Interplay of redox potential and pKa in aminoferrocene

Due to the simplicity of the redox reactions of Fe2+/3+ ions, ferrocenes are popular for electrochemical measurements. While ferrocene itself is rather nonpolar for direct use in aqueous solvents, it’s many derivatives (aminoferrocene, ferroceneboronic acid etc.) are soluble enough to be of use in aqueous solvents. However, improved solubility is not the only change to the ferrocene properties, due to introduction of protonizable group, pH sensitive behavior is expected. The pH-dependent behavior is not completely straightforward as one could expect, electrochemical behavior drastically differs depending on the presence of buffer in solution (figure). On the other hand, not only pH affects the electrochemical behavior, also the electrochemical processes can affect pH-dependent behavior - the pKa of the reduced/oxidized molecules. So, deducing the mechanism behind the effect of buffer hints at very interesting interplay between the pH sensitive group and the redox state Fe2+/3+ ions, where one could be controlled by the state of the other, leading to the possibility of pH/redox switches.