Solvent assisted synthesis, structural characterization and biological evaluation of cobalt(II) and nickel(II) complexes of Schiff bases generated from benzyl carbazate and cyclic ketones

A series of mononuclear cobalt and nickel complexes of the types [M(NCS)(2)(L1)(2)], M = Co 1 or Ni 2, [M(NCS)(2)(L2)(2)]center dot nH(2)O, M = Co (n = 0) 3 or Ni (n = 2) 4 and [M(NCS)(2)(L3)(2)], M=Co 5 or Ni 6, where L1 = benzyl 2-(cyclobutanylidene)hydrazinecarboxylate, L2 = benzyl 2-(cyclopentanylidene)hydrazinecarboxylate and L3 = benzyl 2-(cyclohexanylidene)hydrazinecarboxylate, were synthesized by template reactions of the metal nitrates, ammonium thiocyanate and benzyl carbazate (bc) with cyclobutanone (cb), cyclopentanone (cp) or cyclohexanone (ch). These compounds have been characterized by elemental analysis, infrared, electronic absorption and H-1 NMR spectroscopies, together with single crystal X-ray diffraction. Crystal structures of the complexes 1, 3-6 showed that the Schiff base ligands coordinate to the metal ions via the azomethine nitrogen and carbonyl oxygen atoms. In each case the two thiocyanate ligands coordinate through their nitrogen atoms resulting in complexes with a six coordinate octahedral geometry. The thermal stability of the complexes was investigated by TG-DTA and all of the compounds (1-6) undergo endothermic followed by exothermic decomposition processes to form the respective metal oxides. The antibacterial activity of the cobalt(II) and nickel(II) complexes was examined against Gram positive and Gram negative bacteria at concentrations between 40-80 mu g mL(-1). The binding interaction of human serum albumin (HSA) with compounds 1-6 was explored by multi-spectroscopic techniques and docking studies revealed the complexes bind to HSA through hydrophobic interactions. The cytotoxic activity of all of the compounds was evaluated against the K-562 (leukemia cell line) by MTT assays and, remarkably, all of the complexes were found to show good activity towards this cancer cell line. DAPI (4',6-diamidino-2-phenylindole) staining assay further confirmed that complexes induced leukemia cancer cell death by an apoptosis mechanism. (C) 2018 Elsevier Ltd. All rights reserved.