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 W^VI to W^V[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, ΔH_ms= 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 (t_½ 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.