Polysiloxanic Scaffolds as Ligands and Models to Study Organic and Organometallic Species on the Silica Surface

The use of homogeneous catalytic systems as soluble models of heterogeneous catalysts has become increasingly important for investigations on the structure and reactivity of the active species. Therefore we decided to investigate the reactivity with rhenium complexes of a silanol with an electrophilicity close to that of the silica silanol sites, such as the hydroxysilsesquioxane (c-C6H11)7Si8O12OH. The reaction with [Re(CO)5(O3SCF3)] give {Re(CO)5[OO12Si8(c-C6H11)7]} that is the first [Re(CO)5OR] species stable enough to be characterized. In the search of a poor nucleophile able to stabilize this very elusive species [Re(CO)5(OSiR3)], we decided also to synthesize the previously never described (C6F5)3SiOH. We also studied the synthesis and reactivity of new Ti-silsesquioxanes framework. We decided to synthesize a molecular model of the silica-grafted titanium catalyst by reaction of TiCpCl3 (Cp = η5−C5H5) with endo-C2h-(c-C6H11)6Si6O7(OH)4. The main reaction product is a novel Ti-silsesquioxane containing two titanium atoms connected through an oxygen bridge [(Cy)6Si6O12Ti2(Cp)2] that is a valid model of dimeric (“double”) Ti-O-Ti sites that are supposed to be the active centres on the silica surface where the oxidation of 2,3,6-trimethylphenol takes place selectively to trimethyl-p-benzoquinone (TMBQ). With the aim to dispose of a new model of Titanium-single.site catalyst we also investigated the reactivity of endo-(c-C6H11)8Si8O11(OH)2 with Ti(OCH2CH3)2(O2C6H4). The reaction afforded (Cy8Si8O11O2)2Ti. This species can be viewed as a soluble analogous of titanosilicalites.