Global aromaticity at the nanoscale

Abstract

Aromaticity can be defined by the ability of a molecule to sustain a ring current when placed in a magnetic field. Hückel’s rule states that molecular rings with [4n + 2] π-electrons are aromatic, with an induced magnetization that opposes the external field inside the ring, whereas those with 4n π-electrons are antiaromatic, with the opposite magnetization. This rule reliably predicts the behaviour of small molecules, typically with fewer than 22 π-electrons (n = 5). It is not clear whether aromaticity has a size limit, or whether Hückel’s rule extends to much larger macrocycles. Here, we present evidence for global aromaticity in porphyrin nanorings with circuits of up to 162 π-electrons (n = 40); aromaticity is controlled by changing the constitution, oxidation state and conformation. Whenever a ring current is observed, its direction is correctly predicted by Hückel’s rule. The largest ring currents occur when the porphyrin units have fractional oxidation states.

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Fig. 1: 1H NMR spectra of the aromatic and antiaromatic six-porphyrin nanoring template complexes in oxidation states 2+, 4+ and 6+.
Fig. 2: Magnetic shielding plotted in the xz plane perpendicular to the plane of the nanoring.
Fig. 3: Hückel behaviour in a template-bound eight-porphyrin ring.
Fig. 4: Ring currents in topologically distinct 12-porphyrin ring complexes.
Fig. 5: Summary of the shielding and deshielding of the trihexylsilyl groups across eight different nanorings.

Data availability

All relevant data, including raw computational data from the NICS calculations as well as XYZ coordinates of calculated molecular geometries, are available within the paper and its Supplementary Information files. The NMR data are presented in detail in the main Supplementary Information file and are available upon reasonable request from the authors.

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Acknowledgements

We thank the EPSRC (grants EP/N017188/1, EP/R029229/1 and EP/M016110/1), the ERC (grant 320969), the European Union’s Horizon 2020 research and innovation programme (Marie Sklodowska-Curie grant SYNCHRONICS 643238) and the Swiss National Science Foundation (P300P2_174294) for funding, the National Mass Spectrometry Facility at Swansea University for MALDI mass spectra, the University of Oxford Advanced Research Computing Service (https://doi.org/10.5281/zenodo.22558) and the Australian-government-supported National Computational Infrastructure (NCI) for the provision of high-performance computing. M.J. thanks Oxford University for a Scatcherd European Scholarship. H.G. thanks the Carlsberg Foundation for a Carlsberg Foundation Internationalisation Fellowship.

Author information

M.R., M.J., L.T., H.G., M.D.P., R.H. and H.-W.J. synthesized the compounds; M.R. and M.J. collected and analysed the NMR spectroscopic data; M.J. and M.D.P. performed the DFT calculations; T.D.W.C. assisted with NMR data collection and interpretation; H.L.A., M.R. and M.J. devised the project and wrote the paper; all authors discussed the results and edited the manuscript.

Correspondence to Harry L. Anderson.

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Supplementary information

Supplementary information

Details of experimental procedures, computational methods, synthetic schemes, NMR spectra, electrochemical data and results of DFT calculations testing a range of functionals.

XYZ coordinates

34 XYZ data files. A full set of calculated Cartesian coordinates for each nanoring in a range of oxidation states.

NICS values

42 data files in comma-separated-values (csv) format. The NICSs are organized into five columns, indicating x, y, z coordinates and the corresponding isotropic and zz NICS values (presented as the corresponding chemical shielding values from DFT NMR calculations multiplied by −1). The x, y, z coordinates use the same origin and orientation as the geometries provided in *.xyz files. The NICSs were calculated on structures that were optimized in the presence of templates, but from which the templates were removed before the NICS calculations. The DFT method used for the NICS calculations was LC-ωhPBE (ω = 0.1)/6-31G*. The names of the files indicate the species. For example, NICS for the 8-porphyrin nanoring with all-butadiyne linkers in the 4+ oxidation state, that is, c-P8[b8]4+, are stored in the file named ‘c-P8[B8]_4.csv’.

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Rickhaus, M., Jirasek, M., Tejerina, L. et al. Global aromaticity at the nanoscale. Nat. Chem. (2020). https://doi.org/10.1038/s41557-019-0398-3

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