ScN@ -C as a novel Lewis acid to trap abnormal -heterocyclic carbenes: the unprecedented formation of a singly bonded [6,6,6]-adduct.

The reaction between an -heterocyclic carbene (NHC), namely 1,3-bis(diisopropylphenyl)-imidazol-2-ylene (), and ScN@ -C successfully affords a Lewis acid-base pair (). Single crystal X-ray crystallographic results unambiguously reveal the unexpected structure of where the abnormal carbene center of the NHC is connected to a triple-hexagon-junction (THJ) carbon atom of ScN@ -C a single bond. Theoretical calculations reveal that selective entrapment of the abnormal carbene 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 ScN@ -C indicates that THJ carbons bear relatively low negative charge densities and, accordingly, are easily attacked by the electron-rich NHC to form the singly bonded [6,6,6]-adduct instead of the corresponding [5,6,6]-adduct . It is thus confirmed that the regioselective formation of is a synergistic effect of both cage size and electron density distribution. ScN@ -C, 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.