Generating Flexibility in Inclusion Compounds that Possess Solvent

Article pubs.acs.org/crystal

Generating Flexibility in Inclusion Compounds that Possess SolventAccessible Voids: An Alternative Route to Control Pore Size in ThreeDimensional Nanoporous Molecular Crystals Issam Oueslati,*,†,‡ José A. Paixaõ ,‡ Vítor H. Rodrigues,‡ Kinga Suwinska,§ Barbara Lesniewska,§ Aleksander Shkurenko,§ M. Ermelinda S. Eusébio,∥ Jacques Vicens,⊥ Teresa M. R. Maria,∥ and Amílcar L. Ramalho@ †

Department of Chemistry, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal CEMDRX, University of Coimbra, P-3004-516 Coimbra, Portugal § Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 Kasprzaka, PL-01-224 Warsaw, Poland ∥ Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal ⊥ IPHC-Uds-ECPM-CNRS, 25, rue Becquerel, 67087 Strasbourg, France @ Department of Mechanical Engineering, University of Coimbra, 3030-788 Coimbra, Portugal ‡

S Supporting Information *

ABSTRACT: Exceptionally flexible inclusion compound having three-dimensional (3D) solvent-accessible voids was built from single 1,3-ethylene diamide bridged p-tert-butylcalix[4]arene. The robustness of the crystalline order originates from a fascinating square-grid framework of open molecular capsules that is built entirely from van der Waals forces and weak hydrogen bonding. Yet, its flexibility is induced by a controlled solvent-directed “breathing behavior”. The 3D network of channels generates extrinsic voids sufficiently elastic to trap different sizes of alcohol molecules. Careful extraction of solvent from these voids produces 3D nanoporous molecular crystals with controlled pore size.

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INTRODUCTION

step forward in exploiting directed hydrogen bonding to rationalize the construction of NMCs with high surface area.25 When organic molecules crystallize from solutions with suitable solvents, the host lattices of solvated inclusion compounds (IC) often possess zero-dimensional (0D), onedimensional (1D), two-dimensional (2D), or three-dimensional (3D) solvent-accessible voids. Careful extraction of solvent from these voids may produce NMCs. In such a case, the IC may be regarded as a NMC precursor. The extrinsic porosity describes the empty space (void) generated in the molecular self-assembly. However, intrinsic porosity defines the internal cavity of each molecule. The contribution of the present work is focused on bringing new insights into the understanding and monitoring of extrinsic porosity in molecular crystals. The research methodology to reach this target consists of three steps: study of the effect of intrinsic porosity on the size and shape of the void, investigation of the correlation between van der Waals (vdW) surface area of the guest and size of the void, and study of the consequence of guest removal in intrinsic and/ or extrinsic porosity on crystal molecular architecture. To fulfill these objectives, 1,3-ethylene diamide bridged p-tert-butyl-

There is a growing interest in developing the design/ engineering of nanoporous materials1−6 with molecular-sized pores, which are of great importance in molecular separation, heterogeneous catalysis, gas storage, and carbon dioxide capture.7−9 Conventional nanoporous materials consist of crystalline inorganic frameworks (zeolites) and amorphous structures (silica gel). In recent years, research has been developed in the preparation of nanoporous materials using molecular precursors, particularly metal−organic frameworks (MOFs).3,8 The new generation of nanostructured materials is described as nanoporous molecular crystals (NMCs).10−12 The latter are composed of discrete molecules with weak noncovalent intermolecular interactions. Such materials are quite rare compared to porous network materials.13−16 They combine nanoporosity and the ability to be dissolved and reassembled in solvents and, hence, may lead to the development of novel procedures of solvent-based fabrication.17−21 Owing to their recent appearance, research is still focused on growing and characterizing NMCs.8,12 Controlling their pore size is, indeed, a challenging issue. This was recently achieved for conventional nanoporous materials like zeolitic imidazolate frameworks22 and covalently bonded organic cages.23,24 Mastalerz and Oppel have just made an important © 2013 American Chemical Society

Received: July 2, 2013 Revised: August 13, 2013 Published: August 20, 2013 4512

dx.doi.org/10.1021/cg400996w | Cryst. Growth Des. 2013, 13, 4512−4517

Crystal Growth & Design

Article

NH(CH2)2NH bridge (ESI), evidence relatively similar geometries drawn by rotation about single bonds (Figure 3).

calix[4]arene (compound A, Figure 1) will be examined, since its ability to include an acetonitrile molecule in the intrinsic

Figure 3. View along the C−C bond of the geometry of azacrown bridge in (a) 1, (b) 2, and (c) 3.

The geometry of the bridge in 1 is defined by the 62.5° positive synclinal torsion of the bond C47C48, 88.4° positive anticlinal torsion of the bond N1C47, and 91.5° positive anticlinal torsion of the bond N2C48. The geometry of the bridge in 2 is nearly similar to that in 1 with a small 4.5° negative torsion of the bond C47C48 (which corresponds to 58° positive synclinal torsion of C15C36 in 2) and a 2.6° positive torsion of the bond N1C47 (corresponding to 90° positive synclinal torsion of N1C15 in 2). In contrast, the geometry of the bridge in 3 is comparable to that in 1 after strong negative torsions of the bonds. Actually, a 127.5° negative torsion of the bond N1C47 (corresponding to a 84° negative synclinal torsion of N1C49 in 3) and a 175.5° negative torsion of the bond N2C48 (corresponds to 84° negative synclinal torsion of N2C50 in 3). Packing along the b axis reveals an infinite two-dimensional sheet with unexpected square-grid packing arrangement of interconnected exclusive offset dimeric head-to-head-assembly named open molecular capsule (Figure 4). Contrary to general

Figure 1. Structural formula and molecular structure of A.

porosity and to form methanol accessible voids has been demonstrated.26 Herein, we report the primary results of this work. In particular, the description of the control of the size of pores in NMCs by retaining unchanged intrinsic porosity while including bulkier alcohol molecules (EtOH and iPrOH) in the voids of their precursors ICs.

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RESULTS AND DISCUSSION Crystallization of calixarene A in a 1:1 mixture of acetonitrile/ methanol, acetonitrile/ethanol, or acetonitrile/isopropanol produces good quality single crystals of ICs 1 (A.CH 3 CN.CH 3 OH),26 2 (A.CH 3 CN.C 2 H 5 OH), and 3 (A.CH3CN.C3H7OH) with the monoclinic space group C2/c and a large unit cell (a over 35 Å) that contains eight molecules of A, eight acetonitrile molecules, and eight alcohol molecules. A comparison of the corresponding unit cells (ESI) point toward a similar crystalline order with very little expansion from 1 to 3 (a = 35.5, 36.1, and 36.0 Å; b = 11.8, 12.0, and 12.0 Å; c = 25.7, 26.2, 26.6 Å for 1, 2, and 3, respectively). All calixarene molecules adopt a cone conformation (Figure 2), with no

Figure 4. Packing along the b axis showing (a) open molecular capsule. 2D Array of open capsules in a unit cell of (b) 1, (c) 2, and (d) 3. (e) Prismatic representation.

calixarene molecular capsules,28 open molecular capsules have only so far been described with curcubit[5]uril, where the dihedral angle of the two molecular capsules attains 38°.29,30 Presently, the open capsule accommodates a pair of acetonitrile molecules and is built of two virtually vertical neighboring calixarenes A, with their mean planes making a dihedral angle of 86.8° in 1, 89.5° in 2, and 89.2° in 3. The driving forces of capsular formation are the hydrogen bond N3···H10 2.733(6) and the vdW-type interaction H30A···H35 2.3948 in 1.27 Twodimensional square-grid framework is often found in metallorganic supramolecular networks. In this context, the “node spacer” approach has been remarkably successful in producing predictable network architectures.2,31−36 On the contrary, purely organic supramolecular networks, built entirely from vdW forces linking simple structural units, occasionally adopt such robust microporous architecture.37−39 Here, the squaregrid structure of four open capsules per unit cell is built entirely from vdW forces and hydrogen bonding linking simple structural units (Figure 4). Antidirectional open capsules (black versus magenta) are associated by short intermolecular contacts CH···C (2.81 Å/C−H31C···C49 in 1) and H···H (2.24 Å/C−H14A···H17−C in 1, 2.38 Å/C−H3A···H6−C in 2, and 2.36 Å/C−H24···H28B−C in 3). Additional indirect

Figure 2. Crystal Structures of Compounds (a) 1, (b) 2, and (c) 3.

crystallographic symmetry, able to include in the hydrophobic cavity an acetonitrile molecule via CHπ interactions (H···C about 2.86 Å). This cone conformation is stabilized by intramolecular hydrogen bonds OH···O (1.88−2.00 Å) and NH···O (2.24−2.37 Å).27 Since alcohol molecules lay outside calixarene cavities, we expect little conformational changes. The calixarene shape can be characterized by the values of dihedral angles between phenolic rings and the mean plane defined by the methylene carbon atoms (ESI). The two aromatic rings bound to the bridge appear more vertical than the other two ones. This differentiation tends to vanish from 1 to 3 thus supporting a small distortion decrease in the cone conformation. Additionally the torsion angles, which characterize the geometry of the 4513



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11.8, 7.5, and 4.3° for 1, 2, and 3, respectively) and, therefore, create sufficient space to promote methylene group reorganization. The distance between rings remains unchanged (4.7 Å). As anticipated, the increase of vdWSA of trapped alcohol molecules will enhance the dimension of the channels. In particular, elliptic cylinder channels seem to be more flexible; indeed, 44% increase in the vdWSA of alcohol (from methanol to ethanol) undergoes 5% enhancement of the surface area of every channel. However, a further 40% increase in the vdWSA (from ethanol to isopropanol) experiences disproportional enlargement of 2% of prismatic channels and 13% of elliptic cylinder channels. In this way, the elliptic channels are 3 times (19% vs 6% overall surface area enhancement) more flexible than prismatic ones. On the basis of these results, a competitive experiment of crystallization of A in a 1:1:1 mixture acetonitrile/methanol/ethanol was performed. Only crystals trapping ethanol in their 3D pores were obtained. We may attribute this preference either to molecular electric properties of both alcohol molecules or to geometrical considerations. Indeed, ethanol is less prone to hydrogen bonding (Hansen’s hydrogen-bonding energy δh = 19.4 J/cm3) and polar (Hansen’s dipolar intermolecular force energy δp = 8.8) than methanol (δh 22.3, δp 12.3); hence geometrical considerations based on larger stable channels with less steric hindrance may rule the affinity to trap ethanol. Unlike MOFs, these NMC precursors are soft, showing a balance between robustness and flexibility. Their flexibility by modulating the volume and the shape of the channels upon induced fit of the solvent guests was also described recently for 1D channels of the [HMTA]·[NIS]4 molecular crystals and silver(I) 3D porous coordination polymer.41,42 SEM examination of crystal surfaces of 3 confirms the tubular texture on both faces (“ac” and “bc”). The laminated morphology (Figure 7b) assumes a parallelepipedic structure

association is established in 2 through ethanol molecules (H··· H 2.19 Å/C−H20B···H7−O, CH···O 2.64 Å/C−H50···O7, and 2.68 Å/C−H54C···O3)27. Similar open capsules (black versus black) are interconnected by intermolecular hydrogen bonding of type CHπ (2.76 Å/C−H21A···C24 in 1) and CH···O (2.62 Å/C−H28B···O6 in 1, 2.66 Å/C−H14B···O3 in 2, and 2.67 Å/C−H14B···O6 in 3). Additional indirect interaction via alcohol molecules is found in 2 (2.19 Å/C− H20B···H7−O, 2.69 Å/C−H15A···O7) and 3 (2.69 Å/C− H45A···O1B, 2.69 Å/O−H1B···O6). The spatial arrangement of open capsules gives rise to peculiar 3D nanoporous architecture generated by interconnecting channels along all three axes. Channels along the a and b axes are delimited by two virtually parallel walls of blocks of antidirectional open capsules, thus defining an infinite parallelepipedic channel (2.45, 2.65, and 2.85 Å minimum width in 1, 2, and 3, respectively) (Figures 4 and 5). This

Figure 5. Crystal packing along the a axis in (a) 1, (b) 2, and (c) 3.

channel is considered as a continuous intersection of both vertical parallelogram tunnels. Those along the a axis have a maximum width of 2.3 Å larger, and the maximum width of those along the b axis is 2.7 Å larger. A similar quadrilateral shape was observed for the 1D parallelogram tunnel micropores of trifluorolactate.40 The distances between successive parallelepipedic channels are 9.16, 9.13, and 9.09 Å for 1, 2, and 3, respectively. Channels along the c axis (Figure 6) characterize infinite hydrophobic elliptic cavities and are 3.4 times larger (8.28, 8.50, and 9.67 Å diameter in 1, 2, and 3). The elliptic cylinder cavity is generated by a fundamental supramolecular unit constituted by four adjacent calixarenes belonging to four independent open capsules. Every calixarene supplies an azacrown bridge, one benzene ring, and one methylene group. The distances between subsequent channels are 17.83, 18.16, and 18.14 Å in 1, 2, and 3, respectively. Noteworthy, diameters of the channels are analogous to those of 1,2dimethoxy-p-tert-butylcalix[4]-dihydroquinone 3D NMC,38 which has a density (1.103 g cm−3) comparable to those of 1 (1.172 g cm−3), 2 (1.025 g cm−3), and 3 (1.143 g cm−3). On removal of isopropanol, the reduction of calculated density is 2fold (7% in 3) of that of methanol removal (4% in 1), suggesting enlargement of pores. We notice that an 85% increase in the van der Waals surface area (vdWSA) of alcohol is correlated to a 17% increase in the diameter of 3D channels and changes in the shape of elliptic cylinder channels. Indeed, the ellipse becomes elongated since opposite benzene rings become more parallel (dihedral angle of

Figure 7. SEM images of (a) monocrystal 3, (b) three faces of 3, (c) face “ab”, (d) face “ac”, and (e) face “bc”.

of the channels, where the sheets are approximately 128 nm width and equidistant by approximately 18 nm (Figure 7d). Hence, the mentioned faces match the crystal planes described above. We observe a crossing structure between vertical lines on face “bc”, which evokes the square-grid architecture (Figure

Figure 6. Cross-section view of (a) the network of elliptic cylinder channels and the shape of the channel in (b) 1, (c) 2, and (d) 3. 4514



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without structural alteration as a way to entrap isopropanol. This may indicate that the flexibility described above is more pronounced at the void. The attained volume of 302 Å3 is still inferior to that of the occasionally described voids in purely organic 3D NMCs (98838 and 2066 Å339), optimistically the number of voids per unit cell in 1, 2, and 3 is 2-fold compared to the same examples. IC 3, with the largest channel size, was selected for thermal behavior investigation. The study was monitored by DSC, IR, PXRD, and PLTM techniques (Figure 9). Upon heating 3 up to 220 °C, DSC analysis shows a broad endothermic event from 25 to 90 °C and a more defined peak at the Tonset of ∼140 °C. A much less energetic endothermic peak is also observed at about 100 °C. After heating to 105 °C, no changes were observed in the IR spectrum of 3; however, the peaks in the PXRD pattern become broader. This is in accordance with solvent removal associated to partial loss of crystallinity but not crystalline order, since the parameters of the unit cell of 3 heated to 120 °C (a = 36.2 Å, b = 11.9 Å, c = 26.6 Å, α = 90.0°, β = 115.8°, γ = 90.0°, bravais lattice = monoclinic C) stay similar to those of 3. After heating to 220 °C, the IR spectrum undergoes few spectral changes. Actually, CO stretch (1690 cm−1) becomes more structured and the O−H stretch (3370 cm−1) band is less broad. Additionally, PXRD pattern experiences large changes by appearance of new diffraction peaks at small angles (peaks at 2θ 4.5, 8.6, 9, and 16.3°) and intensity decrease of original reflections, correlated with a gain of crystallinity. These findings suggest complete solvent removal to occur up to 120 °C without destruction of the crystalline order and a crystal-to-crystal phase transition to occur up to 220 °C. PLTM analysis supports these hints and shows no alteration of the surface of 3 up to 75 °C. A small and

7e). It is not clear why this structure is only locally spread. However, SEM magnification is insufficient to observe elliptic channels on face “ab”. The solvent-accessible voids at the points where the channels intersect draw a cagelike structure similar to that of zeolites. We observe four extrinsic voids per unit cell where each is defined by two pairs of antidirectional diagonal calixarenes A and filled with two alcohol guest molecules (Figure 8). The former

Figure 8. Central extrinsic void in the unit cell of 3.

interact with the calixarene host only through vdW-like interactions and weak hydrogen bonds [minimum intermolecular distances are O7A···O6 2.84(7) Å in 1, O52···C22/C23 3.36(4) Å, O52···H22B/H28B 2.80(2) Å, O52···O3 2.76(3) Å in 2, and O1B···H39B 2.65 Å(7) in 3], suggesting that they could easily be removed without disrupting the supramolecular architecture. Interestingly, the developed extrinsic void is sufficiently elastic to trap different size alcohol molecules. Actually, the void is able to undergo 68% volume uptake

Figure 9. (a) DSC trace for 3. (b) νCO and νO−H bands in the IR spectra of 3 at (1) 20 °C and after heating to (2) 105 °C, and (3) 220 °C. (c) PXRD patterns of 3 at (1) 20 °C, (2) 105 °C, and (3) 220 °C. (d) PLTM images of 3 at 20 °C, 113 °C, and 220 °C. 4515



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stable alteration is produced from 85 to 115 °C. Further heating produces dramatic alteration (starting at about 120 °C), indicating the formation of a new solid phase. We ensured that this transition is not reversible either by DSC, PXRD, and PLTM.

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CONCLUSIONS To summarize, ICs were assembled from single calix[4]azacrown A benefiting from a fascinating square-grid framework of open molecular capsules entirely built from vdW forces and hydrogen bonds linking simple structural units. The 3D porous network presents prismatic and elliptic cylinder channels that intersect to produce extrinsic voids, trapping alcohol molecules. More importantly, we demonstrated by a rational approach that control of pore size can be generated by fixing intrinsic porosity, through acetonitrile guest encapsulation, while altering alcohol size in extrinsic porosity. Indeed, 85% increase of the alcohol vdW surface area generates 25% enhancement of the overall surface area of channels and 68% enlargement of the extrinsic voids. This controlled “breathing behavior” confers flexibility to these ICs. The success in preventing destruction of the original crystalline order of the IC by delicately removing solvents on heating, opens this route for production of 3D NMCs and control of their pore size. Variable size NMCs are promising candidates for selective gas adsorption.

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ASSOCIATED CONTENT

S Supporting Information *

Electronic Supporting Information (ESI) available: CCDC reference numbers 916859 and 916860. Crystallographic data in CIF format, DSC, FTIR, powder X-ray, and PLTM data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+351) 239 410 637. Fax: (+351) 239 829 158. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Portuguese Foundation for Science and Technology for financial support. This work is supported by funds from FEDER (Programa Operacional Factores de Competitividade COMPETE) and from FCT-Fundaçaõ para a Ciência e a Tecnologia under the project PEst-C/FIS/ UI0036/2011.

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REFERENCES

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Generating Flexibility in Inclusion Compounds that Possess Solvent

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