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A crystalline doubly oxidized carbene

Nature volume 623, pages 66–70 (2023)Cite this article

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Abstract

The chemistry of carbon is governed by the octet rule, which refers to its tendency to have eight electrons in its valence shell. However, a few exceptions do exist, for example, the trityl radical (Ph3C∙) (ref. 1) and carbocation (Ph3C+) (ref. 2) with seven and six valence electrons, respectively, and carbenes (R2C:)—two-coordinate octet-defying species with formally six valence electrons3. Carbenes are now powerful tools in chemistry, and have even found applications in material and medicinal sciences4. Can we undress the carbene further by removing its non-bonding electrons? Here we describe the synthesis of a crystalline doubly oxidized carbene (R2C2+), through a two-electron oxidation/oxide-ion abstraction sequence from an electron-rich carbene5. Despite a cumulenic structure and strong delocalization of the positive charges, the dicoordinate carbon centre maintains significant electrophilicity, and possesses two accessible vacant orbitals. A two-electron reduction/deprotonation sequence regenerates the parent carbene, fully consistent with its description as a doubly oxidized carbene. This work demonstrates that the use of bulky strong electron-donor substituents can simultaneously impart electronic stabilization and steric protection to both vacant orbitals on the central carbon atom, paving the way for the isolation of a variety of doubly oxidized carbenes.

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Fig. 1: Previous attempts at carbene oxidation.
Fig. 2: Synthesis and characterization of doubly oxidized carbene 12+.
Fig. 3: Doubly oxidized carbene 12+[TfO−]2.
Fig. 4: Reactivity of doubly oxidized carbene 12+ and a reversible 1/12+ redox system.

Data availability

Crystallographic data for this paper (CCDC 2245611–2245613, 2267007, 2267374) are available free of charge via the Cambridge Crystallographic Data Centre. Non-crystallographic data are provided in the Supplementary information file.

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Acknowledgements

This work was supported by the NSF (grant no. CHE-2246948). We thank A*STAR for a postdoctoral fellowship for Y.K.L. We acknowledge the scientific support and HPC resources provided by the Erlangen National High Performance Computing Center (NHR@FAU) of the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU). The hardware is funded by the German Research Foundation (DFG). We thank F. F. Mulks for helpful discussions.

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Authors and Affiliations

  1. UCSD–CNRS Joint Research Laboratory (IRL 3555), Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA, USA

    Ying Kai Loh, Mohand Melaimi, Milan Gembicky & Guy Bertrand

  2. Coordination Chemistry, Saarland University, Saarbrücken, Germany

    Dominik Munz

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Contributions

Y.K.L. conceived and performed the synthetic experiments. M.M. and M.G. performed the X-ray crystallographic analyses. D.M. performed the computational work. G.B. supervised the project. The manuscript was written by Y.K.L., M.M. and G.B.

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Correspondence to Ying Kai Loh or Guy Bertrand.

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Supplementary Figs. 1–30, Tables 1–10, Methods and references.

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Loh, Y.K., Melaimi, M., Gembicky, M. et al. A crystalline doubly oxidized carbene. Nature 623, 66–70 (2023). https://doi.org/10.1038/s41586-023-06539-x

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