Metallohelicate hosts that bind anions internally could potentially be developed as luminescent or redox sensors for anions, according to Sandrine Goetz and Paul E. Kruger at Trinity College Dublin, in Ireland, who have synthesized examples of the complexes (Dalton Trans., published online Jan. 3, dx.doi.org/10.1039/b514580e). The hosts, which consist of three helical strands (shown as red, blue, and yellow), are [Fe2L3]4+ complexes formed by coordination of three bis(amido-2,2'-bipyridine) ligands (L) to two Fe(II) atoms (purple). The water-soluble complexes bind chloride ions (green) in preference to other anions. Because the chloride ions are locked inside the strands, the systems are more stable than previously reported metallohelicates that have external anion receptors. The iron atoms act as a template in the formation of the complexes and introduce positive charges into the ligand scaffold for anion binding. In solution, the complexes exist as a mixture of helical and nonhelical isomers; addition of chloride ions yields a solution containing only helical isomers.