Activation of Hydrogen Peroxide for Oxidation by Copper(II) Complexes

Abstract

Copper(II) complexes of the general formula [LCu(H2O)4]n+ (where L is a bidentate ligand andn = 1 or 2) activate hydrogen peroxide for the oxidation of quinaldine blue, an oxidation indicator. The copper(II) complexes of tri- and tetradentate ligands are shown to be inactive, as are the bis-complexes of bidentate ligands. The proposed mechanism for peroxide activation involves the formation of a copper(II)-hydroperoxide complex, which then rapidly oxidized the substrate. Comparison of reaction rates with different ligand systems, and different ligand-to-metal ratios, lead to the conclusion that the most active complexes are those in which two equatorial coordination positions are occupied by easily displaced water to form the active catalyst. Rate studies are performed which give an experimental rate law which is first order in copper(II) complex, zero order in substrate, and variable order in peroxide. These kinetics are predicated by the rate law derived from our proposed mechanism. The variable order in peroxide can be explained in terms of Michaelis-Menten-type kinetics, as linear Lineweaver-Burk plots of (rate−1) vs ([O2H−]−1) are obtained from our experimental data. This is consistent with our proposed mechanism, as the derived rate law can be rearranged into the Michaelis-Menton equation.