An Organic Knot: When people tie knots, they grasp both ends of a strand and loop them around each other. The task is rather more difficult at the molecular level, when there is no top-down organizing framework, and the strand needs to be coaxed into looping around itself. Recently, chemists have taken advantage of the tight geometrical restrictions of metal-ligand coordination to produce knot morphologies. Ponnuswamy et al. (p. 783 ; see the Perspective by Siegel ) now demonstrate the spontaneous assembly of a trefoil knot from organic fragments without assistance from metal centers. The topology appears to be driven by hydrophobic interactions among aromatic segments that minimize exposure to surrounding water through their overlap.

A new 2,6-diamidopyridinedipyrromethane hybrid macrocycle (1) has been synthesized that shows a high selectivity for dihydrogen phosphate and hydrogen sulfate relative to nitrate in acetonitrile solution as judged from UV-vis spectrophotometric titrations; this leads to the suggestion that this or related systems might find use in nuclear waste remediation applications requiring the selective removal of hydrogensulfate from nitrate-rich waste mixtures.

A new type of neutral donor–acceptor [2]-catenane, containing both complementary units in the same ring was synthesized from a dynamic combinatorial library in water. The yield of the water soluble [2]-catenane is enhanced by increasing either building-block concentrations or ionic strength, or by the addition of an electron-rich template. NMR spectroscopy demonstrates that the template is intercalated between the 2 electron-deficient naphthalenediimide units of the catenane.