Novel scorpionate-type triscarbene ligands and their silver and gold complexes

New silver(I) carbene complexes were obtained starting from the N-heterocyclic carbene ligand precursors {[HB(RImH)3]Br2} (R = Bn, Mes and t-Bu) and {[HC(MeBImH)3](BF4)3}, by treatment of the imidazolium salt with Ag2O. Use of the tris-imidazolylborate precursors resulted in stable, well-characterized trimetallic complexes of general formula {Ag3[HB(RIm)3]2}Br, which were successfully employed as carbene transfer reagents in the synthesis of related gold(I) complexes by transmetallation. The silver complexes also proved to be active catalysts of the coupling of aryl iodides with terminal alkynes (the Sonogashira reaction), although related bimetallic silver complexes were found to exhibit enhanced reactivity.