Although most of the MC-type metallofullerenes (EMFs) tend to form carbide cluster EMFs, we report herein that Lu-containing EMFs LuC (2n = 82, 84, 86) are actually dimetallofullerenes (di-EMFs), namely, Lu@C(6)-C, Lu@C(8)-C, Lu@D(23)-C, and Lu@C(9)-C. Unambiguous X-ray results demonstrate the formation of a Lu-Lu single bond between two lutetium ions which transfers four electrons in total to the fullerene cages, thus resulting in a formal divalent state for each Lu ion. Population analysis indicates that each Lu atom formally donates a 5d electron and a 6s electron to the cage with the remaining 6s electron shared with the other Lu atom to form a Lu-Lu single bond so that only four electrons are transferred to the fullerene cages with the formal divalent valence for each lutetium ion. Accordingly, we confirmed both experimentally and theoretically that the dominating formation of di-EMFs is thermodynamically very favorable for LuC isomers.

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, Lu 3 N@I h (7)-C 80 , Lu 2 @C 3v (8)-C 82 , and Lu 2 @ C 2v (9)-C 82 , 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 Lu 3 N@C 80 reveal that an epoxide adduct (2a) with a specific [6,6,6]-carbon atom of the C 80 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 Lu 2 @C 82 (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.