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

Cationic cyclopentadienyl mixed bis-N-heterocyclic carbene (NHC) nickel complexes of the general formula [Ni(NHC)(NHC′)Cp](PF 6 ) (NHC/NHC′ = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (Mes 2 NHC) a, 1,3-dimethylimidazol-2-ylidene (Me 2 NHC) b, 1-isopropyl-3-methyl-imidazol-2-ylidene ( i Pr-NHC-Me) c; Cp = η 5 -C 5 H 5 ) were prepared by the reaction of the cationic monocarbene complex [Ni(Mes 2 NHC)(NCMe)Cp](PF 6 ) 1a or [Ni(Me 2 NHC)(NCMe)Cp](BF 4 ) 1b with the appropriate free carbene. The new mixed bis(carbene) complexes [Ni(Mes 2 NHC)(Me 2 NHC)Cp](PF 6 ) 2ab, [Ni(Mes 2 NHC)( i Pr-NHC-Me)Cp](PF 6 ) 2ac, and [Ni(Me 2 NHC)( i Pr-NHC-Me)Cp](BF 4 ) 2bc were obtained in moderate-to-high yields and were fully characterized by 1 H and 13 C 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.