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.
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Bai et al. (2005) studied this question.