(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.