Synthetic ionophores .13. Pyridine-diamide-diester receptors: Remarkable effect of amide substituents on molecular organization and Ag+ selectivity

The N-Py... HNamide hydrogen bonding within the macrocyclic cavities in 9, 10, and 13 invokes their symmetrical electron-deficient structures (H-1 NMR) and consequently bind with water. This results in their poor ionophore characters. The steric requirement of methyl/bentyl substituents on am ide N in 11 and 12 takes the substituents out of the cavity and thus positions the amide 0 toward the cavity (H-1, C-13 NMR and X-ray analysis). This arrangement of two pyridine N and two amide 0 (C-13 NMR, IR) binding sites provides an appropriate environment for selective binding toward Ag+ over Pb2+, Tl+, alkali, and alkaline earth cations. The increased spacer length in 14 leads to a lop-sided twist of pyridine rings (X-ray) and disturbs the above arrangement and leads to its poor binding character.