4. Li⁺@C₆₀ Salts: Crystal Structures and Properties

Synchrotron radiation X-ray structural analysis was performed to obtain information about the structure of the isolated Li⁺@C₆₀ salts and evidence for the encapsulation of Li⁺ inside the C₆₀ cage. Two Li⁺@C₆₀ salts, namely [Li⁺@C₆₀]PF\({_{6}}^{ - }\) and [Li⁺@C₆₀]SbCl\({_{6}}^{ - }\), were examined by in-depth crystallographic investigation. [Li⁺@C₆₀]PF\({_{6}}^{ - }\) underwent a phase transition at 370 K from a face-centered-cubic crystal system to a simple cubic system with decreasing temperature. Even at 150 K, the inner Li⁺ ion was delocalized in the C₆₀ cage, while at 22 K, the Li⁺ ion occupied two positions 1.4 Å away from the center of C₆₀. [Li⁺@C₆₀]SbCl\({_{6}}^{ - }\) had an orthorhombic crystal system, and the Li⁺ ion was localized at two off-center positions. Anion exchange reactions of [Li⁺@C₆₀]PF\({_{6}}^{ - }\) was investigated to produce new Li⁺@C₆₀ salts having OTf⁻, TFSI⁻, and TFPB⁻ counter anions (OTf⁻ = CF₃SO\({_{3}}^{ - }\); TFSI⁻ = N(SO₂CF₃)\({_{2}}^{ - }\); TFPB⁻ = B{C₆H₃(CF₃)₂}\({_{4}}^{ - }\)). These salts have two- to tenfold higher solubility than [Li⁺@C₆₀]PF\({_{6}}^{ - }\). X-ray crystal structural analysis of [Li⁺@C₆₀]TFPB⁻ was successfully performed.