Interplay of redox potential and pK[a] in aminoferrocene


                         Jakub Věžník^a,b, Libuše Trnková^a Karel Lacina^b

                ^a Department of Chemistry, Faculty of Science, Masaryk University

             ^b CEITEC - Central European Institute of Technology, Masaryk University

                                      Kamenice 5, 625 00 Brno

                                       jveznik@mail.muni.cz


Due to the simplicity of the redox reactions of Fe^2+/3+ ions, ferrocenes are popular for
electrochemical measurements (calibration, testing, sensing). While ferrocene itself is rather
nonpolar for direct use in aqueous solvents, its many derivatives (aminoferrocene, ferroceneboronic
acid etc.) are soluble enough to be of use in aqueous solvents.

Similarly to aniline, aminoferrocene is slightly acidic (pK[a][ ]= 5.9) due to conjugation of amino
group with the cyclopentadienyl ring. The acidity of the compound is strongly dependent on the
oxidation state of the Fe^2+/3+ ion, as oxidation to Fe^3+ lowers pK[a][ ]to 3.0. This interplay
between redox potential and pK[a] leads to interesting electrochemical behavior, as can be seen in
the figure. Different behavior is observed in buffered and unbuffered solutions. In that case,
aminoferrocene has the ability to locally change pH upon oxidation, which leads to appearance of
two distinct forms in cyclic voltammetry. Next to possible use for pH sensing, this system could be
used for reversible electrochemical switching of pH.

In our contribution we focused mostly on how electrochemical behavior can be used to predict pK[a][
]of oxidized form of aminoferrocene, utilizing measurement in both buffered and unbuffered
solutions and different electrode materials (gold, platinum, boron doped diamond).


Figure: Cyclic voltammetry of aminoferrocene in A) unbuffered (0.1 M KCl) and B) buffered solution
(0.1 M acetate). Gold disk (0.5 mm) electrode, platinum wire and calomel electrode (3 M KCl) were
used as working, counter and reference electrode, respectively.


Acknowledgements

This research has been financially supported by the project CEITEC 2020 (LQ1601), MUNI/A/1359/2018
and by the Czech Science Foundation, grant nr. 19-16273Y.