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Protonation and Anion Binding Properties of Urea Derivatives – Comprehending the Proton Transfer

Knowledge regarding the acid-base behaviour in non-aqueous media has remained relatively scarce in spite of its importance for many aspects of chemistry. Recently, we reported on a detailed study of acid-base properties of dihydrogen phosphate and acetate in aprotic organic solvents (acetonitrile, dimethyl sulfoxide, and dimethylformamide).[1] It was found that several processes, i.e. protonation, homoassociation, and dimerisation play important roles in defining the basicity of these widely important anions. A comprehensive thermodynamic characterisation of the corresponding processes was carried out which provided valuable information for the advancement of host-guest chemistry focusing on highly basic anions. In this field, urea derivatives have been among the most studied receptors, and numerous studies of their complexation with diverse anionic species have been reported. To gather reliable thermodynamic information about the anion binding processes, it is of great importance to identify and characterise the coupled reactions. In the case of anion receptors bearing urea moieties, one of the most commonly encountered side reactions is deprotonation of the NH group, accompanied by protonation of the anion, that is, receptor to anion proton transfer. This process is especially favorable in aprotic organic solvents of lower polarity owing to the drastic increase in basicity of the commonly used anions (e.g., F−, AcO− or H2PO4−) with respect to aqueous medium. As a continuation of our work, a series of aromatic bis-urea derivatives was prepared and their proton dissociation, as well as anion binding properties in DMSO were investigated revealing remarkably high acidities of the compounds (log K1H ≈ 14).[2] Studied receptors were selective for acetate and dihydrogen phosphate among several anions forming complexes of 1:1 and 1:2 (ligand:anion) stoichiometries. Proton transfer was taken into account in the course of data analysis, which was especially important in the case of AcO−. Knowledge regarding protonation properties proved to be essential for reliable quantitative determination of anion binding affinities.

proton transfer ; urea derivatives ; thermodynamic characterisation ; anion binding

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