Goto Yuya, Kamata Keigo, Yamaguchi Kazuya, Uehara Kazuhiro, Hikichi Shiro, Mizuno Noritaka
Department of Applied Chemistry, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
Inorg Chem. 2006 Mar 6;45(5):2347-56. doi: 10.1021/ic052179q.
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 Ti2(mu-OH)2 core connects to the other Ti2(mu-OH)2 core of the paired gamma-Keggin subunit through Ti-O-Ti linkages. The Ti2(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 Ti2(mu-OH)2 cores in 1 are replaced by methoxo groups to give the Ti2(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 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.
通过将钛（IV）离子引入[γ-SiW10O36]8-的二空位缺位γ-凯基型硅钨酸盐中，合成了一种新型的钛取代硅钨酸盐簇[{γ-SiTi2W10O36(OH)2}2(μ-O)2]8-（1）。这种钛取代的多金属氧酸盐1具有二聚体结构。1的γ-凯基片段的一半包含由两个羟基桥连的双核钛中心，所得的Ti2(μ-OH)2核通过Ti-O-Ti键连接到配对的γ-凯基亚基的另一个Ti2(μ-OH)2核。1的Ti2(μ-OH)2核与甲醇反应形成相应的烷氧基衍生物[{γ-SiTi2W10O36(OH)(OMe)}2(μ-O)2]8-（2）。1中Ti2(μ-OH)2核的四个羟基中的两个被甲氧基取代，得到Ti2(μ-OH)(μ-OMe)核，并且连接两个γ-凯基亚基的Ti-O-Ti键在2中得以保留。γ-凯基二钛取代硅钨酸盐1在温和条件下催化单加氧反应，如烯烃的环氧化和用过氧化氢氧化硫化物的反应，而单钛取代硅钨酸盐[α-SiTiW11O39]4-（3）和完全占据的硅十二钨酸盐[γ-SiW12O40]4-则无活性。用1进行的环氧化反应具有立体选择性；顺式和反式烯烃的C=C双键周围的构型在相应的环氧化物中完全保留。对于顺式和反式2-辛烯竞争环氧化反应，与包括[γ-SiW10O34(H2O)2]4-在内的钨酸盐催化剂相比，顺式2,3-环氧辛烷与反式异构体的生成速率之比（R(顺式)/R(反式)）相对较高（21.3）。3-甲基-1-环己烯的环氧化反应具有高度的非对映选择性，得到具有反式构型的相应环氧化物。在催化过程中1的分子结构得以保留，因为氧化反应完成后回收的催化剂的29Si和183W NMR光谱与制备的化合物1的光谱一致。所有这些事实表明在1的多核钛中心上产生的刚性非自由基氧化剂的作用。
