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.

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We describe the first nickel-N-heterocyclic carbene catalyzed Negishi cross-coupling reaction of a variety of unactivated aryl chlorides, heterocyclic chlorides, aryl dichlorides, and vinyl chloride. The mononuclear and binuclear nickel-NHC complexes supported by heteroarene-functionalized NHC ligands are found to be highly efficient for the coupling of unactivated aryl chlorides and organozinc reagents, leading to biaryls and terphenyls in good to excellent yields under mild conditions. For all aryl chlorides, the binuclear nickel catalysts show activities higher than those of mononuclear nickel complexes because of possible bimetallic cooperative effect.

Cobalt(iii) complexes, [CoCl(2)(L1)(2)](PF(6)) (1, L1 = N-methyl-N-(2-pyrimidinyl)imidazolylidene), [CoCl(L2)(2)](PF(6))(2) (, L2 = N-picolyl-(2-pyrimidinyl)imidazolylidene), [Co(L3)(2)](PF(6))(3) (3, L3 = bis(N-2-pyrimidylimidazolylidenyl)methane) and [CoCl(2)(L3)](PF(6)) (4) have been prepared from the corresponding pyrimidine functionalized imidazolium salts [HL1](PF(6)), [HL2](PF(6)), and [H(2)L3](PF(6))(2)via in situ generated silver carbene complexes under mild conditions. These cobalt complexes were characterized by (1)H and (13)C NMR spectroscopy and X-ray diffraction analysis. The cobalt complexes have been found to be good catalyst precursors for Kumada-Corriu cross-coupling reactions of aryl halides and Grignard reagents at room temperature. Complex 1 is more active under the mild conditions.

Ag 4 (L) 4 ](PF 6 ) 4 and [Pd(L) 2 ](PF 6 ) 2 (L ) 3-(2,4-dimethyl-1,8-naphthyrid-7-yl)-1-picolylimidazolylidene) have been prepared and fully characterized. The silver compound consists of a Ag 4 ring with short Ag-Ag contacts displaying a butterfly conformation. The palladium complex exhibits a helical structure with palladium located in a square-planar environment with two carbene ligands and two pyridines in a cis arrangement. The palladium complex has proven to be a highly efficient catalyst for Heck coupling reactions of aryl bromides with n-butyl acrylate. TON values up to 10 6 and TOF values up to 5 × 10 5 h -1 can be achieved.

The nickel salicylaldiminato phosphine complexes [1,2,3-C6H3(9-anthracenyl)O(CH═N(2,6-C6H3(iPr)2)]Ni(Me)[P(4-C6H4R)3] (4; R = a, (CH2)2Rf8; b, (CH2)3Rf8; c, H (Rf8 = (CF2)7CF3)) are prepared from the corresponding phosphines 3a-c and nickel NCMe adduct (46-68%). These are applied as catalysts for ethylene polymerization in toluene and fluorous/toluene liquid/liquid biphasic mixtures. Under the latter conditions, the fluorous phosphines 3a,b that must dissociate to generate the active catalyst migrate to the fluorous phase (partition coefficients 97.5:2.5 and 66.6:33.4 vs <0.5:>99.5 for 4a,b). Catalysts 4a,b show marked accelerations under biphasic conditions, but 4c (which has a lipophilic phosphine ligand) does not. Under all conditions, 4a,b are faster catalysts than the Ni(Ph)(PPh3) analogue, a previously reported benchmark.