The utilization of dioxygen in nature is often mediated by the binding of O₂ to transition metals, such as iron. These metals can then transfer multiple electrons to the bound O₂. The transfer of one electron produces a metal superoxide species, which is proposed as a key intermediate in a number of oxidizing enzymes—including heme enzymes such as the tryptophan- and indoleamine-2,3-dioxygenases, and nonheme enzymes such as the thiol dioxygenases.

David P. Goldberg, Johns Hopkins University, Baltimore, MD, USA, Pierre Moënne-Loccoz, Oregon Health & Science University, Portland, OR, USA, and colleagues have found that an iron corrole compound can bind O₂ to form an iron superoxide species at low temperature (pictured). Metallocorroles are versatile heme analogues, but until now, the canonical FeO₂ adduct of a corrole had not been identified definitively.

The team reduced Fe^III(ttppc) (ttppc = 5,10,15-tris((2,4,6-triphenyl)phenyl)corrolato^3–), with sodium/mercury amalgam to obtain [Fe^II(ttppc)]⁻. A reaction with excess O₂ at −80 °C then gave the superoxo complex [Fe^III(O₂⁻·)(ttppc)]⁻.

The new iron superoxide can carry out biologically relevant oxidations, such as indole dioxygenation and hydrogen-atom abstraction. The work provides insights into the reactivity of biomimetic iron superoxide species, as well as spectroscopic benchmarks for the future identification of such species.

• An Iron(III) Superoxide Corrole from Iron(II) and Dioxygen,
  Jireh Joy D. Sacramento, Therese Albert, Maxime Siegler, Pierre Moënne‐Loccoz, David P. Goldberg,
  Angew. Chem. Int. Ed. 2021.
  https://doi.org/10.1002/anie.202111492