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

Yi-Ning Wang; Kai-Chung Lau; William W. Y. Lam; Wai-Lun Man; Chi-Fai Leung; Tai-Chu Lau

doi:10.1021/ic8015904

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

Yi-Ning Wang, Kai-Chung Lau, William W. Y. Lam, Wai-Lun Man, Chi-Fai Leung, Tai-Chu Lau

Research output: Journal Publications and Reviews › RGC 21 - Publication in refereed journal › peer-review

28 Citations (Scopus)

Abstract

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.

Original language	English
Pages (from-to)	400-406
Journal	Inorganic Chemistry
Volume	48
Issue number	1
DOIs	https://doi.org/10.1021/ic8015904
Publication status	Published - 5 Jan 2009

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@article{be9ec6c08e8f4293bacc4ba37c0e7a2b,

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

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. {\textcopyright} 2009 American Chemical Society.",

author = "Yi-Ning Wang and Kai-Chung Lau and Lam, {William W. Y.} and Wai-Lun Man and Chi-Fai Leung and Tai-Chu Lau",

year = "2009",

month = jan,

day = "5",

doi = "10.1021/ic8015904",

language = "English",

volume = "48",

pages = "400--406",

journal = "Inorganic Chemistry",

issn = "0020-1669",

publisher = "American Chemical Society",

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TY - JOUR

T1 - Kinetics and Mechanism of the Oxidation of Ascorbic Acid in Aqueous Solutions by a trans-Dioxoruthenium(VI) Complex

AU - Wang, Yi-Ning

AU - Lau, Kai-Chung

AU - Lam, William W. Y.

AU - Man, Wai-Lun

AU - Leung, Chi-Fai

AU - Lau, Tai-Chu

PY - 2009/1/5

Y1 - 2009/1/5

N2 - 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.

AB - 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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U2 - 10.1021/ic8015904

DO - 10.1021/ic8015904

M3 - RGC 21 - Publication in refereed journal

C2 - 19061342

SN - 0020-1669

VL - 48

SP - 400

EP - 406

JO - Inorganic Chemistry

JF - Inorganic Chemistry

IS - 1

ER -

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

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