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
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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-bridged mixed-metal Pd/Rh species. This complex represents the first example of a hybrid unsymmetrical dicarbene bridging two different metals.
