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Protonation and anion binding properties of aromatic bis‐urea derivatives – apprehending the proton transfer

A series of aromatic bis‐urea derivatives was prepared and their proton dissociation, as well as anion binding properties in DMSO were investigated. To this end, UV‐Vis and 1H NMR spectroscopies and computational methods were employed. The receptors differed in the relative position of the urea moieties (ortho‐ and meta‐derivatives) and in the functional groups (–H, –CH3, –OCH3, –NO2) in the para‐ position of the pendant phenyl groups. Remarkably high acidities (log K1H ≈ 14), were ascribed primarily to the stabilising effect of the aromatic subunits. Quantum‐chemical calculations corroborated the conclusions drawn from experimental data and provided information from the structural point of view. Knowledge regarding protonation properties proved to be essential for reliable quantitative determination of anion binding affinities. Studied receptors were selective for acetate and dihydrogen phosphate among several anions. Formation of their complexes of 1:1 and 1:2 (ligand:anion) stoichiometries was quantitatively characterised. Proton transfer was taken into account in the data analysis, which was especially important in the case of AcO–. Ortho‐receptors were proven to be more efficient acetate binders achieving coordination with all four NH‐groups. The meta‐ analogues preferred dihydrogen phosphate, which acted as both hydrogen bond donor and acceptor. Allosteric effect detected in the case of 1:2 H2PO4– complexes was assigned to formation of interanionic hydrogen bonds.

urea derivatives ; anion binding ; protonation properties ; thermodynamics ; DFT

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