Isolation and crystallographic characterization of Lu2C2@C2 (2n = 88–92): Internal cluster stretching upon outer cage expansion

Abstract Endohedral metallofullerenes, namely fullerenes encapsulating a variety of otherwise unstable metallic units (such as M3N, M2C2, M2O, etc.), represent as a perfect platform for the study of metal-cage interactions and/or mutual selections. Herein, for the first time, we report the synthesis and isolation of four lutetium-containing carbide cluster metallofullerenes (CCMFs), namely, Lu2C2@Cs(hept)-C88, Lu2C2@C2 (41)-C90, Lu2C2@C2 (43)-C90, and Lu2C2@D3 (85)-C92, respectively. These compounds were fully characterized by vis-NIR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction (XRD) crystallography as well as cyclic voltammetry. Notably, the XRD analysis results unambiguously confirm that the cage size plays a decisive role in determining the geometry of the encased Lu2C2 cluster. Specifically, the butterfly-like Lu2C2 unit is found in the small C88 cage, whereas the Lu2C2 unit exhibits a twisted butterfly configuration in the relatively large C90 cages. As the cage size increases to C92, the Lu2C2 unit finally adopts a stretched zigzag geometry. Our comprehensive characterizations of a series of Lu2C90-94 isomers confirm experimentally that steering the outer cage size can selectively alter the configuration and geometry of the internal carbide clusters, presenting a useful method for the controllable synthesis of novel hybrid molecules.