Synthesis of a new class of fullerene derivative Li⁺@C ₆₀O^-(OH)₇ as a "cation-encapsulated anion nanoparticle"

Metal encapsulation into a cage and chemical modification on the outer surface of fullerenes endow them with some unique characteristic properties. Although the derivatization of endohedral fullerenes holds promise for producing novel new nano-carbon materials, there are few reports about such compounds. Herein, we report the synthesis of lithium encapsulated fullerenol Li ⁺@C₆₀O^-(OH)₇ using a fuming sulfuric acid method from [Li⁺@C₆₀](PF₆^-) and characterization of its structure by IR, NMR, FAB mass spectroscopy, and elemental analysis. The hydroxylation of [Li⁺@C₆₀](PF ₆^-) is site-selective to preferentially give a single isomer (ca. 70%) with two minor isomers in marked contrast to the reaction of empty C₆₀. We conclude from the analysis of radical species produced in the reaction of a C₆₀ cage with fuming sulfuric acid that this unusual site-selective hydroxylation is caused by the lower HOMO level of Li⁺@C₆₀ than that of empty C₆₀. Furthermore, our results clearly indicate that the internal lithium cation is interacted with the introduced hydroxyl groups, and thus the properties of endohedral fullerenes can be controlled by the external modification of a fullerene cage.