Imidazolium Carboxylates as Versatile and Selective N-Heterocyclic Carbene Transfer Agents:  Synthesis, Mechanism, and Applications

N,N‘-Disubstituted imidazolium carboxylates, readily synthetically available, isolable, air- and water-stable reagents, efficiently transfer N-heterocyclic carbene (NHC) groups to Rh, Ir, Ru, Pt, and Pd, to give novel NHC complexes, e.g., [Pd(NHC)₃OAc]OAc and [Pt(NHC)₃Cl]Cl (NHC = 1,3-dimethyl imidazol-2-ylidene). The NHC esters are also effective. Tuning the reaction conditions for NHC transfer can give either mono- or bis-NHCs, or bis- and tris-NHCs. A net N to C rearrangement of the N-alkyl imidazole complex to the corresponding NHC complex was seen with (MeO)₂CO (DMC). DFT calculations identify the steps needed to form the carboxylate from imidazole and DMC:  S_N2 methyl transfer from DMC to imidazole, followed by proton transfer from the imidazolium CH to the carboxylate counterion, produces the free NHC H-bonded to MeOH with a weakly associated CO₂. The nucleophilic NHC attacks CO₂ to form NHC−CO₂. NHC transfer to the metal with loss of CO₂ has been calculated for Rh(cod)Cl. A proposed two-cis-site reactivity model rationalizes the experimental data:  two such vacant sites at the metal are needed to allow coordination of the NHC−CO₂ carboxylate and subsequent CC cleavage with NHC transfer. Partial cod decoordination or chloride loss is thus required for Rh(cod)Cl. Chloride dissociation, calculated to be easier in polar solvent, is confirmed experimentally from the retarding effect of excess chloride.