Ordered Crystals of Fullerenes Produced by Cocrystallization with

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Ordered Crystals of Fullerenes Produced by Cocrystallization with Halogenated Azatriquinacenes David Pham, Jordi Ceron-Bertran, Marilyn M. Olmstead, Mark Mascal,* and Alan L. Balch*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 75-82

Department of Chemistry, UniVersity of California, DaVis, California 95616 ReceiVed July 9, 2006; ReVised Manuscript ReceiVed September 21, 2006

ABSTRACT: The growth and structures of three new cocrystalssC9Cl6Br3N‚C60, C9Cl6Br3N‚C70, and C9Cl6H3N‚3C70‚3C6H5Cls involving fullerenes and halogenated azatriquinacenes are reported. In each case, the fullerene is completely ordered and avoids the orientational disorder found in many crystalline fullerenes and fullerene cocrystals. In each structure, the upper Cl6 surface of the halogenated azatriquinacene molecule cups a fullerene. In C9Cl6Br3N‚C60 and C9Cl6Br3N‚C70, the molecules are arranged alternately in columns with the lower Br3 face of the azatriquinacene also interacting with a neighboring fullerene. In C9H3Cl6N‚3C70‚3C6H5Cl, one of the fullerenes is cupped by the upper surface of the azatriquinacene, while four chlorobenzene molecules also form a belt around it. Introduction The highly symmetrical nature of the fullerenes C60 and C70 and related endohedral fullerenes, that is, carbon cages with included guests, frequently results in the formation of crystalline solids that display high degrees of disorder.1-3 Fullerenes have a proclivity to cocrystallize with a wide range of neutral molecules.4 Some of these either have or can assume complementary shapes that can effectively enclose a section of the curved fullerene surface. Such molecules include the conformationally mobile hosts like calix[5]-, -[6]-, and -[8]arenes,5 cyclodextrins,6 crown ethers,7 and cyclotriveratrylenes.8 However, in many cases, cocrystallization with these hosts produces solids in which there is some degree of disorder in the fullerene orientation. Surprisingly, cocrystallization with the flat surfaces presented by porphyrins frequently results in the formation of crystals with orientationally ordered fullerenes.9 Indeed, cocrystallization with NiII(OEP) has become a widely applicable technique for obtaining structural information on endohedral fullerenes.10 The halogenated derivatives of the rigid, bowl-shaped tricycle 10-azatriquinacene11 shown in Figure 1 are chalice-shaped molecules presenting slightly curved surfaces, which are complementary to the more highly curved fullerenes. Earlier we demonstrated that perchloro-10-azatriquinacene (Cl9ATQ) cocrystallized with C60 and C70 to form solvent-free crystals, Cl9ATQ‚C60 and Cl9ATQ‚C70, with orientationally ordered carbon cages.12 Not only were the upper Cl6 surface and the lower Cl3 surface of Cl9ATQ in close proximity to fullerenes, but the six lateral Cl3 surfaces were also in contact with nearby fullerenes. Consequently, each molecule of Cl9ATQ was surrounded by eight fullerenes. Here we examine the effects of altering the substitution pattern of the azatriquinacene ring system on the interactions between the various surfaces of the molecule and the C60 and C70 fullerenes. The first change we make is to replace three chlorines on the lower surface with bromines to give 3,6,9-tribromo-1,2,4,5,7,8-hexachloro-10-azatriquinacene (Br3Cl6ATQ). We then remove these three halogens altogether to give 1,2,4,5,7,8-hexachloro-10-azatriquinacene (Cl6ATQ), which presents an unsubstituted lower surface. These changes were expected to alter both the charge distribution and the size of the azatriquinacene molecule relative to Cl9ATQ.

Figure 1. Chemical structures of Cl9ATQ, Br3Cl6ATQ, and Cl6ATQ.

Results and Discussion The electrostatic potential surfaces of Cl9ATQ, Br3Cl6ATQ, and Cl6ATQ have been modeled from geometries optimized at the ab initio B3LYP/6-31G(d,p) level of theory. Figure 2 shows the electrostatic potentials mapped onto isodensity surfaces, which approximate the van der Waals surfaces of the molecules. As seen in Figure 2, the surface potentials of Cl9ATQ and Br3Cl6ATQ are similar, although the bromine atoms describe a substantially greater protrusion from the base of the Cl6 surface. The Cl6 face of each molecule shows a ring of moderate electron density along its rim. The lower Cl3 or Br3 faces of these molecules show a somewhat greater degree of electron density. The bottom of Figure 2 shows the electrostatic potential for Cl6ATQ. The picture here is rather different, with more negative potential on the top surface than in Cl9ATQ and Br3Cl6ATQ and a weakly basic nitrogen surrounded by strongly electron-deficient hydrogen atoms on the lower face. Preparation and Structure of Br3Cl6ATQ‚C60. Crystals suitable for X-ray diffraction were obtained by allowing a chlorobenzene solution of Br3Cl6ATQ to diffuse slowly into a saturated carbon disulfide solution of C60. Lustrous black prisms of solvent-free Br3Cl6ATQ‚C60 were obtained. The asymmetric unit consists of an ordered C60 cage and a Br3Cl6ATQ molecule. The two molecular components are arranged into columns. The environment of the Br3Cl6ATQ molecule within a column is shown in Figure 3. One fullerene makes contact with the Cl6 face of Br3Cl6ATQ, while another is positioned below the Br3 face. This arrangement is similar to that found in Cl9ATQ‚C60 earlier, and the two crystals are isostructural. As expected, the larger bromo derivative has larger unit cell lengths and a larger

10.1021/cg060438w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006

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Figure 2. Calculated B3LYP/6-31G(d,p) electrostatic potential surfaces for (top) Cl9ATQ, (middle) Br3Cl6ATQ, and (bottom) Cl6ATQ with surface energy ranging from -18 (red) to +18 (blue) kcal mol-1.

volume: 4040.9(9) Å3 for Br3Cl6ATQ‚C60 versus for 4026.3(4) Å3 for Cl9ATQ‚C60 (at 90(2) K). The shortest contact between a fullerene carbon atom (C29) and a chlorine atom (Cl3) of an adjacent molecule of Br3Cl6ATQ is 3.458(3) Å, which is comparable to the sum (3.45 Å) of the van der Waals radii of carbon and chlorine. As seen in Figure 4, a projection of the Cl6 plane of the Br3Cl6ATQ molecule onto the C60 cage shows that the C3 axis of the Br3Cl6ATQ molecule nearly passes through the center of an adjacent six-membered ring of the fullerene. The lower Br3 face of the Br3Cl6ATQ molecule also makes contact with the fullerene. Here, there are two Br‚‚‚C contacts {dBr2‚‚‚C35a ) 3.433(3) Å; dBr2‚‚‚C45a ) 3.464(4) Å} that are shorter than the sum of the van der Waals radii for carbon and bromine (3.55 Å). For comparison, the closest Br‚‚‚C contacts in the simple solvate C60‚2C6H5Br, fall in the range 3.64-3.92 Å.13 Figure 5 shows an orthographic projection of the Br3Cl6ATQ molecule onto a C60 molecule. The orthographic projection flattens the two molecules and allows their sizes, which are quite similar, to be compared. Figure 6 shows the relationships between two sets of adjacent columns. As this figure demonstrates, not only are there contacts between the upper and lower faces of the Br3Cl6ATQ molecule with C60, but also there are lateral interactions between these two molecules. The periphery of each Br3Cl6ATQ molecule is occupied by six other C60 molecules, which interact with the equatorial Cl2Br faces. These six C60 molecules describe a flattened octahedron about the Br3Cl6ATQ molecule. There are 11 Cl‚‚‚C distances around the equator of the Br3Cl6ATQ molecule that are shorter than the sum of the van der Waals

Figure 3. View of the stacking motif between C60 and the chlorinated and brominated surfaces of Br3Cl6ATQ in crystalline Br3Cl6ATQ‚ C60 showing 50% thermal contours for all atoms.

radii of these atoms. These close contacts range from 3.249(3) to 3.446(4) Å. Preparation and Structure of Br3Cl6ATQ‚C70. Diffusion of a colorless chlorobenzene solution of Br3Cl6ATQ into a brown, saturated carbon disulfide solution of C70 produced dark red prisms of the binary cocrystal Br3Cl6ATQ‚C70. In contrast to the situation with Cl9ATQ‚C60 and Br3Cl6ATQ‚C60, which are isostructural, Cl9ATQ‚C70 and Br3Cl6ATQ‚C70 crystallize in different space groups: Pbca for Cl9ATQ‚C70 and P212121

Ordered Crystals of Fullerenes

Crystal Growth & Design, Vol. 7, No. 1, 2007 77

Figure 4. A projection of the Cl6 surface of Br3Cl6ATQ onto the C60 cage in Br3Cl6ATQ‚C60.

Figure 5. An orthographic projection of Br3Cl6ATQ molecule onto a C60 molecule. The C60 is shown with a gray van der Waals surface, while the Br3Cl6ATQ molecule is shown with a green and red wireframe van der Waals surface. This depicts the similarity in molecular sizes between the two molecules. The orthographic projection flattens the two molecules to allow their sizes to be compared.

for Br3Cl6ATQ‚C70. The asymmetric unit of Br3Cl6ATQ‚C70 consists of an ordered C70 cage and Br3Cl6ATQ molecule. As with the Br3Cl6ATQ‚C60 structure, the molecules of Br3Cl6ATQ and of C70 alternate to form extended columns. The interactions of C70 molecules with the top Cl6 and bottom Br3 faces of a molecule of Br3Cl6ATQ are shown in Figure 7. The Cl6 face of Br3Cl6ATQ abuts the flatter equatorial portion of the neighboring C70 molecule, while the Br3 face of Br3Cl6ATQ interacts with a portion of the more highly curved pole of the adjacent C70 molecule. Figure 8 shows the intermolecular interactions that occur between two adjacent columns in crystalline Br3Cl6ATQ‚C70. Note that there are two positions for the Br3Cl6ATQ molecules

Figure 6. The intramolecular interactions between two adjacent columns in crystalline Br3Cl6ATQ‚C60.

in each column, whereas in Cl9ATQ‚C60, the Cl9ATQ molecules have only one orientation as seen in Figure 6. Figure 9 shows that each Br3Cl6ATQ molecule is surrounded laterally by six additional C70 molecules. These fullerenes interact with the Cl2Br and Cl2Br2 faces of the Br3Cl6ATQ molecule. In these equatorial regions, there are eight Cl‚‚‚C distances around the Br3Cl6ATQ molecule that are shorter than the sum (3.45 Å) of the van der Waals radii of carbon and chlorine. The shortest of these contacts is the Cl2‚‚‚C43 interaction at 3.221(4) Å, and the longest contact is the Cl6‚‚‚C16 interaction at 3.444(3) Å. Preparation and Structure of Cl6ATQ‚3C70‚3C6H5Cl. Using a similar procedure to that which produced solvent-free crystals of Cl9ATQ‚C60, Cl9ATQ‚C70, Br3Cl6ATQ‚C60, and Br3Cl6ATQ‚C70, diffusion of a chlorobenzene solution of

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Figure 7. View of the stacking motif between C70 and the chlorinated and brominated surfaces of Br3Cl6ATQ in crystalline Br3Cl6ATQ‚ C70 showing 50% thermal contours for all atoms. Figure 8. The intramolecular interactions between two adjacent columns in crystalline Br3Cl6ATQ‚C70.

Cl6ATQ into a chlorobenzene solution of C70 produced black blocks of the solvate Cl6ATQ‚3C70‚3C6H5Cl. Attempts to form an adduct between Cl6ATQ and C60 did not produce suitable crystals. The asymmetric unit of Cl6ATQ‚3C70‚3C6H5Cl consists of one molecule of Cl6ATQ, three separate C70 molecules, and three independent molecules of chlorobenzene. Each of these components resides in a general position. Figure 10 shows the interactions between the Cl6ATQ molecule and the C70 molecule closest to it. This C70 molecule is surrounded by four molecules of chlorobenzene in addition to the Cl6ATQ molecule. The ensemble of five encapsulating molecules forms a deep bowl about this particular fullerene. Figure 11 shows a view of the

interaction between the chlorinated surface of Cl6ATQ and the adjacent C70 molecule, along with the positions of the surrounding chlorobenzene molecules. As might be expected, the interaction of the chlorinated surface of Cl6ATQ with the fullerene is similar to that seen in Cl9ATQ‚C70 and Br3Cl6ATQ‚ C70. However, the closest axial contacts from the fullerene cage to the Cl6 surface (dCl4‚‚‚C19C ) 3.551(1) Å; dCl3‚‚‚C20C ) 3.596(1) Å) are longer than the sum of the van der Waals radii of carbon and chlorine (3.45 Å). The shortest C‚‚‚Cl contacts in this solid involve the solvate molecules. There are four C‚‚‚Cl contacts (dCl10‚‚‚C37B ) 3.384(2) Å; dCl10‚‚‚C2C ) 3.407(2) Å; dCl10‚‚‚C48C ) 3.343(2) Å; dCl10‚‚‚C58A ) 3.391(2) Å) with the

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Crystal Growth & Design, Vol. 7, No. 1, 2007 79

Figure 9. View of the interactions of Br3Cl6ATQ with the six C70 molecules that surround it laterally in crystalline Br3Cl6ATQ‚C70.

chlorobenzene molecules that are shorter that the van der Waals distance of 3.45 Å. In Cl6ATQ‚3C70‚3C6H5Cl, the location of the Cl6ATQ molecule relative to other fullerene molecules is also quite different from the situation in Cl9ATQ‚C60, Br3Cl6ATQ‚C60, Cl9ATQ‚C70, and Br3Cl6ATQ‚C70, where eight fullerenes fully surround each azatriquinacene molecule. In Cl6ATQ‚3C70‚ 3C6H5Cl, each molecule of Cl6ATQ is surrounded by only four molecules of C70, one at the upper Cl6 face and three abutting the lateral H2Cl2 surfaces as seen in Figure 12. This arrangement is consistent with the absence of a halogenated lower surface and the deficit of electron density at this end of the aza-

triquinacene molecule. As can be seen in Figure 12b, no fullerene occupies this position. The results reported here and in our earlier work12 demonstrate a consistent pattern of interactions of the upper Cl6 surfaces of Cl9ATQ, Br3Cl6ATQ, and Cl6ATQ with fullerenes. The complementary shape of the top of the “chalice” of these azatriquinacene molecules with that of the more strongly curved fullerene exterior certainly abets the cocrystal formation. However, the interaction of the chlorine atoms on this upper surface with the fullerenes also makes an additional contribution to cocrystal stabilization. Covalently bonded chlorine atoms have been shown to form weak n-donor, charge-transfer interactions

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Figure 10. A drawing of the interactions between the four molecules of chlorobenzene and the molecule of Cl6ATQ that surround one of the C70 molecules in Cl6ATQ‚3C70‚3C6H5Cl with 50% thermal contours for all atoms.

Figure 11. A view the interaction between the chlorinated surface of Cl6ATQ and the adjacent C70 molecule in Cl6ATQ‚3C70‚3C6H5Cl with 50% thermal contours for all atoms.

with electron-deficient acceptors like tetracyanoethylene14 and with fullerenes.15 This type of association is implicated in the general increase in solubility of C60 with degree of chlorination in aliphatic solvents.16 A number of fullerene solvates involving halogenated solvents have been crystallized.13,17 Related noncovalent C-F‚‚‚fullerene interactions have been found in cocrystals formed by fluorinated porphyrins and C60 or C70.18,19 The significance of C-X‚‚‚π interactions in crystal packing has been reviewed.20

In summary, as a result both of the complementarity between the curvature of the surfaces of these haloazatriquinacene molecules and fullerenes and of the presence of significant C-X‚‚‚π interactions, the fullerenes in the crystalline solids Cl9ATQ‚C60, Br3Cl6ATQ‚C60, Cl9ATQ‚C70, Br3Cl6ATQ‚C70, and Cl6ATQ‚3C70‚3C6H5Cl assume fixed orientations. We therefore propose here that cocrystallization of fullerenes with haloazatriquinacenes may constitute a general method for eliminating orientational disorder in the crystallography of

Ordered Crystals of Fullerenes

Crystal Growth & Design, Vol. 7, No. 1, 2007 81 Table 1. Crystal Data and Data Collection Parameters

formula formula weight color and habit crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 T, K Z dcalcd, g‚cm-3 radiation (λ, Å) µ, mm-1 range of transm. factors no. of unique data no. of restraints no. of params refined R1a wR2b a

these carbon cages. Additionally, cocrystallization of haloazatriquinacenes with other highly symmetric molecules such as adamantane that are readily disordered in crystals21 may also produce ordered solids. Experimental Section Preparation of Br3Cl6ATQ. A mixture of Cl6ATQ (10.0 mg, 29.60 µmol),11 freshly recrystallized N-bromosuccinimide (53.0 mg, 298 µmol), and catalytic dibenzoyl peroxide (5 mg) in carbon tetrachloride (15 mL) was heated at 65 °C for 24 h. The mixture was allowed to cool to room temperature, and water (15 mL) was added. The layers were separated, and the aqueous phase was extracted with dichloromethane (×3). The organic fractions were filtered through cotton wool, and the solvent was evaporated. The resulting solid was chromatographed on silica gel (hexanes) to give Br3Cl6ATQ (13 mg, 76%). 13C NMR (CDCl3) δ 132.3, 85.5.

Br3Cl6ATQ‚C70

Cl6ATQ‚ 3C70‚3C6H5Cl

C69Br3Cl6N 1295.13 black prism monoclinic P21/c 10.1657(13) 15.399(2) 26.177(3) 90 99.566(3) 90 4040.9(9) 90(2) 4 2.129 Mo KR (0.71073) 3.453 0.17-0.72

C79Br3Cl6N 1415.23 dark red prism orthorhombic P212121 13.574(3) 15.592(3) 21.473(4) 90 90 90 4544.8(14) 90(2) 4 2.068 Mo KR (0.71073) 3.080 0.31-0.51

C237H18Cl9N 3197.57 black block orthorhombic Pna21 21.8982(6) 27.3098(7) 20.1293(5) 90 90 90 12038.0(5) 90(2) 4 1.764 Mo KR (0.71073) 0.294 0.88-0.95

17261 0 191

13761 0 803

15346 1 2225

0.066 0.066

0.035 0.084

0.034 0.086

For data with I > 2σI, R1 ) (∑||Fo| - |Fc||)/∑|Fo|. b For all data,

wR2 )

Figure 12. Two views of the structure of Cl6ATQ‚3C70‚3C6H5Cl showing the environment about the Cl6ATQ molecule.

Br3Cl6ATQ‚C60

x(∑[w(Fo2-Fc2)2]/(∑[w(Fo2)2]).

Preparation of Crystals. Crystals of Br3Cl6ATQ‚C60 were obtained by layering a solution of 2 mg of Br3Cl6ATQ in 1.0 mL of chlorobenzene over a solution of 2 mg of C60 in 1.0 mL of carbon disulfide in a 7 mm o.d. glass tube at room temperature and allowing the two solutions to diffuse together in the septum capped tube. Crystals of Br3Cl6ATQ‚C70 were similarly obtained using a carbon disulfide solution of C70. Black prisms of Cl6ATQ‚3C70‚3C6H5Cl were obtained from the slow diffusion of a solution of of 2 mg of Cl6ATQ in 1.0 mL of chlorobenzene into a solution of of 2 mg of C70 in 1.0 mL of chlorobenzene at 23 °C. No evidence was found for the existence of more than one phase in each of these crystallizations. X-ray Data Collection. The crystals were removed from the glass tube together with a small amount of mother liquor and immediately coated with a hydrocarbon oil on a microscope slide. Suitable crystals of Br3Cl6ATQ‚C60 and of Br3Cl6ATQ‚C70 were mounted on glass fibers with silicone grease and placed in the cold stream of a Bruker SMART 1000 CCD with graphite monochromated Mo KR radiation at 90(2) K. A full sphere of data was collected. No decay was observed in 50 duplicate frames at the end of the data collection. Data for Cl6ATQ‚3C70‚3C6H5Cl were obtained on a Bruker SMART APEX II. Crystal data are reported in Table 1. A semiempirical absorption correction utilizing equivalents was employed.22 Solution and Structure Refinements. Calculations for the structures were performed using SHELXS-97 and SHELXL-97. Tables of neutral atom scattering factors, f′ and f′′, and absorption coefficients are from a standard source.23 The structures were all solved via direct methods. All atoms except hydrogen atoms were refined anisotropically. All hydrogen atoms were located in difference Fourier maps and included through the use of a riding model. Br3Cl6ATQ‚C60 is a three component twinned crystal {59.1%:33.2%: 7.7%}. The data were integrated using the three domains determined by CELL_NOW and refined using the SHELXL-97 software suite. Interestingly, good solutions were obtained in two monoclinic space groups, P21/c and P21/m. The P21/m solutions position the C60 cage across the crystallographic mirror plane. However, since the crystallographic mirror plane does not coincide with the molecule’s mirror plane, the fullerene cage is disordered with two orientations with equal occupancy {50%:50%}. The P21/c solution does not contain the disordered fullerene cage as observed in the P21/m case. Since the solution of the Cl9ATQ‚C60 analog had nearly identical unit cell parameters and was refined in P21/c, these structures were considered isostructural and P21/c was chosen as the preferred space group for this crystal. All carbon atoms in this structure were refined isotropically, because some atoms became nonpositive definite with anisotropic refinement of the thermal parameters.

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Crystalline Br3Cl6ATQ‚C70 contained two merohedrally twinned components with the major component being 57.88%. The data sets were not integrated with twinned domains. The TWIN command was included in the least-square refinements of the structures. Crystals of Cl6ATQ‚3C70‚3C6H5Cl showed no sign of twinning and refinement of the data proceeded normally.

Acknowledgment. We thank the National Science Foundation (Grants CHE 0070291 and CHE 0448976) for support. The Bruker SMART 1000 diffractometer was funded in part by NSF Instrumentation Grant CHE-9808259.

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Supporting Information Available: X-ray crystallographic files in CIF format for Br3Cl6ATQ‚C60, Br3Cl6ATQ‚C70, and Cl6ATQ‚ 3C70‚3C6H5Cl. This material is available free of charge via the Internet at http://pubs.asc.org.

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