Crystallographic characterization of ErC@C(43)-C, ErC@C(40)-C, ErC@C(44)-C, and ErC@C(21)-C: the role of cage-shape on cluster configuration.

For endohedral metallofullerenes (EMFs), that is, fullerenes encapsulating metallic species, cage size is known to be an important factor for cluster configuration adoption; however, the impact of the cage shape on the cluster geometry fitting remains poorly understood. Herein, for the first time, four dierbium-carbide EMFs with C cages, namely, ErC@C(43)-C, ErC@C(40)-C, ErC@C(44)-C, and ErC@C(21)-C, were successfully synthesized and fully characterized using a combination of mass spectrometry, single-crystal X-ray diffractometry, vis-NIR, Raman and photoluminescence spectroscopies, and cyclic voltammetry. In particular, the fullerene cages of C(43)-C and C(44)-C are crystallographically identified for the first time. Interestingly, the ErEr distance of the major sites in ErC@C(43)-C, ErC@C(40)-C, ErC@C(44)-C, and ErC@C(21)-C is 3.927, 4.058, 4.172, and 4.651 Å, respectively, which increases gradually with an increase in the major axis of the cage. Moreover, the bond length of the inner C-unit decreases progressively with an increase in the ErEr distance, indicating that the inserted C-unit can serve as a molecular spring to support the strong metal-cage interactions within cages with the same size but different shapes. Hence, the role of cage shape on the cluster configuration is unveiled safely for the as-obtained ErC@C isomers.