An Abnormal N-Heterocyclic Carbene-Carbon Dioxide Adduct from Imidazolium Acetate Ionic Liquids: The Importance of Basicity

In the reaction of 1-ethyl-3-methylimidazolium acetate [C(2)C(1)Im][0Ac] ionic liquid with carbon dioxide at 125 C and 10 MPa, not only the known N-heterocyclic carbene (NHC) CO2 adduct I, but also isomeric aNHC-CO2 adducts II and III were obtained. The abnormal NHC-CO2 adducts are stabilized by the presence of the polarizing basic acetate anion, according to static DFT calculations and ab initio molecular dynamics studies. A further possible reaction pathway is facilitated by the high basicity of the system, deprotonating the initially formed NHC-CO2 adduct I, which can then be converted in the presence of the excess of CO2 to the more stable 2-deprotonated anionic abnormal NHC CO2 adduct via the anionic imidazolium-2,4-dicarboxylate according to DFT calculations on model compounds. This suggests a generalizable pathway to abnormal NHC complex formation.