Distinct reactivity of mono- and bis-NHC silver complexes: Carbene donors versus carbene–halide exchange reagents

The synthesis of N-heterocyclic carbene (NHC) complexes of silver is an established technique for their use as carbene transfer reagents. While it is known that both mono-NHC Ag(I) and bis-NHC Ag(I) complexes are accessible, different reactivities of these species have not been explored synthetically. Whereas the commonly used mono-NHC Ag(I) complexes act as NHC donors only, we found that a bis-NHC Ag(I) complex was capable of transferring one carbene under halide abstraction. This is exemplified in the transformation of P–Cl compounds into the respective imidazolium–phosphines by a bis-NHC Ag(I) complex but not by its corresponding mono-NHC Ag(I) complex. When the bis-NHC Ag(I) complex was allowed to react with [M(cod)Cl]2 (M = Rh, Ir), both NHCs were transferred under chloride abstraction to give [M(cod)(NHC)2]+ exclusively, a reaction which is reported to stop at [M(cod)Cl(NHC)] even with an excess of mono-NHC Ag(I) complex in the presence of AgPF6.