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Published May 14, 2015 | metadata_only
Journal Article

Homogeneous Reduction of CO_2 by Photogenerated Pyridinyl Radicals


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.

Additional Information

© 2014 American Chemical Society. Special Issue: Mario Molina Festschrift. Received: September 25, 2014. Revised: November 10, 2014. Publication Date (Web): November 11, 2014. M.R.H. and A.J.C. acknowledge financial support from the National Science Foundation (U.S.A.) Grant AC-1238977. F.R. gratefully acknowledges financial support from the Doctorate School in Chemistry of the University of Milan, Italy. We thank Dr. Nathan Dalleska for expert help with the mass spectrometer.

Additional details

August 20, 2023
August 20, 2023