Synthesis, structural characterization, and catalytic performance of dititanium-substituted γ-Keggin silicotungstate

A novel titanium-substituted silicotungstate cluster of [{gamma-SiTi2W10O36(OH)(2)}(2)(mu-O)(2)](8-) (1) is synthesized by the introduction of titanium(IV) ions into a divacant lacunary gamma-Keggin-type silicotungstate of [gamma-SiW10O36](8-). This titanium-substituted polyoxometalate, 1, exhibits a dimeric structure. One half of the gamma-Keggin fragment of 1 contains a dinuclear titanium center bridged by two hydroxo groups, and the resulting Ti-2(mu-OH)(2) core connects to the other Ti-2(mu-OH)(2) core of the paired gamma-Keggin subunit through Ti-O-Ti linkages. The Ti-2(mu-OH)(2) core of 1 reacts with MeOH to form the corresponding alkoxo derivative, [{gamma-SiTi2W10O36(OH)(OMe)}(2)(mu-O)(2)](8-) (2). Two of four hydroxo groups of the Ti-2(mu-OH)(2) cores in 1 are replaced by methoxo groups to give the Ti-2(mu-OH)(mu-OMe) core, and the Ti-O-Ti linkages connecting two gamma-Keggin subunits are maintained in 2. The gamma-Keggin dititanium-substituted silicotungstate 1 catalyzes mono-oxygenation reactions, such as the epoxidation of olefins and sulfoxidation of sulfides with hydrogen peroxide under mild conditions, while the monotitanium-substituted silicotungstate, [alpha-SiTiW11O39](4-) (3), and the fully occupied silicododecatungstate, [gamma-SiW12O40](4-), are inactive. The epoxidation 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 epoxidation of cis- and trans-2-octenes, the ratio of the formation rate of cis-2,3-epoxyoctane to that of the trans isomer (R-cis/R-trans) is relatively high (21.3) in comparison with those observed for the tungstate catalysts, including [gamma-SiW10O34(H2O)(2)](4-). The epoxidation of 3-methyl-1-cyclohexene is highly diastereoselective and gives the corresponding epoxide with an anti configuration. The molecular structure of 1 is preserved during the catalysis because the Si-29 and W-183 NMR spectra of the catalyst recovered after completion of the oxidation are consistent with those of as-prepared compound 1. All these facts suggest the contribution of rigid nonradical oxidants generated on the multinuclear titanium center of 1.