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

Three metallacrown nickel complexes [Ni3(μ-OH)(L1)3](PF6)2 (1, L1 = 3-((N-methylimidazolylidenyl)methyl)-5-methylpyrazolate), [Ni3(μ-OH)(L2)3](PF6)2 (2, L2 = 3-((N-mesitylimidazolylidenyl)methyl)-5-methylpyrazolate), and [Ni3(μ-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 °C. The same reaction at 80 °C afforded [Ni3(L2)4](PF6)2 (4). The carbene-transfer reaction of the silver–carbene complex with [(η3-C3H5)PdCl]2 yielded the heterotrimetallic complex [AgPd2(η3-C3H5)2(L2)2](PF6) (5), whereas the carbene-transfer reaction with Pt(cod)Cl2 gave [Pt2(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.