Inorganic anion recognition in aqueous solution by coupling nearby highly hydrophilic and hydrophobic moieties in a macrocyclic receptor

Abstract

Receptor L, composed of a tripropylenetetramine chain linking the 2 and 7 positions of an acridine unit via methylene bridges, behaves as a pentaprotic base in aqueous solution. The first four protonation steps occur on the tetra-amine chain, while the acridine nitrogen protonates only below pH 4. The penta-protonated receptor assumes a folded conformation, resulting in a cleft delimited by the aliphatic tetramine and acridine moieties, in which anions of appropriate size can be hosted. Potentiometric titrations reveal that F⁻ forms the most stable complexes, although the stability constants of the Cl⁻ and Br⁻ adducts are unusually only slightly lower than those observed for F⁻ complexes. A remarkable drop in stability is observed in the case of I⁻ adducts. Oxo-anions, including H₂PO₄⁻, NO₃⁻ and SO₄²⁻, are not bound or weakly bound by the protonated receptor, despite the known ability of charged oxygens to form stable O⁻⋯HN⁺ salt bridges. This unexpected stability pattern is explained in the light of the X-ray crystal structures of H₅LCl₅·4H₂O, H₅LBr₅·4H₂O, H₅L(NO₃)₅·3H₂O and H₅L(H₂PO₄)₅·(H₃PO₄)₂·4H₂O complexes, coupled with MD simulations performed in the presence of explicit water molecules, which reveal that Cl⁻ and, overall, Br⁻ possess the optimal size to fit the receptor cleft, simultaneously forming strong salt bridging interactions with the ammonium groups and anion⋯π contacts with protonated acridine. I⁻ and oxo-anions are too large to conveniently fit the cavity and are only partially enclosed in the receptor pocket, remaining exposed to solvent, with a lower entropic stabilization of their complexes. Although F⁻ could be enclosed in the cavity, its smaller size favours the F⁻⋯HN⁺ salt bridging interaction from outside the receptor pocket. The fluorescence emission of the acridinium unit is quenched by anion binding. The quenching ability parallels the stability of the complexes and is related to the relevance of the anion⋯π contacts in the overall host–guest interaction.