CONTROLLING THE ACIDITY OF THE CARBOXYLIC GROUP BY A FERROCENE BASED REDOX SWITCH

The pH dependent oxidation behaviour of the ferrocene subunit in ferrocenylcarboxylic acids of formula Fc(CH,),COOH (n=0, 1, 2) has been investigated through differential pulse voltammetry experiments in 4:1 acetonitrile/water solutions. Different voltammetric responses have been obtained depending whether the pH has been varied through a buffer (a single peak, whose E(1/2) value decreases with the increasing pH) or through an acid-base titration (two distinct peaks, the less anodic corresponding to the oxidation of the FcCOO(-) species, the more anodic corresponding to the oxidation of the FcCOOH species, whose relative intensities vary with the pH). The electrochemically determined pK(a) values for the Fc(CH2)(n)COOH and [Fc(+)](CH2)(n)COOH forms indicate that the Fc-to-Fc(+) oxidation process makes the acidity of the proximate carboxylic group increase. Such a switching effect, electrostatic in nature, decreases as n increases.