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

Reaction of Ag 2 O with the N-heterocyclic carbene (NHC) precursors 3,5-bis[3-(2,4,6-trimethylphenyl)imidazolium-1-ylmethyl]-1 H -pyrazole bishexafluorophosphate ( 1a ) and 3,5-bis[3-(2,6-diisopropylphenyl)imidazolium-1-ylmethyl]-1 H -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 ( 1 H, 13 C, 15 N, 109 Ag, and DOSY experiments), which also provide valuable benchmark data for correlating 1 J AgN couplings and 109 Ag chemical shifts with structural information.