J. Phys. Chem. 1993, 97, 4259-4261
4259
Isolation and Spectroscopic Properties of Sc2 @ C74, SCZ @ C82, and Sc2 @ C84 Hisanori Shinohara,’*+Hiroki Yamaguchi, Naoyuki Hayashi, and Hiroyasu Sato Department of Chemistry for Materials, Mie University, Tsu 51 4, Japan
Masato Ohkohchi and Yoshinori Ando Department of Physics, Meijo University, Nagoya 468, Japan
Yahachi Saito Department of Electrical and Electronic Engineering, Mie University, Tsu 51 4, Japan Received: March 8, I993
The endohedral discandium fullerenes, Sc~@C74,sc2@cg2, and Sc2@Cg4, have been isolated for the first time from soot prepared by the arc burning of Sc~03/graphitecomposite rods. The separation and isolation of the metallofullerenes from various hollow fullerenes have been realized by using a two-stage high-performance liquid chromatography. Laser-desorption time-of-flight mass analyses of the present samples confirm the isolation of the discandium fullerenes. The discandium fullerene, Scz@C74,has been newly found in this study. The UV-vis absorption spectra of the isolated Sc2@C74,scZ@c8Z,and scZ@c84reveal several salient features which are totally absent in those of the corresponding hollow fullerenes such as C76, c78, CgZ, and c g 4 .
Introduction Fullerenes with a metal atom encapsulated (endohedral metallofullerenes) have recently been found in soot prepared by arc burning of metal/graphite composite rods.l-Is The metal atoms encaged by the fullerene c82 so far include La,I-338~~JI-~3 Y,4,5,12,13 Sc,6s7Joand most of the lanthanoid elements.I4 One of the most intriguing and important properties of these novel forms of molecules is that a substantial charge transfer from encaged metal atoms to the fullerene cages is occurring and that these species can formally be expressed as La3+@Cg23-,2J1J2 Y3+@c823-,4’5’12 and
[email protected]’7’10 On the basis of the concentration dependence of electron spin resonance (ESR) intensities, it has been suggested that the metallofullerenes have a tendency to form aggregates or to form adducts involving hollow fullerenes or solvent molecule^.^-^^ Because of these novel properties and also scarcity of the metallofullerenes in solvent extracts, it has not been possible to separate and to “isolate” the metallofullerenes,so that spectroscopicand structural information on the metallofullerenes have been very limited. In this study, we report the first successfulseparation and isolationof endohedral metallofullerenesfrom hollow fullerenes and report spectroscopic properties of isolated metallofullerenes. Of particular interests are scandium fullerenes. Scandium monomer, dimer, and trimer have been found to be encaged by the fullerenes c82 and c84:6’7*’0 sc@c82,sc2@c84,sc2@c82,and sc3@c82. In the previous ~tudies,6,~J’J it has been reported that the sc@c82 and Sc3@C82 fullerenes exhibit respectively 8 and 22 ESR peaks on account of the nuclear hyperfine couplings of scandium nuclei, whereas the Sc2@c84 and Sc2@C82 fullerenes are found to be ESR silent. Furthermore, we have reported that sc@c82 and sc3@c82 can be well separated by the column chromatography by using a deactivated silica gel c01umn.~ However,these studieswere performed for the scandiumfullerenes embedded in various hollow fullerenes which were then major components of the samples. In order to obtain spectroscopicand structural information on these metallofullerenes per se, various spectroscopic measurements should be done on “pure” metallofullerene samples. To whom correspondence should be addressed. address: Department of Chemistry, Faculty of Science, Nagoya University, Nagoya 464, Japan.
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In this Letter, we report the first successful separation and, in particular, “isolation” of the scandium fullerenes. S C ~ @ C ~ ~ , SC2@C82, and sc2@c84, which are the major discandium fullerenes in soot, by using a two-stage high-pressure liquid chromatography (HPLC). The discandium fullerene S C ~ @ C ~ ~ has been newly found in the present study. The isolation has been confirmed by laser-desorption time-of-flight mass and UVvis absorption spectroscopy. The isolated Scz@C74, S C ~ @ C S ~ , and sc2@c84 fullerenesare totally stable in air and also in various solvents including benzene, toluene, and carbon disulfide (even in the complete absence of hollow fullerenes). Electron spin resonance (ESR) measurements for these isolated metallofullerenes suggest that Sc~@C74,SCZ@C82r and sc2@c84 have no ESR spectrum, indicating these discandium fullerenes are diamagnetic in contrast with the mono- and triscandium fullerenes such as La@C82, Y@c82, sC@c82, and SC~@CSZ.
Experimental Section Scandium-containing fullerenes were prepared by arc burning of rods composedof graphite powder (99.995%), Sc2O3 (Kojundo Chemical Laboratory, 99.9% purity), and high-strength pitch (TohoGas).6 We haveused severalcompositerods withdifferent mixing ratios of SqOJgraphite, which ranges from 0.06 to 0.58 by weight. In the present Letter, we report the results for two particular compositerods with a different mixing ratio of Sc2O3/ graphite (0.28 and 0.58; hereafter referred to as rods A and B, respectively) for the arc synthesis of scandium fullerenes. The scandium/graphite rods were cured and baked at 1000 OC for 1 h in vacuum (lC3Torr) and then carbonized at 2000 OC for another 5 h (lC5 Torr) under an Ar flow (1 atm, 1 L/min) condition. We have found that these heat treatments for the composite rods are necessary for a high yield synthesis of endohedral metallofullerenes.8~9The composite rods were used as positive electrodes which were consumed preferentially in the direct current spark mode (220 A) under 50-Torr static He atmosphere. The resulting soot was Soxhlet-extracted by carbon disulfide. The extracts were dissolved in toluene prior to a high-pressure liquid chromatography (HPLC) separation. The separation and isolation of scandium fullerenes were performed by the following two-stage HPLC processes by successively using two different
0022-365419312097-4259%04.00/0 0 1993 American Chemical Societv
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4260 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 I
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:Toluene-Hexane (50:SO v v ) Flow Rate :4.5ml 'mln Wavelength, 320nm Column : Buckyclutcher 1
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Figure 1. (a) An overall high-pressure liquid chromatography (HPLC) spectrum for theC90fraction (see text) whichcontainsvariousdiscandium fullerenes. (b) An isolated HPLC spectrum for the sc2@c84. In (b), only a single peak due to sc2@c84is seen. The experimental conditions are presented in the upper right of the figure.
types of HPLC systems. In the first stage, the toluene solution of the extracts was separated by a preparative recycling HPLC system (Japan Analytical Industry LC-908-C60) with a series of two polystyrene polymer columns (40 mm diameter X 600 mm long for each) with toluene eluent. The six fractions, corresponding taC60, c70, c76/c78,c84, c90, and (&/higher, were collected by this HPLC process. We have found that, for both rods A and B, the scandium fullerenes Sc2@C74, sc2@c82, and sc2@c84 were present in the c90and (&/higher fractions as revealed by mass spectroscopic analyses. It was also found that the yield of the scandium fullerenes was much higher for rod B than for rod A. For a further separation and purification of these metallofullerenes contained in the C90 and (296 fractions, we have used a semipreparative HPLC system (JASCO 875-UV detector; PU987 pump) with a Trident-Tri-DNP columnI6 (10 X 250 mm, Regis Chemical) with a toluene/hexane (5050) mixed eluent. In this second HPLC separation process, the peak due to Sc2@C74 and the peakdue toa mixtureof S C Z @ ~SCZ@CS~. SZ, andSc2@C~ were observed in the vicinity of the hollow C90 and Cg4 peaks, respectively. By collecting the corresponding fractions and by reinjecting these fractions to the column, we were able to purify and finally isolate these metallofullerenes. The isolation of scandium fullerenes was confirmed by laserdesorption time-of-flight(LD-TOF) mass and UV-vis absorption spectroscopy. In the mass analysis, the desorption and ionization laser (Nd:YAG third harmonic, 355 nm) was kept very low fluence below 100 pJ/l-mm-diameter spot on the sample to avoid unnecessaryfragmentationasThe scandium fullerenesas dissolved in toluene were coated onto a quartz plate (20 X 3 X 1 mm), quickly heated to drive off liquid, and then mounted in vacuum in the ion-extraction region of a reflectron type TOF mass spectrometer. The mass spectra confirm the isolation of several scandium fullerenes (Sc2@C74, sc2@c82,and Sc2@Cs4): the various hollow fullerenes (c60, c70, c76, C78r c84, c90-c96, and higher) are almost completely absent in the spectra. The isolated discandium fullerenes are ready to dissolve in benzene, toluene, and carbon disulfide but difficult to dissolve in hexanes. It has been found that the solubility of the discandium fullerenes in toluene is qualitatively the same as those of C ~and O c 9 6 . Electron spin resonance (ESR) spectra were recorded by an ESR spectrometer (Bruker ESP 300E) in an X-band frequency at 9.42 GHz with typically 0.2-mW power. All the ESR measurements were done at 220 K to obtain better spectral resolution.8
Time-of-Flight
Ms
Figure 2. A laser-desorption time-of-flight (LD-TOF) mass spectrum for theisolatedSc2@C84fraction (which corresponds to Figure 1b). Laser desorption and ionization were done at 3 55 nm. Note that various hollow fullerenes are completely absent in the spectrum.
Results and Discussion Figure l a shows an HPLC spectrum for the C90 fraction which containsvariousdiscandiumfullerenes (cf. ExperimentalSection). In addition to the peaks due to the hollow fullerenes, such as c84, C90, and c96, the peaks (shoulders) corresponding to discandium fullerenes are observed, where the intensity of the s C 2 @ c 8 4 peak is particularly salient. The scZ@C84fullerene has been known as one of the most abundant scandium fullerenes in soot prepared by the arc burning of the composite rod~.69~,~0 We have found that the five discandiumfullerenes, Sc2@C74,Sc2@C80,Sc28C82, Sc2@C84,andSc2@C86,arepresent in the retention timebetween 15 and 30 min. All of the peak assignments have been performed by LD-TOF mass spectroscopic analyses of the corresponding fraction. It should be noted that the two new hollow fullerenes, c 8 6 and c92, are also found in the same spectrum. Details for the observation of the new higher fullerenes, including c 8 6 , (288, C92, and C94, will be published e1~ewhere.I~ Figure l b shows an HPLC chromatogram in which a single peak due to sc2@c84 is seen. The spectrum was obtained by a repeated collection/injection (to the column) cycle of the corresponding HPLC fraction. The asymmetrical broadening of the peak to a larger retention time stems from the presence of a small amount of sc2@c86. Figure 2 reveals an LD-TOF mass spectrum for this fraction. The hollow fullerenes such as c60, c70, c76, C78r c82, c84, c90,C96, and higher fullerenes are completely absent in the mass spectrum. This indicates that in the present HPLC analysis the separation of sc2@c84 from the various hollow fullerenes is perfect. Thechromatogram alsoshows the complete absence of fragment ions, indicating that the sc2@c84 fullerene is, on the whole, as stable as hollow fullerenes against laser photolysis. A small peak adjacent to Sc2@Cs4has been identified as sc2@c86, which causes the asymmetric tail of the HPLC peak in Figure 1b. By collecting the tail fraction, we were able to isolate sc2@c86 as well (not shown). In Figure la, there exist at least two other peaks (shoulders) corresponding to the discandium fullerenes, Sc2@C74 and SC2@C82r in the retention time between 15 and 30 min. The peaks (shoulders) appeared at 16 and 26 min are identified as sc*@c74and Sc2@&, respectively. By collecting and further purifying these fractions via the HPLC processes, we are able to obtain the purified S C ~ @ and C ~ sc2@c82 ~ fullerenes. The relevant LD-TOF mass spectra for the purified Sc2@C74 and sc2@c8z fractions are presented in Figure 3, a and b, respectively. Figure 3a confirms the isolation of S C ~ @ Cvarious ~ ~ : hollow and other scandium fullerenes are totally absent (except for a trace amount of C90). Figure 3b shows a distinct peakdue to SCZ@CSZ with several minor peaks due to Sc2@C84,sc2@c86, and c 9 6 . It
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4261
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Time-of-Flight / P S Figure 3. (a) LD-TOF mass spectrum for the isolated Sc2@C74 fraction. (b) LD-TOF mass spectrum for the purified sc2@c82 fraction. In (a), various hollow fullerenes are totally absent. The purified Sc2@C82 sample can be further refined by the present HPLC procedure.
in Sc2@c84 are not seen in the corresponding C84 spectrum. The overall spectral features of S C Z @ C and ~ ~Sc~@Cs2 are similar to those of Sc2@CX4 except for some spectral shift of the peaks. Because of the observed large red shift of the absorption onset (in reference to various hollow fullerenes), the HOMO-LUMO gaps of the discandium fullerenes are much smaller than those of the correspondinghollow fullerenes. It is expected that electric conductivity of these scandium fullerenes are much higher than those of hollow fullerenes. Electron spin resonance (ESR) measurements for these isolated metallofullerenes suggest that SCZ@CW, sc2@c82,and Scz@C84 are totally ESR silent, indicating that thesediscandium fullerenes are diamagnetic in contrast with the mono- and trimetallofullerenes such as La@C82, Y@C82r SC@CSZ, and SCJ@CU In this study, we have established a very powerful separation/ isolation procedure for endohedral metallofullerenes. With this technique we are now isolating various endohedral metallofullerenes which include Y@c82, sc@c82, and scJ@c82. We think that this report will open a new stage for the endohedral metallofullerene study.
Acknowledgment. We express thanks to Dr. Shunji Bandow (Instrument Center, Institute for Molecular Science) for kindly taking ESR spectra of the present samples. We also thank M. Tanahashi, N. Tanaka, Y. Nakano, Y. Yamada, T. Chiba, and M. Inagaki (Mi'e University) for experimental help. H.S. thanks the Japanese Education, Science and Culture (Grant-in-Aid for General Scientific Research No. 0345021 and ScientificResearch on Priority Areas No. 0324621 1) for the support of the present study. References and Notes
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Figure 4. UV-vis absorption spectrum for the isolated Sc2@Cs4 in toluene. Characteristic features appear at 380, 500, and 680 nm, which are not seen in the corresponding c84 spectrum.
is clear that in the HPLC spectrum (Figure la) the retention time for Sc2@C8~ is quite close to those of S C Z @ C SSC2@C86r ~, and c96. For this reason, it will certainly be possible to obtain much purified Sc2@CX2 fullerenes after a further HPLC separation/collection procedure. Figure 4 shows a UV-vis absorption spectrum for the isolated Sc2@C84 fullerene in toluene. The spectral feature is very characteristic as compared, for example, with the corresponding hollow (28.4 fullerene: (1) the observed peaks are broad in the spectral region between 300 and 800nm; (2)the absorption profile continues down to 800 nm; (3) thestrong absorptionpeakobserved at 400nm for C84l7is completely absent in the Sc2@c 8 4 spectrum; and (4)the broad features appearing at 380,500, and 680 nm
(1) 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. (2) Johnson, R.D.; de Vries, M. S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nature 1992,355, 239. (3) Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R.B.;Min, K. S.; Whetten, R. L. J . Phys. Chem. 1991,95, 10561. (4) 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. (5) Shinohara, H.; Sato, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. J . Phys. Chem. 1992, 96, 3571. (6) Shinohara, H.; Sato, H.; Ohkohchi, M.; Ando, Y .;Kodama,T.;Shida, T.; Kato, T.; Saito, Y. Nature 1992, 357, 52. (7) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Inagaki, M.; Saito, Y.; Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H. Mater. Sci. Eng. B, in press. (8) Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H.; Saito, Y.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Shinohara, H. J . Phys. Chem. 1992, 96. 9606. - ---(9) Bandow, S.; Shinohara, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. J . Phys. Chem., submitted for publication. (10) Yannoni, C.S.;Hoinkis, M.;deVries, M. S.; Bethune, D. %;Salem, J. R.;Crowder, M. S.; Johnson, R. D. Science 1992, 256, 1191. (1 1) Suzuki, S.; Kawata, S.; Shiromaru, H.; Yamauchi, K.;Kikuchi, K.; Kato, T.; Achiba, Y. J. Phys. Chem. 1992, 96, 7159. (12) Hoinkis, M.; Yannoni, C. S.; Bethune, D. S.; Salem, J. R.;Johnson, R. D.; Crowder, M. S.; de Vries, M. S. Chem. Phys. Letf. 1992, 198, 461. (13) Ross, M. M.; Nelson, H. H.; Callahan, J. H.; McElvany, S. W. J . Phys. Chem. 1992, 96, 5231. (14) Gillan, E.; Yeretzian, C.; Min, K. S.; Alvarez, M. M.; Whetten, R. L.; Kaner, R.B. J. Phys. Chem. 1992, 96, 6869. (15) Wang, L. S.;Alford, J. M.; Chai, Y.; Diener, M.; Smalley, R.E. Z. Phys. D, in press. (16) Welch, C. J.; Pirkle, W. H. J . Chromatogr. 1992, 609, 89. (17) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Saito, Y. Manuscript in preparation.
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