Preparation, characterization and reactivity of mono- and dinuclear silica-supported titanium(IV) complexes

This thesis deals with the preparation, characterization and reactivity of silica-supported titanium(IV) complexes. The room temperature reactions of excess Ti(OiPr)₄ with the hydroxyl groups of a nonporous silica yield dinuclear surface complexes regardless of the degree of partial dehydroxylation of the silica. The surface reactions were studied by in situ IR transmission spectroscopy, DRUV-vis, 13C CP/MAS NMR, GC/MS and elemental analysis. The spontaneous stoichiometric formation of both 2-propanol and propene during grafting indicates that interaction of Ti(OiPr)₄ with the silica surface induces disproportionation of alkoxide ligands with concomitant formation of a Ti-O-Ti bridge. A synthetic route to mononuclear silica-supported Ti alkoxide complexes was developed by reaction of grafted amide complexes (≡SiO)_nTi(NEt ₂)_4−n (n = 1 or 2) with alcohols. Subsequent reactions of the mononuclear surface alkoxide complexes with Ti(OiPr) ₄ yield dinuclear species identical to those prepared by the direct reaction of Ti(OiPr)₄ with silica.; Both mono- and dinuclear supported alkoxide complexes undergo ligand exchange reactions with tert-butylhydroperoxide, but only the dinuclear alkylperoxo titanium surface complexes react with olefins to generate epoxides, which are formed quantitatively as the exclusive product at short reaction times. At longer contact times, the yield of epoxide decreases, without the appearance of other volatile oxidation products. It was discovered that the epoxide reacts with the catalyst and the product blocks the active site, inhibiting further epoxidation reaction with tert-butylhydroperoxide.