Metal Composition Controls the Confined Reorientation of Nitride Clusters in Endohedral Metallofullerenes

The composition of encapsulated metal clusters plays a critical role in determining their confined motion within the fullerene cages. Here, we investigate the temperature-dependent structural behavior of mixed-metal MSc2N@C80 (M = Nd, Ho) in NiOEP (Ni(II) octaethylporphine) cocrystals using variable-temperature single-crystal X-ray diffraction. Despite the distinct temperature-dependent behavior of the C80 cages, both MSc2N clusters exhibit similar highly restricted librational reorientation upon heating. This behavior contrasts with the larger-amplitude spinning-top-like and tumbling-like reorientations observed for analogous M3N@C80 clusters with different metal compositions (Sc3N, M2ScN, and M3N, where M is a lanthanide). The results demonstrate that the metal composition and distribution within the encapsulated cluster dictate the degree of restricted librational reorientation, providing new structural insight into the interplay between metal composition, cage dynamics, and confined molecular motion in endohedral metallofullerenes.