One of only 13 Archimedean solids, the truncated octahedron (Image: Scott Camazine/Alamy)

There are only 13 “Archimedean solids” – a family of symmetrical, 3D polyhedra attributed to the Greek mathematician. Now chemists have made a molecule-scale version of one of these special structures, known as the truncated octahedron (see picture, right).

The tiny, hollow structure acts as a cage, capable of encapsulating a surprising variety of ions and molecules without falling apart. It also aids the creation of substances that won’t otherwise form.

Michael Ward of New York University, and colleagues, built their cage, which has eight hexagonal and six square faces, by blending two types of carefully-designed molecular “tiles”, one made of chemical groups known as guanidiniums, the other ringed by sulphonate groups. These assembled into the truncated octahedron by forming 72 hydrogen bonds.


Stable cage

The negatively charged cage has encapsulated negative ions as well as positive ones, and neutral molecules. Usually a charged entity would only trap oppositely charged ions, says Ward.

By adding reactants to the tile mixture while the cages were forming, the team also created three metal “complexes” – containing bismuth, lead and mercury – that had never been seen before, inside the cages. The cages can be made to dissolve under mild conditions, so could be used to build and then release such substances. Often the contents of such a cage alters its structure. “But this particular system always finds its way to this same framework,” says Ward.

“The result is fascinating,” says Achim Müller of the University of Bielefeld in Germany. He is impressed that the team managed to create the truncated octahedron as it can be tough to predict the shape of such a structure ahead of time.

Journal reference: Science, DOI: 10.1126/science.1204369