Catalytic trimerization of ethene with highly active cyclopentadienyl-arene titanium catalysts

The mono(cyclopentadienyl-arene)titanium complexes [eta(5)-C(5)H(3)R-(bridge)-Ar]TiCl(3), activated by methylalumoxane (MAO) cocatalyst, form a family of highly active catalysts for the trimerization of ethene, giving 1-hexene as the main product. Concomitant cotrimerization of ethene and 1-hexene, to give mainly 5-methylnon-1-ene, is also observed. The selectivity for trimerization depends on the presence of a pendant arene group on the cyclopentadienyl ligand and the nature of the bridge between these two ligand moieties. In the absence of a pendant arene, polyethene is the main product. The highest activity and selectivity for trimerization was observed for catalysts with a disubstituted C(1) bridge between the cyclopentadienyl and arene ligand moieties. A SiMe(3) substituent on the cyclopentadienyl ligand improves catalyst activity and selectivity, whereas methyl substitution of the arene decreases activity. Nevertheless, combining cyclopentadienyl SiMe(3) substitution with arene Me substitution gives rise to a catalyst with the highest activity and selectivity, evidence of the strongly nonlinear additivity of ligand substituent effects in this system. The cyclopentadienyl-arene ligand is likely to exhibit hemilabile behavior during catalysis, stabilizing intermediates by eta(6) coordination and dissociating or slipping to make room for the incoming substrate. The presence of two pendant arene groups on the cyclopentadienyl ligand diminishes the activity of the catalyst but greatly enhances its stability.