A spider hanging inside a carbon cage: off-center shift and pyramidalization of Sc3N clusters inside C-84 and C-86 fullerene cages

Metal nitride cluster fullerenes (NCFs) are the most intensively studied endohedral fullerenes due to their exceptional structural variety. It is commonly understood that in NCFs, small clusters such as Sc3N favor C-82 and smaller cages, while large clusters (e.g., Tb3N and Gd3N) favor C-84 and larger cages. Endohedral structures with small nitride clusters encaged inside large carbon cages (e.g., C-84 and C-86), although theoretically probed, have never been experimentally obtained. Herein, we report two novel NCFs, Sc3N@C-s(51365)-C-84 and Sc3N@D-3(19)-C-86, which have been successfully synthesized and characterized using MALDI-TOF mass spectrometry, X-ray single-crystal diffraction and UV-vis-NIR spectroscopy. Crystallographic analysis shows that, while in most previously reported cluster fullerenes, clusters tend to take a central position inside fullerene cages, in these two structures, the Sc3N clusters are shifted to one side of the cage and unexpectedly pyramidalized inside the large cages of C-84 and C-86, which resembles a spider hanging inside a carbon cage. These observations, together with the stretched Sc-N bonds, suggest that the M3N cluster can self-adjust not only its configuration but also its position relative to fullerenes to optimize the metal-cage distances as well as cluster-cage interactions, thus promoting the stability of endohedral structures. This work provides new insight into the interaction mechanisms between the clusters and carbon cages of endohedral fullerenes.