Voltammetric and impedance study of the binding of ferrocene derivatives to a sulfonated calix[6]arene host in aqueous solutions

This report describes the effect of host–guest complexation on the thermodynamic and kinetic parameters of ferrocene redox reactions in aqueous media. The formal potentials of the ferrocene/ferrocenium couples examined shifted to less positive values with increasing calixarene concentration, because of the formation of a 1:1 inclusion compound. The order of binding constants was cationic ferrocene≫non-charged ferrocene>anionic ferrocene. The oxidized form with a higher positive charge was bound more strongly to the host than was the corresponding reduced form. The result was contrary to that observed for β-cyclodextrin. The inclusion compounds formed by sulfonated calix[6]arene were greatly stabilized by an electrostatic interaction between the negatively charged host and cationic guest. Fast-scan voltammetric behavior suggested that inclusion-ejection processes practically attain equilibrium in the scan-rate range below 2 V s−1. Randles plots for the Faradaic impedance of the electrode reaction virtually comprised two nearly parallel straight lines with a small separation over the frequency range of 102−10−3 Hz, indicating negligible chemical reaction impedance. As the host concentration increased, the mean diffusion coefficient of the (ferrocenylmethyl)trimethylammonium cation decreased inversely proportionally from 6.0×10−6 cm2 s−1 of the free ion to (2.1±0.2)×10−6 cm2 s−1 of the corresponding inclusion complex. The result of the impedance analysis suggests that the included guest exchanges no electrons directly with the electrode.