Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

doi:10.1073/pnas.0409640102

Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

Xiaopeng Shan and
Lawrence Que, Jr.*

Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, MN 55455

Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved February 22, 2005 (received for review December 22, 2004)

Abstract

The reaction of [Fe₂(μ-OH)₂(6-Me₃-TPA)₂]²⁺ (1) [6-Me₃-TPA, Tris(6-methyl-2-pyridylmethyl)amine] with O₂ in CH₂Cl₂ at –80°C gives rise to two new intermediates, 2 and 3, before the formation of previously characterized [Fe₂(O)(O₂)(6-Me₃-TPA)₂]²⁺ (4) that allow the oxygenation reaction to be monitored one electron-transfer step at a time. Raman evidence assigns 2 and 3 as a diiron–superoxo species and a diiron–peroxo species, respectively. Intermediate 2 exhibits its ν(O–O) at 1,310 cm^–1 with a –71-cm^–1 ¹⁸O isotope shift. A doublet peak pattern for the ¹⁶O¹⁸O isotopomer of 2 in mixed-isotope Raman experiments strongly suggests that the superoxide ligand of 2 is bound end-on. This first example of a nonheme iron–superoxo intermediate exhibits the highest frequency ν(O–O) yet observed for a biomimetic metal–dioxygen adduct. The bound superoxide of 2, unlike the bound peroxide of 4, is readily reduced by 2,4-di-tert-butylphenol via a proton-coupled electron-transfer mechanism, emphasizing that metal–superoxo species may serve as oxidants in oxygen activation mechanisms of metalloenzymes. The discovery of intermediates 2 and 3 allows us to dissect the initial steps of dioxygen binding at a diiron center leading to its activation for substrate oxidation.

Footnotes

↵ * To whom correspondence should be addressed. E-mail: que{at}chem.umn.edu.
Author contributions: X.S. and L.Q. designed research; X.S. performed research; X.S. analyzed data; and X.S. and L.Q. wrote the paper.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: 6-Me₃-TPA, Tris(6-methyl-2-pyridylmethyl)amine; DTBP, 2,4-di-tert-butylphenol.