Monomeric Three-Coordinate N‑Heterocyclic Carbene Nickel(I) Complexes: Synthesis, Structures, and Catalytic Applications in Cross-Coupling Reactions

A series of three-coordinate monovalent nickel halide complexes bearing N-heterocyclic carbene (NHC) ligands, i.e., NiCl­(IPr)­(L) [L = pyridine, P­(OPh)3, bis­(diphenylphosphino)­butane (dppb), IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene], NiX­(IMes)­(PPh3) (X = Cl and Br, IMes = 1,3-bis­(mesityl)­imidazol-2-ylidene), were prepared. The complexes were identified using NMR spectroscopy, superconducting quantum interference device (SQUID), and X-ray crystallography. Additionally, ESR spectra of NiCl­(IPr)­(pyridine) were taken in toluene. These complexes had three-coordinate Y-shaped geometries in both the solid and solution states. The compounds containing IPr showed equilibrium between the monomeric and dimeric forms, with liberation of ligands. Addition of 1,2-bis­(diphenylphosphino)­ethane and 1,3-bis­(diphenylphosphino)­propane to the dinickel­(I) IPr complex instead of dppb resulted in heterolytic cleavage to nickel(0) and nickel­(II) species. Catalysis of Suzuki cross-coupling and Buchwald–Hartwig amination of aryl bromide using the complexes was investigated. The efficiencies in the amination of aryl bromide depended strongly on the additional donor ligands.