Reactions between N‑Heterocyclic Carbene and Lutetium–Metallofullerenes: High Regioselectivity Directed by Electronic Effect in Addition to Steric Hindrance

The Lewis acid–base pairing
reaction between strained N-heterocyclic
carbene (NHC) and endohedral metallofullerenes (EMFs) is an efficient
strategy to obtain stable derivatives in a highly regioselective manner.
Herein, we report an in-depth study on the reactions between 3-dimesityl-1H-imidazol-3-ium-2-ide (1) and three different
EMFs, namely, Lu3N@Ih(7)-C80, Lu2@C3v(8)-C82, and Lu2@C2v(9)-C82, respectively. Only one
monoadduct is obtained for each EMF under certain conditions, demonstrating
surprisingly high regioselectivity and exclusive formation of monoadducts.
X-ray results of the derivatives of Lu3N@C80 reveal that an epoxide adduct (2a) with a specific
[6,6,6]-carbon atom of the C80 cage singly bonded to the
normal carbene center (C2) of the NHC is obtained under ambient condition,
whereas a pure argon atmosphere gives 2b with an abnormal
C5-bonding structure. In contrast, the derivatives of Lu2@C82 (3 and 4) are both normal
C2-bonding [5,6,6]-adducts without oxygen addition, even though air
is involved in the reaction. Our theoretical results confirm that
the remarkably high regioselectivity and the quantitative formation
of monoadducts are direct result from the distributions of molecular
orbital and electrostatic potential on the cage surfaces in addition
to the previously assumed steric hindrance between the fullerene cage
and the NHC moiety.