Chemical Understanding of a Non-IPR Metallofullerene: Stabilization of Encaged Metals on Fused-Pentagon Bonds in La₂@C₇₂

Fullerenes violating the isolated pentagon rule (IPR) are only obtained in the form of their derivatives. Since the [5,5]-bond carbons are highly reactive, they are easily attacked by reagents to release the bond strains. Non-IPR endohedral metallofullerenes, however, still have unsaturated sp2 carbons at the [5,5] bond junctions, which allow their chemical properties to be probed. In this work, La2@C72 was chosen as a representative non-IPR metallofullerene, since it has been experimentally proposed to have either the #10611 or #10958 non-IPR cage structure (J. Am. Chem. Soc. 2003, 125, 7782), while theoretical calculations have suggested that the #10611 cage is more stable (J. Phys. Chem. A 2006, 110, 2231). La2@C72 was modified by photolytic reaction with the carbene reagent 2-adamantane-2,3-[3H]-diazirine. Six isomers of adamantylidene monoadducts were isolated and characterized using various kinds of measurements, including high-performance liquid chromatography, matrix-assisted laser desorption ionization mass spectrometry, UV−vis−near-infrared spectroscopy, cyclic voltammetry, differential-pulse voltammetry, 13C NMR spectroscopy, and single-crystal X-ray diffraction. Electronic spectra and electrochemical studies revealed that the essential electronic structures of La2@C72 are retained in the six isomers and the adamantylidene group acts as a weak electron-donating group toward La2@C72. X-ray structural results unambiguously elucidated that La2@C72 has the #10611 chiral cage (i.e., D2 symmetry) with two pairs of fused pentagons at each pole of the cage and that the two La atoms reside close to the two fused-pentagon pairs. On the basis of these results and theoretical calculations, it is concluded that the fused-pentagon sites are very reactive toward carbene but that the carbons forming the [5,5] junctions are less reactive than the adjacent ones; this confirms that these carbons interact strongly with the encaged metals and thus are stabilized by them.