Polysubstituted ferrocenes as tunable redox mediators.

A series of four ferrocenyl ester compounds, 1-methoxycarbonyl- (1), 1,1'-bis(methoxycarbonyl)- (2), 1,1',3-tris(methoxycarbonyl)- (3) and 1,1',3,3'-tetrakis(methoxycarbonyl)ferrocene (4), has been studied with respect to their potential use as redox mediators. The impact of the number and position of ester groups present in 1-4 on the electrochemical potential E1/2 is correlated with the sum of Hammett constants. The 1/1+ -4/4+ 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 1-4 and 1+ -4+ correlate with the number of ester groups. Paramagnetic 1H NMR redox titration experiments give access to the chemical shifts of 1+ -4+ and underline the fast electron self-exchange of the ferrocene/ferrocenium redox couples, required for rapid redox mediation in organic electrosynthesis.