Synthesis and Structural Characterization of Half-Sandwich Nickel Complexes Bearing Two Different N-Heterocyclic Carbene Ligands

Abstract

Cationic cyclopentadienyl mixed bis-N-heterocyclic carbene (NHC) nickel complexes of the general formula [Ni(NHC)(NHC′)Cp](PF6) (NHC/NHC′ = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (Mes2NHC) a, 1,3-dimethylimidazol-2-ylidene (Me2NHC) b, 1-isopropyl-3-methyl-imidazol-2-ylidene (iPr-NHC-Me) c; Cp = η5-C5H5) were prepared by the reaction of the cationic monocarbene complex [Ni(Mes2NHC)(NCMe)Cp](PF6) 1a or [Ni(Me2NHC)(NCMe)Cp](BF4) 1b with the appropriate free carbene. The new mixed bis(carbene) complexes [Ni(Mes2NHC)(Me2NHC)Cp](PF6) 2ab, [Ni(Mes2NHC)(iPr-NHC-Me)Cp](PF6) 2ac, and [Ni(Me2NHC)(iPr-NHC-Me)Cp](BF4) 2bc were obtained in moderate-to-high yields and were fully characterized by 1H and 13C NMR spectroscopies, elemental analyses, and, in the case of 2ab, by a single-crystal X-ray diffraction study. The monocarbene precursor 1b was also structurally characterized. The mixed bis-NHC complexes 2ab and 2ac were tested as catalysts for the Suzuki–Miyaura cross-coupling of phenylboronic acid with 4′-bromoacetophenone, and their activities were compared to that of related monocarbene CpNi complexes.