Catalytic behavior tuning via structural modifications of silylated-diphosphine Ni(II) complexes for ethylene selective dimerization

The methylene spacers and an uncoordinated diphenylphosphine moiety in the scaffold of the CH₃Si(CH₂)_n(PPh₂)₃ and Si(CH₂PPh₂)₄-type silylated diphosphine Ni(II) complex systems have a marked impact on their catalytic performance in selective ethylene dimerization. Ni(II)-based precatalyst 1, bearing two methylene spacers in its framework, exhibited the highest catalytic activity of 1.29 × 10⁸ g (mol_Ni)^-1 h^-1, while precatalyst 3, with three methylene spacers, affords the highest product selectivity (88%) toward the C₄ fraction. Crystallographic investigations revealed that the precatalyst 3 adopts the mononuclear bidentate binding mode and the steric constraints of its uncoordinated diphenylphosphine moiety may successfully tailor the catalytic environment of the catalyst. The precatalyst 4 may form a dinuclear complex and exhibits high catalytic activity by changing the ligand/Ni molar ratio. The high C₄ selectivity of precatalyst 3 has been rationalized by density functional theory (DFT) calculations and found to be consistent with the experimental results. The study also revealed that designing new systems of Ni(II)-based complexes and their systematic modifications may further provide potential and industrially viable catalyst systems for selective ethylene oligomerization.