Isolation and Characterization of a Carbene Derivative of La@C 82 Yutaka Maeda, † Yoichiro Matsunaga, ‡ Takatsugu Wakahara, ‡ Satomi Takahashi, § Takahiro Tsuchiya, ‡ Midori O. Ishitsuka, ‡ Tadashi Hasegawa, † Takeshi Akasaka,* ,‡ Michael T. H. Liu, | Kisato Kokura, ⊥ Ernst Horn,* ,⊥ Kenji Yoza, X Tatsuhisa Kato, 3 Shingo Okubo, 3 Kaoru Kobayashi, 3 Shigeru Nagase,* ,3 and Kazunori Yamamoto O Department of Chemistry, Tokyo Gakugei UniVersity, Tokyo 184-8501, Japan, Center for Tsukuba AdVanced Research Alliance, UniVersity of Tsukuba, Ibaraki 305-8577, Japan, Graduate School of Science and Technology, Niigata UniVersity, Niigata 950-2181, Japan, Department of Chemistry, UniVersity of Prince Edward Island, Prince Edward Island C1A4P3, Canada, Department of Chemistry, Rikkyo UniVersity, Tokyo 171-8501, Japan, Bruker AXS K. K., Yokohama, Kanagawa 221-0022, Japan, Institute for Molecular Science, Okazaki 444-8585, Japan, and Japan Nuclear Fuel Cycle DeVelopment Institute, Tokai, Ibaraki 319-1100, Japan Received December 10, 2003; E-mail: akasaka@tara.tsukuba.ac.jp Endohedral metallofullerenes encapsulate one or more metal atoms inside a hollow fullerene cage. These fullerenes have attracted special attention because they engender new spherical molecules with unique electronic properties and structures that are unexpected for empty fullerenes. 1,2 The recent successful isolation and purifica- tion of endohedral metallofullerenes have encouraged the investiga- tion of their physical and chemical properties. It would be of interest to understand how the chemical reactivity and selectivity of empty fullerenes change upon endohedral metal doping 3,4 and how the electronic properties of endohedral metallofullerenes change upon reduction, oxidation, 5 and chemical functionalization. Dorn and co- workers reported the selective formation of a diamagnetic Sc 3 N@C 80 adduct via a Diels-Alder reaction. 4b,c Our earlier report indicated that the high reactivity of endohedral metallofullerenes may be ascribed to their electronic properties. 4-7 The ESR spectra measured during the reaction reveal the formation of several regioisomers with different La isotopic splittings. 3,4a In contrast, the regiospecific addition reaction of La@C 82 with 2-adamantane-2,3-[3H]-diazirine (1) affords the first single isomer which has been successfully isolated. We now present the first instance of an isolation and crystallographic characterization of a paramagnetic endohedral monometallofullerene derivative from the selective La@C 82 reac- tion. Irradiation of 1,2,4-trichlorobenzene/toluene solution of La@C 82 8 (4.3 mg, 1.9 × 10 -4 M) and an excess molar amount of 1 in a degassed sealed tube at room temperature using a high-pressure mercury-arc lamp (cutoff < 300 nm) resulted in the formation of the adduct, La@C 82 (Ad) (2, Ad ) adamantylidene), in 80% yield, which was purified by preparative HPLC (eq 1). Figure 1 shows the ESR spectrum and HPLC profile of isolated 2. MALDI-TOF mass spectrometry of adduct 2 (C 92 H 14 La, mass m/z 1257) exhibits a molecular ion peak at m/z 1260-1257 and a peak at m/z 1126- 1123 (La@C 82 ) due to the loss of the Ad group. The UV-visible- near-infrared absorption spectrum of 2 is similar to that of the pristine La@C 82 . These results suggest that 2 retains the essential electronic and structural character of La@C 82 . Finally, the X-ray analysis 9 unambiguously characterizes the isomer type of the fullerene cage and locates the La atom at a single site near the end of the molecule, as shown in Figure 2. The C(1)‚‚‚C(2) distance is 2.097 Å, indicative of the open structure. The La‚‚‚C(1) and La‚‚‚C(2) distances are 2.658 and 2.634 Å, respectively. These agree well with the calculated values; the C‚‚‚C distance is 2.118 Å, and the La‚‚‚C distances are 2.722 and 2.690 Å. 10 Although it has been reported that multiple sites for the metal atoms are found in the structures of the related C 82 endohedrals such as Er@C 82 , 11 interestingly and surprisingly, a comparison of X-ray data collected at 90, 213, and 293 K reveals that the La atom remains in the same position, as shown in Figure 2. Since there are 24 non-equivalent carbons and 19 non-equivalent 6-6 bonds in La@C 82 , addition may take place at several sites to afford a large number of possible monoadduct isomers. The reaction of La@C 82 with disilirane 3 or phenyldiazomethane 4a gave several 1:1 adduct isomers. The remarkable change of selectivity in the † Tokyo Gakugei University. ‡ University of Tsukuba. § Niigata University. | University of Prince Edward Island. ⊥ Rikkyo University. X Bruker AXS K. K. 3 Institute for Molecular Science. O Japan Nuclear Cycle Development Institute. Figure 1. (a) ESR spectrum of isolated La@C82(Ad) (2) (hfcc: 0.89 G, g-value: 2.0018). (b) HPLC profile of isolated La@C82(Ad) (2) (Buckyprep column, φ 4.6 mm × 250 mm, toluene 1 mL/min). Figure 2. ORTEP drawing of one enantiomeric isomer of La@C82(Ad) (2) showing thermal ellipsoids at the 50% probability level. The CS2 molecules are omitted for clarity. Published on Web 05/13/2004 6858 9 J. AM. CHEM. SOC. 2004, 126, 6858-6859 10.1021/ja0316115 CCC: $27.50 © 2004 American Chemical Society