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 0.1 M (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 × 108 M-2 s-1, respectively) were determined by stopped-flow technique, following the decrease in absorbance of the copper(III) form at 550 nm. 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 [Cu II(LH-4)]2-. The determined ΔS ‡ (-41 ± 2 Jmol-1 K-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.
- Catalytic oxidation
- Dioxygen reduction
- l-Ascorbic acid
ASJC Scopus subject areas
- Process Chemistry and Technology
- Physical and Theoretical Chemistry