Encapsulating chemistry

Self-assembling molecular capsules can serve as containers, separation tools and even as reaction vessels and catalysts. Michael Gross is captivated...

From the very hour of its birth - Pedersen’s discovery of crown ethers - supramolecular chemistry has been dealing with molecular containers. Over the decades, the flat crown ethers have given way to three-dimensional shapes including cryptands, cavitands and ultimately capsules, designed to encapsulate a molecular host. Throughout the 1990s, work from Julius Rebek’s laboratory among others has produced a rich variety of self-assembled molecular containers in many different shapes and sizes. 

Now the emphasis is shifting from structure to function, as many researchers are trying to make the nanoscale capsules work for them. Recent work from several laboratories demonstrates that the potential applications are wide-ranging, including structural studies of liquid phases, separation techniques and catalysis.

To make things happen inside their capsules, researchers need to build them somewhat larger than the original constructs, which were often built around a single ion. One technology proven to achieve molecular constructs of spectacular size is the polyoxometalate approach pioneered by Achim Müller at the University of Bielefeld, Germany ( Chem. Br., July 2002, p16). Having constructed some of the biggest supramolecular assemblies known to humankind (6nm in diameter), Müller has now turned his attention to encapsulating ’things’. In a recent paper he has presented a molecular cage built from molybdenum oxide, which can be modified and fine-tuned to present different kinds of interior surface, pores with different sizes and chemical properties and different overall size and charge (see Fig).

Schematic representation of the structure of a molybdenum oxide cage (polyhedra) with encapsulated water and electrolytes (ball and stick models). A sodium ion (violet) is coordinated by sulphate (S orange, O red) and by water (O yellow) molecules. Reproduced with permission from Angew. Chem. Int. Ed., 2003, 42, 2085

Using variations of this capsule to enclose and study a well-defined nanoscale sample of water molecules with and without electrolytes such as sodium ions, the researchers found very different structural properties of this ’water’ depending on the nature of the surrounding capsule. In some cases, water clusters forming highly symmetrical shells-within-shells could be observed by single-crystal X-ray crystallography. While some of the symmetry is induced by the coordination of the molybdenum oxide container, the researchers also observe a few structures that are known or hypothesised to occur in bulk liquid water.

Being able to fine-tune the nature and contents of the cavity with great precision allows researchers to study the effects of many different parameters on the local and mid-range structures of encapsulated water. This may lead to a better understanding not only of the bulk phase, but also of the highly constrained water in the living cell. The experiments also yielded the first example of artificially produced, discrete dodecahedra made of 20 water molecules with 30 hydrogen bonds, which are known to occur naturally in clathrate hydrates (Chem. Br., May 2002, p22).

In a further paper currently in preparation, M?ller’s group will report using the molybdenum oxide assemblies to build a nano-ion chromatograph. Linking up the tunable pores to channels, they find that different kinds of ions have very different mobilities in this nanoscale maze, which could lead to applications in separation technology.

Now that self-assembling containers are big enough to host several molecules, there is also the potential of using them as reaction vessels to isolate and possibly even to catalyse chemical reactions. Six years ago, Rebek’s group demonstrated Diels-Alder reactions involving ternary coordination complexes between the two reactants and the capsule. 

Makoto Fujita and coworkers at Nagoya University and at the Japan Science and Technology Corporation (JST) have now studied the photochemical [2+2] cycloaddition with different olefins in an octahedral capsule built with palladium centres at the corners, coordinated by nitrogen-bearing heterocycles that cover four of the octahedron’s eight sides. 

They found that this capsule can direct the reaction towards high yields of the hetero product, whereas chemistry in free solution would have produced a statistical mixture of homo and hetero dimers. Moreover, the steric hindrance within the cage also controls the stereochemistry of the cycloaddition, such that the reaction of acenaphthylene and naphthochinones results in an astonishing 92 per cent yield of just one product, namely the hetero dimer in a syn conformation, with the ether groups pointing in opposite directions.

For certain combinations of reactants, the Japanese researchers could also demonstrate acceleration of the reaction in the presence of the capsule. In this way, they could induce olefins that would not form photodimers in finite time in free solution to react efficiently. Thus, apart from the reactions happening in vivo and in vitro, we are now beginning to find out more about the new chemistry of reactions ’ in capsula’.

Source: Chemistry in Britain

Acknowledgements

Michael Gross is a science writer in residence at the school of crystallography, Birkbeck College, University of London. He can be contacted via the Prose and passion web page.