Polysubstituted ferrocenes as tunable redox mediators

Abstract

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 E 1/2 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 1 H 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.