4. MOLECULAR AND CRYSTAL STRUCTURES OF METALLOFULLERENES
4.3 Confirmation on endohedral structures as determined by synchrotron X-ray diffraction
4.3.1 Y@C82mono-metallofullerene
Previous experimental evidence including EXAFS (Kikuchi et al., 1994a;
Park et al., 1993) and HRTEM (Beyers et al., 1994) suggested that the metal atoms are inside the fullerenes. Theoretical calculations also indicated that such endohedral metallofullerenes are stable (Andreoni and Curioni, 1996a,b; Chang et al., 1991; Cioslowski and Fleishcmann, 1991;
Guo et al., 1994; Laasonen et al., 1992; Manolopoulos and Fowler, 1991, 1992; Nagase and Kobayashi, 1993; Rosen and Waestberg, 1988, 1989;
Saito and Sawada, 1992; L Wang et al., 1993a). However, the first conclu- sive experimental evidence on the endohedral nature of a metallofuller- ene, Y@C82, was obtained by a synchrotron X-ray diffraction study. The result indicated that the yttrium atom is encapsulated within the C82
fullerene and is strongly bound to the carbon cage (Takata et al., 1995).
The space group was assigned to P21, which is monoclinic for Y@C82. The experimental data were analyzed in an iterative way of a combination of Rietveld analysis (Rietveld, 1969) and the maximum entropy method (MEM) (Bricogne, 1988; Collins, 1982). The MEM can produce an election density distribution map from a set of X-ray structure factors without using any structural model. By the MEM analysis (Kumazawa et al., 1993;
Sakata and Sato, 1990), theRIbecomes as low as 1.5% for Y@C82.
By using the revised structural model based on the previous MEM map and MEM analysis, a series of iterative steps involving Rietveld analysis were carried out until no significant improvement was obtained.
Eventually, theRIfactor improved from 14.4% to 5.9% (RWP¼3.0%). In Figure 7, the best fit of the Rietveld analysis of the Y@C82 is shown. To display the endohedral nature of the Y@C82, the MEM electron density distribution of Y@C82 is shown in Figure 8. There exists a high-density area just inside the C82cage. The density maximum at the interior of the C82 cage corresponds to the yttrium atom, indicating the endohedral structure of the metallofullerene.
It was also found that the cage structure of Y@C82differs from that of the hollow C82fullerene. There are many local maxima along the cage in Y@C82, whereas electron densities of the C82cage are relatively uniform.
This suggests that in Y@C82 the rotation of the C82cage is very limited around a certain axis even at room temperature, whereas that in C82 is almost free.
The MEM election density map further reveals that the yttrium atom does not reside at the center of the C82cage but is very close to the carbon cage, as suggested theoretically (Andreoni and Curioni, 1996a,b; Guo et al., 1994; Laasonen et al., 1992; Nagase and Kobayashi, 1993; Nagase and Kobayashi, 1994a). The ESR (Shinohara et al., 1992a; Weaver et al., 1992) and theoretical (Nagase and Kobayashi, 1993; Schulte et al., 1996) studies suggest the presence of a strong charge transfer interaction between the Y3þion and the C3cage which may cause the aspherical electron density distribution of atoms. The Y–C distance calculated from the MEM map is 2.9(3) A˚ which is slightly longer than a theoretical prediction of 2.55–2.65 A˚ (Nagase and Kobayashi, 1993). The X-ray
3 7
5 7 9 11
2q (deg)
13 15 17 19
6 5 4 3
Intensity (arbitrary units)
2 1 0
Yo − Yc
FIGURE 7 Powder X-ray (synchrotron) diffraction patterns and the corresponding fitting results of Y@C82based on the calculated intensities from the MEM electron density.
(100) (010)
4 Å
FIGURE 8 The MEM electron density distribution of Y@C82for the (001) section.
The density maximum corresponds to Y atom.
study also reveals that the Y@C82 molecules are aligned along the [001]
direction in a head-to-tail (. . .Y@C82. . .Y@C82. . .Y@C82. . .) order in the crystal, indicating the presence of a strong dipole–dipole and charge transfer interactions among the Y@C82fullerenes.
4.3.2 Sc@C82metallofullerene
The endohedral structure of Sc@C82 was also studied by synchrotron X-ray diffraction with MEM analysis (Takata et al., 1998). The Sc@C82 crystal includes solvent toluene molecules and has P21space group as in the Y@C82case. The MEM electron charge density distribution of Sc@C82
is shown in Figure 9. The Sc atom is not at the center of the fullerene but close to one of the six membered rings of the cage. The nearest-neighbor Sc–C distance estimated from the MEM map is 2.53(8) A˚ , which is very close to a theoretical value, 2.52–2.61 A˚ (Nagase and Kobayashi, 1993).
There are in total nine IPR (isolated-pentagon rule) satisfying structural isomers for C82. These areC2(a),C2(b),C2(c),C2v,Cs(a),Cs(b),Cs(c),C3v(a), andC3v(b). The X-ray result indicates that the carbon cage of Sc@C82has C2vsymmetry (Figure 9).
There has been controversy as to whether the encaged Sc atom has a divalent state or a trivalent state (Nagase and Kobayashi, 1993; Ruebsam et al., 1996b; Schulte et al., 1998; Shinohara et al., 1992b). The synchrotron X-ray result shows that the number of electrons around the Sc atom is 18.8e, indicating that the Sc atom in the cage is in a divalent state,
C2
FIGURE 9 Equi-contour density map of the MEM charge density for the side view of Sc@C82molecule. The Sc atom is drawn inside the upper hemisphere of the fullerene.
TheC2axis is indicated.
Sc2þ@C822. The charge state is consistent with an ultraviolet photoelec- tron spectroscopy (UPS) experiment (Hino et al., 1998).
4.3.3 La@C82metallofullerene
The La@C82metallofullerene is one of the first endohedral metallofuller- enes that was macroscopically produced and solvent extracted (Chai et al., 1991). Suematsu et al. (1994) first reported the crystal structure of La@C82 precipitated from CS2 solution via synchrotron X-ray powder diffraction. The composition of the microcrystal is expressed by La@C82(CS2)1.5. The crystal has the cubic structure. The results suggest a molecular alignment in the unit cell, in which the molecules align in the [111] direction with the molecular axis orienting in the same [111] direc- tion. Watanuki et al. (1995, 1996) performed synchrotron X-ray diffraction measurements on solvent-free powder samples of La@C82, and concluded that the major part of the crystal has fcc lattice. Their results strongly suggest the endohedral nature of La@C82where the La atom is displaced from the center of the C82cage by 1.9 A˚ .
The detailed endohedral structure of La@C82was revealed experimen- tally (Nishibori et al., 2000). The electron density distribution of La@C82
based on the MEM analysis of the powder X-ray diffraction data is pre- sented in Figure 10. The result shows that the La atom is encapsulated by theC2visomer of C82as in Sc@C82described above. As is seen from the figure, the La atom is not at rest in the cage but rather is in a floating motion along the nearest six-membered ring at room temperature. The result is different from the Sc@C82and Y@C82 cases in which Sc and Y atoms are almost at a standstill in the cage even at room temperature.
FIGURE 10 The section of the equi-charge density surface of La@C82molecule.
A light metal atom such as Sc seems to be more strongly bound to the fullerene cage than a heavy La atom.
4.3.4 Movement of metal atoms within the cage
Intrafullerene metal motions have been theoretically predicted by Andreoni and Curioni (1996a,b, 1997, 1998) on La@C60 and La@C82 on the basis of molecular dynamics simulations. Experimentally, dynamical motion of metal atoms has been reported on La@C82 (Nishibori et al., 2000), Sc2C2@C82(Miyake et al., 1996) and La2@C80(Akasaka et al., 1995b, 1997). It is noted that La atom is moving in the C82cage at room tempera- ture (Figure 10).
A particularly interesting case has been found in La2@C80. La2@C80
metallofullerene was first produced by Whetten and co-workers (Alvarez et al., 1991) and was first isolated by Kikuchi et al. (1994a). The empty C80
has seven IPR structures (D2,D5d,C2v,C2v,D3,D5h, andIh). A13C NMR study indicated that the most abundant C80hasD2symmetry (Hennrich et al., 1996). However, theoretical calculations (Kobayashi et al., 1995a,b) have shown that encapsulation of two La atoms inside theIh-C80cage is most favorable. This is due to the fact that the Ih-C80cage has only two electrons in the fourfold degenerate highest occupied molecular orbital (HOMO) level and can accommodate six more electrons to form the stable closed-shell electronic state of (La3þ)2@C806
with a large HOMO–LUMO (lowest unoccupied molecular orbital) gap.
On the basis of13C NMR and139La NMR results, Akasaka et al. (1995b, 1997) reported a circular motion of encaged La atoms in the C80 cage.
Two La atoms may circuit the inside of the spherical Ih-C80 cage. The energy barrier for the circuit of the metal cations is very small (about 5 kcal mol1). The dynamic behavior of metal atoms should also be reflected in the139La NMR linewidth, since circulation of two La3þcations produces a new magnetic field inside the cage. Such a linewidth broaden- ing was actually observed with increasing temperature from 305 to 363 K (Akasaka et al., 1997).
A similar but greatly restricted intrafullerene dynamics of encaged metal ions has been reported by Miyake et al. (1996) on Sc2C2@C82. They observed a single45Sc NMR line, indicating that two Sc atoms in the cage are equivalent. However, in contrast to the La2@C80 case, the internal rotation is hindered by a large barrier of about 50 kcal mol1(Nagase and Kobayashi, 1994b).
Nishibori et al. revealed a detailed La dynamic motion in C80cage by synchrotron X-ray powder diffraction (Figure 11, Nishibori et al., 2001), where a perfect pentagonal-dodecahedral charge density of La2was seen in an icosahedral Ih-C80 cage. The characteristic charge density results from a highly selective trajectory of the two La atoms, which hop along the hexagonal rings of the Ih-C80 polyhedral network. This highly
symmetrical hopping of the two La atoms in C80cage was supported and analyzed in detail by a quantum chemical study (Shimotani et al., 2004).
An intrafullerene dynamics of Ce atom in the C82 cage was also studied by time-differential perturbed angular correlation measurements (Sato et al., 1998). The observed angular correlation shows the presence of two different chemical species of Ce@C82. The data at low temperatures reveal that Ce stays at a certain site for one of the species, whereas for the other the atom has an intramolecular dynamic motion.
4.3.5 A di-metallofullerene: Sc2C2@C82
Various metallofullerenes supposed to encapsulate two or three metal atoms within fullerene cages, such as La2@C80 (Akasaka et al., 1995b, 1997; Alvarez et al., 1991; Kikuchi et al., 1994a; Suzuki et al., 1995a,b), La2@C72(Bethune et al., 1994, 1996; Stevenson et al., 1998; van Loosdrecht et al., 1994a,b), Y2@C82(Shinohara et al., 1992a; Weaver et al., 1992), Sc2@C74
(Shinohara et al., 1993a, X Wang et al., 1993c), Sc2@C82 (Shinohara et al., 1992a; Yannoni et al., 1992), Sc2C2@C82(Beyers et al., 1994; Shinohara et al., 1992a,b; Takahashi et al., 1995; Takata et al., 1997; Yamamoto et al., 1996;
Yannoni et al., 1992), Sc3C2@C80(Anderson et al., 1997b; Kato et al., 1995a;
Shinohara et al., 1992b, 1994a; Stevenson et al., 1994a,b; van Loosdrecht et al., 1994a,b; Yannoni et al., 1992) and Er2@C82 (Ding et al., 1997;
Dorn et al., 1995; Macfarlane et al., 1997) have been successfully synthe- sized and purified. Among them the scandium di-metallofullerenes, Sc2C2@C82, are especially interesting, because three structural isomers have been found and isolated so far (Yamamoto et al., 1996).
FIGURE 11 MEM charge density of La2@C80as the equal-density contour surface along theS10axis. The La2dodecahedral charge density is colored in dark and is also additionally shown beside the fullerene molecule.
Ab initio theoretical studies (Nagase and Kobayashi, 1997; Nagase et al., 1996) and the experimental results on Sc2C2@C82 including STM (Shinohara et al., 1993b, X Wang et al., 1993c), TEM (Beyers et al., 1994) and 13C NMR (E Yamamoto et al., 1996) have suggested an endohedral nature. Similar to the mono-metallofullerenes, a synchrotron powder X-ray study is reported on Sc2C2@C82 (isomer III) based on the Riet- veld/MEM analysis (Takata et al., 1997). Figure 12 shows a structural model based on a three-dimensional MEM electron density distribution of Sc2@C84(III), which is now identified as Sc2C2@C82(III) carbide metallo- fullerene as described in Section 5 (Iiduka et al., 2006; Nishibori et al., 2006a,b).
The number of electrons around each maximum inside the cage is 18.8, which is close to that of a divalent scandium ion Sc2þ(19.0). A theoretical study predicted that the formal electronic structure of Sc2C2@C82is well represented by (Sc2C2)22þ@C824, where two 4s electrons of each Sc atom transfer to the C82cage (Nagase and Kobayashi, 1997). The positive charge of the Sc atom from the MEM charge density is þ2.2 which is in good agreement with the theoretical value. Furthermore, Pichler et al. (2000) reported that comparison of the Sc 2p–3d X-ray absorption spectrum with calculated ionic multiplet spectra shows a formal charge transfer to the fullerene cage of 2.6.
C2 Sc
FIGURE 12 The molecular structure of (Sc2C2)@C82(III) carbide metallofullerene [formerly considered to be Sc2@C84(III)].
4.3.6 A tri-metallofullerene Sc3C2@C80
A tri-scandium fullerene, Sc3@C82 (currently identified as Sc3C2@C80, cf. Section 5), has been produced (Shinohara et al., 1992b; Yannoni et al., 1992) and characterized by ESR (see Section 5). A synchrotron X-ray structural study on Sc3@C82 has been reported recently based on Riet- veld/MEM analysis (Takata et al., 1999). The result revealed an intriguing feature of this metallofullerene: three Sc atoms are encapsulated in the form of a triangle Sc3 cluster inside the C3v-C82 fullerene cage.
Furthermore, the charge state of the encaged Sc3cluster is 3þleading to a formal molecular charge state of (Sc3)3þ@C823
as a result of an intra- fullerene electron transfer. This was the first example in which a metal cluster is encaged by a fullerene. The presence of a Sc3trimer in the cage is consistent with an extended Hueckel calculation (Ungerer and Hughbanks, 1993).
As is described in Section 5, the detailed X-ray diffraction studies currently indicate that Sc3@C82should be Sc3C2@C80carbide metalloful- lerene (Iiduka et al., 2005; Nishibori et al., 2006a,b), where a C2molecule is encapsulated in C80-Ih cage. The encapsulated three Sc atoms form a triangle. A spherical charge distribution originating from the C2molecule is located at the center of the triangle. Intraatomic distances between Sc and Sc are 3.61(3) A˚ in the triangle. The distance between Sc and the center of the C2molecule is 2.07(1) A˚ . The molecular structure is shown in Figure 13.
FIGURE 13 The molecular structure model of Sc3C2@C80along theS10axis determined by the MEM/Rietveld method from the synchrotron X-ray power diffraction data. The large atoms forming a triangle are Sc, and the central sphere represents C2molecule averaged by rotation.