Fullerene–Carbene Lewis Acid–Base Adducts

The reaction between a bulky N-heterocylic carbene (NHC) and C60 leads to the formation of a thermally stable zwitterionic Lewis acid–base adduct that is connected via a C–C single bond. Low-energy absorption bands with weak oscillator strengths similar to those of n-doped fullerenes were observed for the product, consistent with a net transfer of electron density to the C60 core. Corroborating information was obtained using UV photoelectron spectroscopy, which revealed that the adduct has an ionization potential ∼1.5 eV lower than that of C60. Density functional theory calculations showed that the C–C bond is polarized, with a total charge of +0.84e located on the NHC framework and −0.84e delocalized on the C60 cage. The combination of reactivity, characterization, and theoretical studies demonstrates that fullerenes can behave as Lewis acids that react with C-based Lewis bases and that the overall process describes n-doping via C–C bond formation.