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 frans- dioxoruthenium(VI) species, frar)s-[Ru^VI(tmc)(O)₂] ²⁺ (tmc = 1,4,8,11-tetramethyl-1,4,8,11 -tetraazacyclotetradecane), has been studied in aqueous solutions under argon. The reaction occurs in two phases: frans-[Ru^VI(tmc)(O)₂]²⁺ + H ₂A → frans-[Ru^IV(tmc)(O)(OH₂)] ²⁺ + A, trans-[Ru^IV(tmc)(O)(OH₂)]²⁺ + H₂A → frans-[Ru^II(tmc)(OH₂) ₂]²⁺ + A. Further reaction involving anation by H ₂A occurs, and the species [Ru^III(tmc)(A ^2-)(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 k ₂ and k₂′ suggest that HA^- is the kinetically active species. Kinetic studies have also been carried out in D ₂O, 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 a hydrogen 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. © 2009 American Chemical Society.