Ho2C2 Cluster with Flexible Configurations inside a Large C-2(61)-C-92 Cage

Carbide clusterfullerenes (CCFs) have been of great concern due to their potential applications in materials science, in which the internal carbide cluster plays vital roles in the stability and properties of CCF. However, there still remains a debate about what configuration is ideal for the internal carbide cluster. In this work, we isolated two isomers (I and II) of Ho2C94 and studied them by means of mass spectrometry, UV-vis-NIR spectroscopy, and cyclic/differential pulse voltammetry. A combined study of single-crystal X-ray diffraction (SC-XRD) and density functional theory (DFT) computation ascertains isomer-I as Ho2C2@C-2(61)-C-92 , in which the Ho2C2 cluster displays variable configurations from planar zigzag to folded butterfly with very small distortion energy (similar to 10 kJ/mol) This study hence confirms that the internal carbide cluster is intrinsically flexible over a broad geometrical range in a relatively large fullerene cage, where the nanoscale compression effect is almost negligible.