Zwitterionic Ni(ii) complexes bearing pyrazolyl-ether-imidazolium ligands: synthesis, structural characterization and use in ethylene oligomerization

Abstract

A series of new tetracoordinated Ni(II) complexes of general formula NiCl₃(L) (Ni1, L = 1-(2-(2-methylimidazole-ethoxy)ethyl)-3,5-dimethylpyrazole; Ni2, L = 1-(2-(1,2-dimethylimizadole-ethoxy)ethyl)-pyrazole; Ni3, L = 1-(2-(1,2-dimethylimizadole-ethoxy)ethyl)-3,5-dimethyllpyrazole; Ni4, L = 1-(2-(2-n-butylimizadole-ethoxy)ethyl)-3,5-dimethylpyrazole) were prepared in high yields. All these complexes were characterized by elemental analysis, and X-ray crystallography was performed for Ni1, Ni2, and Ni3. In the solid state, these nickel complexes are monomeric with the pyrazolyl-ether-imidazolium acting as a monodentate ligand. The positive charge on the imidazolium unit is cancelled out by the negative charge that is provided by the third chloride ion linked to Ni(II), forming a zwitterionic structure. Upon activation with methylaluminoxane (MAO) or ethylaluminum sesquichloride (EASC), these complexes show moderate activity in ethylene oligomerization [TOF = 2100–29 300 (mol C₂H₄)·(mol Ni⁻¹ h⁻¹)] with good selectivities for 1-butene (80.4–89.8 wt% of total products), which vary according to the ligand environment. Under biphasic conditions, the Ni3/[Bmim]·[AlCl₄]/toluene catalytic system proved to be active for ethylene oligomerization with a TOF of 10 700 (mol C₂H₄)·(mol Ni⁻¹ h⁻¹) and highly selective towards production of 1-butene (90.9 wt%).