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

摘要

We employ CCSD(T) coupled-cluster 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 PyCOOH~0 formation where: a) protonated Py (PyH~+) is reduced to PyH~0, b) PyH°then reduces CO2 by one electron transfer (ET) and finally c) proton transfer (PT) from PyH° to CO2 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 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). 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, which agrees with the experimentally determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a pKa for PyH~0 of 31 indicating that PT preceding ET is highly unfavorable, where we show ET must precede PT in PyCOOH~0 formation. 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 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 PyCOOH~0 product through aromatic resonance stabilization.