Crystallographic characterization of Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90: 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 C90 cages, namely, Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90, 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 C2(43)-C90 and C2(44)-C90 are crystallographically identified for the first time. Interestingly, the Er⋯Er distance of the major sites in Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90 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 C2-unit decreases progressively with an increase in the Er⋯Er distance, indicating that the inserted C2-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 Er2C2@C90 isomers.