Aromatic and antiaromatic ring currents in a molecular nanoring

Journal name:
Nature
Volume:
541,
Pages:
200–203
Date published:
DOI:
doi:10.1038/nature20798
Received
Accepted
Published online

Aromatic and antiaromatic molecules—which have delocalized circuits of [4n + 2] or [4n] electrons, respectively—exhibit ring currents around their perimeters1, 2, 3, 4. The direction of the ring current in an aromatic molecule is such as to generate a magnetic field that opposes the external field inside the ring (a ‘diatropic’ current), while the ring current in an antiaromatic molecule flows in the reverse direction (‘paratropic’)5. Similar persistent currents occur in metal or semiconductor rings, when the phase coherence of the electronic wavefunction is preserved around the ring6, 7. Persistent currents in non-molecular rings switch direction as a function of the magnetic flux passing through the ring, so that they can be changed from diatropic (‘aromatic’) to paratropic (‘antiaromatic’) simply by changing the external magnetic field. As in molecular systems, the direction of the persistent current also depends on the number of electrons8. The relationship between ring currents in molecular and non-molecular rings is poorly understood, partly because they are studied in different size regimes: the largest aromatic molecules have diameters of about one nanometre, whereas persistent currents are observed in microfabricated rings with diameters of 20–1,000 nanometres. Understanding the connection between aromaticity and quantum-coherence effects in mesoscopic rings provides a motivation for investigating ring currents in molecules of an intermediate size9. Here we show, using nuclear magnetic resonance spectroscopy and density functional theory, that a six-porphyrin nanoring template complex, with a diameter of 2.4 nanometres, is antiaromatic in its 4+ oxidation state (80 π electrons) and aromatic in its 6+ oxidation state (78 π electrons). The antiaromatic state has a huge paramagnetic susceptibility, despite having no unpaired electrons. This work demonstrates that a global ring current can be promoted in a macrocycle by adjusting its oxidation state to suppress the local ring currents of its components.The discovery of ring currents around a molecule with a circumference of 7.5 nanometres, at room temperature, shows that quantum coherence can persist in surprisingly large molecular frameworks.

At a glance

Figures

  1. Molecular structures of the butadiyne-linked porphyrin oligomers used in this study. l-PN, c-PN and c-P6·T6.
    Figure 1: Molecular structures of the butadiyne-linked porphyrin oligomers used in this study. l-PN, c-PN and c-P6·T6.

    Ar = (3,5-bis(trihexylsilyl))phenyl as shown for l-PN.

  2. Computational data supporting aromaticity and antiaromaticity.
    Figure 2: Computational data supporting aromaticity and antiaromaticity.

    ac, NICS(0)iso grids in the x–y plane of c-P6 (a), c-P64+ (b) and c-P66+ (c). The colour axis is truncated to compare the grids on the same scale; see Supplementary Fig. 8 for grids with individual scales. df, ACID plots for each oxidation state. The yellow iso-surface depicts the anisotropy of the induced current density (isovalue 0.1 a.u.). The neutral oxidation state (a, d) shows ring current effects local to each porphyrin subunit. In contrast, the 4+ and 6+ oxidation states (b, e and c, f) show global ring currents, manifest by sign-reversal of the NICS inside/outside the ring.

  3. Square-wave voltammetry of c-P6·T6.
    Figure 3: Square-wave voltammetry of c-P6·T6.

    The solvent was CH2Cl2 (0.1 M Bu4NPF6). The arrows show the first reduction potential of each oxidant20: ferrocene, diacetylferrocenium, tris(4-bromophenyl)aminium hexafluoroantimonate (BAHAF), thianthrenium hexafluoroantimonate (Thn) and tris(2,4-dibromophenyl)aminium hexafluoroantimonate (DIBAHAF). There are six oxidations in a first manifold, generating oxidation states up to the hexacation (6+). A second manifold contains only a single oxidation wave generating the dodecacation (12+).

  4. NMR spectra of neutral and oxidised c-P6·T6.
    Figure 4: NMR spectra of neutral and oxidised c-P6·T6.

    ad, 1H NMR (500 MHz, CD2Cl2) of neutral c-P6·T6 at 298 K (a); c-P6·T64+ generated by titration with DIBAHAF at 223 K (b); c-P6·T66+ generated by titration with AgSbF6/I2 at 223 K (c); and c-P6·T612+ generated by oxidation with excess DIBAHAF at 223 K (d). The inset shows the molecular structure of the repeat unit of the sixfold symmetric c-P6·T6. The peaks labelled # and * arise from CHDCl2 and neutral oxidant (tris(2,4-dibromophenyl)amine), respectively. Unlabelled resonances are not assigned. is an unidentified impurity. In the neutral state (a), the template resonances (α–δ) probe the local aromaticity of each porphyrin. This aromaticity is reversed in the dodecacation (d), where the template protons report local antiaromaticity. The global aromaticity and antiaromaticity of the tetracation (b) and hexacation (c) are revealed by the large chemical shift difference between similar protons inside and outside the ring. The full spectra, without truncated peaks, are shown in Supplementary Fig. 27.

References

  1. Spitler, E. L., Johnson, C. A., II & Haley, M. M. Renaissance of annulene chemistry. Chem. Rev. 106, 53445386 (2006)
  2. Krygowski, T. M., Cyrañski, M. K., Czarnocki, Z., Häfelinger, G. & Katritzky, A. R. Aromaticity: a theoretical concept of immense practical importance. Tetrahedron 56, 17831796 (2000)
  3. Gleiter, R. & Haberhauer, G. Aromaticity and Other Conjugation Effects (Wiley-VCH, 2012)
  4. Lazzeretti, P. Ring currents. Prog. NMR Spectrosc. 36, 188 (2000)
  5. Gomes, J. A. N. F. & Mallion, R. B. Aromaticity and ring currents. Chem. Rev. 101, 13491384 (2001)
  6. Bleszynski-Jayich, A. C. et al. Persistent currents in normal metal rings. Science 326, 272275 (2009)
  7. Lorke, A. et al. Spectroscopy of nanoscopic semiconductor rings. Phys. Rev. Lett. 84, 22232226 (2000)
  8. Loss, D. & Goldbart, P. Period and amplitude halving in mesoscopic rings with spin. Phys. Rev. B 43, 1376213765 (1991)
  9. Mayor, M. & Didschies, C. A giant conjugated molecular ring. Angew. Chem. Int. Ed. 42, 31763179 (2003)
  10. Wannere, C. S. & von Ragué Schleyer, P. How aromatic are large (4n + 2) π annulenes? Org. Lett. 5, 865868 (2003)
  11. Choi, C. H. & Kertesz, M. Bond length alternation and aromaticity in large annulenes. J. Chem. Phys. 108, 66816688 (1998)
  12. Soncini, A., Fowler, P. W. & Jenneskens, L. W. Ring currents in large [4n + 2]-annulenes. Phys. Chem. Chem. Phys. 6, 277284 (2004)
  13. Soya, T., Kim, W., Kim, D. & Osuka, A. Stable [48]-, [50]-, and [52]dodecaphyrins(1.1.0.1.1.0.1.1.0.1.1.0): the largest Hückel aromatic molecules. Chem. Eur. J. 21, 83418346 (2015)
  14. Toriumi, N., Muranaka, A., Kayahara, E., Yamago, S. & Uchiyama, M. In-plane aromaticity in cycloparaphenylene dications: a magnetic circular dichroism and theoretical study. J. Am. Chem. Soc. 137, 8285 (2015)
  15. Kondratuk, D. V. et al. Supramolecular nesting of cyclic polymers. Nat. Chem. 7, 317322 (2015)
  16. Liu, P. et al. Synthesis of five-porphyrin nanorings by using ferrocene and corannulene templates. Angew. Chem. Int. Ed. 55, 83588362 (2016)
  17. Sprafke, J. K. et al. Belt-shaped π-systems: relating geometry to electronic structure in a six-porphyrin nanoring. J. Am. Chem. Soc. 133, 1726217273 (2011)
  18. Chen, Z., Wannere, C. S., Corminboeuf, C., Puchta, R. & von Ragué Schleyer, P. Nucleus-independent chemical shifts (NICS) as an aromaticity criterion. Chem. Rev. 105, 38423888 (2005)
  19. Geuenich, D., Hess, K., Kohler, F. & Herges, R. Anisotropy of the induced current density (ACID), a general method to quantify and visualize electronic delocalization. Chem. Rev. 105, 37583772 (2005)
  20. Connelly, N. G. & Geiger, W. E. Chemical redox agents for organometallic chemistry. Chem. Rev. 96, 877910 (1996)
  21. Karunanithy, G. et al. Harnessing NMR relaxation interference effects to characterise supramolecular assemblies. Chem. Commun. (Camb.) 52, 74507453 (2016)
  22. Evans, D. F. 400. The determination of the paramagnetic susceptibility of substances in solution by nuclear magnetic resonance. J. Chem. Soc. 20032005 (1959)
  23. Tellgren, E. I., Helgaker, T. & Soncini, A. Non-perturbative magnetic phenomena in closed-shell paramagnetic molecules. Phys. Chem. Chem. Phys. 11, 54895498 (2009)
  24. Dauben, H. J. Jr, Wilson, J. D. & Laity, J. L. Diamagnetic susceptibility exaltation in hydrocarbons. J. Am. Chem. Soc. 91, 19911998 (1969)
  25. Tamura, R., Ikuta, M., Hirahara, T. & Tsukada, M. Positive magnetic susceptibility in polygonal nanotube tori. Phys. Rev. B 71, 045418 (2005)
  26. Yamamoto, Y. et al. Synthesis, reactions, and electronic properties of 16 π-electron octaisobutyltetraphenylporphyrin. J. Am. Chem. Soc. 132, 1262712638 (2010)
  27. Gouterman, M. Spectra of porphyrins. J. Mol. Spectrosc. 6, 138163 (1961)
  28. Lin, V. S.-Y. & Therien, M. J. The role of porphyrin-to-porphyrin linkage topology in the extensive modulation of the absorptive and emissive properties of a series of ethynyl- and butadiynyl- bridged bis- and tris(porphinato)zinc chromophores. Chem. Eur. J. 1, 645651 (1995)
  29. Peeks, M. D., Neuhaus, P. & Anderson, H. L. Experimental and computational evaluation of the barrier to torsional rotation in a butadiyne-linked porphyrin dimer. Phys. Chem. Chem. Phys. 18, 52645274 (2016)
  30. Perrin, C. L. & Dwyer, T. J. Application of two-dimensional NMR to kinetics of chemical exchange. Chem. Rev. 90, 935967 (1990)
  31. Tait, C. E., Neuhaus, P., Peeks, M. D., Anderson, H. L. & Timmel, C. R. Transient EPR reveals triplet state delocalization in a series of cyclic and linear π-conjugated porphyrin oligomers. J. Am. Chem. Soc. 137, 82848293 (2015)
  32. Frisch, M. J. et al. Gaussian 09 Revision D.01 (Gaussian Inc. 2009)
  33. Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 56485652 (1993)
  34. Hehre, W. J., Ditchfield, R. & Pople, J. A. Self-consistent molecular orbital methods. XII. Further extensions of Gaussian-type basis sets for use in molecular orbital studies of organic molecules. J. Chem. Phys. 56, 22572261 (1972)
  35. Rassolov, V. A., Pople, J. A., Ratner, M. A. & Windus, T. L. 6-31G* basis set for atoms K through Zn. J. Chem. Phys. 109, 12231229 (1998)
  36. Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parameterization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 132, 154104 (2010)
  37. Zhao, Y. & Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 120, 215241 (2008)
  38. Chai, J.-D. & Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections. Phys. Chem. Chem. Phys. 10, 66156620 (2008)
  39. Jeener, J., Meier, B. H., Bachmann, P. & Ernst, R. R. Investigation of exchange processes by two-dimensional NMR spectroscopy. J. Chem. Phys. 71, 45464549 (1979)
  40. Green, M. L. H., Wong, L. L. & Sella, A. Relationship between intramolecular chemical exchange and NMR-observed rate constants. Organometallics 11, 26602668 (1992)
  41. Liu, S. et al. Caterpillar track complexes in template-directed synthesis and correlated molecular motion. Angew. Chem. Int. Ed. 54, 53555359 (2015)
  42. Schubert, E. M. Utilizing the Evans method with a superconducting NMR spectrometer in the undergraduate laboratory. J. Chem. Educ. 69, 62 (1992)
  43. Grant, D. H. Paramagnetic susceptibility by NMR. J. Chem. Educ. 72, 3940 (1995)
  44. Piguet, C. Paramagnetic susceptibility by NMR: the “solvent correction” removed for large paramagnetic molecules. J. Chem. Educ. 74, 815816 (1997)
  45. Peeks, M. D., Claridge, T. D. W. & Anderson, H. L. Data for ‘aromatic and antiaromatic ring currents in a molecular nanoring’. http://dx.doi.org/10.5287/bodleian:JVB5KZaD0 (Oxford University Research Archive, 2016)

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Affiliations

  1. University of Oxford, Department of Chemistry, Chemistry Research Laboratory, Oxford OX1 3TA, UK

    • Martin D. Peeks,
    • Timothy D. W. Claridge &
    • Harry L. Anderson

Contributions

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

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The authors declare no competing financial interests.

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Nature thanks M. Bröring and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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    This file contains Supplementary Tables 1-5 and Supplementary Figures 1-27. This file was updated on 11 January 2017 to correct the DOI number.

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