DOI: 10.1021/cg101222w
Metal-Organic Framework Isomers with Diamondoid Networks Constructed of a Semirigid Tetrahedral Linker
2010, Vol. 10 5327–5333
Jian Tian, Radha Kishan Motkuri, Praveen K. Thallapally,* and B. Peter McGrail Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States Received September 15, 2010; Revised Manuscript Received October 19, 2010
ABSTRACT: Solvothermal assembly of a semirigid tetrahedral carboxylate ligand tetrakis[4-(carboxyphenyl)oxamethyl]methane acid (H4X) with Cd(II) ion in different solvent systems yields three novel metal-organic framework isomers (1-3) based on different secondary building units (SBUs). Although all three frameworks have the same dia (diamondoid) topology, complexes 1 and 3 are noncentrosymmetric and complex 2 is centrosymmetric. One of the networks (1) shows cross-linked 3-fold interpenetration of the single dia net and exhibits permanent porosities, as confirmed by Brunauer-Emmett-Teller (BET) and selective CO2 adsorption.
Introduction Metal-organic frameworks (MOFs) are receiving considerable attention in the field of solid state materials,1 owing to their intriguing structural diversity2 and wide applications in gas storage and separation,3 catalysis,4 and magnetism.5 On the basis of the concept of “reticular synthesis”,6 it is possible to choose appropriate metal centers and organic linkers to form networks with predetermined topologies and desired functions. However, in reality, the chemical environment, such as pH value, solvent, temperature, and reagent concentration, exerts a profound and unpredictable impact on the crystallization of MOFs.7 Minor changes of such environmental factors may lead to framework isomerism originated from differences in atomic connectivity or network catenation.8 In this regard, multitopic carboxylate linkers have received increasing attention for the construction of MOF isomers because their versatile coordination modes, from terminal monodentate to various bridging modes, can generate metal-oxygen nodes, rods, or polynuclear clusters.9-13 Recently, we and others have become interested in utilizing flexible tetracarboxylate ligands, such as tetrakis[4-(carboxyphenyl)oxamethyl]methane acid (H4X), for the preparation of MOFs with interesting structures and useful functions.14-20 Several structural topologies including dia (diamond), lon (lonsdaleite), pts (PtS), flu (CaF2), and sxa have been reported, which are commonly seen for MOFs based on 4-connected organic linkers. Strikingly, assembly of the ligand with Zn cation yields five supramolecular stereoisomers based on different Zn clusters as secondary building units (SBUs).15-17 Such a structural richness could be ascribed to both the versatile coordination modes and flexible conformers of the H4X ligand, in which the four carboxylate groups can twist around the central quaternary carbon atoms to meet the coordination environments of different metals. As an effort to further explore the potential of such a versatile ligand, we herein report the synthesis and characterization of three novel MOF isomers assembled from the H4X linker and Cd(II) cation in different
solvent systems, namely, Cd6(X)3(H2O)6 3 Guest (1), Cd2X(H2O)2 3 Guest (2), and Cd2X(H2O)3 3 Guest (3). Experimental Section
*To whom correspondence should be addressed. E-mail: praveen.thallapally@ pnl.gov.
Materials and General Methods. H4X was synthesized according to literature procedures.21 All the solvents and reagents were of analytical grade and used as received without further purification. Thermogravimetric analysis (TGA) was performed on a TG-209F1 from Netzsch Instruments by heating the samples to 500 C with a heating ramp of 5 C/min under a nitrogen flow (20 mL/min). Powder X-ray diffractograms (PXRD) were recorded on a Bruker D8 Discover X-ray diffractometer operating at 45 kV and 35 mA with Cu source. Brunauer-Emmett-Teller (BET) surface area was carried out with an Autosorb IQ surface area analyzer. CO2 and CH4 adsorption isotherms were recorded on a HPVA-100 volumetric gas analyzer. Synthesis of 1. A mixture of H4X (0.05 mmol, 30.8 mg) and Cd(NO3)2 3 6H2O (0.11 mmol, 28.2 mg) was dissolved in 6 mL of DMF/H2O (2:1) in a 20 mL vial. The reaction vial was capped tightly and placed in an oven at 85 C for 24 h. The mixture was then gradually cooled to room temperature at a rate of 10 C/h, and colorless block-shaped crystals of 1 were obtained with 81% yield (based on H4X). Synthesis of 2. A mixture of H4X (0.05 mmol, 30.8 mg) and Cd(NO3)2 3 6H2O (0.11 mmol, 28.2 mg) was dissolved in 5 mL of DMF in a 20 mL vial. The reaction vial was capped tightly and heated at 85 C for 72 h. After being cooled to room temperature, colorless rhombic-shaped crystals of 2 were obtained with 52% yield. Synthesis of 3. A mixture of H4X (0.05 mmol, 30.8 mg) and Cd(NO3)2 3 6H2O (0.11 mmol, 28.2 mg) was dissolved in 5 mL of DMA in a 20 mL vial. The reaction vial was capped tightly and heated at 110 C for 72 h. The mixture was then gradually cooled to room temperature at a rate of 10 C/h, and colorless rectangular crystals of 3 were obtained with 82% yield. Single Crystal X-ray Crystallography. Single crystal X-ray diffraction was performed on a Bruker Kappa CCD diffractometer with graphite-monochromatic Mo-KR1 radiation (λ = 0.71073 A˚) using omega scan mode at 100 or 173 K. Frames were collected using ω scans with 0.2 intervals and a counting time of 10 s per frame. Data integration and reduction were performed using APEX suite software. The structure was solved by direct method and refined by full-matrix least-squares on F2 with anisotropic displacement using SHELX-97.22 Non-hydrogen atoms of the metal and the ligands were refined with anisotropic displacement parameters. The hydrogen atoms on the carbon were calculated in ideal position with isotropic displacement parameters set to 1.2 Ueq of the attached
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atom. Absorption corrections were applied using SADABS after the formula of the compound is determined approximately. The solvent molecules in the structure were highly disordered, and thus their positions were impossible to refine using conventional discreteatom models. The SQUEEZE routine in PLATON was used to remove the scattering from the highly disordered solvent molecules.23 The structure was then refined again using the new . HKL file generated by SQUEEZE. The detailed crystallographic data are shown in Table 1. Table 1. Crystallographic Data and Structural Refinement Summary for Complex 1-3a chemical formula fw space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 T/K Dc/g cm-3 Z μ, mm-1 Rb/ wRc GOF on F2
1
2
3
C99H72Cd6O42 2607.97 P1 15.420(5) 15.842(3) 17.819(3) 112.763(2) 112.203(3) 95.425(3) 3565.6(14) 173(2) 1.215 1 0.943 0.0496/0.1144 0.994
C33H24Cd2O14 869.32 P1 13.398(2) 16.182(1) 17.892(1) 112.924(4) 107.922(5) 97.280(5) 3262.1(6) 100(2) 0.885 2 0.687 0.0687/0.1985 1.169
C33H24Cd2O15 885.32 Pna21 19.517(4) 17.306(4) 18.722(4) 90 90 90 6324.0(2) 100(2) 0.930 4 0.711 0.0417/0.0927 1.026
a Refinement was carried out after the solvent electron density was removed using the SQUEEZE routine in PLATON. b R = Σ[|Fo| - |Fc|]/ Σ|Fo|. c wR = [Σ(|Fo| - |Fc|)2/Σ|Fo|2]1/2.
Results and Discussion Structural Description of 1. Single crystal X-ray analysis revealed that compound 1 is constructed from infinite Cd-O-C rod as its SBUs. 1 crystallizes in the chiral triclinic space group P1. The asymmetric unit consists of six crystallographically unique Cd atoms, three X4-ligand, and six coordinated water molecules. As illustrated in Figure 1A, six Cd atoms have three coordination modes. The odd-numbered Cd atoms are in an octahedral geometry coordinated by two O atoms from two aqua ligands and four O atoms from four different X4- ligands (Cd-O 2.203-2.325 A˚). Cd2 and Cd4 atoms are in a distorted octahedral geometry coordinated by six O atoms from four different X4- ligands (Cd-O 2.185-2.489 A˚). Cd6 atom is in a trigonal bipyramidal geometry coordinated by five O atoms from four different X4- ligands (Cd-O 2.295-2.364 A˚). X4ions link each [Cd6(CO2)12(H2O)6]¥ rod in the a and b directions to nine neighboring rods, resulting in three-dimensional (3D) frameworks with two types of one-dimensional (1D) rhombic channels running along the [1 1 0] direction (Figure 1B,C): one is hydrophilic, with the coordinated aqua ligands aligning the channel walls and a channel size of 4.4 4.0 A˚2. The other is hydrophobic, surrounded by aromatic rings of X4- ligands, with a size of 5.8 5.4 A˚2. The void volume in 1 occupied by the guest solvent molecules is 42.3% calculated by the PLATON/VOID routine. Structure of 2. X-ray analysis reveals that 2 crystallizes in the triclinic space group P1. The asymmetric unit contains one independent X4-, two crystallographically unique cadmium
Figure 1. (A) Infinite 1D Cd(II)-O rod formed by Cd atoms and carboxylate groups of X4- ligands, the blue polyhedron showing the coordination environments of six crystallographically unique Cd atoms in the asymmetric unit of 1. Perspective (B) and space-filling (C) along the [1 1 0] direction, showing two types of 1D rhombic channels.
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Figure 2. Crystallography of 2: (A) local coordination environment of Cd4 cluster, (B) coordination modes of tetrahedral X4- ligand, and perspective views of 2 along (C) a, (D) b axes, showing the rectangular and diamond-shaped channels.
atoms, and two coordinated water molecules. As shown is Figure 2A, the Cd1 ion is in a highly distorted octahedral geometry, coordinated by two chelated and two bridging carboxylate groups from four X4- ions (Cd1-O 2.260-2.423 A˚). The Cd2 ion is also in a distorted octahedral geometry, coordinated by four bridging carboxylate groups from four X4- and two O atoms from two coordinated aqua ligand (Cd2-O 2.230-2.347 A˚). The Cd1 ion is bridged to the neighboring Cd2 by two -O-C-O- bridges to afford a Cd2 unit. Symmetrical expansion of the Cd2 unit yields a Cd4 cluster as the SBUs of the framework. Each Cd4 SBU connects eight X4- ligands, and each X4- ligand binds four Cd4 SBUs to form a 3D (4, 8)-connected net. It is noted that four carboxylate groups in the X4- ligand exhibit four kinds of connection modes: one is in chelating mode, another in bridging mode, the third one in chelating/bridging mode, and the fourth one in an unusual trimetallic bridging mode (Figure 2B). The guest accessible volume of 2 is about 54.1% calculated by the PLATON/VOID routine. Large diamond-shaped and rectangular channels run along three crystallographic axes and are occupied by guest DMF and water molecules (Figure 2C,D). Structure of 3. Compound 3 crystallizes in the orthorhombic space group of Pna21. The asymmetric unit contains two crystallographically unique cadmium atoms, one X4-, and three coordinated water molecules. The Cd1 ion is in a highly distorted octahedral geometry, coordinated by two chelated and two bridging carboxylate groups from four X4- ions (Cd1-O 2.260-2.423 A˚). The Cd2 ion is also in a distorted octahedral geometry coordinated by six O atoms with three O atoms from three coordinated water and three O atoms from three X4- ions (Cd-O 2.203-2.325 A˚). Cd1 are bridged by three -O-C-O- bridges to neighboring Cd2 to afford a dinuclear SBU (Figure 3A). The Cd2 SBU can be considered as 4-connected nodes, and are linked by four
4-coordinated X4- ions to form a (4, 4)-connected dia net (Figure 3C). The whole framework of 3 is a 2-fold interpenetration of the single dia nets generated by a d-glide plane (parallel to the bc plane). Large rhombic channels of ca. 7.1 4.5 A˚2 in diameters exist in the framework along the b axis and are occupied by coordinated water molecules and guest DMA and water molecules (Figure 3D). PLATON/VOID calculations show that the guest accessible volume comprises 57.1% of the unit cell volume of 3. Topologies of 1-3. The structural feature of ligands and coordination modes of metal ion/clusters are two essential factors for investigating the topology of MOFs. In 1-3, X4ligands are all in distorted tetrahedral coordination geometry and each ligand acts as a 4-connected node to connect four metal ion/clusters (Figure 4A). Thus, the coordination modes of metal ion/clusters in each MOFs play a decisive role in the network topology. For 1, the six cadmium atoms in the asymmetric unit could be divided into three discrete Cd2 clusters (Cd1 and Cd2, Cd3 and Cd4, Cd5 and Cd6, respectively) if ignoring the coordination bonds bridging neighboring clusters (Figure 4B). Each Cd2 cluster can be considered as 4-connected nodes and are linked by four 4-coordinated X4- ions to form a (4, 4)-connected dia net. Therefore, the whole framework of 1 will be composed of three dia nets, which are coordinatively linked by the bridging Cd-O bonds between neighboring Cd2 clusters along the infinite rod (Figure 4C,D). The complex 1 can be simplified as noncentrosymmetric 3D 66-dia networks which are 3-fold interpenetrated and further cross-linked via Cd-O connections.24 For complex 2, it could be reduced to a (4, 8)-flu (CaF2) net based on the 4-connecting X4- ligand and 8-connecting Cd4 clusters (Figure S4, Supporting Information), as the case in [Co3(HX)2](DMF)8(H2O)13 with an 8-connecting Co3 cluster reported by Du and co-workers recently.20 However,
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Figure 3. Crystallography of 3: (A) local coordination environment of Cd2 cluster, (B) coordination modes of tetrahedral X4- ligand, (C) perspective view of a single dia net along b axis assembled from Cd2 cluster and X4- ligand, and (D) a three-dimensional 2-fold interpenetrated dia network with open channels. The two nets are shown in red and green colors, respectively.
Figure 4. (A) Schematic representation of the X4- ligand as 4-connecting nodes to connect metal ion/cluster in 1-3. (B) Six crystallographically unique Cd atoms in the asymmetric unit of 1 being divided into three Cd2 cluster, which act as 4-coonecting nodes. Views of the cross-linked 3-fold interpenetrated dia network of 1 along (C) c axis and (D) [1 1 0] direction. The gray and green spheres represent 4-coonecting X4- and Cd2 nodes, respectively.
Article
flu net may not be an accurate description of the topologies of the two above-mentioned MOFs. This is because in both cases the 4-connecting nodes (X4- ligands) are highly distorted from a cubic array that is body-centered by an 8-connecting node, which is a common arrangement of 4-connecting
Figure 5. (A) View of the cross-linked 2-fold interpenetrated dia network of 2 along the c axis. The Cd4 cluster acting as two 4-connecting nodes linked by Cd-O connections. (B) A 2-fold interpenetration of the single dia nets generated by a d-glide plane in 3. The gray and green spheres represent 4-connecting X4- and Cd ion/cluster nodes, respectively.
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nodes in the fluorite network. Therefore, the whole frameworks are highly distorted from ideal fluorite networks if not impossible. In complex 2, as depicted in Figure 5A, each Cd1 ion can be considered as 4-connecting nodes to link 4-connecting X4- ligands into a (4, 4)-connected dia net. The two Cd2 atoms in the Cd4 cluster could be thought as bridging points to link two inversion center-related Cd1 nodes by Cd-O bonds. Expansion of the unit cell reveals that complex 2 exhibits 2-fold interpenetration of dia nets which are related by an inversion center and further cross-linked via Cd-O connections (Figure 5A). As a result, the whole framework of 2 is centrosymmetric. For complex 3, as described in its structural analysis part, its framework is a 2-fold interpenetration of the single dia nets generated by a d-glide plane (parallel to the bc plane, Figure 5B). Thermal Stability. Thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD) studies were conducted to characterize the thermal stability of frameworks in compounds 1-3. TGA of compound 1 shows a two-step weight loss of about 18.2% starting at room temperature (RT) and finishing at 210 C, implying the removal of water molecules first and then the trapped DMF solvent molecules. No further weight loss was observed until 350 C at which the compound starts degrading quickly. Compound 2 shows a rapid weight loss of about 31% between RT and 150 C, corresponding to the loss of DMF and water molecules in 2. No further weight loss was observed until 360 C. Compound 3 shows the loss of coordinated water molecules and guest DMA molecules from RT up to 250 C. The compound starts to decompose at 360 C. The PXRD patterns of 1 before and after activation at 200 C confirm the stability of the host framework upon solvent removal. However, for 2 and 3, both structures collapse after heating the sample at 150 C (see Supporting Information). Gas Adsorption Measurements. The thermal stability of the framework of 1 upon desolvation has encouraged us to examine its gas adsorption properties. The BET surface sorption experiments were performed on desolventized 1 using nitrogen at 77 K and CO2 at 273 K in order to determine its potential use for gas separation and storage purposes. As shown in Figure 6A, the activated 1 displays a reversible type-I adsorption isotherm of CO2, which is typical of microporous materials. Derived from the adsorption data, 1 has the Brunauer-Emmett-Teller (BET) surface and Langmuir surface areas of 231 m2/g and 249 m2/g, respectively. To our surprise, no nitrogen diffusion is observed at 77 K (BET surface area
