CALORIMETRIC STUDIES ON THE COMPLEXATION OF SEVERAL FERROCENE DERIVATIVES BY ALPHA-CYCLODEXTRIN AND BETA-CYCLODEXTRIN - EFFECTS OF UREA ON THE THERMODYNAMIC PARAMETERS

The complexation of several alkyldimethyl(ferrocenylmethyl)ammonium cations, where the alkyl group is -CH3 (1(+)), -(CH2)(6)CH3 (2(+)), or -(CH2)(7)COOH (3(+)), by the hosts alpha- and beta-cyclodextrin (alpha- and beta-CD) was investigated in aqueous media using calorimetric and H-1 NMR spectroscopic measurements. The calorimetric results indicate that the complexation of all the ferrocene-containing guests is enthalpically driven. B-Cyclodextrin binds all the guests primarily by including the ferrocenyl groups into its cavity. By contrast, cx-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 beta-CD and 2(+) by alpha-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.