Synthesis and Catalytic Activity of Group 4 Metallocene Containing Silsesquioxanes Bearing Functionalized Silyl Groups

Abstract

A series of metallocene-containing silsesquioxanes with alkenylsilyl and trimethylsilyl groups, Cp‘2M[(c-C5H9)7Si7O11](OSiMe2R) (2b−d, 3b−d, 4d, 5d: Cp‘ = Cp (cyclopentadienyl), Cp* (pentamethylcyclopentadienyl); M = Ti, Zr, Hf, R = methyl, vinyl, allyl), have been synthesized. The structures of the complexes Cp2M[(c-C5H9)7Si7O11](OSiMe2CH2CHCH2) (M = Zr (3d), Hf (5d)) have been unambiguously established by single-crystal X-ray diffraction analyses. Silylation or germylation of the zirconocene-containing silsesquioxane monosilanol Cp2Zr[(c-C5H9)7Si7O11](OH) (3a) or its hafnocene derivative Cp2Hf[(c-C5H9)7Si7O11](OH) (5a), which can be prepared by the careful reaction of the silsesquioxane trisilanol (c-C5H9)7Si7O9(OH)3 (1a) with zirconocene dichloride or hafnocene dichloride, yields the new series of metallocene-containing silsesquioxanes Cp2Zr[(c-C5H9)7Si7O11](OSiMe2H) (3e), Cp2Zr[(c-C5H9)7Si7O11](OGeMe3) (3f), and Cp2Hf[(c-C5H9)7Si7O11](OSiMe2H) (5e). The reaction of 3e with 2 equiv of hydrogen chloride results in the formation of the dimethylsilyl-containing silsesquioxane disilanol (c-C5H9)7Si7O9(OH)2(OSiMe2H) (1e), indicating the applicability of metallocene moieties as protecting groups of two adjacent silanol groups. Silsesquioxanes bearing alkenylsilyl groups can be easily converted to derivatives with ethoxysilyl groups, Cp2M[(c-C5H9)7Si7O11][OSiMe2(CH2)3SiMe2(OEt)] (M = Ti (2g), Zr (3g)), by the hydrosilylative reaction. The preliminary examination of the catalytic activity of these metallocene-containing silsesquioxanes toward the epoxidation of cyclohexene by tert-butyl hydroperoxide revealed that titanocene-containing silsesquioxanes (2b−d) show modest catalytic activity. The presence of alkenylsilyl groups has been found to accelerate the reactions, especially in the case of titanium-bridged silsesquioxanes.

Ti-bridged silsesquioxanes as precursors of silica-supported titanium oxide catalysts for the epoxidation of cyclooctene

“…Other Ti-containing POSS, 1b and 2 (see Scheme 2), showed similar activities (entries 3 and 4). The activities of Ti-containing POSS as homogeneous catalysts for the epoxidation by organic peroxides have been previously reported [16][17][18][19][20][21][22][23]. Silica-tethered 1a showed a catalytic activity similar to that of 1a, indicating that 1a was tethered without a loss of activity (entry 5).…”

“…Severe non-productive decomposition of hydrogen peroxide was observed in the presence of oxide catalysts such as 1a-SiMe 2 (H)-silica-cal and 1a@SiMe 2 (H)-silica-cal. As reported previously, Ti-POSS do not show activity as homogeneous catalysts when hydrogen peroxide is employed as an oxidizing reagent [23,27]. In aqueous solutions, Ti-POSS molecules severely aggregate, which would be one reason for the lack of the catalytic activity.…”

“…In particular, the catalytic activities of Ti-containing POSS for the epoxidation of alkenes have been examined [16][17][18][19][20][21][22][23]. In addition to their activities as homogeneous catalysts, their application to heterogeneous catalysis using metal-containing POSS as precursors has been examined [10,13,15,[24][25][26][27][28][29][30][31].…”

Preparation of Novel Materials for Catalysts Utilizing Metal-Containing Silsesquioxanes

“…The activity of 2a for the epoxidation of cyclohexene in the presence of t BuOOH as an oxidant was found to be moderate (table 1) [39]. However, the presence of alkenylsilyl groups, which were introduced for other purposes (see below), in the catalyst molecules showed a positive effect on the activities.…”

“…the epoxidation of alkenes [2][3][4][5][6][7][8][9][21][22][23][25][26][27][28] and polymerization of small alkenes [2,3,16,17,[35][36][37][38], have been reported. In the course of our study on the preparation of novel group 4 metal-containing silsesquioxanes, we have successfully synthesized titanocenecontaining silsesquioxanes (2a-c) for the first time (equation 3) [39]. The preparation of 2a-c only proceeded in toluene solutions.…”