Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Isolation of a triplet benzene dianion

Nature Chemistry volume 13, pages 1001–1005 (2021)Cite this article

Subjects

Abstract

Baird’s rule predicts that molecules with 4n π electrons should be aromatic in the triplet state, but the realization of simple ring systems with such an electronic ground state has been stymied by these molecules’ tendency to distort into structures bearing a large singlet–triplet gap. Here, we show that the elusive benzene diradical dianion can be stabilized through creation of a binucleating ligand that enforces a tightly constrained inverse sandwich structure and direct magnetic exchange coupling. Specifically, we report the compounds [K(18-crown-6)(THF)2]2[M2(BzN6-Mes)] (M = Y, Gd; BzN6-Mes = 1,3,5-tris[2′,6′-(N-mesityl)dimethanamino-4′-tert-butylphenyl]benzene), which feature a trigonal ligand that binds one trivalent metal ion on each face of a central benzene dianion. Antiferromagnetic exchange in the Gd3+ compound preferentially stabilizes the triplet state such that it becomes the molecular ground state. Single-crystal X-ray diffraction data and nucleus-independent chemical shift calculations support aromaticity, in agreement with Baird’s rule.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Synthesis and structure of [M2(BzN6-Mes)]n− complexes.
Fig. 2: Electron paramagnetic resonance spectroscopy data.
Fig. 3: Electronic structure analysis.
Fig. 4: Baird aromaticity in [Gd2(BzN6-Mes)]2−.

Similar content being viewed by others

Data availability

Crystallographic data for the structures in this Article have been deposited at the Cambridge Crystallographic Data Centre under deposition numbers CCDC 2012304 (1-Y), 2012306 (2-Y), 2012305 (1-Gd) and 2012307 (2-Gd). Copies of data can be obtained free of charge from www.ccdc.cam.ac.uk/structures. Additional synthetic methods, nuclear magnetic resonance spectra, UV-vis-NIR spectra, single crystal X-ray diffraction data, EPR spectra, magnetism data and computational details are available in the Supplementary Information and Extended Data. Source data for Supplementary Figs. 20 and 22–31 and input files for computations are also provided as Supplementary Data 5 and 6. Source data are provided with this paper.

References

  1. Anslyn, E. V. & Dougherty, D. A. Modern Physical Organic Chemistry (University Science Books, 2006).

  2. Gleiter, R. Aromaticity and Other Conjugation Effects (Wiley-VCH, 2012).

  3. Dewar, M. J. S. Aromaticity and pericyclic reactions. Angew. Chem. Int. Ed. 10, 761–776 (1971).

    Article  CAS  Google Scholar 

  4. Fallon, K. J. et al. Exploiting excited-state aromaticity to design highly stable singlet fission materials. J. Am. Chem. Soc. 141, 13867–13876 (2019).

    Article  CAS  PubMed  Google Scholar 

  5. Randić, M. Aromaticity of polycyclic conjugated hydrocarbons. Chem. Rev. 2003, 3449–3605 (2003).

    Article  CAS  Google Scholar 

  6. Hückel, E. Quantentheoretische Beiträge zum Benzolproblem. I. Die Elektronenkonfiguration des Benzols und verwandter Verbindungen. Z. Phys. 70, 204–286 (1931).

    Article  Google Scholar 

  7. Breslow, R., Brown, J. & Gajewski, J. J. Antiaromaticity of cyclopropenyl anions. J. Am. Chem. Soc. 89, 4383–4390 (1967).

    Article  CAS  Google Scholar 

  8. Heilbronner, E. Hückel molecular orbitals of Möbius-type conformations of annulenes. Tetrahedron 5, 1923–1928 (1964).

    Article  Google Scholar 

  9. Herges, R. Topology in chemistry: designing Möbius molecules. Chem. Rev. 106, 4820–4842 (2006).

    Article  CAS  PubMed  Google Scholar 

  10. Rappaport, S. M. & Rzepa, H. S. Intrinsically chiral aromaticity. Rules incorporating linking number, twist, and writhe for higher-twist Möbius annulenes. J. Am. Chem. Soc. 130, 7613–7619 (2008).

    Article  CAS  PubMed  Google Scholar 

  11. Baird, N. C. Quantum organic photochemistry. II. Resonance and aromaticity in the lowest 3ππ* state of cyclic hydrocarbons. J. Am. Chem. Soc. 94, 4941–4948 (1972).

    Article  CAS  Google Scholar 

  12. Ottosson, H. Exciting excited-state aromaticity. Nat. Chem. 4, 969–971 (2012).

    Article  CAS  PubMed  Google Scholar 

  13. Rosenberg, M., Dahlstrand, C., Kilså, K. & Ottosson, H. Excited state aromaticity and antiaromaticity: opportunities for photophysical and photochemical rationalizations. Chem. Rev. 114, 5379–5425 (2014).

    Article  CAS  PubMed  Google Scholar 

  14. Wan, P. & Krogh, E. Evidence for the generation of aromatic cationic systems in the excited state. Photochemical solvolysis of fluoren-9-ol. J. Chem. Soc. Chem. Commun. 1207–1208 (1985).

  15. Ueda, M. et al. Energetics of Baird aromaticity supported by inversion of photoexcited chiral [4n]annulene derivatives. Nat. Commun. 8, 346 (2017).

    Article  PubMed  PubMed Central  CAS  Google Scholar 

  16. Kim, J. et al. Two-electron transfer stabilized by excited-state aromatization. Nat. Commun. 10, 4983 (2019).

    Article  PubMed  PubMed Central  CAS  Google Scholar 

  17. Kostenko, A. et al. Spectroscopic observation of the triplet diradical state of cyclobutadiene. Angew. Chem. Int. Ed. 56, 10183–10187 (2017).

    Article  CAS  Google Scholar 

  18. Oh, J., Sung, Y. M., Hong, Y. & Kim, D. Spectroscopic diagnosis of excited-state aromaticity: capturing electronic structure and conformations upon aromaticity reversal. Acc. Chem. Res. 51, 1349–1358 (2018).

    Article  CAS  PubMed  Google Scholar 

  19. Hada, M. et al. Structural monitoring of the onset of excited-state aromaticity in a liquid crystal phase. J. Am. Chem. Soc. 139, 15792–15800 (2017).

    Article  CAS  PubMed  Google Scholar 

  20. Sung, Y. M. et al. Reversal of Hückel anti(aromaticity) in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird’s rule. Nat. Chem. 7, 418–422 (2015).

    Article  CAS  PubMed  Google Scholar 

  21. Fratev, F., Monev, V. & Janoschek, R. Ab initio study of cyclobutadiene in excited states: optimized geometries, electronic transitions and aromaticities. Tetrahedron 38, 2929–2932 (1982).

    Article  CAS  Google Scholar 

  22. Ni, Y. et al. 3D global aromaticity in a fully conjugated diradicaloid cage at difference oxidation states. Nat. Chem. 12, 242–248 (2020).

    Article  CAS  PubMed  Google Scholar 

  23. Cha, W.-Y. et al. Bicyclic Baird-type aromaticity. Nat. Chem. 9, 1243–1248 (2017).

    Article  CAS  PubMed  Google Scholar 

  24. Valiev, R. R., Fliegl, H. & Sundholm, D. Bicycloaromaticity and Baird-type bicycloaromaticity of dithienothiophene-bridged [34]octaphyrins. Phys. Chem. Chem. Phys. 20, 17705–17713 (2018).

    Article  CAS  PubMed  Google Scholar 

  25. Falceto, A., Casanova, D., Alemany, P. & Alvarez, S. Distortions of π-coordinated arenes with anionic character. Chem. Eur. J. 20, 14674–14689 (2014).

    Article  CAS  PubMed  Google Scholar 

  26. Breslow, R., Hill, R. & Wasserman, E. Pentachlorocyclopentadienyl cation, a ground-state triplet. J. Am. Chem. Soc. 86, 5349–5350 (1964).

    Article  CAS  Google Scholar 

  27. Wasserman, E., Hutton, R. S., Kuck, V. J. & Chandross, E. A. Dipositive ion of hexachlorobenzene, a ground-state triplet. J. Am. Chem. Soc. 96, 1965–1966 (1974).

    Article  CAS  Google Scholar 

  28. MacDonald, M. R., Bates, J. E., Ziller, J. W., Furche, F. & Evans, W. J. Completing the series of +2 ions for the lanthanide elements: synthesis of molecular complexes of Pr2+, Gd2+, Tb2+, and Lu2+. J. Am. Chem. Soc. 135, 9857–9868 (2013).

    Article  CAS  PubMed  Google Scholar 

  29. Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. The Cambridge Structural Database. Acta. Cryst. 72, 171–179 (2016).

    CAS  Google Scholar 

  30. Astashkin, A. V. & Schweiger, A. Electron-spin transient nutation: a new approach to simplify the interpretation of ESR spectra. Chem. Phys. Lett. 174, 595–602 (1990).

    Article  CAS  Google Scholar 

  31. Bleaney, B. & Bowers, K. D. Anomalous paramagnetism of copper acetate. Proc. Roy. Soc. A. 214, 451–465 (1952).

    CAS  Google Scholar 

  32. Ebata, K. et al. Planar hexasilylbenzene dianion with thermally accessible triplet state. J. Am. Chem. Soc. 120, 1335–1336 (1998).

    Article  CAS  Google Scholar 

  33. Sekiguchi, A., Ebata, K., Kabuto, C. & Sakurai, H. Bis[(dimethoxyethane)lithium(i)] 1,2,4,5-tetrakis(trimethylsilyl)benzenide. The first 6C-8π antiaromatic benzene dianion. J. Am. Chem. Soc. 113, 7081–7082 (1991).

    Article  CAS  Google Scholar 

  34. Demir, S., Jeon, I.-R., Long, J. R. & Harris, T. D. Radical-ligand containing single-molecule magnets. Coord. Chem. Rev. 289–290, 149–176 (2015).

    Article  CAS  Google Scholar 

  35. Diaconescu, P. L., Arnold, P. L., Baker, T. A., Mindiola, D. J. & Cummins, C. C. Arene-bridged diuranium complexes: inverted sandwiches supported by δ backbonding. J. Am. Chem. Soc. 122, 6108–6109 (2000).

    Article  CAS  Google Scholar 

  36. Pampaloni, G. Aromatic hydrocarbons as ligands. Recent advances in the synthesis, reactivity and the applications of bis(η6-arene) complexes. Coord. Chem. Rev. 254, 402–419 (2010).

    Article  CAS  Google Scholar 

  37. von Ragué Schleyer, P., Maerker, C., Dransfeld, A., Jiao, H. & van Eikema Hommes, N. J. R. Nucleus-independent chemical shifts: a simple and efficient aromaticity probe. J. Am. Chem. Soc. 118, 6317–6318 (1996).

    Article  Google Scholar 

  38. Stanger, A. Nucleus-independent chemical shifts (NICS): distance dependence and revised criteria for aromaticity and antiaromaticity. J. Org. Chem. 71, 883–893 (2006).

    Article  CAS  PubMed  Google Scholar 

  39. Stanger, A. Obtaining relative induced ring currents quantitatively from NICS. J. Org. Chem. 75, 2281–2288 (2010).

    Article  CAS  PubMed  Google Scholar 

  40. Gershoni-Poranne, R. & Stanger, A. The NICS-XY-scan: identification of local and global ring currents in multi-ring systems. Chem. Eur. J. 20, 5673–5688 (2014).

    Article  CAS  PubMed  Google Scholar 

  41. Stanger, A. Reexamination of NICSπ,zz: height dependence, off-center values, and integration. J. Phys. Chem. A 123, 3922–3927 (2019).

    Article  CAS  PubMed  Google Scholar 

  42. Kruszewski, J. & Krygowski, T. M. Definition of aromaticity basing on the harmonic oscillator model. Tetrahedron Lett. 13, 3839–3842 (1972).

    Article  Google Scholar 

  43. Klingele, M. H. & Kersting, B. Z. Efficient medium-scale synthesis of 2-bromo-5-tert-butylbenzene-1,3-dicarbaldehyde. Z. Naturforsch. 56b, 437–439 (2001).

    Article  Google Scholar 

  44. Lappert, M. F. & Pearce, R. Stable silylmethyl and neopentyl complexes of scandium(iii) and yttrium(iii). J. Chem. Soc. Chem. Commun. 126 (1973).

  45. Liu, Y., Niu, F., Lian, J., Zeng, P. & Niu, H. Synthesis and properties of starburst amorphous molecules: 1,3,5-tris(1,8-naphthalimide-4-yl)benzenes. Synth. Met. 160, 2055–2060 (2010).

    Article  CAS  Google Scholar 

  46. CrysAlisPro Software System v.1.171.39.7a (Rigaku Corporation, 2015).

  47. SAINT and APEX 2 Software for CCD Diffractometers (Bruker Analytical X-ray Systems Inc., 2014).

  48. Sheldrick, G. M. SADABS v.2.03 (Bruker Analytical X-ray Systems Inc., 2000).

  49. Sheldrick, G. M. SHELXT and SHELXL (University of Göttingen, 2015).

  50. Hassan, A. K. et al. Ultrawide band multifrequency high-field EMR technique: a methodology for increasing spectroscopic information. J. Magn. Reson. 142, 300–312 (2000).

    Article  CAS  PubMed  Google Scholar 

  51. Oyala, P. H. et al. Biophysical characterization of fluorotyrosine probes site-specifically incorporated into enzymes: E. coli ribonucleotide reductase as an example. J. Am. Chem. Soc. 138, 7951–7964 (2016).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  52. Chilton, N. F., Anderson, R. P., Turner, L. D., Soncini, A. & Murray, K. S. PHI: a powerful new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and f-block complexes. J. Comput. Chem. 34, 1164–1175 (2013).

    Article  CAS  PubMed  Google Scholar 

  53. Neese, F. The ORCA program system. WIREs Comput. Mol. Sci. 2, 73–78 (2012).

    Article  CAS  Google Scholar 

  54. Frisch, M. J. et. al. Gaussian 09 Revision A.02 (Gaussian Inc., 2009).

  55. Rahalkar, A. & Stanger, A. The Aroma Package, http://schulich.technion.ac.il/Amnon_Stanger.htm

Download references

Acknowledgements

This work was funded by NSF grant CHE-1800252 (C.A.G., J.R.L.), NSF grant DMR-1610226 (J.M., S.H.) and NIH grant 1R35GM126961 (D.A.M., R.D.B.). High-field EPR data were collected at the National High Magnetic Field Laboratory, which is supported by the NSF (DMR-1644779) and the State of Florida. NMR spectroscopy was collected at UC Berkeley’s NMR facility in the College of Chemistry, which is supported in part by NIH grant S10OD024998. V.V. acknowledges a postdoctoral fellowship from Fonds Wetenschappelijk Onderzoek Vlaanderen (FWO, Flemish Science Foundation) and a V435018N FWO travel grant to UC Berkeley and C.A.G. thanks the NSF Graduate Research Fellowship Program for support. In addition, we thank L. A. Berben for the use of her gloveboxes, R. G. Bergman for insightful discussions and A. Stanger for advice on the Aroma program and NICSπ,zz calculations. We also thank K. R. Meihaus for editorial assistance, N. S. Settineri for assistance with crystallography and A. B. Turkiewicz for keen observations.

Author information

Author notes

  1. Veacheslav Vieru

    Present address: Maastricht Science Programme, Faculty of Science and Engineering, Maastricht University, Maastricht, The Netherlands

Authors and Affiliations

  1. Department of Chemistry, University of California, Berkeley, CA, USA

    Colin A. Gould & Jeffrey R. Long

  2. National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, USA

    Jonathan Marbey & Stephen Hill

  3. Department of Physics, Florida State University, Tallahassee, FL, USA

    Jonathan Marbey & Stephen Hill

  4. Theory of Nanomaterials Group, Katholieke Universiteit Leuven, Leuven, Belgium

    Veacheslav Vieru & Liviu F. Chibotaru

  5. Department of Chemistry, University of California, Davis, CA, USA

    David A. Marchiori & R. David Britt

  6. Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA, USA

    Jeffrey R. Long

  7. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Jeffrey R. Long

Authors
  1. Colin A. Gould
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Jonathan Marbey
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Veacheslav Vieru
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. David A. Marchiori
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. R. David Britt
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Liviu F. Chibotaru
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. Stephen Hill
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. Jeffrey R. Long
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

Synthesis, crystallography and magnetic characterization were performed by C.A.G. High-frequency CW-EPR experiments were performed and analysed by J.M. and S.H. and D-Band EPR experiments were performed and analysed by D.A.M. and R.D.B. DFT calculations and computational analysis of aromaticity were performed by V.V. and L.F.C. The manuscript was written by C.A.G. and J.R.L. and edited by all authors.

Corresponding author

Correspondence to Jeffrey R. Long.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Chemistry thanks Dongho Kim, Amnon Stanger, Dage Sundholm and Jishan Wu for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 UV-Vis-NIR spectra of [M2(BzN6-Mes)]−.

Spectra of 1-Y (maroon) and 1-Gd (orange).

Source data

Extended Data Fig. 2 UV-Vis-NIR spectra of [M2(BzN6-Mes)]2−.

Spectra of 2-Y (blue) and 2-Gd (green).

Source data

Extended Data Fig. 3 Density functional theory calculations on 2-Y.

The two singly-occupied molecular orbitals (SOMOs) obtained for 2-Y by DFT calculations are localized on the central benzene ring of the BzN6-Mes ligand.

Extended Data Fig. 4

CW-EPR spectra of 1-Y (maroon) and 2-Y (blue).

Source data

Extended Data Fig. 5

CW-EPR spectra of 2-Y from 5 to 225 K at 371 GHz.

Source data

Extended Data Fig. 6

Double integrated absorption of the EPR spectrum of 2-Y from 5 to 225 K at 371 GHz.

Source data

Extended Data Fig. 7 Dc magnetic susceptibility measurement for 2-Gd under an applied dc magnetic field of 1000 Oe.

The black line represents a fit to the data using the listed parameters.

Source data

Extended Data Fig. 8 Dc magnetic susceptibility measurement for 2-Gd under an applied dc magnetic field of 5000 Oe.

The black line represents a fit to the data using the listed parameters.

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1–45, Tables 1–10 and Discussion

Supplementary Data 1

Single-crystal X-ray diffraction data for 1-Y.

Supplementary Data 2

Single-crystal X-ray diffraction data for 1-Gd.

Supplementary Data 3

Single-crystal X-ray diffraction data for 2-Y.

Supplementary Data 4

Single-crystal X-ray diffraction data for 2-Gd.

Supplementary Data 5

Source data for Supplementary Figs. 20 and 22–31.

Supplementary Data 6

Input files for computations.

Source data

Source Data Fig. 2

Electron-spin nutation data for 1-Y and 2-Y and temperature dependence of the EPR spin susceptibility for 2-Y.

Source Data Fig. 3

Magnetic susceptibility (d.c.) and magnetization data for 2-Gd and electronic state diagram for 2-Y and 2-Gd.

Source Data Fig. 4

NICSπ,zz calculations for 2-Y.

Source Data Extended Data Fig. 1

UV-vis spectra for 1-Y and 1-Gd.

Source Data Extended Data Fig. 2

UV-vis spectra for 2-Y and 2-Gd.

Source Data Extended Data Fig. 4

CW-EPR spectra of 1-Y and 2-Y.

Source Data Extended Data Fig. 5

CW-EPR spectrum of 2-Y from 5 to 225 K.

Source Data Extended Data Fig. 6

Double integrated absorption of the EPR spectrum of 2-Y from 5 to 225 K.

Source Data Extended Data Fig. 7

Magnetic susceptibility (d.c.) data for 2-Gd under an applied magnetic field of 1,000 Oe.

Source Data Extended Data Fig. 8

Magnetic susceptibility (d.c.) data for 2-Gd under an applied magnetic field of 5,000 Oe.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Gould, C.A., Marbey, J., Vieru, V. et al. Isolation of a triplet benzene dianion. Nat. Chem. 13, 1001–1005 (2021). https://doi.org/10.1038/s41557-021-00737-8

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41557-021-00737-8

This article is cited by

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing