IT IS either the biggest or the smallest wheel of its type, depending on how you look at it. German chemists have made the largest ever wheel-shaped molecule, containing 154 molybdenum atoms. This huge cluster of metal atoms should behave more like a tiny chunk of metal than a conventional molecule, making it the ultimate in miniature wheels.
Achim M眉ller and his colleagues at the University of Bielefeld built their wheel from smaller molecules containing a molybdenum atom surrounded by six atoms of oxygen. The finished structure, which was built around a template made from similar subunits, contains more than 700 atoms in total and has a relative molecular mass of about 24 000 (Angewandte Chemie, vol 34, p 2122). This is many times bigger than other molecular wheels. The best-known example, dubbed the 鈥渇erric wheel鈥�, contains only 10 atoms of iron. 鈥淭his is peanuts compared with our molybdenum giant wheel,鈥� enthuses M眉ller.
M眉ller鈥檚 wheel belongs to a class of compounds called metal clusters, which hover on the cusp between the minuscule domain of molecules and the familiar bulk matter of our daily lives. By studying these compounds, chemists hope to understand how atomic-scale properties develop into the bulk properties of large chunks of matter, such as magnetism and electrical conductivity. They also predict that some metal clusters may have unusual electromagnetic properties, such as the ability to take up and store single electrons within their molecular structure.
Advertisement
M眉ller and his colleagues are still exploring their wheel鈥檚 properties, but theoretical models of the behaviour of metal clusters predict that it should have some of the characteristics of bulk matter.
The wheel is soluble in water, which is unusual for such an enormous molecule. M眉ller attributes this to the molecule鈥檚 shape, which his team has determined using X-ray crystallography. The wheel looks like a tubeless car tyre, with a U-shaped cross-section bent around into a ring, giving it a high surface area relative to its volume. M眉ller says that this brings the hydroxy (OH) groups attached to the metal atoms in contact with water molecules, allowing the wheel to dissolve.
Just as a tyre fits snugly around the hub of a wheel, M眉ller鈥檚 molecule should be able to grab onto and hold other molecules 鈥� either a single molecule of up to two nanometres across, or several smaller ones. As a result, M眉ller speculates, the ring could be used to catalyse reactions by bringing reactants together, or to separate molecules of a particular size from a mixture. It should be possible to produce customised wheels for specific jobs, he adds, as the wheel鈥檚 diameter can be changed by building it around a different template.
