(472g) Mechanistic Studies of the Catalytic Reduction of CO2 to Methanol

We employ quantum chemical calculations to investigate the mechanism of homogeneous CO₂ reduction by pyridine (Py) in the Py/p-GaP system. We find that CO₂ reduction by Py commences with PyCOOH⁰ formation where: a) protonated Py (PyH⁺) is reduced to PyH⁰, b) PyH⁰ then reduces CO₂ by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO₂ and finally c) proton transfer (PT) from PyH⁰ to CO₂ produces PyCOOH⁰. The predicted enthalpic barrier for this proton coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH⁰ to CO₂. However, when PT is mediated by one to three water molecules acting as a proton relay the barrier decreases to 29.5, 20.4 and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6 - 16.5 kcal/mol, depending on the number and configuration of the solvating waters. This agrees with the experimentally determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a pKa for PyH⁰ of 31 indicating that PT preceding ET is highly unfavorable. Moreover, we demonstrate that ET precedes PT in PyCOOH⁰ formation, confirming PyH⁰’s pKa as irrelevant for predicting PT from PyH⁰ to CO₂. Furthermore, we calculate adiabatic electron affinities in aqueous solvent for CO₂, Py and Py•CO₂ of 47.4, 37.9, 66.3 kcal/mol respectively, indicating that the anionic complex PyCOO⁻ stabilizes the anionic radicals CO₂⁻ and Py⁻ to facilitate low barrier ET. As the reduction of CO₂ proceeds through ET and then PT, the pyridine ring becomes aromatic and thus Py catalyzes CO₂ reduction by stabilizing the PCET TS and the PyCOOH⁰ product through aromatic resonance stabilization. Our results suggest that Py catalyzes the homogeneous reductions of formic acid, and formaldehyde en route to formation of CH₃OH through a series of one-electron reductions analogous to the PCET reduction of CO₂ examined here, where the electrode only acts to reduce PyH⁺ to PyH⁰.