Lu2@C2n (2n=82, 84, 86): Crystallographic Evidence of Direct Lu-Lu Bonding between Two Divalent Lutetium Ions Inside Fullerene Cages

Although most of the M2C2n-type metallofullerenes (EMFs) tend to form carbide cluster EMFs, we report herein that Lu-containing EMFs Lu2C2n (2n = 82, 84, 86) are actually dimetallofullerenes (di-EMFs), namely, Lu-2@C-s(6)-C-82, Lu-2@C-3v(8)-C-82, Lu-2@D-2d(23)-C-84, and Lu-2@C-2v(9)-C-86. 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 Lu2C2n isomers.