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

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.