Observation of Li cation encapsulated in C 60 fullerene cage by single-crystal charge density analysis 92 Chemical Science C 60 fullerene is the most promising nanocarbon material for a wide range of practical applications owing to its high productivity and flexible electrical and chemical properties. The encapsulation of atoms and molecule in C 60 fullerene is a promising method of developing application field. Various endohedral C 60 fullerenes have been synthesized since the discovery of the first metallofullerene La@C 60 in 1985. The macroscopic synthesis and molecular structures of the gas molecule endohedral C 60 , H 2 @C 60 and Ar@C 60 , have been reported [1,2]. The physical and chemical properties of H 2 @C 60 and Ar@C 60 are similar to those of empty C 60 . The encapsulation of a metal atom is expected to change the properties of C 60 by metal ionization. However, the isolation and structure determination of M @C 60 ( M : metal) have been precluded by the insolubility and high reactivity of M @C 60 . Recently, the bulk synthesis and complete isolation of Li@C 60 have been successfully carried out by researchers of idealstar Inc. and Tohoku University. A single crystal of [Li@C 60 ](SbCl 6 ) with typical dimensions of 60 × 40 ×10 μ m 3 was grown by the diffusion of CS 2 vapor into solution. However, it is generally difficult to determine the position of a Li cation from small crystals by X-ray diffraction analysis. We observed an encapsulation of a Li cation inside a C 60 cage by a single-crystal X-ray diffraction analysis of [Li@C 60 ](SbCl 6 ) crystals at BL02B1 beamline [3]. The single-crystal X-ray diffraction experiment was performed using a large cylindrical imaging plate camera [4]. The camera rapidly collects diffraction peaks over a wide diffraction angle range (2 θ < 145°) using an imaging plate with a wide dynamic range of 10 6 and a three-axis goniometer (–130 < ω < 220°, –5 < χ < 60°, –180 < φ < 360°). The wavelength of X-rays used is 0.35 Å. The sample temperature was controlled using a He/N 2 gas flow system. The [Li@C 60 ](SbCl 6 ) crystal had a twinned monoclinic structure below room temperature. A phase transition from a twinned monoclinic structure to a single-domain orthorhombic structure was observed at ~320 K. The crystal structure of the orthorhombic [Li@C 60 ](SbCl 6 ) was determined at 370 K (space group Amm 2, a = 12.30 Å, b = 9.95 Å, c = 29.05 Å, V = 3555 Å 3 ). The orientations of C 60 cages and the position of a Li cation inside a cage were determined on the basis of charge density distributions by the maximum entropy method (MEM). A charge density map for a [Li@C 60 ] + is shown in Fig. 1(a). Two charge density peaks (red arrows) inside the C 60 cage were regarded as disordered Li cation occupying two off-centered sites that are crystallographically equivalent. This off-centered structure is quite different from the on-centered structure of the encapsulated gas molecule inside the C 60 cage [1,2]. The distance from the center of the C 60 cage to the Li cation is 1.34 Å, which is close to the theoretically predicted values. The inhomogeneous charge density distribution of the C 60 cage (shown as green in Fig. 1(a)) demonstrates the ratchet-type disorder of the cage with four orientations. In any orientation of C 60 fullerene, the Li cation lies in the vicinity of one of the six-membered rings (Fig. 1(b)) of the cage. This suggests the presence of an attractive force exerted between the six-membered ring and the Li cation. (a) (b) c b c b Sb Cl 6 Fig. 1. (a) MEM charge density map of Li@C 60 cation in [Li@C 60 ](SbCl 6 ) crystal at 370 K. A 0.25 e /Å 3 equi-charge density surface for a Li cation inside a C 60 cage is shown as magenta. A 2.0 e /Å 3 equi-charge density surface is shown in green. The charge density at the two peaks for the Li cation (red arrows) is 0.38 e /Å 3 . (b) Geometrical relationships between Li@C 60 cation and adjacent SbCl 6 anion (orange). The structure of one molecular orientation for a Li@C 60 cation extracted from a disordered structure is shown. The nearest Li-Cl distance is 5.58 Å (red dashed line). 93 An electrostatic attractive interaction between a Li cation and SbCl 6 anions should be crucial to determining the position of the Li cation in the C 60 cage. Figure 1(b) shows the arrangement of an SbCl 6 anion around a Li@C 60 cation. The Li cation is close to a negatively charged Cl atom of the SbCl 6 anion. The electrostatic attractive interaction between Li + and Cl – through the cage should localize the Li cation at the two sites. The restriction of the free rotation of the C 60 cage even at a high temperature of 370 K is regarded as a result of the attractive interaction between the localized Li cation and the six-membered rings. The three-dimensional molecular arrangement of the [Li@C 60 ] + and SbCl 6 – units (Fig. 1(b)) constructs the layered crystal structure, as shown in Fig. 2. The slab structure formed by two-dimensional arrays of the [Li@C 60 ](SbCl 6 ) units (Fig. 1(b)) perpendicular to the c -axis is clearly seen. In the slab, the first- and second-nearest intermolecular distances of the Li@C 60 cations are 9.95 and 10.03 Å, respectively. The shorter distance (9.95 Å) between the adjacent Li@C 60 cations is almost exactly the same as that of the close-packed pristine fcc C 60 structure, whereas the interslab distance of the Li@C 60 cations is 10.30 Å. The positively charged Li@C 60 ions are periodically assembled on the two-dimensional negatively charged sheets of SbCl 6 ions by the electrostatic attractive force between the Li cation and the Cl atom. This unique structural feature may exhibit interesting solid-state properties. The 13 C NMR spectrum of [Li@C 60 ](SbCl 6 ) in solution shows that the Li cation is rapidly moving inside the C 60 cage. These facts suggest that the position of the Li cation can be varied and controlled by adjusting the external field outside the C 60 cage. Such position control by an external field can be widely used in electronics applications such as in single molecular switches and ferroelectric sheets. The most striking feature of Li@C 60 revealed in the present study, which has never been observed in conventional metallofullerenes, is the extremely high tendency of Li@C 60 to form ion-pair states (species) such as [Li@C 60 ](SbCl 6 ). Li@C 60 can only be stabilized significantly under ambient condition when it coexists with an appropriate counteranion. The present methodology for the isolation and crystallization of M @C 60 can be widely applied to the existing Groups 2, 3, and 4 and all lanthanide metallofullerenes such as La@C 60 and Gd@C 60 [5]. c b Fig. 2. Layered crystal structure of [Li@C 60 ](SbCl 6 ). Slabs formed by two-dimensional arrays of [Li@C 60 ](SbCl 6 ) units (Fig. 1(b)) are stacked along the c -axis. Three slabs (one layer in dense color and two outer layers in light colors) are shown in the figure. References [1] Y. Kohama et al. : Phys. Rev. Lett. 103 (2009) 073001. [2] K. Yakigaya et al. : New. J. Chem. 31 (2007) 973. [3] S. Aoyagi, E. Nishibori, H. Sawa, K. Sugimoto, M. Takata, Y. Miyata, R. Kitaura, H. Shinohara, H. Okada, T. Sakai, Y. Ono, K. Kawachi, K. Yokoo, S. Ono, K. Omote, Y. Kasama, S. Ishikawa, T. Komuro and H. Tobita: Nature Chem. 2 (2010) 678. [4] K. Sugimoto et al. : AIP Conf. Proc. 1234 (2010) 887. [5] R.D. Bolskar et al. : J. Am. Chem. Soc. 125 (2003) 5471. Shinobu Aoyagi, Eiji Nishibori and Hiroshi Sawa* Department of Applied Physics, Nagoya University *E-mail: hiroshi.sawa@cc.nagoya-u.ac.jp
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