23 May 2019

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-Å hydrogen bonds originating from the cage’s six 1,2,3-triazoles. Atto-molar 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 3D structure to effect recognition affinity and selectivity: Cl– > Br– > NO3– > I–. This cage shows anti-Hofmeister salt extraction and preliminary corrosion inhibition.

[Image]

Fig. 2 Quantifying and understanding chloride affinity and selectivity.

(A) 1H NMR spectra of cage 1 upon titration with Br– and then by competitive exchange with Cl– ([1]0 = 0.5 mM, 298 K, 500 MHz, DMSO-d6). Peaks associated with free triazolo cage 1 are colored in red, 1•Br– in magenta, and 1•Cl– in blue.

(B) Anion affinities (K1) determined in DMSO-d6 for Cl–, Br–, NO3–, and I– as TBA+salts. Colored lines are drawn to guide the eye. Errors were determined by root-mean-square deviations from the nonlinear fitting (peak shifts) or by averaging multiple data points (peak integral ratios). See figs. S6 to S24 for more details.

(C) Chemical structure of 3D flexible receptor 3.