A highly efficient nickel catalysed method for the direct C2-arylation of N-heterocyclic carbenes (NHCs) is reported. This protocol enables a facile access to C2-arylated imidazolium salts (NHC-Ar)X (X = Cl, Br, I or OTf). Experimental and theoretical studies suggest the viability of a Ni(i)/Ni(ii) catalytic pathway in which the dinuclear Ni(i) species [(NHC)NiX] plays a key role.

Stable N-heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C H )(IPr)] and [(IPr)(C H ) (IPr)] (4 and 5, respectively; IPr=C{N(2,6-iPr C H )} CHCH), are reported. In a nickel-catalyzed double carbenylation of 1,4-Br C H and 4,4'-Br (C H ) with IPr (1), [(IPr)(C H )(IPr)](Br) (2) and [(IPr)(C H ) (IPr)](Br) (3) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC . Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet-triplet energy gap ΔE of 10.7 kcal mol , whereas 4 features more quinoidal character with a rather large ΔE of 25.6 kcal mol . In view of the low ΔE , 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character.

Cationic copper(I) complexes [Cu(aIPr Ph )(IPr)]I (3) and [Cu(aIPr Ph ) 2 ]I (4) featuring an abnormal N-heterocyclic carbene (aNHC) (aIPr Ph = 1,3-bis(2,6-diisopropylphenyl)-2-phenylimidazol-4-ylidene) and/or an NHC (IPr = 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene) ligand(s) are reported. Treatment of Cu(aIPr Ph )I (2) with IPr affords complex 3. Reaction of (IPrPh)I (1) (IPrPh = 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazolium) with CuI in the presence of K{N(SiMe 3 ) 2 } leads to the formation of 4. Complexes 3 and 4 represent rare examples of mixed aNHC-NHC and bis-aNHC metal complexes, respectively. They are characterized by elemental analysis, NMR spectroscopic, and mass spectrometric studies. The solid-state molecular structures of 3 and 4 have been determined by single crystal X-ray diffraction analyses. The catalytic activity of 2, 3, and 4 has been investigated in the [3+2] cycloaddition of alkynes and organic azides, affording triazole derivatives in an almost quantitative yield. Notably, complexes 2, 3, and 4 are excellent catalysts for the three-fold cycloaddition of a tris-azide with various alkynes. This catalytic protocol offers a high yield access to tris-triazoles in a shorter reaction time and considerably reduces the experimental work-up compared to the classical synthetic method.

Stable N-heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons,[ (IPr)(C 6 H 4 )(IPr)] and [(IPr)(C 6 H 4 ) 2 (IPr)] (4 and 5,r espectively;I Pr = C{N(2,6-iPr 2 C 6 H 3 )} 2 CHCH), are reported. In anickel-catalyzed double carbenylation of 1,4-Br 2 C 6 H 4 and 4,4'-Br 2 (C 6 H 4 ) 2 with IPr (1), [(IPr)(C 6 H 4 )(IPr)](Br) 2 (2)a nd [(IPr)(C 6 H 4 ) 2 (IPr)](Br) 2 (3) were generated, whichr espectively afforded 4 and 5 as crystalline solids upon reduction with KC 8 .E xperimental and computational studies support the semiquinoidal nature of 5 with as mall singletÀtriplet energy gap DE SÀT of 10.7 kcal mol À1 ,w hereas 4 features more quinoidal character with ar ather large DE SÀT of 25.6 kcal mol À1 .I nv iew of the low DE SÀT , 4 and 5 may be described as biradicaloids.Moreover, 5 has considerable (41 %) diradical character.