benzimidazoyl Palladium(II) Complex

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Synthesis, Characterization and Catalytic Activity of a Novel 2-(3- aminophenyl)benzimidazoyl Palladium(II) Complex K. Lokesha, S. HariPrasada,*, B. Roopashreea,b and V. Gayathria a

Department of Studies in Chemistry, Central College Campus, Bangalore University, Bangalore-560001, India

b

Department of Chemistry, JSS Academy of Technical Education, Bangalore, Visvesvaraya Technological University, Belgaum- 590018, Karnataka, India Abstract: The synthesis and characterization of a novel aminophenyl benzimidazole Pd (II) complex is reported. The complex proved as an efficient catalyst for the Sonogashira crosscoupling reaction of ten different aryl bromides with trimethylsilylethyne to yield a wide range of aryl trimethylsilylethynes. The reaction involves use of the novel complex and triethylamine

S. HariPrasad

Keywords: Benzimidazole, binuclear complex, cross-coupling reaction, coordination induced shift, trimethylsilylethynes. 1. INTRODUCTION Palladium complexes with substituted benzimidazoles as N-donor ligands are important catalysts with applications in hydrogenation, Heck, Sonogashira and Suzuki cross-coupling reactions [1-4]. The Sonogashira coupling is a fundamental and important reaction for the synthesis of aryl acetylene derivatives [5-8], which in turn can be applied for preparing natural products [9], bioactive compounds [10] and in material sciences [11]. Several examples of Pd-catalyzed Sonogashira reactions in aqueous media have been reported. Many of these reactions are carried out in an aqueous-organic solvent mixture and in some cases, special phosphine ligands and copper salts are required in order to reach high reaction efficiency [12, 13]. Our laboratory is involved in the synthesis and reactions of some novel organosilyl based reagents. A diverse variety of organosilicon based reagents were prepared and their reactions studied [14-18]. In continuation of our investigations on *Address correspondence to this author at the Department of Studies in Chemistry, Central College Campus, Bangalore University, Bangalore-560001, India; Tel: +918022961351; Fax: +918022961331; E-mails: [email protected]

2211-5447/15 $58.00+.00

organosilyl based reagents [19, 20], the synthesis, characterization and catalytic activity of complexes containing transition metals with substituted N-heterocycles were undertaken. Benzimidazole complexes earlier prepared were screened for catalytic activities [1, 21-24] and were found to be efficient catalysts for oxidations and hydrogenations [25-29]. We now report the synthesis and characterization of the novel di-μ-bromodi[2-(3-aminophenyl) benzimidazole]di bromodipalladium(II) complex and its application in the Sonogashira cross coupling reaction of aryl bromides with trimethylsilylethyne under solvent and copper free conditions. 2. MATERIALS AND METHOD All the reagents used were of analytical grade. The solvents used were purified according to standard literature procedures. Microanalyses were carried out on a Finnegan Eager 300 elemental analyzer. IR (nujol mull) spectra were recorded on Agilent Technologies Cary 630 FTIR and Far-IR spectra were recorded on Thermo Nicolet model 6700. Electronic spectra were recorded on Shimadzu UV 3101PC spectrometer. FAB mass spectra were recorded on a JEOL SX102 mass spectrometer using argon/xenon as the FAB gas and mnitrobenzyl alcohol as the matrix. TGA were ©2015 Bentham Science Publishers

2 Current Catalysis, 2015, Vol. 4, No. 2

recorded on Perkin Elmer Diamond TG/DTA with a heating rate of 15 °C min-1 in nitrogen atmosphere. Molar conductivity measurements were made on a Systronic conductivity meter 304-cell type CD-10. 1H and 13C NMR spectra of the complex were recorded on Bruker DRX500 NMR spectrometer with tetramethylsilane (TMS) as the internal standard and DMSO-d6 as the solvent. 1H NMR and 13C NMR of novel cross coupled products were recorded on a Bruker AMX 400 spectrometer using CDCl3 with TMS as internal standard. Chemical shifts are reported in  (ppm downfield from TMS). ESI-MS were recorded on Bruker HCT ultra ETD II. . EXPERIMENTAL Synthesis of Complex [Pd2Br4(m-APB)2] and its Spectral Data PdBr2 (1 mmol) in methanol (10 mL) was treated 1 drop of HBr followed by 2 mmol of the ligand 2-(3-aminophenyl)benzimidazole (m-APB) in methanol (10 mL). The mixture was refluxed for 6 h during which a yellow colored solid separated. The solid was filtered, washed with methanol and dried in vacuo to obtain 0.630 g of complex [Pd2Br4(m-APB)2]. 65% Yield. mp 225 °C. Molar conductivity  (10-3 M solution in DMF) = 27 -1cm2mol-1; IR: 3621, 3418, 3192, 1604, 1587, 1471, 1237, 1175, 869, 785, 739, 689 cm-1; 1 H NMR (CDCl3) : 5.65 (b, 2H, NH2), 6.90 (d, 1H, J = 8.0 Hz), 7.38 (t, 1H, J = 7.6 Hz) 7.45 (t, 1H, J = 7.2 Hz), 7.55 (t, 1H, J = 7.6 Hz), 7.59 (m, 2H), 8.51(d, 1H, J = 8.0 Hz), 8.62 (d, 1H, J = 7.6 Hz) 13.50 (s, 1H, NH) ppm ; 13C NMR (CDCl3) : 112.02, 113.85, 116.90, 119.47, 122.88, 129.15, 129.31, 132.91, 140.90, 148.81, 153.00, 123.78; Anal. Found: C, 31.46; H, 3.10; N, 8.58. Calcd: C, 31.64; H, 2.66; N, 8.51. Mass of [Pd2Br4(mAPB)2] Calcd: 951.6, Found: 951.0; General Procedure for the Sonogashira Reaction and Spectral Data A mixture of aryl bromide 1a-j (1 mmol), trimethylsilylethyne (1.5 mmol), triethylamine (2.0 mmol) and the catalyst [Pd2Br4(m-APB)2] (1.4 mol%) were heated in a sealed tube placed in an oil bath at 100°C for 7 hr. The reaction was

Lokesh et al.

monitored using thin layer chromatography. After the completion of reaction, the reaction mixture was cooled to ambient temperature, diluted with diethyl ether (20 mL), filtered through celite to remove left over solids, and concentrated invacuo. The crude product was then subjected to column chromatography using silica gel (100-200 mesh) and 1:10 ethyl acetate/hexane (60-80 °C fraction) to isolate the aryl substituted trimethylsilylethynes 2a-j in greater than 80% yield. 4-(2-Trimethylsilylethynyl)-2,3-dihydroinden-1one (2a) Solid, mp = 38 °C IR: 3030, 2955, 2899, 2149, 1716, 1575, 1473, 1247, 1045, cm-1; 1H NMR (CDCl3) : 0.29 (s, 9H), 2.70-2.72 (m, 2H), 3.173.19 (2H, t, J = 5.6 Hz), 7.32-7.35 (t, 2H, J = 7.6 Hz), 7.65-7.71 (m, 2H); 13C NMR (CDCl3) : 0.08, 25.47, 36.03, 100.37, 101.01, 122.09,123.60, 127.38, 127.59, 137.10, 157.57, 193.57; ES-MS found: 229.0 Calcd:228.09 5-(2-trimethylsilylethynyl)thiophen-2-yl none (2b)

Etha-

Solid, mp = 59 °C; IR: 3071, 2957, 2899, 2148, 1651, 1434, 1356, 1326, 1268, 1244, 1166, 1036 cm-1; 1H NMR (CDCl3) : 0.027 (s, 9H), 2.54 (s, 3H), 7.18-7.19(d, 1H J = 4 Hz), 7.53-7.54(d, 1H J = 4 Hz); 13 C NMR (CDCl3) : 0.36, 26.78, 96.65, 103.11, 131.11, 131.89, 133.08, 144.53, 190.04; ES-MS found: 222.9 Calcd:222.0 3. RESULTS AND DISCUSSIONS The ligand aminophenylbenzimidazole (mAPB) was synthesized according to literature methods [30]. The palladium complex was synthesized by treating the 2-(3-aminophenyl) benzimidazole with an alcoholic solution of PdBr2. The complex formed was insoluble in common organic solvents but soluble in DMF and DMSO solvents respectively. The complex was characterized by m.p., elemental analysis, IR, electronic, 1H, 13C and 2D NMR spectral studies. A comparative account of ligand and complex are given in Table 1. The thermogravimetric analysis was carried out for the synthesized palladium complex at a heating

Synthesis, Characterization and Catalytic

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Table 1. Coordination induced shift of the [Pd2 Br4(m-APB)2]2H2O complex. C/N-H

1

H NMR

C.i.s

C-atoms

N-H

13.50s

0.86

NH2

5.65b

4

13

C NMR

c.i.s

2

153.00

0.88

0.36

4

123.78

2.0

8.62d

1.07

5

129.15

-0.23

5

7.45t

0.29

6

129.31

-0.07

6

7.38t

0.22

7

122.88

1.1

7

8.51d

0.96

8

140.90

-8.21

2'

7.59m

0.16

9

132.91

2.21

4'

6.90d

0.22

1'

112.02

0.12

5'

7.55t

0.39

2'

148.81

-0.30

6'

7.59m

0.31

4'

113.85

-0.15

5'

119.47

-2.31

6'

116.90

1.32

rate of 15 °C per minute in nitrogen atmosphere to determine the nature of water molecules associated with the complexes. The TGA for the complex was carried out up to 300 °C. The Thermogram of the complex showed % weight loss of 97.97 (theoretical weight loss; 98.18 %) at 73.68 0C and 97.50 % weight loss (theoretical weight loss; 96.3 %) at 93.42 °C which indicated the presence of lattice water molecules in the complex (Fig. 1).

Fig. (1). Thermogram of [Pd2Br4(m-APB)2]2H2O complex.

The IR spectrum of the ligand displayed a broad peak N-H of NH2 at 3350 cm-1 and N-H of benzimidazole at 3209 cm-1. C=N and C=C (ring) vibrations appeared at 1612 cm-1 and these peaks shifted on complexation indicating co-ordination of the ligand through imine nitrogen (C=N) of benzimidazole ring [31]. The far-IR spectrum of the complex showed peaks corresponding to terminal bound as well as bridging bromides in the region 227 and 190 cm-1 respectively. Pd-N for the complex was found at 256 cm-1 (Fig. 2).

Fig. (2). Far IR of [Pd2Br4(m-APB)2]2H2 O.

4 Current Catalysis, 2015, Vol. 4, No. 2

Lokesh et al. H2N 3'

4' 5'

2'

5

6'

1 HN 8

1' 2

3 N

Br Br Pd

9 4 Br

7 6

5

Pd

6 7

4

8 1 NH

9 3 N 2 1'

Br 6'

2'

5'

3' 4'

NH2

Fig. (4). Structure of di-μ-bromodi[2-(3'-aminophenyl) benzimidazole]dibromodipalladium(II) complex.

Fig. (3). Electronic spectrum of [Pd2Br4(m-APB)2] 2H2 O.

The 1H NMR spectrum of the complex displayed positive c.i.s (complex – ligand). The resonance signal of N-H appearing in the range 12.64 ppm in free ligand showed a downfield shift on complexation and appeared at 13.5 ppm. The proton at position 4- of benzimidazole ring also displayed a downfield shift on complexation indicating coordination to central palladium ion through the imine (C=N) nitrogen of benzimidazole ring. The resonance signal of NH2 protons in the ligand was found at 5.29 ppm. On complexation, this proton exhibited a downfield shift of 0.36 ppm. The c.i.s value for other protons also calculated and given in Table 1. The 13C NMR spectrum of the complex exhibited both positive and negative coordination induced shifts (Table 1). The negative c.i.s. may be attributed to greater metal-to-ligand - back donation whereas positive c.i.s. to ligandto-metal  donation. The electronic spectrum of the ligand and the complex were recorded in DMF/nujol mull. The spectrum of the ligand exhibited absorption bands at 300 and 309 nm which were assigned to n* transitions and a band at 332 nm was due to n* transition. The spectrum of the complex exhibited weak absorption band at 430 nm and molar extinction coefficient  was 450 dm3mol-1cm-1 which was assigned to 1A1g1B1g transition arising for square planar geometry of d8 Pd (II) metal ion (Fig. 3).

FAB Mass spectrum of the complex displayed molecular ion peak at 951 and supporting the binuclear nature of the complex. From this data and considering the thermogravimetric/elemental analysis, the molecular formula of the complex was assigned as [Pd2Br4(m-APB)2]2H2O. Based on all the above studies, the complex was assigned binuclear structure as shown in Fig. (4), wherein the ligand displayed monodentate behavior coordinating through imine (C=N) nitrogen of benzimidazole group. 3.1. Sonogashira Cross Coupling Reaction of Aryl Bromides Using [Pd2Br4(m-APB)2]2H2O as Catalyst In continuation of our studies on the synthesis and reactions of some silyl based reagents, we screened the novel catalyst prepared for Sonogashira reaction of aryl bromides. Recently, we reported the synthesis of some novel terminal trimethylsilylacetylene benzoate derivatives showing liquid crystalline property by employing the Sonogashira reaction [20]. In our preliminary experiments we chose the cross coupling reaction of 1.2 mmol 4bromoindanone with 1.8 mmol trimethylsilylethyne in the presence of 2.0 mol% [Pd2Br4(mAPB)2]2H2O catalyst as model reaction under sealed tube condition. We attempted to investigate the optimization of the reaction conditions regarding the base, solvent and temperature and in order to find optimum reaction conditions. When the reaction was carried out by changing the solvent from water to acetonitrile, the desired product was obtained but the reaction took 12 h for completion with 15-20% yields. Replacing the inorganic bases

Synthesis, Characterization and Catalytic

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5

Table 2. Optimization of Sonogashira cross coupling reaction of 1a with trimethylsilylethyne. O

O [Pd2Br4(m-APB)2]2H2O TMS

Et3N

Br

TMS 2a

1a

Entry

Solvent

Base

Temp °C

Time (h)

Yield

1

CH3CN

K2CO3

95

12

15

2

CH3CN

Cs2CO3

95

12

20

3

THF

Et3N

95

9

60

4

CH3CN

Et3N

95

7

85

5

-

Et3N

95

7

95

Arylbromide

TMS

[Pd2Br4(m-APB)2]2H2O, Et3N, 95 oC

Aryl

TMS

sealed tube, 6-7 h 2a-j

1a-j

Scheme (1). Sonogashira Cross-coupling reaction using [Pd2Br4(m-APB)2]2H2 O complex as catalyst. Ar Br 1a-j [Pd2Br4(m-APB)2]2H2O L Pd0

Ar

C

C 2a-j

TMS

Oxidative Addition Br L PdII Ar

Reductive elimination

C L PdII Ar

HC C

C

TMS+ Et3N

TMS C

C

TMS + Et3NH

Et3NHBr

Scheme (2). Schematic representation of a plausible mechanism for Sonogashira reaction.

like K2CO3 and Cs2CO3 by an organic base triethylamine in acetonitrile solvent in a sealed vessel, showed a progress in lowered reaction time from 12 h to 7 h and increased product yield (Table 2). We further investigated the reaction by using different mol% of catalyst [Pd2Br4(m-APB)2] 2H2O. For this, we examined the synthesis of 2a in the presence different mol% of catalyst at various temperatures. We found that 1.4 mol% of [Pd2Br4 (m-APB)2]2H2O was the most appropriate amount of catalyst for achieving the desired conversion (Table S1). Further the reaction was repeated with

ten different aryl bromides 1a-j (Scheme 1, Table 3) and the corresponding pure products 2a-j were isolated by column chromatography and characterized using IR, 1H-NMR, 13C- NMR and ES-MS. The most probable mechanism for the cross coupling reaction is outlined in Scheme (2). The catalyst precursor [Pd2Br4(m-APB)2]2H2O is converted to (m-APB)-Pd (0) in situ [6]. Oxidative addition of (m-APB)-Pd (0) by aryl bromide will form a Pd (II) intermediate. The bromide ion is then be substituted by the trimethylsilyl acetylide group. Reductive elimination forms the products: aryl substituted trimethylsilylacetylene.

6 Current Catalysis, 2015, Vol. 4, No. 2

Lokesh et al.

Table 3. Synthesis of Aryl substituted trimethylsilylethyne (2a-j), their yield and reaction time. Yield (%) Entry

Arylbromides

Products

01

Br

SiMe3

1a

Reported

6

95

-

Novel

7

80

-

Novel

6.5

85

86

[20]

6

90

81

[32]

6.5

85

89

[32]

7

81

80

[33]

7

87

87

[34]

7

83

75

[35]

6

92

-

[36]

2a O

O S

S

02

Br

SiMe3

1b

2b

HO

HO Br

O

SiMe3 O

1c

2c S

S

SiMe3

Br

04 2d

1d N

N Br

05

SiMe3

N

N

2e

1e NC

NC

06

Br

SiMe3

1f

2f NH2

NH2

07

Br

SiMe3

1g

2g OHC

OHC

08

Ref. Isolated

O

O

03

Time (h)

Br

O

O

1h

SiMe3

2h O

O Br

09 O

SiMe3 O

1i

2i

Synthesis, Characterization and Catalytic

Current Catalysis, 2015, Vol. 4, No. 2

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Table 3. contd…

Yield (%) Entry

Arylbromides

Products

10

N Cl

1j

6

[4]

[5]

[6]

CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS Authors wish to thank University Grant Commission, Government of India for financial assistance. Sophisticated Test and Instrumentation Centre (STIC), Cochin University for elemental analyses and TGA. Sophisticated Analytical Instrument Facility, Central Drug Research Centre (CDRI), Lucknow for mass spectra, Sophisticated Analytical Instrument Facility, Nuclear Magnetic Resonance Research Centre, Indian Institute of Science, Bangalore for NMR spectra.

[7] [8] [9] [10] [11]

[12]

REFERENCES

[3]

93

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[37]

2j

The synthesis and characterization of [Pd2Br4(m-APB)2] a novel complex is reported. The synthesized complex exhibited good catalytic activity towards Sonogashira coupling reaction with ten different aryl bromides and trimethylsilylethyne. The method is simple, efficient and involves copper and solvent free reaction conditions for the synthesis of a diverse range of trimethylsilyl aryl acetylenes

[2]

Reported

Cl

CONCLUSION

[1]

Ref. Isolated

SiMe3

Br N

Time (h)

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Received: March 20, 2015

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Revised: April 20, 2015

Accepted: April 08, 2015