(Invited) Crystallographic Characterization of U@C2 n (2n = 82−86): Insights about Metal-Cage Interactions for Mono-Metallofullerenes

Abstract

Endohedral mono-metallofullerenes are the prototypes to understand the fundamental nature and the unique interactions between the encapsulated metals and the fullerene cages. Recently, we succeeded in the synthesis, isolation, and characterization of four new U-based mono-metallofullerenes, including U@Cs (6)-C82, U@C 2(8)-C84, U@Cs (15)-C84, and U@C 1(12)-C86. Furthermore, a new finding concerning the metal positions inside the cages reveals that the encapsulated metal ions are always located on symmetry planes of the fullerene cages, as long as the fullerene cages possess mirror planes. Symmetrical patterns, such as indacene, sumanene, and phenalene, and charge transfer are found to determine the metal positions inside the fullerene cages. DFT calculations support that the metal-fullerene motif interaction determines the stability of the metal position. In fullerenes containing symmetry planes, the metal prefers to occupy a symmetrical arrangement with respect to the interacting motifs, which share one of their symmetry planes with the fullerene. This finding provides new insights into the nature of metal-cage interactions and gives new guidelines for structural determinations using crystallographic and theoretical methods.