Capturing the Missing Carbon Cage Isomer of C via Mutual Stabilization of a Triangular Monometallic Cyanide Cluster.

Monometallic cyanide clusterfullerenes (CYCFs) represent a unique branch of endohedral clusterfullerenes with merely one metal atom encapsulated, offering a model system for elucidating structure-property correlation, while up to now only C and C cages have been isolated for the pristine CYCFs. C is one of the most abundant fullerenes and has 24 isomers obeying the isolated pentagon rule (IPR), among which 14 isomers have been already isolated, whereas the (17)-C isomer has lower relative energy than several isolated isomers but never been found for empty and endohedral fullerenes. Herein, four novel C-based pristine CYCFs with variable encapsulated metals and isomeric cages, including MCN@(13)-C (M = Y, Dy, Tb) and DyCN@(17)-C, have been synthesized and isolated, fulfilling the first identification of the missing (17)-C isomer, which can be interconverted from the (13)-C isomer through two steps of Stone-Wales transformation. The molecular structures of these four C-based CYCFs are determined unambiguously by single-crystal X-ray diffraction. Surprisingly, although the ionic radii of Y, Dy, and Tb differ slightly by only 0.01 Å, such a subtle difference leads to an obvious change in the metal-cage interactions, as inferred from the distance between the metal atom and the nearest hexagon center of the (13)-C cage. On the other hand, upon altering the isomeric cage from DyCN@(13)-C to DyCN@(17)-C, the Dy-cage distance changes as well, indicating the interplay between the encapsulated DyCN cluster and the outer cage. Therefore, we demonstrate that the metal-cage interactions within CYCFs can be steered via both internal and external routes.