Mono- and dinuclear silica-supported titanium(IV) complexes and the effect of TiOTi connectivity on reactivity

The room-temperature reactions of excess Ti((OPr)-Pr-i)(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, C-13 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((OPr)-Pr-i)(4) with the silica surface induces disproportionation of alkoxide Ligands with concomitant formation of Ti-O-Ti bridges. A synthetic route to mononuclear silica-supported Ti alkoxide complexes was developed by reaction of grafted amide complexes (drop SiO)(n)Ti(NEt2)(4-n) (n = 1 or 2) with alcohols. Subsequent reactions of the mononuclear surface alkoxide complexes with Ti((OPr)-Pr-i)(4) yield dinuclear species identical to those prepared by the direct reaction of Ti((OPr)-Pr-i)(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 cyclohexene to generate cyclohexene oxide.