Bioinorganic Relevance of Some Cobalt ( II ) Complexes with Thiophene-2-glyoxal Derived Schiff Bases

Complexes of Co(II) with two new Schiff bases TEAB [2-hydroxy-4{[2-oxo-2-(thiophen-2-yl)ethylidene]amino}benzoic acid] and TEPC [N-[2-oxo2-(thiophen-2-yl)ethylidene]pyridine-3-carboxamide] have been synthesized and characterized with the help of elemental analysis, magnetic, mass, H-NMR, C-NMR, IR and electronic spectral data. IR spectra manifest the coordination of the ligand to the metal ion through the carbonyl oxygen, azomethine nitrogen and thienyl sulphur atoms. With the help of electronic spectral data various ligand field parameters were also calculated. All these studies reveal the distorted octahedral Co(II) complexes. Synthesized compounds have also been screened against some micro organisms viz, Escherichia coli, Proteus vulgaris, Aspergillus niger and Aspergillus flavus with the help of ‘filter paper disc’ technique. It has been observed that the antimicrobial activities of metal complexes are higher than that of the free ligand.


Introduction
In the field of coordination complexes, Schiff base metal complexes 1 have a curious history.Considerable interest attached with the chemistry of Schiff bases 2 derived from heterocyclic aldehydes, glyoxals and primary amines.Various heterocyclic Schiff bases having oxygen, nitrogen and sulphur donor atoms, have been reported by several scientists 3 .In recent years, applications of Schiff bases and their metal complexes in pharmacological 4 and nonpharamcological 5 fields have been receiving increased attention.
In the course of present investigation, complex formation of TEAB [2-hydroxy-4-{[2oxo-2-(thiophen-2-yl)ethylidene]amino}benzoic acid] and TEPC [N-[2-oxo-2-(thiophen-2yl)ethylidene] pyridine-3-carboxamide] Schiff bases with Co(II) ion have been attempted.Magnetic and electronic spectral data of these Co(II) complexes have been discussed on the basis of ligand field theory and the reacting sites in the ligands have been suggested on the basis of IR spectra.Mass and NMR spectral studies have been also discussed for the Schiff bases.In view of biological importance of these ligands and complexes, antibacterial and antifungal activities have also been screened.

Experimental
All the chemicals used were of AR grade and used without further purification.2-acetyl thiophene 6 and 2-thienyl glyoxal 7 were prepared by slightly modified techniques of the literature methods.

Synthesis of TEAB
A mixture of 2-thienyl glyoxal (14 mL, 10 m mol) dissolved in 40 mL acetone and p-amino salicylic acid (15.3 g, 10 m mol) dissolved in 20 mL ethanol was refluxed on water bath for 6 h under stirring.On cooling to room temperature, dark purple brown crystals were obtained.The crystals were filtered and washed with ethanol, recrystallized from acetone and dried at ambient temperature and its purity was confirmed by TLC technique.

Synthesis of TEPC
A mixture of 2-thienyl glyoxal (14 mL, 10 m mol) dissolved in 40 mL acetone and nicotinamide (12 g, 10 m mol) dissolved in 20 mL ethanol was refluxed on water bath for 6 h under stirring.On cooling to room temperature, chocolate brown crystals were obtained.The crystals were filtered and washed with ethanol, recrystallized from acetone and dried at ambient temperature and its purity was confirmed by TLC technique.

Synthesis of complexes
A solution of Co(II) salt (chloride / nitrate 10 m mol) in 20 mL ethanol was directly mixed to the solution of the ligand TEAB / TEPC in the same solvent.The molar ratio of metal: ligand was 1:2 for both TEAB and TEPC Schiff bases.Reflux the above mixture for 3-4 h.On cooling, filter the product and the material was dried in air to get the crystals of metal chelates.The yield varied from 40-60%.

Physical measurements
Percentage of C, N and H were determined micro analytically at RSIC, Punjab Univ.Chandigarh and the percentage of Co(II) in the complexes was determined by the standard method using pyridine and ammonium thiocyanate as precipitants.Melting points of the synthesized compounds were determined in open glass capillaries using melting point apparatus.Magnetic susceptibility measurements were determined at room temperature on 'EG&G model-155' by VSM (vibrating sample magnetometer) method at RSIC, IIT-Madras.
The NMR spectra were recorded on Bruker-300 Ultrashield-300 MHz NMR spectrometer.The mass spectra were recorded at Torrent Pharmaceuticals Ltd., Ahmedabad.Electronic spectra of complexes were carried out on 'Elico SL-171' spectrometer at 25 0 C using ethanol/acetone as solvent at Department of Chemistry, R.B.S. College, Agra.IR spectra were recorded using KBr pellets on 'Perkin Elmer-137 and 577' spectrometer in the region ~4000-200cm -1 at Alembic Ltd.Vadodara and RSIC, Punjab Univ., Chandigarh .

Mass spectral studies
On the basis of the "bar graph" presentation of the Schiff bases, we can observed that the peaks at m/z 154 and m/z 126 are the parent ion peaks of the Schiff bases TEAB and TEPC respectively and the peak at m/z 274 and m/z 244 is corresponding to the molecular weight (M + ) of the TEAB and TEPC respectively.
In the case of TEAB other peaks at m/z 230, 257, 137 and 190 supports the fragmentation of carboxylic group, hydroxyl group, salicylic acid and thiophene ring respectively.Peaks at m/z 150 and m/z 124 are due to the fragmentation at the azomethine linkage.
In the case of TEPC other peaks at m/z 166 and m/z 160 supports the loss of pyridine ring and thiophene ring respectively.Peaks at m/z 121 and m/z 124 are due to the fragmentation at the azomethine linkage.Therefore, the observed fragmented ion peaks for both the Schiff bases are also in confirmation with their structures.
In the spectra of TEPC, peaks due to azomethine carbon found at δ 160.33.Chemical shift due to carbonyl carbon attached to thiophene ring found at δ 184.14 and carbonyl carbon attached to pyridine ring found at δ 173.59.Chemical shift due to the carbon atoms present in the thiophene ring, at position 2, 3, 4 & 5 has been found at δ 150.48, δ 133.12, δ 123.46 and δ 129.22 respectively.Chemical shifts found at δ 152.00, δ 135.26, δ 138.70, δ 125.79 and δ 156.50 are due to the 2, 3, 4, 5 & 6 carbon position of the pyridine ring respectively.By the help of above 1 H and 13 C NMR spectra features, we can propose the structures of both the Schiff bases.

Magnetic susceptibility measurements
The effective magnetic moment values of all the Co(II)complexes lie in the range of 4.5-5.2BM, which suggested a distorted octahedral geometry 8 (Table 1).These values are much higher than the spin only value of 3.87BM.These µ eff values in all the complexes may arise due to electron pairing in the formation of strong covalent bond involving the use of 3d electron of Co(II) ion or spin-spin interaction.
Various other crystal and ligand field parameters 13 for all the Co(II) complexes have been also calculated.The values of these parameters are also in confirmation with distorted octahedral complexes (Table 2).

Infrared spectral studies
In order study the binding mode of the Schiff bases to metal in the complexes, IR spectra 14 of the free ligands are compared to metal complexes.In the IR spectra of TEAB, three sharp bands found at ~1626, ~ 1289 and ~ 1982 cm -1 are due to νC=N , νC-S-C and νC=O bending vibrations respectively.In both the metal complexes of this Schiff base viz.(a) and (b), νC=N band shifts to ~1611 and ~1612 cm -1 respectively, confirms the coordination through azomethine nitrogen 15 .The νC-S-C bands at ~1275 and ~1277 cm -1 respectively indicates the coordination of thienyl sulphur and νC=O bands at ~1701 and ~1708 cm -1 respectively confirms the involvement of carbonyl oxygen in coordination 16 .The values and the position of the νOH band are almost retained +3 cm -1 in these complexes indicates that phenolic oxygen atom do not take part in coordination 17 .
In the IR spectra of TEPC, νC=N , νC-S-C , νC=O and νC-N-C stretching vibrations are at the expected values of ~1618 , ~1272, ~1681 and ~1690 cm -1 respectively.In the complexes (c) and (d), the ligand band of νC=N shifted to ~1603 and ~1610 cm -1 respectively, indicating the coordination of azomethine nitrogen.The νC-S-C , vibrations of these complexes are at ~1275 and ~1273 cm -1 respectively, indicates the non involvement of thienyl sulphur with metal.Appearance of the broad peaks at ~1672 and ~1688 cm -1 respectively, which are due to shifting in νC=O bands, also indicates the coordination through carbonyl oxygen.The position and other characteristics of the band νC-N-C are retained ±1cm -1 in both the above complexes 18 .This indicates that the pyridine nitrogen do not take part in coordination.
In the complexes additional bands are also appear in the region ~287-294 cm -1 and ~1120-1157 cm -1 .These vibrations have been ascribed to νM-Cl and ν 2 band of NO 3 -group respectively indicating coordination.All the above coordination sites are further confirmed by the νM-O , νM-N and νM-S stretching bands at the expected values ~520-545 and ~441-445 cm -1 and ~331-340 cm -1 respectively 19 in all the complexes as we have discussed here.

Microbiocidal assay
Synthesized Schiff bases and their corresponding metal complexes were tested for the growth inhibitory activity against phytopathogenic bacteria viz.Escherichia coli and Proteus vulgaris, and fungi viz.Aspergillus niger and Aspergillus flavus using 'Filter paper disc' method 20 at 250, 500 and 1000 ppm level in acetone/ethanol.For screening antibacterial activity 'Nutrient Agar' and for antifungal activity 'Potato Dextrose Agar' 21 was used as a growth medium.Sterilized filter paper discs of 5 mm diameter were soaked in the solutions of different concentrations of synthesized compounds and introduced on sterilized culture media.The plates were incubated for 24-30 h at 35±2 0 C for bacteria and 72-96 h at 28 0 C for fungi.The zones of inhibition based upon zone size around the discs were measured.The measured zone of inhibition against the growth of various microorganisms is listed in Table 3.It is clear from the microbiocidal screening data that the metal complexes are more toxic in comparison to their parent ligand itself.Hence complexation increases the antimicrobial activity 22 such increased activity of the metal complexes can also be explained on the basis of 'Chelation theory' 23 .

Table 1 .
Analytical, magnetic and physical properties of ligands and their Co(II) complexes.

Table 2 .
Electronic spectral data and compound values of some ligand field parameters for Co (II) complexes Complexes Parameters Co(TEAB) 2 Cl 2 Co(TEAB) 2 (NO 3 ) 2 Co(TEPC) 2 Cl 2 Co(TEPC) 2 (NO 3 ) 2 . Antimicrobial data of synthesized compounds Conclusion All the spectral studies suggest that the Schiff bases are tridentate in nature having O, N and S donor atoms.The complexes having six coordinate system with octahedral geometry.All the compounds are bioactive showing good antimicrobial property.The proposed structures of the complexes are Structure of Complex. Figure 4. Structure of Complex.Co(TEAB) 2 Cl 2 / Co(TEAB) 2 (NO 3 ) 2 Co(TEPC) 2 Cl 2 / Co(TEPC) 2 (NO 3 ) 2