Mechanism of Catalytic O2 Reduction by Iron Tetraphenylporphyrin

Michael L. Pegis, Daniel J. Martin, Catherine F. Wise, Anna C. Brezny, Samantha I. Johnson, Lewis E. Johnson, Neeraj Kumar, Simone Raugei, James M. Mayer

Research output: Contribution to journalArticle

Abstract

The catalytic reduction of O2 to H2O is important for energy transduction in both synthetic and natural systems. Herein, we report a kinetic and thermochemical study of the oxygen reduction reaction (ORR) catalyzed by iron tetraphenylporphyrin (Fe(TPP)) in N,N′-dimethylformamide using decamethylferrocene as a soluble reductant and para-toluenesulfonic acid (pTsOH) as the proton source. This work identifies and characterizes catalytic intermediates and their thermochemistry, providing a detailed mechanistic understanding of the system. Specifically, reduction of the ferric porphyrin, [FeIII(TPP)]+, forms the ferrous porphyrin, FeII(TPP), which binds O2 reversibly to form the ferric-superoxide porphyrin complex, FeIII(TPP)(O2•-). The temperature dependence of both the electron transfer and O2 binding equilibrium constants has been determined. Kinetic studies over a range of concentrations and temperatures show that the catalyst resting state changes during the course of each catalytic run, necessitating the use of global kinetic modeling to extract rate constants and kinetic barriers. The rate-determining step in oxygen reduction is the protonation of FeIII(TPP)(O2•-) by pTsOH, which proceeds with a substantial kinetic barrier. Computational studies indicate that this barrier for proton transfer arises from an unfavorable preassociation of the proton donor with the superoxide adduct and a transition state that requires significant desolvation of the proton donor. Together, these results are the first example of oxygen reduction by iron tetraphenylporphyrin where the pre-equilibria among ferric, ferrous, and ferric-superoxide intermediates have been quantified under catalytic conditions. This work gives a generalizable model for the mechanism of iron porphyrin-catalyzed ORR and provides an unusually complete mechanistic study of an ORR reaction. More broadly, this study also highlights the kinetic challenges for proton transfer to catalytic intermediates in organic media.

Original languageEnglish
JournalJournal of the American Chemical Society
DOIs
Publication statusPublished - Jan 1 2019

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Protons
Iron
Porphyrins
Oxygen
Kinetics
Superoxides
Proton transfer
Dimethylformamide
Temperature
Thermochemistry
Reducing Agents
Protonation
Equilibrium constants
iron tetraphenylporphyrin
Rate constants
Electrons
tetraphenylporphyrin
Catalysts
Acids

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Pegis, M. L., Martin, D. J., Wise, C. F., Brezny, A. C., Johnson, S. I., Johnson, L. E., ... Mayer, J. M. (2019). Mechanism of Catalytic O2 Reduction by Iron Tetraphenylporphyrin. Journal of the American Chemical Society. https://doi.org/10.1021/jacs.9b02640

Mechanism of Catalytic O2 Reduction by Iron Tetraphenylporphyrin. / Pegis, Michael L.; Martin, Daniel J.; Wise, Catherine F.; Brezny, Anna C.; Johnson, Samantha I.; Johnson, Lewis E.; Kumar, Neeraj; Raugei, Simone; Mayer, James M.

In: Journal of the American Chemical Society, 01.01.2019.

Research output: Contribution to journalArticle

Pegis, ML, Martin, DJ, Wise, CF, Brezny, AC, Johnson, SI, Johnson, LE, Kumar, N, Raugei, S & Mayer, JM 2019, 'Mechanism of Catalytic O2 Reduction by Iron Tetraphenylporphyrin', Journal of the American Chemical Society. https://doi.org/10.1021/jacs.9b02640
Pegis, Michael L. ; Martin, Daniel J. ; Wise, Catherine F. ; Brezny, Anna C. ; Johnson, Samantha I. ; Johnson, Lewis E. ; Kumar, Neeraj ; Raugei, Simone ; Mayer, James M. / Mechanism of Catalytic O2 Reduction by Iron Tetraphenylporphyrin. In: Journal of the American Chemical Society. 2019.
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