5. Chemical Modification of Li⁺@C₆₀

The synthesis of lithium-ion-containing [6,6]-phenyl-C₆₁-butyric acid methyl ester (Li⁺@PCBM) was first demonstrated through the organic functionalization of Li⁺@C₆₀. The [5,6]- and [6,6]-isomers of [Li⁺@PCBM]PF₆ ⁻ were successfully prepared, and the structure of [6,6]-[Li⁺@PCBM]PF₆ ⁻ was elucidated by X-ray crystallography. The Diels–Alder reaction of Li⁺@C₆₀ was first investigated using cyclopentadiene as a diene. This reaction was fast, with an equilibrium constant of more than 1000-fold that for the reaction with the C₆₀. The Diels–Alder reaction of [Li⁺@C₆₀]PF₆ ⁻ with 1,3-cyclohexadiene was also experimentally and computationally investigated to precisely determine the kinetic parameters. Li⁺@C₆₀ reacted 2400-fold faster, compared with the empty C₆₀. Li⁺-containing fullerenol was synthesized through cyclosulfation followed by hydrolysis. The reaction proceeded with good regioselectively, and the product was highly soluble, even in polar solvents. Iridium and platinum complexes of Li⁺@C₆₀ were synthesized, and an electrostatic attractive interaction between inner Li⁺ and outer transition metals was demonstrated. A supramolecular complex of [10]cycloparaphenylene and Li⁺@C₆₀ was also synthesized. Charge delocalization of the inner cationic charge on outer [10]cycloparaphenylene through C₆₀ π-conjugated system was suggested.