Kinetics and Mechanism of the Oxidation of Ascorbic Acid in Aqueous Solutions by a trans-Dioxoruthenium(VI) Complex

Research output: Journal Publications and Reviews (RGC: 21, 22, 62)21_Publication in refereed journalpeer-review

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Author(s)

  • Yi-Ning Wang
  • William W. Y. Lam
  • Wai-Lun Man
  • Chi-Fai Leung

Detail(s)

Original languageEnglish
Pages (from-to)400-406
Journal / PublicationInorganic Chemistry
Volume48
Issue number1
Publication statusPublished - 5 Jan 2009

Abstract

The oxidation of ascorbic acid (H2A) by a frans- dioxoruthenium(VI) species, frar)s-[RuVI(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-[RuVI(tmc)(O)2]2+ + H 2A → frans-[RuIV(tmc)(O)(OH2)] 2+ + A, trans-[RuIV(tmc)(O)(OH2)]2+ + H2A → frans-[RuII(tmc)(OH2) 2]2+ + A. Further reaction involving anation by H 2A occurs, and the species [RuIII(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 RuVI 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 RuVI and RuIV 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 RuVI and the O-H bond dissociation energies of HA- and hydroquinones is obtained, which also supports a HAT mechanism. © 2009 American Chemical Society.

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