Structure and stability of cyclodextrin inclusion complexes with the ferrocenium cation in aqueous solution: 1H NMR studies

The interaction of the ferrocenium cation with cyclodextrins has been studied in aqueous and D2O solutions by 1H NMR spectroscopy. The 1:1 binding constants of the β- and γ-cyclodextrin complexes are 15 ± 3 and 18 ± 4 dm3 mol-1, respectively, whereas the binding of α-cyclodextrin is very weak. Although the values for the β- and γ-cyclodextrin complexes are very similar, the guest penetrates the cavity of the latter more deeply than that of the former. The results are compared with literature values of binding constants of ferrocene and substituted ferrocene and ferrocenium complexes of cyclodextrins in terms of the relative abilities of the different cyclodextrins to stabilise the Fe(II) oxidation state with respect to Fe(III).