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Part 6: Redox Potential

This is a basic introduction to the redox potential in electrochemistry.

The topics are listed below:

Redox Potential II: About Ferrocene

Laboratory Of Research & Development, BAS Inc.
Professor Noriyuki Watanabe

In previous article, we discussed how the oxidation-reduction potential changes when the hydrogen of the cyclopentadienyl group of ferrocene was substituted by an acetyl group or a methyl group. Thus, in this article we will talk about ferrocene.

Ferrocene is a stable compound (yellow-orange), and the ferrocenium cation (blue) which undergoes single electron oxidation is also stable. The electron transfer rate between them is extremely fast, and it is a chemically and electrochemically reversible redox system.

Ferrocene is an important compound, not only can be recommended as an internal s redox potential standard in organic solvent system, but also can be used to synthesize various derivatives (e.g. bisferrocene, fulvalene, dendrimer, etc.) and for multiple applications (e.g. catalysts, sensors, media, Molecular labels, etc.).

A compound of which the central iron element replaced by other transition metal element is called metallocene, and it is useful as a reaction intermediate or a catalyst. Ferrocene was found independently in several places around 1951 and shortly after controversy on its structure, Wilkinson et al. determined its sandwich structure7-1), that iron element was pinched by cyclopentadiene group from the upper and lower sides, this discovery pioneered prosperity of organometallic compounds today.

That was an era of polarography using a mercury electrode. The redox potential was measured by using perchlorate (0.31 V vs. SCE) supporting electrolyte in a water-alcohol mixed solvent with a mercury drop working electrode7-2). It was fraudulent that redox potential could be measured well in the potential domain where the anode dissolution of the mercury electrode itself happening.

The accurate redox potential measurement in pure organic solvent systems was carried out in 1959 7-3). This research work was done by three graduate students from University of Kansas, during a short break in Easter holidays. Led by Ted Khuane, and the other two were graduate students from different research labs of the department of organic chemistry.

They two persons were responsible for the synthesis and purification of ferrocene and several ferrocene derivatives, ruthenocene and osmocene. At that time, cyclic voltammetry was not popular as it is now, and Kuwana adopted Chronopotentiometry (CP) which recorded the change of the working electrode potential versus time by applying a constant current flow. Although CP is not often used today, no need for the electrode potential feedback control, and once the electrode potential could be recorded correctly, it is a sufficiently simple method. The redox potential can be obtained from the transition time of CP.

At the beginning, Kuwana used a mercury electrode, however because of the too large background current flow, a platinum electrode was used as an alternative working electrode. At that time, his supervisor was Ralph Adams, a well-known pioneer of solid electrodes research work, replacing the working electrode with a platinum electrode was a natural thing.

Interestingly, there was no name of supervisor in Kuwana's paper. The details including this anecdote can be found in Bill Geiger's reviews 7-4). Recently, I (Watanabe) emailed Kuwana and received the reply from him. A partial of the reply was cited as “We did most of the work over Easter holiday. The faculty graciously let us publish without their names on the papers. Generous and unusual, needless to say.”. It was also an interesting paper that revealed the importance of the redox potential of the ferrocene could be controlled by substituents.

7-1) Wilkinson,G., J.Am.Chem.Soc. 74, 6146, & 6148, (1952)
7-2) Page,J.A., Wilkinson,G. J.Am.Chem.Soc., 74,6149,(1952)
7-3) Kuwana,T.,Bublitz,D.E., Hoh,G. J.Am.Chem.Soc., 82, 5811, (1960)
7-4) Geiger W.E., Organometallics 26, 5738 (2007)