Waves of Halogen–Halogen Bond Formation in the Cocrystallization

Article pubs.acs.org/crystal

Waves of Halogen−Halogen Bond Formation in the Cocrystallization of Hexabromobenzene and 1,2,4,5-Tetrabromobenzene with C70 Kamran B. Ghiassi, Joseph Wescott, Susanne Y. Chen, Alan L. Balch, and Marilyn M. Olmstead* Department of Chemistry, University of California, Davis, 95616, United States S Supporting Information *

ABSTRACT: The single crystal X-ray structures of three well-ordered cocrystals of C70 with brominated benzenes are examined. The crystals are those of C70· C6Br6·C7H8, C70·C6Br6·C6H6, and C70·2(1,2,4,5-tetrabromobenzene)·CS2. While all three structures exhibit extensive van der Waals interactions, the presence of a wavelike C−Br---Br structural motif is a distinctive part of the cocrystal assembly. In these three crystal structures, it is observed that the Br---Br interactions are a cross between type I and II halogen−halogen bonds. In our hands, this structural motif does not occur in structures with C60.

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INTRODUCTION Crystallographically, fullerene structures are notoriously difficult to solve due to disorder, twinning, phase changes, and weak diffraction. A way to circumvent some of these challenges is to cocrystallize fullerenes with other molecules, utilizing intermolecular interactions to help prevent disorder. In the case of C70, these cocrystallization agents have ranged from simple diatomic molecules (e.g., I2)1 or solvent molecules (e.g., toluene and carbon disulfide)2,3 to more shape-complementary curved molecules such as bis(ethylenedithio)tetrathiafulvalene4 or bowl-like corannulenes5 and other large aromatic species.6−8 Overall, few crystal structures of nonderivatized C70 have been well-determined. In many cases, however, crystals with halogenated molecules serving as cocrystallizing agents yield ordered structures.9−11 Typically, cocrystallization agents take advantage of more than one type of intermolecular interaction to accommodate a fullerene. In addition to π---π, C−H---π, and C−X---π interactions with the fullerene, metalloporphyrins may give rise to a donor−acceptor interaction, enhancing the attraction electrostatically.12 A unique type of intermolecular contact with fullerenes that has not been explored is created by the structural prevalence of halogen−halogen bonding. Two major categories of halogen−halogen interactions, denoted type I and type II, have been identified.13−18 Generally, the type of interaction is categorized by comparing the R1−X1---X2−R2 angles of approach. In other words, θ1 is R1−X1---X2 and θ2 is X1--X2−R2; type I interactions have similar θ1 and θ2 angles, whereas type II interactions have θ1 close to 180° and θ2 near 90°. The types of interactions are depicted in Figure 1 for clarity. Although there remains some debate regarding the causes of type I and type II interactions, it is agreed that halogen−halogen bonding is induced by the polarizability of the halogen. Type I interactions are more van der Waals in © 2015 American Chemical Society

Figure 1. A depiction of types I and II halogen−halogen interactions.

nature, whereas type II interactions have an additional electrophilic/nucleophilic electrostatic contribution and are shorter than type I. In this work, we explore the use of brominated benzene derivatives to afford ordered structures of C70. In the three crystal structures we present, we observed that waves of Br---Br interactions are formed between the brominated benzene molecules. In our hands, structures containing C60 with brominated benzene molecules yield ordered structures, but do not show this wavelike bonding motif. The C60 cocrystal structures are reported in Supporting Information.

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RESULTS Structure of C70·C6Br6·C7H8. The structure is triclinic, space group P1,̅ Z = 2. The asymmetric unit is composed of one C70 molecule, one toluene molecule, and two independent half C6Br6 molecules. The C70 and toluene reside on general positions, while the two C6Br6 molecules reside on crystallographic centers of inversion. All components are ordered. Figure 2 shows the molecular components of the structure and key halogen---halogen contacts. On the basis of a van der Waals radius of 1.95 Å for Br,19 two important Br---Br contacts occur in the structure. The two unique Br---Br contacts are 3.6869(4) Received: February 20, 2015 Revised: March 19, 2015 Published: March 25, 2015 2480

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Figure 2. Molecular components of C70·C6Br6·C7H8 drawn with 50% thermal contours looking down the 5-fold rotational axis of C70. Key Br---Br contacts are shown. Symmetry code: A = −x, −y, −z; B = −x, −y, 1 − z.

Figure 4. A diagram of a layer of the structure of C70·C6Br6·C7H8 as viewed down the a-axis.

and 3.7973(4) Å in length between the two different C6Br6 molecules. As a consequence of crystallographic inversion and translation, a wavy pattern of C6Br6 molecules is produced that snakes between C70 cages, as depicted in Figures 3 and 4. In

molecule and cage show a similar distance of 3.129(3) Å. Because there is a symmetry-related toluene molecule occurring at the caps of the C70 cages, there are no short, cage---cage contacts between layers. The shortest contact is 3.381(3) Å. It occurs between inversion-related cages as can be envisioned in the packing diagram, Figure 4. Structure of C70·C6Br6·C6H6. The structure crystallizes in the triclinic setting, space group P1̅, Z = 4. It is related by a doubling of the c-axis to the structure of C70·C6Br6·C7H8, described above, with Z = 2. Thus, the asymmetric unit contains two ordered C70 molecules, one ordered and one disordered benzene molecule, and one ordered hexabromobenzene molecule. In addition, there are two ordered halfmolecules of C6Br6 on crystallographic centers of inversion. The disordered benzene molecule displays a 30° rotation over two orientations with nearly equal occupancies. Figure 5

Figure 3. A display of the shortest π---π interactions (Å) together with Br---Br interactions in the structure of C70·C6Br6·C7H8. Contacts represent distances from ring centroids. The primed atoms are obtained by generation through the center of inversion. Standard uncertainties in the distances are 0.0004 Å for Br---Br and 0.003 Å for π---π.

addition to halogen---halogen contacts, there are short Br--C(cage) contacts ranging from 3.471(3)−3.646(2) Å that are halogen---π in nature. The faces of the C6Br6 and toluene molecules are situated over the waist region of the C70 cage. The close C---C(cage) contacts between the centroids of the two C6Br6 molecules and the cage range from 3.139(3) to 3.241(3) Å. The C---C(cage) contacts between the toluene

Figure 5. Components of the asymmetric unit in the structure of C70· C6Br6·C6H6 drawn with 50% thermal contours. 2481

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shown in Figure 8, which illustrates key intermolecular interactions.

illustrates the molecular components in the asymmetric unit of the structure. The two unique Br---Br contacts link the three independent hexabromobenzene molecules (Figure 6). As

Figure 8. Molecular components of C70·2(1,2,4,5-C6Br4H2)·CS2 drawn with 50% thermal contours featuring important intermolecular interactions.

Figure 6. Significant π---π, and Br---Br interactions (Å) in the structure of C70·C6Br6·C6H6. The standard uncertainties of the Br---Br and π---π distances are 0.0007 and 0.006 Å, respectively.

Figure 9 depicts some of the shortest intermolecular contacts. The 1,2,4,5-tetrabromobenzene molecules are situated

before, waves are formed by crystallographic inversion and translation, as shown in Figure 7. Rather short cage---cage

Figure 9. Significant π---π, cage---cage, and Br---Br interactions (Å) in the structure of C70·2(1,2,4,5-C6Br4H2)·CS2. Standard uncertainties are 0.002 Å for Br---Br interactions and 0.014 for centroid---C(cage).

over 6:6 ring junctions in the waist region of the C70 molecules with the close centroid---C contacts. In addition, there is a short cage---cage contact in the cap region of the C70 cage. Figure 10 shows a layer of the structure and indicates an alternation of the positions occupied by the carbon disulfide and C70. If viewed along the c direction, in a third dimension, two more C70 cages can be seen surrounding the CS2. The position of the CS2 is not symmetrical with respect to the four C70 cages that surround it. It is in an octahedral hole comprised of four C70 molecules parallel to the CS2 line and two 1,2,4,5tetrabromobenzenes that are perpendicular to this line. The unsymmetrical positions of C70 with respect to carbon disulfide are immediately obvious from the C70(centroid)---C (CS2) distances of 7.12(2) Å, 7.68(2) Å, 7.58(2) Å, 7.70(2) Å, listed consecutively around the square plane. Furthermore, the shortest contacts from the C of CS2 are to different regions of the C70 cage. Consecutively around the square of four surrounding C70 cages these are to the centroid of a 5:6 junction near the C70 waist (3.595(14) Å); to a 5:6 junction at

Figure 7. A layer of C70·C6Br6·C6H6 as viewed down the a-axis. The independent C70 molecules in the asymmetric unit are colored red and blue for clarity.

contacts occur at the points of highest pyramidalization of the C70 cages. The shortest of these contacts (3.143(6) Å) occurs between the two independent molecules (red and blue) of C70 and is shown as a single line in Figure 7. A slightly longer distance of 3.175(6) Å occurs via inversion between the C1− C70 cages (blue). Structure of C70·2(1,2,4,5-C6Br4H2)·CS2. The structure is found in the monoclinic space group P21/m with Z = 2. The asymmetric unit consists of half of a C70 cage, two separate halves of 1,2,4,5-tetrabromobenzene, and one-half of a carbon disulfide molecule. Both the C70 and CS2 molecules reside on a crystallographic mirror plane, while the two half 1,2,4,5C6Br4H2 molecules reside on centers of inversion. The structure is fully ordered. The molecular components are 2482

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Hexabromobenzene, 1,2,4,5-tetrabromobenzene, and 1,3,5tribromobenzene were tried; however, no cocrystals were formed with C 70 . In the case of C 70 and 1,2,4,5tetrabromobenzene, a crystal was successfully grown; however, the structure was determined to be incommensurate via singlecrystal X-ray diffraction. We were unable to find a suitable structure solution for this system. In the case of 1,3,5tribromobenzene, we were unable to obtain any satisfactory crystal structure grown by either method. In addition, conductivity measurements were performed on the C70 cocrystals. Much to our chagrin, the cocrystals were nonconducting. For the three C70 cocrystal structures, it seems necessary for solvent molecules to be incorporated into the lattice in order to obtain suitable crystals. By observation of the space-filling models in Figures 11−13, it is clear that the solvent molecules Figure 10. A layer of the structure of C70·2(1,2,4,5-C6Br4H2)·CS2 as viewed down the a-axis.

the C70 cap (3.62(2) Å); to the centroid of a 6:6 junction at the C70 waist (4.254(14) Å); to a 5:6:6 junction near the C70 cap (3.62(2) Å). The two shortest contacts between sulfur and C70 cages occur with the cis-related first and second C70’s, i.e., S1··· C17 = 3.646(8) Å and S1···C12 (symmetry code: 1 + x, y, z) = 3.688(11) Å.

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DISCUSSION All three structures show slipped π−π stacking between the rings of brominated benzene or solvent and the fullerene and demonstrate waves or networks of Br---Br contacts. In our hands, this pattern is only present in the structures containing C70 and not with those of C60. The structures of C60·C6Br6, C60· 0.5C6Br6·C6H6, C60·1,2,4,5-C6Br4H2, and C60·1,2,4,5-C6Br4H2· 2C6H6 are reported in Supporting Information, along with their preparations, and do not feature any Br---Br interactions. This is most likely due to a combination of factors including shapematching with the elongated shape of the C70 cage and the presence of delocalized π-electron density in the C70 waist. This observation is related to the question as to whether there are type I or type II Br---Br interactions in these structures. Referring to the Br---Br distances and C−Br---Br angles collected in Table 1, there is no obvious category, and it is probably the case that the angles are dictated by the packing and the curvature of the fullerene cages and a range of competitive dispersion interactions, rather than any particular type of Br---Br bonding. In addition to cocrystallization via slow diffusion, attempts were made to grow cocrystals by dissolving the fullerene component in the melted brominated benzene.

Figure 11. A space-filling model of C70·C6Br6·C7H8 looking down the 5-fold rotational axis of C70 showing the C6Br6 molecular waves.

Figure 12. A space-filling model of C70·C6Br6·C6H6 showing the wavelike nature of the brominated benzene molecules.

Table 1. Tabulated Br---Br Interactions for C70 Cocrystals C70·C6Br6· C7H8

C70·C6Br6· C6H6

nestle in between voids in each structure. In particular, the CS2 molecule in C70·2(1,2,4,5-C6Br4H2)·CS2 appears to be locked in place by four surrounding 1,2,4,5-tetrabromobenzene molecules. The same “locking” is true for the toluene molecule in C70·C6Br6·C7H8. In regard to the structure of C70·C6Br6· C6H6, one of the benzene molecules is disordered. This may be attributed to the lack of C−H---π interactions and thus a less efficient filling of void space. In the case of C60 cocrystals, we present two structures (see Supporting Information) that do not incorporate solvent molecules: C60·C6Br6 and C60·1,2,4,5C6Br4H2. The former was grown from solution, while the latter

C70·2(1,2,4,5C6Br4H2)·CS2

Br---Br (Å) 3.7973 (4) 3.6869 (4) C−Br---Br (deg) θ1 θ2 θ1 θ2

141.68 113.54 128.44 122.76

(6) (7) (7) (7)

3.7129 (7) 3.6484 (7)

145.77 134.38 144.55 135.39

(11) (10) (11) (10)

3.765 (2) 3.677 (2)

143.4 100.1 149.6 100.5

(3) (3) (3) (3) 2483

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carbon disulfide solvate molecules, are shown to have distinctive crystal packing motifs. In each case, the presence of Br---Br bonding, as well as π---π stacking leads to sinusoidal host-like shapes of the brominated benzene in juxtaposition to the guest C70 cages. Although it is clear that the Br---Br bonding plays a significant structural role, the type I and type II designations do not seem to apply to this particular situation. The π---π interactions primarily occur at the waist of the C70 molecules, whereas some short fullerene cage---cage interactions appear at the more highly pyramidalized regions of C70 near the fullerene caps. Notably, such a wavelike structural motif does not occur in several related cocrystals of brominated benzene molecules with the C60 fullerene molecule.

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Figure 13. A space-filling model of C70·2(1,2,4,5-C6Br4H2)·CS2 looking down the 5-fold rotational axis of C70 showing the packed nature of the system.

EXPERIMENTAL SECTION

Materials. C70 was purchased from SES research with 99% purity. Hexabromobenzene was purchased from TCI America with 99% purity. 1,2,4,5-Tetrabromobenzene was purchased from Sigma-Aldrich with 97% purity. No further purification was performed. Solvents were used as received. Crystal Growth. Crystals were obtained via slow diffusion of fullerene solutions into brominated benzene solutions in various solvents. All solutions were filtered before diffusion. In the preparations with C6Br6, the crystals grew within days and often exceeded 1 mm3 in size. Initial sonication of the solutions was crucial for the rapid growth of some crystals. If sonication was not utilized, the crystals grew within months, presumably because the concentrations of the solutions were lower. C70·C6Br6·C7H8. Eighteen milliliters of toluene was added to 29 mg (0.03 mmol) of C70. Eighteen mL of toluene was added to 39 mg (0.07 mmol) of C6Br6. Both suspensions were sonicated for 1 h. A 1 mL aliquot of the C70 solution was filtered and layered over a 1 mL filtered aliquot of the C6Br6 solution in a glass tube. The tubes were kept in the dark at room temperature. Well-formed crystals appeared within days.

was produced from a melt. In our hands, we could not produce a solvent-free C70 cocrystal via the slow diffusion or melt methods. As stated previously, a crystal was obtained with C70 and 1,2,4,5-tetrabromobenzene, but the structure was incommensurate. This suggests the critical nature of solvent molecules in the crystal packing for the C70 cocrystals. Notice also that the difference between the structures of C70·C6Br6· C7H8 and C70·C6Br6·C6H6 is simply due to a change in the particular solvate molecule present. Moreover, this structural divergence is caused by the simple difference between a hydrogen atom and a methyl group on the solvate molecule present.

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CONCLUSIONS Three cocrystals of the empty cage fullerene C70 with brominated benzenes, together with toluene, benzene, or Table 2. Crystal Data for C70 Cocrystals

chemical formula formula weight radiation source, λ (Å) crystal system space group T (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z dcalc (g cm−3) μ (mm−1) F(000) crystal size reflections collected data/parameters/restraints R(int) R1 [I > 2σ(I)] wR2 (all data) largest difference peak and hole (e Å−3)

C70·C6Br6·C7H8

C70·C6Br6·C6H6

C70·2(1,2,4,5-C6Br4H2)·CS2

C83H8Br6 1484.35 synchrotron, 0.7749 triclinic P1̅ 100(2) 10.8946(4) 13.5002(5) 15.8551(6) 91.327(2) 90.019(2) 90.977(2) 2335.40(15) 2 2.111 6.434 1432 0.05 × 0.05 × 0.04 93463 17787/804/0 0.0359 0.0359 [14229] 0.1050 0.897 and −0.886

C82H6Br6 1470.27 synchrotron, 0.7749 triclinic P1̅ 100(2) 10.1941(11) 13.0528(13) 34.839(4) 92.7676(15) 91.2288(15) 93.6104(15) 4619.8(8) 4 2.114 6.503 2832 0.05 × 0.05 × 0.05 68817 27807/1615/0 0.0687 0.0591 [20580] 0.1779 1.661 and −0.995

C83H4Br8S2 1704.26 sealed tube, 0.71073 monoclinic P21/m 90(2) 10.620(7) 22.401(14) 10.652(7) 90 92.620(9) 90 2531(3) 2 2.236 6.480 1628 0.25 × 0.21 × 0.06 23128 5100/428/324 0.0777 0.0524 [4159] 0.1367 1.183 and −1.334

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C70·C6Br6·C6H6. Twelve milliliters of benzene was added to 84 mg (0.15 mmol) of C6Br6. Twelve milliliters of benzene was added to 24 mg (0.03 mmol) of C70. The suspensions were sonicated for 1 h. A 1 mL aliquot of the C70 solution was filtered and layered over a 1 mL filtered aliquot of the C6Br6 solution in a glass tube. The tubes were kept in the dark at room temperature. Well-formed crystals appeared within days. C70·2(1,2,4,5-C6Br4H2)·CS2. Ten milliliters of carbon disulfide was added to 89 mg (0.23 mmol) of 1,2,4,5-tetrabromobenzene. Ten milliliters of carbon disulfide was added to 36 mg (0.04 mmol) of C70. The suspensions were sonicated for 1 h. A 1 mL aliquot of the 1,2,4,5tetrabromobenzene solution was filtered and layered over a 1 mL filtered aliquot of the C70 solution in a glass tube. The tubes were kept in the dark at room temperature. Well-formed crystals appeared within one month. Crystal Structure Determinations. The black crystals of C70· C6Br6·C7H8 and C70·C6Br6·C6H6 were mounted in the nitrogen cold stream provided by an Oxford Cryostream low-temperature apparatus on the goniometer head of a Bruker D8 diffractometer equipped with an ApexII CCD detector at the Advanced Light Source, Berkeley, CA, beamline 11.3.1. Data were collected with the use of silicon(111) monochromated synchrotron radiation (λ = 0.7749 Å). Data for the black crystal of C70·2(1,2,4,5-C6Br4H2)·CS2 were collected on a Bruker D8 ApexII diffractometer utilizing Mo Kα (λ = 0.71073 Å) and a Cryo Industries low-temperature apparatus. All data sets were reduced utilizing Bruker SAINT,20 and a multiscan absorption correction was applied using SADABS.21 Crystal data are given in Table 2. The structures were solved by direct methods (SHELXS-2008) and refined by full-matrix least-squares on F2 (SHELXL-2014).21 The structure C70·2(1,2,4,5-C6Br4H2)·CS2 was refined as a twin (0 0 1̅ 0 1̅ 0 1̅ 0 0) with twin parameter 0.422(2). The structure C70·C6Br6·C7H8 was refined as a twin (1̅ 0 0 0 1̅ −0.039 0 0 1) with HKLF 5 and twin parameter 0.098(8). Physical Properties. Electrical resistivity measurements were taken on the C70 cocrystals using a physical properties measurement system (Quantum Design PPMS). A single-crystal sample was studied by the standard four-probe AC method from 2 to 300 K, but the resistivity was too high to measure throughout the temperature range.

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REFERENCES

(1) Ghiassi, K. B.; Bowles, F. L.; Chen, S. Y.; Olmstead, M. M.; Balch, A. L. Cryst. Growth Des. 2014, 14, 5131. (2) Bowles, F. L.; Mercado, B. Q.; Ghiassi, K. B.; Chen, S. Y.; Olmstead, M. M.; Yang, H.; Liu, Z. Y.; Balch, A. L. Cryst. Growth Des. 2013, 13, 4591. (3) Troshin, P. A.; Prisyazhnuk, V. V.; Troyanov, S. I.; Boltalina, O. V.; Mackeyev, Y. A.; Kyrikova, M. A. Proc. Electrochem. Soc. 2001, 11, 10. (4) Ghiassi, K. B.; Olmstead, M. M.; Balch, A. L. Chem. Commun. 2013, 49, 10721. (5) Filatov, A. S.; Ferguson, M. V.; Spisak, S. N.; Li, B.; Campana, C. F.; Petrukhina, M. A. Cryst. Growth Des. 2014, 14, 756. (6) Ghiassi, K. B.; Chen, S. Y.; Prinz, P.; de Meijere, A.; Olmstead, M. M.; Balch, A. L. Cryst. Growth Des. 2014, 14, 4005. (7) King, B. T.; Olmstead, M. M.; Baldridge, K. K.; Kumar, B.; Balch, A. L.; Gharamaleki, J. A. Chem. Commun. 2012, 48, 9882. (8) Sygula, A.; Fronczek, F. R.; Sygula, R.; Rabideau, P. W.; Olmstead, M. M. J. Am. Chem. Soc. 2007, 129, 3842. (9) Pham, D.; Bertran, J. C.; Olmstead, M. M.; Mascal, M.; Balch, A. L. Org. Lett. 2005, 7, 2805. (10) Pham, D.; Ceron-Bertran, J.; Olmstead, M. M.; Mascal, M.; Balch, A. L. Cryst. Growth Des. 2007, 7, 75. (11) Olmstead, M. M.; Nurco, D. J. Cryst. Growth Des. 2006, 6, 109. (12) Olmstead, M. M.; Costa, D. A.; Maitra, K.; Noll, B. C.; Phillips, S. L.; Van Calcar, P. M.; Balch, A. L. J. Am. Chem. Soc. 1999, 121, 7090. (13) Bui, T. T. T.; Dahaoui, S.; Lecomte, C.; Desiraju, G. R.; Espinosa, E. Angew. Chem., Int. Ed. 2009, 48, 3838. (14) Desiraju, G. R.; Ho, P. S.; Kloo, L.; Legon, A. C.; Marquardt, R.; Metrangolo, P.; Politzer, P.; Resnati, G.; Rissanen, K. Pure Appl. Chem. 2013, 85, 1711. (15) Desiraju, G. R.; Parthasarathy, R. J. Am. Chem. Soc. 1989, 111, 8725. (16) Mukherjee, A.; Tothadi, S.; Desiraju, G. R. Acc. Chem. Res. 2014, 47, 2514. (17) Pedireddi, V. R.; Reddy, D. S.; Goud, B. S.; Craig, D. C.; Rae, A. D.; Desiraju, G. R. J. Chem. Soc.-Perk. Trans. 2 1994, 2353. (18) Thalladi, V. R.; Weiss, H. C.; Boese, R.; Nangia, A.; Desiraju, G. R. Acta Crystallogr. Sect. B: Struct. Sci. 1999, 55, 1005. (19) (a) Pauling, L. The Nature of the Chemical Bond and the Structure of Molecules and Crystals; Cornell University Press: Ithaca, NY, 1960; p 260. (b) Housecroft, C. E.; Sharpe, A. G. Inorganic Chemistry; Pearson: New York, 2012; p 1127. (20) SAINT; Bruker AXS, Inc.: Madison, WI, 2014. (21) Sheldrick, G. M. University of Göttingen, Göttingen, Germany, 2014.

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format. Tables of crystal data, crystal growth and figures for cocrystals of the C60 fullerene with brominated benzenes: C60·C6Br6, C60·0.5C6Br6·C6H6, C60· 1,2,4,5-C6Br4H2, C60·1,2,4,5-C6Br4H2·2C6H6. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: (530) 7526668. Fax: (530) 752-8995. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (Grant CHE1305125 to A.L.B. and M.M.O.) and the Advanced Light Source, Lawrence Berkeley National Laboratory, for beamtime and a fellowship to K.B.G. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. We thank J. T. Greenfield and K. Kovnir for performing the physical property measurements. 2485

DOI: 10.1021/acs.cgd.5b00256 Cryst. Growth Des. 2015, 15, 2480−2485