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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.

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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.

References

  1. Gomberg, M. An instance of trivalent carbon: triphenylmethyl. J. Am. Chem. Soc. 22, 757–771 (1900).

    Article  Google Scholar 

  2. Grützmacher, H. & Marchand, C. M. Heteroatom stabilized carbenium ions. Coord. Chem. Rev. 163, 287–344 (1997).

    Article  Google Scholar 

  3. Bourissou, D., Guerret, O., Gabbaï, F. P. & Bertrand, G. Stable carbenes. Chem. Rev. 100, 39–91 (2000).

    Article  CAS  PubMed  Google Scholar 

  4. Bellotti, P., Koy, M., Hopkinson, M. N. & Glorius, F. Recent advances in the chemistry and applications of N-heterocyclic carbenes. Nat. Rev. Chem. 5, 711–725 (2021).

    Article  CAS  PubMed  Google Scholar 

  5. Loh, Y. K., Melaimi, M., Munz, D. & Bertrand, G. An air-stable “masked” bis(imino)carbene: a carbon-based dual ambiphile. J. Am. Chem. Soc. 145, 2064–2069 (2023).

    Article  CAS  PubMed  Google Scholar 

  6. Igau, A., Grützmacher, H., Baceiredo, A. & Bertrand, G. Analogous α,α′-bis-carbenoid triply bonded species: synthesis of a stable λ3-phosphinocarbene−λ3-phosphaacetylene. J. Am. Chem. Soc. 110, 6463–6466 (1988).

    Article  CAS  Google Scholar 

  7. Arduengo, A. J. III, Harlow, R. L. & Kline, M. A stable crystalline carbene. J. Am. Chem. Soc. 113, 361–363 (1991).

    Article  CAS  Google Scholar 

  8. Parker, V. D. & Bethell, D. Carbene cation radicals: the kinetics of their formation from diazoalkane cation radicals and their reactions. J. Am. Chem. Soc. 109, 5066–5072 (1987).

    Article  CAS  Google Scholar 

  9. Bethell, D. & Parker, V. D. In search of carbene ion radicals in solution: reaction pathways and reactivity of ion radicals of diazo compounds. Acc. Chem. Res. 21, 400–407 (1988).

    Article  CAS  Google Scholar 

  10. Bally, T., Matzinger, S. & Truttman, L. Diphenyl carbene cation: electronic and molecular structure. J. Am. Chem. Soc. 115, 7007–7008 (1993).

    Article  CAS  Google Scholar 

  11. Stoub, D. & Goodman, G. V. Diarylcarbene cation radicals: generation and chemical reactivity in solution. J. Am. Chem. Soc. 119, 11110–11111 (1997).

    Article  CAS  Google Scholar 

  12. Ramnial, T., McKenzie, I., Gorodetsky, B., Tsang, E. M. W. & Clyburne, J. A. C. Reactions of N-heterocyclic carbenes (NHCs) with one-electron oxidants: possible formation of a carbene cation radical. Chem. Commun. 1054–1055 (2004).

  13. Dong, Z. et al. SET processes in Lewis acid−base reactions: the tritylation of N-heterocyclic carbenes. Chem. Sci. 11, 7615–7618 (2020).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  14. Shaikh, A. C., Veleta, J. M., Moutet, J. & Gianetti, T. L. Trioxatriangulenium (TOTA+) as a robust carbon-based Lewis acid in frustrated Lewis pair chemistry. Chem. Sci. 12, 4841–4849 (2021).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  15. Maiti, A. et al. Disclosing cyclic(alkyl)(amino)carbenes as one-electron reductants: synthesis of acyclic(amino)(aryl)carbene-based Kekulé diradicaloids. Chem. Eur. J. 28, e202104567 (2022).

    Article  CAS  PubMed  Google Scholar 

  16. Song, H. & Lee, E. Revisiting the reaction of IPr with tritylium: an alternative mechanistic pathway. Chem. Eur. J. 29, e202203364 (2023).

  17. Zhang, Q., Lei, H., Zhou, C.-Y. & Wang, C. Construction of N-polyheterocycles by N-heterocyclic carbene catalysis via a regioselective intramolecular radical addition/cyclization cascade. Org. Lett. 24, 4615–4619 (2022).

    Article  CAS  PubMed  Google Scholar 

  18. Liu, L., Zhang, Q. & Wang, C. Redox-neutral generation of iminyl radicals by N-heterocyclic carbene catalysis: rapid access to phenanthridines from vinyl azides. Org. Lett. 24, 5913–5917 (2022).

    Article  CAS  PubMed  Google Scholar 

  19. Courtenay, S., Mutus, J. Y., Schurko, R. W. & Stephan, D. W. The extended borinium cation: [(tBu3PN)2B]+. Angew. Chem. Int. Ed. 41, 498–501 (2002).

    Article  CAS  Google Scholar 

  20. Shoji, Y., Tanaka, N., Mikami, K., Uchiyama, M. & Fukushima, T. A two-coordinate boron cation featuring C−B+−C bonding. Nat. Chem. 6, 498–503 (2014).

  21. Bamford, K. L., Qu, Z.-W. & Stephan, D. W. Activation of H2 and Et3SiH by the borinium cation [Mes2B]+: avenues to cations [MesB(μ-H)2(μ-Mes)BMes]+ and [H2B(μ-H)(μ- Mes)B(μ-Mes)(μ-H)BH2]+. J. Am. Chem. Soc. 141, 6180–6184 (2019).

  22. Franz, D. & Inoue, S. Cationic complexes of boron and aluminum: an early 21st century viewpoint. Chem. Eur. J. 25, 2898–2926 (2018).

    Article  PubMed  Google Scholar 

  23. Piers, W. E., Bourke, S. C. & Conroy, K. C. Borinium, borenium, and boronium ions: synthesis, reactivity, and applications. Angew. Chem. Int. Ed. 44, 5016–5036 (2005).

    Article  CAS  Google Scholar 

  24. Ochiai, T., Franz, D. & Inoue, S. Applications of N-heterocyclic imines in main group chemistry. Chem. Soc. Rev. 45, 6327–6344 (2016).

    Article  CAS  PubMed  Google Scholar 

  25. Goettel, J. T., Gao, H., Dotzauer, S. & Braunschweig, H. MeCAAC=N−: a cyclic (alkyl)(amino)carbene imino ligand. Chem. Eur. J. 26, 1136–1143 (2020).

    Article  CAS  PubMed  Google Scholar 

  26. Huynh, S. et al. Cyclic alkyl(amino)iminates (CAAIs) as strong 2σ,4π-electron donor ligands for the stabilisation of boranes and diboranes(4): a synthetic and computational study. Dalton Trans. 52, 3869–3876 (2023).

    Article  CAS  PubMed  Google Scholar 

  27. Pal, S., Manae, M. A., Khade, V. V. & Khan, S. Reactivity of N-heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, towards heavier halogens (Br2 and I2). J. Indian Chem. Soc. 95, 765–770 (2018).

    CAS  Google Scholar 

  28. Loh, Y. K., Fuentes, M. Á., Vasko, P. & Aldridge, S. Successive protonation of an N-heterocyclic imine derived carbonyl: superelectrophilic dication versus masked acylium ion. Angew. Chem. Int. Ed. 57, 16559–16563 (2018).

    Article  CAS  Google Scholar 

  29. Loh, Y. K. et al. An acid-free anionic oxoborane isoelectronic with carbonyl: facile access and transfer of a terminal B=O double bond. J. Am. Chem. Soc. 141, 8073–8077 (2019).

    Article  CAS  PubMed  Google Scholar 

  30. Januszewski, J. A. & Tykwinski, R. R. Synthesis and properties of long [n]cumulenes (n ≥ 5). Chem. Soc. Rev. 43, 3184–3203 (2014).

    Article  CAS  PubMed  Google Scholar 

  31. Pinter, P. & Munz, D. Controlling möbius-type helicity and the excited-state properties of cumulenes with carbenes. J. Phys. Chem. A 124, 10100–10110 (2020).

    Article  CAS  PubMed  Google Scholar 

  32. Couchman, S. A., Wilson, D. J. D. & Dutton, J. L. Is the perfluorinated trityl cation worth a revisit? A theoretical study on the Lewis acidities and stabilities of highly halogenated trityl derivatives. Eur. J. Org. Chem. 3902–3908 (2014).

  33. Jupp, A. R., Johnstone, T. C. & Stephan, D. W. The global electrophilicity index as a metric for Lewis acidity. Dalton Trans. 47, 7029–7035 (2018).

    Article  CAS  PubMed  Google Scholar 

  34. Zhou, J., Liu, L. L., Cao, L. L. & Stephan, D. W. A phosphorus Lewis super acid: η5-pentamethylcyclopentadienyl phosphorus dication. Chem 4, 2699–2708 (2018).

    Article  CAS  Google Scholar 

  35. Mehlmann, P., Witteler, T., Wilm, L. F. B. & Dielmann, F. Isolation, characterization and reactivity of three-coordinate phosphorus dications isoelectronic to alanes and silylium cations. Nat. Chem. 11, 1139–1143 (2019).

    Article  CAS  PubMed  Google Scholar 

  36. Olah, G. et al. Bridgehead adamantyl, diamantyl, and related cations and dications. J. Am. Chem. Soc. 107, 2764–2772 (1985).

    Article  CAS  Google Scholar 

  37. Kato, T. & Reed, C. A. Putting tert-butyl cation in a bottle. Angew. Chem. Int. Ed. 43, 2908–2911 (2004).

    Article  CAS  Google Scholar 

  38. Légaré, M.-A., Pranckevicius, C. & Braunschweig, H. Metallomimetic chemistry of boron. Chem. Rev. 119, 8231–8261 (2019).

    Article  PubMed  Google Scholar 

  39. Weetman, S. & Inoue, S. The road travelled: after main-group elements as transition metals. ChemCatChem 10, 4213–4228 (2018).

    Article  CAS  Google Scholar 

  40. Martin, D., Melaimi, M., Soleilhavoup, M. & Bertrand, G. Stable singlet carbenes as mimics for transition metal centers. Chem. Sci. 2, 389–399 (2011).

    Article  CAS  PubMed  Google Scholar 

  41. Lipshultz, J. M., Li, G. & Radosevich, A. T. Main group redox catalysis of organopnictogens: vertical periodic trends and emerging opportunities in group 15. J. Am. Chem. Soc. 143, 1699–1721 (2021).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  42. Olah, G. A. My search for carbocations and their role in chemistry (Nobel lecture). Angew. Chem. Int. Ed. 34, 1393–1405 (1995).

    Article  CAS  Google Scholar 

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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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