Issue Date: September 8, 2008
Cagey Iron Reversibly Sequesters Molecules
A novel type of metal-organic cage that traps hydrophobic guest molecules can be reversibly opened, reports a group of researchers led by Jonathan R. Nitschke of the University of Cambridge and Kari Rissanen of the University of Jyväskylä, in Finland (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200803066). The cage self-assembles in water from 4,4′-diaminobiphenyl-2,2′-disulfonate and 2-formylpyridine, which covalently bind together and coordinate with iron(II) to form the edges of a tetrahedron with an iron atom at each apex. Once assembled, the cage can sequester cyclohexane or cyclopentane guest molecules. The researchers found that the cage can be opened up to release its guest by adding a flexible triamine, which displaces the rigid diamines that form the cage edges. That process irreversibly breaks up the cage into mononuclear iron complexes. Alternatively, the cage can be opened by lowering the pH of the solution; the cage re-forms when the solution is made basic again. The researchers propose that the iron cages can be used for drug delivery or as a means to sequester harmful hydrophobic molecules. They are now working to understand the potential reactivity of trapped guest molecules and looking into using longer diamines to make larger cages.
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