The Journal of
Physical Chemistry VOLUME 98, NUMBER 35, SEPTEMBER 1, 1994
0 Copyright 1994 by the American Chemical Society
LETTERS Spectroscopic Properties of Isolated Sc3@ CSZMetallofullerene Hisanori Shinohara' and Masayasu Inakuma Department of Chemistry, Nagoya University, Nagoya 464, Japan
Naohiro Hayashi and Hiroyasu Sat0 Department of Chemistry for Materials, Mie University, Tsu 514, Japan
Yahachi Saito Department of Electrical and Electronic Engineering, Mie University, Tsu 51 4, Japan
Tatsuhisa Kat0 and Shunji Bandow Institute for Molecular Science, Myodaiji Okazaki 444, Japan Received: June 15, 1994'
An endohedral triscandium fullerene, sc3@c82,has been purified and isolated for the first time by the so-called two-stage high-performance liquid chromatography. The isolated Sc3@C82 fullerene has an absorption onset at ca. 1100 nm in toluene solution with several broad features up to 400 nm. The ESR spectrum of the isolated Sc~@C82exhibits 22 perfectly symmetric hyperfine splittings with an inherent line width of 0.77 G at 220 K, which is consistent with a unique structure that three scandium atoms are encaged triangularly within the C3, isomer of C82.
Introduction Fullerenes with various metal atoms encapsulated within the carbon cage have gained extremely wide interest in recent years.l The experimental study of endohedral metallofullerenes was triggered by the Rice group in 1991 when Chai et a1.2 succeeded in producing and extracting a lanthanum fullerene, La@Csz, by using the high-temperature laser-vaporization method. The endohedral metallofullerenes produced and solventextracted so far can encage La,2-4 Y,5,6 Sc,7-10 or most of the lanthanoid elements.l1J2 Experimental evidence including electron spin resonance (ESR)395-10and scanning tunneling microscopy (STM) measurementsl3J4 indicates that the metal atoms are trapped within the carbon cage. Because of the scarcity of endohedral metallofullerenes with respect to normal hollow fullerenes in soot extracts, the purification
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, August 15, 1994. 0022-3654/94/2098-8597$04.50/0
and isolation of the metallofullerenes were not possible. In our recent report,19 several discandium metallofullerenes (Sq@C74, Sc2@cs2, Sc2@cs4) have been purified and isolated, for the first time, by the so-called two-stage high-performance liquid chromatography (HPLC) method. Up to the present, other metallofullerenes such as La@Cs2,I5J6Y @ C S ~ ,G~ ~~@ J ~C S ~ , 'and ~J* Pr@CCs217 have been isolated. Following this study we reported the direct imaging of the isolated discandium fullerene ( S C ~ @ C S ~ ) on silicon clean surfaces in an ultrahigh-vacuum environment by utilizing scanning tunneling microscopy (STM).13J4 The STM results on silicon clean surfaces have revealed nearly spherical scz@c84 fullerenes, spaced 11.7 A apart and stacked in closepacked arrays even at room temperature. The sc3@c82 fullerene has been produced, for the first time, by our group7.8 and also independently by the IBM (Almaden) group9 and known to be the first and the only endohedral metallofullerene so far that can encage as many as three metal atoms. In the present study, the isolation, electronic properties, 0 1994 American Chemical Society
8598
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The Journal of Physical Chemistry, Vol. 98, No. 35, 1994
and the structure of the endohedral triscandium fullerene, Sq@C~gt,are reported. The electron spin resonance (ESR) spectra of the purified Sc3@C82in CS2 solution exhibit 22 narrow (0.770 G) and equally spaced (6.51 G) lines of a perfectly symmetric intensity distribution centered a t g = 1.9985. The results are consistent with the idea that three scandium atoms are trapped in the C3, isomer of c82.
LD-TOF-MS (355
nm) I
Experimental Section We prepared scandium-containing fullerenes by arc burning of a composite rod composed of Sc2O3 (99.9% purity, 2.5 g), graphite powder (99.995%. 4.3 g), and high-strength pitch (5.0 g). The scandium/graphite rods were baked at 1000 OC for 5 h in vacuum (10-3 Torr) and then cured and carbonized at 1600 OC for another 5 h ( l t 5 Torr). These heat treatments of the composite rods have been found to be crucial to an efficient production of endohedral metallofullerenes via the arc-discharge method.20 The composite of Sc203, graphite powder, and pitch was stuffed within the graphite rods (20 X 500 mm) which were used as positive electrodes in the direct current (500 A) spark mode under 50-100 Torr of H e flow conditions. The soot so produced was collected under totally anaerobic conditions to avoid unnecessary degradation of the metallofullerenes produced during the soot collection and handling.21 The resulting soot was Soxhletextracted by carbon disulfide for 12 h. The S C ~c @ 8 2 fullerene was separated and isolated fromvarious hollow (c60-c96) and other types of scandium fullerenes (such as Sc@C82and sc2@c84) by the two-stage HPLC method. The two-stage HPLC methodlg was first successfully applied to the isolation of several discandium fullerenes including Sc2@C84 by the present group. Briefly, in the first HPLC stage, the toluene solution of the extracts was separated by a preparative recycling HPLC system (Japan Analytical Industry LC-908-C60) with a Trident-Tri-DNPcolumn (Buckyclutcher I, 21 X 500 mm; Regis Chemical). In the present study, the Buckyclutcher I column was used with a 100% toluene eluent at a typical flow speed of 10 mL/min. In this HPLC process, the Sq@C82 fullerenecontaining fraction was separated from other fractions including c60, C70, and higher fullerenes (c7&96). The complete purification and isolation of Sc3@c 8 2 were performed in the second HPLC stage by using a Cosmosil Buckyprep Column (10 X 250 mm, Nacalai Tesque)22 with a 100% toluene eluent. The isolation of the S C ~C82 @ metallofullerene was confirmed by laser-desorption time-of-flight (LD-TOF) mass spectrometry as well as the measurements of the 22 ESR hyperfine line^^-^ of the corresponding HPLC fraction.
Results and Discussion The production of the triscandium fullerene, Sc3@C82, was found to be sensitive to the mixing ratio of scandium and carbon atoms in the composite rods for arc discharge; the relative abundance of di- and triscandium fullerene increases as the carbon/scandium ratio decreases. For example, in the carbon/ scandium (atomic) ratio of 86.2, the formation of mono- and discandium fullerenes such as Sc@C82 and Sc2@C84 was dominant, and the production of Sc3@Cs2was almost negligible. In the present study, toenhance the production of the triscandium fullerene, we have used the composite rods with the C/Sc mixing ratio of 9.9. Even with this high mixing ratio, the estimated production ratio of.Sc3@C82/SC2@C84 is only about 0.2. It was observed that the major scandium fullerene produced was Scz@C84 (isomers I and 11) over a wide range of the carbon/ scandium mixing ratio (10-100).7J9 In the second stage of the HPLC separation, a broad fraction which contains a weak series of triscandium fullerenes, Sc3@C, ( n = 84,86,88,90), was seen at much later retention times than that of Scg@C82. The isolated Sc3@C82fullerene has a typical HPLC retention time at 26.3 min by the Buckyclutcher I column
7
BOO
1000
1400
m/ 2 Figure 1. Laser-desorption(355 nm) time-of-flightpositive mass spectrum of the isolated Scj@C82fraction collected in the final HPLC stage (see text). No other fullerenes are observed besides a very weak signal due
to SC3@C84.
sC3@c82
(Isolated)
Figure 2. UV-vis-Near-IR absorptionspectrumof the isolated Sca@C~z in toluene solution. The absorption has the onset at ca. 1100 nm. Broad features are seen around 690 and 1020 nm.
with the conditionsdescribed in the Experimental Section. Figure 1 exhibits a laser-desorption time-of-flight (LD-TOF) mass spectrum of an isolated S C ~ @ Cfraction. S~ As can be seen, no hollow fullerenes or other scandium fullerenes are observed besides a smallamount of Sc3@C84(less than3%ofSc3@C82),indicating the isolation of Sc3@Cg2. Figure 2 shows a UV-vis absorption spectrum of the isolated Sc3@Cs2in toluene solution. The spectrum exhibits a long tail down to 1000 nm. The absorption onset is observed at around 1100 nm, which corresponds to the HOMO-LUMO gap of 1.1 eV. The overall spectral features are broad with the weak bands a t 690 and 1020 nm. Similar broad spectral patterns have been observed in the UV-vis absorption spectra of discandium fullerenes such as S C Z @ C S ~These . ~ ~ spectral features are contrasted to those of monometallofullerenes like La@ C82,15J6Y @ C82,17J8 Gd@C82I7J8 and Pr@C82,17 which have the relatively strong absorption peaks at around 1000 and 1400 nm. These near-IR absorption features can be interpreted by dimer (or cluster) formation of the monometallofullerenes, and this is not the case of di- and triscandium fullerenes. The details of the dimer formation for the monometallofullerenes will be published separately.23 Figure 3 exhibits an ESR spectrum of the isolated S C ~ @ C S ~ in a degassed CS2 solution at 220 K. The spectrum shows perfectly symmetric, equally spaced (6.51 G), 22 narrow (0.770 G) ESR hyperfine splittings (hfs) centered at g = 1.9985, which is a manifestation of the isotropic hyperfine coupling of three scandium nuclei with Z = 7 / 2 in the cage. A similar hfs has been observed already by Nagoya7.8 and IBM (Almaden)g groups in unpurified Sc3@Cg2 samples which contain various hollow and other types of scandium fullerenes such as Sc@C82and S C ~ @ C SA~partial . chromatographic separation of the Sc@Cs2 and Sc3@Cg2 fullerenes has been reported by using a silica-gel column.8 However, due to an imperfect separation the spectral analysis of
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The Journal of Physical Chemistry, Vol. 98, No. 35. 1994 8599
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Figure 3. X-band (9.4360 GHz) ESRspectrum for the isolated SC&CSZ in CS2 solution at 220 K, showing well-resolved, equally-spaced, and perfectly symmetric 22 hyperfine splittings (g = 1.9985, A = 6.51, AH = 0.770 G).
a t 220 K above which the hfs line width increases as temperature increases. A similar temperature dependence has been reported for L a @ C ~ z . ~Even l a t this temperature the hfs line width of Sc3@Cg~is about 20 times as broad as that of Sc@C82. From the temperature dependence we conclude that each hfs line width is homogeneously broadened due to dynamical averaging effects of the encaged scandium atoms. Our preliminary analysis27 suggests that each scandium atom (ion) in C82 is in a certain jumping motion from one local minimum to another. A detailed study on the dynamics of the scandium ions in the CSZcage is in progress. Acknowledgment. We express our thanks to Profs. M. Ohkohchi and Y. Ando (Meijo University) for the use of their high-temperature vacuum oven. Thanks are also due to Prof. S. Nagase for communicating his results prior to publication. H.S. thanks the Japanese Ministry of Education, Science and Culture Grant-in-Aid for Scientific Research on Priority Areas (No. 05233 108) and General Scientific Research (No. 06403006) for the support of the present study.
References and Notes
i o.6'
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' 200
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(1) Bethune,D.S.; Johnson,R. D.;Salem, J. R.;deVries,M. S.;Yannoni, C. S. Nature 1993,366, 123. (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) Johnson, R. D.; de Vries, M. S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nature 1992,355,239. (4) Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R. B.; Min, K. S.;Whetten, R. L. J. Phys. Chem. 1991,95,10561. (5) Weaver, J. H.; Chai, Y.; Kroll, G. H.; Jin, 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. (6) Shinohara, H.; Sato, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. J . Phys. Chem. 1992,96,3571. (7) Shinohara,H.;Sato,H.;Ohchochi,M.;Ando,Y.;Kodama,T.;Shida,
T.; Kato, T.; Saito, Y. Nature 1992,357,52. (8) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Inagaki, M.; Saito, Y.; Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H. Mater. Sci. Eng. 1993,B19,25. (9) Yannoni, C. S.;Hoinkis, M.; de Vries, M. S.;Bethune, D. S.;Salem, J. R.: Crowder. M. S.:Johnson. R. D. Science 1992. 256. 1191. the 22 hfs lines was not possible. The present ESR spectrum of (10) Suzuki,S.;Kawata, S.;Shiromaru, H.; Yamauchi, K.; Kikuchi, K.; the isolated Sc3@C82 sample presents a well-resolved hyperfine Kato, T.; Achiba, Y. J. Phys. Chem. 1992,96,7159. (1 1) Gillan, E.; Yeretzian, C.; Min, K. S.; Alvarez, M. M.; Whetten, R. splitting, which provides us an important structural information L.; Kaner, R. B. J. Phys. Chem. 1992,96,6869. on Sc3@C82. (12) Moro, L.; Ruoff, R. S.; Becker, C. H.; Lorents, D. C.; Malhotra, R. The presence of the perfectly symmetric 22 hfs lines suggests J. Phys. Chem. 1993,97,6801. (13) Shinohara, H. S.;Hayashi, N.; Sato, H.; Saito, Y.; Wang, X.D.; the geometrical equivalency of the three scandium atoms in the Hashizume, T.; Sakurai, T. J. Phys. Chem. 1993,97, 13438. C82 cage.' The W - N M R mea~urements2~ for the hollow C82 (14) Wang, X. D.; Xue, Q.K.; Hashizume, T.; Shinohara, H.; Nishina, fullerene indicate the presence of the, a t least, three structural Y.; Sakurai, T. Phys. Rev. B 1993,48,15492. (15) Kikuchi, K.; Suzuki,S.;Nakao, Y.; Nakahara, N.; Wakabayashi, T.; isomers: C2, C, and C3". At present, no theoretical calculation Shiromaru, H.; Saito, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Letz. 1993, has been reported for Sc3@C82. However, a recent ab initio 216,67. calculationonScz@C84 (theD~isomer)~~reveals that theencaged (16) Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T. J. Phys. two scandium atoms are well separated from each other by 4.05 Chem. 1994,98,2008. (17) Shinohara, H.; Hayashi, N.; Inakuma, M.; Kishida, M.; Nakane, T. 8, along the D2d axis and that a substantial electron transfer from Proceedings of the 185th Electrochemical Society Meeting "Fullerenes: the scandium atoms to the C82 cage is taking place. According Chemistry, Physics and New Directions VI", San Francisco, May 1994. to the ab initio calculation,2sthe formal net charge of the species (18) Kikuchi, K.; Achiba, Y. Private communication. (19) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Ohkohchi, can be described as (Sc2)4.4+@C~4~.'. A similar charge transfer Ando, Y.; Saito, Y. J. Phys. Chem. 1993,97,4259. has been alreadyreported by Nagaseand co-workers on S C @ C S ~ , * ~ M.;(20) Bandow, S.; Shinohara, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. J. which leads to the electronic structure of S C ~ + @ C ~Based ~ ~ - .on Phys. Chem. 1993,97,6101. (21) Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H.; Saito, Y.; the appearance of the perfectly symmetric 22 hfs and the results Yamaguchi, H.; Hayashi, N.; Sato, H.; Shinohara, H. J. Phys. Chem. 1992, of the theoretical calculations described above, three scandium 96, 9606. atoms are separated each from other within the C82 cage so as (22) ,Kimata, K.; Hosoya, K.; Tanaka, N. Presented at the 17th International Symposium on Column Liquid Chromatography, Basel, Switto retain a 3-fold axis as an entire Sc3@C82 molecule. This zerland, May 1993. condition is only satisfied if the three scandium atoms (ions) are (23) Shinohara, H.; Inakuma, M.; Kishida, M.; Yamazaki, S.;Hashizume, situated in a triangular position of the C3, isomer of C82. T.; Sakurai, T. Nature, submitted. (24) Kikuchi, K.; et al. Nature 1992,357, 142-143. The temperature dependence of the 22 hfs lines can provide (25) Nagase, S. Private communication. us with further structural information on Sc3@C82. Figure 4 (26) Nagase, S.;Kobayashi, K. Chem. Phys. Lea. 1993,224,57. shows a temperature dependence of the line width (AH) for one (27) Kato, T.; Bandow, S.;Inakuma, M.; Shinohara, H. J. Phys. Chem., submitted. of the 22 hfs lines of Sc3@C82. The A H value has a minimum
Figure 4. Temperature dependence of a line width of the ESR hyperfine splittings for Sc&~Cgz in CS2 solution. The minimum line width is observed a t 220 K.
