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.