Aminoferrocene:Determination of the acidity of unstable compounds


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Year of publication 2019
Type Article in Proceedings
Conference XIX. Workshop of Biophysical Chemists and Electrochemists
MU Faculty or unit

Faculty of Science

Keywords aminoferrocene; electrochemistry; gold electrode; platinum electrode; glassy carbon electrode; boron doped diamond electrode; pKa constants
Description Ferrocenes are quite popular in electrochemistry, due to the simplicity of redox reactions of iron core. While ferrocene is nonpolar and thus of limited use, its cyclopentadienyl ring opens way for many interesting modifications. One of which is aminoferrocene (FcNH2), the -NH2 group increases solubility in polar solvents, introduces chemical reactivity and most importantly pH sensitivity to the ferrocene complex. Due to the proximity of NH2 group and Fe2+ ion in FcNH2, the redox potential of FcNH2 is influenced by protonation and vice versa the Fe3+ ion changes acidity of the complex upon oxidation. Electrochemical observation of this interplay can be used to deduce the acidity of the Fe3+ complex. Such approach is useful for complexes that are unstable in their oxidized form. pKa constants determined using electrochemical methods are obtained through kinetic processes, therefore great care needs to be taken when interpreting these results. While the limiting redox potential for FcNH2 is easily obtained from electrochemical results (Figure), the limiting potential for FcNH3+ strongly varies depending on the type of electrode used. This is anomalous from other ferrocene derivates, namely ferroceneboronic acid [1] and ferrocenecarboxylic acid [2], where both limiting redox potentials are easily obtainable. In our contribution, we tried to uncover the correct pKa value of aminoferrocene through different electrochemical methods both in buffered and unbuffered solutions. Spectrophotometry was used along to validate our findings.
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