Linear Arrays of Interlocking Calixarenes in Controlling the Interplay of

Jul 8, 2008 - Linear Arrays of Interlocking Calixarenes in Controlling the. Interplay of Fullerene C60 Molecules. Mohamed Makha,* Cameron W. Evans, ...
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CRYSTAL GROWTH & DESIGN

Linear Arrays of Interlocking Calixarenes in Controlling the Interplay of Fullerene C60 Molecules Mohamed Makha,* Cameron W. Evans, Alexandre N. Sobolev, and Colin L. Raston* Centre for Strategic Nano-Fabrication, School of Biomedical, Biomolecular and Chemical Sciences, UniVersity of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

2008 VOL. 8, NO. 8 2929–2932

ReceiVed January 13, 2008

ABSTRACT: p-Benzylcalix[6]arene, 1, and C60 form a fullerene rich inclusion complex in toluene, (C60)3(p-benzylcalix[6]arene)(toluene)4.25, 2, which has been structurally authenticated. The fullerene and toluene molecules shroud linear arrays of the calixarenes, which are dethreaded in the presence of methylene chloride to form an all carbon hcp polymorph of fullerene C60. The ease of dethreading relates to the mode of association of p-benzylcalix[6]arene within the molecular crystal consisting of interlocked calixarenes in linear arrays. Similar association has also been seen in the solid-state structure of p-benzylcalix[6]arene, 1, where one calixarene is associated with another through two modes, a frontal host–guest association involving inclusion of the pendent benzyl arms and back-to-back stacking arrangement. Introduction Inclusion complexes of fullerenes are structurally diverse. They can involve small solvent molecules through to large host molecules such as cavitands and porphyrins and related molecules.1–4 The limiting case is where the fullerenes are completely shrouded by one or more of the included/host molecules.5 This special case aside, the inclusion complexes have fullerene · · · fullerene interplay at the van der Waals limit, although the extent of such interplay is often overlooked not only for complexes of the ubiquitous C60, but also for C70.1,2 Fullerene C60 can assemble into one- and two-dimesional arrays as well as continuous three-dimensional networks including pristine fcc C60 with small molecules included in interstitial sites.6 Calixarenes can form C60 complexes with the nature of the solid state structures depending on the size of the calixarene and the p-substituents on the upper rim, along with the choice of solvent and the presence of other solutes which are important in the crystallization process but yet are not incorporated into the structure.7 Calix[4]arenes usually adopt the cone conformation but the cavity is too small to accommodate the fullerene and thus in any complex the fullerene is exo relative to the calixarene cavity.8 The preferred cone conformation of calix[5]arenes have close to ideal complementarity of size and curvature of the cavity for binding C60, and for all structurally authenticated complexes the fullerenes are endo relative to the cavity of the calixarene.5,7 For larger ring size, and more conformationally flexible calixarenes, a higher level of complexity of the structures is possible. Fullerene C60 forms a 2:1 complex with calix[6]arene, (C60)2(calix[6]arene)(toluene), which has a continuous three-dimensional array of fullerenes, and remarkably it is isostructural with the analogous C70 complex.9 In these complexes the calix[6]arene adopts the double cone conformation, resembling a pincer acting on two fullerenes, one in each of its two shallow cavities. The 1:1 complex of C60 with p-tBu-calix[8]arene features in the purification of the fullerene from fullerite,10 and has a predicted structure based on the calixarenes in the double cone conformation, with three calixarenes shrouding a triangular array of fullerenes in a * Corresponding author. E-mail: [email protected]. Tel: 618 6488 3045. Fax: 618 6488 1005.

Scheme 1. Reaction of p-Benzylcalix[6]arene with Fullerene C60

micelle-like structure.10 Clearly predicting the nature of the complexes with the larger calixarenes is problematic, and simply establishing the stoichiometry of a complex is not necessarily indicative of the nature of association of the fullerenes and their interplay with the calixarene. Herein, we report the synthesis and structure elucidation of the p-benzylcalix[6]arene, 1, complex with C60, [1 · (C60)3] · (C7H8)4.25, 2, Scheme 1, and for comparison we report the crystal structure of unsolvated p-benzylcalix[6]arene. Treatment of solid complex 2 with dichloromethane leads to dethreading the calixarene from the complex, without disolution of the fullerene, affording an all carbon C60 structure. Results and Discussion Crystals of 1 were obtained by slow cooling of a xylene solution close to the boiling point of the solvent, and the structure was determined by X-ray crystallography with the asymmetric unit containing one unique molecule. The calixarene is in a pinched double cone conformation which maximizes the lower rim hydrogen bonding network with O · · · O distances ranging from 2.63 to 2.73 Å. This is a similar conformation to that found in solid-state structures of p-tBu-calix[6]arene.11 The calixarenes form layers involving C-H · · · π phenyl embraces through the aromatic protons of the benzyl groups. It is noteworthy that these layers are comprised of linear arrays of up and down calixarenes running in a parallel fashion, involving the aforementioned C-H · · · π interactions. The calixarenes are oriented face-to-face with their benzyl groups protruding into the aromatic lined pseudo cavity of another calixarene, closest C · · · centroid distance is 3.43 Å. Another type of interlocking involves a second benzyl group of the central calixarene

10.1021/cg800045u CCC: $40.75  2008 American Chemical Society Published on Web 07/08/2008

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Figure 1. Packing diagram of pure p-benzylcalix[6]arene, 1, showing the interdigitation mode of association of calixarene molecules.

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Figure 3. (a) Structure of complex 2 (disordered parts removed for clarity), and (b) the same structure with the calixarene and toluene molecules.

Figure 2. Arrangement of calixarene molecules in the p-benzylcalix[6]arene complex with C60 2, showing the interlocking complementary between calixarenes.

molecule with calixarenes adjacent to its other side, associated with C-H · · · π interaction, with shortest C · · · centroid contact of 3.41 Å, Figure 1. Overall in the structure of 1, the calixarenes show extensive associations with one another through two modes; a frontal host–guest association via the pendent benzyl arms inclusions and back-to-back stacking arrangement involving methylene protons-aromatic interactions (closest C · · · centroid distances 3.63 Å). Such interlocking association of the calixarenes persists in the fullerene complex 2 although now with the calixarenes in a linear array, Figure 2. Black crystals of complex 2 formed by slow evaporation of a toluene solution of C60 and p-benzylcalix[6]arene, and was characterized by single crystal diffraction studies, along with powder XRD and TGA. The overall composition of this fullerene rich complex is (C60)3(p-benzylcalix[6]arene)(toluene)4.25. Solutions containing various mole equivalents of calixarene relative to the fullerene afforded the same complex. This included using excess calixarene which affords a precipitate of the pure calixarene along with complex 2. We note that cyclotriveratrylene (CTV) forms both a 1:1 and 3:2 complex with C60, depending on the ratio of the fullerene to CTV.12 The corresponding p-benzylcalix[5]arene forms a toluene solvated 2:1 complex with fullerene C60 where the calixarene molecules completely shroud a single fullerene molecule.5 Although the p-benzylcalix[4]arene is a small cavity, it forms an inclusion complex with C60 wherein the icosahedral molecules reside exo relative to the calixarene cavity.13 Given the structure of the above-mentioned C60 complex involving calix[6]arene with H-atoms in the para-positions that has the calixarene in the double cone conformation, the structure of the C60/p-benzylcalix[6]arene complex was expected to be based on a fullerene residing in each of the cavities of a double cone conformation calixarene. Furthermore the pendant arms of p-benzylcalix[6]arene could preorganize to create deeper cavities for the double cone conformation, thereby enhancing binding

Figure 4. (a) Structure of complex 2 (disordered parts removed for clarity), and (b) the same structure with the calixarene and toluene molecules removed for clarity and to highlight the virtual porosity (fullerenes are colored purple, toluene molecules are in blue; the calixarenes are shown in ball-and-stick projection down c).

of the fullerene. Indeed, the only isolable fullerene complex is 2, which has each calixarene in the double cone conformation, as predicted, but remarkably the fullerenes are not in the cavities. Rather the pendant arms of other calixarenes reside in the cavities as part of a continuous linear array of calixarenes, Figure 2, with the fullerenes and toluene molecules shrouding these arrays in a flattened honeycomb network, Figures 3 and 4. The asymmetric unit in 2 contains: (i) One molecule of a calixarene located on a 2-fold axis. (ii) One disordered molecule of C60 on a general position with another residing on a center of symmetry. (iii) Disordered toluene molecules in general positions. One of the solvent molecules has 0.25 population located on a 2-fold axis above the calixarene molecule, and

Controlling Interplay of Fullerene C60 Molecules

resides almost at the center of the parallelorgram formed by four C60 molecules having close contacts of 10.07 and 10.25 Å between the centroids of C60 (Figure 3). The intercalation and the snug fit of this toluene molecule between the fullerenes is reflected by C-H · · · π interactions of the meta-hydrogens to the closest six-membered rings of C60 with a C · · · centroid distance of 3.31 Å (3.39 Å to the second disordered C60). The shortest fullerene carbon atom distance to the toluene aromatic centroid is 2.66 Å. Unfortunately, the disorder of the structural components does not allow for meaningful Hirshfeld surface analysis.14 The self-association of the calixarenes through part of the calixarene residing in the cavity of another is intriguing. Interlocking of calixarenes is important in smaller rigid ring systems, notably in the structures of unsolvated calix[4]arene where three cavitands form a globular array, and p-phenylcalix[4 and 5]arenes.15,16 Calixarene association is also seen in the 1:2 complex of o-carborane with calix[5]arene where one calixarene resides in a the cavity of another calixarene.17 p-Benzylcalix[4]arene also forms interlocking linear chains, at least for the pure compound, but now the calixarenes are simply stacked one on top of each other.18 Here the rigid pendant arm of one calixarene reside in the cavity of another and vice versa.2 Interestingly for the 2:1 C70 complex of p-tBu-calix[6]arene, which also has the fullerenes exo to the double cone cavities of the calixarene, the fullerenes are arranged into corrugated sheets with the calixarenes interdigitating in a bilayer array.2 The fullerene organization in 2 consists of a hybrid of corrugated sheets separated orthogonally by a linear array of fullerenes to form a honeycomb type 3D structure. p-Benzylcalix[6]arene molecules in both structures 1 and 2 have essentially the same conformation with minor differences in the orientation of the benzyl para-substituents relative to the phenolic rings. In complex 2, The calixarene hydroxyl groups are engaged in intramolecular H-bonds with O · · · O distances of 2.59–2.67 Å. The calixarene molecules here form chains via inclusion of the benzyl group in the cavity of adjacent calixarene and are held together through C-H · · · π interactions involving CH2 groups and centroids of the phenolic rings with minimum C · · · π distance of 3.41 Å. The fullerene interplay associated with the clefts of the calixarenes involves π · · · π interactions with a minimum distance of 3.35 Å and inter fullerene short contacts with a C · · · C distance of 3.20 Å. In addition to the aforementioned C-H · · · π interactions of toluene molecule in a special positions, toluenes in general positions have such interactions with a short C · · · π contact of 3.75 Å. In the C60 complex with p-H-claix[6]arene, the fullerenes form a 3D array with each fullerene at the vertices of a tetrahedral array of fullerenes, bridging another tetrahedral array, thus with each fullerene surrounded by six other fullerenes.9 This is in contrast to the honeycomb structure in 2 which can be considered as a fullerene rich reverse phase relative to the common arrangement of fullerenes into one-dimensional (i) single column linear or zigzag arrays, (ii) close packed double columns, or (iii) five close packed columns (Z-shape), where the fullerenes are shrouded by a sheath of calixarenes.1a The onlyotherrelevantstructureisthatofthecomplex(C60)2[5,10,15,20tetraphenylporphyrin], also as a fullerene-rich benzene solvate, where the fullerenes form a strictly honeycomb array.19 In the case of complex 2, four fullerenes reside in shallow cavities on the outside surface of the calixarenes and such organization is in consequence of the double cone conformation, and the orientation of the benzyl groups. This fullerene assembly is

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Figure 5. Indexed XRD powder pattern of the dethreaded material, typical of a hexagonal close-packed C60 structure.

somewhat reminiscent of the arrangement of C70 molecules with p-tBu-calix[6]arene.2 Careful examination of the structure in complex 2 revealed the linear arrangement of the calixarenes and virtual porosity of cylindrical space occupied by the calixarenes (ca.16.8 Å). This observation warranted engineering porosity in this material. The porosity of the complex could be realized by removing the calixarene while minimizing disruption to the C60 lattice. Since C60 is virtually insoluble in most solvents, the crystals of complex 2 were immersed in dichloromethane or chloroform which are both good solvents for the calixarene, causing the faces of the crystals to take on a dull, pitted appearance but the overall morphology of the crystals appeared to be retained (ESI). Upon successfully removing the calixarene, the long-range crystalline order of the sample was destroyed making singlecrystal X-ray analysis impossible, and therefore determination of the structure of the dethreaded complex relied on powder X-ray diffraction. The XRD powder analysis shows the C60material to be in a pristine hcp form, Figure 5. The reflection hkl can be observed only for the hcp transformation, but forbidden for the fcc.20 Because the crystal morphology of the dethreaded complex appeared unaffected, it is more likely that the structural organization of the fullerenes is somewhat retained with possible shrinkage but not a total collapse to the fcc phase. Conclusions We have demonstrated that a calixarene can be used to template the formation of a flattened honeycomb like arrangement of fullerenes, and that the fullerenes are not in the cavities of the calixarene. This demonstrates an element of caution in predicting and indeed attempting to oversimplify the nature of the complexes formed on complexation of fullerene C60 with larger cavitand molecules. Moreover the structure is distinctly different from that for the complex of the fullerene with the parent unsubstituted calix[6]arene. The assembly of the fullerenes into a new polymorph induced by the calixarene is noteworthy, and the potential of forming an all carbon porous material via calixarene templation is of interest for guest uptake and storage, a trajectory we are currently pursuing. Experimental Section p-Benzylcalix[6]arene was synthesized according to the literature11 while fullerene C60 was purchased from Bucky USA and used as received. A xylene solution of benzylcalix[6]arene

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(100 mg, 0.085 mmol) is heated and evaporated in vacuo, upon which crystals of compound 1 formed that were suitable for X-ray diffraction studies. X-ray diffracted intensities were measured from a single crystal 0.25 × 0.07 × 0.07 mm3 at 150 K on a Bruker AXS CCD diffractometer using monochromatized Mo-KR (λ ) 0.71073 Å). X-ray Crystallography. (a) Crystal/Refinement Details for 1. C84H72O6, M ) 1177.42, F(000) ) 1248 e, triclinic, P-1, Z ) 2, T ) 150(2) K, a ) 12.953(4) Å, b ) 15.976(5) Å, c ) 16.595(5) Å, R ) 74.555(5), β ) 86.287(6), γ ) 76.784(6)°, V ) 3222.4(17) Å3; Dc ) 1.213 g cm-3; sin θ/λmax ) 0.5955; N(unique) ) 10701 (merged from 20531, Rint ) 0.0866, Rsig ) 0.2188), No (I > 2σ(I)) ) 4269; R ) 0.0773, wR2 ) 0.1687 (A,B ) 0.09, 0.0), GOF ) 0.935; |∆Fmax| ) 0.45(8) e Å-3. CCDC 647919. Synthesis of (C60)3(p-benzylcalix[6]arene)(toluene)4.25, 2: Slow evaporation of a 20 mL toluene solution of p-benzylcalix[6]arene (40 mg, 0.034 mmol) and C60 (20 mg, 0.028 mmol) to ca. 3 mL afforded complex 2 and large black prisms (22 mg, 0.006 mmol, 66% yield). These were collected and washed with hexane (3 × 1 mL) and then dried in air. The X-ray diffracted intensities were measured from a weak-diffracting single crystal 0.16 × 0.16 × 0.12 mm at 100 K on an Oxford Diffraction Gemini-R Ultra CCD diffractometer using Cu-KR (λ ) 1.54178 Å) at the resolution about 1.00 Å. The structures were solved by direct methods and refined on F2 using the SHELX-97 crystallographic package and the X-Seed interface.21 (b) Crystal/Refinement Details for 2. C84H72O6, 3(C60), 4.25(C7H8): C293.75H106O6, M ) 3730.79, F(000) ) 7666 e, monoclinic, C2/c, Z ) 4, T ) 100(2) K, a ) 38.6092(6) Å, b ) 17.7395(3) Å, c ) 24.4054(4) Å, β ) 91.293(2) °,V ) 16711.2(5) Å3; Dc ) 1.483 g cm-3; sin θ/λmax ) 0.5027; N(unique) ) 8625 (merged from 27734, Rint ) 0.0321, Rsig ) 0.0369), No (I > 2σ(I)) ) 6434; R ) 0.1743, wR2 ) 0.4321 (A, B ) 0.2, 260.0), GOF ) 1.338; |∆Fmax| ) 1.35(15) e Å-3. CCDC 647920. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC 647919-CCDC 647920). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (44) 1223–336–033; e-mail: [email protected]). Acknowledgment. We thank the Australian Research Council and the University of Western Australia for support of this work.

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