Electron-reservoir applications of ferrocenes and other late transition-metal sandwich complexes: Flow batteries, sensing, catalysis, and biomedicine

This review presents the applications of first-row late transition-metal sandwich complexes, including derivatives of ferrocene, FeCp₂ (Cp = η⁵-C₅H₅), and of salts of cobalticenium [CoCp₂]⁺ and mixed sandwich complexes [FeCp(η⁶-arene)]⁺ as molecular or macromolecular electron reservoirs. Forty-five years ago, the concept of electron reservoirs was proposed for the permethylated 19-electron d⁷ Fe(I) sandwich complexes, in particular [FeCp(η⁶-C₆Me₆)] and [FeCp*(η⁶-C₆Me₆)] (Cp* = C₅Me₅), because they were the electron richest neutral molecules known, given their extremely low ionization potentials, and their ability to stoichiometrically or catalytically transfer one electron to a variety of organic and inorganic substrates. These processes occurred without any breakdown of their molecular structure, so that the cationic 18-electron d⁶ structure was easily recovered. Thus, in this concept, the robustness of both redox forms of the electron-reservoir redox couple is essential. Later, the concept was extended to isolobal sandwich molecules with various numbers of methyl substituents on the ligands in both the [CoCp₂]⁺ and [FeCp(η⁶-arene)]⁺ salts in order to cover a wide range of redox potentials. It was also extended to electron hole reservoirs with the 17-electron d⁵ cations FeCp₂⁺, FeCp*₂⁺ and [FeCp*(η⁵-C₆Me₆)]²⁺. Along the time, the structural flexibility of these Fe and Co sandwich complexes, bringing about modulation of the redox potentials of the redox systems, enriched the concept and its applications. The latter are summarized here including examples of stoichiometric reactions with monometallic complexes and dendritic metalla-macromolecules, redox-flow metallocene batteries and redox sensing. Catalytic applications include redox catalysis, electrocatalysis, i.e. electron-transfer-chain (ETC) catalysis, and Fenton reaction in nanomedicine of ferrocenes, ferrocene dendrimers and metal-organic-framework (MOF)-ferrocenes, utilized in particular as ferroptosis media. Concluding remarks outline key aspects of applications and offer perspectives.