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

In this work, we\ndeveloped (thio)ureido-calix[4]arene derivatives\nand thoroughly explored their anion-binding properties in acetonitrile.\nA series of anions, including important inorganic ones (Cl<sup>–</sup>, HSO<sub>4</sub><sup>–</sup>, H<sub>2</sub>PO<sub>4</sub><sup>–</sup>, and HP<sub>2</sub>O<sub>7</sub><sup>3–</sup>) and several ever-present carboxylates (acetate, benzoate, and fumarate),\nwere studied. All systems were investigated by several methods (NMR,\nITC, and UV) used in a synergistic fashion, providing their comprehensive\nthermodynamic description. Acidities of the receptors were determined\nprior to the anion-binding studies and considered in the data-handling\nprocedures. Complexes of various stoichiometries were detected and\nthe driving force for their formation elucidated. The correlation\nof the anion structural features and H-bond acceptor properties with\nthe stoichiometries and complexation thermodynamics parameters was\nrationalized. Generally, stability of the complexes followed the trend\ndefined by the basicity of anions. Thiourea and urea analogues exhibited\nsimilar affinities for anion binding except for the H<sub>2</sub>PO<sub>4</sub><sup>–</sup> and HP<sub>2</sub>O<sub>7</sub><sup>3–</sup>, which interacted with the thiourea analogue more strongly. The\nhosts endowed with 4 (thio)urea groups formed species containing two\nreceptor molecules bridged by a fumarate or hydrogen pyrophosphate\nanion. Thermodynamic information provided in this work is applicable\nin further design of supramolecular systems, whereas the presented\napproach to data handling will aid researchers when dealing with multiple\ncoexisting equilibria.