Six-Cyclic Crown Ether-Type Pillar[5]Arene: Enhanced Binding Ability to Bispyridinium Derivatives

A six-cyclic crown ether-type pillar[5]arene was synthesized, and the five ethylene oxide loops were located outside the cavity and not affected by temperature changes which was confirmed by variable-temperature NMR experiment in DMSO-d6 and CDCl₃ and 2D ¹H-¹H NOESY experiment in CDCl₃. The six-cyclic pillar[5]-crown also showed greater binding ability of host-guest with bis(pyridinium) derivatives than conventional alkoxy pillar[5]arenes that illustrated through ¹H NMR titration spectroscopic experiment in acetone-d6/CDCl₃ (1 : 1) and UV-vis titration experiments in CHCl₃ at room temperature. The five benzocrown ethers at the periphery were able to bind metal cations by ¹H NMR titration spectroscopic experiment in CD₂Cl₂/methanol-d4(9 : 1).