Influence of Charge on Anion Receptivity in Amide-Based Macrocycles

Binding and structural aspects of anions with tetraamido/diquaternized diamino macrocyclic receptors containing <i>m</i>-xylyl, pyridine, and thiophene spacers are reported. ¹H NMR studies indicate that the quaternized receptors display higher affinities for anions compared to corresponding neutral macrocycles. The macrocycles containing pyridine spacers consistently display higher affinity for a given anion compared to those with either <i>m</i>-xylyl or thiophene spacers. The <i>m</i>-xylyl- and pyridine-containing receptors exhibit high selectivity for H₂PO₄^– in DMSO-<i>d</i>_<i></i>6 with association constants, <i>K</i>_a = 1.09 × 10⁴ and >10⁵ M^–1, respectively, and moderate selectivity for Cl^– with <i>K</i>_a = 1.70 × 10³ and 5.62 × 10⁴ M^–1, respectively. Crystallographic studies for the Cl^– and HSO₄^– complexes indicate that the <i>m</i>-xylyl-containing ligand is relatively elliptical in shape, with the two charges at ends of the major axis of the ellipse. The anions are hydrogen bonded with the macrocycle but are outside the ligand cavity. In the solid state, an unusual low-barrier hydrogen bond (LBHB) was discovered between two of the macrocycle’s carbonyl oxygen atoms in the HSO₄^– complex. The pyridine-containing macrocycle folds so that the two pyridine units are face-to-face. The two I^– ions are chelated to the two amides adjacent to a given pyridine. In the structure of the thiophene containing macrocycle with two BPh₄^– counterions, virtually no interaction was observed crystallographically between the macrocycle and the bulky anions