VARIABLE DENTICITY OF A MULTIDENTATE TERTHIOPHENE DERIVATIVE TOWARDS Ni(II) AND Zn(II)-STRUCTURAL STUDIES

ABSTRACT. A multidentate ligand, 3,4-bis(2-iminomethylphenol)-2,2’:5,2’’-terthiophene (L), has been synthesized by the condensation of 3,4-diamino-2,2’:5,2’’-terthiophene and salicylaldehyde. The ligand and Ni(II) and Zn(II) complexes were synthesized and characterized by IR, NMR, UV-Vis, AAS, MS, molar conductivity and magnetic susceptibility measurements. The ligand behaves as neutral ONS-ONS bis-chelant towards Ni(II) in [Ni<sub>2</sub>LCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]4H<sub>2</sub>O and as dibasic ONNO donor towards Zn(II) in [ZnL(NH<sub>3</sub>)<sub>2</sub>]. The dinuclear Ni(II) complex exhibits subnormal magnetic moment at room temperature due to metal-metal interaction through extended conjugation. The participation of the ring sulfur in bonding towards Ni(II) and the non-participation of the same towards Zn(II) are notable features. Octahedral geometries are proposed for both complexes. Cyclic voltammetric studies revealed electrochemical polymerization of the free ligand (L) but not of the Ni(II) and Zn(II) complexes.


INTRODUCTION
One of the reasons for widespread use of heterocyclic compounds is that their structures can be subtly manipulated to achieve the required functional modification [1][2][3][4][5][6]. Such variations may include differences in acidity or basicity, susceptibility to attack by electrophiles or nucleophiles and polarity, which are the direct consequences of variation in electronic distribution across the molecular frame [3]. Heterocyclics containing oxygen or sulfur as heteroatoms have been relatively less investigated in metal complex formation in comparison with the nitrogen heterocyclics.
A number of applications have been proposed for conducting polythiophenes, including field effect transistors, electroluminescent devices, solar cells, photochemical resistors, nonlinear optical devices, batteries, and diodes [7,8]. A number of diverse thiophene compounds have been reported in the literature to have germicidal properties and some of them have insecticidal properties [9][10][11].
Terthiophene precursors, which are presently attracting significant investigation for chemical and electrochemical polymerization to generate optically potential materials [2], have not been systematically studied for the synthesis of metal complexes. The presence of metal ions as chelated systems may positively contribute towards modifying the optical properties and also produce desirable magnetic properties for a wider range of applications [12][13][14]. In order to obtain polythiophene-metal chelates of the said desirable properties, the synthesis of precursor monomeric thiophene complexes, followed by chemical or electrochemical polymerization, appears to be a reasonable route. This investigation focuses attention on designing a new multidentate S-heterocyclic system and synthesizing its metal complexes with a view to generate metal chelate precursors suitable for chemical or electrochemical manipulations leading to optical and magnetic applications. This communication, however limits the contents to the synthesis, characterization and voltammetric studies of Ni(II) and Zn(II) complexes.

EXPERIMENTAL
All the chemicals used were of AnalaR grade. The solvents used were purified by distillation prior to use.

Instrumentation
Melting points were determined on Electrothermal IA 92000, digital melting point apparatus. IR spectra were recorded on Perkin-Elmer BTX FT-IR spectrometer in the range of 4000-400 cm -1 using KBr pellets. The metal contents in the complexes were estimated by Buck Model Scientific 210 VGB flame atomic absorption spectrophotometer. The electronic (UV-Vis) absorption spectra were measured on Spectronic Genesy's 2PC UV-Vis spectrophotometer in the 200-800 nm range. NMR data were collected using Bruker Avance 400 spectrometer at 400.13 MHz ( 1 H NMR) and 100.6 MHz ( 13 C NMR). The magnetic susceptibility of the Ni(II) complex was measured by using MSB-AUTO, Sherwood Magnetic Balance. Molar conductivities of complexes in nitrobenzene were recorded at room temperature using EC 214 conductivity meter (Hanna Instrument). Mass spectrum of the ligand was recorded on Thermo Finnegan high temperature insertion probe. The sample was ramped from RT up to 300 o C.

Cyclic Voltammetry
Prior to electropolymerization, the glassy carbon electrode was polished with alumina-water slurry on polishing cloth pads to a mirror-like finish and rinsed with deionized water. After polishing, the electrode was pretreated in 0.5 M H 2 SO 4 by cycling the potential between -0.5 to +1.4 V at 100 mV/s until a reproducible background was obtained.
The cyclic voltammogram of a 0.1 M of blank solution (TEATFB/CH 3 CN solution) was recorded between 0.0 and +1.6 V (vs Ag/AgCl) at a scan rate of 100 mV/s. Electrodeposition of L, and its Ni(II) and Zn(II) complexes were attempted by repeated potential cycling (ten times) of a 5 mM solution of the monomers in the potential range between 0.0-1.6 V (vs Ag/AgCl) at a scan rate of 100 mV/s. After each run, the film was rinsed with the electrolyte to remove excess monomers from the electrode surface.

3,4-bis(2-Iminomethylphenol)-2,2':5,2''-terthiophene (L)
Compound 4 (3 g, 11 mmol) and salicylaldehyde (2.633 g, 23.9 mmol) were dissolved in acetic acid (155 mL) and heated at 60 o C for 8 h. The mixture was then cooled to room temperature and the precipitate was collected by filtration, washed with water and methanol and dried to yield a brownish yellow solid, which was further purified by silica gel column chomatography using petroleum ether:ethyl acetate (9:1) as eluent to obtain the pure ligand L

Ni(II)-L complex
Nickel chloride hexahydrate (NiCl 2 . 6H 2 O) (0.2 g, 0.84 mmol) was dissolved in methanol (20 mL). To the mixture, a solution of the ligand (0.2 g, 0.412 mmol) in dichloromethane (20 mL) was added. The mixture was refluxed for 2 h. Aqueous ammonia diluted with methanol (1:19 v/v) was added to the mixture until the pH was 6 and then the mixture was refluxed for 14 h. The precipitate was filtered off, washed with methanol and finally with dichloromethane and dried in vacuum to afford the Ni(II)-L complex (0.23 g, 68%) as a deep brown solid. Mp: stable up to 350 o C.

Zn(II)-L complex
Anhydrous zinc chloride (ZnCl 2 ) (0.112 g, 0.82 mmol) was dissolved in methanol (10 mL). To the mixture, a solution of the ligand (0.2 g, 0.412 mmol) in dichloromethane (20 mL) was added. The mixture was refluxed for 2 h. Aqueous ammonia diluted with methanol (1:19 v/v) was added to the mixture until the pH was 7-7.5 and then the mixture was refluxed for 6 h. The precipitate was filtered off, washed with methanol and dichloromethane and finally dried in vacuum to afford the Zn(II)-L complex (0.18 g, 70%) as a light yellow solid. Mp: decomposes at 254 o C.

RESULTS AND DISCUSSION
The Ni(II)-L and Zn(II)-L complexes were obtained as powders with high melting points and low solubility in organic solvents. These complexes are colored compounds. Some of the important physical characteristics of the ligand, and the complexes are listed in Table 1.

Estimation of Ni(II) and Zn(II) by AAS
The molar ratios of the metal to ligand of both the complexes were obtained from the determination of metals using atomic absorption spectroscopy ( Table 2).
Based on elemental analysis and AAS data, the metal complexes are formulated as Ni 2 LCl 4 6H 2 O and ZnL(NH 3 ) 2 .  Table 2. Metal determination data.
The 1 H-NMR of the ligand (L) shows nine proton resonances. The two-proton singlet at  8.5 is attributed to the olefinic hydrogens on the imine carbon atoms (C7"'). The thiophene ring protons appeared at 7.16 (d, H3',3"), 7.27 (d, H5',5") and 7.33 (dd, H4',4"). The two-proton doublet of doublets centered at  6.85 and 7.02 are attributed to H-5"' and H-4"', respectively. The remaining two-proton doublets at  6.98 and 7.23, are attributable to H6"' and H3"', respectively ( Table 3). The two-proton broad singlet at  12.5 is due to the phenolic OH protons. The 13 C NMR spectrum of the ligand (L) indicates 13 carbon resonances, of which five are due to quaternary carbon atoms and the remaining eight are due to methine carbons (Table  3). In 1 H-NMR spectrum of the Zn-L complex, the phenolic OH proton signal was absent, confirming deprotonation and its subsequent involvement in coordination. One additional broad singlet signal at  3.25 is due to the NH signal of the coordinated ammonia.

Mass spectrum
The mass spectrum of the ligand (L) displayed a molecular ion peak at m/z 486 corresponding to a molecular formula of C 26 H 18 S 3 N 2 O 2 . Fragment ions appeared at m/z 485, 469, 393, 380, 366, 226, 227, 127, 120, 86, 84, 51 and 49. The base peak at m/z 84 is presumably due to the thiopene radical cation.

Conductivity measurements
The molar conductance values of the Ni(II) and Zn(II) complexes were recorded in nitrobenzene. The molar conductance values of both the complexes are less than 20 S cm 2 mol -1 indicating their non-electrolyte nature. In view of this, the following conclusions are made: (i) all the chlorides in the Ni(II) complex are in the coordination sphere and the ligand is present in the neutral state and (ii) As there are no chlorides in the Zn(II) complex (identified through the absence of precipitation upon decomposition of the complex in nitric acid and subsequent treatment with silver nitrate) [17], the ligand is likely to be in deprotonated and L 2state. This is justified by the fact that the Zn(II) complex separated from the reaction mixture at a higher pH (7.0-7.5), while the Ni(II) complex was formed at pH = 6.

Infrared spectra
Comparison of the IR spectrum of the free ligand with the spectra of the metal complexes (Table 4) reveals that the ligand exhibits different modes of chelation towards Ni(II) and Zn(II). The results show that it behaves as a neutral bis-ONS donor towards Ni(II) and as a dibasic ONNO donor towards Zn(II).
The salient features of the spectra of the metal complexes are summarized below.

Ni(II) complex
A broad band centered as 3400 cm -1 is assignable to OH (phenolic and H 2 O). The bands observed at 3062, 1528, 1223, and 852 cm -1 are assignable to CH (azomethine), (C=N), (C-O) and (C-S-C), respectively. Persistence of OH suggests that the phenolic functions are not deprotonated, but positive shift in (C-O) from 1198 in the free ligand to 1223 cm -1 in the complex supports the involvement of the phenolic oxygen in coordination. Further, the negative shift in (C=N) from 1752 to 1528 cm -1 reveals that the azomethine nitrogen is bonded to the metal. A positive change is noted with respect to (C-S-C), which suggests coordination though the ring sulfur. These observations establish that the ligand is offering two sets of ONS sequences in Ni(II) binding. This is adequately matching with the proposed formula of the complex. Medium intensity bands at 950 and 645 cm -1 , due to rocking and wagging modes, confirm the presence of coordinated water. Other non-ligand bands are assignable to Ni-O and Ni-N stretching. The Ni-S and Ni-Cl are not in the frequency range of the spectrum.

Zn(II) complex
The spectrum does not show the OH (phenoloic), indicating the deprotonation of the phenolic function. It presents characteristic strong bands at 3330 and 3220 cm -1 , assignable to NH 3 . These bands support the presence of ammonia molecules in coordination. The bands observed at 1160 and 645 cm -1 due to rocking and wagging modes further confirm ammonia coordination. The spectrum also reveals bands at 3060, 1547, 1243, and 846 cm -1 assignable to CH (azomethine), (C=N), (C-O) and (C-S-C), respectively. The negative shift of C=N and the positive shift in CO and no shift in CSC suggest chelation though azomethine nitrogen and deprotonated phenolic oxygen. Based on these results it is concluded that the ligand behaves as a dibasic ONNO donor [18,19]. Table 4. Infrared data (cm -1 ) of the ligand and complexes.

Magnetic susceptibility
The magnetic susceptibility of the Ni(II) complex was measured at 22 o C (295 K). The effective magnetic moment of this complex was found to be 2.04 BM. This subnormal magnetic moment (2.04/2 = 1.02 BM) in comparison with the expected moment corresponding to two unpaired electrons is explained on the basis of the possible antiferromagnetic interaction due to superexchange phenomenon. [20]. Further, the back donation of elections due to electron rich metal ions will facilitate spin pairing via the delocalized -electron cloud. The skeletal structure presented in Figure 1 is in support of this conclusion. Binuclearity of Ni(II) complex provides adequate justification for sub-normality of magnetic moments.

Electronic spectra
The electronic spectrum of the ligand shows bands at 42373, 35971, 28818, 25000 cm -1 . These are assignable to * and n* transitions, due to aromatic, phenolic and azomethine functions. The spectra of metal complexes show modifications in these band positions as a consequence of metal ligand interaction. Besides three bands observed in the spectrum of Ni(II) complex are assigned to d-d transitions, which are given in Table 5. A moderately intense band tailing into the visible region (28571-23529 cm -1 ) in the spectrum of the Zn(II) complex is assignable to MLCT. Octahedral geometries are proposed for both complexes (Figure 2).   Figure 3 shows a series of cyclic voltammograms (CVs) recorded for a 5 mM of the ligand L monomer in a 0.1 M Et 4 NBF 4 /CH 3 CN solution, when the potential was cycled between 0.0 and +1.6 V. The CV exhibited three oxidation peaks at +0.63, +1.12 and +1.34 V during the first anodic scan. A reduction peak was observed at +1.18 V in the cathodic scan. The subsequent increase in these peaks indicates the growth of a polymer film on the electrode surface. The CV of the polymer film (poly L) formed on the electrode surface was recorded in a monomer free supporting electrolyte and is shown in Figure 4. Two oxidation peaks and one reduction peak were observed indicating that the polymer film is electrochemically active.    Figure 5 shows series of cyclic voltammograms recorded for a 5 mM Ni(II)-L monomer in a 0.1 M Et 4 NBF 4 /CH 3 CN solution, when the potential was cycled between 0.0 and +1.6 V versus Ag/AgCl. The CV exhibited no oxidation peaks indicating that the monomer is difficult to electropolymerize and deposit at the electrode surface. This could possibly be due to the delocalization of the electrons responsible for the radical formation as shown in Figure 1. Zn(II)-L complex Figure 6 shows a series of cyclic voltammograms (CVs) recorded for a 5 mM Zn(II)-L monomer in a 0.1 M Et 4 NBF 4 /CH 3 CN solution, when the potential was cycled between 0.0 and +1.6 V versus Ag/AgCl. The CV does not exhibit any increase in oxidation peaks indicating the difficulty of electrodepositing a polymer film on the electrode surface. The reason might be due to the Zn(II) present in the blocked extended conjugation lacks any donor groups on the para position relative to phenoxy group to facilitate the polymerization.

CONCLUSIONS
Synthesis and characterization of a multidentate terthiophene based ligand (L) with distinct variability of denticity towards Ni(II) and Zn(II) ions revealed dinuclear ONS-ONS bischelation and mononuclear ONNO chelation, respectively. Subnormal magnetic moment of Ni(II) complex conform potential basis for further investigation and applications. Feasibility of electrochemical polymerization of the free ligand and non-feasibility of polymerization of the metal complexes are notable features, which suggest the influence of metal chelation in the reorganization of ligand based electron denticity.