Role of Pyridine as a Biomimetic Organo-Hydride for Homogeneous Reduction of CO2 to Methanol

TL;DR

This study uses quantum chemical calculations to propose a new homogeneous catalytic mechanism where pyridine acts as a biomimetic organo-hydride, enabling the reduction of CO2 to methanol through sequential proton and electron transfers.

Contribution

It introduces a novel PT-ET-PT-ET mechanism involving pyridine derivatives that mimics NADPH in photosynthesis for CO2 reduction to methanol.

Findings

Pyridine can be transformed into a potent organo-hydride donor (PyH2) via sequential PT-ET steps.

The PyH2/Py redox couple effectively mediates hydride and proton transfers to CO2 and intermediates.

The proposed mechanism aligns with known dihydropyridine formation and mimics biological reduction processes.

Abstract

We use quantum chemical calculations to elucidate a viable homogeneous mechanism for pyridine-catalyzed reduction of CO2 to methanol. In the first component of the catalytic cycle, pyridine (Py) undergoes a H+ transfer (PT) to form pyridinium (PyH+) followed by an e- transfer (ET) to produce pyridinium radical (PyH0). Examples of systems to effect this ET to populate the LUMO of PyH+(E0calc ~ -1.3V vs. SCE) to form the solution phase PyH0 via highly reducing electrons include the photo-electrochemical p-GaP system (ECBM ~ -1.5V vs. SCE at pH= 5) and the photochemical [Ru(phen)3]2+/ascorbate system. We predict that PyH0 undergoes further PT-ET steps to form the key closed-shell, dearomatized 1,2-dihydropyridine (PyH2) species. Our proposed sequential PT-ET-PT-ET mechanism transforming Py into PyH2 is consistent with the mechanism described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process. In particular, in the second component of the catalytic cycle, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first reduction step, i.e. reduction of CO2, are sequential in nature; by contrast, they are coupled in each of the two subsequent hydride and proton transfers to reduce formic acid and formaldehyde.