Synthesis and structures of amido-functionalized N-heterocyclic nickel(II) carbene complexes

Abstract

A series of bis-bidentate nickel(II) complexes [Ni(R-bimy-CH2CONH)2] (bimyH = benzimidazole; R = Me (3), Et (4), Ph (5)) bearing amido-functionalized N-heterocyclic carbene ligands, and pincer-type nickel(II) complexes [Ni(Py-bimy-CH2CONH)X] (X = Cl (6), Br (7)) bearing an amido- and pyridyl-functionalized N-heterocyclic carbene ligand were prepared. These complexes were characterized by NMR (1D and 2D) and single-crystal X-ray diffraction. Complexes 3-5 possess cis configuration, and the carbene ligands bound to the nickel atom through C2 carbon and NH nitrogen in a bis-bidentate coordination mode. In complexes 6 and 7, the pyridyl substituent was also N-bound to the nickel metal center resulting in a pincer-type coordination mode. As observed from the proton NMR spectra, the six-membered chelate rings in complexes 3-5 rendered the protons of the methylene moieties diastereotopic, and the cis configuration made the free rotation of the ethyl substituent in 4 and the phenyl substituent in 5 hampered by the adjacent substituent. The catalytic activity of these nickel complexes in Kumada cross-coupling of phenylmagnesium bromide with aryl chlorides was also investigated. The results indicated that pincer-type complexes 6 and 7 displayed excellent to moderate catalytic activity depending on the aryl chloride used.