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Researchers in Germany are working on an unprecedented high-temperature host-guest super molecule, visualised with NMR spectroscopy. The system might be used to study molecules trapped inside the capsule for hydrophobic and confinement effects. Nature has an abundance of porous materials with holes and channels into which small molecules might fit. Compounds such as zeolites, for instance, can play host to smaller chemical species and so have been used sieves and sensors for a variety of molecular guests. Chemists have spent decades trying to emulate these porous structures with varying degrees of success. Achim Mueller of the University of Bielefeld and his team have taken a different approach to mimicking nature's porous materials. Rather than build a whole crystalline structure like a zeolite filled with pores and channels, they have tried to build just the pores with none of the surrounding scaffold. As such they have constructed nanoscopic, yet enormous on the molecular scale, capsules using sophisticated mixed metal oxides. These singular hosts can welcome inside a large number of different guests, the precise fit and resulting behaviour of which depends on the exact nature of the interior of their molecular capsules. Now, Mueller and Bielefeld colleagues Christian Schaeffer, Hartmut Boegge, Alice Merca, having recently worked with Dieter Rehder and Erhard Haupt (NMR studies) inorganic chemists at the University of Hamburg, Germany, and Ira Weinstock of the Department of Chemistry, at Ben Gurion University of the Negev in Beer Sheva, Israel, have achieved this goal. They designed and synthesised a robust, spherical, porous capsule of the type [{pentagon}12- {linker}30]=[{(Mo)Mo5O21(H2O)6}12{Mo2O4(ligand)}30]n-. The capsules are heat resistant even at the boiling point of water. The team has once again used NMR spectroscopy to track progress in their latest aim of building and testing an organometallic capsule that has an unprecedented structurally well-defined and mostly water-repellent, hydrophobic, interior. The researchers used infrared, Raman, and especially NMR spectroscopy, and single crystal X-ray structure analysis to reveal the atom-by-atom structure in detail. These studies demonstrated that a partly compact spherical aggregate of 24 butyrate groups forms within under the conditions of the synthesis. A detailed study of the butyrate aggregate was undertaken using 1H and 13C NMR spectroscopy. The researchers explain that other bulky organic units might also be used to create different kinds of hydrophobic interiors within related capsules. Additionally, they suggest that the interiors of their capsules might also be modified by simply introducing different chemical species that coordinate, or bond, only weakly to its 30 dinuclear molybdenum-molybdenum linkers. ROESY NMR spectroscopy can then be used to characterise the interiors and the guests trapped within. Crucially, there are in the present case no water molecules encapsulated, although the hydrophobic nature of the interior would according to normal understanding exclude water, regardless. In contrast, a water molecule is found along with an ammonium ion in each of the twenty pores that are evenly distributed around its surface. The ammonium ions are attracted by the high negative charge of the capsule. "I believe that the present paper refers to new options for basic future research," Mueller told SpectroscopyNOW, "The reason is that one can systematically study in the capsule hydrophobic effects in general, (different) chain-chain contacts under confined conditions as well as investigations in hydrophobic cavities, especially with the influence of included water." Mueller points out that such a capsule may have also implications for protein research regarding modelling of hydrophobic cavities. The researchers point out that the investigation of organic molecules encapsulated in such cages is a matter of current interest, sometimes found in the literature under the headlines "nanolabs", "nanotubes", and "molecular flasks".
Related links: The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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 Truly encapsulating |
