Transition-Metal and Rare-Earth-Metal Redox Couples inside Carbon Cages: Fullerenes Acting as Innocent Ligands

Abstract

In endohedral metallofullerenes (EMFs), the carbon cage shields the endohedral species from the surrounding environment and can stabilize unusual clusters that otherwise would not exist. This review is focused on the behavior of the metal/π-system interface in EMFs under electron transfer conditions. We show that the stabilizing role of the fullerene cage can be extended from unusual clusters to the peculiar spin and charge states obtained via endohedral electron transfer. For such redox processes, the role of the fullerene cage can be understood as that of an innocent ligand. This review is specifically focused on four groups of EMFs with different kinds of endohedral redox activity: (i) dimetallofullerenes, (ii) Sc3N@C80 and its derivatives, (iii) titanium-based EMFs, and (iv) cerium-based nitride clusterfullerenes. Frontier orbitals of dimetallofullerenes usually have pronounced metal–metal bonding character, and therefore electron transfer affects the metal–metal bonding character in such molecules. The LUMO of Sc3N@C80 is equally delocalized over three Sc atoms, resulting in a Sc3N-based reduction, whose mechanism can be modified by exohedral derivatization. Titanium is a rare example of a transition metal that can be encapsulated within fullerenes, and we discuss how its valence state in Ti-EMFs can be tuned via electrochemical reactions. Cerium exhibits endohedral redox activity in many nitride clusterfullerenes, allowing for the redox potential of the strain-driven Ce(IV)/Ce(III) redox couple to be tuned by varying the composition of the endohedral cluster and the size of the carbon cage. A discussion of the redox behavior of these EMFs is accompanied by an analysis of their electronic structure and a discussion of their spectroelectrochemical studies.