Anion induced binding electrochemical signal transduction in ferrocenyl benzolimidazolium podands

Abstract

Benzene-based bipodal and tripodal electrochemical-responsive receptors with ferrocenyl arms comprising benzoimidazolium moieties were synthesized and structurally characterized. The two-arm para compound 1·2Br exhibited a cisoid conformation with the two ferrocene moieties positioned in the same side and a templating bromide anion sited inside. The cisoid form was stabilized not only by the (C–H)⁺⋯Br⁻ hydrogen bond, but also cooperated by the C–H⋯Br⁻ hydrogen bonding related to the Cp rings of the ferrocenyl moieties and the CH₂ groups, respectively. The association constants of the receptor 1 for Cl⁻, Br⁻ and I⁻ were determined using ¹H NMR titration in d₆-DMSO as 600 M⁻¹, 440 M⁻¹ and 350 M⁻¹, respectively. The meta three-arm compound 2·3Br adopted a cone conformation with a bromide anion sited inside the molecular cavity and three ferrocene-containing arms positioned on the same side. The cationic heterocycle interacted with the bromide anion through the three-fold C–H⋯Br⁻ hydrogen bonding. ¹H NMR titrations in d₆ DMSO revealed that the association constants of 2 for Cl⁻, Br⁻ and I⁻ were 9.7 × 10³ M⁻¹, 6.8 × 10³ M⁻¹ and 120 M⁻¹, respectively. Electrochemical measurements demonstrated that directed hydrogen bonding between the ferrocenyl moiety and the anion was important for the two-arm receptor 1 exhibiting more efficient binding electrochemical signal transduction.