High Activity Ethylene Oligomerization Using Asymmetric Alkyl P-Substituted Bis(phosphanyl)amine Ni(II) complexes

A series of novel asymmetric alkyl P-substituted bis(phosphanyl)amine (PNP) Ph₂PN (cyclopentyl) PR₁R₂-type L1–L5 ligands and these corresponding Ni(II) precatalysts C1–C5 were synthesized and characterized. The structures of these complexes were confirmed by using ¹H-, ³¹P-, ¹³C-NMR, FT-IR, and elemental analysis. Using ethylaluminum dichloride (EADC) as a cocatalyst, nickel complexes exhibited high activity in elective ethylene oligomerization, with the main products being dimers and a small number of trimers. Under optimized conditions of 60°C and 1.0 MPa ethylene pressure, C4, bearing a diisopropylphosphoryl group, exhibited highest catalytic activity of 1342.9 kg·g Ni⁻¹·h⁻¹ with 85.2% C₄ and 14.8% C₆ products selectivity, while C2, bearing a diethylphosphonyl group, showed catalytic activity of 596.4 kg·g Ni⁻¹·h⁻¹ with 88.2% C₄ and 11.8% C₆ product selectivity. Single crystal analysis offered a more comprehensive insight into the subtle effects of alkyl P-substituted in the scaffold of C2 and C4 ligands on catalytic activity. Density functional theory (DFT) calculations indicated that lower energy of the LUMO in the C4A intermediate enhances the activity of ethylene oligomerization. It provides a new method for purposefully designing ligands for ethylene oligomerization with high catalytic activity.