Exclusive Endo-Cavity Interplay of t-Bu-calix[6]arene with C70

ABSTRACT: Toluene solutions of fullerene C70 and t-Bu-calix[6]arene form a 2:1 inclusion complex that has been structurally authenticated using synchr...
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CRYSTAL GROWTH & DESIGN

Exclusive Endo-Cavity Interplay of t-Bu-calix[6]arene with C70 Mohamed Makha,† Colin L. Raston,*,† Alexandre N. Sobolev,† and Peter Turner‡ School of Biomedical, Biomolecular and Chemical Sciences, UniVersity of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 Australia, and School of Chemistry, UniVersity of Sydney, Sydney, NSW 2006 Australia

2006 VOL. 6, NO. 1 224-228

ReceiVed June 29, 2005

ABSTRACT: Toluene solutions of fullerene C70 and t-Bu-calix[6]arene form a 2:1 inclusion complex that has been structurally authenticated using synchrotron source X-ray diffraction data. Remarkably, the fullerene is not included in the cavity of the calixarene, and this has implications in predicting the structures of fullerene complexes of other calixarenes and container molecules. The fullerenes shroud the exo surface of the calixarene at the van der Waals limit, with the fullerenes arranged into corrugated sheets, with each calixarene interdigitated at the upper rim with the upper rim of two other calixarenes, the macrocycles forming a bilayer separating the sheets of fullerenes. Introduction Fullerenes form complexes in the solid state with a variety of large host molecules, often in association with aromatic solvents. Host-guest and supramolecular chemistry of fullerenes is important in the retrieval of pure fullerenes from fullerite,1 which involves optimizing the interaction between the two components through size, shape, and electronic considerations, at the same time reducing the likelihood of aggregation of fullerenes.2 In general, there is competition between host-guest interplay and aggregation of fullerenes, and controlling the balance between these is a challenge in building up new materials. In this context, varying the nature of the host molecule can profoundly affect the arrangement of the fullerenes. Deep cavity calixarenes can result in fullerenes completely shrouded by host molecules.3,4 While shallow cavity analogues maintain host-guest interplay, the diminished steric demands of the host on the surface of the fullerene allows fullerene-fullerene interactions. In the case of C60 a variety of fullerene arrays have been identified ranging from dimers, trimers, single columns (straight and zigzag) through to complex two-dimensional (2D) and three-dimensional (3D) arrays.2 Under high pressure, such arrays can be converted to covalently linked structures, for example, containing dimeric and polymeric arrays.5 Studies on controlling the aggregation of C60 are extensive, whereas analogous studies for C70 are limited. This presumably relates to the ready availability of C60 relative to C70 but may also relate to the spherical-like shape of C60 allowing more efficient and regular packing. While structural reports based on complexes of C70 highlight the host-guest interplay, they often lack a detailed appraisal and understanding of the extended arrays, including any fullerene-fullerene interactions. Here we report the structure elucidation of a complex of C70 and t-Bu-calix[6]arene which was originally prepared in 1994 and described as a 2:1 complex based on HPLC and microanalysis.1 The complex itself is used in the process for the retrieval of C70 from toluene solutions of fullerite depleted of C60 by complexation with t-Bu-calix[8]arene. Remarkably, we find that for the C70/t-Bu-calix[6]arene complex, the fullerene is not included in the cavity of the calixarene. This has implications for the structures of fullerene * To whom correspondence should be addressed. Fax: (internat.) +618 6488 1005. E-mail: [email protected]. † University of Western Australia. ‡ University of Sydney.

complexes of other calixarenes and container molecules in general, for which there may be an oversimplification of the mode of interaction of the components. For comparison, we also map out herein all the structural types for C70 complexes to date through critical analysis of structural information available from the Cambridge Crystallographic Data Base. Results and Discussion Crystals of composition (C70)2(t-Bu-calix[6]arene)‚toluene were obtained by slow evaporation at room temperature of a brown toluene solution of t-Bu-calix[6]arene and C70 in a 2:1 ratio. Crystallizations were reproducible based on using 50 mg of the fullerene, affording ca. 50% isolated yields of the complex. All crystals were weakly diffracting and too small for acquiring adequate data from a rotating anode X-ray source and CCD diffractometer. Subsequent synchrotron X-ray diffraction data was of sufficient intensity for a meaningful solution and refinement, albeit with some limitations (see below). All samples looked to be of uniform color and crystal morphology, and cell determinations on several crystals were consistent within experimental error with the cell obtained for the crystal used in the synchrotron data collection. In addition, the powder X-ray diffraction results were in agreement with the deconvoluted powder diffraction equivalent from single-crystal diffraction data. The asymmetric unit is comprised of two fullerene C70 molecules, one calixarene, and one toluene molecule. One of the fullerenes is disordered over three positions, and its geometry was constrained using a literature model.6 The calixarene is ordered with the exception of two t-Bu groups, which were refined with disorder over two positions. The calixarene is in the double cone up-up conformation with two shallow cavities on the same side, and flattening of two directly opposed aryl groups which are almost coplanar having a dihedral angle of 19.6°, Figure 1(a). The two crystallographically independent C70 molecules are not located endo with respect to the cavities of the calixarene, as was assumed on the basis of the structure of the 2:1 complex of the fullerene with calix[6]arene.10 Rather, they shroud the exo surface of the calixarene at the van der Waals limit. This involves four fullerenes with the other two beyond the asymmetric unit generated by 1 - x, 0.5 + y, 0.5 z and 1 - x, 1 - y, -z symmetry operations, Figure 1a. The two fullerenes, which are in contact with each other at the van der Waals limit (shortest C‚‚‚C 2.87, 3.10 Å), reside over almost

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Figure 1. Projection of (a) the calixarene and the exo-cavity fullerenes (along the a-axis) and (b) space-filling self-associated calixarenes above a sheet of fullerenes, along with the toluene molecules (green).

Figure 2. Interactions of the t-Bu groups with C70: (a) Side view projection along the b-axis showing CH‚‚‚π interaction (red dashed lines) and (b) top view projection in (a); the pairs of small spheres represent the closest carbon atoms of a fullerene involved in the C-H‚‚‚π interactions.

coplanar opposite aryl groups discussed above (shortest C‚‚‚C 3.30 Å), Figure 1a. The other pair resides in opposite clefts formed by pairs of adjacent aromatic rings, Figure 1a (shortest C‚‚‚C distances 3.63, 3.84 Å and 3.69, 3.31 Å, respectively). This pair of fullerenes are not in contact but are in contact with the other pair (centroid‚‚‚centroid distances 10.40 Å). The toluene molecule is located within the calixarene layer filling the gap created by the interdigitation of the calixarenes. The methyl group of toluene is directed toward the center of the calixarene cavity while the meta hydrogens form a C-H‚‚‚π interaction with C70 (closest CH‚‚‚C contact 3.10 Å), Figures 1b and 2b. A striking feature of the structure is the extensive CH‚‚‚π interactions involving methyl moieties of two adjacent t-Bu groups and the fullerenes. The fullerene interactions associated with the clefts (see above) involve CH‚‚‚π interactions (closest C-H‚‚‚C distances 2.49, 2.65 Å and 2.70, 3.11 Å, respectively for fullerene 1 and fullerene 2), Figure 2a. The other pair of symmetry generated fullerenes similarly involve fullerene‚‚‚ calixarene interactions, in this case with diametrically opposed t-Bu groups (closest C-H‚‚‚C distances 2.63, 3.05 Å and 2.81, 2.99 Å, respectively, for fullerene 1 and fullerene 2), Figure 2b. In the extended structure, the fullerenes form corrugated sheets sandwiched between layers of calixarenes, Figure 3a. The calixarene layer is somewhat reminiscent of the bilayer arrangement or skewed capsules observed with p-sulfonato-calix[4,5]arenes.7 The calixarenes are oriented face-to-face and their t-Bu

groups interdigitated, protruding into the aromatic lined pseudo cavity of each other calixarene, Figure 3b (closest CH‚‚‚Aromatic centriod distances 2.78, 3.03 to 3.40 Å). The calixarene effectively reside in the corrugations of the fullerene sheets, Figure 2b. The shortest C‚‚‚C inter- fullerene distances within each sheet are 2.76, 2.87, 3.07, 3.10, 3.16, 3.23, 3.34, 3.37, 3.38 Å. Structural Relationship with other C70 Complexes. In light of the novel interplay of C70 and “host” molecules, all structures of C70 complexes to date have been analyzed to ascertain the mode of interactions of the components and to establish the ground rules, if any, for designing C70 complexes with specific interplay of the fullerenes and structures of higher complexity. The analysis used the Cambridge Crystallographic Data Base which revealed 26 complexes containing C70; structures based on chemically altered C70 molecules were not considered. The closest contacts between fullerenes can involve different combinations of five- and six-membered rings facing each other, a ring edge facing a ring face or two ring edges facing each other. Interplay of the fullerenes affects the closest intercarbon approach, with C‚‚‚C distances between 2.78 and 3.40 Å, depending on the orientations of the elliptical fullerene. The fullerene usually interact pole directed to the equator, pole-topole, and equator-to-equator (in our structure there is a pseudo pole-to-equator interaction). Seven arrangements of C70 molecules have been identified: (i) encapsulated and isolated fullerenes,16,18,19 (ii) dimers,17 (iii) zigzag columns, (iv) columns,22 (v) close and pseudo-close packed layers, (vi) corrugated

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Figure 3. (a) Space filling packing diagram showing the corrugated sheets of the C70 separated by layers of self-associated calixarenes. (b) Projection showing the interdigitation of the calixarenes relative to the fullerenes. (c) Projection of a layer of interdigitated calixarenes. (d) Schematic diagram of the fullerene layer showing the irregular surface where the dips and summits of the corrugated sheet represent the centroids of the fullerenes.

sheets, and (vii) 3D networks. The type of organization of the fullerenes is influenced by the nature and the geometry of the ligand and/or the solvent used. Encapsulated-Isolated Fullerenes. This is prevalent in complexes of porphyrins where the fullerene is sandwiched between two host molecules,18,19 which includes porphyrins linked via metal complexation.16 One example of a van der Waals dimer also involves complexation with a porphyrin.17 Isolated C70 molecules are also found in a 3D highly porous network built up of hydrogen-bonded (OH)6 rings involving hydroquinone molecules.13 Zigzag Columns. The structure of [(C70)(o-carborane)(CTV)(1,2-dichlorobenzene)2]9 (CTV ) cyclotriveratrylene) has C70 fullerenes associated as trimers in a V-shaped arrangement forming distorted zigzag chains, closest C‚‚‚C contacts 3.27, 3.36, 3.38 and 3.34 Å, with fullerene principal axes at right angles, Figure 4(a). These chains intercalate with each other to form an overall structure of pseudo corrugated sheets of the fullerenes, Figure 4b. The C70 molecules are separated close to the van der Waals limit with the fullerene‚‚‚fullerene centroid distances at 10.79 Å, which is comparable to those in (C70)2(calix[6]arene), 10.53-10.66 Å.10 Another type of a zigzag arrangement is seen in the structure of C70/octaethylporphyrinato-M(II), M ) Co, Ni, Cu), Figure 5.6 The fullerene‚‚‚fullerene closest contacts are 2.8-3.5 Å. Columns. Fullerenes C70 are arranged in a continuous linear chain with a short carbon-to-carbon contact of 3.18-3.37 Å, where the shape of the associated ligand trans-9,9′-bis

Figure 4. Zigzag chains in [(C70)(o-carborane)(CTV)(1,2-dichlorobenzene)2]9 leading to pseudo-corrugated sheets showing (a) the pole-toequator directed interaction of C70 fullerenes within the chain and (b) packing of zigzag chains of the fullerenes along the y-axis.

(telluraxanthenyl) prevents hexagonal close packing and filling the interstitial space between the columns, Figure 6.12 Columnar arrays of C70 are also found in the structure of tris(fullereneC70)tetrakis(bi-(naphtho(1,8-d,e)-1,3-dithin-2-ylidene))tetrakis(benzene).22 Close and Pseudo-Close Packed Layers. This type of fullerene arrangement is common. The flat fullerene layers are generally similar with minor differences in the way the fullerenes are oriented within the layers (z-axis of the fullerene either orthogonal or aligned with the plane of the layer). The structure of C70 fullerene bis-ferrocene11 has a close packed fullerene

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Figure 5. The zigzag chains of fullerenes having pole-to-pole and equator-to-equator interactions in C70/octaethylporphyrinato-M(II), (M ) Co, Ni, Cu).6

Figure 8. Hexakis (octathione) C70 structure:14,15 (a) well-separated fullerene layers; (b) the fullerene layer with z-axis within the plane of the layer.

Figure 6. Columnar array of the fullerene running pole to pole along the a-axis in the structure of trans-9,9′-bis (telluraxanthenyl) fullerene C70. Figure 9. Projection of the corrugated sheet of C70 (a) top view showing the zigzag arrangement within the sheet and (b) side view showing two sheets (all ligands were omitted for clarity).

Figure 7. Structure of ferrocene/C70 complex: (a) Fullerene layers formation intercalated by ferrocene molecules which were omitted for clarity. (b) Hexagonal close-packed C70 arrangement within the layer.11

arrangement forming continuous layers intercalated by ferrocene molecules with π‚‚‚π stacking between the Cp* ring and pentagons of the fullerene (close contact distance of 3.36 Å) and the shortest inter-fullerene close contact is between 3.31 and 3.34 Å, Figure 7b. The fullerene layer is comprised of hexagonal close packed C70 with the z-axis of the fullerene pseudo-orthogonal to the plane of the layer, Figure 7a. In the structure of C70 solvate with hexakis (octathione),14,15 the fullerenes are in hexagonal closed packed layers well separated by solvent molecules, Figure 8a. Each fullerene is surrounded and in close contact with six other fullerenes all having their z-axis parallel and in the fullerene layers, making points of closest contact at 3.49, 3.70, 3.80, and 3.91 Å, Figure 8b. Another type of layer of close packed C70 fullerene is seen in the structure with TiCl4 filling the interstitial space between the layers, and this time the z-axis of the fullerene is orthogonal to the layers’ plane.20

Figure 10. (a) C70 fullerenes resides in the pseudo-cavities of calix[6]arene in the double cone conformation. (b) Schematic diagram of the 3D network of bridged tetrahedral array of four C70 molecules.

Corrugated Sheets. In the structure of C70/NiTMTAA,8 the elliptical fullerene makes close contact with five other fullerenes forming a 2D corrugated sheet consisting of slightly offset fullerene chains, Figure 10a. The z-axes of each C70 are arranged in a zigzag fashion with a five-membered ring of one fullerene and a six-membered ring of another fullerene in the chain π-stacking, albeit with a slight deviation from coplanarity, making a point of closest contact at 3.14 Å, Figure 9b. One of the poles of the fullerene, i.e., the five-membered ring through which the z-axis passes, forms a classical π‚‚‚π stacking interaction with that of an adjacent fullerene such that one of the participating fullerenes is on a ridge, the other in a trough. The fourth and fifth close contact points run at right angles to the grain of the sheet, as defined by the ridges and troughs, and form a chain that has the z-axes aligned in a zigzag fashion and contiguous fullerenes alternating between ridge and trough positions. These points are those of closest contact to each

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fullerene, C‚‚‚C 3.05 Å. Corrugation of fullerenes are also found in the complex of calix[5]arene/C70, where the fullerene is located in the cavity of the calixarene and the overall fullerene interplay consists of corrugated sheets.21 3D Networks. In the structure of [(calix[6]arene)(C70)2]10 fullerene C70 are bound noncovalently to the pseudo-cavities of calix[6]arene in the double cone conformation. The overall structure is complicated and is isostructural with [(calix[6]arene)(C60)2] despite the anisotropic shape of C70. The organization of C70 is not well defined; however, each of the fullerenes is closely packed with three others having close contact distances of 3.17, 3.26, and 3.37 Å. The calixarenes are in the doublecone conformation, and each of the associated shallow cavities is occupied by a fullerene. The overall arrangement resembles the jaws of a pincer acting on two adjacent ellipsoids, and the principal axis of the ellipsoidal C70 molecule is not directed toward the calixarene cavity, Figure 10a. The extended structure is comprised of bridged tetrahedral array of four C70 molecules in close contact that forms a 3D network, Figure 10b.

Makha et al. Crystal/Refinement Details for t-Bu-calix[6] (C70)2 Toluene with a Moiety Formula C66H84O6[C70]2C7H8. C213H92O6, M ) 2746.87, F(000) ) 11344 e, orthorhombic, Pbca, Z ) 8, T ) 123 K, a ) 36.115(3), b ) 18.671(2), c ) 37.497(3) Å, V ) 25284(4) Å3; Dc ) 1.443 g cm-3; sin θ/λmax ) 0.622; N(unique) ) 22395 (merged from 316353, Rint ) 0.1630, Rsig ) 0.1000), No (I > 2σ(I)) ) 13411; R ) 0.2614, wR2 ) 0.5341 (A,B ) 0.122, 0.313), GOF ) 1.131; |∆Fmax| ) 0.040(5) e Å-3.

Acknowledgment. We thank the ARC, the University of Western Australia, for support of this work and Joshua McKinnon for his assistance. Use of the ChemMatCARS Sector 15 at the Advanced Photon Source was supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. ChemMatCARS Sector 15 is also supported by the National Science Foundation/Department of Energy under grant numbers CHE9522232 and CHE0087817 and by the Illinois board of higher education. The Advanced Photon Source is supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract No. W-31-109-Eng-38. References

Conclusion We have structurally authenticated a 2:1 complex of fullerene C70 with t-Bu-calix[6]arene and show that the fullerenes does not reside in the cavity, or cavities for a double cone conformation calixarene, and this result alone demonstrates an element of caution in predicting the nature of such complexes. The association of the fullerenes into corrugated sheets of fullerenes further highlights the role so-called host molecules can play in organizing C70 molecules into various arrays. In addition, the structure is distinctly different to that of the corresponding 2:1 complex with the unsubstituted calix[6]arene, clearly demonstrating that varying the nature of the substituent can control the arrangement of the fullerenes, and area we are currently exploring. Experimental Section Solutions of C70 and t-Bu-calix[6]arene in a 2:1 ratio in toluene were mixed and heated at about 100 °C, and the resulting brown solution was allowed to cool slowly to room temperature over 3 h, followed by standing for a week to afford dark crystals of the title compound. Data were collected at the ChemMatCARS facility at the Advanced Photon Source of the Argonne National Laboratory, Argonne, USA. Double diamond (111) reflections were used to obtain monochromated 0.55 Å radiation from the synchrotron source, and harmonics were eliminated with mirrors. An dark brown needle like crystal was attached with Exxon Paratone N to a short length of fibre supported on a thin piece of copper wire inserted in a copper mounting pin. The crystal was quenched in a cold nitrogen gas stream from an Oxford Diffraction Cryojet. A Bruker three circle diffractometer platform with a SMART 6000 CCD detector was used for the data collection. Data were corrected for Lorentz and polarization effects and absorption correction was applied using multiple symmetry equivalent reflections (µMo ) 0.055 mm-1, Tmin/max ) 0.995, 0.999). The structure was solved by direct methods and refined on F2 using SHELXTL crystallographic package. A full matrix least-squares refinement procedure was used, minimizing w(Fo2-Fc2), with w ) [σ2(Fo2) + (AP)2 + BP]-1, where P ) (Fo2 + 2Fc2)/3. Agreement factors (R ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2 and GOF ) {∑[w(Fo2 - Fc2)2]/(n - p)}1/2 are cited, where n is the number of reflections and p is the total number of parameters refined), CCDC 272525.

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