Synthesis and Characterization of Square-Planar Tetranuclear Silver and Gold Clusters Supported by a Pyrazole-Linked Bis(N-heterocyclic carbene) Ligand

The N-heterocyclic carbene (NHC) precursor 3,5-bis((N-methylimidazoliumyl)methyl)pyrazole bis(hexafluorophosphate) (H3L(PF6)2) reacted with Ag2O to afford the silver complex [Ag4L2](PF6)2·0.5Et2O (5). Reaction of 5 with Au(Et2S)Cl forms the corresponding Au(I) complex, [Au4L2](PF6)2 (6). These complexes were characterized by 1H NMR, 13C NMR, UV−vis, luminescence spectroscopy, elemental analysis, mass spectrometry, and X-ray crystallography. The two isostructural complexes contain tetrametallic cores which are nearly square planar. Each L behaving as anionic tetradentate ligand links four different metal atoms, and each metal atom is coordinated by a carbenic carbon and a pyrazolate nitrogen atom. Both complexes and the ligand precursor are intensely luminescent in the solid state.