Electrochemistry of fullerene/transition metal complexes: Three decades of progress

This review is focused on the electrochemical properties of fullerene complexes of transition metals. In the first part of the review, the coordination properties of fullerenes are briefly overviewed. Metal complexes (polypyridyl complexes of transition metals, metallocenes, metalloporphyrins) that are covalently attached to the fullerene cage through the linkers are also briefly described. The η2-C60 complexes of transition metals exhibit electrochemical activity related to the fullerene cage reduction and metal center oxidation. Upon reduction and oxidation, η2-C60 complexes of transition metals are usually unstable and the metal-fullerene bond is cleaved. More stable electrochemical behavior was reported for η2-C60 complexes of transition metal clusters. Fullerene moieties bonded into dimers through metal clusters can communicate electronically between themselves. The η2-C60 coordination is also responsible for the formation of electrochemically active fullerene coordination polymers. These macromolecular systems show electrochemical activity at negative potentials and a n-doped properties. The metal-fullerene bond in polymers is much more stable under electroreduction conditions in comparison to the η2-C60 complexes. The η5-C60 complexes with a half-sandwich or sandwich structure also exhibit electrochemical activity in negative potential range related to the ferrocene cage reduction and in positive potentials due to the metal center oxidation. In contrast to η2-fullerene complexes, the η5-fullerene complexes of transition metals are much more robust under electrochemical conditions. The electrochemical properties of transition metal complexes in which the metal center is coordinated to the chelating ligand covalently linked to the fullerene moiety are important for understanding the photochemical performance of these systems. The electrochemical behavior of these complexes are usually combination of electrochemical properties of formed dyads and triads. The electronic communication between redox sites in the ground state depends on the complex geometry, distance between electrochemically active centers, and the nature of the linker.