Synthesis and Characterization of Nickel Inverse 9-Metallacrown-3, Palladium-Silver, and Dinuclear Platinum Complexes Containing Pyrazole-Functionalized NHC Ligands

Three metallacrown nickel complexes [Ni-3(mu-OH)(L1)(3)](PF6)(2) (1, L1 = 3-((N-methylimidazolylidenyl)- methyl)-5-methylpyrazolate), [Ni-3(u-OH)(L2)(3)](PF6)(2) (2, L2 = 3-((N-mesitylimidazolylidenyl)methyl)-5-methylpyrazolate), and [Ni-3(mu-OH)(L3)(3)] (PF6)(2) (3, L3 = 3-((N-pyrimidin-2-ylimidazolylidenyl)methyl)-5-methylpyrazolate) were obtained by the reactions of corresponding silver-NHC complexes with Raney nickel powder at 45 degrees C. The same reaction at 80 degrees C afforded [Ni-3(L-2)(4)](PF6)(2) (4). The carbene-transfer reaction of the silver carbene complex with [(eta(3)-C3H5)PdCl](2) yielded the heterotrimetallic complex [AgPd2(eta(3)-C3H5)(2)(L2)(2)] (PF6) (5), whereas the carbene-transfer reaction with Pt(cod)Cl-2 gave [Pt-2(L3)(2)] (PF6)(2) (6). All of these complexes have been fully characterized by ESI-MS, NMR spectroscopy, and elemental analysis. The molecular structures of 1-6 were also studied by X-ray diffraction analysis. In 1-3, three nickel centers are bridged together by three pyrazole NHC ligands and a hydroxide group, forming a 9-metallacrown-3 topology. Complex 4 is paramagnetic, consisting of two square-planar nickel(II) ions and one tetrahedral nickel ion in which three Ni ions are bridged by four pyrazolate units. In the mixed Pd-Ag complex 5, two palladium and one silver centers are bridged by two pyrazole-NHC ligands. Complex 5 showed good catalytic activity in the Sonogashira coupling reaction of aryl bromides and phenylacetylene under mild conditions typically catalyzed by Pd-Cu systems.