Nickel complexes of some thiosemicarbazones: Synthesis, structure, catalytic properties and cytotoxicity studies

Reaction of salicylaldehyde thiosemicarbazone (H2L1), 2-hydroxyacetophenone thiosemicarbazone (H2L2) and 2-hydroxynaphthaldehyde thiosemicarbazone (H2L3) with Ni(ClO4)2·6H2O afforded dimeric complexes of type [{Ni(L)}2]. Reaction of these complexes with triphenylphosphine (PPh3), pyridine (py) and 4,4′-bipyridine (bpy) has yielded complexes of type [Ni(L)(PPh3)], [Ni(L)(py)] and [{Ni(L)}2(bpy)], respectively, which have also been obtained from reaction of the thiosemicarbazones with Ni(ClO4)2·6H2O and PPh3 or pyridine or 4,4′-bipyridine. Structures of the [{Ni(L)}2] complexes have been optimized by DFT calculations. Crystal structures of [Ni(L2)(PPh3)], [Ni(L2)(py)] and [{Ni(L1)}2(bpy)] have been determined. In all these complexes thiosemicarbazone is coordinated to nickel as ONS-donor. All these complexes show characteristic 1H NMR spectra and intense absorptions in the visible and ultraviolet region. Cyclic voltammetry on the complexes shows one irreversible oxidation on the positive side of SCE, and one irreversible reduction on the negative side. The mixed-ligand nickel complexes are found to be efficient catalysts for Heck type C–C coupling reactions. In vitro cytotoxicity screenings of the six mononuclear nickel complexes have been also carried out in a human tumor cell lines, viz. breast carcinoma cell line (MCF-7). [Ni(L3)(py)] shows the lowest LD50 value. An apoptosis study in MCF-7 with all the complexes confirms that at concentrations near LD50 they induce apoptosis.