Sc3N@Ih-C80 as a novel Lewis acid to trap abnormal N-heterocyclic carbenes: the unprecedented formation of a singly bonded [6,6,6]-adduct

Abstract

The reaction between an N-heterocyclic carbene (NHC), namely 1,3-bis(diisopropylphenyl)-imidazol-2-ylene (1), and Sc₃N@I_h-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@I_h-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@I_h-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@I_h-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.