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

Abstract

The oxidation of ascorbic acid (H(2)A) 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 in aqueous solutions under argon. The reaction occurs in two phases: trans-[Ru(VI)(tmc)(O)(2)](2+) + H(2)A --> trans-[Ru(IV)(tmc)(O)(OH(2))](2+) + A, trans-[Ru(IV)(tmc)(O)(OH(2))](2+) + H(2)A --> trans-[Ru(II)(tmc)(OH(2))(2)](2+) + A. Further reaction involving anation by H(2)A 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 H(2)A, with the second-order rate constants k(2) = (2.58 +/- 0.04) x 10(3) M(-1) s(-1) at pH = 1.19 and k(2)' = (1.90 +/- 0.03) M(-1) s(-1) at pH = 1.24, T = 298 K and I = 0.1 M for the first and second phase, respectively. Studies on the effects of acidity on k(2) and k(2)(') suggest that HA(-) is the kinetically active species. Kinetic studies have also been carried out in D(2)O, 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.

Photocatalytic Hydrogen Evolution from Rhenium(I) Complexes to [FeFe] Hydrogenase Mimics in Aqueous SDS Micellar Systems: A Biomimetic Pathway

“…The reason for the presence of water here is that H 2 A is not soluble in pure CH 3 CN. As the irradiation at λ > 400 nm went on for 1 h, H 2 evolution was clearly observed by GC and dehydrogenated ascorbic acid A, the major product when H 2 A loses its electron and proton, was detected by 1 H NMR analysis (Supporting Information Figure S8). Therefore, it is reasonable to think that the photoinduced electron transfer from the Re(I) complexes to the [FeFe] H 2 ases mimics occurs in the aqueous SDS micelle solution even though the electrochemical window of the SDS micelle is too narrow to detect the redox potential of the complexes directly.…”

A Hybrid Photocatalytic System Comprising ZnS as Light Harvester and an [Fe2S2] Hydrogenase Mimic as Hydrogen Evolution Catalyst

“…As shown in Figure 4, photocatalytic H 2 evo- www.chemsuschem.org lution was achieved in the presence of all these five electron donors, demonstrating the versatility of the [Fe 2 S 2 ]/ZnS hybrid catalyst system. The oxidation potentials of these five electron donors are in the order ascorbic acid < TEOA < TEA < lactic acid < acetic acid (that is, 0.71 V [15] < 0.82 V [16] < 0.93 V [17] < 1.34 V [18] < 1.58 V [19] vs. NHE), which means that basic TEOA and TEA are more easily oxidized than lactic acid and acetic acid. However, the photocatalytic activity of the hybrid system in the presence of basic reagents was lower than with the acidic ones (Figure 4).…”

Investigation of the Reactivity between a Ruthenium Hexacationic Prism and Biological Ligands

“…From these NMR experiments, it became apparent that [ 1 ] 6+ can act as a catalyst for oxidation of ascorbic acid. This characteristic is known for Ru-complexes, and it was for instance shown that NAMI-A can be reduced by a variety of agents present in vivo , including ascorbic acid . The ability of [ 1 ] 6+ to oxidize ascorbic acid might have an important biological significance, since ascorbic acid typically reacts with oxidants of the reactive oxygen species (ROS), usually hydroxyl radicals formed from hydrogen peroxide.…”