CRYSTAL GROWTH & DESIGN
Remarkably Stable Porous Assembly of Nanorods Derived from a Simple Metal-Organic Framework
2007 VOL. 7, NO. 2 205-207
D. Krishna Kumar, Amitava Das,* and Parthasarathi Dastidar* Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute (CSIR), G B Marg, BhaVnagar 364 002, Gujarat, India ReceiVed October 3, 2006; ReVised Manuscript ReceiVed NoVember 21, 2006
ABSTRACT: A new strategy has been described to create a microporous metal-organic framework by assembling nanorodlike constructs equipped with a complementary hydrogen-bonding surface. Guest-accessible channels are located in the internanorod space generated by the self-assembly of the nanorodlike constructs via complementary hydrogen bonding. Thus, reaction of a hydrogen-bond-equipped and conformationally flexible ligand such as N,N′-bis(3-pyridyl)urea with CuSO4 resulted in the formation of a stable, guest-free microporous MOF wherein a nanorodlike construct is formed that is further self-assembled into a microporous metal-organic framework via selfcomplementary hydrogen bonding involving a urea backbone and sulfate anion. Because of conformational flexibility, a one-dimensional zigzag coordination polymeric network is also formed. The structures are characterized mainly by single-crystal X-ray diffraction techniques. Porous networks of MOF origin have attracted extensive research attention because of their various potential applications.1 However, it is difficult to control the size and shape of the pores as well as the pore-volume availability. Moreover, collapse of porous networks due to guest removal or high temperature is a serious problem in designing useful porous MOFs. Thus, creation of stable porous network of MOF origin always remains a major challenge. Inorganic and organic nanotubes have received much impetus due to their various potential applications.2 It may be envisaged that assembly of nanotubes devoid of a sticky surface (e.g., absence of complementary hydrogen-bonding sites) are expected to pack in a hexagonal close packing manner, thereby leaving no intertube space in the crystal lattice for occluding any guest species. However, nanotubes with complementary hydrogen-bonding functionality on the surface are expected to self-assemble in a fashion directed by the specific hydrogen-bonding interactions, thereby leaving intertube space for guest molecules to accommodate (Scheme 1). This strategy of creating a stable porous network by assembling sticky nanotubes of MOF origin is hitherto unknown. As a part of our research program in the area of MOFs,3 we have recently reported that the ligand N,N′-bis(3-pyridyl)urea L14 gave a square-grid network, whereas its structural isomer, namely N,N′-bis(4-pyridyl)urea, resulted in a 5-fold interpenetrated diamondoid network in the resulting MOFs.3d It is readily envisaged that L1 is able to display various ligating topologies depending on the relative orientation of the pyridyl rings with respect to the urea functionality.5 Consequently, the resulting MOF architecture is dependent on the ligand conformation. For example, syn-anti conformation should give rise to 1D zigzag polymeric chain, whereas syn-syn or anti-anti conformation may result in the formation of a metal-organic macrocycle (Scheme 2). In the latter case, the metal-organic macrocycle can be further linked to form a nanotubular construct using a suitable bridging ligand such as sulfate.6 Because the ligand L1 has a hydrogenbonding functionality such as urea, which is capable of forming chelate hydrogen-bonding interactions with an oxoanion such as sulfate (urea-sulfate synthon), it is expected that the resulting nanotubular constructs would self-assemble further, directed by these complementary hydrogen-bonding interactions and thereby generating a microporous MOF. Thus, we have reacted L1 with CuSO4 in a 1:1 molar ratio with the hope of creating a nanotubular construct that would self-assemble into a porous network. Crystals of different morphologies (needle 1 and block 2) were obtained from the reaction mixture7 (see the Supporting Information). X-ray structural analyses of these two crystals8 revealed that the crystals * To whom correspondence should be addressed. E-mail: dastidar@ csmcri.org,
[email protected] (P.D);
[email protected] (A.D.). Fax: 91-278-2567562.
Scheme 1
Scheme 2
of 1 and 2 are MOFs of nanorod and zigzag architectures, respectively, as envisaged in Scheme 2. In 1, the ligand adopts a syn-syn conformation with significant molecular planarity (urea/pyridyl dihedral angles of 2.2 and 3.3°). Two such ligands connect the Cu(II) metal center in such a way that a metal-organic macrocyle is formed. The metal center is found to be slightly distorted square-pyramidal (∠N-Cu-O )
10.1021/cg0606693 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/23/2006
206 Crystal Growth & Design, Vol. 7, No. 2, 2007
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Figure 2. Optical micrograph of the crystal of 2 used for data collection (left); 1D polymeric zigzag network of MOF 2 displaying hydrogen-bonded water tetramer (right).
Figure 1. Optical micrograph of the crystal of 1 used for data collection (left); crystal structure illustration of 1; (a) metal-organic macrocycle; (b) polymeric network of nanorod-like construct displaying sulfate (orange) as linker of the macrocycles; (c) self-assembly of the nanorodlike constructs displaying microporous architecture; (d) titled view of assembly of four independent nanorodlike constructs.
85.2(2)-100.1(2)°). The two pyridyl N atoms of L1 occupy the equatorial positions in a trans fashion. One water molecule is found to be coordinated to the other equatorial position. Sulfate, on the other hand, acts as a bridging ligand by coordinating equatorial and apical positions of the metal centers of the adjacent metalorganic macrocycles, thereby forming a 1D infinite nanotubular construct. A space of ∼1.2 × 6.2 Å2 (considering van der Waals radii) is present in the metal-organic macrocycle. However, the macrocycles are arranged on top of each other in a slightly offset fashion, presumably due to the coordinated water molecule that occupies one of the equatorial positions. As a result, a close-ended nanotubular MOF is formed and the space within the nanotube is occupied by the metal-bound water molecule, which is further hydrogen bonded with the CdO oxygen atoms of the urea moiety of each macrocycle (O‚‚‚O ) 2.757(5) Å; ∠O-H‚‚‚O ) 157(9)°). Therefore, the space within the nanotubular construct is not available for further guest occlution. Thus, this nanotubular assembly can best be described as comprising nanorods. However, the outer surface of the nanorod is equipped with complementary hydrogenbonding functionalities arising from urea N-H and sulfate O atoms. In the crystal structure, it can be seen that each nanorod is surrounded by four neighboring nanorods sustained by complementary N-H‚‚‚O hydrogen bonding involving urea and sulfate moieties (N‚‚‚O ) 2.715(6)-2.833(5) Å; ∠N-H‚‚‚O ) 152.3165.3°), resulting in the formation of a 2D hydrogen-bonded network with a continuous internanorod channel running down the a-axis (Figure 1). No electron density peaks could be found within the channel. PLATON SQUEEZE9a calculations indicate that there are no electron densities left in the unit cell.7 This indicates that the assembly of the nanorods through specific hydrogen-bonding interactions results in the formation of a stable, guest-free microporous network. PLATON9b calculations show that the solventaccessible void is 218.4 Å3 per unit-cell volume. A careful examination of the internanorod channel space reveals that the inner wall of the channel is highly hydrophobic. Because the crystals of 1 are obtained from EtOH/water, it is expected that the solvent molecules are not strongly bound in the hydrophobic space of the internanorod channel, thereby leaving the crystal lattice before or during data collection. Thermal analyses7 of 1 confirmed the absence of any guest species that corroborates well with the X-ray structure and the framework in 1 collapses at a temperature as high as 260 °C. On the other hand, block-shaped crystal 2 turned out to be a 1D zigzag polymeric network,7 understandably arising because of the syn-anti conformation of L1 (Figure 2).
It is interesting to note that the CdO stretching bands of 1 and 2 appear at 1673 and 1703 cm-1, respectively. The 30 cm-1 shift of the CdO band may be attributed to the difference in hydrogenbonding environment of the urea carbonyl in these two MOFs; in 1, the urea O atom is hydrogen bonded to two coordinated water molecules in a bifurcated fashion, whereas that in 2 is hydrogen bonded with one solvate water molecule. This is further corroborated with the thermal analyses,7 wherein the water molecules of 1 are released at much higher temperature (136.4°) than that of in 2 (87.2°). It may be noted that both MOFs 1 and 2 are obtained in a single pot. To find out the experimental conditions that result in selective preparation of one of the MOFs, we reacted the components using an instant mixing method.10 XRPD of the resultant microcrystals7 shows an excellent match with that of the simulated pattern obtained from single-crystal data of 2, indicating the exclusive formation of zigzag MOF under this experimental condition, whereas the layering method always resulted in the formation of both nanorod and zigzag MOFs. The complementary hydrogen-bonding surface (urea-sulfate synthon) of the nanorods helps them self-assemble into a highly stable guest-free microporous MOF. Thus, by exploiting the conformational flexibility and hydrogen-bonding backbone (urea) of the ligand L1 and judicial choice of the counterion cum linker ligand (sulfate), we have been able to self-assemble the resulting nanorodlike MOF via complementary hydrogen-bonding interactions involving the urea moiety and sulfate ion into a highly stable, guest-free microporous framework. This strategy of assembling the discrete sticky nanorodlike constructs into a hierarchical microporous framework is the first example in the literature. Acknowledgment. We thank the Department of Science & Technology (DST), New Delhi, India, for financial support. D.K.K. thanks CSIR, New Delhi, India, for a SRF. Supporting Information Available: Syntheses, FT-IR data, crystallographic data (CIF) in CIF format, thermal analyses plot for 1 and 2, XRPD comparison plot for 2. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Saalfrank, R. W.; Demleitner, B. In Transition Metals in Supramolecular Chemistry; Perspectives in Supramolecular Chemistry; J.-P. Sauvage, Ed.; Wiley-VCH: Weinheim, Germany, 1999; Vol. 5, p 1. (b) Fujita, M. Chem. Soc. ReV. 1998, 27, 417. (c) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (d) Schnebeck, R.-D.; Freisinger, E.; Lippert, B. Angew. Chem., Int. Ed. 1999, 38, 168. (e) Matsuda, R.; Kitaura, R.; Kitagawa, S.; Kubota, Y.; Belosludov, R. V.; Kobayashi, R. C.; Kakamoto, H.; Chiba, T.; Takata, M.; Kawazoe, Y.; Mita, Y. Nature 2005, 436, 238. (f) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M. Yaghi, O. M. Science 2003, 300, 1127. (g) Pan, L.; Olson, D. H.; Ciemnolonski, L. R.; Heddy, R.; Huang, X.-Y.; Li, J. Angew. Chem., Int. Ed. 2006, 45, 616. (h) Li, Y.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 726. (i) Kepert, C. J. Chem. Commun. 2006, 695. (2) (a) Su, C.-Y.; Goforth, A. M.; Smith, M. D.; Pellechia, P. J.; zur Loye, H.-C. J. Am. Chem. Soc. 2004, 126, 3576 and references cited therein. (b) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem., Int. Ed. 2001, 40, 988. (3) (a) Krishna Kumar, D.; Ballabh, A.; Jose, D. A.; Dastidar, P.; Das, A. Cryst. Growth Des. 2005, 5, 651. (b) Krishna Kumar, D.; Das, A.; Dastidar, P. Cryst. Growth Des. 2006, 6, 216. (c) Krishna Kumar,
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(4) (5) (6) (7) (8)
D.; Das, A.; Dastidar, P. J. Mol. Struct. 2006, 796, 139. (d) Krishna Kumar, D.; Jose, D. A.; Das, A.; Dastidar, P. Inorg. Chem. 2005, 44, 6933. (e) Krishna Kumar, D.; Das, A.; Dastidar, P. Cryst. Growth Des. 2006, 6, 1903. Krishna Kumar, D.; Jose, D. A.; Das, A.; Dastidar, P. Chem. Commun. 2005, 4059. Custelcean, R.; Moyer, B. A.; Bryantsev, V. S. Hay, B. P. Cryst. Growth Des. 2006, 6, 555. CSD 5.27, Nov. 2005 release: A search motif having sulphate connected to any transition metal in a polymeric fashion resulted in 162 hits. See the Supporting Information. Crystal data for 1: C11H12CuN4O6S, fw ) 391.85, 0.34 × 0.10 × 0.05 mm3, monoclinic, P21/c, a ) 4.9618(14) Å, b ) 16.979(5) Å, c ) 18.354(5) Å, β ) 91.968(5)°, V ) 1545.3(8) Å3, T ) 298(2) K, Z ) 4, Fcalcd ) 1.684 g cm-3, µ ) 1.584 mm-1, F(000) ) 796; 8498 reflections collected. Final residuals (for 213 parameters) were R1 ) 0.0643, wR2 ) 0.1426 for 2534 reflections with I > 2σ(I), and R1 ) 0.0958, wR2 ) 0.1672, GOF ) 1.031 for all 3494 reflections.
Crystal Growth & Design, Vol. 7, No. 2, 2007 207 Max/min residual electron density: 0.930/-0.603 e/Å3. Crystal data for 2: C11H18CuN4O9S, fw ) 445.89, 0.36 × 0.24 × 0.18 mm3, monoclinic, P21/c, a ) 7.5168(8) Å, b ) 21.771(2) Å, c ) 12.1053(10) Å, β ) 122.872(5)°, V ) 1663.8(3) Å3, T ) 100(2) K, Z ) 4, Fcalcd ) 1.780 g cm-3, µ ) 1.496 mm-1, F(000) ) 916; 6502 reflections were collected. Final residuals (for 235 parameters) were R1 ) 0.0478, wR2 ) 0.1162 for 1754 reflections with I > 2σ(I), and R1 ) 0.0607, wR2 ) 0.1230, GOF ) 1.036 for all 2163 reflections. Max/min residual electron density: 0.749/-0.662 e/Å3. (9) (a) Van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, 194. (b) Spek, A. L. PLATON-97; University of Utrecht: Utrecht, The Netherlands, 1997. (10) Reaction was performed by immediate mixing of an ethanolic solution (10 mL) of the ligand L1 (42.8mg, 0.2mmol) with an aqueous solution of Cu(SO4)2‚5H2O (49.8 mg, 0.2 mmol) in a 50 mL RB flask. The mixture was stirred at room temperature for 10 minutes and filtered.
CG0606693
