7. Neutral Li@C₆₀: A Hydrogen-Like Superatom

Lithium-ion-containing [60]fullerene, Li⁺@C₆₀, was electrochemically reduced by constant current electrolysis without the use of any other supporting electrolyte to produce lithium-containing [60]fullerene, Li@C₆₀ in a neutral form. Prior to the electrolysis, the ionic conductivity of Li⁺@C₆₀ was evaluated, and hexafluorophosphate and bis(trifluoromethanesulfonyl)imide salts of Li⁺@C₆₀ in solution showed ionic conductivity similar to that of tetrabutylammonium hexafluorophosphate, a commonly used supporting electrolyte. Li@C₆₀ was obtained from the electrolysis as a black power and characterized by UV–Vis-NIR, NMR, and electron spin resonance spectroscopy and single-crystal X-ray structural analysis. A monomeric form, formally represented as Li⁺@\({{\text{C}}_{60}}^{ \cdot - }\), was likely the predominant species in solution according to the spectroscopic data, but a dimeric structure with coupling of the radicals was found in the crystal structure. An equilibrium might exist between the monomeric and dimeric forms in solution, but the formation of the dimeric structure in the solid state was found to be favored for Li⁺@\({{\text{C}}_{60}}^{ \cdot - }\) compared with empty \({\text{C}_{60}}^{ \cdot - }\) because the positive and negative charges are canceled in Li⁺@\({{\text{C}}_{60}}^{ \cdot - }\), thus weakening the electrostatic repulsion between two molecules. Intriguingly, Li⁺@\({{\text{C}}_{60}}^{ \cdot - }\) can be regarded as a superatom analogous to the hydrogen atom, where Li⁺ is the nucleus and the monovalent \({\text{C}_{60}}^{ \cdot - }\) cage is the electron orbital. In addition, the dimeric form of Li⁺@\({{\text{C}}_{60}}^{ \cdot - }\) can be considered analogous to a hydrogen molecule.