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Isolation of a triplet benzene dianion

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.

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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−.

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.

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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

Authors and Affiliations

Authors

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.

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The authors declare no competing interests.

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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.

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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.

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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

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