Synthesis and Catalytic Activity of Titanium Silsesquioxane Frameworks as Models of Titanium Active Surface Sites of Controlled Nuclearity

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• 作者：Tommaso Giovenzana ; Matteo Guidotti ; Elena Lucenti ; Alessio Orbelli Biroli ; Laura Sordelli ; Angelo Sironi ; Renato Ugo
• 刊名：Organometallics
• 出版年：2010
• 出版时间：December 27, 2010
• 年：2010
• 卷：29
• 期：24
• 页码：6687-6694
• 全文大小：911K
• 年卷期：v.29,no.24(December 27, 2010)
• ISSN：1520-6041

文摘

The synthesis and catalytic activity of some new titanium(IV) complexes with incompletely condensed Cy₆Si₆O₇(OH)₄ and endo-Cy₈Si₈O₁₁(OH)₂ (Cy = c-C₆H₁₁) silsesquioxane cages have been carried out with the aim of investigating new Ti silsesquioxane frameworks as molecular models of titanium active sites of catalysts prepared by grafting Ti(IV) precursors on the surface or in a matrix of mesoporous silica. The silsesquioxane Cy₆Si₆O₇(OH)₄, which bears four potentially reactive OH groups, when reacted with TiCpCl₃ (Cp = η⁵-C₅H₅) affords the new species [Cy₆Si₆O₁₂Ti₂(Cp)₂], with two titanium atoms connected through one μ-oxygen bridge, as confirmed by a XANES and EXAFS spectroscopic investigation. By reaction of endo-Cy₈Si₈O₁₁(OH)₂ with [Ti(o-C₆H₄O₂)(OCH₂CH₃)₂] the new complex [Cy₈Si₈O₁₁O₂]₂Ti was obtained, which exhibits a tetrapodal coordination of Ti, as shown by a X-ray diffraction investigation. The catalytic activity of these two new Ti silsesquioxane complexes was investigated for the catalytic epoxidation of cyclohexene with tert-butyl hydroperoxide and compared with the catalytic activity of the known [Cy₇Si₇O₁₂TiCp], widely used to model very active tripodal titanium single sites. It appears that a tripodal coordination of Ti sites facilitates the catalytic activity that is in contrast quite low for the two new complexes investigated in this work, probably due to the higher stability toward dissociation to generate a tripodal coordination of the Ti−O−Ti bond and to the steric hindrance of Ti tetrapodal coordination.