The oxidation of ascorbic acid by tris(2,2′-bipyridine) complexes of osmium(III), ruthenium(III) and nickel(III) in aqueous media: Applications of the Marcus cross-relation

Abstract

The kinetics of the outer-sphere oxidation of L-ascorbic acid (H 2A) by tris(2,2′-bypyridine) and tris-(4,4′-dimethyl-2,2′-bipyridine) complexes of osmium(III), ruthenium(III), and nickel(III) have been investigated in acidic aqueous media at 9–25 °C. The observed inverse acid dependence, -d[ML 3+ 3]/dt = 2(k 1 + k 2K 1/[H +])(ML 3+ 3][H 2A], suggest a mechanism involving H 2A and HA − pathways, for which the specific rate and activation parameters have been determined. These and previously reported rate constants for cross-reactions involving ascorbate species are correlated in terms of the Marcus relationships. Estimates of the self-exchange parameters for the one-electron couples: H 2A/H 2A +·, k 11 = 2 × 10 3 M − s −1; HA −/HA·, k 11 = 1 × 10 5 M − s −1; A 2−/A−·, k 11 = 2 × 10 5 M − s −1], based on ascorbate radical reduction potentials calculated from proton and redox equilibrium constant, are presented.