Multiple hydrogen bond stabilization of a sandwich complex of sulfate between two macrocyclic tetraamides

A tetraamide macrocycle derived from isophthaloyl dichloride and diethylenetriamine shows extremely high affinities for phosphate and sulfate ions in CDCl3, with log K = 4.66 and 4.50, respectively. Crystallographic findings in the case of sulfate indicate an unusual sulfate sandwich complex, in which a single sulfate ion is held between two macrocyclic amides by hydrogen bonds to eight amides. Two tetra-n-butylammonium ions serve to balance the charge.