Crystal Structure Analysis of Salts and Derivatives of Cationic Lithium Endohedral Fullerene

Cationic lithium endohedral fullerene, Li⁺@C₆₀, has a high tendency to form ion-pair states with anions. The strong intermolecular electrostatic attraction builds unique crystal structures such as the rock-salt structure [1]. The coordinated anions in the crystal affect the position and thermal motion of the encapsulated Li⁺ cation. The endohedral structure of Li⁺ cation should be also affected by chemical functionalization. Recently we synthesized a cyclohexadiene adduct of Li⁺@C₆₀ by the Diels-Alder reaction [2]. The lower LUMO level of Li⁺@C₆₀ accelerates the reaction late 2,400 times faster than that of empty C₆₀. The salt of Li⁺@C₆₀(C₆H₈) with TFPB^- (tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) was crystallized by a vapor diffusion method. The single crystal structure analysis was performed by means of the synchrotron-radiation X-ray diffraction. The crystal structure shows that the addition of C₆H₈ to the C₆₀ cage occurred at the [6,6]-bond to have C_s symmetry. On the other hand, the encapsulated Li⁺ cation is localized under a six-membered ring so that the C_s symmetry of the molecule is broken in the crystal. The peculiar Li⁺ localization would be a result of an electrostatic interaction with the coordinated TFPB^- anions and the addition of C₆H₈. 

[1] S. Aoyagi et al., Angew. Chem. Int. Ed. 2012, 51, 3377-3381.

[2] H. Ueno et al., J. Am. Chem. Soc. 2014, 136, 11162-11167