Porphyrin wheel sets record as largest aromatic ring

A huge molecular wheel has set a new record as the largest aromatic ring (Nat. Chem. 2020, DOI: 10.1038/s41557-019-0398-3). The molecule could be a step toward making even bigger rings that serve as a test-bed for unusual quantum effects.

Aromatic molecules like benzene contain π-electrons that smear into donut-shaped orbitals above and below the ring of carbon atoms. This delocalization produces aromaticity when the ring has 4n + 2 π-electrons, and helps stabilize the molecule.

One of the defining features of aromaticity is that a magnetic field can make those delocalized electrons circulate, creating a ring current that generates its own miniature magnetic field. Inside a benzene ring, this induced field points in the opposite direction to the external magnetic field; outside the ring, it points in the same direction. In an anti-aromatic molecule, which sports 4n π-electrons, the ring current flows in the opposite direction, and the induced magnetization is inverted.

The upshot is that researchers can use nuclear magnetic resonance (NMR) spectroscopy to assess the aromaticity of a molecule because the induced magnetization shifts the signals of certain nuclei in the molecule’s NMR spectrum.

Previously, the largest ring known to be aromatic contained 62 π-electrons (Chem. Eur. J. 2016, DOI: 10.1002/chem.201603121). Because larger molecules are more likely to twist in ways that shut down electron delocalization, Harry L. Anderson at the University of Oxford wanted to explore whether even bigger rings could be aromatic.

So his team made a series of molecular rings containing zinc porphyrins linked together by alkynes, and used template molecules to hold each ring in a neat circle, just like the hub and spokes of a bicycle wheel. Then they removed electrons from the molecule until it had the right number to become aromatic or anti-aromatic.

The largest of these structures was a nanoring with a circumference of 16 nm and containing 12 porphyrin units held in place by a pair of six-spoked templates. In its neutral form, each porphyrin could sustain its own isolated ring current. “But when you change the oxidation state, you can get a global ring current that runs around the entire nanoring,” Anderson says. In its +6 oxidation state, the molecule has 162 π-electrons and shows clear signs of aromaticity, based on the NMR signals of hydrogen and fluorine atoms on the template spokes.

“I’m surprised that at this size, it still works,” says Rainer Herges at Kiel University, who studies aromaticity. “One hundred and sixty-two electrons, that’s quite a lot.”

Anderson’s team also combined the 12-porphyrin ring with a pair of different templates, which made the ring twist into a figure-eight shape. The team thought that with this shape the magnetic field should induce a clockwise current in one ring, and an anticlockwise current in the other, canceling each other out. The NMR spectrum showed that there was indeed no global ring current in this molecule, suggesting that aromaticity can be neatly switched on or off by changing the geometry of the molecule.

The researchers are already trying to make even bigger aromatic rings that could host unusual quantum effects, caused by interference between the quantum wavefunctions of the delocalized electrons. Researchers are investigating micrometer-sized circles of superconducting materials as components in quantum computers. Herges thinks that giant aromatic wheels could offer one way to make similar current loops.

Anderson and his colleagues aren’t the only ones pushing back the boundaries of aromaticity. Jishan Wu at the National University of Singapore last week unveiled a cage-like compound that shows aromaticity is not confined to rings (Nat. Chem. 2020, DOI: 10.1038/s41557-019-0399-2). The molecule contains three chains of thiophene units strung between two bridgehead carbons, forming three connected molecular loops. Its +6 oxidation state is aromatic, and Wu’s team suggests this is because each of the three loops of thiophenes is itself aromatic, with 34 π-electrons in each individual loop.

Both molecules demonstrate that aromatic ring currents exist at a size scale that is far larger than a simple benzene ring, but smaller than a microscopic ring of metal, Herges adds: “It’s extending aromaticity into this mesoscopic scale, and I’m pretty sure we’ll find interesting effects and applications.”