mysterious crystal

Huge crystal baffles chemists

Molecular cluster defies accurate analysis.

Katharine Sanderson

A giant molecular bauble, by far the largest single-molecule metal-cluster ever made, is confounding chemists. Involving almost 500 silver atoms, the crystals are so large and complex that their creators cannot figure out their structure.
Each of the 30 crystals made by Dieter Fenske of the University of Karlsruhe in Germany and his colleagues is thought to contain molecules with 490 silver atoms linked by 188 sulphur atoms and 114 organic groups, Ag₄₉₀S₁₈₈(StC₅H₁₁)₁₁₄. “This is an idealized structure” based on energy calculations, explains Fenske, who published the structure this month (C. E. Anson et al. Angew. Chem. Int. Edn doi:10.1002/anie.200704249 2008).

Atomic mess: the exact structure of this giant crystal may prove impossible to determine.

“With structures that size you are pushing the crystallography technique right to its limit,” says chemist Paul Raithby at the University of Bath, UK. Yet, he adds: “As far as crystallography and mass spectrometry can prove anything, this structure is as definitively proved as you could get.”
Fenske's molecules, described as 'clusters', produce molecules that are about 3 nanometres in diameter. The crystals have a well-defined outer 'shell' that is possible, though not simple, to characterize using X-ray diffraction.
But delve beneath the crystals' outer shell and things get a lot more complicated. Rather than the regular structure seen in a typical crystal, Fenske's clusters contain disorder: a void, filled by silver and sulphur atoms linked together in haphazard disarray. Molecules of silver sulphide (Ag₂S) can be arranged in a number of different geometries, including cubic, octahedral or dodecahedral. The disorder inside Fenske's cluster is so great that he can't tell what the geometry is in any of his samples. He describes the interior as looking “molten”.
The crystals have been tricky to define in more than one way: even though they grow in a crystalline way, the disordered core causes a problem. “If the interior of the particles is 'mobile' then they are not crystals,” says Frank Leusen, a crystallographer at Bradford University, UK. And are they molecules? Some say yes, others are not so sure. “These systems are at the boundary between molecular chemistry and bulk materials chemistry,” says Raithby. “I am at a bit of a loss finding an appropriate term,” adds Leusen.
Because of their huge size, Fenske's clusters may have characteristics that transcend the limits of molecular chemistry and enter the realm of macroscopic particles. For example, they may have interesting electrical properties.
The sheer size and complexity of the clusters mean that their internal structure cannot be revealed using X-ray diffraction. The technique uses X-rays that are reflected off the atoms in a rotating crystal, creating patterns of spots. These spots are converted into a map of the electron density around the atoms, and eventually the molecule's structure is calculated. But with larger molecules the number of spots increases, the spots get closer together and can even overlap. Also the intensity of each spot decreases as molecules get bigger.
Fenske instead calculated likely arrangements for the different internal geometries that Ag₂S could adopt within the known molecular mass. The most tightly packed arrangement, Fenske calculated, involves 490 silver atoms, making it by far the largest cluster reported.

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“The research in this paper is spectacular,” says Bill Clegg, a crystallographer at Newcastle University, UK.

But Fenske is not stopping there. In the same mixture that produced the black crystals he thinks he has a cluster that contains 800 silver atoms, the structure of which he is grappling with at the moment. “I'm pretty sure we can get larger particles,” he says.
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