Synthesis, Characterization, and Catalysis of β₃-[(Co^IIO₄)W₁₁O₃₁(O₂)₄],^10- the First Keggin-Based True Heteropoly Dioxygen (Peroxo) Anion. Spectroscopic (ESR, IR) Evidence for the Formation of Superoxo Polytungstates

Abstract

Reactions of hydrogen peroxide with several lacunary polyoxometalates of the 1:11 series, XW11O39m- (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],9- (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 XW11O39m- 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; ν0-0 = 1040 and 1060 cm-1).