Electrochemical probing of the photoreduction of molybdenum and tungsten Dawson-type polyoxometalates in molecular and ionic liquid media using water as an electron donor

Abstract

The photochemistry of the Dawson-type [Bu₄N]₄[S₂Mo₁₈O₆₂] and [Bu₄N]₄[S₂W₁₈O₆₂] polyoxometalates in molecular solvents and [Bmim][BF₄] and [Bmim][BF₆] ionic liquids with water present as the electron donor is reported. Irradiation with UV (275–320 nm) or white (275–750 nm) light leads to reduction of the [S₂Mo₁₈O₆₂]⁴⁻ anion and concomitant oxidation of water to give dioxygen and protons in all media examined. Oxygen gas also is rapidly evolved when solid [Bu₄N]₄[S₂Mo₁₈O₆₂] in contact with water is irradiated with light. In contrast, photoreduction of [S₂W₁₈O₆₂]⁴⁻ is observed only in “wet” ionic liquids. Reactants and products associated with the photochemical reactions were monitored by a range of electrochemical methods. Substantial shifts in reversible potentials combined with modified structure of water introduced by water–IL interactions are hypothesised to facilitate photooxidation of water in ionic liquid environments. Intriguingly, whilst the polyoxotungstate is preferable as a photosensitizer, the molybdenum analogue is superior for photooxidation of water to dioxygen.