Homogeneous reduction of CO2 by photogenerated pyridinyl radicals

We report that 1-hydropyridinyl radicals (1-PyH•) photogenerated in solution react with dissolved CO2 en route to its 2e− reduction into carboxylic acids. The 254 nm excitation of pyridine (Py) in deaerated 2-PrOH/H2O mixtures saturated with 1 atm of CO2 yields a suite of products, among which we identified Na(HCOO)2−(m/z− = 113), C5H6NCOO− (m/z− = 124), and C5H10O2NCOO− (m/z− = 160) species by electrospray ionization mass spectrometry. These products demonstrably contain carboxylate functionalities that split CO2 neutrals via collisionally induced dissociation. We infer that 1-PyH• [from (1) 3Py* + 2-PrOH → 1-PyH• + •PrOH] adds to CO2, in competition with radical−radical reactions, leading to intermediates that are in turn reduced by •PrOH into the observed species. The formation of carboxylates in this system, which is shown to require CO2, Py, 2-PrOH, and actinic radiation, amounts to the homogeneous 2e− reduction of CO2 by 2-PrOH initiated by Py*. We evaluate a rate constant (2) k2(1-PyH• + CO2 → •Py-1-COOH) ≈ O (10) M−1 s−1 and an activation energy E2 ≥ 9 kcal mol−1 that are compatible with thermochemical estimates for this reaction.