Click synthesis of triazole-linked calix[4]arene ionophores. Potentiometric and ESI-MS screening of ion-selectivity

A series of 1,2,3-triazole-linked calix4]arene ionophores comprised of different O-donor groups (OH, COOEt, CONEt₂) attached either to the lower rim of calix or to the triazole moieties were synthesized to explore their ion-selectivity for the first time in potentiometric transduction. Plasticized PVC membrane electrodes (ISEs) were fabricated, and their potentiometric selectivities were measured toward a series of mono- and divalent metal ions. Structure-ion-selectivity relationship and the structural requirements of the coordination sphere for selective binding were established. ISEs made of calix4]arene-bis-triazoles were found generally to exhibit distinct Ag⁺ selectivity in the order 3 ≈ 4 > 2 > 1, indicating the beneficial effect of the carboxamide or ester groups in the complexing site. In contrast, calix4]arene-tetratriazole 5 comprised only sp ² N-donor atoms displayed excellent Cu²⁺ selectivity over a series of alkali-, alkaline earth- and transition metal ions. A unique feature of the outstanding Ag⁺ selective electrodes made of 3 and 4 was recognized and suggested their potential application as Na⁺ ISEs in systems not containing silver ions. Further, the potential use of competitive ESI-MS experiments for screening the binding affinities of ionophores 3–6 to different metal ions was also demonstrated.