Chemical Reactions of Cationic Metallofullerenes: An Alternative Route for Exohedral Functionalization

The chemistry of cationic forms of clusterfullerenes remain less explored than that of the corresponding neutral or anionic species. In the present work, M₃N@I_h-C₈₀ (M=Sc or Lu) cations were generated by both electrochemical and chemical oxidation methods. The as-obtained cations successfully underwent the typical Bingel–Hirsch reaction that fails with neutral Sc₃N@I_h-C₈₀. Two isomeric Sc₃N@I_h-C₈₀ cation derivatives, [5,6]-open and [6,6]-open adducts, were synthesized, and the former has never been prepared by means of a Bingel–Hirsch reaction with neutral clusterfullerenes. In the case of the Lu₃N@I_h-C₈₀ cation, however, only a [6,6]-open adduct was obtained. Density functional theory (DFT) calculations indicated that the oxidized M₃N@I_h-C₈₀ was much more reactive than the neutral compound upon addition of the diethyl bromomalonate anion. The Bingel–Hirsch reaction of M₃N@I_h-C₈₀ cations occurred by means of an unusual outer-sphere single-electron transfer (SET) process from the diethyl bromomalonate anion to the stable intermediate [M₃N@C₈₀(C₂H₅COO)₂CBr]^.. Remarkably, the diethyl bromomalonate anion was found to act as both a nucleophile and an electron donor.