Synthesis, Structures and Properties of Half-Sandwich Nickel(II) Complexes with Backbone-Modified NHC Ligands

A series of seven nickel complexes [Ni(eta(5)-C5H5)(X)(NHC)] (X = Cl or Br; NHC = N-heterocyclic carbene) containing backbone-modified NHC ligands was prepared by treatment of nickelocene with the corresponding NHC center dot HX salts or, in the case of complex 1h [Ni(eta(5)-C5H5)(Cl)(IMes(O))] (IMes(O) = 4-oxo-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), by deprotection of the corresponding NHC ligand in complex 1g [Ni(eta(5)-C5H5)(Cl)(IMes(OPiv))] [IMes(OPiv) = 4-(pivaloyloxy)-1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]. Single-crystal X-ray structures were determined for complexes 1d [Ni(eta(5)-C5H5)(Cl)(Bn-2-bimy)] (Bn-2-bimy = 1,3-dibenzylbenzimidazolin-2-ylidene), 1g and 1h. The coordination spheres around the nickel atoms resemble those of d(8) NiL4 square-planar complexes, with Ni-C(Cp) bond length differentiation and C-C bond alternation in Cp ligands being taken into account. Desymmetrization of the five-membered heterocyclic ring and a tautomeric equilibrium were observed for complex 1h. All new complexes are efficient precatalysts in three C-C bond-forming reactions (polymerization of styrene, polymerization of methyl methacrylate, Suzuki-Miyaura cross-coupling).