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

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-MIDLINE HORIZONTAL ELLIPSISHN+ salt bridges. This unexpected stability pattern is explained in the light of the X-ray crystal structures of H5LCl5 center dot 4H(2)O, H5LBr5 center dot 4H(2)O, H5L(NO3)(5)center dot 3H(2)O and H5L(H2PO4)(5)center dot(H3PO4)(2)center dot 4H(2)O 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 anionMIDLINE HORIZONTAL ELLIPSIS pi 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-MIDLINE HORIZONTAL ELLIPSISHN+ 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 anionMIDLINE HORIZONTAL ELLIPSIS pi contacts in the overall host-guest interaction.