Synthesis, Characterization, and Catalysis of β₃-[(Co^IIO₄)W₁₁O₃₁(O₂)₄],- the First Keggin-Based True Heteropoly Dioxygen (Peroxo) Anion. Spectroscopic (ESR, IR) Evidence for the Formation of Superoxo Polytungstates

Reactions of hydrogen peroxide with several lacunary polyoxometalates of the 1:11 series, XW₁₁O₃₉^m- (X = Co³⁺, Ga³⁺, Fe³⁺, Si⁴⁺, and P⁵⁺), are reported. Synthetic pathways to new polyoxotungstates incorporating dioxygen moieties (peroxo and/or superoxo) are developed. The key step involves treating lacunary precursors with H₂O₂ in strongly buffered aqueous solutions. Upon reaction of H₂O₂ with α-[Co³⁺W₁₁O₃₉],- (a) the central tetrahedral Co³⁺ is reduced to Co²⁺ and (b) each of the four unshared oxygens surrounding the vacancy are replaced by a peroxide group, yielding salts of the tetraperoxide anion β₃-[(Co²⁺O₄)W₁₁O₃₁(O₂)₄]^10- (1). These results are unequivocally established by a combination of elemental analysis, spectroscopy (UV−Vis−near-IR and IR), magnetic moment determination, and complete X-ray crystal structure analysis of (NH₄)₉K[(Co²⁺O₄)W₁₁O₃₁(O₂)₄]·5H₂O. The dioxygen O−O bonds are 1.41 and 1.44 Å, typical of peroxo complexes. Salts of 1 are excellent stereoselective catalysts for the oxidation/epoxidation by H₂O₂. Reaction of 2-cyclohexenol with H₂O₂ catalyzed by 1 yields cis- and trans-2,3-epoxycyclohexen-1-ol (59.3% and 3.6%, respectively) and 2-cyclohexen-1-one (28.3%). According to ESR and IR spectroscopic results, the reaction of H₂O₂ with other lacunary XW₁₁O₃₉^m- anions (X = P⁵⁺, Si⁴⁺, Ga³⁺, and Fe³⁺) proceeds by a different mechanism which involves the loss of heteroatom and formation of an isopolytungstate containing superoxo moieties (g₁ = 2.039, g₂ = 2.014, g₃ = 2.009; ν_0-0 = 1040 and 1060 cm^-1).