Synthesis and characterisation of titanium silasesquioxane complexes: soluble models for the active site in titanium silicate epoxidation catalysts †

Abstract

Titanium silasesquioxane complexes have been prepared as models for the catalytically active centres in titanium silicate oxidation catalysts. Complexes [TiL(R7Si7O12)] [R = c-C6H11, L = CH2Ph 5, NMe2 6, OSiMe3 7, OPri 8 or OBut 9; R = c-C5H9, L = CH2Ph 13 or OPri 14] were prepared from the reactions of incompletely condensed silasesquioxanes R7Si7O9(OH)3 1, 2 with homoleptic complexes TiL4. Aryloxy derivatives [TiL(R7Si7O12)] [R = c-C6H11, L = OPh 10, O-C6H4F-p 11 or O-C6H4NO2-p 12] were prepared from the reaction of 8 with the corresponding aryl alcohols. The 29Si and 13C NMR spectroscopic data obtained on 5–14 indicate that the local C3v symmetry of the silasesquioxane ligand is retained at titanium, consistent with the formation of monomeric complexes possessing tripodal geometry. The monomeric nature of 7 was confirmed by X-ray crystallography. For complexes 8–12 solution NMR spectroscopy reveals the presence of a dimer, containing µ-alkoxy ligands, in equilibrium with the monomer. The zirconium analogue of 9, [Zr(OBut){(c-C6H11)7Si7O12}] 15, was similarly isolated as a monomer–dimer mixture from the reaction of the incompletely condensed silasesquioxane (c-C6H11)7Si7O9(OH)3 with [Zr(OBut)4]. Reaction of the disilanol (c-C6H11)7Si7O9(OSiMe3)(OH)2 4 with an excess of [Ti(OPri)4] afforded [Ti(OPri)2{(c-C6H11)7Si7O11(OSiMe3)}] 16, containing a bidentate silasesquioxane ligand, while reactions with TiL4 (L = CH2Ph, NMe2 or OSiMe3) afforded [Ti{(c-C6H11)7Si7O11(OSiMe3)}2] 17, independent of the stoichiometry of the reactants. Complexes 5–14 serve as soluble models for putative tripodal (open lattice) sites in titanium silicates, while 16 and 17 represent models for bipodal and tetrapodal (closed lattice) sites, respectively. From a study of the catalytic properties of complexes 5–17 in the epoxidation of oct-1-ene with tert-BuOOH (TBHP), revealing high activity for 5–14 and low activity for 16 and 17, it is concluded that the most active site in titanium silicate epoxidation catalysts corresponds to a four-co-ordinate site possessing tripodal geometry. Studies using IR and NMR spectroscopy show that, in the absence of olefins, putative alkylperoxo complexes formed by the addition of TBHP to tripodal complexes decompose rapidly at ambient temperature. Based on the high TBHP-to-epoxide selectivities observed under epoxidising conditions, it is apparent that the rate of epoxidation is significantly greater than that of alkylperoxo intermediate decomposition.