Journal
DALTON TRANSACTIONS
Volume 39, Issue 31, Pages 7364-7373Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c003248d
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Funding
- Ministry of Education, Culture, Sports, Science and Technology of Japan [20550079]
- Kochi University President's Discretionary Fund
- Kogin Regional Economy Promotion Foundation
- Australian Research Council
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The one electron electrochemical reduction of vanadium(V) substituted-Keggin type polyoxometalate anions [XVVM11O40](4-) ((XVM11)-M-V) where X = P, As; M = Mo or W has been investigated in CH3CN as a function of acid concentration. EPR studies confirm that the product is the highly basic and hence readily protonated V(IV), ((XVM11)-M-IV), species. In the absence of acid or in the presence of a large concentration excess, the V(V/IV) redox couple gives one well defined chemically and electrochemically reversible process under conditions of cyclic voltammetry. In contrast, three well resolved V(V/IV) processes are observed in the presence of a moderate concentration of acid in the case of (XVMo11)-Mo-V but only two with (XVW11)-W-V. NMR and EPR spectra have been obtained as a function of acid concentration when the polyoxometalate anions are in the V(V) and V(IV) oxidation levels respectively. All voltammetric and spectroscopic data indicate that protonation reactions are coupled to the V(V) and V(IV) redox chemistry but that the reduced V(IV) state is much more basic than the V(V) one and that ((XVMo11)-Mo-IV) is more basic than ((XVW11)-W-IV). Digital simulations of voltammograms for reduction of (XVMo11)-Mo-V and (XVW11)-W-V have been undertaken in CH3CN as a function of acid concentration in order to define the thermodynamics and kinetics associated with the V(V/IV) process and the equilibrium constants that accompany the coupled acid-base chemistry. EPR spectra allow an estimation of the relative concentrations of protonated species present in frozen glasses derived from one-electron bulk electrolysis and also allow inferences to be drawn on their nature. This study provides a far more detailed analysis of the coupling of proton and electron transfer than previously available.
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