Proton-coupled electron transfer reactions: kinetic studies on the oxidation of dihydroxybenzenes by heteropoly 10-tungstodivanadophosphate in aqueous acidic medium

Abstract

Kinetic studies on the oxidation of hydroquinone and catechol by the heteropoly 10-tungstodivanadophosphate anion, [PV V V V W 10 O 40 ] 5-, have been carried out in aqueous acidic medium at 25°C by UV-visible spectrophotometry. The oxidation of hydroquinone shows simple second-order kinetics overall, with first-order dependence of the rate on both [oxidant] and [hydroquinone] at constant [H ? ]. For catechol oxidation, the order of the reaction with respect to [oxidant] is unity, while the order with respect to [catechol] is variable; this reaction shows Michaelis-Menten-type kinetics at constant [H ? ]. The rate of the reaction is insensitive to [H ?] in the pH range 1.2-1.7. Rate retardation for deuterated hydroquinone and catechol (C 6 H 4 (OD) 2 ) in D 2 O indicates breaking of the -OH bond in the rate-limiting step. Based on the observed kinetic isotope effect and calculated ground-state free energy change (DG 0 ) values, a hydrogen atom transfer mechanism is suggested for the reaction; i.e., in the ratelimiting step, one electron and one proton are transferred from the reductant to the oxidant in a concerted manner. Rates of oxidation of hydroquinone by this oxidant in neat acetonitrile at 25°C have also been measured. By applying the Marcus equation, the self-exchange rate constant of the oxidant (V V V IV OH/H Á ) in acetonitrile has been evaluated.

Concerted Multiproton–Multielectron Transfer for the Reduction of O2 to H2O with a Polyoxovanadate Cluster

“…To date, there is only a single report describing the BDFE­(O–H) at the surface of a POM. Thermochemical analysis of the monoreduced, monoprotonated form of the heteropolytungsto­divanado­phosphate cluster, [PV 2 W 10 O 40 ] 5– , revealed a BDFE­(O–H) of 75.4 kcal/mol in acetonitrile . Notably, the authors describe selective formation of a bridging hydroxide ligand located between the two adjacent vanadium centers in the cluster as a result of the increased basicity of the reduced vanadium centers located within the assembly.…”

“…While there have been a number of reports focused generally on the acid dependence of the electrochemistry of POMs, [26][27][28][29][30][31][32][33]45 to date there are few studies that investigate the thermochemistry of surface hydroxide moieties of the reduced variants of these metal oxide assemblies. 46 This is likely due to the fact that in addition to positive shifts in potential upon introduction of protons to the surface of the clusters, redox events tend to merge together and form multielectron/multiproton transfer events. [28][29][30][31][32][33]45 Indeed, the high-valent POV cluster reported here suffers a similar fate; acidification of an acetonitrile solution of V 6 O 13 results in the consolidation of single electron redox events (Figure S6).…”

“…where 46 Notably, the authors describe selective formation of a bridging hydroxide ligand located between the two adjacent vanadium centers in the cluster as a result of the increased basicity of the reduced vanadium centers located within the assembly. Studies of a homometallic variant of the polyoxotungstate, [PW 12 O 40 ] 3− , have demonstrated that the cluster is incapable of performing single-site CPET.…”

“…This procedure is adapted from ref . A 5 mL sample was prepared in acetonitrile containing 0.55 mM V 6 O 7 (OH) 6 , 0.27 mM V 6 O 9 (OH) 4 , 0.1 M [ n Bu 4 N]­[PF 6 ] as supporting electrolyte, and 0.05 M buffer of 1:1 1,1,3,3-tetramethylguanidine/​1,1,3,3-tetramethylguanidinium tetrafluoroborate.…”

Coordination-induced bond weakening of water at the surface of an oxygen-deficient polyoxovanadate cluster

“…The few studies in which PCET at POM surfaces has been explicitly investigated has revealed that the uptake of H-atom equivalents primarily occurs at bridging oxide positions. 12,18 An alternative mechanism for the generation of reactive Hatom equivalents at MO x surfaces invokes coordination-induced bond weakening of O-H moieties upon association of water to a surface oxygen-atom (O-atom) vacancy (Figure 1). In this scenario, a coordinatively unsaturated, reduced metal centre at the surface of the material binds to water, resulting in a weakening of the O-H bond of the substrate.…”