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Chloride capture using a C–H hydrogen-bonding cage

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Science  12 Jul 2019:
Vol. 365, Issue 6449, pp. 159-161
DOI: 10.1126/science.aaw5145

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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

Abstract

Tight binding and high selectivity are hallmarks of biomolecular recognition. Achieving these behaviors with synthetic receptors has usually been associated with OH and NH hydrogen bonding. Contrary to this conventional wisdom, we designed a chloride-selective receptor in the form of a cryptand-like cage using only CH hydrogen bonding. Crystallography showed chloride stabilized by six short 2.7-angstrom hydrogen bonds originating from the cage’s six 1,2,3-triazoles. Attomolar affinity (1017 M–1) was determined using liquid-liquid extractions of chloride from water into nonpolar dichloromethane solvents. Controls verified the additional role of triazoles in rigidifying the three-dimensional structure to effect recognition affinity and selectivity: Cl > Br > NO3 > I. This cage shows anti-Hofmeister salt extraction and corrosion inhibition.

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