Light-driven reduction of CO 2 : thermodynamics and kinetics of hydride transfer reactions in benzimidazoline derivatives.

CO 2 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 CO 2 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 CO 2 reduction. We provide a detailed analysis for the chemical CO 2 reduction to the formate anion for 15 different benzimidazoline derivatives in terms of thermodynamic hydricities (Δ G H - ), activation free energies (Δ G ‡HT), and reaction free energies (Δ G 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 -1 i.e. the species possess strong hydride donor abilities required for the CO 2 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.