Ho₂O@D₃(85)-C₉₂: Highly Stretched Cluster Dictated by a Giant Cage and Unexplored Isomerization

For endohedral metallofullerenes (EMFs), it has been well established that the cage shape and size should match those of the endohedral cluster. As a result, sufficient cluster–cage interaction can be achieved, which is essential for mutual stabilization. Nevertheless, how a small endohedral cluster nests in a giant fullerene has been less explored. Herein, we report a pair of large oxide-cluster fullerene (OCF) isomers, denoted as Ho₂O@C₉₂-I and -II. Crystallographic studies reveal that major isomer-I possesses a D₃(85)-C₉₂ cage with a highly stretched Ho₂O cluster inside, which contributes to achieving regular metal–cage contacts. Density functional theory (DFT) computations also reveal the predominant abundance of the D₃(85) isomer relative to the other two possible minor species including C₁(67) and C₂(64) isomers. Moreover, electrochemical (EC) studies verify that the isomers exhibit almost identical redox behaviors, indicating their similar cage structures. On the basis of the remarkable topological similarity of D₃(85) and C₁(67) isomers, isomer-II is likely to be Ho₂O@C₁(67)-C₉₂, though it remains to be confirmed. Our studies thus provide new insights into the cage–cluster interplay and cage isomerization, both contributing to a better understanding of large EMFs.