Oxford collaboration uncovers correct structure of [18]annulene anions

A collaboration between Oxford Chemistry and the University at Albany, State University of New York, USA, have demonstrated in a study published today in Nature Chemistry that the structure originally assigned to the di-anion of [18]annulene by Oth, Woo and Sondheimer in 1973 was incorrect. The di-anion has a completely different geometry to the neutral molecule.

The structures of [18]annulene, its di-anion and tetra-anion. (From Nature Chemistry article.)

The team also discovered that this anti-aromatic di-anion can be reduced further to the aromatic tetra-anion. The structures of these anions, and their (anti)aromaticity, are easily deduced from the 1H NMR spectra, at 500 MHz. However in 1973, Oth et al. only had access to 60 MHz NMR, which did not give sufficient resolution to determine the structure of the di-anion.

The lithium salt of the tetra-anion adopts a sandwich structure with five intercalated lithium cations, both in solution and in the solid state. Furthermore, it forms a mixed sandwich with corannulene. Both sandwich structures were characterised by single-crystal X-ray diffraction.

These findings are fundamentally significant for understanding aromaticity, and the discovery that the [18]annulene tetra-anion forms an intercalation complex with lithium cations suggests potential applications in energy storage.

You can read more about this study in Nature Chemistry.