Synthesis of new N-heterocyclic carbene ligands and related coinage metal complexes

Over the last decade, several reviews have appeared describing the most recent developments with regard to design, structural features and catalytic activity of complexes containing polydentate NHC ligands. This interest toward chelating NHCs is due not only to the formation of more stable metal complexes, but also to the evidence that they provide interesting features that can fine tune the topological properties such as steric hindrance, bite angles, chirality and fluxional behaviour. Recently we described a new methodology for the synthesis of novel hydrophilic pincer carbene ligand precursors based on 1,2,4-triazole and imidazole rings, {H2C(HTzR)2}, {H2C(HImR)2} (R = PrSO3 or EtCOO). The related carbene-silver(I) complexes were synthesized by reaction between the triazolium or imidazolium species with Ag2O; in these metallacycles, of general formula {Na2[H2C(TzR)2]2Ag2} and {Na2[H2C(ImR)2]2Ag2}, every silver atom is coordinated to two triazolin- or imidazolin-2-ylidene rings, belonging to two different dicarbene units. Moreover we have reported the synthesis of trimetallic carbene complexes of general formula {Ag3[HB(ImR)3]2} (R = Bn, Mes and tBu), which were successfully employed, as carbene transfer reagents, in the synthesis of related Au(I) complexes by transmetallation; the Ag(I) complexes also proved to be active catalysts in the Sonogashira reaction. At the present we are developing the chemistry of some new water soluble zwitterionic mono-NHC ligands and of the related Ag(I) and Au(I) carbene complexes {(Im1R,3R)MCl} (M = Ag or Au; R = PrSO3 or CH2COOR′).