Thermodynamics of Anion Binding by (Thio)ureido-calix[4]arene Derivatives in Acetonitrile

In this work, we developed (thio)ureido-calix[4]arene derivatives and thoroughly explored their anion-binding properties in acetonitrile. A series of anions, including important inorganic ones (Cl^-, HSO₄ ^-, H₂PO₄ ^-, and HP₂O₇ ^3-) and several ever-present carboxylates (acetate, benzoate, and fumarate), were studied. All systems were investigated by several methods (NMR, ITC, and UV) used in a synergistic fashion, providing their comprehensive thermodynamic description. Acidities of the receptors were determined prior to the anion-binding studies and considered in the data-handling procedures. Complexes of various stoichiometries were detected and the driving force for their formation elucidated. The correlation of the anion structural features and H-bond acceptor properties with the stoichiometries and complexation thermodynamics parameters was rationalized. Generally, stability of the complexes followed the trend defined by the basicity of anions. Thiourea and urea analogues exhibited similar affinities for anion binding except for the H₂PO₄ ^- and HP₂O₇ ^3-, which interacted with the thiourea analogue more strongly. The hosts endowed with 4 (thio)urea groups formed species containing two receptor molecules bridged by a fumarate or hydrogen pyrophosphate anion. Thermodynamic information provided in this work is applicable in further design of supramolecular systems, whereas the presented approach to data handling will aid researchers when dealing with multiple coexisting equilibria.