Not a drop: buckyball bottles up single water molecule

Ezine

Published: Dec 1, 2010
Author: David Bradley
Channels: NMR Knowledge Base

thumbnail image: Not a drop: buckyball bottles up single water molecule

Bucky bottle

Much of modern chemistry focuses on the incarceration of small molecules in cages and crypts. Now, a team writing in Angewandte Chemie describe how they have added a single water molecule to the world's smallest "water bottle", a fullerene. NMR spectroscopy and X-ray diffraction reveal the details.

During the last ten years or so, chemists have made significant progress in opening up the fullerene molecule to allow other smaller guest molecules to slip inside this spherical carbon host. Fullerenes have a cavity large enough to encapsulate atoms and small molecules, but not so large as to allow clusters of guests to form within. Making a suitable hole in the fullerene cage and then attaching a molecular stopper gives us what might be flippantly termed a "bucky bottle". There are many possible applications for such an entity, according to researchers writing in Angewandte Chemie recently. For example, a stoppered molecule of this kind might be used to carry a radioactive payload, such as tritiated water in radiopharmaceuticals for diagnostic or therapeutic studies.

Qianyan Zhang, Shuming Liu and Liangbing Gan of the Beijing National Laboratory for Molecular Sciences, part of the Chinese Academy of Science working with colleagues in Germany, Tobias Pankewitz and Wim Klopper of the Karlsruhe Institute for Technology, explain that several teams have created openings in fullerenes, but they are now the first to add a stopper to the bucky bottle.

Stoppered molecular cage

Earlier, the team had reported the synthesis of an opened fullerene in which they had successfully encapsulated a water molecule through a peroxide-mediated cage-opening strategy. The team pointed out that carbonyl groups on the rim of the fullerene opening were relatively reactive towards various nucleophiles and now they have treated the complex with triethylphosphite to unexpectedly produce a compound with a phosphate group attached above the opening. They have shown that under basic conditions, the phosphate can be removed by hydrolysis, a process that takes two hours at room temperature compared to the 10 minutes for the forward reaction. The team used carbon-13 nuclear magnetic resonance spectroscopy to confirm the C s symmetry of the structure, while single-crystal X-ray diffraction was able to demonstrate that a single water molecule is trapped within the bucky bottle.

Space-filling models show that with the stopper in place there is no possibility of the water molecule leaking out of the fullerene cage nor of any other molecules or atoms entering. The team also verified that this is indeed the case using deuterated solvent and proton NMR spectroscopy to show that no exchange takes place with the phosphate stopper blocking the opening. The data were also modelled and validated using density functional theory (DFT).

Molecular vessels of this sort might find application as Trojan-type agents for the delivery of small molecules to disease sites in the body or as possible catalysts and sensors. Further research might optimise the "stopper" or the periphery of the opening to make the vessel selective to specific molecules of ions of interest.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.