Isolation and Crystallographic Characterization of Lu₃ N@C_2n (2n=80-88): Cage Selection by Cluster Size

The small Sc₃ N cluster has only been found in such small cages as C_2n (2n=68, 78, 80, 82), whereas the large M₃ N (M=Y, Gd, Tb, Tm) clusters choose those larger cages C_2n (2n=82-88). Herein, concrete experimental evidence is presented to establish the size effect of the internal metallic cluster on selecting the outer cage of endohedral metallofullerenes (EMFs) by using a medium-sized metal, lutetium, which possesses an ionic radius between Sc and Gd. A series of lutetium-containing EMFs have been obtained and their structures are unambiguously determined as Lu₃ N@I_h (7)-C₈₀ , Lu₃ N@D_5h (6)-C₈₀ , Lu₃ N@C_2v (9)-C₈₂ , Lu₃ N@C_s (51365)-C₈₄ , Lu₃ N@D₃ (17)-C₈₆ , and Lu₃ N@D₂ (35)-C₈₈ by single-crystal X-ray diffraction crystallography. It was confirmed that the encaged Lu₃ N cluster always adopts a planar geometry in Lu₃ N@C_80-88 isomers to ensure substantial metal-cage/metal-nitrogen interactions. As a result, the Lu₃ N cluster selects the C_2v (9)-C₈₂ cage, which also encapsulates Sc₃ N, instead of the C_s (39663)-C₈₂ cage which is more suitable for M₃ N (M=Y, Gd, Tb, Tm). However, different from Sc₃ N, Lu₃ N can also template the C_84-88 cages which are absent for Sc₃ N-containing EMFs, confirming clearly the size effect of the internal cluster on selecting the outer cage.