Mechanism of formation of silver N-heterocyclic carbenes using silver oxide:: A theoretical study

The reasons for the efficiency of the silver oxide route in the synthesis of Ag(I) N-heterocyclic carbene complexes have been investigated by means of DFT calculations. A general reaction system that incorporates two N,N-dimethyl imidazolium cations, two iodides as the counterions, Ag2O, and dichloromethane solvent was considered. Exploration of several pathways for the formation of the two silver-NHC cations gives a clear picture for the reaction mechanism. The favored route involves a barrierless and very exergonic deprotonation of the first imidazolium followed by a low barrier and also exergonic metalation, affording the first silver-NHC. The second imidazolium assists these two steps stabilizing intermediates and the transition state by the formation of a strong C-imidazolium - H center dot center dot center dot OAg hydrogen bond. The formed [R2NHC] - AgI then diffuses in the solution, while silver hydroxide deprotonates the second imidazolium salt in a slightly endergonic process. After metalation, the second silver-NHC is obtained. The overall reaction is thermodynamically driven, the conversion of two imidazolium salts to two silver carbenes entailing a system stabilization of more than 70 kcal mol(-1). As the acid-base reaction between the imidazolium salt and the silver base plays a key role in the process, pK(a) calculations were performed to compare the basicity of the N-heterocyclic carbene with those of the silver bases Ag2O, AgOH, and AgOAc. From the pK(a) values obtained, the advantage of silver oxide in the generation of silver carbenes is attributed to its stronger basicity.