Isolation and Crystallographic Characterization of La_2C_2@C_s(574)-C_(102) and La_2C_2@C_2(816)-C_(104): Evidence for the Top-Down Formation Mechanism of Fullerenes

摘要

Tubular higher fullerenes are prototypes of finite-length end-capped carbon nanotubes (CNTs) whose structures can be accurately characterized by single-crystal X-ray diffraction crystallography. We present here the isolation and crystallographic characterization of two unprecedented higher fullerenes stabilized by the encapsulation of a La_2C_2 cluster, namely, La_2C_2@C_s(574)-C_(102), which has a perfect tubular cage corresponding to a short (10, 0) zigzag carbon nanotube, and La_2C_2@C_2(816)-C_(104) which has a defective cage with a pyracylene motif inserting into the cage waist. Both cages provide sufficient spaces for the large La_2C_2 cluster to adopt a stretched and nearly planar configuration, departing from the common butterfly-like configuration which has been frequently observed in midsized carbide metallofullerenes (e.g., Sc_2C_2@C_(80-84)), to achieve strong metal-cage interactions. More meaningfully, our crystallographic results demonstrate that the defective cage of C_2(816)-C_(104) is a starting point to form the other three tubular cages known so far, i.e., D_5(450)-C_(100), C_s(574)-C_(102), and D_(3d)(822)-C_(104), presenting evidence for the top-down formation mechanism of fullerenes. The fact that only the large La_2C_2 cluster has been found in giant fullerene cages (C_(＞100)) and the small clusters M_2C_2 (M = Sc, Y, Er, etc.) are present in midsized fullerenes (C_(80)-C_(86)) indicates that geometrical matching between the cluster and the cage, which ensures strong metal-cage interactions, is an important factor controlling the stability of the resultant metallofullerenes, in addition to charge transfer.