Cooperative Binding by a Bicyclic Calixpyrrole

Significance

The three dipyrrylpyridine binding sites of cryptand-like bicyclic hexapyrrole 3a exhibit positive cooperativity in binding carboxylic acids, as evidenced by UV-Vis and ¹H NMR titration. X-ray crystallography of the 3a·(Cl₂CHCO₂H)₃ complex suggests that binding of the first carboxylic acid increases the basicity of the pyridine of one of the other binding clefts and increases the acidity of the pyrrole of the remaining cleft. The second carboxylic acid to coordinate further increases pyrrole acidity and pyridine basicity, facilitating binding of the third carboxylic acid ligand.