Theoretical design and mechanistic study of the metal-free reduction of CO2 to CO

Abstract

A strategy for the reduction of CO₂ to CO by or catalyzed by metal-free silylboranes has been proposed with the aid of density functional theory (DFT) computations. We showed that one oxygen atom of CO₂ can be abstracted by silylboranes without catalysts or by diboranes in the presence of silylborane catalysts with surprisingly low free-energy barriers so that the reaction can be realized under mild experimental conditions. To achieve this, the reduction mechanism of CO₂ by a hierarchy of silylboranes (R₁)₂BSi(R₂)₃ was systematically investigated. Several rules of thumb were obtained to guide the design of silylboranes with high activity toward CO₂ reduction. After considering many factors, such as side reactions, the stability of the silylboranes, and the solvent effect, two silylboranes, (PFP)₂BSi(CH₂F)₃ and Me₂BSi(CH₂F)₃, suitable for the reduction of CO₂ under mild experimental conditions were designed. The overall free-energy barriers for the reduction of CO₂ by the two silylboranes are just 26.1–27.0 kcal mol⁻¹ and 28.1–28.9 kcal mol⁻¹, respectively, at 298.15 K in solution. We further showed that CO₂ can be reduced to CO by diborane Me₂BBMe₂ using Me₂BSi(CH₂F)₃ as the catalyst. The overall free-energy barrier for this catalytic reaction is just 30.6–30.7 kcal mol⁻¹ at 298.15 K in solution.