Synthesis, structural characterization, and catalytic performance of dititanium-substituted γ-Keggin silicotungstate
Synthesis, structural characterization, and catalytic performance of dititanium-substituted γ-Keggin silicotungstate
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DOI：
10.1021/ic052179q
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发表时间：
2006-03-06
影响因子：
4.6
通讯作者：
Mizuno, N
中科院分区：
文献类型：
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作者：
Goto, Y;Kamata, K;Mizuno, N
通过在双空位γ-Keggin型硅钨酸盐[γ-SiW_（10）O_（36）]（8-）中引入钛离子，合成了一种新型的钛取代硅钨酸盐簇合物[{γ-SiTi_2 W_（10）O_（36）（OH）（2）}（2）（μ-O）（2）]（8-）（1）.该钛取代的聚氧乙烯酸盐1表现出二聚体结构。1的γ-Keggin片段的一半包含由两个羟基桥接的双核钛中心，并且所得的Ti-2（μ-OH）（2）核心通过Ti-O-Ti键连接到成对的γ-Keggin亚基的另一个Ti-2（μ-OH）（2）核心。1的Ti-2（μ-OH）（2）核与MeOH反应形成相应的烷氧基衍生物[{γ-SiTi 2 W10 O36（OH）（OMe）}（2）（μ-O）（2）]（8-）（2）。1中Ti-2（mu-OH）（2）核的四个羟基中的两个被甲氧基取代以得到Ti-2（mu-OH）（mu-OMe）核，并且连接两个γ-Keggin亚基的Ti-O-Ti键保持在2中。γ-Keggin二钛取代的硅钨酸盐1催化单氧化反应，如烯烃的环氧化和硫化物与过氧化氢在温和条件下的磺基氧化，而单钛取代的硅钨酸盐，[α-SiTiW 11 O 39]（4-）（3），和完全占据的硅十二钨酸盐，[γ-SiW 12 O 40]（4-），是无活性的。1的环氧化反应是立体定向的;顺式和反式烯烃的C=C双键周围的构型完全保留在相应的环氧化物中。对于顺式和反式-2-辛烯的竞争性环氧化，顺式-2，3-环氧辛烷的生成速率与反式异构体的生成速率之比（R-cis/R-trans）与在钨酸盐催化剂（包括[γ-SiW 10 O 34（H2O）（2）]（4-）中观察到的那些相比相对较高（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，并得到相应的反式环氧化物。1的分子结构在催化过程中得以保留，因为氧化完成后回收的催化剂的Si-29和W-183 NMR谱与所制备的化合物1的一致。所有这些事实表明，刚性非自由基氧化剂的贡献产生的多核钛中心1。
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.