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 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).

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

“…As will be seen, in the formic acid and formaldehyde reductions, the solvating water molecule(s) play an additional, more active role; they act as a proton relay, for which this mixed explicit/implicit solvation approach 59,60,134 is especially important for an accurate description. 47,[54][55][56][57] For the DHT-1H2O and DHT-2H2O models, we obtain the barriers of ∆G ‡ HT = 17.1 and 14.3 kcal/mol for the CO2 reduction to HCOO -, ~6 and 9 kcal/mol lower than for the DHT model, reflecting the importance of quantum mechanically described water polarization (see Table 1). a All free energies and enthalpies, referenced to separated reactants, are reported in kcal/mol at 298K and 1 atm.…”

“…48 To further improve the reported energies, we performed single point energy calculations at the M06/6-31+G** geometries using 2 nd order Møller-Plesset perturbation theory (MP2) 50 with the extensive aug-ccPVTZ basis sets. 51 We previously found that MP2 accurately reproduces the CCSD(T) reaction and transition state (TS) energies for reactions between pyridine (Py) and CO2, 47 and have further benchmarked this method against CCSD(T) for reactions involving HT to CO2, as summarized in Table S1 of the Supporting Information (SI), section 1.…”

“…In addition to stabilizing the TS, these water molecules also intimately participate in the reaction by acting as a proton relay chain during the proton transfer event. 47,[54][55][56][57][58][59][60][61][62][63][64][65][66][67] We calculate vibrational force constants at the M06/6-31+G** level of theory to: 1) verify that the reactant and product structures have only positive vibrational modes, 2) confirm that each TS has only one imaginary mode and that it connects the desired reactant and product structures via Intrinsic Reaction Coordinate (IRC) calculations, and 3) compute entropies, zero-point energies (ZPE) and thermal corrections included in the reported free energies at 298K.…”

“…ET to reduce PyH + to PyH 0 involves significant energy input to dearomatize the Py ring. 47 (We show in section 3.8 that dearomatization is crucial in driving catalytic HT reactions that concurrently recover pyridine's aromaticity.) Following the discussion of PyH 0 formation, we examine the subsequent PT and ET transfer steps to PyH 0 that produce PyH2 in section 3.2.…”

“…Consequently, PyH2 differs significantly from conventional transition-metal hydrides (M-H) 22,27,127,129 in that C is more electronegative than the transition metals (M), e.g., Co, Ni and Pt. We suggest that the origin of the hydride nucleophilicity of the hydridic C-H bonds of PyH2 lies in the energetics of dearomatization and aromatization of PyH + , 47 a concept similar to one that has been applied to metal-ligand cooperation in catalysis involving transitionmetal complexes. 157,158 During the formation of PyH2, the first reduction of PyH + to PyH 0 dearomatizes PyH + 's ring (Scheme 9a, route II), a destabilization consistent with PyH + 's highly negative E 0 of ~ -1.3V vs. SCE.…”