11120
J. Phys. Chem. 1996, 100, 11120-11121
Electronic Structure of Ce@C82: An Experimental Study Junqi Ding,† Lu-Tao Weng,‡ and Shihe Yang*,† Department of Chemistry and Materials Characterization and Preparation Center, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ReceiVed: March 13, 1996; In Final Form: April 23, 1996X
Ce@C82 has been isolated with a purity of ∼99.5% by an efficient solvent extraction procedure, followed by HPLC separation. The UV-vis-near-IR absorption spectrum of the metallofullerene was measured at a wavelength range from 300 to 2100 nm. X-ray photoelectron spectroscopy (XPS) of Ce@C82 has been studied. The results suggest that, analogous to the La in La@C82, the Ce in Ce@C82 donates three valence electrons to the carbon cage as opposed to two electrons suggested by a recent ab initio calculation.
I. Introduction The successful encapsulation of metal atoms inside fullerene cages sparked considerable interest in the scientific community.1 Of all the endohedral metallofullerenes, M@C82 (M ) metal atoms) has been subjected to the most extensive investigation.2 The reason for this is because M@C82 is the first extracted endohedral fullerenes and one of the few endohedral fullerenes that survive the extraction process.1b For chemists, one of the first and foremost questions about this compound is the oxidation state of the metal atom, which to a large extent determines the chemical behavior of the compound. The EPR spectra of Sc@C82, Y@C82, and La@C82 all show small hyperfine constants and near free-spin g values.3 These have been interpreted in terms of an electronic structure described formally as M3+@C823- which results from the transfer of three valence electrons on M (d1s2) to C82. The oxidation state of +3 for La in La@C82 was confirmed by XPS.4 It has been proposed, based on laser-desorption and thermal-desorption data as well as thermodynamic calculations, that lanthanide atoms such as Ce, Pr, Nd, Tb, Ho, Er, and Lu exhibit a +3 oxidation state.5 However, a recent ab initio calculation showed that the electronic structure of Ce@C82 should be formally described as
[email protected] Although this proposal from the calculation seems plausible, experimental confirmation has yet to be made. If true, the different electronic structure of Ce@C82 as compared to La@C82 may have important effects on its properties and chemical reactivities. Our recent efforts to develop efficient methods to separate endohedral metallofullerenes7 allowed us to characterize Ce@C82 by UV-vis-near-IR absorption spectroscopy and X-ray photoelectron spectroscopy (XPS). The results from these measurements show that the oxidation state of Ce in Ce@C82 is +3 rather than +2 as proposed by the aforementioned ab initio calculation.6 II. Experimental Section Details of our separation methods for preparing Ce@C82 have been given elsewhere.7 In brief, soot containing metallofullerenes was produced by the standard arc vaporization method using a composite anode which contains graphite and cerium in its oxide or carbide form. The rod was then subjected to a dc discharge under a He atmosphere of 50 Torr. The raw soot * To whom correspondence should be addressed. † Department of Chemistry. ‡ Materials Characterization and Preparation Center. X Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00771-X CCC: $12.00
was collected and extracted in a Soxhlet extractor using N,Ndimethylformamide (DMF, 99.9%, BDH) at its boiling temperature for 8 h. After removal of the DMF by evaporation, a black powder (∼1% of the raw soot) was obtained. The soluble fraction (∼40%) was dissolved in toluene and injected into an HPLC. A buckyprep column (4.6 mm × 250 mm; Cosmosil, Nacalai Tesque Inc.) similar to a PYE column was used, with toluene as the mobile phase. The injection volume was 1 mL, and the elution rate was 1 mL/min. The purity of the Ce@C82 sample obtained from the HPLC collection was checked by DCI negative-ion mass spectrometry (Finnigan TSQ7000). The UV-vis-near-IR absorption spectrum of Ce@C82 in toluene solution was recorded with a Perkin-Elmer spectrometer (Lambda 19). For XPS study, we prepared Ce@C82 film on a polycrystalline Au substrate. To obtain a clean Au surface, we used a town gas flame to pretreat a small piece of gold foil. After heating, the Au foil was dipped into methanol and dried in N2 gas. Several drops of a concentrated Ce@C82 solution in toluene were transferred to the Au foil. The evaporation of the solvent left a uniform film of Ce@C82. This film was dried and washed with n-hexane. XPS measurements were performed with a Perkin-Elmer PHI 5600 multitechnique spectrometer using monochromatized Al KR (hυ ) 1486.6 eV) as the X-ray source. The spectrometer was operated in the fixed analyzer transmission mode. The pass energy was 188 eV for the survey scan and 23 eV for the detailed scan of each XPS peak. Binding energies were referenced to Au 4f7 (84.00 eV). Peak deconvolution was done by an iterative least-squares fitting routine using a linear baseline. III. Results and Discussion Figure 1 shows the UV-vis-near-IR absorption spectrum of Ce@C82 in toluene. The spectrum exhibits salient absorption peaks at 380, 631, 1014, and 1413 nm (broad). Overall, the spectrum manifests a remarkable similarity to that of La@C82 in the sense that all the peaks (except the one at 380 nm, which has not been measured for La@C82) have a one-to-one correspondence.3b,8,9 This similarity has been shown before by Kappes et al. in the 400-1100-nm range. We extended the spectrum further to the near-infrared wavelength and revealed the broad peak at 1413 nm which is characteristic of
[email protected] The similarity in the absorption spectra of Ce@C82 and La@C82 strongly suggests that the spectral features are derived from the cage with an open-shell electronic structure and that the cage accepts roughly the same amount of electrons from the metals; i.e., endohedral metallofullerenes have the isoelectronic structure M3+@C823-. © 1996 American Chemical Society
Electronic Structure of Ce@C82
J. Phys. Chem., Vol. 100, No. 26, 1996 11121 the higher-energy peak to the lower-energy peak sensitively depends on the electronegativity of the ligands.14 Our analysis shows that the electronegativity of the fullerene cage is somewhat smaller than that of Cl. A similar conclusion was obtained for La@C82 by Weaver et al.15 IV. Conclusions Our experimental results suggest that the Ce in Ce@C82 actually possesses an oxidation state of +3 rather than +2. Its UV-vis-near-IR absorption spectrum from 300 nm up to 2100 nm displays a remarkable similarity to that of La@C82, which is widely known to be of the form La3+@C823-. More importantly, its XPS pattern shows that Ce in Ce@C82 exists in the form of Ce3+ and that the fullerene has a smaller electronegativity than chlorine as a ligand to the rare-earth-metal cation.
Figure 1. UV-vis-near-IR absorption spectrum of Ce@C82 in toluene solution.
Acknowledgment. We are indebted to Rowena Leung for her kind technical assistance in the experiments. We thank Dr. M. Yang for using his instrument to measure the UV-visnear-IR spectrum. This work is supported by an RGC grant (HKUST601/95P) administrated by the UGC of Hong Kong. References and Notes
Figure 2. XPS pattern of Ce 3d in Ce@C82. The smooth line is a fitting of the spectral peaks. The two separate features (3d3/2, higher binding energy feature; 3d5/2, lower binding energy feature) are due to spin-orbital splitting of the 3d core level.
Figure 2 shows the XPS spectrum of Ce@C82 in the region of the Ce 3d3/2 and 3d5/2 core levels. Both core level features show a strong peak and a weak shoulder. For the 3d5/2 feature, the stronger peak at the higher binding energy side (886.57 eV) corresponds to the screening by only the surrounding fullerene orbitals, while the weak shoulder at the lower binding energy (881.83 eV) originates from screening by charge transfer to the 4f shell.10 It is known that Ce4+ has a characteristic XPS (3d3/2) peak at the binding energy of ∼914 eV, which corresponds to a transition from the initial state 3d104f0 to the final state 3d94f0.11 The intensity of this peak has been used to estimate the Ce4+ concentration of unknown samples.12 On the basis of our data, we can easily rule out the existence of Ce4+ in Ce@C82 since we did not observe the characteristic peak at a binding energy of ∼914 eV. Since no characteristic peak for Ce2+ is known in the 3d core level region, we cannot rule out Ce2+ in the way we ruled out Ce4+. However, the peak position and shape of the Ce 3d XPS peak of Ce@C82 are rather similar to those of cerium trihalides,13 suggesting that the oxidation state of Ce is +3. It is well-known that the peak intensity ratio of
(1) (a) Heath, J. R.; O’Brien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Tittel, F. K.; Smally, R. E. J. Am. Chem. Soc. 1985, 107, 7779. (b) Chai, Y.; Guo, T.; Jin, C. M.; Haufler, R.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, M.; Smalley, R. E. J. Phys. Chem. 1991, 95, 7564. (c) Johnson, R. D.; de Vries, M. S.; Salem, J. R.; Bethune, D. S.; Yannoni, C. S. Nature (London) 1992, 355, 239. (2) (a) Edelmann, F. T. Angew. Chem., Int. Ed. Engl. 1995, 34, 981. (b) Bethune, D. S.; Johnson, R. D.; Salem, J. R.; de Vries, M. S.; Yannoni, C. S. Nature 1993, 366, 123. (c) Fuchs, D.; Rietschel, H.; Michel, R. H.; Benz, M.; Fischer, A.; Kappes, M. M. in Proc. IWEPNM 95, Fink, J., Kuzmani, H., Mehring, M., Roth, S., Eds.; World Scientific: Singapore, 1995. (3) (a) Kikuchi, K.; Suzuki, S.; Nakao, Y.; Nakahara, N.; Wakabayashi, T.; Shiromaru, H.; Saito, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Lett. 1993, 216, 67. (b) Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T.; Suzuki, T.; Maruyama, Y. J. Phys. Chem. 1994, 98, 12831. (c) Funasaka, H.; Sakurai, K.; Oda, Y.; Yamamoto, K.; Takahashi, T. Chem. Phys. Lett. 1995, 232, 273. (d) Kikuchi, K.; Kobayashi, K.; Sueki, K.; Suzuki, S.; Nakahara, H.; Achiba, Y. J. Am. Chem. Soc. 1994, 116, 9775. (e) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Ohkohchi, M.; Ando, Y.; Saito, Y. J. Phys. Chem. 1993, 97, 4259. (4) Poirier, D. M.; Knupfer, M.; Weaver, J. H.; Andreoni, W.; Laasonen, K.; Parrinello, M.; Bethune, D. S.; Kikuchi, K.; Achiba, Y. Phys. ReV. B 1994, 49, 17403. (5) Moro, L.; Ruoff, R. S.; Becker, C. H.; Lorents, D. C.; Malhotra, R. J. Phys. Chem. 1993, 97, 6801. (6) Nagase, S.; Kobayashi, K. Chem. Phys. Lett. 1994, 228, 106. (7) Ding, J. Q.; Yang, S. H. In preparation. (8) Kikuchi, K.; Suzuki, S.; Nakao, Y.; Nakahara, N.; Wakabayashi, T.; Shiromaru, H.; Saito, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Lett. 1993, 216, 67. (9) Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T. J. Phys. Chem. 1994, 98, 2008. (10) (a) Schneider, W. D.; Delley, B.; Wuilloud, E.; Imer, J.-M.; Baer, Y. Phys. ReV. B 1985, 32, 6819. (b) Imada, S.; Jo, T. Phys. Scr. 1990, 41, 115. (11) (a) Fujimori, A. Phys. ReV. B 1983, 27, 3992. (b) Fujimori, A. Phys. ReV. B 1983, 28, 2281. (c) Lasser, R.; Fuggle, J. C.; Beyss, M.; Campagna, M.; Steglich, F.; Hulliger, F. Phys. B. 1980, 102B, 360. (12) Shyu, J. Z.; Weber, W. H.; Gandhi, H. S. J. Phys. Chem. 1988, 92, 4964. (13) Suzuki, S.; Ishii, T.; Sagawa, T. J. Phys. Soc. Jpn. 1974, 37, 1334. (14) Imada, S.; Jo, T. J. Phys. Soc. Jpn. 1989, 58, 2665. (15) 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.
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