Cobalt(II), copper(II), zinc(II) and palladium(II) Schiff base complexes: Synthesis, characterization and catalytic performance in selective oxidation of sulfides using hydrogen peroxide under solvent-free conditions

Abstract

An asymmetric bidentate Schiff-base ligand (HL: 2-tert-butyliminomethyl-phenol) was prepared from the reaction of salicylaldehyde and tert-butylamine. Cobalt(II), copper(II), zinc(II) and Pd(II) complexes, CoL2, CuL2, ZnL2 and PdL2, were synthesized from the reaction of CoCl2·6H2O, CuCl2·2H2O, Zn(NO3)2·6H2O and PdCl2 with the bidentate Schiff base ligand HL in methanol. The ligand and its metal complexes were characterized by elemental analysis (CHN), FT-IR and UV-Vis spectroscopy. In addition, 1H and 13C NMR techniques were employed for characterization of the ligand (HL) and the diamagnetic complexes (ZnL2 and PdL2). The molecular structures of all the complexes were determined by the single crystal X-ray diffraction technique. The crystallographic data reveal that in all the complexes the metal centers are four-coordinated by two phenolate oxygen and two imine nitrogen atoms of two Schiff base ligands. The geometry around the metal center in the CoL2, CuL2 and ZnL2 complexes is a distorted tetrahedral and for PdL2 it is square-planar.The catalytic activity of these complexes has been evaluated for the selective oxidation of sulfides with the green oxidant 35% aqueous H2O2 under solvent free conditions. For all the catalysts, using optimized reaction conditions, different sulfides were converted to the corresponding sulfones. ZnL2 showed a higher catalytic performance for the oxidation of the different sulfides to the corresponding sulfones.