Polysubstituted ferrocenes as tunable redox mediators.

A series of four ferrocenyl ester compounds, 1-methoxycarbonyl- (), 1,1'-bis(methoxycarbonyl)- (), 1,1',3-tris(methoxycarbonyl)- () and 1,1',3,3'-tetrakis(methoxycarbonyl)ferrocene (), has been studied with respect to their potential use as redox mediators. The impact of the number and position of ester groups present in - on the electrochemical potential is correlated with the sum of Hammett constants. The / -/ redox couples are chemically stable under the conditions of electrolysis as demonstrated by IR and UV-vis spectroelectrochemical methods. The energies of the C=O stretching vibrations of the ester moieties and the energies of the UV-vis absorptions of - and - correlate with the number of ester groups. Paramagnetic H NMR redox titration experiments give access to the chemical shifts of - and underline the fast electron self-exchange of the ferrocene/ferrocenium redox couples, required for rapid redox mediation in organic electrosynthesis.