Hydrogen bonding between carboxylic acids and amide-based macrocycles in their host-guest complexes

For 12- and 13-membered macrocycles in which two amide linkages are integrated in the macrocyclic ring systems, the formation of 1:1 host-guest complexes with acetic and benzoic acids has been confirmed by NMR titrations. The complex formation occurs with the formation constants of 8-27M-1, under competition with the dimerisation of acid molecules. Benzoic acid tends to form more stable complexes than acetic acid. The binding force is due to a pair of hydrogen bonds, Ocarboxyl-H⋯O=Camide and C=O carboxyl⋯H-Namide, between the carboxyl group of a guest molecule and the amide group of a host molecule. The former bond is stronger than the latter, and defines the stability of the complexes. The formation of the pair of hydrogen bonds is accompanied by the conformational conversion of the amide group from the trans-form to the cis-form. The influence of such a conversion on the internal molecular motion is observed as a slight broadening of signal width.