Endohedral metallofullerenes (EMFs) with a composition of M₂C_2n may exist as di-EMFs, M₂@C_2n, or as carbide cluster metallofullerenes (CCMFs), M₂C₂@C_2n-2. Thus care needs to be taken to distinguish between these two possibilities. However, there are rare examples for M₂C₂@C_90-104 and the configuration of the carbide cluster is not clear in these cages.

Herein we report the isolation and single-crystal X-ray crystallographic characterization of eight unprecedented EMFs containing a La₂C₂ cluster, namely, La₂C₂@C₂(41)-C₉₀, La₂C₂@D₃(85)-C₉₂, La₂C₂@C₁(132)-C₉₄, La₂C₂@C₂(157)-C₉₆, La₂C₂@C₁(175)-C₉₈, La₂C₂@D₅(450)-C₁₀₀,  La₂C₂@C_s(574)-C₁₀₂, and La₂C₂@C₂(816)-C₁₀₄. It is evident that the cage length increases roughly along with increasing of the carbon atom number, resulting in an increase of the La-La distance and the Ni···cage distance as well. This is a clear indication that giant fullerenes tend to grow along the long axis to achieve a tubular appearance. Encapsulation of a large La₂C₂ cluster inside D₅(450)-C₁₀₀ induces a 5% axial compression of the cage, as compared with the structure of La₂@D₅(450)-C₁₀₀. Moreover, it is demonstrated that the defective cage of C₂(816)-C₁₀₄ is a starting point to form the other three tubular cages known so far, i.e., D₅(450)-C₁₀₀, C_s(574)-C₁₀₂, and D_3d(822)-C₁₀₄, presenting evidence for the top-down formation mechanism of fullerenes. Overall, our experimental results have unambiguously confirmed that La₂C_92-106 tend to form CCMFs La₂C₂@C_90-104 instead of di-EMFs La₂@C_92-106.

References：

[1] Lu, X.*; Akasaka, T.*; Nagase, S. Acc. Chem. Res. 2013, 46, 1627-1635.

[2] Cai, W.; Bao, L.; Zhao, S.; Xie, Y-P.; Akasaka, T.; Lu X. J. Am. Chem. Soc. 2015, 137, 10292-10296.

[3] Cai, W.; Li, F-F.; Bao, L.; Xie, Y-P.; Lu X*. J. Am. Chem. Soc. 2016, 138, 6670-6675.