✍ Scribed by József S. Pap; Łukasz Szywriel; Magdalena Rowińska-Żyrek; Konstantin Nikitin; Igor O. Fritsky; Henryk Kozłowski
No coin nor oath required. For personal study only.
The anionic [Cu III (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 lascorbic 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
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 K 1 at pH 8 (8.78 ± 3.11 × 10 -3 ) and k values for the forward and backward reactions (1.18 ± 0.68 × 10 6 and 1.46 ± 0.82 × 10 8 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 S ‡ (-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.
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