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

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 OBu^t 9; R = c-C₅H₉, L = CH₂Ph 13 or OPrⁱ14] 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 C_3v 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(OBu^t){(c-C₆H₁₁)₇Si ₇O₁₂}] 15, was similarly isolated as a monomer-dimer mixture from the reaction of the incompletely condensed silasesquioxane (c-C₆H₁₁)₇Si₇O₉(OH) ₃ with [Zr(OBu^t)₄]. Reaction of the disilanol (c-C₆H₁₁)₇Si₇O ₉(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)₇Si₇O ₁₁(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.