J. Phys. Chem. 1994, 98, 12831-12833
12831
Isolation and Characterization of an ESR-Active La @ c 8 2 Isomer Kazunori Yamamoto," Hideyuki Funasaka, and Takeshi Takahashi Nuclear Fuel Technology Development Divison, Tokai Works, Power Reactor & Nuclear Fuel Development Corporation, Tokai, Ibaraki, 319-11, Japan
Takeshi Akasaka Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki, 305, Japan
Toshiyasu Suzuki and Yusei Masuyama Institute for Molecular Science, Myodaiji, Okazaki, 444, Japan Received: October 13. 1994@
A minor isomer of La@C82 has been isolated with an efficient HPLC method. The isolated minor La@C82 was well identified as a stable metallofullerene molecule. Both mass and ESR spectra confirmed that it was La@& with a 0.836-G 139Lahyperfine splitting. Visible and near-IR absorption spectra, cyclic and differential pulse voltammograms, and 13C hyperfine splitting structures in an ESR spectrum for the sample showed features quite different from those for the major isomer of La@C82 with a 1.159-G 139Lahyperfine splitting. We concluded that isolated species is a carbon-cage isomer of the major La@&.
There has been a growing interest in the characterization of r-metallofullerenes, especially ESR-active ones.' Very recently, two groups independently succeeded in isolating a major isomer of La@&, which shows an electronic spectrum with lowenergy absorptions as well as an ESR spectrum of eight equallyspaced octet lines with a 1.15-G (139La)hyperfine coupling.2 However, previous ESR measurement of lanthanofullerene mixtures showed another minor octet signal with a smaller hyperfine ~ p l i t t i n g . ~In . ~ this Letter, we describe the first isolation and characterization of a minor octet species, a cage isomer of La@&. Since the minor octet species is much more air-sensitive than t the major 0ne,2b,3c the isolation has been achieved by manipulating single-stage high-performance liquid chromatography (HPLC) with an on-line degasser. A peak fraction containing a minor octet species and empty fullerenes was eluted after a major La@& on a 2-( 1-pyreny1)ethylsilylated silica gel column4 (SPYE,Nacalai Tesque Co.) with a toluene eluent at 20 "C. Repeated chromatography at 0 "C purifed the minor octet species,s which is dark brown in toluene.6 The ESR spectrum of the sample (Figure 1A) shows one set of equally spaced octet lines with a 0.836-G hyperfine splitting, which is the same as that in the crude extracted m i x t ~ r e . ~ ~ . ~ 3360 3364 3368 3372 3376 This octet signal was stable in a degassed solution for more [GI than 6 months. The relative signal intensity remained unFigure 1. (A) ESR spectrum of the isolated minor La@Csz isomer. changed in degassed 1,2,4-trichlorobenzene,even at 200 "C for Spin parameters obtained are as follows: La hyperfine coupling with 1 h. Note that no other octet signals appeared during the 0.836 G and AHpp= 0.165 G. (B) ESR spectrum of the isolated major La@& isomer for comparisomZbSpin parameters: La hyperfiie experiments. The positive ion fast atom bombardment (FAB) coupling with 1.159 G and AHpp= 0.154 G. Both spectra were mass spectrum of the sample indicates that it contains La@Csz measured at room temperature in 1,2,4-t1ichlorobenzene. alone as shown in Figure 2. Both the ESR spectrum and this mass spectral result conclude that it is surely an isomer of structure.2b The new absorption peaks at 708, 1093, 1675, and La@Csz. 1800 nm are observed in spectrum A. Figure 3 shows the visible and near-IR absorption spectrum The redox property of the La@C82 isomer was measured by of the isolated minor isomer (A) as well as the major isomer cyclic (CV) and differential pulse voltammograms (DPV).8 The (B) in carbon disulfide.' Both isomers have near-IR absorption CV shows one reversible oxidation and four reversible reducbands down to 2300 nm due to their open-shell electronic tions (E1,z = +197, -207, -1138, -1743, and -2146 mV vs AglAgNO, reference electrode) as well as an irreversible oxidation (+1352 mV by DPV) as shown in Figure 4. The @Abstractpublished in Advance ACS Abstracts, November 15, 1994. 0022-365419412098-12831$04.50/0
0 1994 American Chemical Society
Letters
12832 J. Phys. Chem., Vol. 98, No. 49, 1994
I
I 700
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Figure 2. Positive FAB mass spectrum of a minor isomer of La@(& from 700 to 1200 amu. Inset: expanded views of the 1075-1165 amu region.
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Figure 3. (A) Visible and near-IR absorption spectrum from 500 to 2500 nm for the isolated minor La@& isomer in CSz at mom temperature. (B) Absorption spectrum for the major La@C82 isomer under the same conditions.
Figure 4. Cyclic and differential pulse voltammograms of the minor isomer of La@& in 1,2-dichlorobenzene containing 0.1 M (n-Bu)dNPF6. The rest potential is +87 mV vs Ag/AgNOs. The dotted line shows the differential pulse voltammogram of the major isomer of La@Cs2under the same conditions. CV: scan rate, 20 mVls. DPV: pulse amplitude, 50 mV; pulse width, 50 ms; pulse period, 200 ms.
following are the salient features: (1) The f i s t oxidation potential shifts negatively by 143 mV relative to that of the major isomer.8 This is in agreement with the observation that the minor isomer is more sensitive to o ~ y g e n ? ~(2) . ~Each ~ of the isomers has a small potential difference between the first oxidation and reduction (major, 494 mV;8 minor, 404 mV). (3) By DPV, the peak current intensity of the second reduction (-1117 mV) is twice that of each of the redox peaks. In contrast, the major isomer shows two sequential one-electron reductions (-1084 and -1256 mV by DPV).*
The observed differences between major and minor isomers, such as 13Chyperfine structures in the ESR spectra, absorption spectra, and three voltammograms, could be attributed to the difference in (282 cages. Indeed, Kikuchi et al. suggested from their 13C NMR measurement that their c 8 2 sample consists of at least four cage isomer^.^ For both isomers of La@C82, the electronic structure should be described as La3+C823- experimentallylo and theoretically." Another possibility might be that the La atom is in the same cage but at different positions, Le., conformers of the same m e t a l l ~ f u l l e r e n e . ~Based ~ ~ ~ ~on the
Letters model calculations showing that such a less stable species isomerizes to a stable one with a barrier of 30 kcal/mo1,llbthe observed stability of the minor isomer might also exclude the latter possibility.
Acknowledgment. This work was partly supported by the Japanese Ministry of Education, Science and Culture of Japan (Nos. 05233204 and 05233231). We express thanks to Mr. K. Sugiyama (PNC), K. Sakurai (Zuiho Sangyo Co.), T. Ishiguro (Genshiryoku Gijyutsu Co.), and Y. Kano (GGC) for their experimental help. K.Y. thanks Dr. S. Suzuki for helpful discussions. References and Notes (1) For recent reviews on metallofullerenes, see, e.g.: Bethune, D. S.; Johnson, R. D.; Salem, J. R.; de Vries, M. S.; Yannoni, C. S. Nature 1993, 366, 123. (2) (a) Kikuchi, K.; Suzuki, S.; Nakao, Y.; Nakahara, N.; Wakabayashi, T. Shiromaru, H.; Saito, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Lett. 1993, 216, 67. (b) Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T. J. Phys. Chem. 1994, 98, 2008. (3) (a) Hoinkis, M.; Yannoni, C. S.; Bethune, D. S.; Salem, J. R.; Johnson, R. D.; Crowder, M. S.; de Vries, M. S. Chem. Phys. Lett. 1992, 198,461. (b)Suzuki, S.; Kawata, S.; Shiromm, H.; Yamauchi, K.; Kikuchi, K.; Kato, T.; Achiba, Y. J . Phys. Chem. 1992, 96, 7159. (c) Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H.; Saito, Y .; Yamaguchi, H.; Hayashi,
J. Phys. Chem., Vol. 98, No. 49, 1994 12833 N.; Sato, H.; Shinohara, H. J. Phys. Chem. 1992, 96, 9609. (4) Kimata, K.; Hosoya, K.; Araki, T.; Tanaka, N. J. Org. Chem. 1993, 58, 282. (5) CSSwas also isolated; details will be reported elsewhere. (6) Reaction of the metallofullerene with a trace amount of oxygen cannot be avoided during the purification of the minor isomer. Therefore, the recovery was less than 5%. (7) The absorption spectrum A was changed in a few hours under aerobic conditions. The changed spectrum, however, does not superpose on spectrum B. (8) Suzuki,T.; Maruyama, Y.; Kato, T.; Kikuchi, K.; Achiba, Y. J. Am. Chem. SOC. 1993, 115, 11006. (9) Kikuchi, K.; Nakahara, N.; Wakabayashi, T.; Suzuki,S.; Shiromaru, H.; Miyake, Y.; Saito, K.; Ikemoto, I.; Kainosho, M.; Achiba, Y. Nature 1992, 357, 142. (10) (a) Johnson, R. D.; de Vries, M. S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nature 1992, 355, 239. (b) Weaver, J. H.; Chai, Y.; Kroll, G. H.; Jim,C.; Ohno, T. R.; Haufler, R. E.; Guo,T.; Alford, J. M.; Conceicao, J.; Chibante, L. P. F.; Jain, A.; Palmer, G.; Smalley, R. E. Chem. Phys. Lett. 1992, 190, 460. (11) (a) Laasonen, K.; Andreoni, W.; Parrinello, M. Science 1992,258, 1916. (b) Nagase, S.; Kobayashi, K.; Kato, T.; Achiba, Y. Chem. Phys. Lett.1993,201,475. (c) Nagase, S.; Kobayashi, K. Chem. Phys. Lett. 1993, 214, 57. (d) Poirier, D. M.; Knupfer, M.; Weaver, J. H.; Andreoni, W.; Laasonen, K.; Parrinello, M.; Bethune, D. S.; Kikuchi, K.; Achiba, Y. Phys. Rev. B . 1994,49, 17403. (12) Yannoni, C. S.; Wendt, H. R.; de Vries, M. S.; Siemens, R. L.; Salem, J. R.; Lyerla, J.; Johnson, R. D.; Hoinkis, M.; Crowder, M. S.; Brown, C. A.; Bethune, D. S.; Taylor, L.; Nguyen, D.; Jedrzejewski, P.; Dom, H. C. Synth. Met. 1993, 59, 279.
