Unsymmetrical Dicarbenes Based on N-Heterocyclic/Mesoionic Carbene Frameworks: A Stepwise Metalation Strategy for the Generation of a Dicarbene-Bridged Mixed-Metal Pd/Rh Complex

A pair of linked imidazolium/triazolium salts have been prepared using copper-catalyzed azide-alkyne cycloaddition (CuAAC or click chemistry) and methylation protocols, producing a precursor for bidentate N-heterocyclic carbene (NHC)/mesoionic carbene (MIC) ligands, representing a rare example of an NHC/MIC hybrid. Metalation of one-half of this dicationic species using the basic ligand-containing [Pd(OAc)(2)] in the presence of potassium iodide or half an equivalent of [Rh(mu-OMe) (COD)](2) yields NHC-anchored/pendent triazolium species of Pd or Rh, respectively. The pendent Pd species can be further functionalized through iodide substitution by various monophosphines, which preferentially adopt a cis or trans arrangement depending on the bulk of the anchored NHC substituent. Combining these internal-base and pendent strategies, the pendent MIC(H)(+) arm of the trans-triethylphosphinefunctionalized Pd species can be metalated by [Rh(mu-OMe)(COD)](2), resulting in the generation of a hybrid NHC/MIC-bridged mixed-metal Pd/Rh species. This complex represents the first example of a hybrid unsymmetrical dicarbene bridging two different metals.