Robust metal-pentagon interactions in the Th-based endohedral metallofullerenes revealed by DFT calculations

Endohedral metallofullerenes (EMFs) all feature obvious charge transfer from the metallic core to the carbon shell with the donated electrons largely accepted by the cage pentagons. In this work, a series of Th@C-2n (2n = 64-88) were thoroughly investigated by means of density functional theory calculations. Interestingly, we found that the tetravalent thorium atom mainly coordinates to three pentagonal rings with the metal-pentagon interactions independent on the distribution and distance among these pentagons. This coordination pattern is not only in sharp contrast to that of common organometallic complexes, where four pentagons are indispensable for stabilizing Th(IV), but also different from that of Ti-containing fullerenes, whose valence state highly depends on the pentagon distribution. The specificity of Th-based EMFs was rationalized by the synergetic effect of small metal ion size, low electronegativity, strong metal-cage electrostatic attractions and effective orbital overlap between the metal and cage orbitals. Our work highlights the role of cage pentagons in the Th-cage interactions, and points out the fundamental difference between EMFs and common organometallic complexes.