DOI: 10.1021/cg901347p
Porous Metal-Organic Frameworks Containing Alkali-Bridged Two-Fold Interpenetration: Synthesis, Gas Adsorption, and Fluorescence Properties
2010, Vol. 10 1301–1306
Ruqiang Zou,*,† Amr I. Abdel-Fattah,† Hongwu Xu,† Anthony K. Burrell,† Toti E. Larson,† Thomas M. McCleskey,† Qiang Wei,† Michael T. Janicke,† Donald D. Hickmott,† Tatiana V. Timofeeva,‡ and Yusheng Zhao*,† †
Earth and Environmental Sciences Division, Los Alamos Neutron Science Center, Materials Physics and Applications Division, and Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and ‡Department of Chemistry, New Mexico Highlands University, Las Vegas, New Mexico 87701 Received October 28, 2009; Revised Manuscript Received December 22, 2009
ABSTRACT: Solvothermal reactions of Zn(NO3)2 3 6H2O and alkali (Na, K) chloride with the trigonal-planar ligand benzene-1,3,5-tribenzoic acid (H3BTB) gave rise to two new crystalline porous metal-organic frameworks (MOFs), [Zn3Na2O(BTB)2(DMF)2](DMF)(H2O) and [Zn2K3(BTB)2(HCOO)(DMF)3](DMF)3(H2O)2, respectively. Both phases have Zn3Na2(μ4-O) and Zn2K2(HCOO) clusters as molecular building block nodes, and they form similar alkali-bridged 2-fold interpenetrated, (3,6)-connected nets with the mineral rtl-c topology. The alkali-bridged interpenetration reduces the flexibility of their interpenetrated nets, affording permanent porosity and high thermal stability. These two MOFs also exhibit high capacities of hydrogen uptake and strong solid fluorescent emissions.
Introduction The rapid development of a new-style porous material based on metal-organic frameworks (MOFs) is spurring researchers in chemistry, materials science, and crystal engineering.1 The remarkable chemical and physical properties of MOFs make them intriguing functional materials for numerous applications such as storage/separation,2 catalysis,3 sensing/detection,4 luminescence,5 and nonlinear optics (NLO).6 Their extreme porosity and high surface areas make them ideal for applications in gas storage and gas separation, which has many applications in energy security and environmental remediation. The potential advantages of MOFs for gas storage and separation applications are based upon several of their characteristics: (1) High surface areas (10-200 times those of other porous materials such as zeolite) and mutually isolated organic linkers making the frameworks more accessible to adsorbates,7 thereby increasing the potential amount of stored guest molecules; (2) crystalline nature allowing highprecision computational modeling of their structures and thereby prediction of favorable guest binding sites; (3) a wide range of organic ligands and multiple cost-effective synthetic approaches allowing preparation of MOFs with tailorable structures (including postsynthesis modifications) with optimized steric and chemical properties to enhance guestMOF interactions; and (4) favorable adsorption-desorption kinetics indicative of the potential of both high flux and high separation factors for MOFs. Synthesis of any new-class of materials by traditional methods needs to address the general limitations of control over the physical characteristics of these solid.8 This is directly related to the structural instability of the starting moieties during the reaction, leading to poor correlation between
reactants and the resulting crystalline precipitate. In particular, the predesigned large channels in MOFs are often fraught with an interpenetrating disturbance during synthesis which reduces the channel sizes of the MOF products. Moreover, the flexibility of the interpenetrated nets used to construct MOFs can disrupt their channels upon removal of the guest molecules. To overcome these limitations, we present a new strategy of using alkali-bridged interpenetration to reduce the flexibility of MOFs host frameworks, thereby leading to permanent porosity and high thermal stability. We employ the trigonal-planar ligand benzene-1,3,5-tribenzoic acid (H3BTB) to promote the formation of interpenetrated nets employing the π-π stacking interactions of the BTB ligand pairs.9 We have taken advantage of the soft alkali ions to form the interpenetrated nets and create permanent porosity. Solvothermal reactions of Zn(NO3)2 3 6H2O and alkali (Na, K) chloride with H3BTB gave rise to two porous MOFs, [Zn3Na2O(BTB)2(DMF)2](DMF)(H2O) (1) and [Zn2K3(BTB)2(HCOO)(DMF)3](DMF)3(H2O)2 (2), respectively (DMF = N,N0 -dimethyformamide). These two MOFs exhibit similar alkali-bridged 2-fold interpenetrated, (3,6)connected nets with distinct metal clusters as molecular building blocks (MBBs). In this contribution, we report their synthesis, crystal structures, gas adsorption, and fluorescent emission properties of the two new MOFs. Experimental Section
*To whom correspondence should be addressed. E-mail:
[email protected] (R.Z.);
[email protected] (Y.Z.).
Materials and General Methods. All the solvents and reagents for synthesis were commercially available and used as received. H3BTB was synthesized according to previously described procedures.10 The infrared (IR) spectra were recorded on a NEXUS 870 FTIR spectrometer operating at a spectral resolution of 2 cm-1 accumulating 64 scans. Thermogravimetric analysis (TGA) was carried out using a DuPont Instruments 951 (thermogravimetric analyzer DTG-50) from room temperature to 600 °C with a ramp rate of 10 °C/min in a flowing nitrogen atmosphere. Powder X-ray
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Table 1. Crystallographic Data and Structural Refinement Summary for Complexes 1 and 2 1a
2
Zn3Na2C63H57N3O19 1402.21 Pnma 17.108(4) 21.644(5) 25.258(6) 90 90 90 9353(4) 296(2) 0.996 4 0.823 0.1181/0.3362
Zn3Na2C54H30O13 1128.87 Pnma 17.108(4) 21.644(5) 25.258(6) 90 90 90 9353(4) 296(2) 0.802 4 0.807 0.0519/0.1446
Zn2K3C73H84N6O22 1645.50 P1 16.3656(18) 17.979(2) 19.232(2) 65.452(2) 66.275(2) 69.154(2) 4589.5(9) 296(2) 1.191 2 0.723 0.0995/0.2930
2a
diffractograms (PXRD) were recorded on a Siemens D-500 X-ray diffractometer operating at 45 kV and 35 mA and using Cu KR radiation. Emission spectra were taken on a Photon Technology International QuantaMaster spectrofluorometer. BET surface area and hydrogen adsorption measurements were carried out with an ASAP 2020 surface area analyzer. Synthesis of 1. A mixture of H3BTB (0.1 mmol, 48.3 mg), Zn(NO3)2 3 6H2O (0.15 mmol, 45 mg), and NaCl (0.1 mmol, 6 mg) was dissolved in 15 mL of DMF/H2O (9:1) while being stirred in a 25 mL Parr acid digestion autoclave. After dissolution of the reagents, 0.5 mL of deionized water was added. The tightly capped Parr Bomb was placed in an oven and the temperature was increased at a rate of 3 °C/min, to 110 °C and held this temperature for 48 h, after which time it was cooled back to 25 °C (the ramp rate = 0.5 °C/min). After being decanted and rinsed with DMF, the colorless block crystals were washed with DMF to yield pure 1 with the formula [Zn3Na2O(BTB)2(DMF)2](DMF)(H2O). The product was immersed in methanol and dichloromethane for 7 days to allow exchange of the high boiling-point DMF guest molecules. The solvent-exchange samples were then further activated under Ar atmosphere at 120 °C for 5 h to remove the low boiling-point methanol and dichloromethane. IR: 2960 w, 2890 m, 1667 s, 1635 vs, 1360 vs, 1256 m, 1063 m, 850 m, 797 s, 659 m, 491 m. Synthesis of 2. The procedure is similar to that of 1 except for the use of KCl (0.1 mmol, 7 mg) instead of NaCl. The resultant colorless crystals were washed with DMF to give pure 2 with the formula [Zn2K3(BTB)2(HCOO)(DMF)3](DMF)3(H2O)2. IR: 2816 w, 1656 s, 1603 vs, 1553 m, 1367 vs, 859 m, 763 s, 661 m, 475 m. Single-Crystal X-ray Crystallography. Single-crystal X-ray diffraction was carried out on a Bruker Apex2 CCD diffractometer with graphite-monochromatic Mo KR1 radiation (λ = 0.71073 A˚) using the ω-scan mode at 100 K. Crystals of 1 and 2 were glued on a polymer fiber, transferred to the diffractometer, and cooled in a nitrogen stream. Frames were collected with 0.2° intervals in j and ω for 10 s per frame such that a hemisphere of data was collected. Lattice parameters were initially determined from least-squares analysis of more than 100 centered reflections and were then refined using all the data. None of the reflections showed evidence of significant crystal damage during data collection. Raw data collection and cell refinement were carried out with SMART, and data reduction with SAINTþ to account for Lorentz and polarization effects.11 Absorption corrections were applied using the SADABS routine. Space group assignment was based on systematic reflection absences, E-statistics, and successful refinement of the structures. Structures were solved by the direct method using SHELXTL and were refined by full-matrix least-squares on F2 using SHELX-97.2.12 Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles except for the occluded DMF molecules. Hydrogen atoms were placed in calculated positions with isotropic displacement parameters set to 1.2 Ueq of the attached atom. Solvent molecules in the structure were randomly dispersed, and thus their positions were impossible to refine using conventional discrete-atom models. To resolve these
)
Zn2K3C55H31O14 1163.84 P1 16.3656(18) 17.979(2) 19.232(2) 65.452(2) 66.275(2) 69.154(2) 4589.5(9) 296(2) 0.842 2 0.696 0.0521/0.1381 P P a Refinement after the solvent electron density was removed using the SQUEEZE routine in PLATON.13 b R = ( Fo| - |Fc )/ | P was2carried2out c 2 P 2 1/2 Fo|. wR = [ (|Fo| - |Fc| ) / (Fo )] . )
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(Mo-KR)/mm-1 Rb/wRc
1
issues, the contribution of solvent electron density was removed by the SQUEEZE routine in PLATON.13 Further details for structural analysis are summarized in Table 1.
Results and Discussion Sample Preparation and Characterization. The H3BTB ligand is insoluble in water at room temperature but quite soluble in common organic solvents such as acetonitrile, ethanol, and DMF. To obtain pristine crystals of 1 and 2, a slight stirring of the mixture to form a clear solution was necessary before adding the solution to the Parr autoclave. Crystals suitable for X-ray structure determination were obtained using the solvothermal approach, a standard method in MOF synthesis. It tends to involve in situ organic reactions including ligand oxidative coupling, hydrolysis, and substitution.14 In this work, hydrolysis of DMF was observed in preparation of phase 2, in which the hydrolysis product HCOO- acts as a participant in the formation of 2 to maintain the charge balance.15 It should be noted that both phases are air- and moisture-stable. Description of the Structures of 1 and 2. 1. Single crystal X-ray analysis indicates that complex 1 crystallizes in an orthorhombic space group Pnma and consists of BTB ligands coordinated to Zn3Na2O clusters in a 2:1 ratio with DMF and water molecules either coordinated to Na(I) ions or trapped in molecular cavities. As shown in Figure 1, there are two crystallographically independent Zn(II) and Na(I) sites (Figure 1a). The Zn1 site is located on a 2-fold symmetry axis and bound to a μ4-O and four BTB ligands via a uniform κ2-mode carboxylate. The Zn2 site adopts a four-coordinated tetrahedral geometry via coordinating to a μ4-O, a monodentate and two κ2-carboxylate groups from four separate BTB ligands. All the Zn-O bond lengths fall into the normal Zn-O ranges.16 The two Na(I) ions are bridged together by a μ2-O of two DMF molecules with a short Na 3 3 3 Na separation of 3.388 A˚. Each Zn3Na2O cluster as a MBB links six carboxylate groups of the trigonal-planar BTB ligands (Figure 1b), resulting in a three-dimensional (3,6)-connected net. In this framework, the structure of the Zn3Na2O cluster is similar to that of Zn4O cluster that occurs in a series of isoregular MOFs first reported by Yaghi et al.17 Remarkably, such a zinc/sodium mixed cluster not only decreases the density of the framework but also enhances its stability in moisture in comparison to the previously reported Zn4O cluster.18
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Figure 1. Crystallography of 1: (a) local coordination environment of Zn3Na2(μ4-O) MBB (symmetric codes: A = x, -y þ 2.5, z), (b) coordination modes of staggered BTB ligand pairs, and (c) a three-dimensional sodium-bridged 2-fold interpenetrated network with open channels.
The 2-fold interpenetrated topological structure of 1 is similar to that of MOF-39,19 in which the protons attached to μ3-OH and carboxyl groups of BTB ligands in MOF-39 are replaced by sodium ions. The two interpenetrated nets are bridged together by sodium ions to shorten their separations. 2. Complex 2 crystallizes in a triclinic space group P1 and consists of BTB and formate ligands coordinated to Zn(II) and K(I) ions in a 2:1:2:3 ratio with DMF and water molecules either coordinating to K(I) centers or trapped in molecular cavities. As shown in Figure 2a, there are two crystallographically independent Zn(II) and three K(I) sites. Zn1 and Zn2 sites both adopt a four coordinated tetrahedral geometry via coordinating to four carboxyl oxygen atoms from a formate and three BTB ligands in a uniform κ2-mode, whereas K1 and K2 are bound to a DMF and four carboxylate groups from four BTB ligands, respectively. The Zn2K2(HCOO) cluster is linked by six 3-connected BTB ligands, resulting in a three-dimensional 2-fold interpenetrated, (3,6)-connected network with Zn2K2(HCOO)(COO)6 anion MBBs as 6-connected nodes (Figure 2b). Two adjacent MBBs are linked by a pair of K3 atoms via the strong K 3 3 3 K interaction to achieve the charge balance (Figure 1a). K1, K3, and their symmetric units of K1A (A = -x þ4, -y, -z) and K3A form a unique parallelogram with the short K1-K3 and K1-K3A distances of 3.132(1) and 3.144(1) A˚, respectively. Furthermore, the 2-fold interpenetrated networks of 2 are linked together by the K3 pairs to form a three-dimensional porous framework (Figure 2c).
Topologies of 1 and 2. There are two essential components that are necessary to investigate the topology of MOFs: the structural feature of ligands and coordination modes of metal ion/clusters.20 From the above structure description, we can clearly see that each trigonal-planar BTB ligand links three MBBs to form a uniform 3-fold bridging mode in both 1 and 2. Although complexes 1 and 2 exhibit distinct MBBs, the two types of MBBs are both bound to six carboxylate groups from six separate BTB ligands, leading to a threedimensional (3,6)-connected net (Figure 3).21 Two such (3,6)-nets are mutually interpenetrated due to the staggered BTB ligand pairs to form a rtl-c topological structure. The centroid-to-centroid separations of the two benzyl rings of BTB ligand pairs in 1 and 2 fall into the range of 3.6823.738 A˚, similar to a separation of 3.677(2) A˚ in the free H3BTB compound (Figure 4),22 which implies strong intermolecular π-π stacking interactions. It is worth noting that the two 2-fold interpenetrated nets in compounds 1 and 2 are bridged together by Na(I) or K(I) ions, respectively, which significantly reduces the flexibility of their interpenetrated nets and affords permanent porosity. Furthermore, the staggered BTB ligand pairs resulting from strong π-π interactions also lead to 2-fold interpenetrated nets enclosed into each other. By viewing the ligand pairs parallel to the centers of the two π-stacked benzyl rings, they resemble a hexagonal 6-connected linker with each of the six carboxylate groups extending outward in different directions. Using the BTB ligand pairs and the metal clusters as nodes, complexes 1 and 2 show a similar 6-connected topological net with the Schl€ afli symbol of (48.66.8)(414.6) (Figure 5).
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Figure 2. Crystallography of 2: (a) local coordination environment of Zn2K2(HCOO) MBB, (b) coordination modes of staggered BTB ligand pairs, and (c) three-dimensional potassium-bridged 2-fold interpenetrated network with open channels.
Figure 3. Alkali-bridged 2-fold interpenetrated (3,6)-connected net with rtl-c topology of 1 and 2.
Thermal Stability. Thermogravimetric analyses (TGA) were conducted to determine the thermal stability of 1 and 2 (Figure 6), an important issue for porous MOFs. Compound 1 exhibits a two-step weight loss starting at room temperature and finishing at 260 °C, implying removal of the trapped water and then the DMF solvent molecules. The host framework remains stable up to 390 °C at which it collapses rapidly. Compound 2 shows the loss of free solvent molecules from room temperature up to 105 °C and then the loss of coordination solvent molecules up to 250 °C. The host framework starts to decompose above 370 °C. Note that the loss of solvent molecules in 1 and 2 occurs over wide temperature ranges. Solvent-exchange methods were utilized to activate samples 1 and 2 for adsorption experiments. 1a and 2a are the materials obtained by pretreatment of 1 and 2
using the solvent-exchanged method described in Experimental Section. No weight losses were observed from room temperature to 300 °C with either of the solvent-exchanged samples 1a and 2a. Gas Adsorption Measurements. To determine the permanent porosities of 1 and 2, gas adsorption measurements were carried out using desolventized samples 1a and 2a. The nitrogen physisorption measurements of 1a and 2a display classic isotherms (Figure 7) typical of microporous materials. The adsorption and desorption branches are closed with a slight hysteresis. Analysis of the isotherms reveals a specific BET surface area of 527 m2 g-1 for 1a and 457 m2 g-1 for 2a. The Langmuir surface areas are fitted as 772 m2 g-1 for 1a and 667 m2 g-1 for 2a. Low-pressure volumetric hydrogen adsorption analysis reveals a remarkable H2 uptake of 1.3 and
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Figure 4. Staggered BTB ligand pairs resulting from strong π-π interactions in free ligand, 1 and 2.
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Figure 7. N2 adsorption isotherms of 1 and 2.
Figure 5. 6-connected topological net of 1 and 2 using staggered BTB ligand pairs as 6-connect nodes.
Figure 8. H2 adsorption isotherms of 1 and 2 (P0 = 1 bar).
Figure 6. TGA curves of 1, 2 and their desolventized samples.
0.6 wt % at 77 K and 0.78 bar for 1a and 2a, respectively (Figure 8). The higher H2 uptake in 1 is consistent with its larger surface area compared with 2. Fluorescent Emission. The emission properties of 1 and 2 were investigated at room temperature. Compound 1 exhibits intense fluorescent emission maxima at λem = 408 nm (λex = 355 nm), while 2 displays maximum emission at λem = 383 nm (λex = 340 nm) (Figure 9). The strong fluorescent emissions of 1 and 2 may be ascribed to the conjugation of the staggered BTB ligand pairs, in which the increased conjugation of ligands prevents ligand-to-metal charge transfer in 1 and 2.5 The difference in emission properties between 1 and 2 is due to the rigidity difference of their host frameworks as well as the coordination diversity of metal ions. Furthermore, the size of the metal, the structure of the
Figure 9. Room temperature solid fluorescent excitation and emission spectra of 1 and 2.
molecular building blocks, and the orientation of the linkers all affect the degree of isolation of the linkers from each other in the structure, which may also have a significant effect on the emission properties of 1 and 2. Concluding Remarks. Two novel MOFs have been solvothermally prepared and structurally characterized.
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Different alkali cations (Na or K) led to different molecular building blocks and framework structures. Given these differences, the two MOFs possess the same topology. Interestingly, the alkali-bridged interpenetration reduces the flexibility of MOFs and thus affords their permanent porosity and high thermal stability. To the best of our knowledge, this kind of porous MOF containing coordinatively linked interpenetration is rare.9 These interpenetrated MOFs with permanent porosity will continue to grow in functionality and provide a useful platform for further studies. Acknowledgment. We especially acknowledge Professor Bernard De Jong for his invaluable suggestions and comments. R.Z. sincerely thanks LANL for a Director’s Postdoctoral Fellowship. This work was financially supported by LANL Director’s funded postdoc LDRD Project No. 20080780PRD2. TVT is grateful for NSF support via DMR/ PREM program grant No. 0934212. Supporting Information Available: Powder X-ray diffraction data and crystallographic information in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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