Ionic Liquid-Enhanced Photooxidation of Water Using the Polyoxometalate Anion [P2W18O62]6- as the Sensitizer

Simple polyoxometalate anions are known to be photoreduced in molecular solvents in the presence of 2-propanol or benzyl alcohol. The use of ionic liquids (ILs) as the solvent is now reported to also allow the photooxidation of water to be achieved. In particular, the photochemistry of the classic Dawson polyoxometalate salt K-6[P2W18O62] has been studied in detail when water is present in the aprotic IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and the protic IL, diethanolamine hydrogen sulfate (DEAS). In these and other ILs, irradiation with white light (wavelength 275-750 nm) or UV light (wavelength 275-320 nm) leads to overall reduction of the [P2W18O62](6-) anion to [P2W18O62](7-) and concomitant oxidation of water to dioxygen and protons. The modified structure of bulk water present in ILs appears to facilitate its oxidation. Analogous results were obtained in aqueous solutions containing the protic IL as an electrolyte. The photoproducts (reduced polyoxometalate anion, dioxygen, and protons) were identified by, respectively, voltammetry, a Clark electrode, and monitoring of pH. The formal reversible potentials E-F(0) for [P2W18O62](6-/7-/8-/9-/10-) couples are much more positive than in molecular solvents. The [P2W18O62](8-) and more reduced anions, if formed as intermediates, would efficiently reduce photoproducts H+ or dioxygen to produce [P2W18O62](7-), rather than reform to [P2W18O62](6-). Thus, under photoirradiation conditions [P2W18O62](7-) acts as a kinetic sink so that in principle indirect splitting of water to produce dioxygen and dihydrogen can be achieved. The equivalent form of photooxidation does not occur in liquid water or in molecular solvents such as MeCN and MeCN/CH2Cl2 containing added water, but does occur for solid K-6[P2W18O62] in contact with water vapor.