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

We employ quantum chemical calculations to investigate the mechanism of homogeneous CO(2) reduction by pyridine (Py) in the Py/p-GaP system. We find that CO(2) reduction by Py commences with PyCOOH(0) formation where: (a) protonated Py (PyH(+)) is reduced to PyH(0), (b) PyH(0) then reduces CO(2) by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO(2), and finally (c) proton transfer (PT) from PyH(0) to CO(2) produces PyCOOH(0). The predicted enthalpic barrier for this proton-coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH(0) to CO(2). 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 pK(a) for PyH(0) of 31 indicating that PT preceding ET is highly unfavorable. Moreover, we demonstrate that ET precedes PT in PyCOOH(0) formation, confirming PyH(0)'s pK(a) as irrelevant for predicting PT from PyH(0) to CO(2). Furthermore, we calculate adiabatic electron affinities in aqueous solvent for CO(2), Py, and Py·CO(2) of 47.4, 37.9, and 66.3 kcal/mol respectively, indicating that the anionic complex PyCOO(-) stabilizes the anionic radicals CO(2)(-) and Py(-) to facilitate low barrier ET. As the reduction of CO(2) proceeds through ET and then PT, the pyridine ring becomes aromatic, and thus Py catalyzes CO(2) reduction by stabilizing the PCET TS and the PyCOOH(0) product through aromatic resonance stabilization. Our results suggest that Py catalyzes the homogeneous reductions of formic acid and formaldehyde en route to formation of CH(3)OH through a series of one-electron reductions analogous to the PCET reduction of CO(2) examined here, where the electrode only acts to reduce PyH(+) to PyH(0).