Ho₂O@C₇₄: Ho₂O Cluster Expands within a Small Non-IPR Fullerene Cage of C₂(13333)-C₇₄

Steering the cluster configuration inside a fullerene cage has been one of most interesting topics in the field of fullerenes, since the physical property of a cluster fullerene may be modified accordingly. It has been well-recognized that the cluster configuration can be tuned via altering the cage size. Typically, the carbide cluster and the oxide cluster are experimentally seen to be curled up within a small fullerene cage whereas they are expanded in a large cage. In this work, a new oxide cluster fullerene Ho₂O@ C₂(13333)-C₇₄ is prepared and isolated. The single-crystal X-ray diffraction (XRD) study reveals that the Ho₂O cluster, however, expands within the small non-IPR cage of C₂(13333)-C₇₄ with a Ho-O-Ho angle of >170°, indicating that cluster configuration is highly related to the cage shape and cage structure as well. The DFT computation demonstrates that the cluster-to-cage electron-transfer obviously enhances the aromaticity of the motif containing the fused-pentagon pair and hence stabilizes the non-IPR cage of C₂(13333)-C₇₄. In addition, the electrochemical and magnetic properties of Ho₂O@ C₂(13333)-C₇₄ are studied to further investigate the effect of endohedral Ho₂O cluster.