J. Phys. Chem. B 2001, 105, 2971-2974
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Structural Determination of the La@C82 Isomer Takeshi Akasaka,*,†,‡ Takatsugu Wakahara,† Shigeru Nagase,*,§ Kaoru Kobayashi,§ Markus Waelchli,| Kazunori Yamamoto,⊥ Masahiro Kondo,† Shingo Shirakura,† Yutaka Maeda,† Tatsuhisa Kato,‡ Masahiro Kako,∇ Yasuhiro Nakadaira,∇ Xiang Gao,∞ Eric Van Caemelbecke,∞ and Karl M. Kadish*,∞ Graduate School of Science and Technology, Niigata UniVersity, Niigata 950-2181, Japan, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan, Department of Chemistry, Graduate School of Science, Tokyo Metropolitan UniVersity, Hachioji, Tokyo 192-0397, Japan, Bruker Japan, Tsukuba, Ibaraki 305-0051, Power Reactor & Nuclear Fuel DeVelopment Corporation, Tokai, Ibaraki 319-1100, Japan, Department of Applied Physics and Chemistry, The UniVersity of Electro-Communications, Chofu, Tokyo 182-8585, Japan, and Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: October 25, 2000
A stable diamagnetic monoanion of the La@C82 isomer was electrochemically prepared and isolated in order to disclose its cage symmetry. By measuring the 13C NMR spectrum of the anion, it was determined for the first time that the isomer has Cs symmetry, as was also confirmed by density functional calculations.
Introduction Endohedral metallofullerenes (fullerenes with metal atoms encapsulated inside hollow carbon cages) have attracted special interest as new spherical molecules which have unique properties that are not seen for empty fullerenes.1 Among these, La@C82 has been recognized as the prototype of endohedral metallofullerenes since the first successful extraction in 1991 by Smalley and co-workers.2 Interestingly, it has been found from electron spin resonance (ESR) measurements that two isomers of La@C823 can be extracted and isolated and this is also the case for Sc@C823b,4a and
[email protected],4 We have very recently verified, by observing the 13C NMR spectrum of its anion, that the major isomer of La@C82 (La@C82-A) has C2V symmetry,5 while the C2V structure has also been found for Sc@C82-A from an X-ray powder diffraction study by a maximum entropy method,6 and both were predicted from theoretical calculations.7 It has been reported that the second isomer of M@C82 (M@C82-B, where M ) La, Y, and Sc) is considerably less stable in air or solution than
[email protected],4 It was once assumed that M@C82-A and M@C82-B had the same cage structure but differed in their metal positions.8 However, it is now generally considered that they have different cage structures.1b-d Although several attempts have been made to elucidate the structural feature, its cage symmetry is still unknown for any M@C82-B isomer. The structural determination of endohedral metallofullerene isomers is an important subject, since this knowledge can provide a key clue in elucidating formation mechanisms and in developing new routes to bulk production of endohedral metallofullerenes. In this context, it is of interest to determine * Corresponding authors. Present address for T.A.: Center for Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba, Ibaraki 3058577, Japan; e-mail
[email protected]. † Niigata University. ‡ Institute for Molecular Science. § Tokyo Metropolitan University. | Bruker Japan. ⊥ Power Reactor & Nuclear Fuel Development Corporation. ∇ The University of Electro-Communications. ∞ University of Houston.
the structures of representative isomers. We now report the 13C NMR structural determination of the La@C82-B isomer, which is the first example in the M@C82 series (M ) La, Y, and Sc). Experimental Section La@C82-B was prepared and separated according to our recently developed method.3f Electrochemical-grade tetra-nbutylammonium perchlorate (TBAP), purchased from Wako, was recrystallized from absolute ethanol and dried in a vacuum at 313 K prior to use. 1,2-Dichlorobenzene was distilled over P2O5 under vacuum prior to use. UV-visible spectra were recorded on Perkin-Elmer Lambda 19 and Hewlett-Packard model 8453 spectrophotometers. Near-IR absorption spectra for metallofullerenes were measured with a Perkin-Elmer model 330 spectrometer. ESR measurements were performed on a conventional X-band ESR spectrometer equipped with a variable-temperature apparatus (Bruker ER 100D). Positive-ion fast atom bombardment (FAB) mass spectral data were obtained on a JEOL JMS-SX102A mass spectrometer with m-nitrobenzyl alcohol as the matrix. Cyclic voltammetry and controlled-potential bulk electrolyses in an o-dichlorobenzene (ODCB)/1,2,4-trichlorobenzene(TCB) (3:1) mixture were carried out with an EG & G Princeton Applied Research (PAR) 263 potentiostat/galvanostat and a BAS CV-50W. A conventional three-electrode cell was used for CV measurements and consisted of a platinum working electrode, a platinum counterelectrode, and a saturated calomel reference electrode (SCE). Controlled-potential bulk electrolyses were performed with an H-type cell consisting of two platinum gauze electrodes (working and counter electrodes) that were separated by a sintered glass frit. Solutions containing the anion and cation of La@C82-B [La@C82-B(-) and La@C82-B(+)] were obtained in ODCB/TCB (3:1) containing 0.2 M TBAP by setting the applied potential at values 150-250 mV more negative or more positive than E1/2 for the La@C82(n+1)-/La@C82n- and La@C82n+/ La@C82(n-1)+ redox couples, respectively. Cyclic voltammetric measurements were carried out immediately after bulk electrolysis. The electrogenerated La@C82-B(-) and La@C82-B(+)
10.1021/jp003930d CCC: $20.00 © 2001 American Chemical Society Published on Web 03/24/2001
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Akasaka et al.
were then transferred from the bulk cell to a 1.00 cm quartz cuvette under an argon atmosphere. Near-IR and UV-vis measurements were carried out under an Ar atmosphere. After electrolysis of La@C82-B, ESR measurements were carried out at 133 K. The sample (ca. 2 mg) prepared by bulk electrolysis was dissolved in carbon disulfide and acetone (1:1) under an argon atmosphere, with a capillary tube of D2O as an external lock solvent. [Cr(acac)3] (acac ) acetylacetonate) was added as a relaxant for 13C NMR measurements. 13C NMR spectra were obtained at 125 MHz on a Bruker DRX500 spectrometer with a CryoProbe system. Chemical shifts were expressed downfield of the signal for the carbon atom in carbon disulfide as an internal standard (δ ) 195.0). The 139La NMR spectrum was measured at 300 K in ODCB-d4 at 70.6 MHz on a Bruker DMX500 spectrometer. The 139La chemical shift was calibrated with 0.6 M LaCl3/D2O as an external reference (δ ) 0). Results and Discussion The C82 fullerene has nine distinct isomers [C3V(a), C3V(b), C2V, C2(a), C2(b), C2(c), Cs(a), Cs(b), and Cs(c)] that satisfy the so-called isolated pentagon rule9 (see ref 10 for these structures).13C NMR measurements of C82 have shown that only one isomer with C2 symmetry is abundantly produced.11 Accordingly, it has been calculated that the C2(a) isomer is the most stable of the isomers.10 However, a three-electron transfer from La to the C82 cage markedly changes the relative stability of the isomers.7 The C2(a) isomer is highly destabilized upon accepting three electrons and becomes the third most unstable isomer in the case of C823-. Instead, the C2V, C3V(b), and Cs(c) isomers (that are highly unstable for C82) become most stable for C823-. It has been predicted that encapsulation of La inside these C2V, C3V(b), and Cs(c) cage isomers is much more favorable than that inside the C2(a) cage or the other isomers, and this leads to endohedral structures with C2V, C3V, and Cs symmetry, respectively.7 These structures are close to each other in energy and have 24 [17(4) + 7(2)], 17 [11(6) + 5(3) + 1(1)], and 44 [38(2) + 6(1)] nonequivalent carbons, respectively, where the values in parentheses denote the relative intensities. By observing 24 [17 + 7 (half intensity)] 13C NMR lines for La@C82A(-), we have recently verified that La@C82-A has C2V symmetry.5 This is consistent with the fact that the C2V endohedral structure is energetically most stable.7 It is therefore most likely that La@C82-B has either C3V or Cs symmetry. Figure 1a compares the redox potentials observed for La@C82-A and
[email protected] La@C82-B has a lower oxidation (Eox) and a higher reduction (Ered) potential than La@C82-A. To provide insight into the redox properties, the C2V, C3V, and Cs endohedral structures of La@C82 were optimized with nonlocal density functional theory at the BLYP level12 by use of the Gaussian 98 program13 to calculate the vertical ionization potentials (Ip) and electron affinities (Ea).14 The calculated Ip and Ea values are shown in Figure 1b. The C3V structure has larger Ip (6.39 eV) and Ea (3.49 eV) values than the C2V structure (Ip ) 6.07 eV and Ea ) 3.15 eV). In contrast, the Ip (5.97 eV) and Ea (3.02 eV) of the Cs structure are smaller than those of the C2V structure, this trend agreeing with the redox potentials observed for La@C82-A and La@C82-B. These results predict that La@C82-B has Cs symmetry. The electronic absorption spectra of La@C82-B(-) and its neutral and cationic forms are shown in Figure 2. The absorption spectrum of La@C82-B has broad absorption bands ranging over the entire near-IR region and this agrees with its open-shell electronic structure that is described formally as La3+C823-.
Figure 1. (a) Plot of oxidation (Eox) and reduction (Ered) potentials of La@C82-A and La@C82-B and (b) plot of ionization potentials (Ip) and electron affinities (Ea) calculated at the BLYP level for C2V, C3V, and Cs endohedral structures of La@C82.
Figure 2. Vis-near-IR absorption spectra of La@C82-B(-), La@C82B, and La@C82-B(+).
Actually, neutral La@C82-B has near-IR absorption bands down to 2300 nm, while its anion has an onset of a band at around 1600 nm. La@C82-B(-) exhibits broad visible absorption bands at 770, 660, and 580 nm. No color change is observed during reduction of La@C82-B. An interesting finding is that the La@C82-B(-) anion is stable in air for at least 1 week, unlike paramagnetic
[email protected] In contrast, the absorption spectrum of La@C82-B(+) remained unchanged only for several hours at room temperature under an argon atmosphere. No ESR signal was observed for either La@C82-B(-) or La@C82-B(+), confirming that both are diamagnetic. The high stability and diamagnetic nature of La@C82-B(-) allowed NMR measurements to be made. The 139La NMR
Structural Determination of the La@C82 Isomer
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2973
Figure 4. Two views of the optimized structure of La@C82-B (Cs) at the BLYP level.
Figure 3. (a) 13C NMR spectrum of La@C82-B(-) and (b) its expanded views.
spectrum exhibited a single peak in o-dichlorobenzene-d4 at 300 K with a line width of ∼1600 Hz. The chemical shift observed at -520 ppm is close to that of La@C82-A(-), which has a resonance at -470 ppm. This may suggest that La@C82-A(-) and La@C82-B(-) have a similar charge on the La atom. Figure 3 shows the 125 MHz 13C NMR spectrum of La@C82B(-) observed in carbon disulfide and acetone with Cr(acac)3 as a relaxant. A total of 44 13C NMR signals is observed in the range 139-168 ppm.15 This range is slightly wider than that observed for both empty C82 (131-151 ppm)11 and La@C82A(-) (135-158 ppm).5 As is clearly shown in Figure 3, the spectrum of La@C82-B (-) consists of 38 distinct lines of nearequal intensity and six lines of half-intensity, verifying that La@C82-B(-) (and also La@C82-B as discussed below) has Cs symmetry. This agrees with the prediction made from comparison of redox potentials with Ip and Ea values (Figure 1). It may be argued that the C3V structure of La@C82 can be deformed to have lower Cs symmetry upon reduction. However, this possibility was ruled out because vibrational frequency calculations show that C3V symmetry is maintained even after reduction and corresponds to an energy minimum on the potential energy surface. The optimized Cs structure of La@C82-B is shown in Figure 4. This La@C82-B(Cs) structure is energetically most stabilized when the La atom in the Cs plane approaches the junction between two hexagonal carbon rings in C82. The same trend was also calculated for the anion with Cs symmetry, identified as an energy minimum by frequency calculations. This confirms that La@C82-B also has Cs symmetry, as does La@C82-B(-).
Figure 5. Orbital diagrams of La@C82-B (Cs) and its anion at the BLYP level in electronvolts.
The distances between La and the near carbons in La@C82B(Cs) were calculated to be 2.541 and 2.566 Å. These distances differ little from the values of 2.539 and 2.563 Å in the anion, which indicates that the La position is changed little even after reduction. Three valence electrons on La (5d16s2) are transferred in La@C82-B (Cs) to the LUMO and LUMO + 1 level of the C82 cage. The electronic structure is then formally described as La3+C823-, despite significant back transfer from C82. Consequently, La@C82-B(Cs) has an open-shell structure, as does La@C82-A(C2V). The addition of one electron to La@C82 leads to La@C82(-), in which the singly occupied LUMO + 1 level is filled up to form a closed-shell electronic structure, as shown in Figure 5. The resultant anion has no radical character, as evidenced by the lack of an ESR signal. Therefore, it is not surprising that La@C82-B(-) is less air-sensitive than La@C82B. The positive charge of 2.73 on La in La@C82-B differs little from that of 2.74 in La@C82-B(-). This indicates that the reduction takes place on the carbon cage of La@C82-B. Conclusion By utilizing the high stability and diamagnetic nature of La@C82-B(-), it has been successfully determined from 13C NMR measurements that La@C82-B has Cs symmetry. It is expected that Sc@C82-B and Y@C82-B also have Cs symmetry. At present, it is still very difficult to prepare good single crystals of endohedral metallofullerenes. Therefore, NMR measurements of diamagnetic anions are the most useful method for structural determination of paramagnetic endohedral metallofullerenes.
2974 J. Phys. Chem. B, Vol. 105, No. 15, 2001 Acknowledgment. This work was supported in part by a grant from the Asahi Glass Foundation and by a grant from the Ministry of Education, Science, Sports, and Culture of Japan. K.M.K. also acknowledges support of the Robert A. Welch Foundation (Grant E-680). References and Notes (1) For recent reviews, see the following: (a) Bethune, D. S.; Johnson, R. D.; Salem, J. R.; de Vries, M. S.; Yannoni, C. S. Nature 1993, 366, 123. (b) Nagase, S.; Kobayashi, K.; Akasaka, T. Bull. Chem. Soc. Jpn. 1996, 69, 2131. (c) Nagase, S.; Kobayashi, K.; Akasaka, T. J. Comput. Chem. 1998, 19, 232. (d) Nagase, S.; Kobayashi, K.; Akasaka, T.; Wakahara, T. In Fullerenes: Chemistry, Physics and Technology; Kadish, K., Ruoff, R. S., Eds.; John Wiley & Sons: New York, 2000; pp 395-436. (2) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, J. M. Smalley, R. E. J. Phys. Chem. 1991, 95, 7564. (3) (a) Johnson, R. D.; de Vries, M. S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nature 1992, 355, 239. (b) Suzuki, S.; Kawata, S.; Shiromaru, 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, N.; Sato, H.; Shinohara, H. J. Phys. Chem. 1992, 96, 9609. (d) Kikuchi, K.; Suzuki, S.; Nakao, Y.; Nakahara, H.; Wakabayashi, T.; Shiromaru, H.; Saito, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Lett. 1993, 216, 67. (e) Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T. J. Phys. Chem. 1994, 98, 208. (f) Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T.; Suzuki, T.; Maruyama, Y. J. Phys. Chem. 1994, 98, 12831. (4) (a) Inakuma, M.; Ohno, M.; Shinohara, H. In Fullerenes: Chemistry, Physics and Technology; Kadish, K. M., Ruoff, R. S., Eds.; John Wiley & Sons: New York, 1995; pp 330-342. (b) 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. (5) Akasaka, T.; Wakahara, T.; Nagase, S.; Kobayashi, K.; Waelchli, M.; Yamamoto, K.; Kondo, M.; Shirakura, S.; Okubo, S.; Maeda, Y.; Kato, T.; Kako, M.; Nakadaira, Y.; Nagahata, R.; Gao, X.; Van Caemelbecke, F.; Kadish, K. M. J. Am. Chem. Soc. 2000, 122, 9316. (6) Takata, M.; Nishibori, E.; Umeda, B.; Sakata, M.; Inakuma, M.; Shinohara, H. Chem. Phys. Lett. 1998, 298, 79. (7) Kobayashi, K.; Nagase, S. Chem. Phys. Lett. 1998, 282, 325.
Akasaka et al. (8) (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) 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.; Dorn, H. C. Synth. Met. 1993, 59, 279. (9) Fowler, P. W.; Manolopoulos, D. E. An Atlas of Fullerenes; Clarendon: Oxford, U.K., 1995. (10) Kobayashi, K.; Nagase, S. Chem. Phys. Lett. 1997, 274, 226. (11) Achiba, Y.; Kikuchi, K.; Aihara, Y.; Wakabayashi, T.; Miyake, Y.; Kainosho, M. Mater. Res. Soc. Symp. Proc. 1995, 359, 3. (12) For the BLYP method, see the following: (a) Becke, A. D. Phys. ReV. 1988, A38, 3098. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785. For the relativistic effective core potential on La, see Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. The basis sets employed were (5s5p3d)/[4s4p3d] for La and 3-21G for C. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998. (14) The adiabatic Ip and Ea values were 6.04 and 3.19 eV for C2V, 6.39 and 3.52 eV for C3V, and 5.92 and 3.06 eV for Cs, respectively. (15) Line positions (parts per million, ppm) and relative intensities (in parentheses) observed in the 13C NMR spectrum of La@C82-B(-) are 163.82(1), 154.56(2), 154.22(1), 151.67(2), 150.82(2), 150.56(2), 150.50(2), 150.23(2), 150.19(2), 149.61(2), 149.37(2), 147.84(2), 147.70(2), 147.61(2), 147.13(2), 147.08(2), 146.81(2), 145.99(2), 145.84(2), 145.62(2), 145.58(2), 145.40(2), 145.16(2), 144.75(1), 144.55(2), 143.97(1), 143.21(1), 143.02(2), 143.01(2), 142.85(2), 142.67(2), 141.54(2), 141.51(2), 141.08(2), 140.59(2), 140.38(1), 139.80(2), 139.05(2), 138.42(2), 138.06(2), 137.72(2), 137.38(2), 137.33(2), and 135.97(2). These sp2 carbons on the C82 cage were identified by means of measurements with and without proton decoupling.
