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Mono- and Dinuclear Silica-Supported Titanium(IV) Complexes and the Effect of TiOTi Connectivity on Reactivity

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journal contribution
posted on 27.07.1999, 00:00 by Abdillahi Omar Bouh, Gordon L. Rice, Susannah L. Scott
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, 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 Ti−O−Ti bridges. A synthetic route to mononuclear silica-supported Ti alkoxide complexes was developed by reaction of grafted amide complexes (⋮SiO)nTi(NEt2)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 cyclohexene to generate cyclohexene oxide.