Synthesis, characterization and antibacterial activity of cobalt(III) complexes with pyridine-amide ligands.

The ligands 2-(N-(X-pyridyl)carbamoyl)pyridine (X=2, 3 or 4 for HL1-HL3, respectively) and 2,6-bis(N-(Y-pyridyl)carbamoyl)pyridine (Y=2, 3 or 4 for H2L4-H2L6, respectively) in their mono- and di-deprotonated forms have been used to synthesize kinetically stable cobalt(III) compounds [Co(L1-3)3] (1-3) and Na[Co(L4-6)2] (4-6), respectively. The Co(III) ion is in octahedral environment and is surrounded by three bidentate ligands in complexes 1-3 and two tridentate ligands in complexes 4-6. Ligands coordinate the cobalt center via amidic-N and pyridine-N centers forming a 5-membered chelate ring. Complexes 1-6 have thoroughly been characterized by the various spectroscopic analyses (1H NMR, 13C NMR, UV-vis, IR, mass), elemental analysis, and conductivity measurement. All complexes have been assayed for in vitro antimicrobial activity against clinically isolated resistant strains of Pseudomonas, Proteus, Escherichia coli and standard strains of Pseudomonas aeruginosa (MTCC 1688), Shigella flexneri (MTCC 1457), Klebsiella planticola (MTCC 2272). All cobalt compounds show mild to moderate activity. However, complexes [Co(L1)3] (1) and Na[Co(L4)2] (4) were found to have potent activity against standard and pathogenic resistant bacteria used in the study. Their MIC ranged from 2.7 to 187 microg/ml. In vitro toxicity tests demonstrated that all complexes were less cytotoxic than that of gentamycin on HEK cell lines and the results reveal that these complexes can act as potent antimicrobial agents.