Synthesis and structure of nickel(ii) thiocarboxamide complexes: effect of ligand substitutions on DNA/protein binding, antioxidant and cytotoxicity

Four low spin d⁸ nickel(ii) square planar complexes with general formula [Ni(L)₂] (where L = monobasic N, S bidentate thiocarboxamides) have been synthesized from the reaction of Ni(OAc)₂·4H₂O with 2 equivalent of thiocarboxamide ligands in ethanol. The complexes have been fully characterized by analytical, spectral (FT-IR, UV-Vis, ¹H and ¹³C NMR) and single crystal X-ray methods. Molecular structure of one of the complexes indicates a mono anionic bidendate coordination of thiocarboxamide ligands to the nickel via pyridine nitrogen and thiolate sulphur and reveal a square planar geometry. The binding interaction of nickel(ii) thiocarboxamide complexes with calf-thymus DNA (CT-DNA) was studied by electronic and emission spectroscopic methods revealed that complexes 1–4 could interact with CT-DNA via intercalation mode. DNA cleavage experiment shows that all the complexes cleave pUC19 supercoiled DNA in the presence of an activator like H₂O₂. The CD spectral study shows that binding of the complexes to DNA does not lead to any significant changes in the conformation of CT-DNA. Further, the protein binding ability of the nickel(ii) thiocarboxamide complexes with the BSA was investigated by UV-Vis, fluorescence and synchronous fluorescence methods and a static quenching mechanism was observed for their interaction with BSA. The free radical scavenging ability of all the complexes was evaluated by in vitro antioxidant assays involving DPPH radical, hydroxyl radical and nitric oxide radical and was found to be excellent. Moreover, the efficiency of complexes 1–4 to arrest the growth of HeLa and MG-63 cell lines has been studied along with the cell viability test against the non-cancerous cells NIH-3T3 under in vitro condition. Complex 3 was found to be the highest anticancer activity among the nickel complexes.