Roles of the Lewis Acid and Base in the Chemical Reduction of CO<sub>2</sub> Catalyzed by Frustrated Lewis Pairs LimChern-Hooi HolderAaron M. HynesJames T. MusgraveCharles B. 2013 We employ quantum chemical calculations to discover how frustrated Lewis pairs (FLP) catalyze the reduction of CO<sub>2</sub> by ammonia borane (AB); specifically, we examine how the Lewis acid (LA) and Lewis base (LB) of an FLP activate CO<sub>2</sub> for reduction. We find that the LA (trichloroaluminum, AlCl<sub>3</sub>) alone catalyzes hydride transfer (HT) to CO<sub>2</sub> while the LB (trimesitylenephosphine, PMes<sub>3</sub>) 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 CO<sub>2</sub>, 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·CO<sub>2</sub> complex relative to the LA dimer, free CO<sub>2</sub>, and free LB. This greatly increases the concentration of the reactive complex in the form FLP·CO<sub>2</sub> and thus increases the rate of reaction. We expect that the principles we describe will aid in understanding other catalytic CO<sub>2</sub> reductions.