Intermolecular Proton Transfer Enabled Reactive CO2 Capture by the Malononitrile Anion

Task-specific ionic liquids employing carbanions represent a new class of ionic liquids for carbon capture. The deprotonated malononitrile carbanion, [CH(CN)2]−, has shown close to equimolar capacity for reactive CO2 capture. Although 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 is unclear. 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. The insight of reactive CO2 capture enabled by intermolecular proton transfer will be useful in designing novel carbanions and ionic liquids for carbon capture and conversion.