Nano the Hedgehog

By: David Bradley

Forget Sonic. Molybdenum is the prickly element of the day, thanks to German chemists who have designed and synthesised a super structure containing 368 molybdenum atoms with a rather spiky profile.

Achim Mueller and colleagues at the University of Bielefeld have for many years been developing ways to make increasingly complexity and geometric complexes. Previously, they synthesised a metal wheel with a mere 154 Mo atoms. Then they moved on to spheres, which were highlighted in The Alchemist in 1999. These metal "buckyballs" had some 500 atoms. Now, the Bielefeld team has curled up what the whimsically call a nano-hedgehog.

This compound uses the team's increasingly well-known approach to building structures from the molybdate (molybdenum oxide) building block. Mo has several properties that make it easy to exploit in building new compounds the size and shape of which can be tailored simply by changing the boundary conditions under which it is made. For example, the bonding to Mo can be altered readily as too can its coordination number. It readily exchanges water as a ligand and Mo-O-Mo type bridging bonds can form, allowing for split-and-link processes. In turn, the formation of terminal Mo-O groups provides a boundary on the otherwise potentially unlimited growth of a complex.
 

The new compound is a giant molybdenum oxide based nanocluster. It is approximately the size of haemoglobin protein molecule (diameter approximately 6 nm), which is far bigger than other structurally well-defined synthetic species. It contains 368 metal (1880 non-hydrogen) atoms formed by the linking of 64 {Mo1}-, 32 {Mo2}-, and 40 {Mo-(Mo5)}-type sub-units. The team reveals how the cluster forms as a result of a "remarkable symmetry-breaking process". This, they add is revealed by the surface geometry of the compound, which takes on a spiked appearance. Hence the whimsical monicker.

Mueller sees this unusual complex as a springboard from which chemists can dive into the pool of similarly sized materials with tailorable properties.

Ref: Angew. Chem. 2002, 114(7).