Solution photochemistry of tetrakis(tetrabutylammonium) decatungstate(VI) and catalytic hydrogen evolution from alcohols

Abstract

Photoexcitation of the oxygen-to-metal charge-transfer band (323 nm, ε= 1.35 × 104 dm3 mol–1 cm–1) of [NBu4]4[W10O32] in acetonitrile results in the reduction of WVI to WV[quantum yield, φ=(1–2)× 10–2] with accompanying formation of 1-butene in less than stoicheiometric yield (70%). Prolonged irradiation leads to the accumulation of two-electron reduced species. The e.s.r. spectrum (g= 1.84, ΔHms= 20 G) of the photolyte is in good agreement with that of [W10O32]5–. Flash photolysis experiments reveal that [W10O32]5– is produced by electron transfer from [NBu4]+ to [W10O32]4– within the lifetime of the photoflash (t½ 50 µs) and that upon protonation [W10O32]5– undergoes disproportionation to yield the two-electron reduced and diprotonated species [H2W10O32]4–. The addition of water to the solvent results in a decrease in the rate of disproportionation of [HW10O32]4–, a shift in the reduction potential of [W10O32]4– to more positive values, and the formation of butyraldehyde as an additional photoproduct. The significance of the highly negative oxidation potential for the photoreduced decatungstates [W10O32]5– and [H2W10O32]4– is considered in connection with the photocatalytic evolution of H2 from alcohols and their net oxidation to the corresponding aldehydes. U.v. irradiation of [W10O32]4– in the presence of heterogeneous catalysts such as RuO2, IrO2, and Pt leads to the evolution of H2 from butanol (φ= 0.03) with a turnover of more than 100 at room temperature.