What Jonathan Nitschke of the University of Cambridge and colleagues made shouldn’t be stable. But it is. After two years of work by postdoc Masahiro Yamashina, now at the Tokyo Institute of Technology, the team reports a nanosized cage with antiaromatic walls that boasts some peculiar magnetic properties (Nature 2019, DOI: 10.1038/s41586-019-1661-x).
To understand why this nanocage’s stability is surprising, you have to remember Hückel’s rule. According to the rule, a molecule is aromatic if it has 4n + 2 π electrons in a system of rings containing conjugated double bonds. That aromaticity increases the stability of the compound. But when a cyclic conjugated compound has 4n π electrons, it is antiaromatic. These molecules are typically unstable and reactive and the rings have a paramagnetic ring current that can be seen with nuclear magnetic resonance spectroscopy.
To build the nanocage, Yamashina used some relatively stable antiaromatic Ni(II) norcorrole building blocks and then added substituents and iron ions until the conditions were right for the molecule to self-assemble into a tetrahedral shape that can hold guest molecules inside.
“To my best knowledge, this is the first molecular architecture where an antiaromatic unit has been built into a cage,” says Norbert Jux, who works on metalloporphyrins at Friedrich Alexander University Erlangen-Nürnberg. “It is also very intriguing that the host can encapsulate more than one guest, and also different guests at the same time.”
Joost Reek at the University of Amsterdam had seen the molecule presented at a lecture before publication and thinks the synthesis is impressive. One feature that he finds interesting is the effect of the ring currents on any guest molecules inside the cage. These effects cause NMR shifts that are “hugely different” from those caused by aromatic cages. “Ring current effects usually result in upfield shifts,” Reek explains. But “for these novel cages, a huge downfield shift is observed.”
That shift is caused by what Nitschke describes as a “magnetically weird environment” inside the cage. He says future studies of this cage and similar ones will further explore what properties the weird environment might induce and how it might react with different guests.
For Xiaopeng Li at the University of South Florida, the significance of the new work is how it demonstrates parts of antiaromatic theory that chemists hadn’t seen before, like the magnetic effects of the ring currents in 3-D. “This is curiosity-driven research,” he says, “and it will prompt more curiosity.” For example, he wonders what would happen if an antiaromatic guest were put in the new cage.