Microwave Synthesis , Spectral , Thermal and Antimicrobial Studies of Some Co ( II ) , Ni ( II ) and Cu ( II ) Complexes Containing 2-Aminothiazole Moiety

Some new Schiff base metal complexes of Co(II), Ni(II) and Cu(II) derived from 4-chlorobenzylidene-2-aminothiazole (CAT) and 2nitrobenzylidene-2-aminothiazole (NAT) have been synthesized by conventional as well as microwave methods. These compounds have been characterized by elemental analysis, FT-IR, FAB-mass, molar conductance, electronic spectra, ESR, magnetic susceptibility, thermal, electrical conductivity and XRD analysis. The complexes are coloured and stable in air. Analytical data revealed that all the complexes exhibited 1:2 (metal: ligand) ratio with coordination number 4 or 6. FAB-mass and thermal data show degradation pattern of the complexes. The thermal behavior of metal complexes shows that the hydrated complexes loses water molecules of hydration in the first step; followed by decomposition of ligand molecules in the subsequent steps. The crystal system, lattice parameter, unit cell volume and number of molecules in unit cell in the lattice of complexes have been determined by XRD analysis. XRD patterns indicate crystalline nature for the complexes. The solid state electrical conductivity of the metal complexes has also been measured. Solid state electrical conductivity studies reflect semiconducting nature of the complexes. The Schiff base and metal complexes show a good activity against the Gram-positive bacteria; Staphylococcus aureus and Gram-negative bacteria; Escherichia coli and fungi Aspergillus niger and Candida albicans.


Introduction
A large number of Schiff bases and their metal complexes have been found to possess important biological and catalytic activity 1 .It is well known that N and S atoms play a key role in the coordination of metals at the active sites of numerous metallobiomolecules 3 .They used as models for biological systems and find applications in biomimetic catalytic reactions 3,4 .Various heterocycles, especially thiazoles, occupy an important place owing to their versatile bioactivities due to the presence of multifunctional groups.2-aminothiazoles are a remarkably versatile group of compounds that have many biological activities and found recent applications in the drug development.Benzothiazoles are used for production of dyes with photosensitizing properties.Metal complexes with these ligands are becoming increasingly important as biochemical, analytical and antimicrobial reagents in the design of molecular models and material chemistry 4,5 .Thiazoles and their derivatives form a part of vitamin B 1 and coenzyme carboxylase.They represent a very interesting class of compounds because of their wide applications as antimicrobial, anti-inflammatory, anti-degenerative and anti-HIV activities 6,7 .
Microwave-assisted synthesis is a branch of green chemistry.Microwave-assisted synthesis has gained much attention in recent years.The applications of microwave irradiation are used for carrying out chemical transformations, which are pollution free, ecofriendly, low cost and offer high yields together with simplicity in processing and handling.The salient features of microwave approach are shorter reaction times, simple reaction conditions and enhancements in yields [8][9][10][11][12] .Reports on the synthesis of metal complexes by microwave methods have been comparatively less.In this study we report the synthesis, physicochemical characterization and biological significances of Co(II), Ni(II) and Cu(II) complexes with ligands derived from 4chlorobenzylidene-2-aminothiazole (CAT) and 2-nitrobenzylidene-2-aminothiazole (NAT) (Fig. 1).The metal complexes formed with these two new ligands may be used as precursors for the synthesis of new compounds.Some of them may exhibit interesting physical, chemical and biological properties.

Experimental
Melting points were taken in open glass capillaries and are uncorrected.Progress of reaction was monitored by silica gel-G coated TLC plates using MeOH:CHCl 3 system (1:9 A thermocouple device was used to monitor the temperature inside the vessel of the microwave.The microwave reactions were performed using on/off cycling to control the temperature.

Conventional synthesis of Schiff bases
The Schiff bases (CAT and NAT) have been synthesized by adding the ethanolic solution of 4-chlorobenzaldehyde and 2-nitrobenzaldehyde with ethanolic solution of 2-aminothiazole in equimolar ratio.The reaction mixture was then refluxed on water bath for about 4-6 hours.The condensation product was filtered, thoroughly washed with ethanol and ether, recrystallized and dried in vacuo.The Purity of the synthesized compound was monitored by TLC using silica gel G (Yield: CAT -83%; NAT -80%).

Microwave method for the Synthesis of Schiff bases
The equimolar (1:1) ratio of 4-chlorobenzaldehyde and 2-nitrobenzaldehyde with 2aminothiazole were mixed thoroughly in a grinder.The reaction mixture was then irradiated by the microwave oven by taking 3-4 mL solvent.The reaction was completed in a short time (4-5 min.)with higher yields.The resulting product was then recrystallized with ethanol and finally dried under reduced pressure over anhydrous CaCl 2 in a desiccator.The progress of the reaction and purity of the product was monitored by TLC using silica gel G (yield: 88-90%).

Conventional preparation of Schiff Base Metal Complexes
The metal complexes have been prepared by mixing the ethanolic solution of CoCl A coloured product appeared on standing and cooling the above solution.The complex was filtered, washed with ether and dried under reduced pressure over anhydrous CaCl 2 in a desiccator.It was further dried in an electric oven at 50-70°C.

Microwave method for the Synthesis of metal complexes
The ligand and the metal salts were mixed in 1:2 (metal:ligand) ratio in a grinder.The reaction mixture was then irradiated by the microwave oven by taking 3-5 mL solvent.The reaction was completed in a short time (6-8 min) with higher yields.The resulting product was then recrystallized with ethanol and ether and finally dried under reduced pressure over anhydrous CaCl 2 in a desiccator.The progress of the reaction and purity of the product was monitored by TLC using silica gel G (yield: 78-86%).

Biological activity
The in-vitro biological activity of the investigated Schiff base and its metal complexes was tested against the bacteria Escherichia coli and Staphylococcus aureus by disc diffusion method using nutrient agar as medium and streptomycin as control.The antifungal activities of the compounds were also tested by the Well diffusion method against the fungi A. niger and C. albicans, on potato dextrose agar as the medium and Miconazole as control.Each of the compounds was dissolved in DMSO and solutions of the concentrations (25, 50 and 100 ppm) were prepared separately.In a typical procedure, a well was made on agar medium inoculated with microorganism.The well was filled with the test solution using a micropipette and the plate was incubated 24 h for bacteria at 37 °C and 72 h for fungi at 30 °C.During this period, the test solution diffused and the growth of the inoculated microorganism was affected.The inhibition zone was developed, at which the concentration was noted.

Results and Discussion
As a result of microwave assisted synthesis, it was observed that the reaction was completed in a short time with higher yields compared to the conventional method.In the microwave method homogeneity of reaction mixture was increased by the rotating of reaction platform tray.The confirming of the results was also checked by the repeating of the synthesis process.
All the metal complexes are coloured, solid and stable towards air and moisture at room temperature.They decompose on heating at high temperature and more or less soluble in common organic solvents.The comparison study data of microwave and conventional methods, with analytical and physical data of the compounds are given in the Table 1.Analytical data show that metal chelates have 1:2 (metal:ligand) stoichiometry.The observed molar conductance of the complexes in DMSO at room temperature are consistent with the non-electrolytic nature of the complexes while Co(II) complex of CAT Schiff base ligand show electrolytic nature.

FAB-mass spectra
The FAB-mass spectra suggested that all the complexes have a monomeric nature.These complexes show molecular ion peaks in good agreement with the empirical formula suggested by elemental analyses.
The FAB mass spectra of the ligand (CAT) and its [Co(CAT)  O complexes showed a molecular ion peak at m/z 613 and 610 respectively, confirm the stoichiometry of (CAT) and its complexes as ML 2 type.It is good agreement with the microanalytical data 13,14 .).

IR spectra
The IR spectra of the complexes were compared with those of the free ligand in order to determine the involvement of coordination sites in chelation.Characteristic peaks in the spectra of the ligand and complexes were considered and compared.
The IR spectra of the ligand CAT exhibits a band at 1602 cm -1 due to the azomethine ν(C=N) group.This band shifts to lower frequency by 20-25 cm -1 in the complexes, indicating the bonding of nitrogen of the azomethine group to the metal ion and this can be explained by the donation of electrons from nitrogen to the empty d-orbital of the metal atom.A sharp band at 833 cm -1 is due to ν(C-S-C) group.Its position in the complexes has been observed at 818-822 cm -1 indicating the involvement of ring sulphur atom in coordination.An intense band at 1570 cm -1 due to C=N cyclic of thiazole ring does not shift in the spectra of complexes.This rules out the participation of ν(C=N cyclic) group in coordination.Thus the Schiff base acts as bidentate ligand.The appearance of bands at 3310-3313 cm -1 (stretching mode) supports the presence of water molecules in all the three complexes.However, the appearance of band at 723 cm -1 (rocking mode), in cobalt(II) complex suggest the coordinated water molecule.In the low frequency region, the band of weak intensity observed for the complexes in the region 472-478 cm -1 is attributed to ν(M-N), and that the region 402-406 cm -1 to ν(M-S).
IR spectrum of the NAT ligand exhibits the ν(C=N) band at 1612 cm -1 has shifted to lower wave numbers (1580+10 cm -1 ) in the complexes, suggesting the metal coordination with the azomethine nitrogen.The ligand exhibited a sharp band at 827 cm -1 due to ν(C-S-C) group.Its position in the spectra of complexes has been observed at 803-807 cm -1 indicating the involvement of ring sulphur atom in coordination.An intense band at 1572 cm -1 in Schiff base ligand due to ν(C=N, thiazole ring), does not alter in the spectra of complexes.This indicates the non-involvement of ring nitrogen in coordination.A band around 3405-3409 cm -1 in the spectra of complexes is assignable to ν str of water.In the low frequency region, the band of weak intensity observed for the complexes in the region 481-487 cm -1 is attributed to ν(M-N), and that the region 403-410 cm -1 to ν(M-S) [15][16][17][18][19] .
The IR data of both the Schiff base and its metal complexes show that the Schiff bases (CAT, NAT) are coordinated to the metal ion in bidentate manner with NS donor sites of azomethine nitrogen and thiazole sulfur.

Electronic spectra and magnetic moment
The nature of the ligand field around the metal ion has been deduced from the electronic spectra.

ESR spectra
The X-band ESR spectra of Cu(II) complexes were recorded in the solid state at room temperature and their g ║, g ┴ , Δg, g av and G have been calculated.The values of ESR parameters g ║, g ┴, g av, Δg and G for Cu(II) complex of CAT are 2.3932, 2.2724, 2.3126, 0.1208 and 1.4467 respectively.Similarly, the corresponding values for Cu(II) complex of NAT are 2.2145, 2.1995, 2.2045, 0.0150 and 1.0759 respectively.
ESR spectra of the complexes revealed two g values (g ║ and g ┴ ).Since the g ║ and g ┴ values are closer to 2 and g ║ > g ┴ suggesting a tetragonal distortion around the Cu(II) ion.The trend g ║ > g ┴ >g e (2.0023) shows that the unpaired electron is localized in d X 2 -Y 2 orbital in the ground state of Cu(II) and spectra are characteristic of axial symmetry.
The exchange coupling interaction between two Cu(II) ions is explained by Hathaway expression G = (g ║ -2.0023)/(g ┴ -2.0023).According to Hathaway 24 , if the value G is greater than four (G>4.0), the exchange interaction is negligible; whereas when the value of G is less than four (G<4.0) a considerable exchange coupling is present in solid complex.The G values for the Cu(II) complexes are less than four indicating, considerable exchange interaction in the complexes 25 .

Thermal analyses
The thermal behavior of metal complexes shows that the hydrated complexes lose molecules of hydration first; followed by decomposition of ligand molecules in the subsequent steps.The thermal degradation behavior of the Co(II) complex of NAT has been studied by thermogravimetric analysis.The thermogram of the complex shows a weight loss between 80-130 °C.This corresponds to loss of two lattice water molecules in the complex (Remaining Wt.%, Obs./Calcd., 94.7/94.31).After 260 °C, a gradual weight loss has been observed in general upto 490 °C, corresponding to the loss of partially decomposed ligand part from the complex (Remaining Wt.%, Obs./Calcd., 45.5/41.26).After this temperature a weight loss has been observed up to 620 °C, suggesting the elimination of the remaining part of the ligand.Above 620 °C, a constant weight region has been observed due to metal oxide, as a final pyrolysis product (Remaining Wt.%, Obs./Calcd., 18.6/16.86).
The thermogram of the Ni(II) complex of NAT shows that the complex loss in the weight during the temperature range 80-120 °C, approximately equal to two water molecules, assignable to lattice water in the complex (Remaining Wt.%, Obs./Calcd., 95.5/94.31).On increase in temperature above 240 °C, a speedy weight loss has been observed upto 430 °C, corresponding to the loss of partially decomposed ligand part from the complex (Remaining wt.%, obs./cal., 45.60/44.29).Above this temperature loss in weight has been occurs up to 600 o C.This indicates the elimination of the remaining thermally degradable part of the complex.After 600 °C a horizontal curve has been observed which corresponds to a mixture of metal oxide as an ultimate pyrolysis product (Remaining Wt.%, Obs./Calcd., 19.2/17.82) 26,27 X-ray diffraction study X-ray diffraction was performed of metal complexes.The XRD patterns indicate crystalline nature for the complexes.X-ray powder diffractogram of the complexes were recorded using CuKα as source in the range 5°-90° (2θ).X-ray crystal system has been worked out by trial, error methods for finding the best fit between observed, calculated sin 2 θ.The diffractogram of Co(II) complex of CAT has recorded 23 reflections with maxima at with maxima reflection of 2θ =15.684 which corresponds to d= 5.6457Å.The complex crystallized in cubic system.Sin 2 θ and hkl values for different lattice planes have been calculated.Crystal data for complex, a=19.37 Å; V = 7267.56Ǻ 3 , Z = 9, D obs = 1.3298 g/cm 3 , D cal = 1.3890 g/cm 3 .The observed and calculated values of density and sin 2 θ show good agreement.The diffractogram of Ni(II) complex of CAT has recorded 10 reflections with maxima at with maxima reflection of 2θ =16.046 which corresponds to d=5.5189 Å.The complex crystallized in cubic system.Sin 2 θ and hkl values for different lattice planes have been calculated.Crystal data for complex, a=20.84Å; V = 9052.92Ǻ 3 , Z = 13, D obs = 1.4809 g/cm 3 , D cal = 1.5235 g/cm 3 .The observed and calculated values of density and sin 2 θ show good agreement 28,29 .

Electrical conductivity
The temperature dependence of the solid state conductivity (σ) of the compounds in their compressed pellet form have been measured at fixed frequency 1KHz in the temperature range 297-413 K. the values of the solid state electrical conductivity of the Schiff base, its complexes increases with increasing temperature, decreases upon cooling over the studies temperature range indicating their semiconducting behavior.The general behavior of electrical conductivity follows the Arrehenius equation: where Ea is the thermal activation energy of conduction, σ o is the conductivity constant, K is the Boltzman constant.The lots of σ vs 1000/T for all the compounds are found to be linear over a studies temperature range.The room temperature electrical conductivity of all the compounds lies in the range 1.241×10 -7 -3.525×10 -9 ohm -1 cm -1 .These values show their semi-conducting nature.The electrical conductivity at room temperature for the complexes of CAT are Co>Cu>Ni and for the metal complexes of NAT are Co>Cu>Ni.The activation energy of the compound lies in the range 0.252-0.786 30,31 The confirming of the temperature dependence conductivity of the compounds was also checked by the repeating of the conductivity measurements.

Antimicrobial activities
The in-vitro Antimicrobial activity of the synthesized Schiff base ligands and their corresponding metal complexes on selected bacteria E. coli and S. aureus and two fungi A. niger and C. albicans was carried out.All of the tested compounds showed good biological activity against microorganism.On comparing the biological activity of the Schiff base and its metal complexes with the standard bactericide and fungicide, it is show that the metal complexes have moderate activity as compared to the standard but all the complexes are more active than their respective ligands.The higher inhibition zone of metal complexes than those of the ligands can be explained on the basis of Overtone's concept and Chelation theory.On chelation, the polarity of the metal ion will be reduced to greater extent due to the overlap of the ligand orbital and partial sharing of the positive charge of the metal ion with donor groups.Further, it increases the delocalization of π-electrons over the whole chelating ring and enhances the penetration of the complexes into lipid membranes and blocking of the metal binding sites in the enzymes of microorganisms.There are other factors which also increases the activity are solubility, conductivity and bond length between the metal and ligand 32- 34 .
The minimum inhibition values were determined using the filter paper disc diffusion method and the concentrations have been used in ppm.All the synthesized compounds have been screened in-vitro for their antibacterial activity against bacteria E. coli and S. aureus and antifungal activity against two fungi A. niger and C. albicans.Standards for antibacterial and antifungal activity Streptomycin and miconazole were used respectively.Standards also screened under the similar conditions for comparison.The diameter of the inhibition zones in (mm) are given in Tables 3 and 4.

Conclusion
In the present research studies, our efforts are synthesized of some newly compounds from the conventional as well as microwave methods.These synthesized compounds Characterized by various physicochemical and spectral analyses.The synthesized Schiff base ligands bind with the metal ions in a bidentate manner, with SN donor sites of thiophene-S and azomethine-N.FAB-mass and thermal data show degradation pattern of the complexes.Thermogravimetric studied of the complexes also helped to characterize of the complexes.The XRD patterns indicate crystalline nature of the complexes.The antimicrobial data show that the metal complexes to be more biological active compared to those parent Schiff base ligand against all phathogenic species.The compounds also inhibit the growth of fungi and bacteria to a greater extent as the concentration is increased.
).The spot was visualized by exposing dry plate at iodine vapours chamber.All the used chemicals and solvents were of Anal R grade.All the reagents used for the preparation of the Schiff bases argon/xenon (6 kV, 10 mA) as the FAB gas.The accelerating voltage was 10 kV and the spectra were recorded at room temperature.Electronic spectra (in DMSO) were recorded on Perkin Elmer Lambda-2B-spectrophotometer.Molar conductance measurements were conducted using 10 -3 M solutions of the complexes in DMSO on Elico-CM 82 Conductivity Bridge at room temperature.Magnetic susceptibility measurements were carried out on a Gouy balance at room temperature using CuSO 4 .5H 2 O as the calibrant.Diamagnetic corrections were applied in compliance with Pascal's constant.FT-IR spectra were recorded in KBr pellet on a Perkin Elmer RX1 spectrophotometer in wave number region 4000-400 cm -1 .X-band EPR spectra were recorded on a Varian E-112 spectrometer at room temperature operating at the X-band region with 100 kHz modulation frequency, 5 mw microwave power and 1 G modulation amplitude using TCNE as the internal standard.Thermogravimetric analysis was carried out under atmospheric condition with a heating rate 10°C min -1 on TGA Q500 universal V4.5A TA instrument.Powder X-ray diffraction (XRD) patterns were recorded on a RINT2000 wide angle goniometer.X-ray diffractometer, operated at 40 kV and 30 mA generator using the CuKα line at 1.54056 Ǻ as the radiation sources.Sample was scanned between 5° to 90° (2θ) at 25 °C.The solid state electrical conductivity has been measured by impedance spectroscopic method using HIOKI 3532-50 LCR Hitester at fixed frequency 1 KHz in the temperature range of 298-413 K. Microwave assisted synthesis were carried out in open glass vessel on a modified microwave oven model 2001 ETB with rotating tray and a power source 230 V, microwave energy output 800W and microwave frequency 2450 MHz.
were obtained from Sigma Aldrich.Metal salts were purchased from Loba Chemie.Elemental analyses were performed on an Elemental Vario EL III Carlo Erba 1108 analyzer.FAB-mass spectra were recorded on a JEOL SX 102/DA 6000 Mass Spectrometer using 2 Cl 2 ]2H 2 O and [Ni(CAT) 2 Cl 2 ]2H 2 O complexes were recorded and they are used to compare their stoichiometry composition.The Schiff base (CAT) shows a molecular ion peak at m/z 223 respectively.The [Co(CAT) 2 Cl 2 ]2H 2 O and [Ni(CAT) 2 Cl 2 ]2H 2

Table 1 .
The comparative results of conventional and microwave methods, analytical, physical data and magnetic moment values of the compounds.

Table 2 .
IR bands of Schiff base ligands and their complexes.

Table 3 .
Antibacterial screening data for the ligands and their complexes.

Table 4 .
Antifungal screening data for the ligands and their complexes.
Compound Diameter of inhibition zone (mm); Concentration in ppmA.