Combining Pyrazolate and N-Heterocyclic Carbene Coordination Motifs: Synthesis and Characterization of a Double-Crowned Silver Complex

Reaction of Ag2O with the N-heterocyclic carbene (NHC) precursors 3,5-bis[3-(2,4,6-trimethylphenyl)imidazolium-1-ylmethyl]-1H-pyrazole bishexafluorophosphate (1a) and 3,5-bis[3-(2,6-diisopropylphenyl)imidazolium-1-ylmethyl]-1H-pyrazole bishexafluorophosphate (1b) yielded silver(I) complexes 2 and 3, respectively. X-ray crystallography revealed a tetranuclear dimeric solid state structure in the case of 2, whereas an unprecedented octanuclear tetrameric “double-crown”-like structure was found for 3, presumably due to the slightly bulkier ligand. Unlike 2, complex 3 is not stable in solution but in equilibrium with tetranuclear and hexanuclear complexes 4 and 5, respectively. The identity of those species could be elucidated by detailed NMR spectroscopic studies (1H, 13C, 15N, 109Ag, and DOSY experiments), which also provide valuable benchmark data for correlating 1JAgN couplings and 109Ag chemical shifts with structural information.