Structure of Silver–N-Heterocyclic Carbenes in Solution: Evidence of Equilibration in DMSO at Very Different Time Scales by 1H NMR Experiments

Abstract

Ag–N-heterocyclic carbene (NHC) complexes can occur as neutral [(NHC)AgX] and/or ionic [(NHC)2Ag][AgX2] species in solution. Herein, the behavior of four complexes bearing common bulky N,N′-diaryl NHCs was examined in DMSO. 1H NMR experiments performed systematically in CDCl3 and in DMSO-d6 were used to detect DMSO or chloride ion induced equilibration between the neutral and ionic forms at very different time scales. We observed that solubilization of IMesAgCl and SIMesAgCl in DMSO at 300 K led in less than a few minutes to a complete equilibration. Here, magnetization exchange rate constants were determined from EXSY experiments. In contrast, IPrAgCl and SIPrAgCl, which possess more bulky NHCs, were found to exhibit much lower exchange rates (or no exchange) at 300 K. IPrAgCl was shown to slowly generate an increasing amount of homoleptic cationic complex in DMSO for several hours before reaching equilibrium. In this case, the initial rate of formation of the cationic complex was determined.