Electron transfer ferrocene-ferricenium systems

Reversible one-electron oxidation of ferrocene and its derivatives to cation radicals (so-called ferricenium cations) is a well-known reaction. The cation radical center is localized at the iron atom. In contrast to this statement, the hole transfers though conjugated systems were proven for the bis(ferrocenyl) ethylene cation radical (Delgado-Pena et al. 1983) and the cation radical of bis(fulvaleneiron) (LeVanda et al. 1976). Scheme 1-53 depicts these structures. 

The results presented in the following three reports are on systems in which the electron transfer step to Ce(IV) is rapid and the subsequent chemistry is complex. In Holecek et al. (1979) the first step in the ceric oxidation of ferrocene produces unstable ferricenium cations which subsequently decompose with oxidation of the ligand. In Soria et al. (1980) and Chum and Helene (1980) the initial electron transfer process results in both metal-centered oxidation of the tris(2-pyridinial-a-methyl-(methylimine))-Fe(II) complex as well as oxidation of the a-methyl group to an aldehyde group, with no change in the oxidation state of iron. 

Photochemical studies of iron-group metallocene substrates have received much attention, e.g. quenching of excited uranyl ion luminescence, formation of charge-transfer complexes with TCNE, redox reactions of octamethyl-ferrocene and of carboxylate anions of the types FcCOj and Fc(X)COr (X = saturated or unsaturated alkyl chain). Enzyme-catalysed one-electron oxidation of ferrocene derivatives has also been studied. Ferrocene-ferricenium cation redox systems have been incorporated into electrochemical and photo-electrochemical cells, and have found use for catalysis of photo-addition of MeOH to Bu NCO. Inter-valence electron-transfer properties of partially oxidized Fc(C C)nFc ( = 0—2), meso-tetraferrocenylporphyrin, and poly(vinylferrocene) have been assessed. 

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