Nickel complexes of a bis(benzimidazolin-2-ylidene)pyridine pincer ligand with four- and five-coordinate geometries

Abstract

The four- and five-coordinate complexes [(CNC)NiX₂] (X = Cl, Br, I), [(CNC)NiX]PF₆ (X = Cl, Br) and [(CNC)NiCl]Cl·H₂O have been isolated, where CNC is the bis(N-butylbenzimidazolin-2-ylidene)-2,6-pyridine pincer ligand. A five-coordinate geometry is rare for this class of complex. Where amenable, the complexes have been structurally characterised by single crystal X-ray diffraction studies and in solution by NMR, UV-vis and MS studies. The five-coordinate dibromo complex [(CNC)NiBr₂] is readily prepared on the gram-scale from the benzimidazolium salt precursor and Ni(OAc)₂·4H₂O in DMSO without the exclusion of air. Halide exchange and salt metathesis reactions using [(CNC)NiBr₂] afford the other four- and five-coordinate complexes. [(CNC)NiBr₂] displays very low solubility, and upon dissolution affords solutions of the four-coordinate [(CNC)NiBr]⁺. Factors that influence the formation of four- or five-coordinate complexes with this ligand class are discussed.