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) (3)Py* + 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.
†Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, California 91125, United States.|‡Department of Chemistry, University of Milan, via Golgi 19, 20133 Milano, Italy.
Francesca
F
Riboni
‡Department of Chemistry, University of Milan, via Golgi 19, 20133 Milano, Italy.
Elena
E
Selli
†Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, California 91125, United States.
M R
MR
Hoffmann
†Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, California 91125, United States.
A J
AJ
Colussi
D002245:Carbon Dioxide / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry*; D055668:Photochemical Processes; D011725:Pyridines / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D014867:Water / Q000737:chemistry
D002264:Carboxylic Acids; D005609:Free Radicals; D011725:Pyridines; D014867:Water; D002245:Carbon Dioxide