Trapping Metallic Oxide Clusters inside Fullerene Cages

Abstract

The sub-nanometer sized void inside a fullerene cage permits the accommodation of a single atom, atomic cluster, or even small molecule, resulting in the formation of endohedral fullerenes. Particularly, clusterfullerenes can be formed by encapsulating multiple metallic ions in most cases along with nonmetal ions (i.e., N^3-, C₂^2-, S^2-, O^2-) inside the fullerene cage. Such an association makes clusterfullerene more functional than empty fullerenes and conventional mono-metallofullerenes. To date, a variety of clusterfullerenes have been reported, including metal nitrides, carbides, oxides, sulfides, cyanides, and so on. Among them, oxide clusterfullerenes (OCFs) can contain variable oxide clusters (i.e., M₄O₂, M₄O₃, M₃O, and M₂O; M = Sc or other metal), yielding one of the most versatile families. Thus, OCFs may provide a more convenient platform for developing new functional molecules and for studying previously less-explored topics such as formation mechanisms of clusterfullerenes. In this Account, we review recent progress in the field of OCFs, including their synthesis, isolation, and structural and electrochemical studies as well as the preliminary exploration into their potential functions and applications. Thanks to the concrete crystallographic results of an OCF series, we can track the transition of endohedral cluster and fullerene cage. It is suggested that the configuration and internal dynamics of the oxide cluster are highly dependent on not only the cage size but also cage structure. On the other hand, based on the experimental observations, two competitive transformation pathways are established for the majority of OCFs, verifying the bottom-up or top-down formation mechanism. It is also found that the redox behaviors of OCFs are more or less comparable to their isoelectronic species with common cage structure and similar cluster geometry but varied greatly with the cluster variety (i.e., Sc₂O vs Sc₄O_2-3). The mechanism behind such phenomena has been discussed. In addition, the potential of Dy-based OCFs as single molecular magnets (SMMs) is presented theoretically. Nevertheless, experimental advance remains to be achieved.