Pyrazolylimine iron and cobalt, and pyrazolylamine nickel complexes: Synthesis and evaluation of nickel complexes as ethylene oligomerization catalysts

A series of tridentate (O^N^N) and bidentate (N^N) iron, cobalt and nickel complexes were prepared from {2,4-di-tert-butyl-6-[(2-(3,5-RR-pyrazol-1-yl- ethylimino)methyl]-phenol}, R = H (L1), Me (L2), Ph (L3), [(3,5-di-tert-butyl-2- ethoxy-benzylidene)-[2-(3,5-dimethyl-pyrazol-1-yl)-ethyl]amine (L4), and (4-(4-tert-butyl-benzylidene)-[2-(3,5-dimethyl-pyrazol-1-yl)ethyl]-imine) (L5). Ligands L1-L5 were reacted with MX₂ (M = Fe, Co, Ni; X = Cl, Br) to form three types of complexes. Whereas tridentate O∧N∧N bound complexes were formed with Fe(II) and Co(II) salts (1-6), reactions involving Ni(II) salts resulted in hydrolysis of the Schiff base ligands used to form a novel tetranuclear cubane-like cluster [Ni₄(3,5-Me₂pza) ₄Cl₈] (7) (pza = (3,5-dimethylpyrazol-1yl)ethylamine) and mononuclear [Ni(3,5-R₂pza)Br₂] (R = H (8); R = Me (9) complexes. Molecular structures of five complexes (1, 5-7 and 9), determined by single-crystal X-ray diffraction, confirmed that ligand hydrolysis occurred in the nickel reactions. All three nickel complexes showed good to excellent activity towards ethylene oligomerization with activities up to 10 726 kg (mol Ni h)^-1.