Synthesis of new dinuclear dicopper(II) and dinickel(II) complexes. The kinetics of catechol oxidase and electrochemistry of a dicopper(II) complex

Abstract

A new dinuclear ligand L′, ethylene[OO′-bis-salicylidene-β-diketone] bearing two symmetrical coordination sites was synthesized by the condensation of salicylaldehyde and acetylacetone, L, with 1,2-dibromoethane under reflux. The ligand L′ in a 1:1 ratio was treated with CuCl2 and NiCl2 to yield the complexes, tetrachloro bis[OO′-bis- salicylidene-β-diketone copper(II)] and bis[OO′-bis-salicylidene-β-diketone nickel(II)] chloride. The complexes were subsequently characterized by spectroscopic techniques, elemental analysis, i.r., 1H-n.m.r., 13C-n.m.r., u.v.–vis., e.p.r. spectroscopy, and conductance measurements. The conductance measurements in DMF reveal that the CuII complex is covalent while the NiII complex is ionic and the spectral data support the CuII complex to be distorted octahedral whereas the NiII complex has square-planar geometry. The dioxygen binding was studied spectrophotometrically by the oxidation of tetrachloro bis[OO′-bis-salicylidene-β-diketone copper(II)] with pyrocatechol in the presence of oxygen. The kinetic experiments were performed with the copper complex in DMF by monitoring the increase in absorbance over time at pH 8.0 in the presence of pyrocatechol at 25 °C. The kinetic parameters Vmax and KM were determined on the Michaelis–Menten Approach. Redox behavior of the dinuclear copper(II) complex was investigated by cyclic voltammetry in the presence of O2 with the pyrocatechol (substrate) and also without the substrate. The large difference in potentials E0 is indicative of reversible oxygen binding of the complex and distinct catalytic activity.