A ligand influence on the stability of heterobimetallic complexes containing the Ti(mu-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 mu-oxo-bridged Ti(IV) complexes has been studied. Reaction of LTiCl3 (1) and LAlMe(OLi) (L = HC(CMeN(2,6-iPr(2)C(6)H(3)))(2), "NacNac") afforded intermediate LTiCl2(mu-O)AlMeL (5) in solution, which was converted to LTiCl(mu-O)(2)TiClL (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(mu-O)AlMeL ( 7). Complex 7 decomposes in solution giving the titanium oxo complex LTiMe(O) (8) and LAlMe2. The calculated Delta G(298) for this reaction is -128 kJ mol(-1). The degradation of LTiMe2(mu-O)AlMeL is slow and follows first order kinetics with k(2) = 4.09(7) x 10(-7) s(-1). The dimeric complex LTiMe(mu-O)(2)TiMeL-toluene (9a) was isolated from the reaction of 3 with LAlMe(OH) in toluene and LTiMe(mu-O)(2)TiMeL-hexane (9b) from hexane. The dimerization of 8 yielding LTiMe(mu- O)(2)TiMeL (9) is marginally endothermic, with a calculated Delta G(298) of +27 kJ mol(-1). The formation of the solid 9 is due to the lattice stabilization. The solid mu-oxo-bridged complex 9 and Mes(3)Ga were obtained from the reaction of LTiMe3 with Mes(2)Ga(OH)-THF in toluene, and 9 was also isolated from the reaction of LTiMe3 with 1 equiv. of H2O in toluene. Compounds LTiCl3 (1), LTiCl(mu-O)(2)TiClL (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.