N-Heterocyclic Carbene Bound Nickel(1) Complexes and Their Roles in Catalysis

The new paramagnetic complexes Ni(1)(IMes)(2)X (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) were prepared from the reaction of Ni(IMes)(2) with aryl halides. Products that would arise from oxidative addition were not observed. En contrast, Ni(II)(tmiy)(2)(X)(Ar) complexes were formed from the oxidative addition of aryl halides to Ni bound by a stencally less hindered NHC ligand, tmiy (tetramethylimidazol-2-ylidene). The paramagnetic Ni(1)(IMes)(2)X complexes were compared to known Ni(0) and Ni(II) catalysts for Kumada and Suzuki coupling reactions. Stoichiometric reactions between the Ni(1)(IMes)(2)X complexes with aryl halides and transmetalating agents were also evaluated.