Influence of Charge on Anion Receptivity in Amide-Based Macrocycles

Binding and structural aspects of anions with tetraamido/diquaternized
diamino macrocyclic receptors containing m-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 m-xylyl
or thiophene spacers. The m-xylyl- and pyridine-containing
receptors exhibit high selectivity for H2PO4– in DMSO-d6 with association constants, Ka = 1.09 × 104 and >105 M–1, respectively, and moderate selectivity for Cl– with Ka = 1.70 × 103 and 5.62 × 104 M–1, respectively.
Crystallographic studies for the Cl– and HSO4– complexes indicate that the m-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.