The observation of ion-pairing effect based on substituent effect of ferrocene derivatives

The ion-pairing effect was investigated based on the substituent effect of ferrocene (Fc) derivatives using cyclic voltammetry. It was shown that the presence of ion-pairing strongly affected the electrochemical redox behavior in the organic solvent. The formal redox potential (E^0′, the average of anodic and cathodic peak potential) shifted negatively with the increasing ion-pairing effect. That was because the formation of ion pair (Fc⁺·ClO₄^-) was beneficial to equilibrium shift from Fc to Fc⁺ in thermodynamics. In this work, electron-donating and electron-withdrawing substituents of ferrocene derivatives were employed for a deep study of ion-pairing effect, respectively. It is confirmed that both ion-pairing effect and electron-donating substituent effect facilitated the negative shift of E^0′ for ferrocene derivatives, showing the positive cooperativity. While the electron-withdrawing substituent effect resulted in the positive shift of E^0′ for ferrocene derivatives and was unfavorable for the oxidation of Fc derivatives, reflecting the negative cooperativity with ion-pairing effect. In addition, the reversal phenomenon of weak electron-withdrawing substituent was revealed when the ion-pairing effect was stronger than the electron-withdrawing substituent effect, indicating that the ion-pairing function has a significant effect on electrochemical behavior of ferrocene derivatives.