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Substantial π-aromaticity in the anionic heavy-metal cluster [Th@Bi12]4−

Abstract

The concept of aromaticity was originally defined as a property of unsaturated, cyclic planar organic molecules like benzene, which gain stability by the inherent delocalization of 4n + 2 π-electrons over the ring atoms. Since then, π-aromaticity has been observed for a large variety of organic and inorganic non-metal compounds, yet, for molecules consisting only of metal atoms, it has remained restricted to systems with three to five atoms. Here, we present the straightforward synthesis of a metal 12-ring that exhibits 2π-aromaticity and has a ring current much stronger than that of benzene (6π) and equivalent to that of porphine (26π), despite these organic molecules having (much) larger numbers of π-electrons. Highly reducing reaction conditions allowed access to the heterometallic anion [Th@Bi12]4−, with interstitial Th4+ stabilizing a Bi128− moiety. Our results show that it is possible to design and generate substantial π-aromaticity in large metal rings, and we hope that such π-aromatic heavy-metal cycles will eventually find use in cluster-based reactions.

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Fig. 1: Survey of different classes of experimentally secured molecules exhibiting 4n + 2 π-aromaticity.
Fig. 2: Molecular structure of the cluster anion [Th@Bi12]4− in compound 2.
Fig. 3: Frontier orbital region of the molecular orbital schemes of anions based on 12-atomic polybismuthide rings.
Fig. 4: HOMO of the cluster anion [Th@Bi12]4− and localized molecular orbitals (LMOs) from a Boys localization procedure in top and side views.
Fig. 5: Calculated ring currents in [Th@Bi12]4−.

Data availability

All data generated or analysed during this study are included in this Article and its Supplementary Information files. The structures of compounds 14 were determined by single-crystal X-ray diffraction. The crystallographic data have been deposited with the Cambridge Crystallographic Data Centre under CCDC numbers 1983070 (1), 1983072 (2), 1983073 (3) and 1983071 (4).

The optimized structures of all studied compounds are part of the Supplementary Information (separate zip file ‘Supplementary-Computational-Data.zip’): the coordinates of the optimized structures shown in Fig. 1, as well as their NICS values and ring current strengths, are provided in the Supplementary File ‘Fig1-OptimizedStructures-GIMIC-NICS.txt’. All calculated coordinates of the optimized structures of 2A and the compounds mentioned explicitly in the main text or Supplementary Information are provided in a Supplementary File ‘OptimizedStructures.txt’. The files comprise all necessary data for reproducing the values. All non-default parameters for the computational studies are given in the Supplementary Information together with the corresponding references of the used methods. For the default parameters of TURBOMOLE, such as the convergence criteria for structure optimizations, please see the manual at https://www.turbomole.org (retrieved 29 August 2020).

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Acknowledgements

We thank the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for financial support within the framework of GRK 1782. We thank J.L. Vasco and M. Pyschik for help with the synthesis, S. Ivlev, B. Weinert, M. Marsch and R. Riedel for help with the diffraction experiments, and M. Hellwig for measuring the EDX spectra of 1. We also thank K. Reiter and F. Dehnen for discussions. N.L. acknowledges a grant from Marburg University Research Academy (MARA). Y.J.F. is grateful to Fonds der Chemischen Industrie for general support of his PhD. studies (Kekulé fellowship), to the German Academic Exchange Service (Deutscher Akademischer Austauschdienst, DAAD) for a fellowship (grant no. 57438025) and to F. Furche for hosting. R.C. acknowledges support from the University of Bordeaux, the CNRS, the Region Nouvelle Aquitaine, the MOLSPIN COST action CA15128 and the GdR MCM-2.

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Contributions

A.R.E., N.L., R.J.W. and H.L.D. conceived and performed the synthetic experiments, collected single-crystal X-ray crystallographic data, solved and refined the structures, performed ESI-MS and prepared samples for further analyses. R.C. performed and analysed the magnetic measurements. F.W. performed the computational structure optimization and orbital analysis, and Y.J.F. studied the aromaticity and performed the TD-DFT calculations as well as the structure optimizations for Fig. 1. S.D., F.K. and F.W. supervised the work. All authors co-wrote the paper.

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Correspondence to Florian Weigend or Stefanie Dehnen.

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Supplementary information

Supplementary Information

Supplementary discussion on the formation of compound 2, supplementary information on X-ray diffraction (including Supplementary Table 1 and Supplementary Figs. 1–5), supplementary information on micro-X-ray fluorescence spectroscopy (µ-XFS) (including Supplementary Table 2 and Supplementary Figs. 6–8), supplementary information on electrospray ionization (ESI) mass spectrometry (including Supplementary Figs. 9–11), supplementary information on magnetic measurements of compound 2 (including Supplementary Fig. 12), supplementary details on quantum chemical investigations (including Supplementary Figs. 13–16 and Supplementary Tables 3–12).

Supplementary Data

Crystallographic information file for compound 1.

Supplementary Data

Crystallographic information file for compound 2.

Supplementary Data

Crystallographic information file for compound 3.

Supplementary Data

Crystallographic information file for compound 4.

Supplementary Data

The zip archive comprises two ASCII files entitled ‘Fig01-OptimizedStructures-GIMIC-NICS.txt’ (providing the coordinates of the optimized structures shown in Fig. 1, as well as their NICs values and ring currents) and ‘OptimizedStructures.txt’ (providing all coordinates of the optimized structures of 2A and the compounds mentioned explicitly in the manuscript or the Supplementary Information.pdf file).

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Eulenstein, A.R., Franzke, Y.J., Lichtenberger, N. et al. Substantial π-aromaticity in the anionic heavy-metal cluster [Th@Bi12]4−. Nat. Chem. 13, 149–155 (2021). https://doi.org/10.1038/s41557-020-00592-z

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