A Simple and Rational Approach for Binodal Metal–Organic

Article pubs.acs.org/crystal

A Simple and Rational Approach for Binodal Metal−Organic Frameworks with Tetrahedral Nodes and Unexpected Multimodal Porosities from Nonstoichiometric Defects Hyungphil Chun,*,† Woojeong Bak,† Keunil Hong,† and Dohyun Moon‡ †

Department of Applied Chemistry, College of Science and Technology, Hanyang University, 55 Hanyangdaehak-ro, Ansan 426-791, Republic of Korea ‡ Beamline Division, Pohang Accelerator Laboratory, Pohang, Kyungbuk 790-784, Republic of Korea S Supporting Information *

ABSTRACT: The fact that asymmetrically substituted dicarboxylate is capable of forming two different coordination modes is exploited to synthesize a binodal metal−organic framework containing tetrahedral nodes, and thus [H2N(CH3)2]2[Zn5(hbdc)6(dabco)2] (1) (hbdc =2-hydroxyterephthalate, dabco =1,4-diazabicyclo[2.2.2]octane) possesses the (8,4)coordinated fluorite topology. This simple approach is verified by using symmetrically substituted ligand, 2,5-dihydroxyterephthalate to synthesize a uninodal, 4-coordinated net whose tetrahedral nodes lead to the rare lonsdaleite topology. It has been found that 1 readily loses some of the neutral ligand and cations during the activation processes without collapsing the whole framework, and multimodal porosities are observed due to the nonstoichiometric defects.

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INTRODUCTION Metal−organic frameworks (MOFs) occupy a somewhat unique position in network solids in that available building blocks are not only infinite in varieties, but they also vary greatly in terms of their chemical and physical properties. As a result, possibilities for practical applications have been proposed in numerous areas, such as separation, storage, catalysis, electronics, and the delivery of biological molecules.1 The highly versatile nature of the crystalline porous materials is probably best exemplified by myriad variations in framework types; however, as O’Keeffe and co-workers have pointed out,2 only those MOFs having simple and high-symmetry topologies are plausible targets for designed synthesis. The targeted synthesis of porous MOFs with a highly symmetric, binodal topology presents intricate challenges due to greater uncertainties of nodal geometries compared to uninodal systems. Therefore, without the ability to predict crystal structures a priori, practical approaches for the designed and rational synthesis of such MOFs should rely on experiencebased knowledge of simple and well-defined building blocks, such as di-, tri-, or tetranuclear metal carboxylate clusters. We have recently initiated systematic investigations toward new binodal MOFs, and reported a highly symmetric (8,6)coordinated net unprecedented in MOFs.3 The MOF was obtained by linking two heterometallic secondary building units (SBUs) with an asymmetrically substituted dicarboxylate ligand. We hereby describe an even simpler method using © 2014 American Chemical Society

only one kind of metal, along with a similar ligand, 2hydroxyterephthalic acid (H2hbdc). This ligand, we expected, is capable of forming two different SBUs within one framework when a coordinatively versatile metal, such as Zn2+, is used (Scheme 1a). This approach is verified by synthesizing a uninodal 4-coordinated net with symmetrically substituted ligand, 2,5-dihydroxyterephthalic acid (H2dhbdc) (Scheme 1b).

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EXPERIMENTAL SECTION

Materials and Methods. All the reagents and solvents were commercially available and used as received. The synthesis described below is the result of optimizations with many different parameters,

Scheme 1. Expected Connectivities for Substituted Dicarboxylate Ligands

Received: January 23, 2014 Revised: February 24, 2014 Published: March 6, 2014 1998

dx.doi.org/10.1021/cg500116f | Cryst. Growth Des. 2014, 14, 1998−2002

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Article

thus the flux of the incident beam) and detector distances; however, no diffraction was observed beyond the resolution limit of 1.1 Å in any case (SI Figure S1). The structure was solved with the space group P63/m and refined to reveal the framework atoms, albeit with unusually large anisotropic thermal parameters. The cations were not located, probably due to the low resolution and severe disorder in the solvent-occupied regions (78% of the total crystal volume), and therefore assumed to be dimethylammonium as in the case of 1. The final refinements were carried out after applying the SQUEEZE routine without the cations.

including heating profiles, solvents, additives, and stoichiometries between reagents. A simultaneous differential scanning calorimetrythermal gravimetric analysis (DSC-TGA) was carried out on a SCINCO S-1000 instrument with a heating rate of 5 °C/min in air. Fourier-transform nuclear magnetic resonance spectra were obtained using a Bruker 400 MHz spectrometer. [H2N(CH3)2]2[Zn5(hbdc)6(dabco)2] (1). To a solution containing Zn(NO3)2·6H2O (133.0 mg, 0.45 mmol), H2hbdc (81.4 mg, 0.45 mmol) and 1,4-diazabicyclo[2.2.2]octane (dabco) (16.6 mg, 0.15 mmol) in DMF (3.00 mL) were added, along with a small amount of deionized water (5 μL). The solution mixture was stirred at room temperature for 1 h, and then heated to 85 °C for 16 h in a sealed glass vial. After filtering off a small amount of polycrystalline residue, the solution was allowed to stand at room temperature in a capped vial to produce truncated octahedron-shaped single crystals. The product was collected after 5−7 days and soaked in dichloromethane before drying under vacuum overnight (51.0 mg, 40%). Calcd: C, 43.67; H, 3.89; N, 4.77; Zn, 18.57. Found: C, 43.53; H, 3.65; N, 4.91; Zn, 18.99%. [H2N(CH3)2]2[Zn2(dhbdc)3(dabco)] (2). A solution containing Zn(NO3)2·6H2O (63.7 mg, 0.21 mmol), H2dhbdc (61.9 mg, 0.31 mmol),d and dabco (11.7 mg, 0.10 mmol) in DMF (3.0 mL) was sealed in a glass vial and heated to 105 °C for 6 h. The solution was quickly filtered while warm, and then allowed to stand at room temperature in a capped vial. Large hexagonal block-shape crystals began to form within several hours. The product was collected after 1 d and soaked in dichloromethane before drying under vacuum overnight (25.6 mg, 27%). Calcd: C, 44.22; H, 4.37; N, 6.07. Found: C, 43.86; H, 4.01; N, 5.82%. Gas Sorption Studies. Gas sorption isotherms were measured with a Belsorp Mini-II at 77 (liquid nitrogen) or 273 K (slush baths of ice−water). The gases used were of the highest quality available (N60 for H2, N50 for CO2 and N2, N45 for O2, and N35 for CH4). Typically, 100−200 mg of solvent-exchanged sample was evacuated under a dynamic vacuum at temperatures between 25 and 80 °C for 12 h. The equilibrium criteria were set consistent throughout all the measurements (change in adsorption amounts less than 0.1 cm3/g within 180 s). X-ray Powder Diffractions. X-ray powder diffraction patterns were recorded at the 2D SMC beamline of the Pohang Accelerator Laboratory, Korea. Crystalline samples of 1 and 2 were thoroughly ground in an agate mortar and packed in a capillary tube (0.3 mm diameter). Debye−Scherrer diffraction data were collected on an ADSC Quantum-210 detector with a fixed wavelength (λ = 1.000 43 and 1.200 41 Å for 1 and 2, respectively) and an exposure of 60 s. The ADX program4 was used for data collection, and Fit2D program5 was used to convert the 2D to 1D patterns. X-ray Crystallography. Single-crystals of 1 were directly picked up from the mother liquor with a cryoloop attached to a goniohead, and transferred to a cold stream of liquid nitrogen (−173 °C). The data collection was carried out using synchrotron X-ray on a ADSC Quantum 210 CCD detector with a silicon (111) double-crystal monochromator at 2D SMC beamline of the Pohang Accelerator Laboratory, Korea. The ADSC Quantum-210 ADX program4 was used for data collection, and HKL3000sm (Ver. 703r)6 was used for cell refinement, data integration, and absorption correction. After space group determination, the structures were solved by direct methods and subsequent difference Fourier techniques (SHEXLTL).7 All the nonhydrogen atoms were refined anisotropically, and hydrogen atoms were added to their geometrically ideal positions. The hydroxyl group was found disordered over two positions (80:20) and was left without the hydrogen atom. The diffused electron densities in the void space could not be modeled properly and were removed from the reflection data using the SQUEEZE routine of PLATON.8 The results of the SQUEEZE process were attached to the CIF file. The crystal data and results of structure refinements are summarized in Supporting Information, SI, Table S1. The Connolly surface diagram of 1 was generated using Materials Studio v. 4.3 (Accelrys Software, Inc. 2008) with the probe radius of 1.4 Å. For 2, more than 10 different crystals of varying sizes (0.1−0.5 mm) were examined in the same beamline by varying the wavelengths (and

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RESULTS AND DISCUSSION On the basis of our experience of MOFs synthesized from M2+, simple dicarboxylate and diamine ligands,9 we expected that new MOFs with mixed nodal figures of either paddlewheel and tetrahedron or pinwheel and tetrahedron could be obtained from the asymmetrically substituted hbdc ligand (Scheme 1a). Therefore, Zn2+ was reacted with H2hbdc and dabco under solvothermal conditions in DMF, and truncated-octahedron shaped crystals of an apparently singular phase were obtained. X-ray crystallography on the single crystals establishes the formula as [H2N(CH3)2]2[Zn5(hbdc)6(dabco)2] (1) for the framework. Analysis for the contents of C, H, N, and Zn corroborates with this formulation, and X-ray powder diffraction studies further support the bulk purity of the product (Figure 1).

Figure 1. Debye−Scherrer diffraction patterns for 1 (λ = 1.000 43 Å).

The crystal structure of 1 was solved with the cubic space group Pa-3, in which the asymmetric unit contains 5/6 Zn2+ ion over three sites, one hbdc dianion, 1/3 dabco, and 1/3 dimethylammonium cation. Two of the three independent Zn2+ ions constitute a trinuclear pinwheel SBU with the general formula [Zn3(O2C)6(dabco)], and the last Zn2+ ion forms a mononuclear tetrahedral node supported by three carboxylates and a nitrogen of the dabco ligand. In accordance with our expectation, the two different SBUs are bridged by hbdc ligand (Figure 2a). Topologically, the 8-connecting pinwheel SBU adopts the nodal geometry of a cube, as shown in Figure 2b, and on average, there exist two tetrahedral nodes for every pinwheel SBU. The overall packing structure of the new MOF (Figure 2c) can thus be simplified as a binodal (8,4)-net with the cubic and tetrahedral nodes in a 1:2 ratio. The default net having this connectivity is that of fluorite (CaF2) with the net symbol flu in the RCSR database,10 and flu is indeed the topology underlying 1999

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Figure 3. (a) X-ray powder diffraction patterns for 2 (λ = 1.20041 Å). (b) Coordination environment around the tetrahedral Zn2+ center in 2. (c) Partially expanded net showing the lonsdaleite topology. (d) Perspective view of the hexagonal channels shown without cations and solvent molecules.

Figure 2. X-ray structure of 1. (a) Two SBUs linked by hbdc ligand. (b) Nodal geometry of the 8-connecting pinwheel SBU. (c) Overall packing in the unit cell and the Connolly surface. (d) The network structure simplified according to (b).

observed at around 250 °C according to simultaneous DSCTGA analysis (SI Figure S2). The activated solid of 1 was subject to nitrogen sorption measurements at 77 K, and the results, shown in Figure 4, revealed rather unusual features.

the net of 1 (Figure 2d). The flu net is one of the most fundamental and frequently observed structure types in binary ionic solids, and has first been replicated in MOFs by one of us.11 Since then, many other MOFs have been reported as having the flu topology;12 however, all the examples known to date contains multitopic ligands playing the role of either 4- or 8-connecting node. Therefore, to the best of our knowledge, 1 is the first case in which two unique SBUs define both nodes of the fluorite net. Our synthetic approachthe derivation of mononuclear tetrahedral center through the 2-substituted carboxylate moiety− may be adopted as a general strategy for Zncarboxylate-based new MOFs.13 In order to demonstrate this, we reacted Zn2+, dabco and symmetrically substituted ligand, 2,5-dihydroxyterephthalate (dhbdc2‑) under a condition similar for 1 (Scheme 1b), and were able to isolate large hexagonal block shape crystals as the only product (Figure 3a). The X-ray diffractions by these single crystals are extremely weak and poor, even at 100 K under a synchrotron radiation. According to our best interpretation of the data with a limited resolution,14 the new MOF is built upon tetrahedral Zn2+ coordinated by three carboxylates and a dabco nitrogen atom, as in the case of 1 (Figure 3b). The uninodal 4-coordinated net has an anionic framework with the formula [Zn2(dhbdc)3(dabco)]2− (2), and possess the topology of lonsdaleite (lon), also known as the “hexagonal diamond” (Figure 3c,d).15 The charge-balancing dimethylammonium cations in 1 are believed to be a decomposition product of DMF,16 and are found at locations close to the pore walls. The solventaccessible voids estimated after considering the cations is 55% of the total crystal volume.8 The Connolly surface of 1 created with the probe radius of 1.4 Å is depicted in Figure 2c. The figure clearly shows that adjacent pores are interconnected through windows and channels. Solvent molecules occupying these voids can be readily replaced by low-boiling solvents, such as dichloromethane, and removed under mild conditions. The onset of the thermal decomposition of the framework is

Figure 4. Gas sorption isotherms for 1. Filled and open symbols denote adsorption and desorption, respectively.

The overall shape is type I typical for microporous materials; however, hysteresis and unlimited adsorption are observed at high P/P0, which are the characteristics of meso- or macroporous materials.17 Because the microporous nature of 1 is unambiguously established by X-ray crystallography, we attributed the atypical features to physical defects in the framework caused by the activation processes. In order to verify this hypothesis, we measured 1H NMR spectra of 1 after digesting the solid in NaOH/D2O solutions (Figure 5). For comparison, samples were prepared by slightly different activation processes, such as the number of solvent-exchange steps and evacuation temperatures. To our surprise, the results 2000

dx.doi.org/10.1021/cg500116f | Cryst. Growth Des. 2014, 14, 1998−2002

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SBUs are linked by asymmetrically substituted ligands to form the fluorite topology. Although this material readily loses some of the ligands that constitute the framework, the stability as a porous solid is maintained and multimodal porosities are observed. The fact that tetrahedral Zn nodes can readily be reproduced using the 2-hydroxycarboxylate moiety may be a useful guide for the designed synthesis of new MOFs with highsymmetry topologies.

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ASSOCIATED CONTENT

S Supporting Information *

Plots of TGA and N2 sorption data for 1, and snapshots of single-crystal diffractions by 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-31-400-5506; fax: +82-31-400-5457; e-mail: [email protected].

Figure 5. 1H NMR spectra of 1 measured after digestion in NaOH/ D2O. See the text for detailed activation processes for samples in (a)− (c). The integrations are referenced against the most deshielded aromatic protons. Asterisks denote protons from residual solvents, such as DMF or CH2Cl2.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (2012R1A1A2004333).

were inconsistent from measurement to measurement, even though only hand-picked single crystals were used to ensure the purity of the samples. More specifically, the integrated ratio of protons from dabco with respect to the hbdc dianion varies significantly depending on the activation temperatures (60, 80, and 100 °C for Figure 5b, a, and c, respectively). This is possible when the neutral diamine ligand had been partially removed from the framework by the heating processes. Note that all atoms of the dabco ligand are well-defined with no sign of a partial occupancy or disorder in the original crystal structure. The integration of protons for the dimethylammonium cations also shows large variations in which the numbers decrease as the solvent-exchange steps increase from Figure 5a−c.18 Therefore, it can be concluded that the framework of 1 readily loses some of the ligands during the activation processes, and the meso- and macropores created in the framework are responsible for the unusual hysteresis and unlimited uptake in the nitrogen sorption isotherms. It may be interesting to note that despite the nonstoichiometric nature of the defects in the framework, the nitrogen sorption profile is highly reproducible with almost identical features over the whole pressure range. This was confirmed by N2 sorptions measured for four independent samples (SI Figure S3). The fact that some ligands in porous MOFs can be removed without collapsing the whole framework may be a general phenomenon that has been overlooked in the past, and underlines the versatility of MOFs as porous materials. The highest values of the BET and Langmuir surface areas for 1 estimated from the nitrogen adsorption are 1042 and 1168 m2/g, respectively. The uptake capacity for H2 at 77 K and 1 bar is modest with 166 cm3/g (STP) or 1.5 wt %. At 273 K, the sorption of CO2 reaches to 16.7 wt % at 1 bar, and is selective against CH4, N2, or O2 at that temperature. For comparison, the highest uptake of CO2 at 298 K and 1 bar has been reported for a series of materials known as MOF-74 or CPO-27 (20−28 wt %).19 In conclusion, a simple and rational approach has been adopted to synthesize a new MOF in which two independent

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REFERENCES

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