J. Phys. Chem. B 2000, 104, 7595-7599
7595
Temperature-Dependent EPR Studies on Isolated Scandium Metallofullerenes: Sc@C82(I, II) and Sc@C84 Masayasu Inakuma and Hisanori Shinohara* Department of Chemistry, Nagoya UniVersity, Nagoya 464-8602, Japan ReceiVed: January 28, 2000; In Final Form: April 24, 2000
Two structural isomers of Sc@C82(I, II) and one isomer of Sc@C84 have been produced and isolated for the first time. EPR hyperfine spectra together with the UV-vis-NIR absorption spectra indicate that the electronic structures are different between Sc@C82 isomers I and II. The absorption spectra of these scandium metallofullerenes are different from the other group-3 monometallofullerenes, such as La3+@C823- and Y3+@C823-. The satellite 13C hyperfine structure of Sc@C84 shows fewer structural features than that of Sc@C82(I), which suggests the cage symmetry of Sc@C84 is lower than C2V symmetry of Sc@C82(I). EPR hyperfine splitting of Sc@C82(I, II) at room temperature shows that the isotropic hyperfine interaction of Sc@C82(II) is one-third of that of Sc@C82(I). One of the main reasons for the large difference of the anisotropic g tensors is due to a difference in excitation energies of the unpaired electron spin of each Sc@C82 isomer, which is reflected in the HOMO-LUMO gap deduced from the observed absorption spectra.
1. Introduction Endohedral metallofullerenes are known to have novel structural properties suited for new functional materials in electronic and biological applications.1 To explore further applications, it is necessary to study the structural and electronic properties of the metallofullerenes. Because electron paramagnetic resonance (EPR) is one of the best techniques to investigate the electronic properties of paramagnetic species, EPR hyperfine structures (hfs) can provide much information on the electronic and structural properties of EPR-active metallofullerenes.2-24 The analyses of the temperature-dependent line width of hfs of metallofullerenes have been performed to explain their molecular dynamics via continuous-wave, Fourier-transform, and pulsedand 2D-EPR measurements.8,12,14-20,22-24 During the past 8 years, many Sc metallofullerenes have been produced, isolated, and characterized in detail by various experimental techniques via 13C NMR,25 45Sc NMR,26 ESR,5,6,8,12,13,16,23,24,27-33 HR-TEM,34 UV-vis-NIR,13,28,29,31,35,36 STM,29,37,38 XPS/UPS,39,40 X-ray diffraction,41-43 and IR/Raman spectroscopy.44-51 Endohedral Sc metallofullerenes are particularly interesting in view of the fact that a series of fullerene cages (C72-C100) can encapsulate from one to four Sc atoms, depending on the fullerene cage. Recently, we reported a synchrotron X-ray structural study on a triscandium fullerene, Sc3@C82. A Sc3 cluster has been found to form a Sc trimer having an equilateral geometry with an interatomic distance of 2.4 Å.42 The Sc3 trimer has been encaged in a C3V-C82 fullerene. The observed structure is consistent with the previous EPR results which showed symmetric 22 hfs lines, which suggests a geometric equivalence of the three Sc atoms within the carbon cage.27,28 This is one of the best examples that EPR studies of metallofullerenes provide us with important information not only on the electronic state but also on their structure. In the present study, we report isolation and EPR characterization of two isomers of Sc@C82(I, II) and a major isomer of * To whom correspondence should be addressed. E-mail: nori@ nano.chem.nagoya-u.ac.jp.
Sc@C84. The analysis of the temperature-dependent line width of EPR hfs of two Sc@C82(I, II) isomers has been performed. UV-vis-NIR absorption spectra of two Sc@C82(I, II) isomers have also been characterized. The observed EPR hyperfine splittings reveal that the difference in electronic structure for each encapsulated Sc atom is reflected in the observed difference of the anisotropic g tensor. 2. Experimental Section Details of production of endohedral metallofullerenes are reported elsewhere.13,25,27,36 Briefly, soot containing various Sc monometallofullerenes was produced by the DC arc-discharge method (40-50 Torr He flow, 350 A, and 25 V). Graphite/ scandium composite rods (0.8 atom %, Toyo Tanso Co., Ltd.) baked at 1600 °C were used for the negative electrode. The resulting soot was collected and dissolved into carbon disulfide under dry N2 conditions. Following CS2 extraction, the residue was extracted by pyridine to further recover the Sc fullerenes. The isolation of the two isomers of Sc@C82(I, II) and an isomer of Sc@C84 was accomplished by employing the multistage HPLC (high-performance liquid chromatography) method. In the first stage, the CS2 extract was separated into several fractions by a preparative HPLC system (PU-987 and UV-970, Jasco) with a 5-PBB column (ø20 × 250 mm, Nacalai Tesque) with CS2 eluent. The fraction containing Sc monometallofullerenes was further separated into several subfractions containing Sc@C82(I, II) and Sc@C84 by a preparative recycling HPLC system (LC-908-C60, Japan Analytical Industry) with a Buckyprep column (ø20 × 250 mm, Nacalai Tesque) with toluene eluent. At this stage, we obtained fractions containing Sc@C82(I), Sc@C82(II), and Sc@C84. Finally, Sc@C82(I, II) and Sc@C84 were isolated from empty fullerenes by using recycling HPLC with a Buckyclutcher-I column (ø21 × 500 mm, Regis Chemical Co.) with toluene eluent. Because these metallofullerenes are somewhat reactive in toluene, the toluene had to be replaced by CS2 as soon as they were isolated by HPLC. Sc@C82(I, II) and Sc@C84 were characterized by laserdesorption (LD) TOF mass spectrometry, UV-vis-NIR ab-
10.1021/jp0003508 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/20/2000
7596 J. Phys. Chem. B, Vol. 104, No. 32, 2000
Inakuma and Shinohara
Figure 2. EPR spectra of Sc@C82(I, II) and Sc@C84 at room temperature: (a) Sc@C82(I), (b) Sc@C82(II), and (c) Sc@C84. Figure 1. Isolation scheme for Sc@C82(I, II) and Sc@C84 by using the multistage HPLC method: (a) first stage of the multistage HPLC for the pyridine extracts using a PBB column in 18 mL/min CS2 eluent; (b) second HPLC for the β1 fraction using a Buckyprep column in 18 mL/min toluene eluent, Sc@C82(I) elute with C88, Sc@C82(II), and Sc@C84 exist in the former C90 fraction; (c) isolated Sc@C82(I) in the final HPLC stage using Buckyclutcher-I column in 10 mL/min toluene eluent.
sorption, and EPR spectroscopy. The UV-vis-NIR spectra were measured between 400 and 2000 nm in CS2 solution by using a UV-vis-NIR scanning spectrophotometer (UV3101PC, SHIMADZU). Various EPR parameters were obtained using an EPR spectrometer (ESR300E, Bruker). The temperature dependence of EPR spectra was measured by using an EPR spectrometer (JES-RE1X, JEOL) with a temperature-control unit. The samples for the EPR measurements were prepared using CS2 solutions degassed by a freeze-pump-thaw cycle and sealed in quartz tubes. 3. Results and Discussion 3.1. Extraction and Isolation of Sc@C82(I, II) and Sc@C84. We have found that carbon disulfide extracts of soot contain discandium fullerenes (Sc2@C74, Sc2@C76, Sc2@C84, etc.), whereas pyridine extracts abound in mono- and triscandium fullerenes such as Sc@C76, Sc@C80, Sc@C82, Sc@C84, Sc3@C82, and Sc3@C84. The EPR spectra of the pyridine extracts (not given) exhibited several sets of hfs octet lines from monoscandium metallofullerenes and a symmetric 22 hfs due to Sc3@C82. Furthermore, aniline extractions with sonication can even extract such Sc fullerenes as Sc@C60, Sc@C70, Sc@C72, and Sc@C74 that are normally difficult to extract by CS2. In the first HPLC stage, using a PBB column in CS2 eluent, we separated the extracts into four fractions, R, β1, β2, and γ (Figure 1a). Fraction R contained mainly higher fullerenes from C76 to C88 and Sc@C82(I). Fraction β1 contained Sc@C82(I, II) and Sc@C84. A further separation of fraction β1 on a Buckyprep column in toluene eluent (the second HPLC stage) confirmed that Sc@C82(I) and Sc@C82(II)/Sc@C84 exist in C88 and C90 fractions, respectively (Figure 1b). Finally, these Sc monometallofullerenes were fully separated from the C88 and C90 in
TABLE 1: EPR Parameters (isotropic hfc, isotropic g factor, line width of hfs) of Sc Monometallofullerenes Obtained at Room Temperature.a species
hfc/G
g value
∆HPP/G
Sc@C82(I) Sc@C82(II) Sc@C84 Y@C82(I) La@C82(I)
3.82 1.16 3.78 0.48 1.20
1.9999 2.0002 1.9993 2.0004 2.0008
0.036 0.019 0.017 0.087 0.055
a The parameters of Y@C (I) and La@C (I) are listed for com82 82 parison.
the third HPLC stage on a Buckyclutcher-I column in toluene eluent. An HPLC chromatogram for Sc@C82(I), which was separated and isolated easily from C88, is shown in Figure 1c. 3.2. EPR Hyperfine Structure of Sc@C82(I, II) and Sc@C84. The EPR spectra of Sc@C82(I, II) and Sc@C84 at room temperature are shown in Figure 2. All spectra show equally spaced octet hfs due to a nuclear spin of the Sc atom (I ) 7/2) with weak satellites of 13C hfs. The isotropic hyperfine coupling (hfc) parameters of these metallofullerenes are summarized in Table 1, together with those of Y@C82(I) and La@C82(I) for comparison. Small hfc constants for the Sc metallofullerenes suggest a low density of an unpaired electron in the 4s orbital of the encapsulated Sc atom. According to synchrotron X-ray diffraction41 and ultraviolet photoelectron spectroscopic40 studies, the formal charge state of Sc@C82(I) is represented by Sc2+@C822-, which indicates that an unpaired electron is localized in the Sc atom. An unpaired electron of Sc@C82(I), therefore, is lying in the 3d orbital of the encapsulated Sc atom, which is consistent with the previous studies.8,52,53 The hfc of Sc@C82(II) (1.16 G) is one-third of that of isomer I (3.82 G) (also cf. Figure 2a,b). This large difference arises from the differing orbital angular momentum of the encapsulated Sc atom in each isomer. The hfc constant of Sc@C84 (3.78 G) is comparable to that of Sc@C82(I) (also cf. Figure 2c); however, the appearances of the 13C hfs satellites of these metallofullerenes are quite different from each other. The 13C hfs depends on the geometric and electronic structure of the metallofullerenes.23,54 The 13C hfs of Sc@C82(I) has a salient feature which is due to the nuclear spin of the carbon atoms of
EPR Studies on Isolated Scandium Metallofullerenes
J. Phys. Chem. B, Vol. 104, No. 32, 2000 7597
Figure 3. UV-vis-NIR spectra of isolated Sc@C82(I, II) in CS2 solution. The energy of the onset of Sc@C82(II) (
