Inclusion complexes of ferrocenes and β-cyclodextrins. Critical appraisal of the electrochemical evaluation of formation constants

We report the results of a systematic electrochemical study of the host-guest supramolecular adducts between ferrocene (Fc), ferrocenium cation (Fc⁺), and other mono- and disubstituted ferrocene derivatives with different β-cyclodextrins (CD) in mixed organic-aqueous media. The influence on the formation constants (K_f) of the organic cosolvent, the different substituents on Fc, and the type of CDs are evaluated. NMR and conductometry responses of ferrocenium cation solutions in the presence of CD confirm the weak propensity of Fc⁺ to enter into the cyclodextrin cavity. The K_f value generally decreases as the steric bulk and the rigidity of Fc substituents increases, consistent with an inclusion model in which the Fc fits into the CD cavity in an axial mode while the substituent protrudes out. Interestingly, the addition of sulfated β-CD shifts the redox Fc/Fc⁺ couple toward cathodic values, indicating that the oxidized, cationic form Fc⁺ is more strongly bound to the sulfated cyclodextrin than neutral Fc, probably by means of electrostatic interaction with the external -SO₃^- functionalities.