The imidazolium salts bis(N-pyridylimidazoliumyl)methane hexafluorophosphate ([H2L1](PF6)2) and bis(N-picolyl)benzimidazoliumyl)methane hexafluorophosphate ([H2L2](PF6)2) as the precursors of potentially tetradentate NHC ligands were synthesized in 63–98% yields. Their reactions with Ag2O afforded silver−NHC complexes [Ag3(L1)2(CH3CN)2](PF6)3 (1) and [Ag4(L2)2(CH3CN)2](PF6)4 (2), respectively. Further reactions of these silver complexes as carbene transfer reagents yielded square-planar nickel(II) complexes [NiL1](PF6)2 (3) and [NiL2](PF6)2 (4), respectively. These silver and nickel complexes have been fully characterized by 1H and 13C NMR spectroscopy and X-ray diffraction analysis. Both 3 and 4 catalyzed Suzuki-type cross-coupling of aryl chlorides and bromides containing electron-withdrawing and electron-donating substituents. Complex 3 is a more active catalyst under mild conditions.

Ni 2 (3,5-bis(N-methylimidazolylidenylmethyl)pyrazolate) 2 ](PF 6 ) 2 (1), [Ni 2 (µ-OH)(3,5-bis(N-pycolylimidazolylidenylmethyl)pyrazolate)](PF 6 ) 2 (2), and [Ni 2 (µ-OH)(3,5-bis(N-pyridylimidazolylidenylmethyl)pyrazolate)](PF 6 ) 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 (µ-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.

The Kumada cross-coupling reaction of a variety of unactivated aryl chlorides, vinyl chlorides, and heteroaryl chlorides catalyzed by nickel(II) complexes containing pyridine-functionalized NHC ligands is described. The catalysts are so active that the reactions proceed at room temperature in excellent yields.