THE world鈥檚 tiniest packing crate has been built by chemists in Illinois. A perfectly formed cube, the crate has only enough space inside for a single metal ion. It is little more than a curiosity at present, but its makers hope variants could be used to detect specific molecules or ions.
The box, made by Tom Rauchfuss and his team at the University of Illinois in Urbana-Champaign, has edges 0.51 nanometres long and a volume of just 0.13 cubic nanometres. It is not the smallest that has ever been made. That honour goes to a molecule called cubane, first made in 1964, which has carbon atoms at each corner and edges just 0.16 nanometres long.
Cubane, however, is too small for anything to fit inside. Rauchfuss claims that the Illinois box is the smallest that will actually accommodate something solid. 鈥淐aesium sits perfectly in the middle, but potassium rattles around a bit, so there鈥檚 probably a bit too much room for it,鈥� says Rauchfuss. It鈥檚 also the first soluble box, opening up the possibility of trapping molecular targets in solution.
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There are hundreds of types of molecular 鈥渃ages鈥� for trapping ions, atoms and molecules, but these are usually suspended solids such as zeolites, which are widely used in industry and in household items such as washing powders. Zeolites sift out certain molecules from the solution without themselves dissolving. 鈥淭hese are often referred to as molecular sieves,鈥� says Rauchfuss.
Working with Kevin Klausmeyer and Scott Wilson in the university鈥檚 school of chemical sciences, Rauchfuss made the miniature box from a mixture of metals and organic substances. The corners of the box are occupied by rhodium or cobalt atoms, and are joined by cyanonitrile groups, which consist of a carbon atom bound to a nitrogen atom. 鈥淭he CN link is very robust and doesn鈥檛 break apart, so the box is very rugged,鈥� says Rauchfuss.
The box is held in a cubic arrangement by pentagon-shaped molecules called cyclopentadienyl groups, which fan out from each corner. The box shape works because each metal atom can adopt its preferred octahedral configuration. That is, each corner adopts the proper shape before it is chemically assembled into the box. 鈥淓ach corner is pre-organised,鈥� says Rauchfuss.
He says that the box is similar in structure to the pigment Prussian blue, which is also made of subunits of metals and CN bonds. The fact that Prussian blue is often used to 鈥渕op up鈥� caesium pollution inspired him to try fitting caesium into the box. Full details appear in Angewandte Chemie International Edition (vol 37, p 1694).
