Nickelacycles with anionic C–N–N′ terdentate α-diimine based ligands. Reaction with ethylene

Abstract

A series of chloro-substituted 1,4-diaza-1,3-butadiene (a–e) and pyridine-imine (f, g) ligands have been prepared. The oxidative addition of five of the bidentate ligands to [Ni(cod)₂] gave a group of neutral complexes containing an anionic terdentate ligand [NiCl(CNN′)] with five-and six-membered nickelacycles (1a, 1b, 1e, 1f, 1g). Two isomers were obtained with the 1,4-diaza-1,3-butadiene ligands a and b due to a 1,3-prototropic rearrangement. The reaction of the neutral complexes (1) with TlBF₄ in the presence of 2,4,6-trimethylpyridine yields well-defined cationic complexes [NiL(CNN′)]BF₄ (2). The low solubility of the neutral compounds (1) complicates their characterisation which could be completed only after preparation of the ionic complexes (2). The molecular structure of compound 2a [NiL[(2-Ph)CH₂–NC(CH₃)C(CH₃)N–CH₂–(2-ClC₆H₄)]]BF₄ has been determined by X-ray diffraction methods. Four ionic complexes [Ni(NCCH₃)(CNN′)]BF₄ (3) have been obtained and tested as precursors of catalytic species in the reaction of ethylene oligomerisation. The catalytic results are limited, as expected, due to the structural type of the nitrogenated ligands. However it is possible to use these relatively stable nickelacycles as one-component precursors for the oligomerisation reaction. Similar palladacycles are not suitable for this purpose.