Entrapped Planar Trimetallic Carbide in a Fullerene Cage: Synthesis

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Entrapped Planar Trimetallic Carbide in a Fullerene Cage: Synthesis, Isolation, and Spectroscopic Studies of Lu3C2@C88 Wei Xu,†,‡ Tai-Shan Wang,† Jing-Yi Wu,†,‡ Yi-Han Ma,†,‡ Jun-Peng Zheng,†,‡ Hui Li,†,‡ Bao Wang,†,‡ Li Jiang,† Chun-Ying Shu,*,† and Chun-Ru Wang*,† Key Laboratory of Molecular Nanostructure and Nanotechnology, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China, and Graduate School of Chinese Academy of Sciences, Beijing 100049, China ReceiVed: September 14, 2010; ReVised Manuscript ReceiVed: NoVember 28, 2010

We report the preparation, isolation, and spectroscopic characterizations of a novel endofullerene Lu3C2@C88, where a planar trimetallic carbide cluster Lu3C2 is encapsulated by D2-symmetric C88 cage. Meanwhile, the trimetallic nitride endofullerene Lu3N@C88 was also isolated and characterized for comparison. The results revealed that both metal carbide Lu3C2 and metal nitride Lu3N share the same D2-symmetric C88 cage. Theoretical studies revealed that the internal moieties supply the C88 cage with six electrons in both Lu3N@C88 and Lu3C2@C88 cases; however, Lu3N@C88 has a closed-shell electronic structure and Lu3C2@C88 owns an unpaired electron localized on the internal [Lu3C2]6+ moiety. The different molecular electronic structures of Lu3C2@C88 and Lu3N@C88 lead to distinct differences on both the UV-vis absorption spectral features and their electrochemical redox properties. Introduction The internal spaces of fullerene cages enable them to encapsulate various species, e.g., one metal ion, two metal ions, and complex metal clusters, such as M3N, M2C2, M3C2, M4C2, M4O2, M4S2, etc., to form the so-called endohedral metallofullerenes (EMFs).1-7 In the past decades, EMFs have attracted extensive interest because of their unique structures, novel electronic properties, and potential applications in a variety of fields, such as nanoelectronics, nanotechnology, and biomedical applications.2c Among them metal carbide and metal nitride EMFs have induced special interests due to their relative high yields and excellent variety in structures. The two families of metallic nitride and metal carbide EMFs were first found in 1999 and 2001, respectively; since then, dozens of metal nitride metallofullerenes and metal carbide fullerenes have been successfully synthesized.3a,b For metal nitride EMFs, the internal species are usually M3N, but for metal carbide EMFs, the internal moiety may be M2C2 such as Y2C2@C82, Ti2C2@C78, Sc2C2@C68, Sc2C2@C82, Gd2C2@C92, M3C2 such as Sc3C2@C80, or M4C2 such as Sc4C2@C80, etc.4a,5a,c,e,9-16 Undoubtedly, the variability of metal carbide EMFs would bring them more novel structures and properties; for example, the recently reported Sc4C2@C80 (Ih) has a unique Russian doll-type structure C2@Sc4@C80,5e,g and Sc3C2@C80 possesses a rare open-shell electronic configuration with alluring ESR properties.5a,c,g,17-19 Therefore, it is of great significance to find and study more EMFs with M3C2 and M4C2 internal moieties to enlarge the metal carbide family. Herein, we report the preparation, isolation, and spectroscopic studies of two C88based EMFs, i.e., Lu3C2@C88 and Lu3N@C88. By various comparative studies, an open-shelled electronic structure of * Corresponding authors. E-mail: [email protected] (C.-Y.S.); crwang@ iccas.ac.cn (C.-R.W.). † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences.

Lu3C2@C88 was unambiguously revealed together with interesting optical and electrochemical properties. Experimental Section Lu3C2@C88 and Lu3N@C88 were prepared by the Kra¨tschmerHuffman arc discharged method and isolated by the multistage high-performance liquid chromatography (HPLC) strategy as we previously reported.5f,15 Briefly, graphite rods were coredrilled and subsequently packed with a mixture of Lu/Ni2 alloy and graphite powder in a weight ratio of 2:1. Lu3C2@C88 was produced by vaporizing the Lu-doped graphite rods at 160 A direct current (dc) under 600 Torr He atmosphere, and Lu3N@C88 was generated under the same conditions except for 6 Torr of N2 being introduced in the atmosphere. The two kinds of resulting soot were collected and Soxhlet-extracted with toluene for 24 h, and HPLC was performed to isolate Lu3C2@C88 and Lu3N@C88 with two complementary columns, i.e., the Buckyprep and the Buckyprep-M. The purity of the samples was confirmed by the matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry and HPLC profiles. To spectroscopically characterize Lu3C2@C88 and Lu3N@C88, the two samples were dissolved in carbon disulfide and then deposited on a copper substrate for Raman measurements in a Renishaw Invia Raman microscope (laser wavelength 633 nm). Electrochemical experiments were carried out in o-DCB solvent with a 2 mm glassy carbon disk as the working electrode as well as Pt wire and Ag wire as the counter and reference electrodes, respectively. The potentials were referred to the E1/2 value of the Fc/Fc+ redox couple measured in the sample solution. Results and Discussion Purified Lu3C90 and Lu3N@C88 were isolated by the multistage HPLC technique. Figure 1 shows the HPLC traces of these two metallofullerenes on Buckyprep column. The MALDI-TOF

10.1021/jp1087459  2011 American Chemical Society Published on Web 12/14/2010

Entrapped Trimetallic Carbide in a Fullerene Cage

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Figure 3. DFT-optimized structure (a), isodensity surface plots for the SOMO (b), and LUMO (c) of Lu3C2@C88. DFT-optimized structure (a′), isodensity surface plots for the HOMO (b′), and LUMO (c′) of Lu3N@C88. Green balls represent the Lu atoms, yellow balls are the carbon atoms of the inner carbide moiety, and the blue ball is the nitrogen atom of the inner nitride moiety.

Figure 1. HPLC traces of Lu3C2@C88 (a) and Lu3N@C88 (b) in Buckyprep column. The inset shows the positive-ion MALDI-TOF mass spectra as well as the calculated isotope distributions.

Figure 2. Low-energy Raman spectra of Lu3C2@C88 and Lu3N@C88 (633 nm laser excitation, 2 h accumulation time).

mass spectra exhibit a single molecular ion peak m/z at 1605 for Lu3C90 and m/z at 1595 for Lu3N@C88, respectively. The measured isotopic distributions of Lu3C90 and Lu3N@C88 are in well agreement with their theoretical distributions, confirming the assignment of their compositions (see Figure 1 inset). It is well-known that Raman spectroscopy is a powerful tool to reveal the geometric structure of endohedral fullerenes.20-22 The low-energy Raman spectra of Lu3C90 and Lu3N@C88 were recorded as shown in Figure 2, in which Lu3C90 exhibits a group of Raman vibrations in the tangential cage mode region between 1000 and 1600 cm-1, almost identical to that of Lu3N@C88 as well as that of previously studied Y3N@C88 (D2) and Gd3N@C88 (D2).23 This strongly suggested that Lu3C90 and Lu3N@C88 share the same C88 (D2) cage though the encaged species are Lu3C2 and Lu3N, respectively. In other words, Lu3C90 should be denoted as a metal carbide endohedral metallofullerene, Lu3C2@C88 (D2). Furthermore, the other parts of the Raman

spectra, e.g., the radial carbon cage modes between 200 and 815 cm-1 and the Lu-cage stretching modes between 200 and 100 cm-1, revealed also high similarity for the two molecules, strongly supporting the identical carbon cage for Lu3C2@C88 and Lu3N@C88. DFT studies were performed to study the electronic structures of Lu3C2@C88 and Lu3N@C88 at a GGA-PBE/DNP theoretical level.24 Figure 3 shows the optimized structure of Lu3C2@C88 and Lu3N@C88. According to the experimentally determined structure of previous reported Y3N@C88 (D2), Gd3N@C88 (D2), and Tb3N@C88 (D2: 35),23 both Lu3C2@C88 and Lu3N@C88 were assumed to have a D2 (35) symmetry, which are quite different from previously reported C68-based endohedral fullerenes Sc2C2@C68 and Sc3N@C68, where the metal carbide Sc2C2 and the metal nitride Sc3N were encaged in two different C68 cage isomers C2V and D3, respectively.3c,15 The Lu3C2 cluster is the second M3C2 endohedral moiety beyond the famous Sc3C2 cluster in Sc3C2@C80 (Ih); however, the optimized Lu3C2 cluster in Lu3C2@C88 demonstrates a near-planar geometry which is quite different from the trifoliate Sc3C2 cluster in Sc3C2@C80 case. This difference may be attributed to the D2 symmetry of the C88 (D2: 35) cage in comparison with that of C80 (Ih:7) cage. Based on the optimized structures of Lu3C2@C88 and Lu3N@C88, their electronic structures were also investigated as shown in Figure 3. Similar to Sc3C2@C80 (Ih), the Lu3C2@C88 has also an open-shell electronic structure as (Lu3C2)6+@(C88)6-, and the singly occupied molecular orbital (SOMO) and LUMO are mainly localized on the internal [Lu3C2]6+ moiety and the outer C88 cage, respectively. This is primarily attributed to the covalent dative bondings between the atomic orbitals of Lu3+ cations and the π* orbital of the C2 moiety. Hence, the C2 moiety in the encased cluster can be described as C23-, and accordingly, the valence state of Lu3C2@C88 can be described as (Lu3+)3(C2)3-@C886-. Addition or subtraction of only one electron on the C2 carbide moiety made its charge states flexible. By comparison, Lu3N@C88 has an analogous charge state of (Lu3+)3N3-@C886- though Lu3C2@C88 and Lu3N@C88 demonstrate large difference on the frontier orbitals (Figure 3). For Lu3N@C88, either HOMO or LUMO is localized on the outer C88 cage. Therefore, the optical and electronic properties related to electron transition must be different between these two molecules.

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Figure 4. UV-vis absorption spectra of the purified Lu3C2@C88 (solid line) and Lu3N@C88 (dashed line) in toluene.

Figure 4 shows the UV-vis absorption spectra of Lu3C2@C88 and Lu3N@C88. The two spectra are distinctly different even they own the same C88 fullerene cage. It was observed that the spectrum of Lu3N@C88 highly resembles other trimetallic nitride endofullerenes such as Nd3N@C88, Gd3N@C88, Pr3N@C88, and Ce3N@C88, which have closed-shell electronic structures.25 However, for Lu3C2@C88 case, it does not exhibit obvious absorption peaks owing to its open-shell electronic structure, as a similar featureless spectrum for previously revealed Sc3C2@C80 with open-shell electronic structure.5a It should be noted that the HPLC retention time and UV-vis spectrum of Lu3C2@C88 were unchanged even after 3 months storage, indicating that this open-shelled molecule possesses considerable chemical stability due to the existence of C2 moiety. Electrochemical cyclic voltammetry (CV) was employed to further characterize the molecular electronic properties of Lu3C2@C88 and Lu3N@C88. The CV of Lu3C2@C88 at a scan rate of 100 mV s-1 showed three reversible reduction processes and one reversible oxidation step. In comparison, the CV of Lu3N@C88 exhibits three reversible reduction steps and two reversible oxidation steps (Figure 5), which is similar to the reported electrochemical redox properties of [email protected] Obviously, the nearly identical reduction potentials of Lu3N@C88 and Lu3C2@C88 are due to their identical fullerene cage C88. The reduction potential of Lu3N@C88 was comparable to that of Lu3C2@C88, but for the former, the first oxidation with potential at 0.019 V was negatively shifted, making it much easier to be oxidized than Lu3C2@C88 case with the first potential at 0.307 V (Table 1). This suggested that the higher oxidation potential for Lu3C2@C88 may benefit from the endohedral SOMO, where the unpaired electron is embedded and significantly affects the molecular oxidation potential. Conclusions In conclusion, for the first time we have prepared and characterized two C88-based EMFs, Lu3C2@C88 and Lu3N@C88, which share an identical fullerene cage C88 (D2). Raman spectroscopic characterizations in combination with DFT calculations confirmed the encapsulating nature of a planar trimetallic carbide Lu3C2 cluster inside C88 cage, which is another example with M3C2 internal moiety besides Sc3C2@C80 (Ih). Theoretical studies revealed that the Lu3C2@C88 has an unpaired electron localized on the internal [Lu3C2]6+ moiety, and it is different from the closed-shell electronic structure of

Figure 5. Cyclic voltammeties of Lu3C2@C88 and Lu3N@C88 in NBu4PF6/o-DCB with ferrocene as the internal standard (100 mV s-1 scan rate).

TABLE 1: Half-Wave Potentials (E1/2) vs Fc+/Fc of the Reduction and Oxidation Potentials of Lu3C2@C88 and Lu3N@C88 in o-DCB EMF Lu3C2@C88 Lu3N@C88

E1/2,ox(2)

E1/2,ox(1)

E1/2,red(1)

E1/2,red(2)

E1/2,red(3)

0.437

0.307 0.019

-1.341 -1.318

-1.700 -1.650

-2.150 -2.152

Lu3N@C88. UV-vis absorption spectra and electrochemical studies of the two molecules confirm the different electronic structures of Lu3C2@C88 and [email protected] is a peculiar example that different endohedral species can function as a template to stabilize the same carbon cage but form dissimilar endofullerenes in characters. Acknowledgment. This work was supported by NSFC (20821003, 20702053), CMS (CX200913, CX200910), Important National Science &Technology Specific Projects (2008ZX05013-004), and ICCAS. References and Notes (1) Endofullereres: A New Family of Carbon Clusters; Akasaka, T., Nagase, S., Eds.; Academic Publishers: Dordrecht, 2002. (2) (a) Dunsch, L.; Yang, S. F. Phys. Chem. Chem. Phys. 2007, 9, 3067. (b) Dunsch, L.; Yang, S. F. Small 2007, 3, 1298. (c) Chaur, M. N.; Melin, F.; Ortiz, A. L.; Echegoyen, L. Angew. Chem., Int. Ed. 2009, 48, 7514. (3) (a) Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan, M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; Maitra, K.; Fisher, A. J.; Balch, A. L.; Dorn, H. C. Nature 1999, 401, 55. (b) Wang, C. R.; Kai, T.; Tomiyama, T.; Yoshida, T.; Kobayashi, Y.; Nishibori, E.; Takata, M.; Sakata, M.; Shinohara, H. Angew. Chem., Int. Ed. 2001, 40, 397. (c) Stevenson, S.; Fowler, P. W.; Heine, T.; Duchamp, J. C.; Rice, G.; Glass, T.; Harich, K.; Hajdu, E.; Bible, R.; Dorn, H. C. Nature 2000, 408, 427. (4) (a) Yang, H.; Lu, C. X.; Liu, Z. Y.; Jin, H. X.; Che, Y. L.; Olmstead, M. M.; Balch, A. L. J. Am. Chem. Soc. 2008, 130, 17296. (b) Huang, H. J.; Yang, S. H.; Zhang, X. X. J. Phys. Chem. B 2000, 104, 1473. (c) Zuo, T. M.; Xu, L. S.; Beavers, C. M.; Olmstead, M. M.; Fu, W. J.; Crawford, D.; Balch, A. L.; Dorn, H. C. J. Am. Chem. Soc. 2008, 130, 12992. (d) Popov, A. A.; Dunsch, L. J. Am. Chem. Soc. 2008, 130, 17726. (e) Mercado, B. Q.; Beavers, C. M.; Olmstead, M. M.; Chaur, M. N.; Walker, K.; Holloway, B. C.; Echegoyen, L.; Balch, A. L. J. Am. Chem. Soc. 2008, 130, 7854. (5) (a) Iiduka, Y.; Wakahara, T.; Nakahodo, T.; Tsuchiya, T.; Sakuraba, A.; Maeda, Y.; Akasaka, T.; Yoza, K.; Horn, E.; Kato, T.; Liu, Michael,

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