We use quantum chemical calculations to elucidate a viable homogeneous mechanism for pyridine-catalyzed reduction of CO2 to methanol. In the first phase 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). Highly reducing electrons are present in both photo-electrochemical p-GaP system (ECBM ~ -1.5V vs. SCE at pH= 5) and photochemical [Ru(phen)3]2+/ascorbate system to populate PyH+'s LUMO (E0calc ~ -1.3V vs. SCE) to form the solution phase PyH0. Our results predict that PyH0 undergoes further PT-ET steps to form a closed-shell, dearomatized 1,2-dihydropyridine (PyH2 ) species. Our proposed sequential PT-ET-PT-ET steps that transform Py to PyH2 is consistent with the mechanism described in the formation of related dihydropyridines e.g. 1,6-dihydronicotinamide and acridans. Because it is driven by its proclivity to regain aromaticity, PyH2 becomes a potent renewable 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 phase of the catalytic cycle, we predict that the PyH2 /Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (mediated by a proton relay chain) to CO2 and its two succeeding intermediates to ultimately form CH3OH. Figure 1. <img src='https://acs-archive-assets2.s3.amazonaws.com/241nm/uploads/75720_final_figure.jpg' alt='' />

Role of pyridine as a biomimetic organo-hydride for homogeneous reduction of CO2 to methanol

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.