Sc3N@Ih-C80 as a novel Lewis acid to trap abnormal N-heterocyclic carbenes: the unprecedented formation of a singly bonded 666-adduct

摘要

The reaction between an N-heterocyclic carbene (NHC), namely 1,3-bis(diisopropylphenyl)-imidazol-2-ylene (>1), and Sc3N@Ih-C80 successfully affords a Lewis acid–base pair (>2a). Single crystal X-ray crystallographic results unambiguously reveal the unexpected structure of >2a where the abnormal carbene center of the NHC is connected to a triple-hexagon-junction (THJ) carbon atom of Sc3N@Ih-C80via a single bond. Theoretical calculations reveal that selective entrapment of the abnormal carbene >1 is caused by the steric hindrance between the normal NHC moiety and the fullerene cage, which precludes the formation of normal carbene adducts. Furthermore, the analysis of the electronic density distribution on the cage of Sc3N@Ih-C80 indicates that THJ carbons bear relatively low negative charge densities and, accordingly, are easily attacked by the electron-rich NHC >1 to form the singly bonded [6,6,6]-adduct >2a instead of the corresponding [5,6,6]-adduct >2b. It is thus confirmed that the regioselective formation of >2a is a synergistic effect of both cage size and electron density distribution. Sc3N@Ih-C80, although with a highly charged cage, is proven to show excellent Lewis acidity, opening a wide avenue toward carbon-based Lewis acids taking into account the diversity of endohedral metallofullerenes.