Hemilabile Ligand Induced Selectivity:  a DFT Study on Ethylene Trimerization Catalyzed by Titanium Complexes

Abstract

In this computational study, we propose a detailed mechanism, which has been explored by density functional theory simulations, for the trimerization reaction of ethylene to give selectively 1-hexene using a [(η5-C5H4CMe2C6H5)TiCl3/MAO] catalyst. For ring-opening reactions we distinguish between agostic assisted β-hydrogen transfer and hydride formation. With the B3LYP functional it was found that the rate-determining step is the ring-opening reaction of the seven-membered metallacycle, exhibiting a barrier ΔG⧧(298.15 K) of 18.4 kcal/mol. It appears that the selectivity of the reaction results from two effects: the stabilizing effect of the hemilabile phenyl ligand and the ring size of the metallacycle. Upon interchange of the phenyl group by the labile methyl group, the calculations predict the formation of polyethylene, which is in agreement with the experimental data.