Sulfoniumcalixpyrrole: the decoration of a calix[4]pyrrole host with positive charges boosts affinity and selectivity of anion binding in DMSO solvent

Abstract

With the goal to improve host–guest binding affinity, while retaining guest selectivity, the parent calixpyrrole artificial receptor unit was decorated with four sulfonium moieties (e.g.3b) placed rigidly in symmetrical fashion remote from the core binding site. The determination of binding constants using isothermal titration calorimetry (ITC), in conjunction with trend analyses in a series of related anions, reveals the occurrence of several specific and unspecific binding steps. The former predominate in affinity by 20–1000 fold depending on guest charge and structure. The dissection of the free energy of association into its enthalpic and entropic parts, in combination with structural variation in the guest anion, offers insight into the nature and diversity of the guest binding modes. On these grounds, the supplementation of parent host scaffolds with rigidly positioned positive charge centers, holds promise as a general concept for anion binding in competitive media.