Development of Catalytic Reaction Fiekl From Nano-to Subnano-Scale by Utilizing Properties of Large Oxide Cluster

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The dinuclear peroxotungstate, K2[{W(=O)(O_2)_2(H_2O)}_2(μ-O)]・2H_2O (W2), could act as an effective epoxidation catalyst using H_2O_2 in water under mild reaction conditions. Further, we synthesized organic-inorganic hybrid support materials by covalently anchoring dihydroimidazorium based ionic liquid fragments onto the SiO_2 to heterogenize W2 for the development of recoverable and recyclable epoxidation catalysts.The epoxidation with H_2O_2 catalyzed by lacunary silicodecatungstate ([γ-SiW_<10>O_<34>(H_2O)_2]^<4->) proceeded with 【greater than or equal】99% selectivity to epoxide, 【greater than or equal】99% efficiency of H_2O_2 utilization, high stereospecificity.A novel S-shaped disilicoeicosatungustate, [(γ-SiW_<10>O_<32>(H_2O)_2)_2(μ-O)_2]^<4->, is obtained by dehydrative condensation of a partially protonated divacant lacunary Keggin-type polyoxometalate, [γ-SiW_<10>O^<34>(H_2O)_2]^<4->, in an anhydrous organic solvent with CF_3SO_3H. In contrast, the dimerization of ([γ-SiW_<10>O_<34>(H_2O)_2]^<4-> without CF_3SO_3H results in the formation of a closed-shape cluster, ([γ-SiW_<10>O_<32>_2(μ-O)_4]^<8->. While ([γ-SiW_<10>O_<34>(H_2O)_2]^<4-> and ([γ-SiW_<10>O_<32>_2(μ-O)_4]^<8-> with the common [SiW_<10>O^<32>] fragment are almost inactive, the S-shaped cluster ([γ-SiW_<10>O_<34>(H_2O)_2]^<4-> efficiently promotes the H_2O_2-based Baeyer-Villiger oxidations of cycloalkanones.On the other hand, a novel titanium-substituted silicotungstate cluster of ([γ-SiTi_2W_<10>O_<36>(OH)_2}_]^<8-> is synthesized by the introduction of titanium(IV) ions into a divacant lacunary γ-Keggin-type silicotungstate of ([γ-SiW_<10>O_<36>]^<8->. This titanium-substituted polyoxometalate exhibits a dimeric structure The epoxidation with ([γ-SiTi_2W_<10>O_<36>(OH)_2]^<8-> is stereospecific.