Voltammetric Determination of the Reversible Potentials for [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10- over the pH Range of 2-12: Electrolyte Dependence and Implications for Water Oxidation Catalysis

Voltammetric studies of the Ru-containing polyoxometalate water oxidation molecular catalyst [{Ru4O4(OH)(2)(H2O)(4)}(gamma-SiW10O36)(2)](10-) ([1(gamma-SiW10O36)(2)](10-) where 1 represents the {Ru4O4(OH)(2)(H2O)(4)} core and 1(0) stands for its initial form with all ruthenium centers in the oxidation state IV) have been carried out in aqueous media over a wide range of pH (2-12 using Britton-Robinson buffer) and ionic strength. Well-defined voltammograms in buffered media are only obtained when Frumkin double layer effects are suppressed by the presence of a sufficient concentration of additional supporting electrolyte (LiNO3, NaNO3, KNO3, Ca(NO3)(2), Mg(NO3)(2), MgSO4, or Na2SO4). A combination of data derived from dc cyclic, rotating disk electrode, and Fourier transformed large amplitude ac voltammetry allow the assignment of two processes related to reduction of the framework and the complete series of Ru-III/IV and Ru-IV/V redox processes and also provide their reversible potentials. Analysis of these data reveals that K+ has a significantly stronger interaction with 1(1) (the number inside bracket stands for the number of electrons removed from 1(0)) than found for the other cations investigated, and hence its presence significantly alters the pH dependence of the 1(0)11(1) reversible potential. Comparison of experimental data with theory developed in terms of equilibrium constants for process 1(0)11(1) reveals that both H+ and K+ interact competitively with both 1(0) and 1(1). Importantly, reversible potential data reveal that (i) proton transfer does not necessarily need to be coupled to all electron transfer steps to achieve catalytic oxidation of water, (ii) the four-electron oxidized form, 1(4), is capable of oxidizing water under all conditions studied, and (iii) under some conditions, the three-electron oxidized form, 1(3), also exhibits considerable catalytic activity.