Dinickel(II) Complexes of Bis(N-heterocyclic carbene) Ligands Containing [Ni2(μ-OH)] Cores as Highly Efficient Catalysts for the Coupling of Aryl Chlorides

[Ni-2(3,5-bis(N-methylimidazolylidenylmethyl)pyrazolate)(2)](PF6)(2) (1), [Ni-2(mu-OH)(3,5-bis(N-pycolylimidazolylidenylmethyl)pyrazolate)](PF6)(2) (2), and [Ni-2(mu-OH)(3,5-bis(N-pyridylimidazolylidenylmethyl)pyrazolate)](PF6)(2) (3) have been prepared from the corresponding imidazolium salts via in situ generated silver-carbene complexes. The complexes and imidazolium salts were characterized by elemental analyses and NMR spectroscopy. The structures of 1-3 were identified by X-ray diffraction analysis. In complex 1, two nickel(II) ions are sandwiched by two 3,5-bis(N-methylimidazolylidenylmethyl)pyrazolates behaving as anionic tetradentate ligands. Complexes 2 and 3 contain Ni-2(mu-OH) cores with two nickel centers bridged by anionic hexadentate imidazolylidene ligands. Complexes 2 and 3 show excellent catalytic activities in Suzuki-Miyaura and Kumada-Corriu coupling reactions of various aryl chlorides. The cross-coupling reactions of deactivated aryl chlorides with arylboronic acids and Grignard reagents have been accomplished in excellent yields at low catalyst loadings.