A C–H bonding trap for chloride: Part of the reason salt dissolves so well in water is that the polarized O–H bonds attract the negatively charged chloride ions. It has therefore been common to include O–H or N–H bonds in molecular receptors designed to capture anions such as chloride. Liu et al. report the surprising finding that sufficiently polarized C–H bonds can work even better (see the Perspective by Bowman-James). They designed a cage with triazole rings that directed C–H bonds inward to encapsulate chloride with a remarkable attomolar affinity in dichloromethane. Science , this issue p. 159 ; see also p. 124

Contrary to expectations, protic hydroxyanions show a surprisingly reliable facility to dimerize or oligomerize together by hydrogen bonding.

Synthetic foldamers with helical conformation are widely seen, but controllable interconversion amongst different geometries (helical structure and sense) is challenging. Here, a family of oligourea (tetra-, penta-, and hexa-) ligands bearing stereocenters at both ends are designed and shown to switch between single and double helices with concomitant inversion of helical senses upon anion coordination. The tetraurea ligand forms a right-handed single helix upon chloride anion (Cl⁻) binding and is converted into a left-handed double helix when phosphate anion (PO₄ ³⁻) is coordinated. The helical senses of the single and double helices are opposite, and the conversion is further found to be dependent on the stoichiometry of the ligand and phosphate anion. In contrast, only a single helix is formed for the hexaurea ligand with the phosphate anion. This distinction is attributed to the fact that the characteristic phosphate anion coordination geometry is satisfied by six urea moieties with twelve H-bonds. Our study revealed unusual single–double helix interconversion accompanied by unexpected chiroptical switching of helical senses.

The sulfate anion (SO₄^2–) is known as an end metabolite of cysteine and methionine, and its proper concentration is associated with the expression of key functions in the physiological system. Thus, maintaining sulfate concentration at a precise level is of great significance for biology, environments, and industrial productions. Fundamental research for sulfate anion chemistry can help understand sulfate-associated physiological processes and related applications, for example, remediation. In this minireview, we summarized recent research progresses in sulfate recognition and separation using crystallization and liquid–liquid extraction. We focused on the studies wherein molecular recognition is the key element and is considered the driving force for selective sulfate separations from aqueous solution.

Hexaurea-based receptors enable highly efficient sulfate extraction (over 97%) via liquid–liquid extraction, and a controllable release of the bound sulfate is achieved by acidifying the solution, as demonstrated in the U-tube experiment.