DFT Study on the Formation Mechanism of Normal and Abnormal N-Heterocyclic Carbene-Carbon Dioxide Adducts from the Reaction of an Imidazolium-Based Ionic Liquid with CO2.

To illustrate the formation mechanism of normal and abnormal N-heterocyclic carbene-carbon dioxide adducts (NHC-CO2 and aNHC-CO2), we implement density functional theory calculations on the reactions of two imidazolium-based ionic liquids ([C2C1Im][OAc] and [C2C1Im][CH3SO3]) with CO2. The reaction of [C2C1Im][OAc] with CO2 is mimicked using the gas phase model, implicit solvent model, and combined explicit-implicit solvent model. In the gas phase, the calculated barriers at 125 °C and 10 MPa are 12.1 kcal/mol for the formation of NHC-CO2 and 22.5 kcal/mol for the formation of aNHC-CO2, and the difference is significant (10.4 kcal/mol). However, the difference becomes less important (1.5 kcal/mol) as the solvation effect is considered more realistically using the combined explicit-implicit solvent model, rationalizing the experimental observation of aNHC-CO2 adduct in the [C2C1Im][OAc]-CO2 system. The anion of the ionic liquid is shown to play a substantial role, which can adjust the reactivity of imidazolium cation toward CO2: upon replacement of the basic [OAc]- anion with a less basic [CH3SO3]- anion, the reaction becomes very difficult, as indicated by high free energy barriers involved (41.4 kcal/mol for the formation of NHC-CO2 and 39.2 kcal/mol for the formation of aNHC-CO2). This is in agreement with the fact that neither NHC-CO2 or aNHC-CO2 is formed in the [C2C1Im][CH3SO3]-CO2 system, emphasizing the important dependence of the reactivity on the basicity of the anion of imidazolium-based ionic liquids for the formation of NHC- and aNHC-CO2 adducts.