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

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.