Cocrystallization of C60 or C70 with the Bowl-Shaped Hydrocarbon

Article pubs.acs.org/crystal

Cocrystallization of C60 or C70 with the Bowl-Shaped Hydrocarbon Hexakis[(E)‑3,3-dimethyl-1-butenyl]benzene To Form Chains of Clamshell Assemblies Kamran B. Ghiassi,† Susanne Y. Chen,† Peter Prinz,‡ Armin de Meijere,*,‡ Marilyn M. Olmstead,*,† and Alan L. Balch*,† †

Department of Chemistry, University of California, Davis, California 95616, United States Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany

‡

S Supporting Information *

ABSTRACT: The bowl-shaped hydrocarbon hexakis[(E)-3,3-dimethyl-1-butenyl]benzene (HB) has been cocrystallized with either C60 or C70 to form clamshell-like assemblies with the fullerenes residing within the concave surfaces of two HB molecules. The structures of these two crystals have been determined by singlecrystal X-ray diffraction. The HB molecules exhibit back-to-back stacking and close contacts in both cocrystals. This trait is remarkably similar to the packing seen in cases in which fullerenes cocrystallize with MII(OEP), where OEP is the dianion of octaethylporphyrin and M is generally Ni or Co. The C60 structure features an ordered cage with a disordered solvent position, while the C70 structure exhibits two orientations of the cage, all other components being ordered.

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INTRODUCTION The smooth, nearly isotropic outer surface of Buckminsterfullerene, C60, makes it an ideal component in the creation of a molecular form of a ball-and-socket joint. There are a number of molecules that have the concave inner surfaces to act as molecular sockets.1 Suitable concave molecules include calix[n]arenes,2 corannulene,3 cyclotriveratrylene,4 cyclic paraphenyleneacetylenes,5 and tetrabenzoquadrannulene.6 Many of these molecules possess aromatic rings that can take advantage of π−π interactions to stabilize the ball-and-socket arrangement. The initial crystallographic evidence of the association of arene rings with fullerene cages through π−π contact came from studies of organometallic adducts such as C70{Ir(CO)Cl(PPhMe2)2}2 and C60Ir(CO)Cl(PPh2CH2C6H4OCH2C6H5)2.7,8 In the latter case, the -CH2C6H4OCH2C6H5 arms of the phosphine ligands formed a bowl-like cavity that surrounded the neighboring fullerene in the crystalline state. The π−π interactions that stabilize such ball-and-socket arrangements also can accommodate movement of the ball relative to the socket. In the crystalline state, this phenomenon frequently manifests itself as disorder in the position of the fullerene. For example, the fullerene portion in the cocrystal C60·corannulene is disordered with four sites that were refined to fractional occupancies of 0.431(6), 0.254(7), 0.169(7), and 0.148(6).9 A double concave buckycatcher, which incorporates two corannulene units to form a concave cavity, forms a cocrystal with C60, and again the fullerene is disordered over two positions.10 Conical cyclotriveratrylene © 2014 American Chemical Society

forms cocrystals with C60, but the fullerene portion is badly disordered.4,11 Hexakis[(E)-3,3-dimethyl-1-butenyl]benzene (HB), the structure of which is shown in Figure 1, is a novel hydrocarbon with a curved side depicted in the space filling drawing in Figure 1 and a back side, which is nearly flat.12−14 The bowlshaped face seems appropriate for hosting a fullerene. While this face involves an arene ring at its base, much of the curvature is created by the disposition of the six substituents. All of these substituents reside on a common side of the central arene plane. It can be proposed that these groups will interact with a fullerene in a similar fashion. HB can be prepared from hexabromobenzene through a 6-fold Suzuki coupling reaction. HB is different from many other curved molecules used to interact with fullerenes because of the presence of the six arms with numerous C−H groups positioned to interact with the fullerene. In that regard, HB somewhat resembles another cocrystallization agent, MII(OEP) (OEP is the dianion of octaethylporphyrin, and M usually is Ni or Co), the structure of which is shown in Figure 1. MII(OEP) is not intrinsically curved, but when all eight ethyl groups are directed to one side of the porphyrin plane, the molecule assumes a shape that is complementary to that of a fullerene. Although the high degree of symmetry of many fullerene cages makes crystallographic Received: April 28, 2014 Revised: June 21, 2014 Published: July 7, 2014 4005

dx.doi.org/10.1021/cg500603c | Cryst. Growth Des. 2014, 14, 4005−4010

Crystal Growth & Design

Article

Table 1. Crystal and Experimental Data for Fullerene·HB 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 no. of reflections collected data/parameters/ restraints R(int) R1a [I > 2σ(I)] wR2b (all data) largest difference peak and hole (e Å−3)

Figure 1. Chemical structure of hexakis[(E)-3,3-dimethyl-1-butenyl]benzene (HB), a space-filling model of HB (drawn from the crystallographic data of ref 12), and chemical structures of MII(OEP) and MII(Etio-I).

disorder a prevalent issue in the determination of their structures, cocrystallization with MII(OEP) has proven to be an effective means of obtaining crystals of fullerenes and endohedral fullerenes with sufficient order to allow their structural identification.15,16 Here, we report the formation of two new cocrystals involving HB and either C60 or C70 and compare their structures to those of related cocrystals involving MII(OEP).

C60·2HB·0.94C6H6·0.06CS2

C70·2HB·C6H6

C149.70H137.64S0.12 1940.46 synchrotron, 0.77490 monoclinic P21/n 100(2) 14.6166(6) 32.4939(14) 22.8644(9) 90 94.194(2) 90 10830.4(8) 4 1.190 0.082 4151 0.13 × 0.09 × 0.09 199599

C160H138 2060.70 synchrotron, 0.77490 triclinic P1̅ 100(2) 13.3072(5) 20.3766(8) 21.6407(8) 73.832(2) 83.605(2) 83.898(2) 5583.4(4) 2 1.226 0.081 2196 0.13 × 0.09 × 0.09 103646

36132/13/1402

36757/1591/2108

0.0806 0.0633 0.1830 0.897 and −0.759

0.0589 0.0672 0.2227 1.137 and −0.307

For data with I > 2σ(I), R1 = (∑||FO| − |FC||)/(∑|FO|). bFor all data, wR2 = {∑[w(FO2 − FC2)2]}/{∑[w(FO2)2]}1/2.

a

There are close contacts between the faces of C60 and the adjacent HB molecules. However, there are no close contacts with neighboring fullerene molecules. The shortest C60 centroidto-centroid distance is 13.111 Å. Because the asymmetric unit consists of two independent molecules of HB, there are two sets of HB−fullerene interactions, and the fullerene is somewhat asymmetrically positioned between the two HB molecules. However, in both cases, a benzene ring of HB is positioned near a pentagon on the surface of the C60 molecule. The ranges of distances for these close contacts between carbon atoms for the two HB−fullerene arrangements are 3.266−3.387 and 3.295− 3.354 Å. Each HB molecule has idealized, noncrystallographic C6 symmetry. Thus, individual HB molecules are chiral. The two molecules that embrace one fullerene in C60·2HB· 0.94C6H6·0.06CS2 are enantiomers of each other as shown in Figure 4. Figure 4 shows that the packing within C60·2HB·0.94C6H6· 0.06CS2 involves chains of C60 and HB molecules, which stack about centers of symmetry in a back-to-back fashion. These pairs of HB molecules alternate with C60 molecules to form the chain. The distances between the planes of the benzene rings are 3.66 and 3.64 Å for the two different pairs, but the overlap of these rings is limited to contact between a pair of carbon atoms in adjacent molecules at distances of 3.691 and 3.726 Å. For comparison, the distance between the planes of the benzene rings in pristine HB is longer, 3.851 Å.12 The arrangement in these chains is similar to the supramolecular organization found in C60·2CoII(OEP)·CHCl3, as shown in Figure 4.15 In this crystalline solid, two porphyrin molecules surround each fullerene in a clamshell fashion. As with C60·2HB·0.94C6H6·0.06CS2, the

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RESULTS Crystals of C60·2HB·0.94C6H6·0.06CS2 and C70·2HB·C6H6 were grown by slow diffusion of solutions of the fullerene and HB. C60 and C70 were dissolved in carbon disulfide, while HB was dissolved in benzene. These solutions were filtered and layered over one another. Black crystals suitable for X-ray diffraction grew within 1 month. The use of benzene as a solvent was critical during crystal growth, because benzene is incorporated into both crystals. Interestingly, replacing benzene with toluene did not yield suitable crystals, and the sole use of carbon disulfide led to the rapid precipitation of both components individually. Crystal data for the two new cocrystals are listed in Table 1. Structure of C60·2HB·0.94C6H6·0.06CS2. The asymmetric unit consists of an ordered C60 molecule, two molecules of HB, and a disordered solvent site containing two positions for a benzene molecule and one position for a carbon disulfide molecule with occupancies of 0.766, 0.174, and 0.060, respectively. Figure 2 shows the molecular components of the asymmetric unit with only the major solvent site shown. Two HB molecules surround the fullerene in a clamshell manner. The packing of the structure, as seen in Figure 3, shows back-to-back contacts of two molecules of HB along the crystallographic b direction with benzene molecules residing between two adjacent fullerenes. The stacking of two molecules of HB is parallel because of the presence of a crystallographic inversion center between the two molecules. Benzene molecules are situated between fullerene cages throughout the lattice. 4006



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fullerene is ordered in C60·2CoII(OEP)·CHCl3. The CoII(OEP) molecules, like the HB molecules, pack in a back-to-back

manner with an interplanar spacing of 3.21 Å. An additional similarity comes from the angles between the planes of the pairs of HB and CoII(OEP) molecules in the two structures. For the HB cocrystal, the angle between the planes of the two benzene rings is 55.13°, while for the CoII(OEP) cocrystal, the corresponding angle between the planes of the two porphyrins is 48.24°. There are some close contacts between the carbon atoms of the fullerene and the C−H groups of the HB molecules that surround the cage. For one of the two HB molecules, there are six contacts in the range of 2.909−3.188 Å for the alkenyl hydrogen atoms and five contacts in the range of 2.955−3.159 Å for the hydrogen atoms of the tert-butyl groups. For the other HB molecule, there are five contacts in the range of 2.806− 3.146 Å for the alkenyl hydrogen atoms and four contacts in the range of 2.999−3.191 Å for the hydrogen atoms of the tert-butyl groups. For C60·2CoII(OEP)·CHCl3, there are only four contacts in the range of 3.067−3.274 Å between hydrogen atoms on the ethyl groups and carbon atoms of the fullerene. Structure of C70·2HB·C6H6. The asymmetric unit consists of two independent HB molecules, a disordered site for the C70 molecule, which occupies two orientations with populations of 0.568 and 0.432, and two halves of benzene molecules that reside on centers of inversion. Figure 5 shows the clamshell arrangement of the two HB molecules and the manner in which they surround the C70 cage in its two orientations. Unlike the situation in the HB/C60 cocrystal, in which the two HB molecules surrounding a C60 molecule are enantiomeric, in the HB/C70 cocrystal the two HB molecules that form a single clamshell have the same configuration. However, as one passes down a chain of 2HB/C70 molecules, the configuration of the HB molecules in each clamshell motif alternates, because adjacent HB molecules pack about centers of symmetry. Focusing on the major orientation, we find there are close carbon−carbon contacts between HB and the fullerene. As with the C60/HB cocrystal, the HB molecules are not identical crystallographically in the C70/HB cocrystal. Therefore, two different ranges of carbon−carbon interactions are expected between the HB molecules and the C70 molecule. These two ranges are 3.336−3.660 and 3.435−3.591 Å. These ranges refer

Figure 2. Top: Molecular components of C60·2HB·0.94C6H6·0.06CS2 with 30% thermal contours. For the sake of clarity, hydrogen atoms have been omitted and only the major solvate component. Bottom: A space-filling drawing of the two HB molecules surrounding a C60 molecule in the C60·2HB unit.

Figure 3. Two HB molecules that embrace a C60 molecule in C60·2HB·0.94C6H6·0.06CS2. The HB molecules are seen from the flat side that points away from the fullerene and are enantiomers of each other. Hydrogen atoms are colored yellow and carbon atoms gray. 4007



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Figure 4. Stacking of chains of alternating sets of C60 and pairs of HB molecules in C60·2HB·0.94C6H6·0.06CS2 (top) and corresponding stacking of C60 and pairs of CoII(OEP) molecules in C60·2CoII(OEP)·CHCl3 (bottom) (drawn from the data of ref 15). Hydrogen atoms and solvate molecules have been omitted for the sake of clarity.

to the HB being situated under five-membered and sixmembered rings of the fullerene cage, respectively. Figure 6 illustrates the packing of the structure. As with C60·2HB·0.94C6H6·0.06CS2 and C60·2CoII(OEP)·CHCl3, the C70/2HB units in C70·2HB·C6H6 form chains with back-toback contacts of two HB molecules. The distances between the planes of the two benzene rings are 3.68 and 3.72 Å for the two different types of pairwise contacts. In addition to the close contacts between the faces of the C70 and HB molecules, there are close contacts between adjacent C70 molecules. Each fullerene possesses interactions with two neighboring fullerenes with close carbon−carbon contacts of 3.302 and 3.349 Å.

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DISCUSSION We have demonstrated that the bowl-like shape of HB allows it to cocrystallize with both C60 and C70. Both cocrystals possess a ratio of two HB molecules to each fullerene, and these components are arranged in a similar, chainlike fashion. In each structure, there are back-to-back arrangements of HB molecules resulting in close contacts due to slipped π−π stacking. It is evident that in both structures, the clamshell HB− fullerene motif is quite similar to those reported for cocrystals formed from MII(OEP) and various fullerenes and endohedral fullerenes. In general, a 1:1 ratio of NiII(OEP) to fullerene is found in these cocrystals, as is the case when NiII(OEP) cocrystallizes with the egg-shaped, non-IPR endohedral fullerene-Gd3N@Cs(39663)-C82,17 with the four isolated isomers of Sm@C90,18 and with the cluster-containing fullerene Sc2(μ2-O) @Cs(6)-C82.19 However, in several cases, including cocrystallization with very large cages, Sm2@D3d(822)-C104,20 La2@ D5(450)-C100,21 and Tm@C3v-C94,22 a 2:1 ratio of NiII(OEP) to fullerene is observed. With both types of stoichiometry, there is a general predilection to have the NiII(OEP) molecules arranged in a back-to-back fashion. This arrangement also facilitates the alignment of all eight ethyl groups so that they are directed toward the fullerene, as shown in the packing diagram in Figure 4.

Figure 5. Molecular components of C70·2HB·C6H6 with 30% thermal contours showing the major orientation of the cage (top) and the minor cage orientation (bottom). Hydrogen atoms have been omitted for the sake of clarity.

The shape of the bowl-like HB molecules in these new cocrystals is similar to that of pristine HB reported some time 4008



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Figure 6. Packing motif for C70·2HB·C6H6. Hydrogen atoms have been omitted for the sake of clarity. layered over a 1 mL sample of a filtered solution of 16.0 mg (19.0 μmol) of C70 in 15 mL of carbon disulfide. Black crystals formed in a 10% yield within 1 month. Determination of Crystal Structures. Each crystal was mounted in the nitrogen cold stream provided by an Oxford Cryostream lowtemperature apparatus on the goniometer head of a Bruker D8 diffractometer equipped with an ApexII CCD detector at beamline 11.3.1 of the Advanced Light Source (Lawrence Berkeley National Laboratory, Berkeley, CA). Data were collected with the use of silicon(111) monochromated synchrotron radiation (λ = 0.77490 Å). All data sets were reduced with the use of Bruker SAINT, and a multiscan absorption correction was applied with the use of SADABS. The structures were determined by direct methods and refined by full-matrix least squares on F2 (SHELXL-2014).24 For both structures, all non-hydrogen atoms were refined anisotropically. Some low-angle reflections were omitted because of overload or beamstop blockage. For the structure of C60·2HB·0.94C6H6·0.06CS2, two atoms were modeled using ISOR commands and a SUMP command was used to model the solvent position disorder. For the structure of C70·2HB·C6H6, a SIMU command was used to model the two C70 cage orientations.

ago by de Meijere and co-workers.12 In all cases, the arrangement of the six substituents on one face of the central benzene ring produces the concave surface of the HB molecule. These six peripheral substituents appear to be locked into a fixed orientation because of the steric interactions between the eight tert-butyl groups. This situation differs from that in MII(OEP) where the ethyl groups are free to assume positions on either side of the porphyrin plane but generally line up so that all eight arms are directed toward the fullerene when cocrystallization occurs. It appears that the many contacts between the C−H groups in HB or in MII(OEP) with the carbon cage contribute to the ordering of the fullerene cage in these cocrystals. Despite the considerable success achieved in cocrystallizing NiII(OEP) and CoII(OEP) with C60 and many other fullerenes, we have not succeeded in cocrystallizing NiII(Etio-I) (see Figure 1) with C60. The otherwise trivial loss of four methyl groups seems to inhibit cocrystallization because fewer C−H group contacts can be made with C60. Formation of crystals clearly required the use of benzene as a solvent, because it was incorporated into both solids. The critical role of the solvent and solvate in modulating the nature of the interaction between C70 and another bowl-like molecule, bis(ethylenedithio)tetrathiafulvalene, has been observed.23 Synchrotron radiation was imperative for data collection. It was discovered that conventional in-house X-ray sources were not sufficiently intense to achieve high-angle reflection intensity. This is a common problem with organic structures.

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ASSOCIATED CONTENT

S Supporting Information *

CIF data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

EXPERIMENTAL SECTION

Notes

Materials. Hexakis[(E)-3,3-dimethyl-1-butenyl]benzene was prepared as previously described.12 C60 and C70 were purchased from SES research with purities of 99.5 and 99%, respectively. No further purification was performed. Benzene and carbon disulfide were purchased commercially and used as received. Crystal Growth. C 60 ·2HB·0.94C 6 H 6 ·0.06CS 2 . An 18.1 mg (25.1 μmol) sample of C60 was dissolved in 14 mL of carbon disulfide and filtered. A 1 mL aliquot of this solution was layered over a 1 mL sample of a filtered solution of 16.4 mg (29.2 μmol) of HB in 12 mL of benzene. Black crystals formed in a 13% yield within 1 month. C70·2HB·C6H6. A 16.4 mg (29.2 μmol) sample of HB was dissolved in 12 mL of benzene and filtered. A 1 mL aliquot of this solution was

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (Grants CHE1305125 and CHE-1011760 to A.L.B. and M.M.O.) and the Advanced Light Source, Lawrence Berkeley National Laboratory, for support. 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-AC0205CH11231. 4009



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Cocrystallization of C60 or C70 with the Bowl-Shaped Hydrocarbon

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