Light-driven reduction of CO2: thermodynamics and kinetics of hydride transfer reactions in benzimidazoline derivatives

Abstract

CO₂ capture, conversion and storage belong to the holy grail of environmental science. We therefore explore an important photochemical hydride transfer reaction of benzimidazoline derivatives with CO₂ in a polar solvent (dimethylsulfoxide) by quantum-chemical methods. While the excited electronic state undergoing hydride transfer to formate (HCOO⁻) shows a higher reaction path barrier compared to the ground state, a charge-transfer can occur in the near-UV region with nearly barrierless access to the products involving a conical intersection between both electronic states. Such radiationless decay through the hydride transfer reaction and formation of HCCO⁻via excited electronic states in suitable organic compounds opens the way for future photochemical CO₂ reduction. We provide a detailed analysis for the chemical CO₂ reduction to the formate anion for 15 different benzimidazoline derivatives in terms of thermodynamic hydricities (ΔG_H⁻), activation free energies (ΔG^‡_HT), and reaction free energies (ΔG_rxn) for the chosen solvent dimethylsulfoxide at the level of density functional theory. The calculated hydricities are in the range from 35.0 to 42.0 kcal mol⁻¹i.e. the species possess strong hydride donor abilities required for the CO₂ reduction to formate, characterized by relatively low activation free energies between 18.5 and 22.2 kcal mol⁻¹. The regeneration of the benzimidazoline can be achieved electrochemically.