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

Abstract

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(μ-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 -triethylphosphine-functionalized Pd species can be metalated by [Rh(μ-OMe)(COD)] 2, resulting in the generation of a hybrid NHC/MIC-bridgedmixed-metal Pd/Rh species. This complex represents the first example of a hybrid unsymmetrical dicarbene bridging two different metals.