Endohedral Metallofullerenes Based on Spherical Ih-C80 Cage: Molecular Structures and Paramagnetic Properties

Fullerenes are carbon cages assembled from fused hexagons and pentagons that have closed networks and conjugated pi systems. The curve of the fullerene structure requires that the constituent carbon atoms take on a pyramidal shape and produces extra strain energy. However, the highly symmetrical geometry of the fullerene decreases the surface tension in these structures, so highly symmetrical fullerenes are usually very stable. For example, C-60 with icosahedral symmetry (I-h) is the most stable fullerene molecule. However, another highly symmetrical fullerene, I-h-C-80, is extremely unstable. The reason for this difference is the open-shell electronic structure of I-h-C-80, which has a 4-fold degenerate HOMO occupied by only two electrons. Predictably, once the degenerate HOMO of I-h-C-80 accepts six more electrons, it forms a dosed-shell electronic structure similar to I-h-C-60 and with comparable stability. Because the hollow structure of fullerenes can encapsulate metal atoms and those internal metals can transfer electrons to the fullerene cage, the encapsulation of metal clusters may provide an ideal technique for the stabilization of the I-h-C-80 fullerenes. In this Account, we focus on the molecular structures and paramagnetic properties of spherical I-h-C-80 endohedral fullerenes encaging a variety of metal moieties, such as metal atoms (M-n), metal nitride (M3N), metal carbide (MnC2), metal carbonitride (M3CN), and metal oxides (M4Om). We introduce several types of endohedral metallofullerenes such as Sc4C2@I-h-C-80, which exhibits a Russian-doll-like structure, and Sc3CN@I-h-C-80, which encapsulates a planar metal carbonitride duster. In addition, we emphasize the paramagnetic properties of I-h-C-80-based metallofullerenes, such as Sc3C2@I-h-C-80, Y-2@C79N, and M3N@I-h-C-80, to show how those spin-active species can present a controllable paramagnetism. This Account highlights an Inspiring molecular world within the spherical I-h-C-80 cages of various metallofullerenes.