Urea-, squaramide-, and sulfonamide-based anion receptors:a thermodynamic study

In this work, we compare the anion-binding capabilities of receptors 1-5, characterized by similar structures, but possessing different hydrogen-bond-donor moieties (urea, squaramide, and sulfonamide). The presence of chromophoric substituents on the receptor's skeleton allowed the detn. of assocn. consts. by performing UV/Vis titrns. with the investigated anions on solns. of the receptors in pure acetonitrile. Addnl. quant. studies of the anion-binding properties of receptors 1-5 were performed by isothermal titrn. calorimetry (ITC). The exptl. results indicated that 1 and 2 formed 1:1 hydrogen-bonded complexes with most of the anions investigated. In the case of receptors 3-5, the formation of the 1:1 adduct was obsd. only with anions of low basicity (i.e., chloride, bromide, iodide, and hydrogen sulfate). With more basic anions (i.e., acetate and dihydrogen phosphate), both spectrophotometric and ITC titrns. accounted for the deprotonation of the sulfonamide group, involving the formation of the conjugated base of the receptor.