New Mixed Ligand Cobalt(II), Nickel(II) and Copper(II) Complexes of 2,2'-Bipyridine-3,3'-Dicarboxylic acid (bpdc) with 2-Mercapto-5-Phenyl-1,3,4-Oxadiazole (phozSH) and Their Antioxidant activity REZAN ALI SALEH1*, HIKMAT ALI MOHAMMAD2 and SALIM NAJM

The mixing of one mole of 2,2'-bipyridine-3,3'-dicarboxylic acid (bpdc) with two mole of potassium hydroxide (KOH) in methanol were refluxed for (half hour), followed by addition of one mole methanol solution of MCl2.nH2O (where M=Co, Ni or Cu). The mixture was refluxed for (2 hours) to give colored complexes of the metal ions of [M(bpdc)(H2O)4]. The [M(bpdc)(H2O)4] were reacted with one mole of 2-Mercapto-5-phenyl-1,3,4-oxadiazole (phozSH) producing the colored mixed ligand complexes with general formula [M(bpdc)(phozSH)(H2O)3] in which the metal ions coordinated to the ligand through O-atoms of carboxyl group in (bpdc) and N-atom of (phozSH) ligand. The ligands and complexes are well identified by using Furrier transform infrared spectroscopy, 1H-NMR, 13C-NMR, Electronic spectroscopy, CHNS analysis, Melting point, conductivity measurement. The Antioxidant activity were screened for all the complexes by the use of 2, 2-diphenyl-1-picrylhydrazyl (DPPH) method.

In the present work, we give an account of the preparation and identification of new mixed ligand Co(II), Ni(II) and Cu(II) complexes comprising the two ligands of 2, 2'-bipyridine-3, 3'-dicarboxylic acid (bpdc) and 2-Mercapto-5-phenyl-1,3,4-oxadiazole (phozSH). We found that bpydc ligand behave as a bidentate chelate that connected to the metal centreions by oxygen atom of the carboxyl group while the phozSH ligand work as a monodentate that link to the metal ions by nitrogen atom.

FT-IR spectra for the synthesized complexes
The IR spectra of Cobalt(II), Nickel(II) and Copper(II) complexes, contain broad band at (3398 and 3421, 3088) cm -1 and occurrence of a new weak peaks at (630 and 651 and 638) cm -1 were ascribed to ν(O-H) of coordinated water molecule to the metal ions 14 . The (SH) stretching band at (2567) cm -1 of (phozSH) was shifted to a higher frequency to (2953) cm -1 in complex 1 and (2852) cm -1 in complex 2 and 3 15-18 . A new weak band was appeared at (443, 513, 542) cm -1 in the spectra of Cobalt(II), Nickel(II) and Copper(II) complexes were respectively indicated to the linkage of oxadiazole nitrogen to the metal ions. The broad band of (O-H) group of (bpdc) at (2576) cm -1 was disappeared in all complexes and a new weak peak was observed at (480, 450 and 420) cm -1 were indicated to O-coordination of (bpdc) ligand to (Cobalt(II), Nickel(II) and Copper(II)) metals correspondingly [19][20][21][22] . Furthermore, disappearance of the singlet carboxyl protons of (bpbc) ligand at (11.0) ppm indicated the coordination of O-atom of bpdc to Cobalt(II), Nickel(II) and Copper(II) metals. By contrast, the thiol proton of (phozSH) ligand was shifted to a higher chemical shift and appeared at (2.59, 3.49, 3.70) ppm were assigned to N-bound coordination of oxadiazole nitrogen to Co, Ni and Cu(II) metals, respectively 23-26 .

Fig. 5. IR spectrum of [Cu(κ 2 -bpdc)(κ 1 -phozSH)(H 2 O) 3 ] complex 1 H-NMR Spectra for the synthesized Complexes
The 1 H-NMR spectra of (1, 2) complexes have been recorded in DMSO-d 6 and complex (3) in CDCl 3 solvent. The assignment of all signals have been attained by comparing the 1 H-NMR spectra of the ligands registered in literature. Each sort of signal has a particular chemical shift range that can be applied for initial assignment.

Elemental analysis for the synthesized complexes
The elemental analysis (C, H, N, S) data for all the synthesized complexes are listed in Table  1. These numbers are coherent with the proposed stoicheometries. Other physical properties such as colors, molecular weight (M.Wt.) and melting points (M. P.) of the synthesized complexes are also written.
The spectrum of Ni(II) complex (2) exhibited two d-d transitions at 23809 and 30303 cm -1 were respectively attributed to 1 A 1g → 1 A 2g and 1 A 1g → 1 B 1g , and the Cu(II) complex also exhibit two transition bands at 23809 and 29411 cm -1 were due to 2 B 1g → 2 A 1g and 2 B 1g → 2 B 2g transitions. The electronic transitions for all metal complexes indicate an octahedral geometry (Table 2) 28,29 .

Molar conductivity for the synthesized complexes
The molar conductivities of the synthesized complexes (1-3) were taken for (10 -3 M) solution in DMSO at (25°C). It was deduced that all the synthesized complexes are non-electrolyte as demonstrated in Table 3. Antioxidant assay (DPPH free radical scavenging activity) The metal complexes were monitored for free radical scavenging activity by the 2,2-diphenyl-1picrylhydrazyl (DPPH) method.
The different concentrations of the tested complexes (25, 50, 75 ppm) and standard vitamin-C were received in separate test tubes, and by addition of DMSO solvent the volume of each tube was settled to 2 milliliter. To all sample solution tubes in DMSO, a methanolic solution of DPPH (2 milliliter). The tubes were permitted to stand for 30 minute. The control experiment was fulfilled by the same method but without the addition of test samples. The absorbance was taken at 517 nm. Radical scavenging activity was calculated by the subsequent formula 30,31 . The results of free radical scavenger activity of the tested compounds at various concentrations are exhibited in Fig. 17. All complexes 1-3 showed comparable or slightly lower activity than the standard, ascorbic acid.
Co(II), Cu(II) complexes have demonstrated a good free radical scavenging activity with (IC 50 = 0.920, 5.075 d mol dm -1 ) were the most effective than Ni(II) complex, Fig. 18. Whereas Nickel(II) complex with (IC 50 = 31.683 d mol dm -1 ) has showed less activity. The metal complexes of Copper(II), Cobalt(II) were displayed higher scavenging activity than the standard, while Ni(II) complex has lower activity than ascorbic acid. The synthesized complexes scavenged the DPPH radical in a concentration dependent manner.

CONCLUSION
This work comprises synthesis of new mixed ligand Cobalt(II), Nickel(II) and Copper(II) complexes with 2-Mercapto-5-phenyl-1,3,4-oxadiazole and 2,2'bipyridine-3,3'-dicarboxylic acid ligands. Based upon Infrared, UV-Visible, 1 H-NMR, 13 C-NMR and (CHNS) analysis, we concluded that all the prepared complexes have an octahedral shape in which (bpdc) ligand coordinated to the metals through oxygen atoms, whereas the phozSH ligand connected to the metals via oxadiazole nitrogen atom. In accordance with the molar conductivity data, it has been recommended that all complexes are non-electrolyte. Finally, on the basis of Antioxidant assay, in which metal complexes were screened for free radical scavenging activity by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method, it can be conclude that all the Cobalt(II), Nickel(II) and Copper(II) complexes have a good free radical scavenging activity.