Anion Exchange of Li⁺@C₆₀ Salt for Improved Solubility

Anion exchange of [Li⁺@C₆₀]PF₆⁻ salt was performed to obtain three highly soluble Li⁺@C₆₀ salts, namely, the trifluoromethanesulfonate (OTf⁻), bis(trifluoromethylsulfonyl)imide (NTf₂⁻), and tetrakis{3,5-bis(trifluoromethyl)phenyl}borate (TFPB⁻). We investigated the solubility of [Li⁺@C₆₀]OTf⁻, [Li⁺@C₆₀]NTf₂⁻, and [Li⁺@C₆₀]TFPB⁻ and found that, compared with [Li⁺@C₆₀]PF₆⁻, the NTf₂ and TFPB salts had substantially higher solubility. X-ray crystal structure analysis of the TFPB salt revealed that the Li⁺@C₆₀ units spaced further apart because of the large anions in the crystal packing. We attribute the improved solubility of the salts to primarily the increased inter-fullerene distance of the Li⁺@C₆₀ cations.