Roles of
the Lewis Acid and Base in the Chemical Reduction of CO<sub>2</sub> Catalyzed by Frustrated Lewis Pairs
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Abstract
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