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)center dot 0.5Et(2)O (5). Reaction of 5 with Au(Et2S)Cl forms the corresponding Au(I) complex, [Au4L2](PF6)(2) (6). These complexes were characterized by H-1 NMR, C-13 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.