Quantitative and Qualitative Photoelectrochemical Studies on the 18-Molybdodisulfate Anion [S₂Mo₁₈O₆₂]⁴⁻

The heteropoly anion clusters [S₂Mo₁₈O₆₂]⁴⁻ and [S₂Mo₁₈O₆₂]⁵⁻ may be photochemically activated to generate extremely powerful oxidants. Quantitative photoelectrochemical measurements using laser excitation at 325 nm of acetonitrile solutions of [S₂Mo₁₈O₆₂]⁴⁻ containing the electron donors (ED), toluene and tetrahydrofuran, with hydrodynamic voltammetric monitoring at a platinum channel electrode are consistent with the following CECE mechanism: [S₂Mo₁₈O₆₂]⁴⁻ + ED (k₁(hv)) → [S₂Mo₁₈O₆₂]⁵⁻ + ED+ (C); [S₂Mo₁₈O₆₂]⁵⁻ → [S₂Mo₁₈O₆₂]⁴⁻ + e⁻ (E); [S₂Mo₁₈O₆₂]⁵⁻ +ED (k₂(hv)) ⇌ [S₂Mo₁₈O₆₂]⁶⁻ + ED⁺ (C); [S₂Mo₁₈O₆₂]⁶⁻ ⇌ [S₂Mo₁₈O₆₂]⁴⁻ + 2e⁻ (E); ED⁺ (fast) → products. Photochemical oxidation of other electron donors such as methanol, [BPh₄]⁻, ferrocene, and Cr(CO)₃ (arene) also occurs to generate [S₂Mo₁₈O₆₂]⁵⁻ and [S₂Mo₁₈O₆₂]⁶⁻ but not the more highly reduced [S₂Mo₁₈O₆₂]⁷⁻ and [S₂Mo₁₈O₆₂]⁸⁻ anions. It is concluded on the basis of these studies that [S₂Mo₁₈O₆₂]⁴⁻ is a potentially valuable photooxidant in the synthetic sense, since it may be readily synthesised in large quantities from inexpensive materials.