6049
2005, 109, 6049-6051 Published on Web 03/11/2005
Synthesis and Characterization of Exohedrally Silylated M@C82 (M ) Y and La) Michio Yamada,† Lai Feng,† Takatsugu Wakahara,† Takahiro Tsuchiya,† Yutaka Maeda,‡ Yongfu Lian,† Masahiro Kako,§ Takeshi Akasaka,*,† Tatsuhisa Kato,| Kaoru Kobayashi,⊥ 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 184-5801, Japan, Department of Applied Physics and Chemistry, The UniVersity of Electro-Communications, Chofu, Tokyo 182-8585, Japan, Department of Chemistry, Josai UniVersity, 1-1 Keyakidai, Sakado, Saitama 350-0295, Japan, Department of Theoretical Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan ReceiVed: February 5, 2005; In Final Form: March 2, 2005
The silylation of endohedral mono-metallofullerenes (Y@C82 and La@C82) and isolation of the corresponding adducts by HPLC separation have been accomplished. The redox properties of the silylated monometallofullerene were first clarified by CV and DPV measurements, indicating that the bis-silylated monometallofullerenes have lower oxidation and higher reduction potentials than the parent mono-metallofullerenes. These results reveal that bis-silylation is very effective for producing the electronegatively monometallofullerene derivatives as well as empty fullerenes.
To design new fullerene derivatives for applications in material science1 and biochemistry,2 one should control their electronic properties, namely their redox properties.3 For this purpose, there are three methods to modify the electronic properties of fullerenes: (1) exohedral chemical functionalization,3 (2) endohedral metal doping (endohedral metallofullerene),4 and (3) substitution of one or more cage carbon atoms by heteroatoms (heterofullerene).5 We carried out a comparative electrochemical study of various organofullerene derivatives with oxygen, carbon, and silicon-containing groups.6 We found the silylated C60 has higher reduction and lower oxidation potentials than C60 itself. This result can be confirmed by a theoretical calculation indicating that almost one electron flows into C60 from the silicon substituents.6 Meanwhile, endohedral metal-doping changes drastically the electronic states of carbon cages, because of significant electron transfer from the metal to the carbon cage.7 In this context, it is a very interesting challenge to modify the electronic property of endohedral metallofullerenes by silylation. Recently, we have succeeded in preparing the carbene and the Prato derivatives of La@C82,8-11 whose electronic properties are very similar to that of the pristine La@C82, indicating that carbon substituents on the fullerene cages do not change the electronic property of metallofullerenes efficiently, as in the case of C60.6 We herein report the synthesis and isolation of bis-silylated M@C82 (M ) Y and La), in which their electronic properties, for the first time, were greatly modified by the silylation. * To whom correspondence should be addressed. Tel & Fax: +81-29853-6409. E-mail:
[email protected]. † University of Tsukuba. ‡ Tokyo Gakugei University. § The University of Electro-Communications. | Josai University. ⊥ Institute for Molecular Science.
10.1021/jp050647b CCC: $30.25
SCHEME 1
Although empty fullerenes such as C60 and C70 react only photochemically with disilirane,12 metallofullerenes such as M@C82 (M ) La,13 Pr,14 Ce,15 and Gd16) can be exohedrally functionalized with disilirane in both thermal and photochemical ways due to their low reduction potentials.13-16 We adopted thermal reaction to obtain the silylated La@C82-A in a large scale. A toluene solution of La@C82-A and 1,1,2,2-tetramesityl1,2-disilirane (1) was heated at 80 °C for 13 h to afford a mixture of the monoadduct isomers. The main isomer (La@C82(Mes2Si)2CH2-I, named as La-isomer-I) of the monoadducts was successfully isolated by a HPLC separation. An EPR spectrum of La-isomer-I displays only one set of an equally spaced octet hyperfine, indicating that a successful isolation from other isomers has been performed (Figure 1). The hyperfine coupling constant (hfcc) of La-isomer-I (1.04 G) was a little smaller than that of the pristine La@C82-A (1.15 G). The small difference between the hfccs of La@C82 and La-isomer-I may reflect the change of the La position. A mass spectrum of La-isomer-I shows a peak at m/z 1669 with La@C82 at m/z 1123 because of the loss of the disilirane part from La-isomer-I. In addition, a fragment peak at m/z 1390 corresponding to La@C82(SiMes2) was also observed as a result of the loss of CH2SiMes2 from La-isomer-I during the laser desorption process. An absorption spectrum of La-isomer-I shows an absorption maximum at 1146 nm, whereas that of La@C82 exhibits an absorption maximum at 1003 nm, as shown © 2005 American Chemical Society
6050 J. Phys. Chem. B, Vol. 109, No. 13, 2005
Letters
Figure 1. EPR spectra of (a) La@C82-A (hfcc, 1.15 G; g value, 2.0012), (b) isolated La-isomer-I (hfcc, 1.04 G; g value, 2.0012).
TABLE 1: Redox Potentialsa of M@C82 (M ) Y and La) and Their Monoadducts compd. La-isomer-I Y-isomer-I Y-isomer-II La@C82(Ad)b La@C82-Ac Y@C82 C60(Mes2Si)2CH2 d C60
ox
E2
0.10 1.01 1.07
ox
E1
-0.07 -0.10 -0.03 -0.01 0.07 0.10 0.60 1.21
red
E1
-0.50 -0.55 -0.42 -0.49 -0.42 -0.34 -1.29 -1.12
red
E2
red
E3
-1.71 -1.36
-1.75
-1.44 -1.37 -1.33 -1.67 -1.50
-1.79 -1.53 -1.33 -2.18 -1.95
a Values are in volts relative to a ferrocene/ferrocenium couple and obtained by DPV. Conditions: working electrode, Pt disk (1 mm diameter); counter electrode, Pt wire; reference electrode, saturated calomel reference electrode (SCE); supporting electrolyte, 0.1 M (nBu)4NPF6 in 1,2-dichlorobenzene. CV: scan rate, 20 mV/s. DPV: pulse amplitude, 50 mV; pulse width, 50 ms; pulse period, 200 ms; scan rate, 20 mV/s. b Reference 9. c Reference 17. d Reference 12b.
Figure 2. Absorption spectra of La@C82-A and its derivatives in CS2 solution.
in Figure 2. This difference in the absorption spectrum between La@C82 and La-isomer-I demonstrates that the π electronic state of the fullerene cage is changed by the silylation. In contrast, the absorption spectra of La@C82 adducts such as La@C82(Ad)9 and La@C82(CH2)2NCH3,10 which have carbon substituents, are similar to that of the pristine La@C82. These results reveal that silylation is effective for tuning the electronic character of monometallfullerenes as well as empty fullerenes.12 We also carried out the thermal silylation of Y@C82 with disilirane 1. Several monoadduct isomers were detected in the mixture by EPR measurement. The most abundantly produced isomers, Y@C82(Mes2Si)2CH2-I and Y@C82(Mes2Si)2CH2-II named as Y-isomer-I and Y-isomer-II, respectively, were successfully isolated after the two-step HPLC separation. The hfccs of Y-isomer-I and Y-isomer-II were measured as to be 0.49 and 0.44 G, respectively. Their small changes from that of the pristine Y@C82 (0.51 G) may be due to the change of the Y position, as well as the case of La@C82. The redox properties of La-isomer-I, Y-isomer-I, and Yisomer-II were investigated by cyclic (CV) and differential pulse voltammetry (DPV) measurements. In the CV and DPV measurements, it was found that La-isomer-I and Y-isomer-I show the irreversible curves to afford the parent M@C82 (M ) Y and La). Actually, the catalytic oxidation process took place
during the measurement. It is noteworthy that Y-isomer-II is fairly stable owing to its lower oxidation potential. The oxidation and reduction potentials of M@C82 and their derivatives are summarized in Table 1. As compared to pristine M@C82, the reduction and oxidation potentials of silylated metallofullerenes were cathodically shifted to 80-210 and 130-200 mV, respectively. Since the fullerene cages of silylated metallofullerenes are electron-rich as a result of the electron donation from the disilirane part, they are reluctant to undergo reduction and prefer oxidation. In the case of La@C82(Ad) possessing a carbon substituent, the reduction and oxidation potentials were only cathodically shifted to 70 and 80 mV, respectively. The large difference in oxidation potentials suggests that the silylated M@C82 has a considerably higher-lying HOMO level than La@C82(Ad). The reduction and oxidation potentials of the Gd@C82 adduct with benzyne are cathodically and anodically shifted, respectively,18 indicating that its HOMO-LUMO gap is wider than that of pristine Gd@C82. In this context, it is worthy to note that the silylated M@C82s have the smaller HOMO-LUMO gaps than those of M@C82s. This is one of the characteristic features of the silylated metallofullerenes. In conclusion, the silylated M@C82 derivatives, La-isomerI, Y-isomer-I, and Y-isomer-II, were successfully synthesized, and their electronic properties were characterized by CV and DPV techniques, indicating that the bis-silylated mono-metallofullerenes have lower oxidation and higher reduction potentials than the parent mono-metallofullerenes. They are the first examples, showing that their electronic properties are tunable by exohedral addition. These results reveal that bis-silylation is very effective for producing the electronegatively mono-
Letters metallofullerene derivatives as well as empty fullerenes. The silicon derivatives of endohedral metallofullerenes will constitute an important stepping-stone on the way to their material, catalytic, and biological applications. Acknowledgment. This study was partly supported by a Grant-in-Aid, the 21st Century COE Program “Promotion of Creative Interdisciplinary Materials Sciences,” Nanotechnology Supporting Project, and NAREGI Nanoscience Project from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Supporting Information Available: Complete refs 9 and 15, HPLC profiles, and MALDI-TOF mass and EPR spectra of the silylated M@C82 (M ) Y and La). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Shi, S.; Khemani, K. C.; Li, Q.; Wudl, F. J. Am. Chem. Soc. 1992, 114, 10656. (b) Geckeler, K. E.; Hirsch, A. J. Am. Chem. Soc. 1993, 115, 3850. (2) Wharton, T.; Kini, V. U.; Morties, R. A.; Wilson, L. J. Tetrahedron Lett. 2001, 42, 5159. (3) (a) Hirsch, A. The Chemistry of Fullerenes; Thime, Verlag: Stuttgart, 1994. (b) The Chemistry of Fullerenes; Taylor, R., Ed.; World Scientific: Singapore, 1995. (c) Fullerenes and Related Structures; Hirsch, A., Ed.; Springer: Berlin, 1999. (d) Prato, M.; Martin, N. J. Mater. Chem. 2002, 12. (4) Akasaka, T.; Nagase, S. Endofullerenes: A New Family of Carbon Clusters; Kluwer Academic Publisher: Dordrecht, The Netherlands, 2002; p 231. (5) (a) Prato, M.; Li, Q.; Wudl, F.; Lucchini, V. J. Am. Chem. Soc. 1993, 115, 1148. (b) Hummelen, J. C.; Prato, M.; Wudl, F. J. Am. Chem. Soc. 1995, 117, 7003. (c) Hummelen, J. C.; Knight, B.; Pavlovich, J.; Gonzalez, R.; Wudl, F. Science 1995, 269, 1154. (d) Andreoni, W.; Curioni,
J. Phys. Chem. B, Vol. 109, No. 13, 2005 6051 A.; Holczer, K.; Prassides, K.; Keshavarz, M.; Hummelen, J. C.; Wudl, F. J. Am. Chem. Soc. 1996, 118, 11335. (e) Bellavia-Lund, C.; Gonzalez, R.; Hummelen, J. C.; Hicks, R. G.; Sastre, A.; Wudl, F. J. Am. Chem. Soc. 1997, 119, 2946. (f) Akasaka, T.; Okubo, S.; Wakahara, T.; Kobayashi, K.; Nagase, S.; Kako, M.; Nakadaira, Y.; Kato, T.; Yamamoto, K.; Funasaka, H.; Kitayama, Y.; Matsuura, K. Chem. Lett. 1999, 945. (6) Suzuki, T.; Maruyama, Y.; Akasaka, T.; Ando, W.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 1994, 116, 1359. (7) Nagase, S.; Kobayashi, K. Chem. Phys. Lett. 1993, 214, 57. (8) Two isomers of La@C82 have been isolated. In this paper, we have used a major isomer of La@C82 (named as La@C82-A): Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T. J. Phys. Chem. 1994, 98, 2008. A chemical functionalization of a minor isomer of La@C82 (La@C82-B) was reported: Tagmatarchis, N.; Taninaka, A.; Shinohara, H. Chem. Phys. Lett. 2002, 355, 226. (9) Maeda, Y.; et al. J. Am. Chem. Soc. 2004, 126, 6858. (10) Cao, B.; Wakahara, T.; Maeda, Y.; Han, A.; Akasaka, T.; Kato, T.; Kobayashi, K.; Nagase, S. Chem. Eur. J. 2004, 10, 716. (11) iLaC82 provided by the IUPAC nomenclature, for instance, is named as La@C82 for convenience. (12) (a) Akasaka, T.; Ando, W.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 1993, 115, 1605. (b) Akasaka, T.; Ando, W.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 1993, 115, 10366. (c) For a review, see: Akasaka, T.; Wakahara, T.; Nagase, S.; Kobayashi, K. J. Synth. Org. Chem. Jpn. 2000, 58, 1066. (13) (a) Akasaka, T.; Kato, T.; Kobayashi, K.; Nagase, S.; Yamamoto, K.; Funasaka, H.; Takahashi, T. Nature 1995, 374, 600. (b) Kato, T.; Akasaka, T.; Kobayashi, K.; Nagase, S.; Kikuchi, K.; Achiba, Y.; Suzuki, T.; Yamamoto, K. J. Phys. Chem. Solids 1997, 58, 1779. (14) Akasaka, T.; Okubo, S.; Kondo, M.; Maeda, Y.; Wakahara, T.; Kato, T.; Suzuki, T.; Yamamoto, K.; Kobayashi, K.; Nagase, S. Chem. Phys. Lett. 2000, 319, 153. (15) Wakahara, T.; et al. J. Am. Chem. Soc. 2004, 126, 4883. (16) Akasaka, T.; Nagase, S.; Kobayashi, K.; Suzuki, T.; Kato, T.; Yamamoto, K.; Funasaka, H.; Takahashi, T. J. Chem. Soc., Chem. Commun. 1995, 1343. (17) Suzuki, T.; Maruyama, Y.; Kato, T.; Kikuchi, K., Achiba, Y. J. Am. Chem. Soc. 1993, 115, 11006. (18) Lu, X.; Xu, J.; He, X.; Shi, Z.; Gu, Z. Chem. Mater. 2004, 16, 953.
