Calorimetric studies on the complexation of several ferrocene derivatives by α- and β-cyclodextrin. Effects of urea on the thermodynamic parameters

The complexation of several alkyldimethyl(ferrocenylmethyl)ammonium cations, where the alkyl group is -CH₃ (1⁺), -(CH₂)₆CH₃ (2⁺), or -(CH₂)₇COOH (3⁺), by the hosts α- and β-cyclodextrin (α- and β-CD) was investigated in aqueous media using calorimetric and ¹H NMR spectroscopic measurements. The calorimetric results indicate that the complexation of all the ferrocene-containing guests is enthalpically driven. β-Cyclodextrin binds all the guests primarily by including the ferrocenyl groups into its cavity. By contrast, α-cyclodextrin binds guests 2⁺ and 3⁺ by threading the aliphatic chains of these guests through its cavity. The zwitterionic, deprotonated form (3) of the latter guest is bound by partial inclusion of the ferrocenyl group, a mechanism similar to that prevalent in the complexation of 1⁺. The complexation processes of 1⁺ by β-CD and 2⁺ by α-CD were also investigated in water-urea mixtures. Our calorimetric data reveal that large concentrations of urea cause a substantial decrease in the binding constant for the complexation process. The presence of urea considerably diminishes the enthalpic stabilization of both complexes.