Department of Chemical and Biological Engineering, University of Colorado at Boulder, Boulder, Colorado 80309, United States.
J Am Chem Soc. 2013 Jan 9;135(1):142-54. doi: 10.1021/ja3064809. Epub 2012 Dec 21.
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).
我们采用量子化学计算方法研究了在吡啶（Py）/p-GaP 体系中 CO2 均相还原的机制。我们发现，Py 引发的 CO2 还原起始于 PyCOOH（0）的形成，其中：（a）质子化的 Py（PyH（+））被还原为 PyH（0），（b）然后，PyH（0）通过其 N 孤对电子对 CO2 的 C 的亲核攻击，通过单电子转移（ET）还原 CO2，最后（c）从 PyH（0）到 CO2 的质子转移（PT）产生 PyCOOH（0）。对于这种质子耦合 ET（PCET）反应，从 PyH（0）直接到 CO2 的预测焓垒为 45.7 kcal/mol。然而，当 PT 通过一个到三个水分子作为质子中继介导时，该障碍分别降低到 29.5、20.4 和 18.5 kcal/mol。水质子中继降低了过渡态（TS）中的应变，并促进了更完全的 ET。对于通过三个水分子质子中继介导的 PT，向核心反应体系中添加水分子以使反应完全溶剂化，可将势垒降低至 13.6-16.5 kcal/mol，具体取决于溶剂化水的数量和构型。这与实验确定的 16.5±2.4 kcal/mol 的势垒一致。我们计算出 PyH（0）的 pKa 为 31，表明 ET 之前的 PT 非常不利。此外，我们证明了在 PyCOOH（0）形成中 ET 先于 PT，这证实了 PyH（0）的 pKa 与预测从 PyH（0）到 CO2 的 PT 无关。此外，我们分别计算了 CO2、Py 和 Py·CO2 在水溶液中的绝热电子亲和能为 47.4、37.9 和 66.3 kcal/mol，表明阴离子配合物 PyCOO（-）稳定了阴离子自由基 CO2（-）和 Py（-），从而促进了低势垒 ET。由于 CO2 的还原通过 ET 然后是 PT 进行，吡啶环变得芳香化，因此通过芳香共振稳定化稳定 PCET TS 和 PyCOOH（0）产物，吡啶（Py）催化 CO2 的还原。我们的结果表明，在通过一系列单电子还原将甲酸和甲醛还原为 CH3OH 的过程中，Py 通过类似于此处研究的 CO2 的 PCET 还原，通过稳定的 PCET TS 和 PyCOOH（0）产物，催化 CO2 的均相还原，其中电极仅用于将 PyH（+）还原为 PyH（0）。
