Over the past few decades, several molecular cages, hosts and nanoporous materials enclosing nanometre-sized cavities have been reported1,2,3,4,5, including coordination-driven nanocages6. Such nanocages have found widespread use in molecular recognition, separation, stabilization and the promotion of unusual chemical reactions, among other applications3,4,5,6,7,8,9,10. Most of the reported nanospaces within molecular hosts are confined by aromatic walls, the properties of which help to determine the host–guest behaviour. However, cages with nanospaces surrounded by antiaromatic walls have not yet been developed, owing to the instability of antiaromatic compounds; as such, the effect of antiaromatic walls on the properties of nanospaces remains unknown. Here we demonstrate the construction of an antiaromatic-walled nanospace within a self-assembled cage composed of four metal ions with six identical antiaromatic walls. Calculations indicate that the magnetic effects of the antiaromatic moieties surrounding this nanospace reinforce each other. This prediction is confirmed by 1H nuclear magnetic resonance (NMR) signals of bound guest molecules, which are observed at chemical shift values of up to 24 parts per million (ppm), owing to the combined antiaromatic deshielding effect of the surrounding rings. This value, shifted 15 ppm from that of the free guest, is the largest 1H NMR chemical shift displacement resulting from an antiaromatic environment observed so far. This cage may thus be considered as a type of NMR shift reagent, moving guest signals well beyond the usual NMR frequency range and opening the way to further probing the effects of an antiaromatic environment on a nanospace.
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All data needed to evaluate the conclusions given in the paper are present in the paper and Supplementary Information. Any additional data related to this paper may be requested from the authors. Crystallographic data for the structure reported in this paper has been deposited at the Cambridge Crystallographic Data Centre (deposition number 1893553) and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.
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This study was supported by the European Research Council (695009) and the UK Engineering and Physical Sciences Research Council (EPSRC, EP/P027067/1). M.Y. acknowledges the Japan Society for the Promotion of Science (JSPS) for an Overseas Research Fellowship. R.L. was funded by a Fondation Wiener-Anspach postdoctoral fellowship. M.P. acknowledges support from the Danish Council for Independent Research (DFF 4181-00206). We thank the NMR service of the University of Cambridge Chemistry Department for NMR experiments. We thank Diamond Light Source for providing time on Beamline I19 (MT15768), and D. Allan and L. Saunders for assistance. The computations were performed at the Research Center for Computational Science, Okazaki, Japan. We appreciate discussions with H. Shinokubo (Nagoya University), M. Yoshizawa (Tokyo Institute of Technology) and T. Soya (Kyoto University).
The authors declare no competing interests.
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Extended data figures and tables
When 1 was treated with 1,3-dibromo-5,5-dimethylhydantoin, inseparable mono-, di- and tribrominated products were obtained. As bromination randomly occurred at positions 3, 7, 12 and 16 (that is, adjacent to the mesityl moieties), the 3,12- and 3,16-substituted dibromonorcorroles trans-1a and cis-1a were obtained as the main products (Supplementary Figs. 1, 2). Subsequently, disubstituted trans-1b and cis-1b were obtained by Suzuki–Miyaura cross-coupling at 43% yield as a mixture of two regioisomers (Supplementary Figs. 3, 4). Finally, 3,12-substituted subcomponent 2 could be isolated as a single regioisomer at 46% yield following reduction of the NO2 groups and chromatography (Supplementary Figs. 5–9). Reagents and conditions: (i) 1,3-dibromo-5,5-dimethylhydantoin, CH2Cl2, −78 °C, 1 h, 94% (mixture of trans-1a, cis-1a and other brominated species). (ii) 4-Nitrophenylboronic acid pinacol ester, Pd(PPh3)4, K3PO4, dry THF, 70 °C, 1 h, 43% (mixture of trans-1b and cis-1b). (iii) SnCl2·H2O, dry THF, 70 °C, overnight, 46% (isolated 2).
This file contains detailed additional synthetic procedures, host–guest studies, extended methods, full characterisation data, Supplementary Figures 1–74, Supplementary Tables 1–7 and additional references – see content page for details.
This file contains the crystallographic data for compound 3.
Three-dimensional NICSiso grid within the antiaromatic-walled nanospace. Yellow, orange and red dots represent medium to strong magnetic deshielding regions (NICSiso = 3–6 for yellow, 6–9 for orange and >9 for red). Regions outside the walls and for NICSiso<3 are not shown for clarity. Calculation details are provided in the Supplementary Information.
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Yamashina, M., Tanaka, Y., Lavendomme, R. et al. An antiaromatic-walled nanospace. Nature 574, 511–515 (2019) doi:10.1038/s41586-019-1661-x