No abstract

No abstract

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 (g 1 = 2.039, g 2 = 2.014, g 3 = 2.009; ν0 - 0 = 1040 and 1060 cm-1).

No abstract

Synthesis, Characterization, and Catalysis of β 3 -[(Co (II) O 4 )W 11 O 31 (O 2 ) 4 ] 10− , the First Keggin-Based True Heteropoly Dioxygen (Peroxo) Anion. Spectroscopic (ESR, IR) Evidence for the Formation of Superoxo Polytungstates.-Compounds (IV) represent the first true heteropolymetallates containing dioxo (peroxo) moieties in a Keggin-related structure. The structure is unequivocally established by a combination of spectroscopic methods and X-ray analysis of the mixed salt (IVb) (space group P2 1 /m, Z = 2). Salts (IV) prove to be effective catalysts for the oxidation/epoxidation of alcohols and olefins with H 2 O 2 . Similar reaction of H 2 O 2 with other lacunary XW 11 O 39 m− anions (X: Si IV , P V , Ga III , Fe III ) yields a novel isopolytungstate containing superoxo moieties, whose structure and composition is independent of the heteroatom present in the lacunary precursor.