A study of Ni(ii) catalyzed ethylene oligomerization and dimerization with various N∩N ligands: structure/reactivity/selectivity relationships

Abstract

The Cossee–Arlman mechanism of nickel-catalyzed oligomerization and dimerization with various N^∩N-type ligands was studied using the DFT method. For the L1 ligands (L1 = 2,9-disubstituted 1,10-phenanthroline), the dimerization energy barriers for R = Me and R = Ph are about 17.9 and 15.8 kcal mol⁻¹, respectively. However, the presence of Ph substituents at positions 2 and 9 of the L1 ligand reduces the free energy barriers for the dimerization and trimerization processes by approximately 2.1 and 8.1 kcal mol⁻¹, respectively. Therefore, the Ph substituents prevent the removal of β-H from 1-butene, thus promoting the formation of 1-hexene. This result is consistent with experimental observations. In the case of the L2 ligands (L2 = substituted 2-benzoxazole-pyridines) with R¹ = H and R² = H or Me, and R³ = H, the calculated dimerization energy barriers for the systems (H-H) and (H-Me) are about 12.8 and 20.5 kcal mol⁻¹, respectively. The energy barrier for the system with R¹ = H and R² = R³ = Me (Me-Me) is about 20.9 kcal mol⁻¹. Thus, the activity of these systems towards the formation of 1-butene can be arranged as (H-H) > (H-Me) > (Me-Me). As for L3 ligands (L3 = 2-benzimidazolylpyridine derivatives), six systems with R¹ = H, Me, ^tBu; R² = H, Me, were studied. The calculated dimerization energy barrier values indicate that the (H-H), (H-Me), and (H-^tBu) systems (10.8, 11.1, and 10.9 kcal mol⁻¹, respectively) are more active than the (Me-H), (Me-Me) and (Me-^tBu) systems (18.8, 19.1 and 19.0 kcal mol⁻¹, respectively). Finally, for the L4 ligand (L4 = 2-benzoimidazol-8-alkylquinoline derivatives), nine systems were studied, divided into three groups: A system (R¹ = H and R² = H or Me or Et), B system R¹ = Me with identical R² substituents, and C system (R¹ = Et with identical R² substituents). Based on the values of the calculated energy barriers for dimerization and trimerization, the B system shows higher activity than the other systems.