Catenane Packs On Positive Charge

Stringing together two molecular rings, each bearing four positive charges, seems like it should defy the laws of physics. After all, those laws dictate such charges repel one another, so attempting to corral two of these molecules should be a chemist’s folly.

Imagine Jonathan Barnes’s surprise then, when he managed to make this new molecule, an octacation, on his first try (Science, DOI: 10.1126/science.1228429). The key, says the Northwestern University graduate student, was to knit together positively charged radicals to form the interlocked rings. The radicals’ propensity to pair up overcame the repelling forces, and Barnes was then able to link the two halves of the molecule together.

The compound, which is known to chemists as a catenane, is the latest addition to the menagerie of mechanicallyinterlocked molecules to come out of J. Fraser Stoddart’s laboratory at Northwestern. As synthesized, the new catenane exists as a tetraradical tetracation, but upon exposure to air, it oxidizes to a mixture of a 6+ diradical and a 7+ monoradical. Such stable radicals are rare, Stoddart points out. His research team managed to get X-ray crystal structures for both aforementioned radical states as well as the tetraradical tetracation.

The team characterized six of the nine possible oxidation states for the species, including the octacation. This state is prepared via electrochemical oxidation and crams eight positive charges into a space approximately the size of a cubic nanometer, a feature that could make the molecule useful as an electron storage system.

“An interesting aspect of the work is how such a multiple-charged molecular species, when formed, does not want to literally find a way to fall apart. If you would have drawn the compound on a piece of paper and asked if it could be made, I would have said, ‘No,’ ” says Andrew Benniston, an expert in molecular photonics at England’s Newcastle University. “Sometimes flying in the face of opinion is where real breakthroughs are made.”