Structure of [60]fullerene with a mobile lithium cation inside

The structure of crystalline [60]fullerene with a lithium cation inside (Li⁺@C₆₀) was determined by synchrotron radiation X-ray diffraction measurements to understand the electrostatic and thermal properties of the encapsulated Li⁺ cation. Although the C₆₀ cages show severe orientation disorder in [Li⁺@C₆₀](TFPB^-)·C₄H₁₀O and [Li⁺@C₆₀](TFSI^-)·CH₂Cl₂, the Li⁺ cations are rather ordered at specific positions by electrostatic interactions with coordinated anions outside the C₆₀ cage. The Li⁺@C₆₀ molecules in [Li⁺@C₆₀](ClO₄^-) with a rock-salt-type cubic structure are fully disordered with almost uniform spherical shell charge densities even at 100 K by octahedral coordination of ClO₄^- tetrahedra and show no orientation ordering, unlike [Li⁺@C₆₀](PF₆^-) and pristine C₆₀. Single-bonded (Li⁺@C₆₀^-)² dimers in [Li⁺@C₆₀^-](NiOEP)⋅CH₂Cl₂ are thermally stable even at 400 K and form Li⁺-C bonds which are shorter than Li⁺-C bonds in [Li⁺@C₆₀](PF₆^-) and suppress the rotational motion of the Li⁺ cations.