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Reply to: [{Th(C8H8)Cl2}3]2− is stable but not aromatic

Nature volume 603, pages E21–E22 (2022)Cite this article

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The Original Article was published on 23 March 2022

replying to B. J. R. Cuyacot & C. Foroutan-Nejad Nature https://doi.org/10.1038/s41586-021-04319-z (2022).

In their Comment1, Cuyacot & Foroutan-Nejad question our conclusion that the thorium (Th)–Th bond in our reported complex [{Th(C8H8)Cl2}3{K(THF)2}2] is aromatic2. Despite being a concept researched for around 200 years, there is no single, universally accepted definition of aromaticity3. As aromaticity is not a physical observable, it can be a controversial concept, yet one that holds instinctive meaning to chemists4. Many types of aromaticity have been described, each drawing on distinct criteria that produce divergent classificational frameworks. These frameworks to some extent constrain and favour the resulting labels that can become subjective. Indeed, as has been noted5, “Throughout recent decades, several dozen types of chemical aromaticity have been reported in the literature … Every now and then new quantitative criteria of chemical aromaticity are introduced … Different criteria, on which the quantifiers are based and their sometimes inconsistent predictions has led to aromaticity concept reputation being somewhat tarnished and often criticized, both in its conceptual and methodological layer.” Put another way, different methods will probe the phenomena of aromaticity in different ways and so can sometimes agree and other times give different interpretations, especially as there is no direct relationship between the ground-state electronic structure and the magnetic response, so there is no universal right or wrong when discussing aromaticity.

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Fig. 1: The highest occupied molecular orbital of [{Th(C8H8)Cl2}3K2].

References

  1. Cuyacot, B. J. R. & Foroutan-Nejad, C. [{Th(C8H8)Cl2}3]2− is stable but not aromatic. Nature https://doi.org/10.1038/s41586-021-04319-z (2022).

  2. Boronski, J. T. et al. A crystalline tri-thorium cluster with σ-aromatic metal–metal bonding. Nature 598, 72–75 (2021).

  3. Krämer, K. The search for a grand theory of aromaticity. Chemistry World 18, 20–25 (2021).

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  4. 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, 3842–3888 (2005).

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  5. Szczepanik, D. W. et al. The electron density of delocalised bonds (EDDB) applied for quantifying aromaticity. Phys. Chem. Chem. Phys. 19, 28970 (2017).

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  6. Báez-Grez, R., Ruiz, L., Pino-Rios, R. & Tiznado, W. Which NICS method is most consistent with ring current analysis? Assessment in simple monocycles. RSC Adv. 8, 13446 (2018).

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Authors and Affiliations

  1. Department of Chemistry and Centre for Radiochemistry Research, The University of Manchester, Manchester, UK

    Josef T. Boronski, John A. Seed, Adam W. Woodward, Ashley J. Wooles, Louise S. Natrajan, Nikolas Kaltsoyannis & Stephen T. Liddle

  2. Institute of Physical Chemistry, University of Stuttgart, Stuttgart, Germany

    David Hunger & Joris van Slageren

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  1. Josef T. Boronski
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S.T.L. and N.K. wrote the Reply with all other authors commenting on the draft to produce the final version. All authors agreed on the final version.

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Correspondence to Nikolas Kaltsoyannis or Stephen T. Liddle.

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Boronski, J.T., Seed, J.A., Hunger, D. et al. Reply to: [{Th(C8H8)Cl2}3]2− is stable but not aromatic. Nature 603, E21–E22 (2022). https://doi.org/10.1038/s41586-021-04320-6

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  1. Cina Foroutan-Nejad

    I am pleased to read your response to our MA article. However, I would like to point out a few points.
    - In general, I have no objection that we might consider this molecule structural, energetic(?), and electronic aromatic. However, NICS is not a separate class of aromaticity, different from ring current analysis. NICS is a local measure of chemical shielding that is inferior to current density analysis.
    - In the article you mentioned that in the outer edge of the molecule red ring current is diamagnetic that is consistent with NICS characterization, i.e. aromaticity. Unfortunately, you neglected green (stronger) current in the inner rim of the molecule that is paramagnetic, i.e., antiaromatic.
    - In aromatic species like benzene the stronger diatropic current forms on the outer edge of the molecule. I left benzene in the SI for comparison. I presumably should have left it in the manuscript to avoid this misunderstanding. See the figure below.
    - The only way to assess aromaticity is summing up total current passing through interatomic regions, or middle of bonds. However, chemical bonds are not straight lines between two nuclei. For instance, the QTAIM analysis performed by both of us suggests that in this case Th-Th bonds are curved inward. Nevertheless, the bond is not a single "line". The integrated current passing through the whole bonded region clearly suggests dominance of paramagnetic ring current. https://uploads.disquscdn.c...
    -
    https://uploads.disquscdn.c...

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