CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 5 1059-1061
Communications Rational Synthesis and Characterization of Robust Microporous Metal-Organic Frameworks with Base Functionality Dong Mok Shin,† In Su Lee,† and Young Keun Chung* Intelligent Textile System Research Center and Department of Chemistry, College of Natural Sciences, Seoul National UniVersity, Seoul 151-747, Korea ReceiVed August 24, 2005; ReVised Manuscript ReceiVed March 27, 2006
ABSTRACT: Employing pyrimidine-containing ligands such as 1-(4-pyridyl)-2-(4-pyrimydyl)ethene (L1) and 1-(4-pyridyl)-2-(3-methyl4-pyridyl)ethene (L3) in assembly with Co(SCN)2 affords robust porous coordination polymers with uncoordinated nitrogen atoms on the channel walls. Metal-organic framework (MOF) solids comprise a group of materials having channels and pores analogous to those found in zeolites. MOFs are typically synthesized by the assembly of organic “spacers” and metal ion “nodes”, and these compounds have the advantage to perform gas storage, guest inclusion, catalysis, and ion exchange.1-5 With the goal to realize size-, shape-, and chemical-selective guest adsorption and catalysis, we and others have pursued the synthesis of porous frameworks with specific chemical affinity.4,6 While several selective guest-absorbing MOFs with acidic functional groups in their channels have been reported,7 examples with base functionality are quite rare.4c,6c The difficulty in the rational incorporation of basic moieties can be attributed to the coordination of these groups to metal ions during the assembly. Recently, we demonstrated that pyrimidine-containing ligands can be utilized to access two-dimensional-framework-based channel structures with basic nitrogen atoms on their walls.8 To obtain robust structures with base-functionalized channels, 1-(4-pyridyl)-2-(4pyrimidyl)ethene (L1) was used as a spacer and reacted with Co(NCS)2. L1 is an analogue of 2-bis(4-pyridyl)ethane (L2), which is one of the most frequently employed ligands in MOF synthesis. We anticipated that the assembly reaction of L1 and Co(SCN)2 would afford a porous framework that is isostructural with that obtained using L2, with uncoordinated pyrimidyl nitrogen atoms on the channel walls.9 In the course of assembly study, we encountered a polymorphism involving a porous structure generated by the interpenetration of two-dimensional grids and a threedimensionally networked structure. The unique topology of the three-dimensional framework found in this work is unprecedented, to the best of our knowledge. We set out to control the polymorphism to get porous solids as the only product. In addition, the use of mixtures of ligands of L1 and 1-(4-pyridyl)-2-(3-methyl-4pyridyl)ethene (L3) provided access to an isostructural solid with half the number of base moieties in the channel. The organic ligands used in this work are depicted in Chart 1. Here we present their synthesis, characterization, and guest absorption properties. The assembly reaction was carried out by a slow diffusion of a * To whom correspondence should be addressed. E-mail: ykchung@ snu.ac.kr. † Both of these authors contributed equally to this work.
Figure 1. 1-D chain structure of the coordination polymer 1, [Co(L1)(DMF)2(NCS)2]∞.
Figure 2. (a) 2-D grid layer of coordination polymer 2. (b) 3-D porous structure of 2 with square channels formed by perpendicular interpenetration of 2-D grids. Uncoordinated pyrimidyl nitrogen atoms are shown in red.
Chart 1.
Structures of the Organic Ligands Used in This Work
DMF solution containing L1 into an aqueous solution of Co(NCS)2, which was maintained at a temperature of 90 °C over several days. Long blocklike orange crystals of 1, needlelike red crystals of 2, and blocklike red crystals of 3 were obtained by slow evaporation of the solvent. X-ray crystallographic analysis of 110 reveals the formation of a chain structure. In the structure, each cobalt center is coordinated by two L1, two SCN, and two DMF ligands. Thus, it serves as a linear node for extending a one-dimensional coordination-polymer chain with the formulation [Co(L1)(DMF)2(NCS)2]∞ (Figure 1).
10.1021/cg0504399 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/13/2006
1060 Crystal Growth & Design, Vol. 6, No. 5, 2006
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Figure 4. 2-D grid layer of coordination polymer 4, [Co(L1)(L3)(NCS)2]∞, formed by assembly of Co(SCN)2 with mixed ligands of L1 (green) and L2 (red). SCN ligands are omitted for clarity. Table 1. Guest Binding Data for Desolvated Host of 2 and 4a,b host
guest
Kf, M-1
[BS]0/ω, mmol g-1
guest inclusion capacity,c mol
2
MeOH EtOH MeOH EtOH
26 ( 5 37 ( 6 31 ( 6 16 ( 5
3.6 ( 0.3 1.4 ( 0.1 2.2 ( 0.2 1.8 ( 0.3
1.9 0.76 1.2 1.0
4
a Measurements were performed according to the method in ref 17. b K f and [BS]0/ω indicate the binding constant and the binding capacity of the host solid with guest molecules, respectively. c Per formula unit of desolvated host (fw ) 542 (2), 555 (4)).
Figure 3. (a) 3-D framework structure of coordination polymer 3. SCN ligands are omitted for clarity. (b) Schematic representation of the framework of 3. (c) Threefold interpenetration mode of 3-D frameworks of 3.
The single-crystal X-ray analyses of 2 and 311 reveal isomeric coordination-polymer network structures with the chemical composition [Co(L1)2(NCS)2]∞.1d The octahedral coordination environments of two axial SCN and four equatorial L1 ligands of 2 and 3 are equivalent with each other. The two isomers differ in the rotational placement of metal nodes. Each of the Co(SCN)2 nodes of 2 are linked in parallel with each other through L1 to afford two-dimensional rhombic grids (Figure 2a). The grids of 2 are perpendicularly interpenetrated, resulting in large square onedimensional channels with an effective size of 5.4 × 5.4 Å (Figure 2b).12 As anticipated, the structure of 2 is isostructural with that found in [Co(L2)2(NCS)2]∞, with uncoordinated pyrimidyl 2-N atoms situated on the channel walls to give the base functionality. On the other hand, each Co(SCN)2 node of 3 is connected with three equivalent neighbors and with one twisted at 90°, generating a three-dimensional framework structure with unique topology (Figure 3a). Despite the infinite number of possible topologies by the combination of linear spacers and square-planar nodes, only a handful of three-dimensional coordination-polymer frames have been reported to date, including the cubic NbO, the tetragonal CdSO4, and the “dense net”.13 The topology found in 3 has never been described in this context. There exist three independent frames in a crystal, and they are mutually interpenetrating to avoid the presence of a large void volume (Figure 3c). The extent of polymorphism varied, depending on the ratio of Co(SCN)2 and L1 used in the assembly reaction. While the concomitant formation of 1-3 occurred at ratios of 1:2 and 1:2.5, the desired crystals of 2 were afforded as a unique product at a ratio of 1:3 (see the Supporting Information). In another attempt to control the polymorphism, solids of 1 that were obtained from the reaction of L1 and Co(SCN)2 in a 1:1 ratio were redissolved into the solution and reacted with additional L1. In these studies, a similar dependence on the reagent ratio was observed. In this case, crystals of 2 were exclusively formed even at a 1:1 ratio of 1 and L1. The source of this dependence is not clear at this time; however, it may be reasoned that excess L1 molecules can serve to fill the
void of 2 and favor the formation of 2 at a high L1 to Co(SCN)2 ratio. The X-ray powder diffraction pattern of solids of 2 measured after the desolvation is in agreement with a simulated pattern obtained on the basis of the single-crystal structure. This indicates the porous framework of 2 is robust even after the removal of solvent guest, presumably due to the stability provided by the interpenetrated grids.14 Following the successful synthesis of 2, the same approach was employed to prepare an isostructural framework with half the number of base units in the channels. When L1, L3, and Co(SCN)2 were mixed into a solution for the assembly, powder X-ray diffraction of the ensuing solids indicated the formation of a mixture including 1, 2, and incompletely identified complexes from the assembly of L3 and Co(SCN)2 (see the Supporting Information). On the other hand, when an isolated solid of 1 was reacted with L3 using a “step by step” process, crystals of [Co(L1)(L3)(NCS)2]∞ (4) were obtained as the sole product. Similar to the case for 2, the structure of 4 has square channels generated by the perpendicular interpenetration of grids.15 However, because the grids in 4 are formed from mixed ligands of L1 and L3, the number of nitrogen atoms in the channels is half of the number present in 2 (Figure 4).16 At this time, further studies are required to figure out the formation mechanism of 4 through a “step by step” process. However, the existence of oligomeric species in the solution of 1 and their connection by L3 to finish the grid may be a possible explanation. The guest absorption efficacies of desolvated solids of 2, 4, and [Co(L2)2(NCS)2]∞ (which have similar porous frameworks but different functional groups in their channels), were investigated by examining the change of alcohol concentration after immersing the solids into toluene solutions containing alcohols.17 While [Co(L2)2(NCS)2]∞ did not show any measurable change in alcohol concentration, 2 and 4 absorbed small alcohol molecules such as methanol and ethanol from the solution, exhibiting Langmuir isotherm curves (see the Supporting Information). The inclusion capacities of methanol estimated from the numbers of binding sites ([BS]0/ω), 1.9 mol (2) and 1.2 mol (4), are consistent with numbers of uncoordinated nitrogen atoms in the molecular formulas of 2 (2.0) and 4 (1.0) (Table 1). On the basis of these observations, it seems apparent that the uncoordinated nitrogen atoms in the channels of 2 and 4 are unique sites for the binding of methanol, probably via hydrogen-bonding interactions. While 4 has similar inclusion
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capacities for methanol and ethanol, the inclusion capacity of 2 is much lower for ethanol than for methanol. This may be due to the shielding of 2-N atoms by guest molecules that are bound to nearby 2-N atoms in the channels of 2. In conclusion, we have successfully demonstrated the syntheses of robust porous solids with basic functional groups using a rational design of the molecular building block and the control of polymorphism during the assembly. We have also demonstrated a unique three-dimensional framework with noteworthy topological features. Furthermore, we verified the guest absorption ability rendered by the base functionalization of the channels. We anticipate that porous materials with utility in separation and inclusion of specific guest molecules can be engineered using this strategy. Our research will be continued along these lines. Acknowledgment. We thank the Korea Science and Engineering Foundation (KOSEF) for sponsoring this research through the SRC/ERC program of MOST/KOSEF (Grant No. R11-2005-065). Supporting Information Available: Text giving experimental details, Figures S1 and S2 giving powder XRD data, Figure S3 giving the guest binding isotherm, Figure S4 giving a top view of 3, and CIF files giving X-ray crystallographic data for 1-4. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) For recent reviews on microporous MOFs, see: (a) Kitagawa, S.; Kitaura, R.; Noro, S.-i. Angew. Chem., Int. Ed. 2004, 42, 2334. (b) Janiak, C. Dalton Trans. 2003, 2781. (c) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature (London, U.K.) 2003, 423, 705. (d) Moulton, B.; Zaworotko, M. J. Chem. ReV. 2001, 101, 1629. (2) For gas storage, see: (a) Chae, H. K.; Siberio-Pe´rez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.; Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature (London, U.K.) 2004, 427, 523. (b) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science (Washington, DC) 2003, 300, 1127. (c) Kitaura, R.; Kitagawa, S.; Kubota, Y.; Kobayashi, T. C.; Kindo, K.; Mita, Y.; Matsuo, A.; Kobayashi, M.; Chang, H,-C.; Ozawa, T.; Suzuki, M.; Sakata, M.; Takata, M. Science (Washington, DC) 2002, 298, 2358. (3) For selective inclusion, see: (a) Kosal, M. E.; Chou, J.-H.; Wilson, S. R.; Suslick, K. S. Nat. Mater. 2002, 1, 118. (b) Maji, T. K.; Uemura, K.; Chang, H.-C.; Matsuda, R.; Kitagawa, S. Angew. Chem., Int. Ed. 2004, 43, 3269. (4) For catalysis, see: (a) Pan, L.; Liu, H.; Lei, X.; Huang, X.; Olson, D. H.; Turro, N. J.; Li, J. Angew. Chem., Int. Ed. 2003, 42, 542. (b) Cui, Y.; Evans, O. R.; Ngo, H. L.; White, P. S.; Lin, W. Angew. Chem., Int. Ed. 2002, 41, 1159. (c) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K. Nature (London, U.K.) 2000, 404, 982.
(5) For ion exchange, see: (a) Jung, O.-S.; Kim, Y. J.; Kim, K. M.; Lee, Y.-A. J. Am. Chem. Soc. 2002, 124, 7906. (b) Min, K. S.; Suh, M. P. J. Am. Chem. Soc. 2000, 122, 6834. (6) (a) Kitaura, R.; Onoyama, G..; Sakamoto, H.; Matsuda, R.; Noro, S.-i.; Kitagawa, S. Angew. Chem., Int. Ed. 2004, 43, 2648. (b) Shin, D. M.; Lee, I. S.; Chung, Y. K. Inorg. Chem. 2003, 42, 8838. (c) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wacher, J.; O’Keeffe, M.; Yaghi, O. M. Science (Washington, DC) 2002, 295, 469. (7) (a) Kitagawa, S.; Uemura, K. Chem. Soc. ReV. 2005, 34, 109 and references therein. (b) Lu, J. Y.; Babb, A. M. Chem. Commun. 2002, 1340. (8) Lee, I. S.; Shin, D. M.; Chung, Y. K. Chem. Eur. J. 2004, 10, 3158. (b) Shin, D. M.; Lee, I. S.; Chung, Y. K.; Lah, M. S. Inorg. Chem. 2003, 42, 5459. (9) Park, S. H.; Kim, K. M.; Lee, S.-G.; Jung, O.-S. Bull. Korean Chem. Soc. 1998, 19, 79. (10) Crystal and refinement parameters for 1: C19H23CoN7O2S2 (293 K). Mw ) 504.50, triclinic, P1h, a ) 9.2868(4) Å, b ) 10.3422(4) Å, c ) 13.2609(5) Å, R ) 86.8385(15)°, β ) 72.350(2)°, γ ) 78.4268(15)°, V ) 1189.00(8) Å3, Z ) 2, Fcalcd ) 1.409 g/cm3, GOF ) 0.908, R1 ) 0.0416, wR2 ) 0.0899 (I > 2σ(I)). (11) Crystal and refinement parameters for 2: C30H40CoN10O6S2 (293 K), Mw ) 759.77, orthorhombic, Pccn, a ) 15.721(1) Å, b ) 15.828(1) Å, c ) 15.414(1) Å, V ) 3835.5(4) Å3, Z ) 4, Fcalcd ) 1.316 g/cm3, GOF ) 1.101, R1 ) 0.0950, wR2 ) 0.2958 (I > 2σ(I)). Crystal and refinement parameters for 3: C54H50Co2N18O2S4 (293 K), Mw )1229.22, triclinic, P1h, a ) 13.7195(8) Å, b ) 13.6998(8) Å, c ) 18.1247(11) Å, R ) 77.634(3)°, β ) 77.225(3)°, γ ) 63.562(3)°, V ) 2948.6(3) Å3, Z ) 2, Fcalcd ) 1.384 g/cm3, GOF ) 1.024, R1 ) 0.0719, wR2 ) 0.1441 (I > 2σ(I)). (12) For the intepenetration of 2-D grids, see: Zaworotko, M. J. Chem. Commun. 2001, 1 and references therein. (13) (a) Barnett, S. A.; Blake, A. J.; Champness, N. R.; Wilson, C. Chem. Commun. 2002, 1640. (b) Carlucci, L.; Cozzi, N.; Ciani, G.; Moret, M.; Proserpio, D. M.; Rizzato, S. Chem. Commun. 2002, 1354. (c) Carlucci, L.; Ciani, G.; Macchi, P.; Proserpio, D. M. Chem. Commun. 1998, 1837. (d) Power, K. N.; Hennigar, T. L.; Zaworotko, M. Z. Chem. Commun. 1998, 595. (14) (a) Ganesan, P. V.; Kepert, J. C. Chem. Commun. 2004, 2168. (b) Chen, B. Eddaoudi, M.; Hyde, S. T.; O’Keeffe, M.; Yaghi, O. M. Science (Washington, DC) 2001, 291, 1021. (15) To date, only two papers have reported on the generation of grid polymers from two different neutral ligands: (a) Biradha, K.; Fujita, M. Chem. Commun. 2001, 15. (b) Tong, M.-L.; Chen, X.-M.; Yu, X.-L.; Mak, T. C. W. J. Chem. Soc., Dalton Trans. 1998, 5. (16) Crystal and refinement parameters for 4: C26H21CoN7S2 (293 K), Mw ) 554.55, orthorhombic, Pccn, a ) 16.604(1) Å, b ) 14.532(1) Å, c ) 16.097(1) Å, V ) 3884.0(4) Å3, Z ) 4, Fcalcd ) 0.948 g/cm3, GOF ) 1.034, R1 ) 0.0898, wR2 ) 0.2749 (I > 2σ(I)). (17) Min, K. S.; Suh, M. P. Chem. Eur. J. 2001, 7, 303.
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