The oxidation of ferrocenecarboxylic acid in the presence of β-cyclodextrin (β-CD) has been investigated by cyclic voltammetry in pH 9.2 aqueous solution in which the ferrocene is present as the anion, FCA-. Binding of FCA- by β-CD is indicated by changes in the UV-vis absorption spectrum and the appearance of induced circular dichroism. The latter data also show that FCA- is complexed by β-CD in a configuration with the Cp-Fe-Cp axis parallel to the axis of the β-CD cavity. In slow scan rate cyclic voltammetry, β-CD causes a decrease in the peak current and a positive shift in the peak potential for FCA- oxidation. Each of these effects was quantitatively evaluated, giving a formation constant for FCA--β-CD of 2200 M-1 at 20 °C and 1H =-2.8 kcal/mol, ΔS = 5 cal/(K·mol). No binding of the oxidized form of FCA- by β-CD could be detected. Fast scan rate studies revealed that the inclusion complex is oxidized via a CE scheme in which the complex first dissociates to FCA- and β-CD followed by oxidation of the free FCA-. Quantitative evaluation of the data gave 2.1 X 104s-1 for the dissociation rate constant, and variable temperature studies yielded an activation enthalpy for dissociation of 15 kcal/mol. Significantly, no direct oxidation of FCA--β-CD at the electrode could be detected. Possible explanations are discussed, and the significance of the present results to earlier electrochemical synthetic studies with cyclodextrins is highlighted.
ASJC Scopus subject areas
- Colloid and Surface Chemistry