A DFT Chemical Descriptor to Predict the Selectivity in α-Olefins in the Catalytic Metallacyclic Oligomerization Reaction of Ethylene According to the (Hemi)labile Ligand Coordinating to Titanium

Density functional theory (B3LYP functional) has been applied to investigate the role of hemilabile ligands composed of a cyclopentadienyl group tethered to a pendent R-group, which are used in titanium-based catalysts to selectively trimerize ethylene into 1-hexene. In order to fairly compare the influence of the R-group on the reaction outcome, it was assumed in this study that, independent of the nature of the R-group, the catalyst enters the metallacyclic mechanistic pathway. The identified competing reaction pathways that determine the selectivity, i.e., the opening of the seven-membered ring to yield 1-hexene and the uptake and insertion reaction of the fourth ethylene molecule, have been explored for 15 catalytic systems. The theoretical results show that the nature of the R-group hardly affects the energy barrier for the ring-opening reaction. On the other hand, the uptake and insertion reaction of the fourth ethylene molecule is favored by more labile R-groups. The lability of the R-group has been quantified by calculating the dissociation energy, using isodesmic reactions. Correlating the barrier heights of the two competing reaction pathways and taking into account the experimental data of catalysts 1 − 7, a model is derived that predicts the formation of 1-hexene when the dissociation energy is larger than 15 kcal/mol. In the case of smaller dissociation energies, multiple insertion reactions are likely to occur. Next, the model has been applied to six catalysts ( 8 − 13 ) and found to correctly predict the major product for five catalysts. The dissociation energy thus appears as a relevant DFT chemical descriptor for the research of new analogous hemilabile ligands. For catalysts 14 and 15, where no experimental data are available, the model foresees respectively 1-hexene and longer oligomers, resulting from multiple insertion reactions, as the major products.