Mechanism of Homogeneous Reduction of CO₂ by Pyridine: Proton Relay in Aqueous Solvent and Aromatic Stabilization

Abstract

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 p<i>K</i> _a 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 p<i>K</i> _a 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, and 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