Steric Bulkiness-Dependent Structural Diversity in Nickel(II) Complexes of N-Heterocyclic Carbenes: Synthesis and Structural Characterization of Tetra-, Penta-, and Hexacoordinate Nickel Complexes

A family of tetra-, penta-, and hexacoordinate nickel complexes of functionalized N-heterocyclic carbene (NHC) ligands have been prepared via carbene transfer reactions of Ni(PPh3)(2)Cl-2 and silver-NHC complexes in situ generated from corresponding imidazolium salts and Ag2O. These nickel(II) complexes have been characterized by H-1 NMR, (CNMR)-C-13, and ESI spectroscopy and X-ray diffraction analysis. Structural analysis revealed that [Ni(L1)(3)](PF6)(2) (L1 = 3-butyl-1-(5,7-dimethyl-1,8-naphthyridin-2-yl)imidazolylidene, 3) is a rare example of a nickel complex with trigonal-bipyramidal geometry. [Ni(L4)(2)](PF6)(2) (L4 = 1-butyl-3-((7-methyl-1,8-naphthyridin-2-yl)methyl)imidazolylidene, 5) and [Ni(L20)(2)](PF6)(2) (L20 = 3-(pyridin-2-ylmethyl)-1-(pyrimidin-2-yl)imidazolylidene, 21) are normal square-planar. [Ni(L8)(3)](PF6)(2) (L8 = 3-methyl-1-(pyrimidin-2-yl)imidazolylidene, 9), [Ni(L10)(2)(CH3CN)](PF6)(2) (L10 = 3-ethyl-1-(pyrimidin-2-yl)imidazol-ylidene, 11), [Ni(L12)(2)(CH3CN)](PF6)(2) (L12 = 3-butyl-1(pyrimidin-2-yl)imidazolylidene, 13), [Ni(L14)(2)(CH3CN)](PF6)(2) (L14 = 3-benzyl-1-(pyrimidin-2-yl)imidazolylidene, 15), and [Ni(L16)(2)(CH3CN)](PF6)(2) (L16 = 3-isopropyl-1-(pyrimidin-2-yl)imidazolylidene, 17) are pentacoordinate, displaying unusual square-pyramidal geometry. [Ni(L12)(3)](PF6)(2) (L9 = 3-butyl-1-(pyrimidin-2-yl)imidazolylid-ene, 19), [Ni(L22)(2)(CH3CN)(2)](PF6)(2) (L22 = 3-(2,6-diisopropylphenyl)-1 -(pyrimidin-2-yl)imidazolylidene, 23), and [Ni(L24)(CH3CN)(H2O)] (L22 = 3-(2,4,6-trimethylphenyl)-1 -(pyrimidin-2-yl)imidazolylidene, 25) are octahedral complexes. The structural diversity of these nickel(II) complexes originating from steric pressure of the ligands is discussed.