Cationic charges have long been used in the design of receptors to bind anions. Our fundamental understanding of the underlying driving forces that control binding affinities and selectivities has been limited to polar solvents, which are used to simplify the distribution of species by dissociating ion pairs and circumventing the competition from the counter ions. To help address this limited solvent window, and to evaluate the role of solvation on ion-ion interactions in anion binding, we aim to quantify the association constants of a charged receptor. This quantification is performed using a positively-charged, partially-organized crescent receptor and delineating a set of binding equilibria that presumably exist in solution. We compared the affinities to uncharged but pre-organized receptors to evaluate which effect dominates in which solvents. Our experimental analyses deconvolute the chloride binding equilibrium from ion-pairing in solvents ranging from non-polar dichloromethane (εr = 8.9) to polar acetonitrile (εr = 37.5). Across these solvents, the ion-ion interaction was found to strengthen anion binding affinity of the receptor compared to preorganization. This effect was consistent even in high polarity solvents which is counterintuitive in comparison to the dielectric screening observed in neutral hosts. Owing to the redox active methyl-quinolinium reporter in the receptor, voltammetric responses were used to corroborate the binding events seen in both the solvents (acetonitrile and dichloromethane) upon addition of chloride. These findings help lay the groundwork for understanding how solvent impacts recognition chemistry of cationic receptors and thereby aid towards an informed receptor design.

Quantifying anion binding to cationic receptors across solvents

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