DFT Investigation into the Role of Superbases as the Auxiliary Groups in CO2 Reduction

Non-metallic hydride donors have emerged as an interesting, highly tunable class of compounds capable of CO2 reduction, with benzimidazoles being simple, yet efficient and regenerable, representatives. In this work, the role of superbases as auxiliary groups attached to the benzimidazole framework was investigated using the CPCM(CH3CN)/ωB97xD/aug-cc-pVTZ//CPCM(CH3CN)/ωB97xD/6-31+G(d,p) approach. Three modes of operation were assessed through hydricity calculations and the modeling of two different CO2 reduction mechanisms. Among the superbases considered, phosphazene substituents yielded the largest increase in the hydride donation ability, lowering hydricity by 6 kcal mol−1 relative to 2-methylbenzimidazole, with the α-substitution exerting a stronger effect than β-substitution. For most systems, changes in hydricity correlate with changes in aromaticity, except in systems where steric congestion limits optimal substituent alignment. CO2 activation pathways encompassing guanidine/CO2 hydrogen bonding and guanidinium carboxamidine formation were modeled. In the former, transition state structures were significantly stabilized, and the overall exergonicity of the reduction is enhanced. Also, utilizing the longer and more flexible linker additionally decreases the barrier for the reaction. The carboxamidine pathway is disfavored because of the high stability of the carboxamidine intermediate and low barrier for the C–N bond cleavage, which reverses the mechanism to the reduction of isolated CO2.