A novel Ti-substituted silicotungstate cluster of [{ﾎｳ-SiTi2W10O36(OH)2}2(ﾎｼ-O)2]8- (1) was synthesized by the introduction of Ti(IV) ions into a divacant lacunary ﾎｳ-Keggin-type silicotungstate of [ﾎｳ-SiW10O36]8-. This Ti-substituted polyoxometalate, 1, exhibits a dimeric structure. One half of the ﾎｳ-Keggin fragment of 1 contains a dinuclear Ti center bridged by two hydroxo groups, and the resulting Ti2(ﾎｼ-OH)2 core connects to the other Ti2(ﾎｼ-OH)2 core of the paired ﾎｳ-Keggin subunit through Ti-O-Ti linkages. The Ti2(ﾎｼ-OH)2 core of 1 reacts with MeOH to form the corresponding alkoxo deriv., [{ﾎｳ-SiTi2W10O36(OH)(OMe)}2(ﾎｼ-O)2]8- (2). Two of four hydroxo groups of the Ti2(ﾎｼ-OH)2 cores in 1 are replaced by methoxo groups to give the Ti2(ﾎｼ-OH)(ﾎｼ-OMe) core, and the Ti-O-Ti linkages connecting two ﾎｳ-Keggin subunits are maintained in 2. The ﾎｳ-Keggin dititanium-substituted silicotungstate 1 catalyzes mono-oxygenation reactions, such as the epoxidn. of olefins and sulfoxidn. of sulfides with H2O2 under mild conditions, while the monotitanium-substituted silicotungstate, [ﾎｱ-SiTiW11O39]4- (3), and the fully occupied silicododecatungstate, [ﾎｳ-SiW12O40]4-, are inactive. The epoxidn. with 1 is stereospecific; the configurations around the C:C double bonds of the cis- and trans-olefins are completely retained in the corresponding epoxides. For the competitive epoxidn. of cis- and trans-2-octenes, the ratio of the formation rate of cis-2,3-epoxyoctane to that of the trans isomer (Rcis/Rtrans) is relatively high (21.3) in comparison with those obsd. for the tungstate catalysts, including [ﾎｳ-SiW10O34(H2O)2]4-. The epoxidn. of 3-methyl-1-cyclohexene is highly diastereoselective and gives the corresponding epoxide with an anti configuration. The mol. structure of 1 is preserved during the catalysis because the 29Si and 183W NMR spectra of the catalyst recovered after completion of the oxidn. are consistent with those of as-prepd. compd. 1. All these facts suggest the contribution of rigid nonradical oxidants generated on the multinuclear Ti center of 1. [on SciFinder(R)]
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