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A Stable Hydrogen-Bonded Coordination Network with Removable Guests Greg A. Hogan, Nigam P. Rath, and Alicia M. Beatty* Department of Chemistry and Biochemistry and the Center for Nanoscience, University of Missouri-St. Louis, St. Louis, Missouri 63121, United States
bS Supporting Information ABSTRACT: Zinc(II) coordination complexes, each having two linearly oriented peripheral carboxylate units, combine with 3,30 dimethylbenzidene pillars to create channeled structures that host small organic guests. The porous solids reported here are connected solely via strong, charge-assisted hydrogen bonds and contain guests that vary in size, shape, and degree of hydrophobicity. The hydrogen-bonded framework is maintained upon guest loss, under vacuum and up to ∼200 °C. The strength and flexibility of these frameworks make them ideal candidates for molecular storage, separations, guest exchange, and guest transport.
A
lthough it is common to presume that hydrogen-bonded materials are too weak to withstand processes such as guest loss, for many years it has been known that intermolecular forces work in concert to create stable molecular networks in crystalline solids. In fact, we and others have demonstrated that solids comprising hydrogen-bonded metalorganic networks1 are reproducible,2 thermally stable, and resilient upon undergoing dynamic processes.3 Very recently, it was demonstrated that even van der Waals forces alone can lead to porous frameworks that remain intact after guest loss.4 Strong hydrogen bonds have been used in creating layered hostguest materials for both organicand metal-organic framework solids.58 Hydrogen-bonded networks resulting from molecular components that can participate in strong hydrogen bonds, such as charge-assisted hydrogen bonds between ammonium and carboxylate substituents, can be surprisingly stable.3,6,9 Creating frameworks that can accommodate a variety of guests is of great interest in both the organic10 and inorganic11 solid state communities because these materials have the potential for properties such as separations,5 storage,12 and catalysis.13 Porous frameworks that are created by combining molecular components that are easily modified, that are thermally stable, that can host a number of different types of guests, and that remain intact after the loss of these guests, may satisfy the requirements that exhibit these desirable properties. Synthesis of pillared, porous two-dimensional (2-D) metal organic hybrids have been limited to the use of organic pillars with metal-oxides and metal-phosphates14 and hydrogenbonded metal-hexaammines,6 all of which have fixed distances between pillars; therefore, the metrics of pores or channels are limited to the intermetal distances. However, in the study reported here, modifiable pyridine dicarboxylate ligands are used to create the metal-containing layer, and a modifiable diamine is used as the pillar (Figure 1). This means that the metrics and the r 2011 American Chemical Society
chemical nature of the pores/channels are readily changeable by increasing the length, width, and substituents of the organic ligands and pillars. It is important to note also that the hydrogenbonded framework described here does not depend on water ligands or other small ligands or counterions; the strong ammonium-carboxylate hydrogen bonds connect in three dimensions to form the framework, as designed. We have already demonstrated that metalorganic dicarboxylic acids (i.e., a coordination complex with peripheral carboxylic acid moieties) combined with organic primary amines yield guest-containing layered solids upon crystallization.3 Here we report that using the same approach with diamines has allowed us to create porous hostguest solids that encapsulate a variety of guest molecules and that are stable to guest loss up to 200 °C. The synthesis, thermal behavior, and stability of the framework, along with the crystal structures of four hostguest solids are detailed. Synthesis of the host is accomplished by using a metal-containing dicarboxylic acid [1, Zn(2,4-HPDCA)2(H2O)2], where 2,4-H2PDCA is 2,4-pyridinedicarboxylic acid. When 1 is combined with 3,30 -dimethylbenzidine, the neutral framework 2 [(3,30 dimethylbenzidinium)][Zn(2,4-PDCA)2(H2O)2], is produced. A full list of the guest molecules that were incorporated successfully, along with a list of molecules that were not found to crystallize inside the framework, can be found in the Supporting Information. Here we report crystal structures of 2 with four different guest molecules: 2 3 p-xylene (2a), 2 3 nitrobenzene (2b), 2 3 hexanol (2c), and 2 3 acetone (2d), as well as evidence of the remarkable stability of the framework, which maintains its structural integrity upon guest loss and reuptake. Received: June 23, 2011 Revised: July 12, 2011 Published: July 19, 2011 3740
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Figure 1. Schematic of the metal-containing layer component 1 (top) and the organic pillar 3,30 -dimethylbenzidene (bottom) used to assemble the channeled, hydrogen-bonded framework 2.
Figure 3. Space-filled representation of the crystal structure of 2b (guest = nitrobenzene) with views down the channel (top) and perpendicular to the channel (bottom).
Figure 2. Stick representation of the structure of 2c (guest = hexanol), showing the location of the hydroxyl group in red, pointed toward the layer (ab plane).
Compounds 2a2d were synthesized by allowing the neutral dicarboxylic acid 1 to react with dimethylbenzidine in the presence of guest molecules, or in the case of 2d, solvents of crystallization. Zn(HPDCA)2(H2O)2 (0.025 g, 0.06 mmol) was suspended in 2 mL of methanol followed by the addition of o-tolidine (0.012 g, 0.06 mmol) in 2 mL of methanol. The cloudy suspension became clear upon the addition of 1 mL of both dimethylformamide and water. Finally, 1 mL of the guest molecule was added and allowed to sit until crystals formed. The crystal structures show the presence of one diammonium counterion per metal dianion.1518 The host frameworks in each compound 2a2d are virtually identical (Figure 2). The dicarboxylate and diammonium ions are present in a 1:1 ratio. The tilted metal dianions comprise sheets that are held together by ammonium-carboxylate NH+ 3 3 3 O hydrogen bonds [r(N 3 3 3 O) 2.70 2.77 Å]. This hydrogen-bonded layer is identical to that seen previously with monoammonium counterions, although these previous compounds were close-packed, with no guest inclusion.3a In this case, the sheets are connected to each other by the linear diammonium cations, with interlayer distances of 19.6319.75 Å, as measured from the Zn(II) to
Zn(II) interlayer distances along the cell axes. This results in a three-dimensional (3-D) hydrogen-bonded porous network. The diammonium pillars are arranged orthogonal to the plane of the hydrogen-bonded sheet, resulting in a void space that accommodates a variety of guests ranging in size and shape (Figure 3, top). The aryl rings of the cations are arranged in a louvered fashion, so that the sides of each channel are walled off (Figure 3, bottom). Therefore, in contrast to many porous/ channeled metalorganic structures, molecular transport can only take place in one direction. The distance between channel walls varies from 8.27 to 8.51 Å as measured from the pillar nitrogen atoms along the cell axes. The channel width varies by up to 3% from structure to structure, demonstrating that the hydrogen bonds allow some flexibility to accommodate guest molecules. Each guest has disorder imposed by a crystallographic inversion center and can be so badly disordered that the identity of the guest cannot be confirmed by single crystal X-ray diffraction and must be used in tandem with results from thermogravimetric analysis (TGA). Because all guest molecules are located on an inversion center, each has at least two orientations. The most disordered guest reported here is p-xylene (2a), and this structure also exhibits the largest channel size (8.51 Å), which is presumably required to accommodate a mobile hydrophobic guest that is not anchored to any part of the channel. Ironically, crystals of 2a were of the best quality by far and formed readily overnight. Therefore, the quality of the crystal structure of 2a exceeds that of the other compounds. The nitrobenzene guest in 2b exhibits less disorder and is oriented with the nitrobenzene substituent directed toward the hydrophilic layer. Similarly, hexanol (2c) is well ordered and is arranged with the hydroxyl group pointing toward the layer. Interestingly, use of simple linear alkanes as guest templates in 3741
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Table 1. Results of TGA Analysis, Theoretical Weight Loss Versus Actual Weight Loss for 2a2d theoretical
actual weight
weight loss
loss
temperature at
(1 eq guest and 2
(1 eq of
which guest and
compound
H2O) (%)
guest and 2 H2O) (%)a
water loss begins and is completed (°C)
2a (p-xylene)
18.9
21.1
100, 212
2b (nitrobz)
20.7
19.1
50, 215
2c (hexanol)
18.5
18.9
125, 212
2d (acetone)
13.4
5.8
25b, 170
a
Averaged from two (2a, 2b) or three (2c, 2d) trials. b Acetone is mostly gone before the TGA experiment commences.
solution have not yielded crystals containing those alkanes, but the corresponding 1-alcohols have been co-crystallized, presumably due to the anchoring effect of the hydroxyl substituent. Acetone (2d), on the other hand, is oriented with the carbonyl oxygen atom directed down the channel. The presence of the guests and the thermal stability of the framework can be demonstrated using TGA (Table 1).19 The observed weight loss occurs stepwise and begins at different temperatures, depending on the guest. Up to 130 °C both the guest and the water ligands are lost, sometimes in two stages. It can be seen in Table 1 that when the guest is less volatile (p-xylene, nitrobenzene, and hexanol) the values for the theoretical loss of guest and the observed loss are quite similar. It should be noted that the number and type of incorporated guests is extensive (Tables S.1 and S.2 in Supporting Information). On the basis of a visible color change upon decomposing (during melting point determinations), we assume that the color change corresponds with a collapse of the framework at around 210 °C for each compound. The derivative plots of the TGA data (Figures S.7S.9) show events at between 212 and 215 °C that match with what can be seen visually upon heating. These materials are stable to air, as established by handling of the samples for X-ray diffraction (XRD). Powder XRD patterns were also determined for each compound 2a2d, Figure 4. The powder patterns are very similar to each other and to the theoretical pattern for 2d, which is shown as a comparison. This is expected, as the structures obtained from single-crystal X-ray show virtually identical hydrogen-bonded frameworks and nearly equal cell parameters for all four compounds. It must be noted that the acetone-containing compound, 2d, has been shown by TGA to have little or no guest remaining after grinding vide supra. This has little effect on the structure, as evidenced by the powder pattern, demonstrating that the framework is maintained after guest loss. It is clear that there is a flexibility of the hydrogenbonded framework that gives rise to the slightly shifted peaks. The 0 0 1 peak shifts to higher 2θ, consistent with a flattening out of the tilted metal-dicarboxylate component (significantly shrinking the c axis from 19.6 to 17.7 Å). The b axis also decreases slightly (from 8.1 to 7.9 Å), while the a axis increases slightly from 5.1 to 5.5 Å. The structure containing the nitrobenzene guest (2b) also shows evidence of an empty framework combined with a filled framework, with both the original peaks and the shifted peaks showing up together. Efforts are currently underway to model the structure of the empty framework by density functional theory (DFT) calculations and powder structure solution.
Figure 4. Powder X-ray patterns for 2ad (guests = p-xylene, nitrobenzene, hexanol, and acetone, in order from top down), including the theoretical pattern for 2d (bottom).
Further stability studies of the guest-free framework were accomplished in the following manner: Crystals of 2d were ground and heated under a vacuum to remove the acetone guest molecules. The solid was then sonicated in ethyl acetate for 1 h, dried, and analyzed by powder XRD. The XRD showed that the porous framework was filled upon sonication in ethyl acetate (Figure S.1 in Supporting Information). The solid was then placed in a small round-bottom flask, and a vacuum was applied overnight with slight heat (∼80 °C) to remove the guest molecules. The sample was again analyzed by XRD, this time showing the empty framework (Figure S.2). This process of guest insertion and removal was repeated a total of seven times, and little or no loss of crystallinity was observed. The implications of this framework stability are significant: these frameworks and those made from similar components have the potential for guest exchange, guest transport through the channels, and separations of small molecules based on the frameworks’ molecular preferences. Here we have reported the successful use of a metalorganic dicarboxylic acid in combination with a rigid organic diamine to create a channeled layered solid that is stable to guest loss. We have demonstrated that the use of molecular building blocks containing peripheral hydrogen-bonding substituents is an effective way to build networks containing channels or pores, and in the future we will be able to increase the metrics of channel and/ or pore sizes by using longer, wider ligands and longer diamines for construction. Using the host 2, a significantly large void volume allows for a variety of small molecular guests to be crystallized inside the channeled host framework. Further, the channel is in fact one-dimensional — close packed aryl substituents prevent penetration of the walls, so molecular transport is possible in only one dimension. Most significantly, though held together “only” by hydrogen bonds, the host framework withstands the repeated incorporation and loss of guest molecules, a phenomenon that is a testament to the stability that can be imparted on metalorganic frameworks by using charge-assisted 3742
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’ ASSOCIATED CONTENT
bS
Supporting Information. Further experimental details, thermal analysis, and X-ray diffraction information; crystallographic information file. This material is available free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the University of Missouri-St. Louis and NSF-Career #0645758 for financial support. ’ REFERENCES (1) (a) Beatty, A. M. Coord. Chem. Rev. 2003, 246, 131. (b) Dalrymple, S. A.; Parvez, M.; Shimizu, G. K. H. Inorg. Chem. 2002, 41, 6986. (c) Braga, D. Dalton 2000, 3705. (d) Braga, D.; Maini, L.; Polito, M.; Grepioni, F. Struct. Bonding (Berlin) 2004, 111, 1. (e) Shimizu, G. K. H.; Vaidhyanathan, R.; Taylor, J. M. Chem. Soc. Rev. 2009, 38, 1430. (2) Burrows, A. D.; Chan, C.-W.; Chowdhry, M. M.; McGrady, J. E.; Mingos, D. M. P. Chem. Soc. Rev. 1995, 24, 329. (3) (a) Beatty, A. M.; Helfrich, B. A.; Hogan, G. A.; Reed, B. A. Cryst. Growth Des. 2006, 6, 122. (b) Chen, C.-L.; Beatty, A. M. J. Am. Chem. Soc. 2008, 130, 17222. (c) Aakeroy, C. B.; Beatty, A. M.; Leinen, D. S. Angew. Chem., Int. Ed. 1999, 38, 1815. (4) (a) Msayib, K. J.; Book, D.; Budd, P. M.; Chaukura, N.; Harris, K. D. M.; Helliwell, M.; Tedds, S.; Warren, J. E.; Xu, M.; McKeown, N. B. Angew. Chem., Int. Ed. 2009, 48, 3273–3277. (b) Bezzu, C. G.; Helliwell, M.; Warren, J. E.; Allan, D. R.; McKeown, N. B. Science 2010, 327, 1627–1628. (5) (a) Ward, M. D.; Russell, V. A. NATO ASI Ser., Ser. C 1997, 499, 397–407. (b) Pivovar, A. M.; Holman, K. T.; Ward, M. D. Chem. Mater. 2001, 13, 3018. (c) Holman, K. T.; Pivovar, A. M.; Ward, M. D. Science 2001, 294, 1907. (6) (a) Reddy, D. S.; Duncan, S.; Shimizu, G. K. H. Angew. Chem., Int. Ed. 2003, 42, 1360. (b) Dalrymple, S. A.; Shimizu, G. K. H. J. Am. Chem. Soc. 2007, 129, 12114–12116. (7) Swift, J. A.; Ward, M. D. Chem. Mater. 2000, 12, 1501–1504. (8) (a) Dechambenoit, P.; Ferlay, S.; Kyritsakas, N.; Hosseini, M. W. J. Am. Chem. Soc. 2008, 130, 17106–17113. (b) Brunet, P.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1997, 119, 2737–2738. (c) Demers, E.; Maris, T.; Wuest, J. D. Cryst. Growth Des. 2005, 5, 1227–1235. (d) Trolliet, C.; Poulet, G.; Tuel, A.; Wuest, J. D. J. Am. Chem. Soc. 2007, 129, 3621–3626. (e) Yaghi, O. M.; Li, H.; Groy, T. L. J. Am. Chem. Soc. 1996, 118, 9096–9101. (f) Aakeroy, C. B.; Beatty, A. M.; Leinen, D. S. Angew. Chem., Int. Ed. 1999, 38, 1815–1819. (g) Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Tejada, J.; Rovira, C.; Veciana, J. J. Am. Chem. Soc. 2004, 126, 730–731. (h) Uemura, K.; Kitagawa, S.; Fukui, K.; Saito, K. J. Am. Chem. Soc. 2004, 126, 3817–3828. (i) Wan, C.-Q.; Li, G.-S.; Chen, X.-D.; Mak, T. C. W. Cryst. Growth Des. 2008, 8, 3897–3901. (j) Wuest, J. D. Chem. Commun. 2005, 5830–5837. (9) (a) Dechambenoit, P.; Ferlay, S.; Kyritsakas, N.; Hosseini, M. W. J. Am. Chem. Soc. 2008, 130, 17106–17113. (b) Wuest, J. D. Chem. Commun. 2005, 5830–5837. (10) (a) Russell, V. A.; Etter, M. C.; Ward, M. D. J. Am. Chem. Soc. 1994, 116, 1941. (b) Biradha, K.; Dennis, D.; MacKinnon, V. A.;
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Sharma, C. V. K.; Zaworotko, M. J. J. Am. Chem. Soc. 1998, 120, 11894. (c) Aakeroy, C.; Beatty, A. M. Aust. J. Chem. 2001, 54, 409. (11) (a) Robson, R. Dalton Trans. 2008, 5113. (b) Spokoyny, A. M.; Kim, D.; Sumrein, A.; Mirkin, C. A. Chem. Soc. Rev. 2009, 38, 1218. (c) Maji, T. K.; Kitagawa, S. Pure Appl. Chem. 2007, 79, 2155. (d) Adelani, P. O.; Albrecht-Schmitt, T. E. Inorg. Chem. 2009, 48, 2732. (12) (a) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Acc. Chem. Res. 1998, 31, 474. (b) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127. (13) Endo, K.; Koike, T.; Sawaki, T.; Hayashida, O.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1997, 119, 4117. (14) Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Tejada, J.; Rovira, C.; Veciana, J. J. Am. Chem. Soc. 2004, 126, 730–731. (15) Crystal data for 2a, C36H38N4O10Zn: Mr 752.07, triclinic, space group P1 (No. 2), a = 5.1842(5), b = 8.5087(9), c = 19.5787(19) Å, R = 89.508(6), β = 85.077(7), γ = 80.522(6)°, V = 848.69(15) Å3, Z = 1, dcalcd = 1.472 Mg/m3, λ(Mo KR) = 0.7107 Å, crystal size 0.31 0.25 0.10 mm3. Data were collected on a Bruker Kappa Apex II CCD diffractometer using Mo KR radiation at 100 K. A total of 14 549 reflections with (2.43 < θ < 35.30°) were processed and considered significant with Inet > 2σ(Inet). Structure solution, refinement, and molecular graphics were carried out with SHELXTL-PLUS software package.17 Final residuals for Inet > 2σ(Inet) were R1 = 0.0407 and wR2 = 0.0981 (GOF = 1.031) for 289 parameters. (16) Crystal data for 2b, C34H33N5O12Zn: Mr 769.02, triclinic, space group P1 (No. 2), a = 5.2035(2), b = 8.3292(3), c = 19.7543(7) Å, R = 90.0910(10), β = 94.6460(10), γ = 100.0870(10)°, V = 840.06(5) Å3, Z = 1, dcalcd = 1.520 Mg/m3, λ(Mo KR) = 0.7107 Å, crystal size 0.28 0.28 0.15 mm3. Data were collected on a Bruker Kappa Apex II CCD diffractometer using Mo KR radiation at 100 K. A total of 6322 reflections with (2.07 < θ < 27.59°) were processed and considered significant with Inet > 2σ(Inet). Structure solution, refinement, and molecular graphics were carried out with SHELXTL-PLUS software package. Final residuals for Inet > 2σ(Inet) were R1 = 0.0362 and wRq2 = 0.0939 (GoF=1.050) for 293 parameters. (17) Crystal data for 2c, C34H42N4O11Zn: Mr 748.09, triclinic, space group P1 (No. 2), a = 5.2571(8), b = 8.2727(13), c = 19.632(4) Å, R = 91.072(9), β = 96.028(9), γ = 99.886(8)°, V = 835.9(2)Å3, Z = 1, dcalcd = 1.486 Mg/m3, λ(Mo KR) = 0.7107 Å, crystal size 0.41 0.13 0.11 mm3. Data were collected on a Bruker Kappa Apex II CCD diffractometer using Mo KR radiation at 100 K. A total of 5710 reflections with (3.13 < θ < 27.86°) were processed and considered significant with Inet > 2σ(Inet). Structure solution, refinement, and molecular graphics were carried out with SHELXTL-PLUS software package. Final residuals for Inet > 2σ(Inet) were R1 = 0.0834 and wR2 = 0.2107 (GOF = 1.016) for 255 parameters. (18) Crystal data for 2d, C31H34N4O11Zn: Mr 703.99, triclinic, space group P1 (No. 2), a = 5.1750(9), b = 8.3571(13), c = 19.712(4)Å, R = 90.174(7), β = 95.130(7), γ = 100.135(7)°, V = 835.7(3) Å3, Z = 1, dcalcd = 1.399 Mg/m3, λ(Mo KR) = 0.7107 Å, crystal size 0.26 0.14 0.12 mm3. Data were collected on a Bruker Kappa Apex II CCD diffractometer using Mo KR radiation at 100 K. A total of 15 678 reflections with (2.70 < θ < 23.88°) were processed and considered significant with Inet > 2σ(Inet). Structure solution, refinement, and molecular graphics were carried out with SHELXTL-PLUS software package. Final residuals for Inet > 2σ(Inet) were R1 = 0.0733 and wR2 = 0.1957 (GOF = 1.042) for 242 parameters. (19) TGA results were obtained using a method in which the samples are ground and then heated to and held at 40 °C for 5 min in order to ensure that any solvent or volatile guest molecules located on the surface of the crystals are driven off before the experiment is performed.
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