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

Abstract

Abstract A series of seven nickel complexes [Ni(η 5 ‐C 5 H 5 )(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 · HX salts or, in the case of complex 1h [Ni(η 5 ‐C 5 H 5 )(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(η 5 ‐C 5 H 5 )(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(η 5 ‐C 5 H 5 )(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 NiL 4 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).