Roles of the Lewis Acid and Base in the Chemical Reduction of CO₂ Catalyzed by Frustrated Lewis Pairs

We employ quantum chemical calculations to discover how frustrated Lewis pairs (FLP) catalyze the reduction of CO₂ by ammonia borane (AB); specifically, we examine how the Lewis acid (LA) and Lewis base (LB) of an FLP activate CO₂ for reduction. We find that the LA (trichloroaluminum, AlCl₃) alone catalyzes hydride transfer (HT) to CO₂ while the LB (trimesitylenephosphine, PMes₃) actually hinders HT; inclusion of the LB increases the HT barrier by ∼8 kcal/mol relative to the reaction catalyzed by LAs only. The LB hinders HT by donating its lone pair to the LUMO of CO₂, increasing the electron density on the C atom and thus lowering its hydride affinity. Although the LB hinders HT, it nonetheless plays a crucial role by stabilizing the active FLP·CO₂ complex relative to the LA dimer, free CO₂, and free LB. This greatly increases the concentration of the reactive complex in the form FLP·CO₂ and thus increases the rate of reaction. We expect that the principles we describe will aid in understanding other catalytic CO₂ reductions.