An efficient copper(III) catalyst in the four electron reduction of molecular oxygen by l-ascorbic acid

Abstract

The anionic [CuIII(LH−4)]− complex (L=8,17-dioxa-1,2,5,6,10,11,14,15-octaaza-tricyclo[13.3.1] eicosane-3,4,12,13-tetrone) is one of the rare copper(III) compounds, which exhibits high stability in basic aqueous solution. This copper(III) compound catalyses the reduction of molecular oxygen by l-ascorbic acid at pH 7.97, in Tris–HCl buffer solution at the ionic strength of 0.1M (NaCl). Stoichiometry of the reaction (2:1.07 for l-ascorbic acid:dioxygen) indicates formation of water and dehydroascorbic acid as primary products. Based on detailed kinetic measurements, the rate equation–d[AscH−]/dt=kobs[AscH−]0.5[CuIII(LH−4)][O2]0.5 was obtained. The proposed mechanism includes a fast redox pre-equilibrium between the copper(III) centre and its reduced, copper(II) form, induced by the presence of l-ascorbate. The equilibrium constant K1′ at pH 8 (8.78±3.11×10−3) and k values for the forward and backward reactions (1.18±0.68×106 and 1.46±0.82×108M−2s−1, respectively) were determined by stopped-flow technique, following the decrease in absorbance of the copper(III) form at 550nm. In the presence of molecular oxygen, re-oxidation of the copper(II) form of the catalyst takes place, based on cyclic voltammetry (CV) measurements. The decrease of the Cu(II)→Cu(III) oxidation and the subsequent increase of the Cu(III)→Cu(II) reduction current peaks in the CV spectrum, when argon is exchanged to dioxygen atmosphere, indicate a relatively fast oxidation rate for [CuII(LH−4)]2−. The determined ΔS‡ (−41±2Jmol−1K−1) for the catalytic reaction indicate an associative mechanism for the formation of the catalytically active copper–oxygen species that will react with the l-ascorbate to yield dehydroascorbate as product.