Kinetics and mechanism of the oxidation of ascorbic acid in aqueous solutions by a trans-dioxoruthenium(VI) complex

The oxidation of ascorbic acid (H₂A) by a trans-dioxoruthenium(VI) species, trans-[Ru^VI(tmc)(O)₂]²⁺ (tmc =1,4,8,11-tetramethyl-1,4,8,11 -tetraazacyclotetradecane), has been studied inaqueous solutions under argon. The reaction occurs in two phases: trans-[Ru^VI(tmc)(O)₂]²⁺ + H₂A→trans-[Ru^IV(tmc)(O)(OH₂)]²⁺ + A, trans-[Ru^IV(tmc)(O)(OH₂)]²⁺ + H₂A → trans-[Ruˡˡ(tmc)(OH₂)₂]²⁺+ A. Further reaction involving anation by H₂A occurs, and the species [Ruˡˡˡ(tmc)(A²⁻)(MeOH)]⁺ can be isolated upon aerial oxidation of the solution at the end of phase two. The rate laws for both phases are first-order in both Ru^VI and H²A, with the second-order rate constants k₂ = (2.58 ± 0.04) × 10³ M⁻¹ s⁻¹ at pH = 1.19 and k₂´ = (1.90 ± 0.03) M⁻¹ s⁻¹ at pH = 1.24, T = 298 K and / = 0.1 M for the first and second phase, respectively. Studies on the effects of acidity on k₂ and k₂´suggest that HA⁻ is the kinetically active species. Kinetic studies have also been carried out in D₂O, and the deuterium isotope effects for oxidation of HA⁻ by Ru^VI and Ru^IV are 5.0 ± 0.3 and 19.3 ± 2.9, respectively, consistent with ahydrogen atom transfer (HAT) mechanism for both phases. A linear correlation between log(rate constants) for oxidation by Ru^VI and the O-H bond dissociation energies of HA⁻ and hydroquinones is obtained, which also supports a HAT mechanism. Copyright © 2008 American Chemical Society.