Nickelacycles with anionic C–N–N′ terdentate ligands containing (5,5), (5,6), (5,7) and (6,5) membered fused rings. Reaction with ethylene

Abstract

A series of nickelacycle complexes with di-Schiff bases acting as terdentate CNN⁻ anionic ligands has been prepared. Oxidative addition of one C–X bond in mono- or di-ortho-halogenoaryl substituted ligands, N,N′-dibenzylidenealkane-1,2-diamines (ethane, propane and butanediamines) and N,N′-bis(1,2-diphenylethylidene)ethane-1,2-diamine, to [Ni(cod)₂] gives neutral complexes [NiX(CNN)] containing (5,5), (5,6), (5,7) and (6,5)-membered fused chelate rings (NiCN and NiNN respectively). Complexes from monohalogenoaryl substituted ligands are obtained as mixtures of two non-interchanging configurational isomers (E or Z stereochemistry about the non-metallated imine group). Cationic complexes [NiL(CNN)]BF₄ are generated by halide abstraction from these neutral complexes by using TlBF₄ and 2,4,6-Me₃C₅H₂N or CH₃CN as stabilizing ligand. In ionic compounds from monohalogenoaryl substituted ligands the formation of isomers by restricted rotation around the C_phenyl–C_methinic bond is also observed. The ¹H NMR spectra of complexes with (5,5), (5,6), (6,5) fused rings show fluxional behavior, the data being consistent with interconversion of two chelate ring conformations that are associated with the relative position of the non-metallated methinic moiety in the lower or the upper side of the coordination plane. Molecular structures of three ionic complexes [Ni(2,4,6-Me₃C₅H₂N)(CNN)]BF₄, were determined by X-ray single-crystal diffraction, revealing the effects of chelate sizes on the distortion of the coordination sphere around nickel. Ionic complexes [Ni(CH₃CN)(CNN)]BF₄ were tested as precursors of active species in the reaction with ethylene under pressure. The influence of the nickelacycle ring size on the insertion reaction is discussed, with moderate activities being observed only when the Ni–C bond is involved in a six-membered ring.