The ingenuity of chemists has produced molecules that mimic a range of mechanical devices, from rotors and shuttles to switches, turnstiles and ratchets. Hiraoka, Shiro and Shionoya have now created a molecular ball-bearing, an assemblage of two disc-shaped molecules that rotate relative to each other on silver ions (Ag+) sandwiched between them (J. Am. Chem. Soc. 126, 1214–1218; 2004).

As shown in the figure, the disc-shaped molecules consist of a central benzene ring carrying either alternating thiazolyl and p-tolyl rings (top) or six thiazolyl rings (bottom). Mixing solutions of these molecules with silver ions results in the spontaneous formation of the ball-bearing complex. This is most stable when each silver ion is linearly coordinated to two nitrogen atoms, one each from a thiazolyl group in the upper and lower molecules.

Trigonal coordination is also possible, between a silver ion and two neighbouring thiazolyl nitrogen atoms in the lower molecule and one thiazolyl nitrogen atom in the upper molecule. The complex can shift from linear coordination to trigonal to linear again, as each silver ion exchanges its original partner on the lower molecule for a nitrogen atom on a neighbouring thiazolyl group of the same molecule. During each exchange, the two discs rotate through 60° relative to each other.

Hiraoka et al. use NMR spectroscopy to watch the ball-bearing in action. Close to room temperature, the complex is in full swing, like a spinning carousel, and the coordination exchanges are faster than the timescale of NMR response. But if the temperature is lowered, the coordination exchange, and hence the rate of rotation, slows down. Then the NMR data clearly reveal that the six thiazolyl groups of the lower molecule experience two distinct chemical environments, with half of the groups coordinated to silver ions whereas the other half remain uncoordinated.

The task now is to control the speed and direction of the rotation — and put the device to practical use.