Angewandte Chemie International Edition
Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2000, 39 (18), 3273 - 3276
Deflated Soccer Ball
containing a benzene structural element
In the early nineties an unusual molecule, whose structural determination resulted in a Nobel Prize in 1996, caused a furore: buckminsterfullerene, a molecule made exclusively of carbon. Its 60 C atoms are arranged exactly like the corners of the leather panels of a soccer ball, leaving a cavity on the inside. Since then, a whole range of these round carbon cages has been discovered.
Two Russian groups and British chemists working with Roger Taylor have now structurally characterized an unusual fluorine-containing fullerene, discovered in 1996 at Moscow University. What is so unusual about this molecule? First let us return to our classic soccer ball molecule: as described above, its 60 C atoms describe a system of five- and six-membered rings. The six-membered rings each contain three double bonds, which at first glance make them resemble benzene rings. Is this a super-aromatic molecule? On the contrary, the aromaticity of fullerenes is barely existent. In order for the electron pairs of the double bonds to interact with each other and spread out as an electron cloud - a characteristic feature of aromatic molecules - the ring system must be planar. This requirement is not met by fullerenes. The cage structure would undergo too much strain if regions of it were flattened.
The new fluorofullerene is different: C60F18 is the first fullerene that contains a "real" aromatic benzene ring. By using X-ray structure analysis, the researchers found out that the molecule looks like a soccer ball with the air let out. On one side it is spherical, and on the other it's flat. In the middle of the flat side rests the "benzene ring". On the flattened side, the 18 attached fluorine atoms arrange themselves like a crown around this special six-membered ring. But why is the ball flat? The binding of the fluorine causes a number of the double bonds to be broken. This results in a belt of C atoms that are coupled to their neighbors by only single bonds. This is substantially more flexible than the hemispherical side of the "ball" and allows the strain-free formation of a completely planar benzene structure.
"The unusual electronic properties of the fluorinated fullerene cage make this molecule a particularly interesting candidate for future applications in photonics and photovoltaics" foresees Taylor.