Homogeneous Reduction of CO_2 by Photogenerated Pyridinyl Radicals

Abstract

We report that 1-hydropyridinyl radicals (1-PyH^•) photogenerated in solution react with dissolved CO_2 en route to its 2e^– reduction into carboxylic acids. The 254 nm excitation of pyridine (Py) in deaerated 2-PrOH/H_2O mixtures saturated with 1 atm of CO_2 yields a suite of products, among which we identified Na(HCOO)_2^– (m/z^– = 113), C_5H_6NCOO– (m/z^– = 124), and C_5H_(10)O_2NCOO– (m/z^– = 160) species by electrospray ionization mass spectrometry. These products demonstrably contain carboxylate functionalities that split CO_2 neutrals via collisionally induced dissociation. We infer that 1-PyH^• [from (1) ^3Py^* + 2-PrOH → 1-PyH^• + ^•PrOH] adds to CO_2, 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 CO_2, Py, 2-PrOH, and actinic radiation, amounts to the homogeneous 2e^– reduction of CO_2 by 2-PrOH initiated by Py^*. We evaluate a rate constant (2) k_2(1-PyH^• + CO_2 → •Py-1-COOH) ≈ O (10) M^(–1) s^(–1) and an activation energy E_2 ≥ 9 kcal mol^(–1) that are compatible with thermochemical estimates for this reaction.