Intermolecular Proton Transfer Enabled Reactive CO₂ Capture by the Malononitrile Anion

Task-specific ionic liquids (ILs) employing carbanions represent a new class of ILs for carbon capture. The deprotonated malononitrile carbanion, [CH(CN)₂]⁻, has shown close to equimolar capacity for reactive CO₂ capture. Although the formation of the [C(CN)₂COOH]⁻ carboxylic acid was found to be the final product, how the hydrogen atom on the [CH(CN)₂]⁻ carbanion transfers to the carboxylate group as a proton has not been fully understood. In this work, we employ density functional theory calculations with an implicit solvation model to investigate the proton transfer mechanisms in forming carboxylic acid from the reaction of the [CH(CN)₂]⁻ carbanion with CO₂. We find that the intramolecular proton-transfer pathway in [CH(CN)₂COO]⁻ to form [C(CN)₂COOH]⁻ is unlikely due to the high energy barrier of 152 kJ/mol. Instead, the intermolecular proton transfer pathway between two [CH(CN)₂COO]⁻ anions is more feasible to form two molecules of [C(CN)₂COOH]⁻, with a significantly lower activation energy of 50 kJ/mol. Moreover, the [C(CN)₂COOH]⁻ dimer is further stabilized by the intermolecular hydrogen bonds of the two -COOH groups in the Z-configuration of the π-conjugated planar geometry. This insight of reactive CO₂ capture enabled by intermolecular proton transfer will be useful in designing novel carbanions and ILs for carbon capture and conversion.