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

Abstract

The N_Py…HN_amide hydrogen bonding within the macrocyclic cavities in 9, 10, and 13 invokes their symmetrical electron-deficient structures (¹H NMR) and consequently bind with water. This results in their poor ionophore characters. The steric requirement of methyl/benzyl substituents on amide N in 11 and 12 takes the substituents out of the cavity and thus positions the amide O toward the cavity (¹H, ¹³C NMR and X-ray analysis). This arrangement of two pyridine N and two amide O (¹³C 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.