2)

89.29(2)

Cl(1)−Re(1)−O(4)

103.18(7)

Cl(1)−Re(1)−O(1)

95.42(4)

O(1)−C(1)−C(11)

109.1(3)

O(1)−C(1)−C(11)

107.9(2)


303

Chapter 8

J.Mukiza

Table 8.8: Selected bond lengths (Å) and angles (°) for complex 3 and 4 Bond lengths 3 Re(1)−Cl(1)

2.425(1)

4 Re(1)−O(1)

2.144(4)

Re(1)−O(2)

1.702(2)

Re(1)−C(3)

1.904(6)

Re(1)−P(1)

2.447(1)

Re(1)−C(5)

1.915(5)

O(1)−C(1)

1.399(5)

O(1)−C(1)

1.419(6)

N(2)−C(2)

1.455(6)

N(1)−C(12)

1.341(7)

Re(1)−Cl(2)

2.349(1)

Re(1)−N(1)

2.190(4)

Re(1)−O(1)

1.914(2)

Re(1)−C(4)

1.898(6)

Re(1)−N(1)

2.145(4)

Re(1)−O(1i)

2.175(3)

N(2)−C(1)

1.381(6)

O(2)−C(1)

1.381(7)

N(1)−C(11)

1.350(6)

N(1)−C(11)

1.347(6)


168.7(1)

4 O(1)−Re(1)−C(4)

169.5(2)

O(1)−Re(1)−O(2)

161.7(1)

O(1i)−Re(1)−C(3)

174.3(2)

O(1)−Re(1)−N(1)

75.7(2)

C(3)−Re(1)−C(5)

85.6(2)

P(1)−Re(1)−O(2)

88.43(8)

N(1)−Re(1)−C(4)

98.4(2)

Cl(1)−Re(1)−N(1)

87.2(1)

O(1i)−Re(1)−C(4)

97.2(2)

Cl(2)−Re(1)−O(1)

93.29(9)

O(1)−Re(1)−N(1)

74.0(2)

Cl(1)−Re(1)−P(1)

177.05(3)

N(1)−Re(1)−C(5)

172.2(7)

O(2)−Re(1)−N(1)

86.5(1)

O(1i)−Re(1)−N(1)

88.9(1)

P(1)−Re(1)−N(1)

91.7(1)

O(1)−Re(1)−C(5)

100.0(2)

Cl(1)−Re(1)−Cl(2)

90.38(4)

O(1)−Re(1)−C(3)

100.5(2)

Cl(2)−Re(1)−O(2)

104.6(1)

O(1)−Re(1)−O(1i)

75.1(1)

O(1)−C(1)−C(11)

108.3(4)

O(1)−C(1)−C(11)

109.0(4)


304

Chapter 8

J.Mukiza

Table 8.9: Selected bond lengths (Å) and angles (°) for complex 5 Bond lengths Re(1)−Br(2)

Re(1)−Br(1)

2.4935(6)

Re(1)−O(3)

1.699(2)

Re(1)−O(1)

1.918(2)

Re(1)−P(1)

2.4523(8)

Re(1)−N(1)

2.154(3)

O(1)−C(1)

1.404(4)

O(2)−C(1)

1.382(4)

N(1)−C(15)

1.347(4)

N(1)−C(11)

1.355(4)

Br(1)−Re(1)−N(1)

Bond angles 169.50(7) Br(2)−Re(1)−P(1)

176.62(2)

O(1)−Re(1)−O(3)

161.9(1)

Br(1)−Re(1)−O(3)

103.83(7)

P(1)−Re(1)−N(1)

91.63(7)

P(1)−Re(1)−O(1)

88.00(6)

Br(2)−Re(1)−O(3)

93.86(7)

Br(1)−Re(1)−Br(2)

91.01(2)

P(1)−Re(1)−O(3)

89.06(7)

O(1)−C(1)−C(11)

108.4(3)

Br(2)−Re(1)−O(1)

88.70(6)

Re(1)−O(1)−C(1)

124.8(2)


2.5796(4)

305

Chapter 8

J.Mukiza


2.143(1)

Bond lengths Re(1)−O(21)

Re(1)−N(11)

2.182(2)

Re(1)−C(50)

1.901(3)

Re(1)−O(51)

1.915(5)

Re(1)−C(52)

1.888(2)

Re(2)−O(11)

2.184(2)

Re(2)−O(21)

2.137(2)

Re(2)−O(21)

2.137(2)

Re(2)−N(21)

2.179(2)

Re(2)−C(60)

1.915(3)

Re(2)−C(61)

1.891(3)

O(22)−C(28)

1.196(4)

O(12)−C(18)

1.207(4)

O(11)−C(16)

1.423(3)

O(21)−C(26)

1.420(3)

N(21)−C(21)

1.351(3)

N(11)−C(15)

1.350(3)

2.174(2)

O(11)−Re(1)−C(50)

Bond angles 169.66(9) O(21)−Re(1)−C(52)

173.7(1)

N(11)−Re(1)−C(51)

175.1(1)

N(11)−Re(1)−C(52)

95.5(1)

O(21)−Re(1)−C(51)

92.03(9)

O(21)−Re(1)−N(11)

86.03 (6)

C(50)−Re(1)−C(51)

89.2(1)

C(51)−Re(1)−(52)

85.9(1)

O(11)−Re(1)−O(21)

75.01(6)

O(11)−Re(1)−N(11)

75.34(7)

O(11)−Re(2)−C(61)

176.15(9)

O(21)−Re(2)−C(62)

167.41(9)

N(21)−Re(2)−C(60)

174.83(9)

C(60)−Re(2)−C(61)

86.3(1)

N(21)−Re(2)−C(61)

91.6(1)

O(11)−Re(2)−N(21)

88.36(6)

O(12)−C(18)−C(17)

120.9(3)

O(22)−C(28)−C(27)

120.2 (3)

O(21)−C(26)−C(27)

109.4(2)

O(11)−C(16)−C(17)

110.0 (2)


306

Chapter 8

J.Mukiza

Table 8.11: Selected bond lengths (Å) and angles (°) for complexes 7and 8 Bond length 7

8

Re−Cl(1)

2.4708(7)

Re−Cl(1)

2.476(2)

Re−N(1)

2.173(2)

Re−N(11)

2.230(2)

Re−N(3)

2.198(2)

Re−N(11i)

2.230(3)

Re−C(2)

1.932(3)

Re−C(1)

1.913(2)

Re−C(3)

1.932(3)

Re−C(1i)

1.913(2)

Re−C(4)

1.902(3)

Re−C(2)

1.875(6)

C(1)−N(3)

1.286(3

N(11)−C(11)

1.346(3)

Bond angles 7 Cl(1)−Re−C(2)

175.75(8)

8 Cl(1)−Re−C(2)

178.6(2)

N(1)−Re−C(3)

173.76(9)

N(11)−Re−C(1i)

178.14(8)

N(3)−Re−C(4)

172.20(9)

N(11i)−Re−C(1)

178.14(8)

C(3)−Re−N(3)

100.61(9)

N(11)−Re−N(11i)

85.58(7)

N(3)−Re−N(1)

74.33(7)

Cl(1)−Re−N(11)

92.25(6)

Cl(1)−Re−C(4)

92.17(8)

N(11)−Re−C(1)

92.56(8)

Cl(1)−Re−N(1)

83.66(5)

C(1)−Re−C(2)

87.1(2)

C(2)−Re−C(3)

90.7(1)

N(11i)−Re−C(2)

92.7(2)

N(3)−C(1)−C(11)

118.6(2)

C(1i)−Re−C(2)

92.5(2)


307

Chapter 8

J.Mukiza

Table 8.12: Selected bond lengths (Å) and angles (°) for complex 9 Re−I(1)

Bond lengths 2.7332(3) Re−I(2)

Re−O(1)

1.694(3)

Re−N(1)

2.189(3)

Re−N(2)

2.189(3)

Re−N(11)

2.149(4)

Re−N(21)

2.171(4)

N(1)−C(1)

1.323(6)

N(2)−C(3)

1.324(5)

C(1)−C(2)

1.387(6)

C(2)−C(3)

1.381(6)

N(2)−C(21)

1.381(5)

I(1)−Re−I(2)

Bond angles 168.45(1) N(11)−Re−N(21)

168.9(1)

O(1)−Re−N(1)

143.44(1)

O(1)−Re−N(2)

143.4(1)

N(1)−Re−N(2)

73.1(1)

N(1)−Re−N(11)

58.8(1)

N(1)−Re−N(21)

132.2(1)

N(2)−Re−N(11)

131.9(1)

N(2)−Re−N(21)

59.1(1)

O(1)−Re−N(11)

84.6(1)

O(1)−Re−N(21)

84.4(1)

I(1)−Re−O(1)

96.3(1)

I(1)−Re−N(1)

85.47(8)

I(2)−Re−O(1)

95.2(1)

I(2)−Re−N(11)

88.41(1)

C(1)−N(1)−C(11)

123.1(3)

C(3)−N(2)−C(21)

123.1(3)

C(1)−C(2)−C(3)

122.3(4)


2.7338(4)

308

Chapter 9

J.Mukiza

Chapter 9 Conclusion and Future Work This study presents the successful synthesis of rhenium(I), (III), (IV) and (V) complexes with the various multidentate ligands containing the amino, imino, hydroxyl and thiol groups. These complexes have a variety of structural diversity, such as monomers, ligand-bridged dimers and metal-metal multiply bonded species. They also display exceptional stability and some of them have shown unusual chemical properties. Chapter 3 presents the dimeric and monomeric rhenium complexes with orotic acid and 2-mercapto-orotic acid, and these ligands have shown unusual coordination modes to rhenium. This work can be extended further by repeating the reactions by using benzimidazole, benzothiazole and carboxamide derivatives of orotic acid and 2-mercapto-orotic acid (Figure 7.1). These ligands can be obtained by methods in the literature for the synthesis of benzimidazole, benzothiazole and carboxamide derivatives.

R

R N

X

HN

N

X

HN

O

N

HN N

HN

O O

N H

O

HS

N

OH

O

O

N H

HS

OH

N H

Carboxamide derivatives

X = NH, benzimidazole derivatives

R = alkyl or functionalised groups

X = S, benzothiazole derivatives Figure 7.1: The chelating ligand derivatives of orotic acid and 2-mercapto-orotic acid proposed to be used in the future


309

Chapter 9

J.Mukiza

Chapter 4 reports the reactivity of the 5-amino-orotic acid (H2aoa) and its Schiff base derivative 5-(2-hydroxybenzylideneamino)-1,2,3,6-tetrahydro-2,6-dioxopyrimidine-4carboxylic acid (H2soa) to oxorhenium(V), as well as its carboxamide derivative 5-(5aminopyrimidine-2,4(1H,3H)-dioxamido)-1,2,3,6-tetrahedro-2,6-dioxopyrimidine-4carboxylic acid (H2ampa) to [ReI(CO)5Cl]. For extension of the work in Chapter 4, the synthesis of more Schiff bases and carboxamide derivatives of H2aoa as well as their reactions with oxorhenium(V) and [ReI(CO)5Cl] are recommended. Hydroxypicolinic acids as ligands have attracted attention since they display a variety of bonding modes. In Chapter 5, the reactions of trans-[ReOX3(PPh3)2] (X =Cl, Br) and cis-[ReO2I(PPh2)3] with 6-hydroxypicolinic acid was investigated, and led to the rhenium(III) and (V) monomers as well as rhenium(IV) dimers, and all of these complexes have shown triphenylphosphine as ligand. This work can be extended by reacting the 6-hydroxypicolinic acid with the rhenium(V) precursors which do not contain the triphenlyphosphine group, such as trans-[ReO2(py)4]Cl and (nBu4N)[ReOCl4]. These will undoubtedly result in rhenium complexes different from those reported in Chapter 5. This work can also be extended by the synthesis of benzothiazole, benzimidazole and carboxamide derivatives of 6-hydroxypicolinic acid (Figure 7.2), and reacting them with the various oxorhenium(V) and (I) precursors.

X

O

OH N

N

OH

HN

N

R

X = NH, benzimidazole derivative X = S, benzothiazole derivative

Carboxamide derivatives R = alkyl or functionalised groups

Figure 7.2: The chelating ligand derivatives of 6-hydroxypicolinic acid proposed to be used in the future Dithizone (H2dtz), phenothiazine and thiourea derivatives as well as their rhenium complexes have biological and medicinal applications. Chapter 6 reports the coordination modes of these ligands to the [ReO]3+ and fac-[Re(CO)3]+ cores. This work can be extended by the further synthesis of a variety of thiourea derivatives and Nelson Mandela Metropolitan University

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reacting them with various rhenium(V) and (I) precursors. These could lead to the new rhenium complexes and enhance the advanced attachment of biological molecules. For the typical reaction of dithizone with trans-[ReOBr3(PPh3)2] in the presence of KSCN, the ligand was decomposed and the decomposition product reacted with KSCN and triphenylphosphine from rhenium starting material, giving a new N,N-donor ligand triphenylphosphazenomethinimino-N-mercaptobenzenamine Htmmb, and both dtz2− and tmmb− anions coordinate to oxorhenium(V). For extending this work, repeating this reaction using potassium cyanide KCN as well as thiourea H2NCSNH2, instead of KSCN, is recommended. In Chapter 7, the coordination mode of aroylhydrazone-based ligands to the [ReO]3+, [Re2O3]4+ and fac-[Re(CO)3]+ cores was investigated. These ligands exhibit a variety of physiological and biological activities in the treatment of several diseases. This work can be extended by synthesis of bipodal and tripodal aroylhydrazone-based ligands (Figure 7.3) and reacting them with various rhenium(V) and (I) precursors. These reactions may lead to multinuclear rhenium complexes. H N

R3 O

N

O

N

N R1

N H

O

N H

R2

H N

H N

R2

O Bipodal aroylhydrazone derivatives

R1 N

N

O

Tripodal aroylhydrazone derivatives

Figure 7.3: Aroylhydrazone-based ligands proposed to be used in the future Pyrimidine and pyridine are heterocyclic compounds and display a variety of biological activities, together with some of their transition metal complexes. Chapter 8 reports the rhenium complexes of (pyridin-2-yl)methanol derivatives, 2aminopyrimidine and its Schiff bases drerivatives. This work can be extended by the further synthesis of Schiff bases derivative of 2-aminopyrimidine using various aldehydes and ketones, and reacting them with various rhenium(V) and (I) precursors in alcohol, ketone and toluene.


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