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Trapping of a phenoxyl radical at a non-haem high-spin iron(II) centre

Nature Chemistry volume 16, pages 658–665 (2024)Cite this article

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Abstract

The activation of dioxygen at haem and non-haem metal centres, and subsequent functionalization of unactivated C‒H bonds, has been a focal point of much research. In iron-mediated oxidation reactions, O2 binding at an iron(II) centre is often accompanied by an oxidation of the iron centre. Here we demonstrate dioxygen activation by sodium tetraphenylborate and protons in the presence of an iron(II) complex to form a reactive radical species, whereby the iron oxidation state remains unaltered in the presence of a highly oxidizing phenoxyl radical and O2. This complex, containing an unusual iron(II)-phenoxyl radical motif, represents an elusive example of a spectroscopically characterized oxygen-derived iron(II)-reactive intermediate during chemical and biological dioxygen activation at haem and non-haem iron active centres. The present report opens up strategies for the stabilization of a phenoxyl radical cofactor, with its full oxidizing capabilities, to act as an independent redox centre next to an iron(II) site during substrate oxidation reactions.

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Fig. 1: Reactions and complexes.
Fig. 2: Mechanistic scheme.
Fig. 3: EPR and Mössbauer characterizations of 2.
Fig. 4: rRaman spectra of 2.
Fig. 5: MCD characterization.
Fig. 6: Molecular orbitals.

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Data availability

Source data are provided with this paper. They include text, Excel or pdf files containing data for Figs. 3a,b, 4a and 5a–c and Extended Data Fig. 1a,b. Computed structures included in Fig. 6 and Extended Data Table 1 are attached as .xyz files. Coordinates for all structures are also present in Supplementary Information. Cube files for the MOs in Fig. 6. are included. Supplementary Information also contains all instrumental specification and additional experimental procedures. Electrochemical, EPR, NMR, ESI–MS, rRaman, SQUID and magnetic Mössbauer data are also included, together with further reactivity studies. Source data for supplementary figures and tables can be made available upon request.

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Acknowledgements

This work was funded by the Deutsche Forschungsgemeinschaft (DFG) (1) under Germany’s Excellence Strategy—EXC 2008-390540038—UniSysCat. (to K.R. and P.H.) and (2) via a Heisenberg-Professorship (to K.R.), Alexander von Humboldt Foundation and COST-STSM-CM1305-39979 to T.C., and the National Science Foundation (CHE-1900380 to N.L.). V.A.L. acknowledges support from a University of Michigan Rackham Predoctoral Fellowship. T.L. is also grateful to DFG for support under Project No. LO 2898/1-1. We also thank W. Browne (University of Groningen), G. Rajaraman (Indian Institute of Technology, Mumbai), M. Swart (University of Girona), A. Schnegg (Max Planck Institute for Chemical Energy Conversion, Mulheim an der Ruhr, Germany), A. R.-Rivera (University of Girona) and R. Kumar (Indian Institute of Technology, Mumbai) for valuable suggestions.

Author information

Author notes

  1. These authors contributed equally: Dustin Kass, Virginia A. Larson.

  2. Deceased: Eckhard Bill.

Authors and Affiliations

  1. Department of Chemistry, Humboldt-Universität zu Berlin, Berlin, Germany

    Dustin Kass, Teresa Corona, Thomas Lohmiller & Kallol Ray

  2. Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor, MI, USA

    Virginia A. Larson & Nicolai Lehnert

  3. Department of Chemistry, Technische Universität Berlin, Berlin, Germany

    Uwe Kuhlmann & Peter Hildebrandt

  4. EPR4Energy Joint Lab, Department Spins in Energy Conversion and Quantum Information Science, Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany

    Thomas Lohmiller

  5. Max-Planck-Institut für Chemische Energiekonversion, Mülheim an der Ruhr, Germany

    Eckhard Bill

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Contributions

K.R. and N.L. conceived the project; D.K., V.A.L. and T.C. performed all the experiments. V.A.L. and N.L. performed the computational studies, and MCD measurements. U.K., P.H., V.A.L. and N.L. performed rRaman measurements. T.L. and E.B. performed EPR. E.B. performed SQUID and magnetic Mössbauer studies. All authors contributed to data analysis. D.K., T.C., K.R., T.L., V.A.L., P.H. and N.L. wrote the manuscript.

Corresponding authors

Correspondence to Nicolai Lehnert or Kallol Ray.

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Nature Chemistry thanks Thomas Brunold and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Spectral changes associated with the formation of 2, 2a and 2b.

a, UV/Vis spectral changes associated with the formation of 2 (bold line) starting from 1 (dashed line) by addition of excess O2, NaBPh4 (1.3 eq) and HClO4 (0.5 eq) in CH3CN at 0°C. Inset: Time traces for the formation of 2 with different amounts of ferrocene (Fc) showing a sigmoidal feature with an increase of the induction time with increasing amount of Fc. b, Comparison of the UV/vis spectra of 2 (black), 2a (dotted) and 2b (dashed) Inset: time traces of the decay of the characteristic ~660 nm band associated with 2, 2a and 2b at 10 °C.

Source data

Extended Data Table 1 Theoretical Calculations
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1–36 and Tables 1–19.

Source data

Source Data Fig. 3

Excel file for the measured EPR and pdf file for the measured Mössbauer data and simulation.

Source Data Fig. 4

Excel file with the measured 16O and 18O rRaman data.

Source Data Fig. 5

Excel file with the MCD data and the simulations.

Source Data Fig. 6

All the cube files are included as a zip folder.

Source Data Extended Data Fig. 1

Excel file for all the measured data.

Source Data Extended Data Table 1

.xyz files of all structures are included in the zip folder.

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Kass, D., Larson, V.A., Corona, T. et al. Trapping of a phenoxyl radical at a non-haem high-spin iron(II) centre. Nat. Chem. 16, 658–665 (2024). https://doi.org/10.1038/s41557-023-01405-9

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