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]­(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.