A hemilabile 2-(2′-pyridyl)-imidazole based nickel(ii) complex: proton-coupled-electron-transfer, bactericidal and cytotoxicity studies

Abstract

A 2-(2′-pyridyl)-imidazole (L) based water soluble nickel(II) complex of the type NiL₂Cl₂ (1) has been synthesized and characterized by spectroscopic, electrochemical, analytical and X-ray diffraction studies. Structural characterization of 1 shows a distorted octahedral environment around the nickel(II) center coordinated by two L ligands and two chloro ligands cis to each other. Intermolecular hydrogen-bonding, π⋯π interactions and C–H⋯π interactions have been investigated in detail. The irreversible electrochemical behaviour of complex 1 has been explained by considering the L-based proton-coupled-electron-transfer (PCET) mechanism. The labile nature of L and the chloro ligands in 1 is verified by substituting a biologically relevant quinonoid ligand 2,4-di-tert-butyl-6-(phenylamino)phenol, which leads to an inner-metallic chelate, 2. Furthermore, the antibacterial activity of the complexes 1 and 2 is tested against a few clinical bacteria suggesting their significant bactericidal properties relative to a standard antibiotic, ciprofloxacin. Prominent anticancer activity of 1 and 2 against human colorectal adenocarcinoma (HT-29) cancerous cell lines is also observed with IC₅₀ 33.5 and 45 μM, respectively, exhibiting the potential to produce promising cost-effective metallo-therapeutic agents.