Credit: © 2009 Wiley

Polyoxometalate (POM) clusters can be assembled into a wide variety of architectures, with electronic properties that largely depend on their localized and itinerant d electrons, and functionalized with magnetic groups such as iron or vanadyl ions. Among those species, polyoxomolybdate Keplerate-type structures — spherical species consisting of 12 pentagonal {Mo(Mo)5} clusters bridged by 30 linkers — have attracted particular attention. Now, Paul Kögerler from the Forschungszentrum Jülich and co-workers in Germany, Belgium and the US have used1 Raman spectroscopy to monitor the formation of a partially reduced Keplerate system with altered intramolecular magnetic interactions.

The researchers first formed Keplerate species featuring Fe-, V- and Mo-based linkers by precipitation. They then added KCl to the remaining solution before leaving it exposed to air for two weeks, which led to the partial reduction of the Mo(VI)-based units. The Keplerate clusters that were subsequently collected by precipitation were shown to be Mo(V)/Mo(VI)-based clusters linked only by iron-based units.

These clusters had a similar structure to that of previously reported Mo(VI)-based clusters, but were highly charged. They therefore stacked into linear chains in the solid state, through interactions with potassium ions. A weak antiferromagnetic coupling, mediated by O–Mo–O exchange pathways, occurred between neighbouring iron centres, resulting in a highly symmetrical geometric frustration despite no changes in the overall architecture.