Synthesis, Structural Characterization, and Catalytic Performance of Dititanium-Substituted γ-Keggin Silicotungstate

A novel titanium-substituted silicotungstate cluster of [{γ-SiTi2W10O36(OH)2}2(μ-O)2]8- (1) is synthesized by the
introduction of titanium(IV) ions into a divacant lacunary γ-Keggin-type silicotungstate of [γ-SiW10O36]8-. This titanium-substituted polyoxometalate, 1, exhibits a dimeric structure. One half of the γ-Keggin fragment of 1 contains a
dinuclear titanium center bridged by two hydroxo groups, and the resulting Ti2(μ-OH)2 core connects to the other
Ti2(μ-OH)2 core of the paired γ-Keggin subunit through Ti−O−Ti linkages. The Ti2(μ-OH)2 core of 1 reacts with
MeOH to form the corresponding alkoxo derivative, [{γ-SiTi2W10O36(OH)(OMe)}2(μ-O)2]8- (2). Two of four hydroxo
groups of the Ti2(μ-OH)2 cores in 1 are replaced by methoxo groups to give the Ti2(μ-OH)(μ-OMe) core, and the
Ti−O−Ti linkages connecting two γ-Keggin subunits are maintained in 2. The γ-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,
[α-SiTiW11O39]4- (3), and the fully occupied silicododecatungstate, [γ-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 (Rcis/Rtrans) is relatively high (21.3) in comparison
with those observed for the tungstate catalysts, including [γ-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 29Si and 183W 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.