Multi-Site CO2 Fixation in Triazolates: Cooperative O,N Binding Enhanced by Solvation and Counter-Ion Effects

Abstract

The cooperative interaction of nitrogen and oxygen centers in aromatic heterocycles provides an effective pathway for charge-assisted CO2 capture. Building on the pioneering work of Luo et al. on hydroxy-pyridine systems and our recent PCCP study on hydroxy-substituted N-heterocycles, this work extends the O,N-cooperative binding concept to triazolate frameworks. Density functional theory (DFT) and molecular electrostatic potential (MESP) analyses were performed on neutral, monoanionic, and dianionic 1,2,3- and 1,2,4-triazoles to elucidate how charge, counter ion and solvation govern CO2 adsorption. Deprotonation generates anionic and dianionic triazolates with enhanced negative potential at N and O sites, enabling O-carboxylate and N-carboxylate formation. While gas-phase dianions capture multiple CO2 molecules through cooperative charge delocalization and carbonate-chain growth (up to three CO2 with ΔGad ≈ -54 kcal mol-1), the inclusion of ethanol solvation and tetramethylphosphonium counter-ions reveals that these species remain strongly exergonic. Reaction modeling shows that both mono- and dianionic triazolates undergo spontaneous CO2 fixation in solution (ΔG ≈ -24 to -61 kcal mol-1), forming ion-paired poly(carboxylate) complexes with up to six CO2 molecules. These results demonstrate that even under polar, solvated conditions, counter-cation-stabilized triazolates preserve high-affinity, multi-site CO2 capture, identifying them as realistic and promising building blocks for molecular CO2-activation strategies.