Perfectly Ordered Two-Dimensional Layer Structures Found in Some

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Perfectly Ordered Two-Dimensional Layer Structures Found in Some Endohedral Metallofullerenes Sachiko Maki,† Eiji Nishibori,*,† Yutaka Kitamura,∥ Ryo Kitaura,∥ Masayuki Ishihara,‡ Takayuki Aono,∥ Shinobu Aoyagi,§ Masaki Takata,† Makoto Sakata,⊥ and Hisanori Shinohara∥ †

RIKEN SPring-8 Center, RIKEN 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan Department of Applied Physics, Nagoya University, Nagoya, 464-8603, Japan § Department of Information and Biological Sciences, Nagoya City University, Nagoya 467-8501, Japan ⊥ Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ∥ Department of Chemistry and Institute for Advanced Research, Nagoya University, Nagoya 464-8602, Japan ‡

S Supporting Information *

ABSTRACT: A two-dimensional (2D) arrangement of metallofullerenes for both M@C82 (isomer I) (M = Y, La, Ce, Pr) and (M2Cy)@C82 (isomer III) (M = Er, y = 0, 2; M = Sc, y = 2) is crystallized with a 1:2 fullerene/toluene molecular ratio. The determined crystal is a 2D layered structure, which is composed of metallofullerene layers sandwiched by the toluene layers. Although the M@C82(I) arrangement in a 1:1 crystal is almost identical to that of (M2Cy)@C82(III), the M@C82(I) arrangement in a 1:2 crystal is different from (M2Cy)@ C82(III). It is found that the difference of the molecular arrangement strongly correlates with an orientation of metallofullerenes in a 2D layer. Our findings suggest the existence of a subtle but important interaction between the confined atoms/molecules and the solvent molecules/toluene through the carbon cage, which leads to a guiding principle of crystallization control with a solvent ingredient for metallofullerenes.

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one toluene molecule.9 The study suggests that all the (MxCy@ C2n−y)(C6H5CH3) (M = La, Y, Sc, Lu, Ti, Eu, Er, Hf, Sc3N; x = 1, 2; y = 0, 2; 34 < n < 43) crystals have similar molecular arrangements with each other, the so-called sphere packing model, first reported by Kawada et al.10 This structure is derived from the fcc crystal by simply displacing molecules by half a lattice constant along the cubic [110] direction. The displacement produces channels in close packing which contain one toluene per fullerene molecule. Consequently, an insertion of a single toluene molecule into the channel maintains 2D fullerene layers in the fcc crystal. The fact provides an important insight for production of 2D layered metallofullerene structure by solvent ingredient control. Here, we report a new crystal structure with a 2D arrangement of metallofullerenes, M@C82(I) (M = Y, La, Ce, Pr) and (M2Cy)@C82(III) (M = Er, y = 0, 2; M = Sc, y = 2), by the co-crystallization with double toluene molecules. The crystals have been structurally characterized by synchrotron radiation (SR) X-ray powder diffraction. The structure of M@ C82(I) crystals is different from those of the (M2Cy)@C82(III) crystals, although 1:0 and 1:1 crystal packings of M@C82(I) are almost identical to those of (M2Cy)@C82(III).

INTRODUCTION Nanoapplications of endohedral metallofullerenes have attracted wide interest in the last two decades owing to their unique structure, which is compatible with super molecules, and to the varieties of properties caused by confined atoms/ molecules.1 A significant recent progress in nanoapplication is the development of various metallofullerene devices fabrication using thin films,2−4 carbon nanotube peapods,5,6 and solar cell applications.7 Those studies elaborately fabricate one- and twodimensional (1D and 2D) systems of metallofullerene molecules in crystals to create and control novel properties of the endohedral character. The physical properties of metallofullerenes based on low-dimensional arrangement are still far from controllable due to the lower perfectness of crystals, nonuniqueness of structures for crystal specimens, and the preparation method dependent quality of the crystals. Thus, in order to investigate the firm relationship of physical properties associated with a low-dimensional metallofullerene arrangement, a stable and homogeneous metallofullerene crystal with low-dimensional arrangements is highly required. Various endohedral metallofullerenes have been reported to form closed packed crystal such as face-centered cubic (fcc).8 Many of them are also found to co-crystallize with solvents such as toluene and CS2. Systematic investigation of the molecular arrangement of metallofullerenes has been carried out by the present group in the case of co-crystallization with © 2013 American Chemical Society

Received: April 23, 2013 Revised: June 9, 2013 Published: June 17, 2013 3632

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Figure 1. Powder profiles of (a) Y@C82(I)-0, Y@C82(I)-1, and Y@C82(I)-2 and (b) Sc2C2@C82(III)-1 and Sc2C2@C82(III)-2. Temperature dependence of powder profiles for (c) Sc2C2@C82(III) and (d) Pr@C82(I).

Table 1. List of the Lattice Constants

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material

a [Å]

b [Å]

c [Å]

α [deg]

β [deg]

γ [deg]

V [Å3]

Y@C82(I)-0 Y@C82(I)-1 Y@C82(I)-2 La@C82(I)-2 Ce@C82(I)-2 Pr@C82(I)-2 Er2@C82(III)-1 Er2@C82(III)-2 (Er2C2)@C82(III)-1 (Er2C2)@C82(III)-2 (Sc2C2)@C82(III)-1 (Sc2C2)@C82(III)-2

15.7647(2) 11.22205(6) 11.2459(4) 11.2505(4) 11.2484(4) 11.2540(3) 11.25122(12) 19.4614(3) 11.24625(16) 19.5040(3) 11.24868(12) 19.47289(19)

15.7647(2) 34.8174(2) 11.2515(4) 11.2599(4) 11.2524(4) 11.2629(3) 34.8171(5) 11.27123(15) 34.8589(6) 11.27410(17) 34.8419(4) 11.28981(10)

15.7647(2) 11.22569(7) 11.5288(3) 11.5291(4) 11.5338(4) 11.5809(4) 11.23733(14) 11.58716(13) 11.24395(18) 11.57057(14) 11.23344(13) 11.57649(9)

90 90 99.298(3) 99.297(3) 99.224(3) 99.782(2) 90 90 90 90 90 90

90 90 94.058(4) 94.188(4) 94.190(4) 93.178(3) 90 104.0154(13) 90 103.7451(16) 90 103.8963(8)

90 90 119.782(3) 119.779(2) 119.756(2) 119.795(1) 90 90 90 90 90 90

3917.93(2) 4386.13(7) 1229.9(1) 1230.8(1) 1230.9(1) 1238.44(8) 4402.05(10) 2466.02(6) 4407.99(12) 2471.39(6) 4402.7(7) 2470.55(4)

experiments were carried out at SPring-8, BL02B2 beamline.11 An imaging plate, 2D detector, was used for achieving high counting statistics. The powder profiles are collected at room temperature. Details of experimental conditions are listed in the Supporting Information.

EXPERIMENTAL SECTION

Soot-containing metallofullerenes were produced in a DC arcdischarge of metal-graphite composite rods (Toyo Tanso Co. Ltd.). Monometallofullerenes, Y@C82(I), Pr@C82(I), La@C82(I), and Ce@ C82(I), and dimetallofullerenes, (Sc2C2)@C82(III), (Er2C2)@C82(III), and Er2@C82(III), were isolated by the multistage high performance liquid chromatography (HPLC) method.1 Laser desorption mass spectrometry and HPLC analysis showed that the sample purities were greater than 99%. Powder crystals were grown from toluene solution using a rotary evaporator with a heated water bath. The samples, 50 mL toluene solutions of the fullerene (0.1 mg/mL for Y@C82(I), Pr@C82(I), La@ C82(I), and Ce@C82(I), and 0.05 mg/mL (Sc2C2)@C82(III), (Er2C2) @C82(III) and Er2@C82(III)), were loaded into a 200 mL separable flask. The pressure of the flask was controlled at 55 hPa during evaporation by a vacuum controller equipped with solvent recovery unit for all the samples. We controlled the temperature of the water at 303 and 323 K. The powder samples with 1:1 co-crystallized with toluene molecule denoted as “-1” were obtained at 323 K. The corresponding crystals are hereafter denoted as MxCy@C82-1. The powder samples with 1:2 co-crystallized with toluene molecule denoted as “-2” were obtained at 303 K. A nonsolvent fcc crystal of Y@C82(I) denoted as “-0” was also obtained by sublimation of the powder sample. The powder samples were sealed in a glass capillary (0.4 mm internal diameter) for X-ray analysis. The SR powder diffraction

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RESULTS AND DISCUSSION Figure 1 shows powder profiles of (a) Y@C82(I)-0, Y@C82(I)1, and Y@C82(I)-2 and (b) (Sc2C2)@C82(III)-1 and (Sc2C2)@ C82(III)-2. The figures clearly show the difference of powder pattern between MxCy@C82-0, MxCy@C82-1, and MxCy@C82-2 crystals. Peak positions of Y@C82(I)-0 can be indexed by fcc lattice which is the same as La@C82(I).7 Peak positions of Y@ C82(I)-1 are almost identical to those of (Sc2C2)@C82(III)-1, and the lattice constants are a = 11.22205(6) Å, b = 34.8174(2) Å, c = 11.22569(7) Å for Y@C82(I)-19 and a = 11.2487(12) Å, b = 34.8419(4) Å, c = 11.2334(13) Å for (Sc2C2)@C82(III)-1.12 Both crystals have an orthorhombic unit cell as reported previously.9,12 Crystal packing of Y@C82(I)-1 is identical to that of (Sc2C2)@C82(III). Peak positions of Y@C82(I)-2 are different from those of Sc2C2@C82(III)-2. We determined lattice constants of Y@ C82(I)-2 and Sc2C2@C82(III)-2 using the DICVOL06 program.13 The lattice constants are refined by Le Bail analysis.14 3633

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The lattice constants of Y@C82(I)-2 are a = 11.2459(4) Å, b = 11.2515(4) Å, c = 11.5288(3) Å, α = 99.298(3)°, β = 94.058(3)°, and γ = 119.781(3)°, which are triclinic. The lattice constants of (Sc2C2)@C82(III)-2 are a = 19.4729(2) Å, b = 11.2898(1) Å, c = 11.57649(9) Å, α = 90°, β = 103.8963(8)°, and γ = 90.0°, which are monoclinic. We also determined lattice constants of Pr@C82(I)-2, La@C82(I)-2, Ce@C82(I)-2, (Er2C2) @C82(III)-2, and Er2@C82(III)-2. Figure 1c,d shows the temperature dependence of powder profiles for (c) (Sc2C2)@C82(III) and (d) Pr@C82(I). In some cases, we prepared a mixed phase of powder crystals. In Figure 1c, the 300 K data of (Sc2C2)@C82(III) is a mixture of (Sc2C2) @C82(III)-1 and (Sc2C2)@C82(III)-0. Most of (Sc2C2)@ C82(III)-1 phase transformed to (Sc2C2)@C82(III)-0 at 630 K. The data of Pr@C82(I) at 300 K is a mixture of Pr@C82(I)-1 and Pr@C82(I)-2. The Pr@C82(I)-2 crystal completely transformed to Pr@C82(I)-1. Similar temperature dependences were also found in the Y@C82(I), La@C82(I), Ce@C82(I), (Er2C2)@ C82(III), and Er2@C82(III). We found that the 1:2 crystal was transformed to the 1:1 fullerene/toluene closed packed structure and, then, further transformed to non-toluene fcccrystal by heating treatment. Table 1 shows lattice constants of MxCy@C82 crystals. The corresponding unit cell volumes, V, are also listed. The V of the “-1” crystals are almost identical to each other. The fcc crystals contain four fullerene molecules. A “-1” crystal contains four toluene and fullerene molecules. We calculated the volume of toluene molecule, Vt, in “-1” crystal using the V of Y@C82(I)-0 and Y@C82(I)-1. The Vt is 117.05 Å3. The V of M@C82(I)-2 (M = Y, La, Ce, Pr) crystals are almost a quarter of that of Y@ C82(I)-1. The V of M@C82(I)-2 can contain one fullerene molecule. Subtracting the V of one Y@C82(I)-0 from V of Y@ C82(I)-2 provides 250.4 Å3, which is almost twice as large as Vt. We found the “-2” crystals contain two toluene molecules per one metallofullerene from the unit cell volume. We determined crystal structures from powder diffraction data. The structures of Y@C82(I)-1,15 La@C82(I)-1,16 and (Sc2C2)@C82(III)-1, (Er2C2)@C82(III)-1, and Er2@C82(III)-19 have been reported previously. Crystal structures of Y@C82(I)0, Y@C82(I)-2, Pr@C82(I)-2, La@C82(I)-2, Ce@C82(I)-2, (Sc2C2)@C82(III)-2, (Er2C2)@C82(III)-2, and Er2@C82(III)-2 were determined by a combination of the maximum entropy method (MEM) and Rietveld analysis, the so-called MEM/ Rietveld method, which is utilized for powder analysis of endohedral metallofullerenes. Details of the analysis have been described elsewhere.17 Figure 2 shows crystal structures of (a) Y@C82(I)-0, (b) Y@ C82(I)-1, and (c) Y@C82(I)-2. Extended unit cells are shown to exhibit the molecular packing. The fcc crystal has a disorder structure with 6 directions for metal atoms and 48 directions for carbon atoms. One of the molecular orientations is extracted and shown in Figure 2a. Two-dimensional fullerene molecular layers along the c-axis divided by toluene layers are seen in the Figure 2c. In addition, fullerene molecules are ordered in the crystal. We have successfully produced a 2D ordered metallofullerene crystal by 1:2 co-crystallization with two toluene molecules. Molecular arrangement in the layer is a triangular lattice which is very similar to the (111) plane of the fcc crystal as shown in Figure 2d,e. Intermolecular distances of the triangular lattice for Y@C82(I)-2 are 11.286 Å, 11.246 Å, and 11.251 Å, which are approximately 0.1 Å longer than that of Y@C82(I)-0 (11.147 Å). The same crystal structures are also obtained in Pr@C82(I)-2, La@C82(I)-2, and Ce@C82(I)-2.

Figure 2. Crystal structures of (a) Y@C82(I)-0, (b) Y@C82(I)-1, and (c) Y@C82(I)-2. The triangular lattice of (d) Y@C82(I)-0 and (e) Y@ C82(I)-2.

Figure 3 shows the crystal structure of (Sc2C2)@C82(III)-2. A 2D layer similar to Y@C82(I)-2 can be seen in Figure 3a. A

Figure 3. (a) Crystal structure of (Sc2C2)@C82(III)-2. (b) The triangular lattice of (Sc2C2)@C82(III)-2.

triangle lattice of fullerene molecules is also found as shown in Figure 3b. Intermolecular distances of the triangular lattice, 11.254 Å and 11.290 Å, are close to those of Y@C82(I)-2. The same crystal structures are also obtained in (Er2C2)@C82(III)-2 and Er2@C82(III)-2. We also produced 2D dimetallofullerene crystals by 1:2 co-crystallization with two toluene molecules. The layered structure of C82-based mono- and dimetallofullerene can be crystallized with two toluene molecules. The triclinic lattice of M@C82(I)-2 is different from the monoclinic (M2Cy)@C82(III)-2. Figure 4 shows a schematic molecular arrangement in the crystal for (a) M@C82(I)-2 (M = Y, La, Ce, Pr) and (b) (M2Cy)@C82(III)-2 (M = Er, y = 0, 2; M = Sc, y = 2). Metallofullerene molecules are shown as green balls, and toluene molecules are not shown for clarity. In (M2Cy)@C82(III)-2, the angle indicated by red lines in Figure 4b is 90°, indicating completely overlapped layers. In M@ C82(I)-2, a similar angle indicated by red lines in Figure 4b is different from 90° showing a shift of layers. We investigated the origin of the difference between the two metallofullerene crystals. 3634

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molecule. The interaction induces the ordered layer structure of M@C82(I)-2. Our findings suggest the existence of a subtle but important interaction between the encapsulated atoms and the solvent toluene molecules through the carbon cage.

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CONCLUSION We have reported the fabrication and structural determination of ordered 2D layer structures of monometallofullerenes, Y@ C82(I)-2, Pr@C82(I)-2, La@C82(I)-2, and Ce@C82(I)-2 and dimetallofullerenes, (Sc2C2)@C82(III), (Er2C2)@C82(III) and Er2@C82(III). The solvent-free metallofullerene thin films may have disordered structures as the present Y@C82(I)-0. For the first time, fabrication of thin films using the present cocrystallization technique provides an ordered 2D layer thin film of endohedral metallofullerenes. The ordered 2D layer structure can be highly reflected by the details of the endohedral molecular structure, and it enables us to obtain information on the essential properties of the molecules.

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Figure 4. Schematic molecular arrangement of the crystal structures for (a) M@C82(I)-2 and (b) (M2Cy)@C82(III)-2. Positions of metal atom for (c) M@C82(I)-2 (purple balls) (d) (M2Cy)@C82(III)-2 (sky blue balls).

ASSOCIATED CONTENT

S Supporting Information *

Details of sample preparations, SR X-ray diffraction experiments, results of Rietveld refinements, crystal structures of Pr@ C82(I)-2, La@C82(I)-2, Ce@C82(I)-2, (Er2C2)@C82(III)-2, and Er2@C82(III)-2. This information is available free to charge via Internet at http://pubs.acs.org/.

Figure 4c,d shows the positions of the metal atoms for M@ C82(I)-2 (purple balls) and (M2Cy)@C82(III)-2 (sky blue balls), respectively. In M@C82(I)-2, the metal atom is located upside of the layers, whereas two metal atoms are symmetrically located both up- and downside in (M2Cy)@C82(III)-2. This strongly suggests that M@C82(I)-2 has polar layers and that the interaction exerted among the polar layers causes the stacking shift. We found that the difference of molecular arrangement between M@C82(I)-2 and (M2Cy)@C82(III)-2 is due to the difference of molecular structures. The metallofullerene crystal structures via 1:2 cocrystallization toluene molecules highly reflect the position(s) of metal atoms of the metallofullerenes. Lattice structures of Y@C82(I)-2, Pr@C82(I)-2, La@C82(I)-2, and Ce@C82(I)-2 are very similar to each other, and deviations of lattice constants are less than 0.01 Å for a, b, c and 0.14° for α, β, γ. These fullerenes have almost the same molecular packing including molecular orientations. In contrast, M−C nearest neighbor distances vary more than the lattice constants. The M−C distances for Y@C82(I)-2, Pr@C82(I)-2, La@C82(I)2, and Ce@C82(I)-2 are 2.16, 2.30, 2.32, and 2.32 Å, respectively. The maximum deviation of M−C distances is more than 0.15 Å. Since the charge state of a metallofullerene can be varied depending on the kind of metal atoms encapsulated (i.e., M2+ or M3+), the endohedral metal atom can be regarded as a control of polarization for 2D layers. Furthermore, since the ionic radius M3+ can be varied by an order of 0.2 Å, one can also control the corresponding molecular polarization by the metal atom encaged. This might be one of the great advantages for a future fabrication of functional layers and ferroelectric thin films. The metal atom of M@C82(I) is generally located on the C2 axis of C82-C2v(9). The C2 axis of M@C82(I) in the “-2” crystal is almost perpendicular to C5H6 plane of toluene molecules as shown in Figures 2c, S3.2, S3.3, S3.4, and S3.5. The major sites of two metal atoms in (M2Cy)@C82(III) are located on the C3 axis of C82-C3v(8). The C3 axis of (M2Cy)@C82(III) in the ”-2” crystal is also almost perpendicular to C5H6 plane of toluene molecules as shown in Figures 3a, S3.6, S3.7, and S3.8. There are π electrons on the C5H6 plane. The metal atom(s) in fullerene seems to be attracted π electrons of the toluene

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grant-in-Aid for Young Scientists (A) (No. 17686003) and Scientific Research (B) (No. 20360006) of MEXT, Japan, for partial support of the present study. We also thank Dr. K. Kato and Dr. J. E. Kim for experimental help. The synchrotron radiation experiments were performed at SPring-8 with approval of the Japan Synchrotron Radiation Research Institute (JASRI).

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