Bulky and Electron‐Deficient α‐Iminocarboxamidato‐Nickel(II) Complexes: A Study of the Steric and Electronic Effects on Ethylene Activation

Abstract

Two α-iminocarboxamidato-nickel(II) complexes containing ligands with several CF₃ groups were synthesized and characterized by NMR spectroscopy, elemental analysis, and density functional theory (DFT) calculations. Surprisingly, the prepared complexes were inactive toward ethylene oligo/polymerization reactions upon activation attempts with common lewis acid co-catalysts such as B(C₆F₅)₃ and BF₃. Quantum chemistry calculations were employed to reveal that adduct formation is thermodynamically favored for small Lewis acids such as BF₃ due to the sterically demanding ligand environment of the complex, confirming the experimental findings. DFT results associate the lack of polymerization activity with a highly unfavorable steric environment, undesirable London dispersion interactions between the ligands, and a strong electrostatic stabilization caused by the employed ligands. Our findings should help future researchers to identify necessary electronic and steric requirements for the compounds to generate active Ni(II) catalysts for ethylene oligo/polymerization activated by suitable boron Lewis acids.