J. Phys. Chem. B 2006, 110, 22517-22520
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Selective Extraction and Purification of Endohedral Metallofullerene from Carbon Soot Takahiro Tsuchiya,† Takatsugu Wakahara,† Yongfu Lian,† Yutaka Maeda,‡ Takeshi Akasaka,*,† Tatsuhisa Kato,§ Naomi Mizorogi,| and Shigeru Nagase*,| Center for Tsukuba AdVanced Research Alliance, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan, Department of Chemistry, Tokyo Gakugei UniVersity, Koganei, Tokyo 184-8501, Japan, Department of Chemistry, Josai UniVersity, Sakado, Saitama 350-0295, Japan, and Department of Theoretical Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan ReceiVed: August 7, 2006; In Final Form: September 2, 2006
A preferential extraction of endohedral metallofullerenes (EMFs) from carbon soot through the use of reduction in the extraction process and a convenient isolation of endohedral metallofullerene anions (EMFs-) and empty fullerenes utilizing their difference in solubility are accomplished. EMFs are easily isolated by one-stage high-performance liquid chromatography after chemical oxidation of the extracted endohedral EMFs-.
Introduction Endohedral metallofullerenes (EMFs) have attracted much attention as new molecules and materials with novel properties and reactivities.1 They have lower reduction potentials than empty fullerenes because of the electron transfer from encapsulated metal(s) to the carbon cage.1 For the isolation of EMFs, the multistage high-performance liquid chromatography (HPLC) method is widely used.2 However, this method is timeconsuming because of the coexistence of numerous empty fullerenes, which makes it difficult to obtain purified EMFs in macroscopic quantities. Thus, the development of a convenient separation method is urgently required for applying EMFs to materials sciences. Effective methods for the enrichment of EMFs have been developed in the past several years.3 Among those, extractions of carbon soot by pyridine,3a N,N-dimethylformamide (DMF),3a,b or mixed solvents such as triethylamine/ acetone3c are known as a simple approach to enhance the extraction efficiency of EMFs. The specific affinity of these solvents to EMFs is associated with the dipole moments of solvents.3b However, the isolation of pure EMFs has not yet been accomplished by these methods. Recently, we demonstrated a convenient separation of EMFs from extracts of soot by coupling its selective reduction and one-stage HPLC.4 The reduction method was performed by utilizing the bulk electrolysis after 1,2,4-trichlorobenzene (TCB) extraction. In this context, we report an improved isolation of EMFs from carbon soot through the use of the reduction in the extraction process and different solubilities between neutral empty fullerenes and endohedral metallofullerene anions (EMFs-). Experimental Method Materials. The reagents such as acetone, pyridine, DMF, toluene, TCB, carbon disulfide (CS2), and n-Bu4NClO4 were obtained commercially and used as received. * Corresponding authors. Tel./fax: +81 298 53 7289 (T.A.). E-mail:
[email protected] (T.A.),
[email protected] (S.N.). † University of Tsukuba. ‡ Tokyo Gakugei University. § Josai University. | Institute for Molecular Science.
Generation of Carbon Soot. Soot containing metallofullerenes was produced by the standard arc vaporization method2a,b with a φ 4.6 × 130 mm composite anode containing graphite and lanthanum oxide in an atomic ratio of La/C ≈ 0.008. The composite rod was then subjected to an arc discharge as an anode under a helium atmosphere of 100 Torr. After that, ca. 2 g of soot was obtained. Extraction of La-Metallofullerenes. The raw soot was collected and extracted twice with TCB, pyridine, or DMF (30 mL/1 g of soot) at its boiling temperature for 3 h under an argon atmosphere. The resultant solution was thoroughly filtered off with the extracting solvents (10 mL/1 g soot). Isolation of La-Metallofullerenes from DMF Extracts. After the addition of 100 mg of n-Bu4NClO4 to the combined extracts from 1 rod, the extracting solvent was removed by a rotary evaporator, and the precipitate was washed by 50 mL of acetone/CS2 () 4:1) to extract [La@C82(C2V)]-, [La@C82(Cs)]-, and [La2@C80]-. The residue was further extracted with 50 mL of CS2. The addition of 5 mg of CHCl2COOH to the acetone/CS2 solution yielded La@C82(C2V), La@C82(Cs), and La2@C80 as dark brown solids, which could be extracted with 50 mL of CS2. The CS2 solution of La@C82(C2V), La@C82(Cs), and La2@C80 was replaced with 25 mL of a toluene solution, and it was injected into the HPLC with a PYE column (φ 20 × 250 mm; eluent, toluene; flow rate, 10 mL/min). After onestage HPLC separation, 2.5, 1.8, and 2.3 mg of pure La@C82(C2V), La@C82(Cs), and La2@C80 were isolated, respectively. Results and Discussion TCB, Pyridine, and DMF Extraction. A comparison of the La-metallofullerene extraction behaviors was carried out using different solvents. In this experiment, soot from the same arcdischarge batch (Figure 1a) was equally divided into three parts, and each part was extracted twice with TCB, pyridine, and DMF. Laser-desorption time-of-flight mass spectrometry (LDTOF) mass spectra and HPLC profiles of the extracts are shown in Figures 1b-d and 2a-c, respectively. HPLC peak areas of empty fullerenes C60 and C70 and La-metallofullerenes La@C82(C2V), La@C82(Cs), and La2@C80 in the extracts are also listed in Table 1. Figure 2a-c and Table 1 show that the contents of La-metallofullerenes increase in the order of TCB, pyridine, and
10.1021/jp0650679 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/13/2006
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Tsuchiya et al.
Figure 2. HPLC profiles of (a) TCB, (b) pyridine, and (c) DMF first extracts from raw soot and (d) sample 1 and (e) sample 3 in Scheme 1 [Buckyprep column (ø 4.6 × 250 mm), toluene as eluent, flow rate 1 mL/min, 40 °C]. The volume of those samples and the injection volumes were aligned. Figure 1. Negative ion laser desorption mass spectra of (a) raw soot, (b) TCB, (c) pyridine, and (d) DMF extracts and (e) sample 1, (f) sample 2, and (g) sample 3 in Scheme 1.
DMF extracts, and the contents of empty fullerenes decrease in the same order. Figure 3 shows an absorption maximum around 930 nm in the visible-near-infrared (vis-NIR) spectrum of the DMF extracts, which corresponds to that of the La@C82(C2V) anion {[La@C82(C2V)]-}.5 Figure 3 suggests that the other EMFs having similar low reduction potentials are also reduced to anion forms. We reported that the electrochemically produced diamagnetic [La@C82]- shows the 13C NMR signal in CS2/ acetone-d6.5 To verify the reduction during the DMF extraction, the formation of [La@C82]- in DMF was confirmed by 13C NMR measurement in CS2/acetone-d6. Because EMFs- are oxidized to neutral EMFs after removing the extracting solvents such as DMF,3a,6 n-Bu4NClO4 was added as a countercation source (n-Bu4N+) for [La@C82(C2V)]-, [La@C82(Cs)]-, and [La2@C80]- before concentrating the DMF extracts. Figure 4a shows the 13C NMR spectrum of La@C82(C2V) cocrystallized with n-Bu4NClO4 after refluxing in DMF, which is consistent with that of [La@C82(C2V)]- prepared by electrochemical
TABLE 1: HPLC Peak Areas of Empty Fullerene and La-Metallofullerenes Found in First and Second Extracts Using TCB, Pyridine, and DMF C60
C70
La@C82(C2V) La@C82(Cs) La2@C80
TCB Extraction first extracts 194.4 37.8 10.6 second extracts 20.8 5.0 4.1 total 215.2 42.8 14.7
2.7 1.0 3.7
1.9 0.9 2.8
Pyridine Extraction first extracts 166.4 21.9 9.5 second extracts 39.5 6.6 3.4 total 205.9 28.5 12.9
5.4 1.5 6.9
1.4 0.7 2.1
8.3 1.8 10.1
1.9 0.8 2.7
first extracts second extracts total
34.7 18.9 53.6
DMF Extraction 2.6 11.6 1.1 4.5 3.7 16.1
reduction (Figure 4b). Solodovnikov and co-workers also proposed, the formation of [La@C82]- from the dissolvation of La@C82 in pyridine or DMF.7 Considering the thermodynamics, it is unlikely that La@C82 is reduced by DMF.6a Recently, we reported the behavior of La@C82 with macrocyclic
Endohedral Metallofullerene from Carbon Soot
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Figure 3. Vis-NIR absorption spectrum of DMF extracts.
Figure 5. Orbital levels of La@C82(C2V), Me2NH, and DMF in eV.
SCHEME 1
Figure 4. 13C NMR spectra of (a) La@C82(C2V) cocrystallized with n-Bu4NClO4 from DMF and (b) electrochemically generated [La@C82(C2V)]-. Measurements were performed in CS2/acetone-d6 () 1:1). Insets: 139La NMR spectra.
and acyclic amines in solution.8 In this system, electron transfer between La@C82 and amines readily occurred even at ambient temperature, which is due to the low reduction and oxidation potentials of La@C82 and amine, respectively. It is well-known that the thermolysis of DMF at its boiling temperature yields dimethylamine and formaldehyde.9 Theoretical calculation shows that the electron transfer from dimethylamine to La@C82 is much more feasible than that from DMF, as shown in Figure 5.10 Therefore, endohedral metallofullerenes having low reduction potentials may be considered to be reduced to form anions by amine formed during the DMF refluxing process.11 The reaction of C60 with amine to afford an adduct is reported;12 however, no adduct such as La@C82NR2H is observed in the mass spectrum of DMF extracts. The differences in the extraction efficiency of TCB, pyridine, and DMF might be caused by their electric dipole moments; boiling points (extraction temperature); highest occupied molecular orbital levels; and the solubilities of La@C82(C2V), La@C82(Cs), and La2@C80 and their anions toward solvents. Isolation of La-Metallofullerene. Recently, we reported the electrochemical reduction of M@C82 (M ) La,5,13 Ce,14 Pr,15 and Y16). The obtained [M@C82]- was revealed to be soluble in polar solvents and insoluble in nonpolar solvents. This sharply contrasts with the fact that neutral fullerenes are insoluble in polar solvents and soluble in nonpolar solvents. Therefore,
EMFs- and neutral empty fullerenes could be separated by utilizing their different solubilities. On the basis of this finding, we examined the isolation of La-metallofullerenes following Scheme 1 by using the DMF extracts. After the addition of n-Bu4NClO4, which is a countercation source (n-Bu4N+) for [La@C82(C2V)]-, [La@C82(Cs)]-, and [La2@C80]-, to the DMF extracts, the solution was concentrated. The resulting residue was washed by diethyl ether to remove the remaining DMF and extracted by CS2 and acetone/CS2 () 4:1). Figures 1e and 2d show the LD-TOF mass spectrum and HPLC profile of the CS2 solution (sample 1 in Scheme 1), respectively. It can be seen that both La@C82 and La2@C80 are removed as compared to what is seen in Figures 1d and 2c. On the other hand, the LD-TOF mass spectrum of the acetone/CS2 solution (sample 2 in Scheme 1, Figure 1f) indicates that the empty fullerenes having a large-band gap are eliminated and that La@C82 and La2@C80 are concentrated. A vis-NIR spectrum of sample 2 showed a characteristic absorption maximum of [La@C82(C2V)]- at 934 nm. The oxidation of [La@C82(C2V)]-, [La@C82(Cs)]-, and [La2@C80]- to their neutral forms by weak acids was carried out.4,5 The absorption spectrum of [La@C82(C2V)]- is unchanged not only in water but also in an o-dichlorobenzene (ODCB) solution of phenol (pKa ) 10), thiophenol (pKa ) 8), pnitrophenol (pKa ) 7), acetic acid (pKa ) 5), and 2,4dinitrophenol (pKa ) 4). [La@C82(C2V)]- was oxidized to afford La@C82 in an ODCB solution of dichloroacetic acid (pKa ) 1).5 The addition of dichloroacetic acid to the acetone/CS2 solution (sample 2 in Scheme 1) led to the precipitation of a dark brown solid. The precipitates were collected by filtration
22520 J. Phys. Chem. B, Vol. 110, No. 45, 2006 and washed with acetone. In this process, impurities such as excess amounts of n-Bu4NClO4 and dichloroacetic acid could be removed. The resulting precipitates were extracted with CS2. A vis-NIR spectrum of the CS2 solution of the precipitate (sample 3 in Scheme 1) shows a characteristic absorption maximum of La@C82(C2V) at 1010 nm. Its LD-TOF mass spectrum (Figure 1g) was almost the same as that in sample 2. These results indicate that reduced EMFs can be converted back to their neutral forms by oxidation with dichloroacetic acid.17 Figure 2e shows a HPLC profile of sample 3 in which empty fullerenes are almost undetectable. La@C82(C2V), La@C82(Cs), and La2@C80 can be easily isolated following a one-stage HPLC separation of the sample. Obviously, the present purification technique leads to much larger amounts of pure La@C82(C2V), La@C82(Cs), and La2@C80 than the routine HPLC purification method and bulk electrolysis method reported previously.3,4 Thus, a most effective and simple method to purify La@C82(C2V) in large quantities from carbon soot has been developed. The successful isolation of EMFs in microscopic amounts by using this method is an important stepping stone on the way to developing future material sciences, as well as catalytic and biological applications using these materials. Conclusions EMF-enriched extracts were obtained by DMF extraction. The extracted EMFs were presumably reduced in the extraction process. By the addition of organic salts such as n-Bu4NClO4 to the extracted solution, EMFs- survived even if DMF was removed. A development of convenient separation methods of EMFs and empty fullerenes from carbon soot was accomplished utilizing the difference in solubility between EMFs- and neutral empty fullerenes. EMFs were easily isolated by one-stage HPLC separation after the chemical oxidation of EMFs-. Acknowledgment. This work was supported in part by a Grant-in-Aid, the 21st Century COE Program, Nanotechnology Support Project, NAREGI Nanoscience Project from the Ministry of Education, Culture, Sports, Science, Technology of Japan and grants from the Kurata Memorial Hitachi Science and Technology Foundation and Mitsubishi Chemical Corporation Fund. References and Notes (1) (a) Endofullerenes: A New Family of Carbon Clusters; Akasaka, T., Nagase, S., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2002. (b) 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. (c) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843. (2) (a) Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T. J. Phys. Chem. 1994, 98, 2009. (b) Yamamoto, K.; Funasaka, H.; Takahashi,
Tsuchiya et al. T.; Akasaka, T.; Suzuki, T.; Maruyama, Y. J. Phys. Chem. 1994, 98, 12831. (c) Suzuki, T.; Maruyama, Y.; Kato, T.; Kikuchi, K.; Nakao, Y.; Achiba, Y.; Kobayashi, K.; Nagase, S. Angew. Chem., Int. Ed. 1995, 34, 1094. (3) (a) Lian, Y.; Shi, Z.; Zhou, X.; Gu, Z. Chem. Mater. 2004, 16, 1704. (b) Bubnov, V. P.; Laukhina, E. E.; Kareev, I. E.; Koltover, V. K.; Prokhorova, T. G.; Yagubskii, E. B.; Kozmin, Y. P. Chem. Mater. 2002, 14, 1004. (c) Kodama, T.; Higashi, K.; Ichikawa, T.; Suzuki, S.; Nishikawa, H.; Ikemoto, I.; Kikuchi, K.; Achiba, Y. Chem. Lett. 2005, 34, 464. (d) Bolskar, R. D.; Alford, J. M. Chem. Commun. 2003, 1292. (e) Sun, B.; Gu, Z. Chem. Lett. 2002, 1164. (f) Sun, B.; Feng, L.; Shi, Z.; Gu, Z. Carbon 2002, 40, 1591. (g) Diener, M. D.; Alford, J. M. Nature 1998, 393, 668. (h) Fuchs, D.; Rietschel, H.; Michel, R. H.; Fischer, A.; Weis, P.; Kappes, M. M. J. Phys. Chem. 1996, 100, 725. (4) Tsuchiya, T.; Wakahara. T.; Shirakura, S.; Maeda, Y.; Akasaka, T.; Kobayashi, K.; Nagase, S.; Kato, T.; Kadish, K. M. Chem. Mater. 2004, 16, 4343. (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, E.; Kadish, K. M. J. Am. Chem. Soc. 2000, 122, 9316. (6) (a) Koltover, V. K.; Logan, J. W.; Heise, H.; Bobnov, V. P.; Estrin, Y. I.; Kareev, I. E.; Lodygina, V. P.; Pines, A. J. Phys. Chem. B 2004, 108, 12450. (b) Koltover, V. K.; Bubnov, V. P.; Estrin, Y. I.; Lodygina, V. P.; Davydov, R. M.; Subramoni, M.; Manoharan, P. T. Phys. Chem. Chem. Phys. 2003, 5, 2774. (7) Solodovnikov, S. P.; Tumanskii, B. L.; Bashilov, V. V.; Lebedkin, S. F.; Sokolov, V. I. Russ. Chem. Bull. Int. Ed. 2001, 50, 2242. (8) (a) Tsuchiya, T.; Sato, K.; Kurihara, H.; Wakahara, T.; Nakahodo, T.; Maeda, Y.; Akasaka, T.; Ohkubo, S.; Fukuzumi, S.; Kato, T.; Mizorogi, N.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 2006, 128, 6699. (9) Robertson, G. P.; Mikhailenko, S. D.; Wang, K.; Xing, P.; Guiver, M. D.; Kaliaguine, S. J. Membr. Sci. 2003, 219, 113. (10) Geometries were optimized using the Gaussian 03 program at the B3LYP level. The effective core potential (ECP) and LANL2DZ basis set were used for La. The split-valence d-polarized 6-31+G(d) basis set were used for C, N, O, and H. (11) Formation of [La@C82(C2V)]- in pyridine might be due to minute impurities such as amine. (12) (a) Davey, S. N.; Leigh, D. A.; Moody, A. E.; Tetler, L. W.; Wade, F. A. J. Chem. Soc., Chem. Commun. 1994, 397. (b) Hirsch, A.; Li, Q.; Wudl, F. Angew. Chem., Int. Ed. 1991, 30, 1309. (13) Akasaka, T.; Wakahara, T.; Nagase, S.; Kobayashi, K.; Waelchli, M.; Yamamoto, K.; Kondo, M.; Shirakura, S.; Maeda, Y.; Kato, T.; Kako, M.; Nakadaira, Y.; Gao, X.; Van Caemelbecke, E.; Kadish K. M. J. Phys. Chem. B 2001, 105, 2971. (14) Wakahara, T.; Kobayashi, J.; Yamada, M.; Maeda, Y.; Tsuchiya, T.; Okamura, M.; Akasaka, T.; Waelchli, M.; Kobayashi, K.; Nagase, S.; Kato, T.; Kako, M.; Yamamoto, K.; Kadish, K. M. J. Am. Chem. Soc. 2004, 126, 4483. (15) Wakahara, T.; Okubo, S.; Kondo, M.; Maeda, Y.; Akasaka, T.; Waelchli, M.; Kako, M.; Kobayashi, K.; Nagase, S.; Kato, T.; Yamamoto, K.; Gao, X.; Caemelbecke, E. V.; Kadish, K. M. Chem. Phys. Lett. 2002, 360, 235. (16) Feng, L.; Wakahara, T.; Tsuchiya, T.; Maeda, Y.; Lian, Y.; Akasaka, T.; Mizorogi, N.; Kobayashi, K.; Nagase, S.; Kadish, K. M. Chem. Phys. Lett. 2005, 405, 274. (17) Ferrocenium hexafluorophosphate (FcPF6) was also available for the oxidation of EMFs- to their neutral one.3g The oxidation utilizing La@C82(C2V) was confirmed by vis-NIR absorption, 13C NMR, and ESR measurement,5 and the clustering of EMF was not observed in this process.18 (18) (a) Nath, S.; Pal, H.; Sapre, A. V.; Bubnov, V. P.; Estrin, Y. I.; Parnyuk, T. A.; Koltover, V. K. Fullerenes, Nanotubes, Carbon Nanstruct. 2004, 12, 53. (b) Nath, S.; Pal, H.; Sapre, A. V.; Mittal, J. P. J. Photosci. 2002, 10, 105. (c) Nath, S.; Pal, H.; Sapre, A. V. Chem. Phys. Lett. 2000, 327, 422.
