The Role of Co-Activation and Ligand Functionalization in Neutral Methallyl Nickel(II) Catalysts for Ethylene Oligomerization and Polymerization

A detailed quantum chemical study that analyzed the mechanism of ethylene oligomerization and polymerization by means of a family of four neutral methallyl Ni^II catalysts is presented. The role of the boron co-activators, BF₃ and B(C₆F₅)₃, and the position of ligand functionalization (ortho or para position of the N-arylcyano moiety of the catalysts) were investigated to explain the chain length of the obtained polymers. The chain initialization proceeded with higher activation barriers for the ortho-functionalized complexes (≈19 kcal mol⁻¹) than the para-substituted isomers (17–18 kcal mol⁻¹). Two main pathways were revealed for the chain propagation: The first pathway was favored when using the B(C₆F₅)₃ co-activated catalyst, and it produced long-chain polymers. A second pathway led to the β-hydrogen complexes, which resulted in chain oligomerization; this pathway was preferred when the BF₃ co-activated catalysts were used. Otherwise, the termination of longer chains occurred via a stable hydride intermediate, which was formed with an energy barrier of about 14 kcal mol⁻¹ for the B(C₆F₅)₃ co-activated catalysts. Significant new insights were made into the reaction mechanism, whereby neutral methallyl Ni^II catalysts act in oligomerization and polymerization processes. Specifically, the role of co-activation and ligand functionalization, which are key information for the further design of related catalysts, were revealed.