Unsymmetrical NCN-pincer mononuclear and dinuclear nickel(ii) complexes of N-heterocyclic carbene (NHC): synthesis, structure and catalysis for Suzuki–Miyaura cross-coupling

Abstract

This report describes the synthesis and characterization of a family of unsymmetrical NCN pincer Ni(II) complexes 2–8 with NHC-triazole arms. All of these complexes have been fully characterized by X-ray single crystal analysis, NMR spectroscopy, and elemental analysis. Complexes 2 and 4–6 were square planar with a chloride trans to the carbene carbon atoms. Complex 3 was a paramagnetic octahedral complex with its central metal surrounded by two NCN pincer ligands. Complexes 7 and 8 contain [(NHC)₂Ni₂-OH] moieties bearing a OH bridge. Both the [(NHC)₂Ni₂-OH] complexes 7 and 8 and [(NC_NHCN)Ni-Cl] complexes 2 and 4–6 were synthesized similarly via the reactions of the in situ formed Ag–NHCs from the corresponding imidazolium salts with [NiCl₂(PPh₃)₂]. The catalytic activities of all complexes for Suzuki–Miyaura cross-coupling were examined. Under the optimized conditions, complex 4 was active in the Suzuki–Miyaura cross-coupling reactions of aryl iodides and aryl bromides at 110 °C. Aryl chlorides were successfully coupled in the presence of triphenylphosphine as an additive.