A ligand influence on the stability of heterobimetallic complexes containing the Ti(µ-O)Al skeleton. Transformation of heterometallic systems to the homometallic Ti(iv) and Al(iii) complexes

Abstract

The influence of the ligands on the formation and stability of µ-oxo-bridged Ti(IV) complexes has been studied. Reaction of LTiCl₃ (1) and LAlMe(OLi) (L = HC(CMeN(2,6-iPr₂C₆H₃))₂, “NacNac”) afforded intermediate LTiCl₂(µ-O)AlMeL (5) in solution, which was converted to LTiCl(µ-O)₂TiClL (6) and LAlMeCl within 2 days. The decomposition of 5 was estimated to be thermodynamically favorable. The interaction of LTiMe₃ (3) and LAlMe(OH) yielded the intermediate LTiMe₂(µ-O)AlMeL (7). Complex 7 decomposes in solution giving the titanium oxo complex LTiMe(O) (8) and LAlMe₂. The calculated ΔG²⁹⁸ for this reaction is −128 kJ mol⁻¹. The degradation of LTiMe₂(µ-O)AlMeL is slow and follows first order kinetics with k₂ = 4.09(7) × 10⁻⁷ s⁻¹. The dimeric complex LTiMe(µ-O)₂TiMeL–toluene (9a) was isolated from the reaction of 3 with LAlMe(OH) in toluene and LTiMe(µ-O)₂TiMeL–hexane (9b) from hexane. The dimerization of 8 yielding LTiMe(µ-O)₂TiMeL (9) is marginally endothermic, with a calculated ΔG²⁹⁸ of +27 kJ mol⁻¹. The formation of the solid 9 is due to the lattice stabilization. The solid µ-oxo-bridged complex 9 and Mes₃Ga were obtained from the reaction of LTiMe₃ with [Mes₂Ga(OH)]₂–THF in toluene, and 9 was also isolated from the reaction of LTiMe₃ with 1 equiv. of H₂O in toluene. Compounds LTiCl₃ (1), LTiCl(µ-O)₂TiClL (6), 9a and 9b have been characterized by X-ray single crystal structure, NMR, IR, EI-MS and elemental analysis. Complexes 5, 7 and 8 have been characterized by NMR. Compounds 3, 6 and 9 possess moderate catalytic activity in the polymerization of ethylene.