Intermolecular\nProton Transfer Enabled Reactive CO<sub>2</sub> Capture by the Malononitrile\nAnion

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