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Arvind Kumar. Academia Sinica, Institute of Chemistry, 128 Academia Road, Sec. 2, Nankang, Taipei, 115 Taiwan, Republic of China. Received 31 July 2006; ...

 Springer 2007

Transition Metal Chemistry (2007) 32:481–493 DOI 10.1007/s11243-007-0189-3

Synthesis and spectral characterization of zinc(II), copper(II), nickel(II) and manganese(II) complexes derived from bis(2-hydroxy-1-naphthaldehyde) malonoyldihydrazone Ram A. Lal* and Debajani Basumatary Department of Chemistry, North-Eastern Hill University, Shillong, Meghalaya 793022, India Arjun K. De Department of Chemistry, Tripura University, Suryamaninagar, Tripura 799130, India Arvind Kumar Academia Sinica, Institute of Chemistry, 128 Academia Road, Sec. 2, Nankang, Taipei, 115 Taiwan, Republic of China Received 31 July 2006; accepted 11 January 2007

Abstract The present study shows that the reaction of different salts of the same metal with sterically crowded dihydrazone bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (CH2LH4) in ethanol/aqueous media gives complexes of different stereochemistry. While the reaction of zinc(II) and copper(II) sulphate with dihydrazone yields tetrahedral complexes, the zinc(II) and copper(II) chlorides give square pyramidal and distorted octahedral complexes, respectively. On the other hand, nickel(II) sulphate and chloride, both give high-spin octahedral complexes with dihydrazone, manganese sulphate gives low-spin octahedral and manganese(II) chloride gives high-spin octahedral complexes. The reaction of these complexes with KF has been investigated. All of the products have been characterized by analytical, magnetic moment and molar conductivity data. The structures of the complexes have been established by spectroscopic studies.

Introduction The chemistry of polyfunctional metal complexes has become a fascinating area of research in contemporary coordination chemistry following the discovery of multinuclear sites in several enzymes [1–4] and development of novel functional materials showing molecular ferromagnetism [5] and specific catalytic properties [6]. These properties may be modified by the character of the bridging species. Polyfunctional ligand-containing o-hydroxy aromatic aldehydes and ketones, azomethine and amide functions are of interest. Such ligands form complexes with appropriate transition metals displaying intramolecular magnetic exchanges in the dinuclear compounds. Magnetic exchange involving a M–O–M bridge has been postulated to result from a r- and p-overlap superexchange pathway involving r-/p-electrons of o-hydroxy aromatic aldehydes and ketones. Moreover, there have been numerous reports of transition metal complexes containing polyfunctional ligands derived from aromatic aldehydes and ketones [7] with two nitrogen and two oxygen donor atoms as * Author for correspondence: E-mail: [email protected]ffmail.com

well as other similar ligands [8] in which one or both aryl rings are electron-withdrawing. Dihydrazones derived from condensation of acyl, aroyl-, pyridoyldihydrazones with o-hydroxy aromatic aldehydes and ketones are related ligands possessing four oxygen and four nitrogen donor atoms [9]. As the growth of interest in the use of polyfunctional ligands containing electron-withdrawing bulky fragments in their molecular skeleton becomes more significant [7, 10], we are interested in the synthesis and characterization of the metal complexes derived from polyfunctional ligands containing electron-withdrawing bulky naphthyl fragments in their molecular skeleton and to see as if they could readily be prepared and how the chemical reactivity of these complexes varies relative to their corresponding salicylaldehyde dihydrazone complexes. In addition, as some of these complexes have potential to show several types of properties, it is interesting to explore how this feature could be modified by the presence of electron-withdrawing groups. The ligand bis(2-hydroxy-1naphthaldehyde)malonoyldihydrazone, an example of polyfunctional dihydrazones, has been selected in the present study. The ligand has been derived from condensation of 2-hydroxy-1-naphthaldehyde with malonoyldihydrazones and possesses as many as eight

482 oxygen and nitrogen donor atoms and a bulky electron withdrawing naphthyl fragment in its molecular skeleton. The naphthyl fragment, being a bulky group as compared to the salicyl group, is capable of introducing steric crowding in the molecule leading to the formation of complexes with discrete molecularity. The ligand is capable of giving rise to monometallic [11], homobimetallic [9] and heterobimetallic complexes [11]. Although, the ligand is a potential polyfunctional ligand, even then the work done on its metal complexes is quite meagre. 1:1 (metal:ligand) complexes of ZnII, CuII, NiII and MnII containing the bridging hexadentate CH2LH4 ligand in its enol form have been described [12]; it seems that no complexes containing ligand in keto form with the title metal ions are yet known. Moreover, a reactivity study of the isolated complexes has not been done previously, to the best of our knowledge. Accordingly, the present paper describes the synthesis, characterization and stereochemical investigation of metal complexes derived from reaction of zinc(II), copper(II), nickel(II) and manganese(II) sulphates and chlorides with the title ligand (CH2LH4, Figure 1). The reaction of the complexes have also been investigated with KF and the products have also been characterized.

Experimental The metal salts, diethyl malonate, hydrazine hydrate and 2-hydroxy-1-naphthaldehyde were of E-Merck grade. Malonoyldihydrazine was prepared by reacting diethyl malonate (1 mol) with hydrazine hydrate (2 mol). Bis(2-hydroxy-1-naphthaldehyde)malonoyldi hydrazone (CH2LH4) was prepared by slight modification of the procedure given by Narang and Singh [12]. The ligand was prepared by reacting a warm dilute ethanol solution of malonoyldihydrazide (1 mol) with 2-hydroxy-1-naphthaldehyde (2 mol) and was suction filtered, washed with EtOH and dried over anhydrous CaCl2 (m.p. 265 C) (Found: C, 68.5; H, 4.6; N, 13.0). Required for C25H20N4O4,C, 68.2; H, 4.6; N, 12.7(%). The estimations of zinc, copper, nickel and manganese were done following the standard literature procedures [13]. Carbon, hydrogen and nitrogen were determined microanalytically. Molecular weights of the

H C

O

H C

N

N

H H

H C 2 C

N

H O

O O

N C H

Fig. 1. Bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (CH2LH4).

complexes were determined in spectroscopic grade DMSO solution by the freezing point depression method. DMSO was kept over 4 A˚ molecular sieves prior to use. Magnetic susceptibility measurements were carried out on a vibrating sample magnetometer. The molar conductance of the complexes at 10)3 M dilution in DMSO solution were measured on a Direct Reading Conductivitymeter-303 with a dip type conductivity cell at room temperature. Infrared spectra were recorded on a paragon 500 model infrared spectrophotometer in the range 4000–350 cm)1 in KBr discs. The 1 H-NMR spectra were recorded on a EM-390, 90 MHz spectrometer and 13C-NMR spectra were recorded on Model Brucker ACF-300 at a frequency 75.16 MHz spectrometer in DMSO-d6, respectively, using TMS as an internal standard. The electronic spectra of the complexes were recorded on a Milton Roy Spectronic-21 spectrophotometer. The ESR spectra of the complexes, in powdered form as well as in CH3CN + DMSO solution at room temperature and LNT, were recorded at the X-band frequency on a Varian E-112X/Q-band spectrometer using DPPH (g = 2.0036) as an internal field marker. Preparation of the complexes [MII(CH2LH2)] [(M = ZnII (1) and CuII (4)] and [MII(CH2LH2)(H2O)2] [MII = NiII (7) and MnII (9)] A typical procedure for the preparation of these complexes is given below. NiSO4(H2O)6 (1.58 g, 6.00 mmol) in water was added to a homogeneous suspension of CH2LH4 (0.88 g, 2.0 mmol) in hot water-ethanol (50 cm3, 90:10) accompanied by gentle stirring for 20 min. The reaction mixture was than refluxed for 6 h. The precipitate so obtained was filtered hot, washed with a hot H2O–EtOH mixture, EtOH and dried over anhydrous CaCl2. Yield: 0.80–0.70 g. The ZnII, CuII and MnII complexes were also prepared essentially by the above procedure using ZnSO4 Æ 7H2O, CuSO4 Æ 5H2O, MnSO4 Æ 7H2O instead of NiSO4 Æ 6H2O, respectively. Preparation of the complexes [ZnII(CH2LH2)(H2O)](2), [MII(CH2LH2)(H2O)2] [MII = CuII(5), NiII(7), MnII(10)] These complexes were prepared by essentially the same procedure. Hence a typical procedure for their preparation is given below. ZnCl2 Æ 2H2O (1.02 g, 6.0 mmol) in EtOH (20 cm3) was added to a suspension of dihydrazone (CH2LH4) (0.83 g, 2.0 mmol) in EtOH (20 l) accompanied by gentle stirring for 20 min. The reaction mixture was then refluxed for 6 h. The precipitate so obtained was then filtered under hot conditions, washed with EtOH and dried over anhydrous CaCl2. Yield: 0.90–0.75 g.

483 Preparation of K2[Zn(CH2L)(H2O)] (3) and K2[M(CH2L)(H2O)2] [where M = Cu(6), Ni(8) and Mn(11)] The complexes were prepared by the following general procedures using the complexes (2), (5), (7) and (10) as precursors. The precursor complex (2.0 mmol) was suspended in ethanol (20 cm3) and stirred gently for 20 min to get a homogeneous slurry. To this suspension, KF (1.0 g, 10 mmol) in H2O (10 cm3) was added. The resulting mixture was refluxed for 3 h. The precipitate so obtained was filtered, washed with hot H2O, EtOH, Et2O, dried over anhydrous CaCl2 and collected. Yield: 0.85–0.73 g.

Results and discussion In the present study, the complexes have been prepared by reaction of metal salts (sulphates and chlorides) with the title ligand in a 3:1 (metal:ligand) molar ratio, either in aqueous or ethanol media. All of the complexes have 1:1 (metal:ligand) molar ratios. It is imperative to mention that while the reaction of metal sulphates with the corresponding salicylaldehyde dihydrazone, in a 3:1 molar ratio in aqueous medium, gives metal complexes having 1:1 and 2:1 (copper sulphate) molar ratio (metal:ligand) [14], the reaction of metal chlorides with ligands in a 3:1 molar ratio in ethanol yields complexes having a 2:1 (metal:ligand) molar ratio. Narang and Singh [12] synthesized polymeric complexes of the title ligand having a 1:1 (metal:ligand) molar ratio from solid–solution reactions of metal salts (M = FeII, MnII, CoII, NiII, ZnII, CdII and HgII) in a 1:1 molar ratio in ethanol, and subsequent reactions of the resulting complexes with the excess metal salts, while some monometallic and heterobimetallic complexes have been described by us [11]. The complexes together with their characterization data are given in Table 1. All of the complexes are coloured. On the basis of various elemental analyses, molecular weight determinations and various physicochemical studies, the complexes have been suggested to have 1:1 (metal:ligand) stoichiometry. They are all insoluble in water and common organic solvents such as ethanol, methanol, acetone, CCl4, CHCl3, benzene and ether while completely soluble in highly coordinating solvents such as DMSO and DMF. However, freshly prepared complexes are slightly soluble in CH3CN. The complexes are air stable and decompose above 300 C except the copper(II) complexes (4) and (5) which melt with decomposition at 292 C and 280 C, respectively. The complexes (1) and (4) show weight loss neither at 110 C nor at 180 C ruling out the possibility of presence of water molecules either in the lattice structure or in the first coordination sphere of the complexes. The remaining complexes also do not show loss of weight at 110 C, dismissing the possibility of

presence of water molecules in their lattice structure. On the other hand, the zinc(II) complexes (2) and (3) show loss of weight corresponding to one water molecule while the remaining complexes (5) to (11) show loss of weight corresponding to two water molecules at 180 C suggesting their coordination to the metal centre. The complexes (1), (2), (4), (5), (7), and (10) show molar conductance values in the region 1.8–0.5 ohm)1 cm2 mol)1 at 10)3 M dilution, consistent with their non-electrolytic nature [15]. On the other hand, the molar conductance value for the remaining complexes (3), (6), (8) and (11) lies in the region 38.6–43.3 ohm)1 cm2 mol)1. The values suggest that the complexes are ionic in nature but these values do not conclusively suggest their electrolytic nature. The molar conductance values are inconsistent with the 2:1 electrolytic nature of the complexes. The molar conductance values lower than those required for 2:1 electrolyte might be attributed to larger size of the anionic coordination sphere which again might have low ionic mobility [16]. In order to establish the structure of the complexes unequivocally with the help of X-ray crystallography, an effort was made to crystallize the complexes in various solvent systems under different experimental conditions. Both saturated and dilute solutions of the complexes in various solvent systems such as DMSO, DMF, DMSO–CH3CN, DMSO–CH2Cl2, DMF–CH3CN and DMF–CH2Cl2 each were kept for half, one and two months at ambient temperature to grow crystals. Further, the solutions were gently evaporated at 40 C, 50 C and 60 C in a hot air oven to promote crystal growth. Moreover, an effort was made to grow the crystals from the reaction mixture by layering a solution of the metal salts with a solution containing the ligand in ethanol. Solutions of the metal salts were also layered with a solution containing ligand in DMSO and DMF. Again, the metal chloride solutions mixed with ligand solutions in DMSO and DMF were also layered with diethyl ether and the resulting solution in a small beaker was kept in a big beaker containing n-hexane. Unfortunately, in all our efforts only amorphous compounds precipitated which prevented analysis of the complexes by X-ray crystallography.

Molecular weight The molecular weight for the complexes were determined in DMSO by the freezing point depression method and the results are given in Table 1. The experimentally determined values for the complexes (1), (2), (4), (5), (7), (9) and (10) are very close to the theoretical values calculated on the basis of monomer formulation indicating their monomer character. The values of molecular weights for the complexes (1) and (4) are very close to the theoretical values calculated on the basis of their monomer formulation.

>300

>300

670±24 (529)

675±20 (529)

290±10 (605.2)

(9) [Mn(CH2LH2) (H2O)2] Yellow

(10) [Mn(CH2LH2) (H2O)2] Brown (11) K2[Mn(CH2LH2) (H2O)2] Yellow

>300

>300

325±15 (608.90)

(8) K2[Ni(CH2L) (H2O)2] Brown

>300

>300

280±12 (613.75)

680±30 (550.69)

280

685±25 (537.55)

(7) [Ni(CH2LH2) (H2O)2] Yellow

(5) [Cu(CH2LH2) (H2O)2] Dark green (6) K2[Cu(CH2L) (H2O)2] Dark green

292

525±20 (501.55)



>300

12.12 (12.9)





12.00 (12.8)



12.51 (12.7)





13.4 (13.1)



9.31 (9.1)

10.12 (10.4)

10.61 (10.4)

9.72 (9.7)

10.80 (11.0)

10.00 (10.4)

12.53 (11.8)

13.00 (12.7)

10.7 (10.9)

12.1 (12.5)

13.2 (12.9)

Elemental analysis found (Calcd)% K M

>300

>300

510±10 (503.38) 660±20 (521.38)

(1) [Zn(CH2LH2)] Yellow (2) [Zn(CH2LH2) (H2O)] Yellow (3) K2[Zn(CH2L) (H2O)] Yellow (4) [Cu(CH2LH2)] Leaf green

D.P. (C)

300±10 (597.58)

Mol. wt expl (Calcd)

Sl. No. complex (colour)

49.05 (49.4)

57.08 (56.7)

56.30 (56.7)

49.70 (49.3)

56.00 (56.3)

49.50 (48.9)

56.50 (55.8)

59.50 (59.8)

50.6 (50.2)

57.1 (57.5)

60.0 (59.6)

C

3.58 (3.2)

4.12 (4.2)

4.20 (4.2)

3.25 (3.3)

4.18 (4.1)

3.2 (3.3)

4.13 (4.1)

3.62 (3.6)

3.3 (3.4)

3.8 (3.8)

3.6 (3.6)

H

9.51 (9.2)

10.72 (10.9)

11.12 (10.9)

9.22 (9.2)

10.30 (10.5)

8.9 (9.1)

10.21 (10.4)

11.32 (11.2)

9.2 (9.4)

11.0 (10.7)

11.4 (11.1)

N

5.98

5.90

1.85

2.90

3.16

1.80

1.90

2.30

dia

dia

dia

lB

38.6

1.5

1.3

43.3

0.7

40.6

1.8

0.6

39.8

0.5

0.9

Molar conductance ^ M (ohm)1cm2mol)1)

325 400(10780) 450 440(5370) 740 750(239) 805 810(290) 330 350(9750) 440 440(8940) 600 710(35) 340 340(9780) 435 430(10780) 500(3270) 680 700(50) 330 350(8970) 360 400 410(10240) 600 630(58) 940 960(45) 370 340(8970) 450 390(10680) 630 650(45) 950 960(30) 325 330(10750) 360 400 400(10580) 440 450(1750) 550 520(970) 670 680(320) 350 410 400(10370) 480 490(560) 360 350(12350) 440 435(9750) 500 510(870)

340 350(9670) 395 460(980)

360 365(151300) 405 410(10370) 460 450(7350)

Electronic spectral bands (kmax (emax)) solid solution

Table 1. Colour, decomposition point, analytical data, magnetic moment, molar conductance and electronic spectral data for Zinc(II), Copper(II), Nickel(II) and Manganese(II) complexes of Bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone

484

485 Much more closeness of the experimental values with the theoretical values for these complexes suggests that they are monomeric, that their fundamental features in the solid state are retained even in the highly coordinating solvents DMSO, and that the highly coordinating solvent molecules do not interfere with the structure. However, the experimental values of the molecular weights for the complexes (2), (5), (7), (9) and (10) were found to be more than those calculated theoretically on the basis of monomer formulation. The molar conductance values for these complexes lie in the non-electrolytic region which rules out the possibility of their ionic character. In view of the nonionic nature of these complexes and proximity of the experimental molecular weights with the theoretical values for the monomer formulation, these complexes are also suggested to be monomer. The higher molecular weight values than the theoretical values may be attributed to the substitution of water molecules by highly coordinating molecules in the coordination sphere. On the other hand, the experimental values of the molecular weights for the complexes (3), (6), (8) and (11) are much less than the theoretical values for the monomer formulation. Such a behaviour of these complexes from points of view of their molecular weight suggests that they are highly dissociated in solution. This is also consistent with their molar conductance values in DMSO solution which fall in the ionic range. 1

H-NMR spectra

The complexes (1) to (3) have been characterized by 1 H-NMR spectroscopy. The 1H-NMR spectrum of CH2LH4 has been recorded in DMSO-d6 as it is insoluble in CCl4 and CHCl3. The signals have been assigned to various types of protons in the light of literature records [17]. Four and two proton signals observed in the d 11.11–12.96 and d 8.40–9.48 regions downfield of TMS have been assigned to dOH + dNH and d-CH=N– protons, respectively. Further, two signals are observed at d 3.60 and d 3.90 ppm which are assigned to methylene protons [18]. The multiplet due to aromatic protons appears in the d 7.03–8.30 ppm region. The appearance of methylene protons in the form of two signals indicates that the dihydrazone exists in keto–enol equilibrium in solution involving methylene protons as shown below. While the signal at d3.60 ppm is attributed to arise due to methylene protons (–CH2–), that at d3.90 ppm is attributed to the methine proton (=CH–). The most crucial features of the 1H-NMR spectrum of the dihydrazone are the four signals in the d 11.11–12.96 ppm and d 8.40–9.48 ppm regions each, respectively. While the signals in the d11.11–12.96 ppm region might have composite character due to

naphtholic –OH and secondary –NH protons, those in the d8.40–9.48 ppm region arise from azomethine protons only. If the dihydrazone exists in the syn-cisconfiguration or staggered configuration, the dOH, dNH and d–CH=N–, resonances, each should appear as a singlet. However, the appearance of dOH + dNH and d-CH=N– proton resonances each, in the form of four signals (doublet of doublet) rules out the possibility of existence of the dihydrazone either in the staggered configuration or in the syn-cis-configuration. Alternatively this suggests the existence of dihydrazone in the anti-cis-configuration. The first factor which is responsible for the existence of dihydrazone in the anti-cis-configuration is provided by strong intramolecular hydrogen bonding existing in the molecule which inhibits free rotation of the hydrazone groupings about the C–C single bond. Another factor which contributes towards hindrance for free rotation of the two hydrazone parts about the C–C bond is the > C=C < double bond, which is created as a result of enolization of the dihydrazone in solution involving methylene protons. As a result of the anti-cis-configuration of the ligand, one hydrazone grouping attains axial position while the other hydrazone grouping remains in the equatorial plane. Hence, the equatorial protons appear upfield as compared to axial protons. Further, coupling between axial protons and equatorial protons occurs which ultimately leads to the splitting of their signal resulting in a doublet for each. It is pertinent to mention that stereospecific long range coupling has been reported in hydrazones [19]. The essential features of the 1H NMR spectra of the complexes (1) and (2) are similar to that of the free dihydrazone but there are significant differences too. Thus four signals appear in the region d 10.77–12.80 ppm similar to that in the free dihydrazone, two of which together may be considered as a doublet corresponding to equatorial and axial protons. The signals in the region d 10.77–12.80 ppm are weak and broad in the complex (2), as compared to those in complex (1) as well as in the free dihydrazone in which they are relatively intense indicating that they arise due to secondary –NH protons [20]. This suggests coordination of the naphtholic –OH groups to the metal centre via deprotonation in the complex (1), but with a protonated –OH group in the complex (2). The average position of the signals is upfield shifted by 0.15 ppm in the complex (1) ruling out the possibility of involvement of secondary –NH group in bonding. On the other hand, the average position of these signals in downfield shifted by 0.12 ppm in the complex (2). This indicates bonding of naphtholic –OH group in the complex (2). On the other hand in the zinc complex K2[Zn(CH2L)(H2O)] (3), the signal in the region d 11.00–13.00 ppm disappears indicating disappearance of secondary –NH and naphtholic –OH protons as a result of enolization of dihydrazone and its coordination through naphtholate oxygen atoms via deprotonation. Similar to dOH + dNH resonances,

486 Table 2. 1H-NMR Spectral Data (in d) for bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone and its Zinc(II) Complexes Sl. no.

Complex/Ligand

d (–CH2–)

d(naphthyl)

d(–CH = N–)

dOH + dNH

CH2LH4

3.90 3.60 3.80 3.47 3.77 3.52 3.75 3.67

8.30–7.03(m)

9.31 (d, 32.90) 8.60 (d, 32.90) 9.23(d, 35.1) 8.78(d, 35.1) 9.27(d, 33.2) 8.83(d, 33.2) 10.02(d, 26.6) 9.24(d, 26.6)

12.65 (d, 63.00) 11.50 (d, 63.00) 12.58(d, 56.7) 11.35(d, 56.7) 12.72(d, 55.9) 11.59(d, 55.9) ———–

(1)

[Zn(CH2LH2)]

(2)

[Zn(CH2LH2)(H2O)]

(3)

K2[Zn(CH2L)(H2O)]

Table 3. 25–16 MHz proton noise decoupled one(CH2LH4) and Zinc(II) complexes Carbon atoms

CH2LH4 dx

13

8.57–7.18(m) 8.55–7.14(m) 8.52–7.12(m)

C-NMR spectral data (DMSO–d6) for bis(2-hydroxy-1-naphthaldehyde)malonoyldihydraz-

[Zn(CH2LH2)] (1)

[Zn(CH2LH2)(H2O)] (2)

dx C(2a), C(2b) C(2a¢), C(2b¢) C(13a), C(13b) C(3a), C(3b) C(10a), C(10b) C(5a), C(5b) C(9a), C(9b) C(7a), C(7b) C(8a), C(8b) C(6a), C(6b) C(4a), C(4b) C(12a), C(12b) C(11a), C(11b) C(1b) C(1a)

168.3, 162.6, 157.8, 146.0, 142.9, 132.8, 131.4, 128.9, 127.8, 123.5, 121.0, 118.1, 108.4, 101.1 41.5

167.8 162.2 156.7 145.6 142.5 132.4 131.1 128.6 127.6 123.3 120.8 118.0 108.3

168.3, 162.6, 171.6 163.0, 146.0, 134.8, 132.2, 128.8, 128.0, 125.0, 120.9 118.1, 108.3, 95.3 19.0

Ddy 167.8 162.2 161.1, 160.0, 157.8, 156.7 145.6 134.4 131.4, 131.2 128.6 127.7 123.5, 123.3 118.0 104.9

0.0 0.0 ) 14.35 ) 11.17 ) 3.10 ) 1.0 ) 0.35 +0.05 ) 0.15 ) 0.50 0.0 0.0 +1.05 +14.8 +22.5

the d-CH=N– signals also appear in the form of four resonances in all of the complexes. The average position of the d-CH=N– signal shifts downfield by 0.72–0.37 ppm suggesting involvement of azomethine nitrogen atoms in coordination to the metal centre [21]. Another important feature of 1H-NMR spectra of the complexes is the upfield shift shown by methylene protons which appear in the d 3.46–3.80 ppm region although the possibility of interference with these signals by the signals arising from water absorbed by DMSO-d6 cannot be ruled out. These signals shift upfield by 0.20–0.11 ppm indicating an increase in electron density on methylene protons. This further rules out the possibility of involvement of > C=O groups in coordination. All of these pieces of evidence indicate that probably the N2O2 coordination chamber is occupied by a metal ion. This is possible only if the hydrazone coordinates to the metal centre in the anti-cis configuration.[22]. This is also supported by the fact that the d(NH) and d(CH=N)– signals each appear as a doublet of doublets in the form of four resonances. 13

C-NMR spectra

Only zinc complexes (1) to (3) have been characterized by 13C-NMR spectroscopy due to their diamagnetic character. The chemical shifts d (ppm from SiMe4) and the chemical shift changes, D d (ppm)

dx 167.8, 162.4, 170.2, 163.4, 145.8, 134.8, 132.2, 128.9, 128.0, 125.2, 121.0, 118.0, 108.2, 95.3 22.7

Ddy 168.0, 167.6 162.2 171.2 161.0, 160.3, 159.2 145.4 134.4 131.6, 131.0 128.6 127.8 123.4, 123.2, 122.0 120.80 117.80 106.7

+0.25 +0.10 ) 13.45 ) 15.25 ) 2.90 ) 2.30 ) 0.35 0.0 ) 0.20 ) 0.40 0.0 +0.15 +0.90 +14.8 +15.8

K2[Zn(CH2LH2) (H2O)] (3) dx Ddy 168.2, 162.5, 171.9, 163.1, 146.4, 134.8, 132.1, 129.0, 128.2, 125.0, 120.8, 117.8, 108.1, 90.8 –

167.6 162.3 170.5 161.8 145.2 134.5 131.2 128.5 127.9 124.0 120.6 117.5 106.3

+0.15 0.0 ) 13.95 ) 16.70 ) 3.10 ) 2.35 ) 0.40 0.0 ) 0.35 ) 0.60 +0.20 +0.40 +1.15 +19.3 –

accompanying the coordination of the ligand in the complexes are set out in Table 3. The assignments for the ligand have been deduced, taking into account the shift in the resonances of naphthyl ring carbon atoms caused by the substituents azomethine group and naphtholic –OH group [23] . As a result of coordination, many signals are split resulting in a greater number of signals than those in the free ligand. The effect of metal ions on carbon resonances of the naphthyl ring thus shifts downfield the signals for C(6), C(8) and C(9) by 0.15 to 0.60 ppm, respectively. On the other hand, the signals due to C(7) remains either almost unshifted in position [in complexes (2) and (3)] or shift to higher position by 0.05 ppm in the complex (1), while the signal due to C(11) shows upfield shift by 0.90–1.15 ppm. Further, the signals due to C(2), C(4) and C(12) remain almost unshifted in position or shift upfield by 1.0–0.40 ppm. The non-shifting or shift to higher field of the signal due to C(2) rules out the possibility of coordination of æC=O/ ‡ C–O group to the metal ion. It is therefore, reasonable to expect that the C(13) and C(3) resonances are shifted downfield even more since they are closer to the coordinated oxygen and nitrogen atoms. As a consequence, the signals for C(13) which in the free ligand were at 157.8 ppm and 156.7 ppm, respectively, could be either in the 163.4–156.7 ppm region in the complexes, or in the 171.6–170.5 ppm region. The latter assignment is much more likely since it gives

487 a deshielding of d14.35–13.45 ppm, whereas the former assignment gives only a small deshielding of 5.20–2.47 ppm. On this basis, the signals in the 163.0–156.7 ppm region are assigned to the azomethine carbon atoms, C(3), alone which in the free ligand were at 146.0 and 145.6 ppm giving a chemical shift change in the region 16.70–11.17 ppm. The signals in the 146.0–145.2 ppm region in the complexes (1) to (3) could be assigned to C(10) while in the 134.8–134.4 ppm region to C(5) carbon atoms. These carbon atoms absorb at 142.9, 142.5 and 132.8, 132.4 ppm respectively, in the free dihydrazone giving chemical shift changes of 3.10–2.90 and 2.35–1.0 ppm, respectively. The signal in the 121.0–120.6 ppm region is assigned to C(4a) and C(4b) carbon atoms which in the free ligand appeared at d 121.0 and 120.8 ppm giving either an upfield shift of ca. 0.20 ppm as in complex (3) or no chemical shift change as in complexes (1) and (2). Such a feature associated with C(4a) and C(4b) resonances may be attributed to combined effect of drainage of electron density from azomethine nitrogen atoms and naphtholate oxygen atoms in opposite directions. There are two points that need mentioning in respect of the 13C-NMR spectra of ligand as well as complexes. 13C-NMR spectra of the complexes (1) and (2) show more signals than those in the free hydrazone, while in the complexes (3) all signals appear as pairs similar to that in the uncoordinated dihydrazone. The additional signal in the complexes

(1) and (2) may be attributed to the effect of coordination of the metal centre to the ligand molecules. Such features of 13C-NMR spectra of the complexes support the anti-cis configuration of the ligand in the free state as well as in the complexes as deduced from consideration of 1H-NMR spectra [24]. Further, it is to be noted that the signals due to methylene carbon atoms appear in the form of two resonances in the free ligand as well as in the complexes (1) and (2) while as a single resonance in the complex (3). This suggests that the free ligand and that coordinated in the complexes (1) and (2) exists in the keto-enol equilibrium while that in the complex (3) exists in only one form, i.e., in the enol form.

Infrared Some structurally significant IR bands for the free dihydrazone and the monometallic complexes are set out in Table 4. A comparison of the IR spectrum of the free dihydrazone with those of the complexes suggests that it is present in the keto form in complexes (1), (4), (7) and (9) while in the enol form in the remaining complexes. The present ligand shows strong broad bands centred at 3467, 3217 and 3019 cm)1 assigned to stretching vibrations of naphtholic –OH and secondary –NH

Table 4 Important infrared spectral band for dihydrazone and its complexes Sl. No.

Complex/Ligand

m(OH + mNH3)

mCO

m(C=N)

amideII + m(CO) (naphtholic)

m(C=O)

m(N–N)

npmhH4

3467(sbr) 3217(sbr) 3019(sbr) 3508(sbr) 3000(s) 3458(sbr) 3032(s) 3500 3000(sbr) 3421(s) 3200(s) 3047(s) 3530 3000(sbr) 3200(sbr) 3412(s)

1684(vs) 1667(vs)

1622(s) 1602(s)

1532(m)

1282(s)

1032(m)

1692(vs) 1672(vs) – – – – 1695(s) 1663(s)

1616(s) 1600(s) 1614(s) 1602(s) 1617(vs) 1600(vs) 1618(s) 1598(s)

1539(s)

1286(s)

1032(w)

1542(m)

1285(m)

1032(w)

1540(vs)

1290(w)

1033(w)

589(m) 561(m) 592(m) 565(m) 568(w)

1534(s)

1285(m)

1032(w)

532(w)

1685(s) 1670(s)

1622(vs) 1604(vs)

1540(vs)

1284(vs)

1042(w)

581(m)



1540(vs)

1297(w)

1036(w)

531(w)

1694(s) 1666(s)

1618(vs) 1603(vs) 1616(vs) 1597(s)

1537(s)

1286(s)

1032(w)

530(m)

– – 1702(s) 1694(s)

1616(vs) 1600(vs) 1622(s) 1594(s)

1536(s)

1287(w)

1040(w)

1534(s)

1286(s)

1032(w)

572(w) 540(w) 534(m)



1622(s) 1599(vs) 1616(vs) 1600(vs)

1538(vs)

1304(s)

1033(w)

538(m)

1536(s)

1287(w)

1035(w)

572(w)

(1)

[Zn(CH2LH2)]

(2)

[Zn(CH2LH2)(H2O)]

(3)

K2[Zn(CH2L)(H2O)]

(4)

[Cu(CH2LH2)]

(5)

[Cu(CH2LH2)(H2O)2]

(6)

K2[Cu(CH2L)(H2O)2]

(7)

[Ni(CH2LH2)(H2O)2]

(8)

K2[Ni(CH2L)(H2O)2]

(9)

[Mn(CH2LH2)(H2O)2]

(10)

[Mn(CH2LH2)(H2O)2]

(11)

K2[Mn(CH2LH2)(H2O)2]

3403(s) 3200(s) 3027(s) 3530 3000(sbr) 3418(s) 3189(s) 3040(s) 3183(s) 3550 3000(sbr)

– –

m(M–O) (naphtholic)

488 group. The IR spectra of the complexes (1), (4), (5) (7) and (9) show two to three bands in this region similar to that of the free ligand indicating the keto form of the ligand. The position of these bands remains almost unshifted as compared to that in the free ligand. Hence, it has not been possible to derive inference regarding involvement of NH in bonding or otherwise. On the other hand, in the remaining complexes, the above bands are replaced by a strong broad band in the region 3550–3000 cm)1. Such a feature of the IR spectra of these complexes indicates destruction of the NH group on complexation. The band in this region appears to arise due to the stretching vibration of the naphtholic –OH group or to coordinated water molecules. The amide I band appearing at 1684 and 1667 cm)1 in free dihydrazone, on average shifts to higher frequency by 3.5–12.5 cm)1. The almost unaltered intensity of these bands and their shift to higher frequency in the complexes (1), (4), (5), (7) and (9) dismisses the possibility of coordination of æC=O group to the metal centre. In the remaining complexes, the m(C=O) bands disappear suggesting collapse of amide structure of free dihydrazone and its coordination to the metal centre in the enol form. The present ligand shows two very strong bands at 1617 and 1596 cm)1 which are assigned to stretching vibration of æ C=N group. The average position of the mC=N band shifts to lower frequency by ca. 3 to 7 cm)1 in the complexes (1) to (4), (7) to (9) and (11), while in the remaining complexes, it remains almost unshifted in position. Such a feature associated with the m(C=N) band is due to the difference of bonded species (H+ or M2+) to the æC=N group. The shift of m(C=N) band to higher frequency in the complexes (5), (6) and (10) may indicate flow of naphthyl ring electron density to the metal centre through azomethine group [25] . The appearance of m(C=N) stretching vibration in the form of two bands in the IR spectra of the complexes similar to that in the dihydrazone indicates that the two æ C=N groups are inequivalent. This inequivalency of the two æ C=N groups suggests that the two azomethine nitrogen-tometal bonds are of unequal length. Such an inequivalency in the strength of the M ‹ N bands may be related to coordination of dihydrazone to the metal centre in the anti-cis configuration to the same metal centre. In this configuration, one hydrazone arm attains axial position while the other hydrazone arm remains in the equatorial position [26]. The free dihydrazone shows a strong band at 1532 cm)1. This band is assigned to have composite character due to mixed contribution of the amide II and m(C–O)(naphtholic) bands. This band shifts to higher frequency by 2–10 cm)1 in the complexes. Such a small positive shift of this band indicates coordination of naphtholate atoms to the metal centre but rules out the possibility of involvement of naphtholate oxygen atom in bridging. On examining, the spectra of

ligand and its complexes below 600 cm)1, the new bands appearing in the 532–592 cm)1range are tentatively assigned to the m(M–O)(naphtholic) stretching vibration [27]. Thus, IR, 1H-NMR and 13C-NMR spectral evidence indicate probably that N2O2 coordination chamber is occupied by metal.

Magnetic moment The lB values for the complexes isolated in the present study have been given in Table 1. The lB value for zinc(II) complexes (1) to (3) are zero consistent with its d10 configuration. Copper(II) complexes (4) to (6) show lB values in the 2.30–1.80 BM range. These values are close to the spin-only value of 1.75–2.20 BM. This magnetic moment range indicates the absence of any appreciable spin–spin coupling between unpaired electrons belonging to different molecules. According to Figgis [28], a magnetic moment value greater than 1.90 BM indicates tetrahedral stereochemistry, while a lB value less than 1.90 BM is indicative of square planar as well as tetrahedral stereochemistry. This shows that the magnetic susceptibility is not of much use in deciding on the stereochemistry of copper(II) complexes. Nickel(II) complexes (7) and (8) have lB values equal to 3.16 and 2.90 BM corresponding to two unpaired electrons characteristic of nickel(II) in octahedral environment. The manganese(II) complexes (9), (10) and (11) have lB values equal to 1.85, 5.90 and 5.98, respectively. The lB value for the manganese(II) complex (9) indicates that it is a low-spin complex, while the lB values for the remaining manganese(II) complexes suggests that they are high-spin complexes. The value of 1.85 BM lies within the range reported for Mn(II) complexes in the low-spin state (t2g5, s = 1/2). Very few low-spin complexes of divalent manganese are currently known. The low-spin Mn(II) complexes with cyano ligand [29], phosphine ligands [30] and oxime ligands [31] have been reported.

Electronic spectra The electronic spectral bands for the dihydrazone ligand (CH2LH4) and monometallic complexes along with molar extinction coefficients have been given in Table 1. The free dihydrazone shows two bands at 320 nm (emax , 950 dm3 cm)1 mol)1) and 390 nm (emax, 15700 dm3 cm)1 mol)1) in the region 300–400 nm. The band at 320 nm is assigned to intraligand n fi p* transition while the band at 390 nm is assigned to the p fi p* transition which is characteristic of the naphthaldimine part [32]. The electronic spectra for the complexes show two to four bands in the 330–500 nm region. The ligand bands at 320 nm and 390 nm show red shift on complexation. The bands appearing in the

489 region 330–370 nm in the complexes are attributed to the ligand band at 320 nm. On the other hand, the band appearing in the region 390–430 nm may be attributed to the ligand band at 390 nm. The red shift of the ligand bands provides good evidence for chelation by dihydrazone to the metal centre. The magnitude of shift of ligand bands on complexation indicates strong bonding between the ligand and the metal centre. All of the complexes show a strong band in the 330–450 nm region which is indicative of their discrete molecularity [33]. All of the complexes show a new band in the 430–490 nm region which has a very high molar extinction coefficient. In view of the very high molar extinction coefficient of this band, it is assigned so as to have its origin in the ligand-to-metal charge-transfer transition. This band arises, most probably, from charge-transfer transition from naphtholate oxygen atoms to the metal centre [34]. This ligand-to-metal charge-transfer band, which is strongly influenced by the chemical nature of the ligand within a given stereochemistry, is responsible for the appearance of the colour of the complexes. The copper(II) complex (4) shows two bands at 740 and 810 nm in acetonitrile solution. These bands are assigned as ligand-field transitions. These bands are significantly red shifted compared to those for N2O2 chromophore having almost planar geometry [35]. Tetra-coordinate copper(II) can exist in a large number of stereochemistries [36]. As the geometry distorts from planarity towards tetrahedral for the same chromophore, d-orbital splitting decreases and consequently the ligand field bands shift towards lower energies [37]. Band positions and spectral features suggest that the complex has pseudo-tetrahedral stereochemistry. Somewhat higher intensities observed for the ligand field bands in the present case are possibly due to intensity borrowing from nearby charge-transfer transition. The electronic spectra of this complex in DMSO solution showed almost similar features as in the case of CH3CN. Therefore, it indicates that if there is any solvent coordination, it might be very weak. The electronic spectra of the copper(II) complexes (5) and (6) are entirely different from that of complex (4). These complexes show a band at 690 nm in the visible region as the d9 configuration is highly susceptible to Jahn–Teller distortion and such a distortion leads, in the extreme case, to square planar structures and the band due to 2Eg fi 2T2g transition is considerably blue shifted, as has been reported in some olive green square planar copper(II) chelates [38]. This band splits, in three principal bands [39], but only in few complexes have these bands been resolved either by Gaussian analysis or by single crystal polarization studies and are assigned to transitions 2B1g fi 2A1g , 2 B1g fi 2B2g and 2B1g fi 2Eg in order of increasing energy. The band positions together with molar extinction coefficients in these complexes suggest that they have tetragonally distorted octahedral stereochemistry.

Nickel (II) complexes show three bands in octahedral environment, 3A2g(F) fi 3T2g (F), (m1),3T1g (F)(m2) and 3T1g (P)(m3). The first two low energy bands observed in the 500–1000 nm range in the complexes (7) and (8) are characteristic of nickel (II) in octahedral environment. Ligand field parameters have been calculated by the equation given by Lever [40]. The energy of the first transition for the complexes is 10420 cm)1, which is equal to 10 Dq. The evaluation of B from the expression for m2 gives 626 and 598 cm)1 which are very low compared to the free ion values. This indicates that both the complexes have considerable covalent character. The nephelauxetic ratio, b, for the complexes is 0.579 and 0.553, respectively. The percentage lowering of energy of ‘P’ state in the complexes compared to its value in the free gaseous ion is 42.1% and 44.7%, which show a high degree of covalency. The electronic spectra of the low-spin manganese(II) complex (9) are dominated by strong charge-transfer bands in the region 500–700 nm [41]. Any d–d band occurring in the visible region is masked by strong charge transfer bands. The high-spin manganese (II) complexes (10) and (11) do not show any band in the visible region which may be assigned to d–d band. Any weak band occurring in the visible region is masked by either the ligand band or charge-transfer band, as is typical for high-spin manganese(II) complexes. Electron spin resonance As is evident from the magnetic moment value, the copper(II) complex (4) has a discrete molecularity. The ESR spectra are rhombically distorted. The ESR spectrum in the polycrystalline phase is isotropic while that in the CH3CN–DMSO glass is anisotropic. The solvent molecules are not bonded to copper(II) in the complex as the analysis of electronic spectral data showed. The highest symmetry appropriate for the CuN2O2 chromophore that should show rhombic splitting is C2v. In this point group, both dx2 y2 and dz2 orbitals transform to the A1 representation [41]. Therefore, the metal part of the ground state wave function should be a mixture of both of these orbitals. In this symmetry, the g-shifts (from the free electron value) [42] are given by: gx ¼

2kk2 xða  31=2 bÞ2 Exz

ð1Þ

gy ¼

2kk2 xða þ 31=2 b2 Þ Eyz

ð2Þ

gz ¼

8kk2 zða2 Þ Exy

ð3Þ

Here, kx, z, y are the orbital reduction factors [43], k is the spin-orbit coupling of the copper(II) ion, ‘a’ and ‘b’

490 are the coefficients of the dx2 y2 and dz2 in the ground state wave function, respectively, while the denominators represent d–d transition energies. Exy is the energy difference between the ground state and dxy and so on. As these equations indicate three possible mechanisms can split gx and gy which are (1) difference in the Exy and Eyz values (2) difference in reduction factors (excited state delocalization) and (3) magnitude of ‘a’ and ‘b’ (dz2-mixing into the ground state wave functions). Detailed calculations on Cu(II)-doped Zn(1,2-dimethylimidizole)2Cl2 host lattice having a pseudotetrahedral structure [44] suggest that for a difference of ca. 125 nm between Exz and Eyz values, the difference (gx ) gy) can be as much as 0.004. In the absence of polarized electronic spectral data, it is not possible to designate the two ligand field transitions observed. In any case, the band positions put an upper limit of 70 nm between the two transition energies, namely Exy and Eyz. Therefore point (1) above cannot account for such a large splitting in the g-values. The major contributing factor(s) towards the rhombic splitting for complex (1) should be either due to dz2 mixing into the ground state wave function or extensive excited state delocalization or both. In probing the EPR spectral characteristics of plastocyamin, it has been argued in favour of anisotropic delocalization over the cysteine–sulphur pp-orbital [45]. However, the extent of g-splitting in plastocyanin can not be accounted for considering orbital reduction factors due to their small effects. On the other hand, this splitting is quite sensitive to the amount of d2z mixing into the ground state. For stellocyamin, the observed g-split could be explained satisfactorily taking into consideration about 3% mixing of d2z into the ground state. This amount of mixing can also account for the difference between, Ax and Ay values observed in stellocyamin. To evaluate the extent of d2z mixing neglecting the orbital reduction factors, a parameter Rg has been defined by Belford et al. [46]. Rg ¼

gx  gy 0:5ðgx þ gy Þ  2:0023

where Rg is thus a measure of the degree of rhombic splitting in the g-values. For stellocyanin, Rg is 0.9 while for Cu(II)-doped Zn (1,2-dimethylimidizole)2Cl2 this value is 1.1 and for complex (9), the value of Rg is calculated to be 1.5. This parameter is steep function of the extent of dz2 mixing in the ground state which implies that rhombic splitting should be sensitive to the amount of dz2 mixed into the ground state. For complex (9), a mixing of the order of 4–5% can satisfactorily account for the split observed. For the copper(II) doped complex above, ca. 3% mixing is suggested for the observed g split (0.102). For a 4–5% mixing, the differences Ax)Ay should be about 60  10)4 cm)1. However, the difference, Ax ) Ay, could not be calculated in the present case as the spectrum did not show metal-hyperfine splitting in the gx region. The ESR spectrum of copper(II) complexes (5) and (6) show entirely different features as compared to the complex (4). While the complex (5) shows an isotopic spectrum in the polycrystalline state with gav value equal to 2.089, the complex (6) shows an anisotropic spectrum with gav and g1 values equal to 2.234 and 2.089 and A11 value equal to 180G. On the other hand both the complexes show anisotropic spectrum at LNT. The parallel and perpendicular Cu features are resolved in the complex (5) at LNT. The magnetic parameters deduced from the analysis of the spectra are given in Table 5. Kivelson and Neiman [47] have reported that the gjj value in a copper complex can be used as a measure of the nature of the metal–ligand bond. If this value is more than 2.3, the environment is essentially ionic and values less than this limit are indicative of a covalent character. The fact that g2jj value in both the complexes are less than 2.3 indicates that the metal–ligand bond has considerable covalent character. Also the shape of the ESR lines indicate that the geometry around the copper(II) ions is not trigonal bipyramidal in these complexes; since the lower side of the ESR spectrum is less intense than the high field side and the order of the g-values is not in accordance with the range suggested for trigonal bipyramidal

Table 5. EPR parameters for copper(II) and manganese(II) complexes Sl. no.

Complex

Temp (state)

gav

g1 or g11

g2

g3 or g1

Aav

A1 or A11

A2

A3 or A1

(4)

[Cu(CH2LH2)]

(5)

[Cu(CH2LH2)(H2O)2]

(6)

K2[Cu(CH2L)(H2O)2]

(9)

[Mn(CH2LH2)(H2O)2]

RT (solid) LNT (solid) RT (solid) LNT (CH3CN–DMSO) RT (solid) LNT (CH3CN–DMSO) RT (solid) LNT (CH3CN–DMSO)

(10)

[Mn(CH2LH2)(H2O)2]

(11)

K2[Mn(CH2LH2)(H2O)2]

2.146 2.098 2.089 2.106 2.13 2.13 2.013 4.439 1.97 4.05 2.023 2.026 2.055 2.033

– 2.236(gz) – 2.189 2.234 2.234 – – – – – – – –

– 2.050(gy) – – – – – – – – – – – –

– 2.009(gx) – 2.064 2.078 – – – – – – – – –

– – – 62 – – – – 92 – – 94 100 94

– 170(Az) – 136 178 178 – – – – – – – –

– 19(Ay) – – – – – – – – – – – –

– – – 25 – – – – – – – – – –

RT (solid) LNT (CH3CN–DMSO) RT (solid) LNT (CH3CN–DMSO)

491 8a 9a 10a 11a

7a 6a 5a H 2C

HC 2a C

H N

3a

N

O

N

K2

H

O

H C

HC

N N

N

CH

O

N

N H

O

OH 2

Zn

H

2b C

OC

C

12a 4a 13a

2-

CH2

-O

O

O 3b

4b 13b 12b

5b

11b 10b

6b 7b

Fig. 4. Tentative structure for the complex K2[Zn(CH2L)(H2O)] (3).

9b 8b

8a

CH2

9a 10a 6a 11a 5a 12a HC 4a 13a 3a H2O OH N 7a

H 2C 1 2a C

H N

O

O

O C

C HN H C

OH2

N

N

O N

N H

O

CH

M

Zn 2b C

O

OH

OH2

HC 3b 4b 13b 12b 5b 11b 6b 10b 7b 9b 8b

Fig. 5. Tentative structure for the complexes [M(CH2LH2)(H2O)2] [where M = CuII (5), NiII (7) and MnII (9)].

Fig. 2. Numbering scheme of the carbon atoms in CH2LH4 and [Zn(CH2LH2)(H2O)] (2).

2CH2

-O

complexes (2.00 > gjj gj ). The magnetic parameters indicates gjj gj free-spin (2.0023), which shows that the unpaired electron is in the dx2 y2 orbital of the copper(II) centre. The gjj and g? values depart considerably from the free ion value. The presence of strong broad band centred at 700 nm in the visible region of the electronic spectra of these compounds suggest their distorted octahedral stereochemistry. The covalency parameters a2Cu for the complexes is calculated from gjj , g? and Ajj by the following equation [47]. a2Cu ¼ ðA11 =PÞ þ ðg  2Þ þ 3=7ðg1  2Þ þ 0:04

ð3Þ

where P is equal to 0.036 cm)1. The a2Cu value has been found to be 0.63 and 0.86 for the complexes (5) and (6), respectively. The a2Cu values for the

O C

CH2

HN N

C

O

NH

M N

O

CH

O

Fig. 3. Tentative structure for the complex [M(CH2LH2)] [where M = ZnII (1) and CuII (4)].

HN K2

H C

OH 2

NH N

N

CH

M O

OC

C

H C

NH

O OH2

Fig. 6. Tentative structure for the complexes K2[M(CH2L)(H2O)2] [where M = CuII (6), NiII (8) and MnII (11)].

complexes suggest considerable covalent character (37 and 14%) in r-bonding involving metal ion and ligand. It is also evident that the metal-ligand bond is more covalent in neutral complex (5) than that in the anionic complex (6) as expected [48]. The manganese(II) complex (9) has a leff value equal to 1.85 BM indicating its low-spin nature. This complex shows two ESR signals, a strong one at g = 2.031 and a weak one at g = 4.439 at LNT in the solid state. The CH3CN–DMSO solution gives a well-resolved EPR spectrum with six hyperfine lines {Mn, I = 5/2} with metal hyperfine coupling constant equal to 92 G. In the low spin Mn(II) complexes ‘A’ values span the range 75–105G, the 90–105G domain being more frequented [49]. The hyperfine split signal in this complex is more intense than those in the high spin manganese(II) complexes, a feature of low-spin manganese(II) complexes.

492 On the other hand, the manganese(II) complexes (10) and (11) have magnetic moments equal to 5.90 and 5.98 BM, indicating their high-spin character. These complexes show isotropic spectra with g  2.0 at RT as well as at LNT. No other signal is observed in the 8000 G scan spectra as would have been expected for a non-cubic Mn(II) complexes with an appreciable zero field splitting. Such a simple one line spectrum devoid of any hyperfine structure has been interpreted as arising from a Mn(II) complex with a neighbouring atom magnetic interaction [50]. However, in the CH3CN–DMSO solution, a six-line spectrum is obtained with Mn hyperfine splitting constant equal to 94 G. This hyperfine splitting is characteristic of Mn(II) complexes rather than for Mn(IV) complexes. At LNT, between every pair of the six–hyperfine lines at the g  2.0 resonance, there is a pair of relatively weak forbidden transitions. The central resonance in complex (11) is split into five components with average nitrogen superhyperfine splitting constant AN equal to 12 G, indicating coordination of two nitrogen atoms to the manganese metal centre.

Conclusion All of the complexes prepared by different methods either in aqueous ethanol or ethanol media have 1:1 (metal:ligand) stoichiometry and are monomeric. The bulky dihydrazone gives tetrahedral zinc(II) and copper(II) complexes hitherto unknown in dihydrazone coordination chemistry when metal sulphates are used for synthesis of the complexes in aqueous-alcoholic medium, while the low spin octahedral manganese(II) complex, also unknown in dihydrazone coordination chemistry, is also obtained when manganese(II) sulphate is used for the reaction. The complexes, having square pyramidal and distorted octahedral geometry, are obtained when metal chlorides are used in the reaction in ethanol medium. The ligand coordinates to the metal centre in the keto form, as well as enol form as a tetradentate ligand only as compared to its hexadentate character in enol form in polymeric monometallic complexes as suggested by Narang and Singh [12] and in heterobimetallic complexes suggested by us [11]. The dihydrazone coordinates to the metal centre through azomethine nitrogen and naphtholic oxygen atoms only. The > C = O group coordinates to the metal centre neither in keto form nor in enol form. The dihydrazone exists in the anti-cis-configuration in the free state as well as in the coordinated state in the complexes in the present study. The metal centres occupy the N2O2 coordination chamber of the ligand. The complexes readily react with KF giving ionic complexes, devoid of fluoride, in which the ligand exists in the enol form retaining its conformation similar to that in the precursor. The ease with which KF reacts with the complexes rules out their polymeric character.

On the basis of various physico-chemical and spectral data presented and discussed above, the complexes may tentatively be suggested to have structures shown in Figures 2–6.

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