Synthesis, Characterization and Antimicrobial Activity of Cu(II), Co(II) and Ni(II) Complexes with O, N, and S Donor Ligands

The complexes of the type ML2 [where M = Cu(II), Co(II), and Ni(II) ] L = 1-phenyl-1-ene-3-(2-hydroxyphenyl)-prop-2-ene with 3substituted-5-mercapto4-amino-1,2,4-triazoles. Schiff base ligands have been prepared by reacting 3-(2hydroxyphenyl)-1-phenylprop-2-en-1-one and 3-phenyl/pyridyl-4-amino-5-mercapto1,2,4-triazoles in an alcoholic medium. The complexes are non-electrolytes in DMF. The resulting complexes were characterized by elemental analysis, magnetic measurements, conductivity measurements and spectral studies. The Schiff base acts as a tridentate dibasic and coordinating through the deprotonated oxygen, thioenolic sulphur and azomethine nitrogen atoms. It is found that Cu(II), Co(II), and Ni(II) complexes exhibited octahedral geometry. The antimicrobial activities of ligands and its complexes were screened by cup plate method.


Introduction
There is great interest in synthesis and characterization of ligands which contain O, N, Ssequence and their metal complexes. Chalcones were synthesized by the condensation of acetophenones with aromatic aldehyde in the presence of acidic 1 and basic 2 media. Chalcones were found to be pharmacologically and physiologically active 3 . Chalcone derivatives are associated with some important biological activities such as antitubercular, anthelmintic 4 , fungicidal 5 , antitumor 6 and antibacterial activity 7 . The presence of a reactive α,β unsaturated keto in chalcones is found to be responsible for their antimicrobial activity. The tridentate Schiff bases with heterocyclic amines containing O, N, S donor sequence have been tried for complexation with transition metals 8 .The search for a new sequence has resulted by the condensation of chalcone and triazoles. The neutral tridentates are shown to form octahedral complexes with transition metal, where two ligands encompass the metal ion in an octahedral array 9 . In this paper we wish to report the synthesis and characterization of Cu(II), Co(II) and Ni(II) complexes with the Schiff bases obtained from the condensation of 1-phenyl-1-ene-3-(2-hydroxyphenyl)-prop-2-ene with 3-Substituted-5-mercapto-4-amino-1,2,4-triazoles resulted as shown in Figure 1.

Experimental
All the chemicals are reagent grade. Solvents were dried and distilled before use according to standard procedures 10 . The metal salts used were in their hydrated form.

Synthesis of ligands
One mole of chalcone in 30 mL of ethanol was taken in round bottom flask, to this one mole of 3-phenyl/pyridyl-5-mercapto-4-amino 1, 2, 4 triazoles were added. The reaction mixture was refluxed for 6-7 hours, evaporated the solvent on water bath and solid separated was collected, the product was washed with absolute alcohol, the product was crystallized from rectified spirit to obtain yellow coloured crystals.

Preparation of complex
Warm ethanolic solutions of metal(II)chloride (0.01M) were added to ethanolic solution of ligands H 2 L 1 and H 2 L 2 (0.02M) in about 30 mL of ethanol. The resulting solutions were refluxed for about 6 hours. The complex thus formed was filtered and washed with alcohol and dried in vacuum over fused CaCl 2. The metal estimation was carried out by standard methods, nitrogen by Kjeldahl method and sulphur 11 in the complexes estimated as BaSO 4 . The CHN analyses were carried out by STIC Cochin. The conductance was measured in DMF and DMSO solvent on an Elico CM-82 conductivity bridge. The magnetic susceptibility measurements at room temperature were made on Gouy balance at room temperature using HgCo(NCS) 4 as calibrant. The IR spectra of ligand and its complexes were recorded on a Perkin-Elmer instrument in KBr pellets in the range of 4000-350 cm -1 . UV-Visible spectra were recorded on an Elico SL 164 double beam UV-Visible spectrometer in the range 200-1200nm. 1 HNMR spectra were recorded on an AMX-400 NMR spectrometer using TMS as internal standard and DMSO as a solvent. Electron spin resonance spectra complexes in polycrystalline state were recorded on Varian E-4x-band ESR spectrometer using DPPH free radical as 'g' marker (g=2.0027) at room temperature.

Results and Discussion
The elemental analysis shown in Table 1 indicates that, all the metal complexes have 1:2 stoichiometry and are dark colored amorphous substances, soluble in DMF and DMSO. The molar conductance values obtained for these complexes at the concentration of 10 -3 m are in the range of 20-30 ohm -1 mol -1 cm 2 . These values are too low to account for any dissociation of the complexes in DMF. Hence these complexes can be regarded as nonelectrolytes.

Electronic spectra
The electronic spectral data of Cu(II), Co(II) and Ni(II) complexes of the ligands H 2 L 1 and H 2 L 2 were recorded in DMF as shown in Table 2. The electronic spectra of Co(II) complexes exhibit bands in the region of 8895 -21000 cm -1 and 15000 -18000 cm -1 are attributed to 4 T 1g → 4 A 2g (F) (ν 1 ) and 4 T 1g (F) → 4 T 1g (p)(ν 3 ) transitions respectively. The bands due to the 4 T 1g (F) 4 A 2g (F) (ν 2 ) transition could not observed because of its very low intensity. However the position of the ν 2 band has been computed (15549 cm -1 ) by the equation.ν 2 = ν 1+ 10Dq. The Co(II) complex under present investigation possess interelectronic repulsion parameter (B ' ) 749 cm -1 . The Racah parameter (B) is less than free ion value (971) suggesting a considerable orbital overlap and delocalization of electrons on the metal ion. The nephelauxetic ratio (β) for the present Co(II) complex (0.77). This is less than one, suggesting partial covalency in the metal ligand bond. The values Dq, β %, LFSE and ν 2 /ν 1 ( Table 2) suggest the octahedral geometry for Co(II) complex. 13,14 . These observations suggest that Co(II) complexes have octahedral geometry. Ni(II)complexes exhibit three bands at 8990-13810 cm -1 , 16088-16335 cm -1 and 24964-25210 cm -1 are assigned to 3 A 2g (F)→ 3 T 2g (F) (ν 1 ) , 3 A 2g (F)→ 3 T 1g (F)(ν 2 ) and 4 A 2g (F)→ 3 T 1g (p)(ν 3 ) transitions respectively are in confirmatory with the octahedral geometry for the Ni(II) ion. The Table 2 shows the ligand field parameter such as Dq, B ' ,β, β % and LFSE have been calculated by using Band-fitting equation given by Underhill and Billing 15 . Racah parameter B ' is less than the free ion value of 1040 cm -1 indicating the covalent character of the complex. The ratio ν 2 /ν 1 and β % are further support the octahedral geometry around the Ni(II) ion 16 . The Cu(II) complexes exhibits three bands in the region 11330cm-1 (ν 1 ) , 17300 cm-1 (ν 2 )and 19500-21000 cm -1 (ν 3 ) are of equal energy and giving rise to single broad band which may be assigned to the transitions 2 B 1g ν→ 2 A 1g (ν 1 ) , 2 B 1g → 2 B 2g (ν 2 ), 2 B 1g → 2 Eg(ν 3 ) respectively. The broadness of the band is due to the ligand field and the John-Teller effect 17 . These observations favor the octahedral geometry for the Cu(II) ion.

Infrared spectra
The important infrared frequencies exhibited by the ligands H 2 L 1 and H 2 L 2 and its complexes are given in Table 3. The ligand molecule exists in both thiol and thione forms due to tautomerism. A medium intensity band around 2560cm -1 due to ν(S-H) indicates the thiol form of the ligand. A comparison of IR spectra of ligand with those of complexes is based on earlier studies of similar ligand 18 . The ligand molecule shows a band at 700-820 due to ν(C=S). The coordination via thioketo sulphur atom causes the decrease in frequency of the ν(C=S). The complexes show a new band at 650-700cm-1 due to conversion of to νC=S into C-S-band indicates the thione ↔ thiole tautomerism followed by deprotonation of thiol group and consequent coordination of sulphur atom 19,20 and is indicated by absence of band at 2560 (due to SH) in the spectra of complexes. The infrared spectra of ligands exhibit high intensity band around 1612 cm -1 is due to ν(C=N) and the band around 3450 cm -1 due to phenolic OH. In the complexes, the low frequency shift (15-20cm -1 ) are observed around 1594cm -1 are due to ν(C=N) is suggestive of coordination through nitrogen of the azomethine group 21 to the metal(II) Chlorides. The band around 3450 cm -1 due to phenolic OH, which is observed in ligands, disappears in complexes, this indicates the ligands co-ordinate to the metal ion through phenolic oxygen atom 21 of OH group via deprotonation. The appearance of three new bands 550-520cm -1 , 450-420cm -1 , 380-415cm -1 are due to M-N, M-S and M-O bonds respectively.

H NMR spectra
The 1 H NMR spectrums of ligands H 2 L 1 and H 2 L 2 and its complexes were recorded in DMSO d 6 in the range 0-15δ (ppm). The multiplets were observed around δ6.9-8.6 (m.14, Ar-H) are due to phenyl protons. The protons due to CH=CH are observed around δ 7.4-7.8 (d 2H). In the ligand H 2 L 2 Signals due to pyridine ring protons occurs as multiplets between δ8.03-9.10 (m.4H Pyridine-H). A proton due to -OH group at 2-position of salicylaldehyde ring has resonated as a singlet at δ11.20 and δ11.34 (S.1H, OH) respectively. Signals in therange of δ 9.6 and 9.5 (S 1H, SH) are due to SH protons 18 . In the 1 H NMR specrum of Cd (L 1 ) and Zn (L 2 ) complexes, the OH moity of salicylaldehyde and SH which was observed in the ligands disappears in case of complexes indicating the involvement of phenolic oxygen 18 and sulphur in the coordination via deprotonation.

ESR spectra
The ESR spectra of the polycrystalline sample of the complexes were recorded at room temperature (  22 . G = (g || -2)/ (g ┴ -2) which measures the exchange interaction between copper(II) centers. According to Hathaway 23 if the G value is greater than 4, the exchange interaction is negligible, while a value of less than 4 indicates a considerable exchange interaction in the complexes. As G= 4.4 and 4.6 for the present complexes indicates that there is no spin exchange interaction in the copper complexes and hence distorted octahedral geometry proposed for the Cu(II) complex.

Antimicrobial activity
The biological and medicinal potency of coordination compounds has been established by their antitumor, antiviral and antimalarial activities. This characteristic property has been related to the ability of the metal ion to form complexes 24 with ligand containing sulfur, nitrogen and oxygen donor atoms. The synthesized ligands and its complexes were screened for their antibacterial activity 25 against E.coli and S.aurious and antifungal activity against 26 A.niger and A.flavous at 100 µg/0.1 cm 3 concentration The standard drugs streptomycin and chlotrimazole were also tested for their antibacterial and antifungal activity at the same concentration under the conditions similar to that of the test compounds concentration The zones of inhibitions of the antimicrobial activity have been presented in Table 5. The results of antibacterial activity of ligands and its complexes Cu(II), Co(II) and Ni(II) shows weak activity against E.coli and S.aurious when compared with standard streptomycin. The antifungal activity results revealed that the ligands and its Cu(II), Co(II) and Ni(II), complexes have exhibited weak to good activity against A.niger and A.flavous. The ligand and its Cu(II) and Co(II) complexes shows weak activity when compared to the standard drug chlotrimazole

Conclusion
The elemental analysis, magnetic susceptibility, electronic, IR, 1 H NMR and ESR spectral observations suggest the octahedral geometry for the Cu(II), Co(II) and Ni(II) complexes and exhibit coordination number six.