Synthesis, Characterization, and Catalysis of β<sub>3</sub>-[(Co<sup>II</sup>O<sub>4</sub>)W<sub>11</sub>O<sub>31</sub>(O<sub>2</sub>)<sub>4</sub>],- 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, XW<sub>11</sub>O<sub>39</sub><i><sup>m</sup></i><sup>-</sup> (X = Co<sup>3+</sup>, Ga<sup>3+</sup>, Fe<sup>3+</sup>, Si<sup>4+</sup>, and P<sup>5+</sup>), are reported. Synthetic pathways to new polyoxotungstates incorporating dioxygen moieties (peroxo and/or superoxo) are developed. The key step involves treating lacunary precursors with H<sub>2</sub>O<sub>2</sub> in strongly buffered aqueous solutions. Upon reaction of H<sub>2</sub>O<sub>2</sub> with α-[Co<sup>3+</sup>W<sub>11</sub>O<sub>39</sub>],- (a) the central tetrahedral Co<sup>3+</sup> is reduced to Co<sup>2+</sup> and (b) each of the four unshared oxygens surrounding the vacancy are replaced by a peroxide group, yielding salts of the tetraperoxide anion β<sub>3</sub>-[(Co<sup>2+</sup>O<sub>4</sub>)W<sub>11</sub>O<sub>31</sub>(O<sub>2</sub>)<sub>4</sub>]<sup>10-</sup> (<b>1</b>). 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 (NH<sub>4</sub>)<sub>9</sub>K[(Co<sup>2+</sup>O<sub>4</sub>)W<sub>11</sub>O<sub>31</sub>(O<sub>2</sub>)<sub>4</sub>]·5H<sub>2</sub>O. The dioxygen O−O bonds are 1.41 and 1.44 Å, typical of peroxo complexes. Salts of <b>1</b> are excellent stereoselective catalysts for the oxidation/epoxidation by H<sub>2</sub>O<sub>2</sub>. Reaction of 2<i>-</i>cyclohexenol with H<sub>2</sub>O<sub>2</sub> catalyzed by <b>1</b> yields <i>cis-</i> and <i>trans</i>-2,3-epoxycyclohexen<i>-</i>1<i>-</i>ol (59.3% and 3.6%, respectively) and 2<i>-</i>cyclohexen<i>-</i>1<i>-</i>one (28.3%). According to ESR and IR spectroscopic results, the reaction of H<sub>2</sub>O<sub>2</sub> with other lacunary XW<sub>11</sub>O<sub>39</sub><i><sup>m</sup></i><sup>-</sup> anions (X = P<sup>5+</sup>, Si<sup>4+</sup>, Ga<sup>3+</sup>, and Fe<sup>3+</sup>) proceeds by a different mechanism which involves the loss of heteroatom and formation of an isopolytungstate containing superoxo moieties (<i>g</i><sub>1</sub> = 2.039, <i>g</i><sub>2</sub> = 2.014, <i>g</i><sub>3</sub> = 2.009; ν<sub>0</sub><sub>-</sub><sub>0</sub> = 1040 and 1060 cm<sup>-1</sup>).