Macrocycle Stars In Anion-Grabbing Action

There’s a star on the rise in the world of supramolecular chemistry. Cyanostar, a new star-shaped macrocycle (shown), can pull weakly coordinating anions, which previously defied capture, out of solution (Nat. Chem. 2013, DOI: 10.1038/nchem.1668). The compound’s anion-sequestering abilities could find applications in environmental remediation of perchlorate and molecular sensing of biological phosphates. Although cyanostar is a neutral molecule, it possesses polarizable cyanostilbene-based C–H bonds that can coordinate large anions. It does so by forming sandwichlike structures, with the anion in the center capped by two cyanostars. The cyanostar, invented by Indiana University, Bloomington, chemists Amar H. Flood, Semin Lee, and Chun-Hsing Chen, is also easy to make. Its synthesis makes use of multiple Knoevenagel condensations, wherein benzylic nitriles react with aromatic aldehydes, followed by dehydration. The reaction proceeds with yields in excess of 80%, even on multigram scales, via a one-pot procedure. Such ease of synthesis is rare in the world of macrocycles, the Indiana chemists point out. The chemists also use cyanostar’s anion-binding powers to prepare a rotaxane with two cyanostars threaded on a dialkylphosphate moiety. Such structures are precursors to molecular machines.