Synthesis and Structures of Nickel Halide Complexes Bearing Mono- and Bis-coordinated N-Heterocyclic Carbene Ligands, Catalyzing Grignard Cross-Coupling Reactions

Substitution of phosphine ligands in nickel(II) halide complexes by now attractive N-heterocyclic carbene
(NHC) ligands is one of the well-known organometallic reactions. New, simple, and easy-to-prepare
nickel(II) halides bearing both a phosphine and an NHC ligand, [NiX2(PPh3)(NHC)], were synthesized
by the reaction of [NiX2(PPh3)2] (X = Cl and Br) with 1 equiv of a bulky NHC ligand. Rather small
NHC ligands did not form NHC/PR3 mixed complexes. Controlling the amount of the NHC ligand and
purification led to successful isolation in good to moderate yields and structural determination of these
carbene complexes. Studies on catalytic Grignard cross-coupling reactions using three complexes, the
NHC/PPh3 mixed complex, a “bis”-carbene complex, and [NiCl2(PPh3)2], revealed that the monocarbene
complex catalyzes reactions with the highest activity, but, in comparison, catalysis does not proceed
well using the latter two complexes.