Synthesis, Structure, and Gas Sorption Studies of a Three

Jul 12, 2010 - A new tetracarboxylate ligand with two alkyne functionalities has been synthesized and used to form a three-dimensional (3D) metal−or...
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DOI: 10.1021/cg100046j

Synthesis, Structure, and Gas Sorption Studies of a Three-Dimensional Metal-Organic Framework with NbO Topology

2010, Vol. 10 3405–3409

Bing Zheng, Zhiqiang Liang, Guanghua Li, Qisheng Huo, and Yunling Liu* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China Received January 13, 2010; Revised Manuscript Received June 28, 2010

ABSTRACT: A new tetracarboxylate ligand with two alkyne functionalities has been synthesized and used to form a threedimensional (3D) metal-organic framework {[Cu2(BDDC)(H2O)2] 3 DMF 3 3H2O}n (H4BDDC = 5,50 -(buta-diyne-1,4-diyl) diisophthalic acid) (DMF = N,N0 -dimethylformamide). The single-crystal structure analysis reveals the topology is based on the NbO net, constructed by 4-connected rectangular ligands and 4-connected square Cu2(CO2)4 secondary building units (SBUs). The compound has permanent porosity with a large Langmuir surface area of 3111 m2/g, and shows excess and total H2 uptake as high as 3.98 and 4.60 wt %, respectively, at 77 K and 17 bar.

Introduction Metal-organic frameworks (MOFs) have attracted much attention because of their potential applications and fascinating structural diversities.1 The applications of MOFs are being realized in areas such as magnetism, nonlinear optics, catalysis, gas storage, etc.2 Recently, the approach of utilizing molecular-building-blocks (MBBs) has proven a powerful strategy and has attracted much attention in designing functional MOFs. The inorganic building blocks, such as dimeric MBBs ([M2(CO2)4] square paddle-wheels), trimeric MBBs ([M3O(CO2)6]), and tetrameric MBBs ([M4O(CO2)6]), can be targeted under suitable reaction conditions at the design stage. The organic carboxylate ligands, including ditopic, tritopic, and tetratopic ligands, can be utilized as linear linkers, triangular linkers, and square-planar or tetrahedral linkers to bridge inorganic MBBs, respectively. Numerous studies have been reported in past few years relating to high porosity and high hydrogen storage of MOFs because hydrogen is considered a most promising energy that could replace fossil fuels in mobile vehicles. Hydrogen storage capacity in MOFs can be enhanced in various ways, such as introducing open metal sites, increasing surface area and pore volume, functionalizing organic linkers, doping of alkali elements onto the organic linker parts, and utilizing catenation.2f,3 In recent years, tetracarboxylate ligands, especially elongated tetracarboxylic acids, have been widely used in the construction of MOFs, including not only stable MOFs with large pores but also MOFs that are applicable in adsorption and storage of nitrogen, hydrogen, and methane.4,5 MOF-505 [Cu2(bptc)(H2O)2(dmf)3(H2O)], based on 3,30 ,5,50 -biphenyltetracarboxylate as the ligand, is a suitable material for high hydrogen storage at 77 K and 1 atm.4a Another compound, PCN-11 or Cu2(sbtc)(H2O)2 3 3DMA, used trans-stilbene-3,30 ,5,50 -tetracarboxylate as the linker and is applicable to both hydrogen and methane storage.4b Schr€ oder and co-workers have also reported a series of isostructural MOF materials with both exposed metal sites and organic linkers of varying length and functionalization.

These materials have large pore volumes and exhibit significant hydrogen uptake, specifically NOTT-103 having achieved 77.8 mg/g (7.22 wt %) total H2 adsorption at 77 K and 60 bar.5c The research group of Eddaoudi used 3,30 ,5,50 azobenzenetetracarboxylic acid to synthesize a porous MOF with soc topology and high hydrogen storage.6 Some other groups have used various kinds of flexible tetracarboxylate linkers as the organic ligands to exhibit distorted network topologies.7 Usually, these tetracarboxylate ligands are expected to act as rigid rectangular-planar 4-connected nodes, while inorganic dimeric, trimeric, or tetrameric MBBs, formed in situ, could serve as square-planar or tetrahedral 4-connected nodes or trigonal 6-connected nodes. Then, the ligands and the inorganic MBBs could self-assemble into NbO, PtS, or soc topologies. Recently, our group has focused on developing and designing novel ligands having alkyne functionality. In this paper, we designed a new tetracarboxylate ligand named 5,50 -(butadiyne-1,4-diyl) diisophthalic acid (H4BDDC) to treat with inorganic MBBs. There are a multitude of reasons for exploring H4BDDC as the ligand. First, there have been no reports of metal-organic frameworks from H4BDDC up to now, as far as we know. Second, compared with many other elongated tetracarboxylates linkers, it is more rigid and, as a result, is ideally suited as a rectangular-planar 4-connected ligand. Third, but not last, it has both aromatic ring and alkyne functionalities, which can potentially serve as specific sites for certain molecular recognition, catalysis applications, or unique gas sorption properties with gases such as acetylene or hydrogen. Herein, we report a MOF, having the NbO net topology, self-assembled from the aforementioned ligand and Cu2(CO2)4, formed in situ. Considering the unique nature of the ligand and the large pores of the resulting MOF, we investigated its N2 and H2 sorption behavior. Experimental Section

*To whom correspondence should be addressed. Fax: þ86-431-85168624. E-mail: [email protected].

Materials and Methods. Commercially available reagents were used without further purification. Powder X-ray diffraction (PXRD) data was collected on a Rigaku D/max-2550 diffractometer with Cu KR radiation (λ = 1.5418 A˚). The elemental analyses were performed on a Perkin-Elmer 2400 element analyzer. The

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a Reaction conditions: (a) KF 3 2H2O, EtOH, rt, 4 h; (b) pyridine, MeOH, Cu(OAc)2 3 H2O, 80 °C, 3 h; (c) MeOH, KOH, 80 °C, 5 h.

infrared (IR) spectra were recorded within the 400-4000 cm-1 region on a Nicolet Impact 410 FTIR spectrometer using KBr pellets. The thermal gravimetric analyses (TGA) were performed on NETZSCH STA 449C thermogravimetric analyzer used in air with a heating rate of 10 °C min-1. N2 sorption isotherm measurement of the compound was carried out on a Micromeritics ASAP 2020 instrument. High-pressure hydrogen uptake measurment was performed by gravimetric methods (Rubotherm). Synthesis of 5,50 -(Buta-diyne-1,4-diyl)diisophthalic Acid (H4BDDC) (see Scheme 1) Diethyl-5-(trimethylsilylethynyl)isophthalate (1). Compound 1 was synthesized according to the literature procedures.8 Diethyl-5-(ethynyl)isophthalate (2). KF 3 2H2O (5.90 g, 62.7 mmol) was added to a solution of 1 (8.00 g, 25.1 mmol) dissolved in absolute ethanol (80 mL). The reaction mixture was stirred for 4 h at room temperature; then, it was extracted with ethyl acetate, washed with brine, and dried over anhydrous MgSO4. Evaporation and flash column chromatography on silica gel (petroleum ether/ethyl acetate =25:1) gave 2 (4.98 g, 20.2 mmol, 81% yield) as a pale yellow solid. 1H NMR (300 MHz, CDCl3): δ = 8.57 (t, J = 0.9 Hz, 1H), 8.25 (d, J = 1.2 Hz, 2H), 4.35 (q, J = 4.2 Hz, 4H), 3.11 (s, 1H), 1.35 (t, J = 4.2 Hz, 6H). Diethyl-5, 50 -(buta-diyne-1,4-diyl)diisophthalate (3). Cu(OAc)2 3 H2O (6.00 g, 30.1 mmol) was added to a solution of 2 (4.95 g, 20.1 mmol) dissolved in a mixture of pyridine (25 mL) and methanol (25 mL). The reaction mixture was stirred at 80 °C for 3 h; then, it was quenched with water (20 mL), and concentrated aqueous HCl (12 N) was added to neutralize pyridine. The precipitated solid was separated by filtration and washed successively with water and ethyl acetate to give pure 3 (4.33 g, 8.8 mmol, 88%) as a pale white solid. 1H NMR (300 MHz, CDCl3): δ = 8.67 (t, J = 1.5 Hz, 2H), 8.39 (d, J = 1.2 Hz, 4H), 4.44 (q, J = 6.9 Hz, 8H), 1.44 (t, J = 6.9 Hz, 12H). 13C NMR (75 MHz, CDCl3): δ = 165.0, 136.4, 131.4, 130.5, 123.4, 89.0, 61.6, 14.2. Anal. Calcd for C28H26O8 (490.50): C 68.56, H 5.34. Found: C 68.06, H 5.72. 5,50 -(Buta-diyne-1,4-diyl)diisophthalic acid (4). KOH (4.95 g, 88.4 mmol) was added to a solution of 3 (4.33 g, 8.8 mmol) dissolved in a mixture of methanol (30 mL) and water (30 mL). The mixture was stirred under reflux for 5 h; water (20 mL) was then added to dissolve solids, and the resulting solution was acidified to pH 1-2 with concentrated aqueous HCl (12 N). The precipitated solid was separated by filtration and washed successively with water to give 4 (2.68 g, 7.1 mmol, 81%) as a pale yellow solid. 1H NMR (400 MHz, DMSO): δ = 13.52 (bs, 4H), 8.48 (t, J = 1.2 Hz, 2H), 8.35 (d, J = 1.2 Hz, 4H). 13C NMR (100 MHz, DMSO): δ = 166.6, 136.7, 132.8, 131.0, 123.6, 89.8. Anal. Calcd for C20H10O8 (378.29): C 63.50, H 2.66. Found: C 63.16, H 2.52. Synthesis of {[Cu2(BDDC)(H2O)2] 3 DMF 3 3H2O}n. Cu(NO3)2 3 3H2O (10 mg, 0.040 mmol), H4BDDC (5 mg, 0.013 mmol), DMF (1 mL), EtOH (0.5 mL), H2O (0.5 mL), and HNO3 (0.2 mL, 2.7 M in DMF) were added to a 20 mL vial; then the solution was heated to 85 °C for

Table 1. Crystal Data for the Compound empirical formula Cu2C23H23NO14 formula weight 664.50 T/K 296(2) wavelength /A˚ 0.71073 crystal system hexagonal space group R3m a/A˚ 18.7762(5) c/A˚ 40.787(2) 3 12453.0(8) V /A˚ Z 9 0.797 Dcalcd/g m-3 -1 0.803 μ/mm F(000) 3042 θ range/deg 1.50-25.94 reflections collected 25548 2930 (0.0715) reflections unique (Rint) 1.055 goodness-of-fit on F2 0.0627 R1a [I > 2σ(I)] 0.1975 R2b [all data] P P P P a b R1 = Fo| - |Fc / |Fo|. R2 = {[ w(Fo2 - Fc2)2]/[ w(Fo2)2]}1/2. )

Scheme 1. Synthesis of 5,50 -(Buta-diyne-1,4-diyl)diisophthalic Acid (H4BDDC)a

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12 h to give green, block-shaped crystals (5.7 mg, 64% yield based on H4BDDC). The agreement between the experimental and simulated powder X-ray diffraction patterns indicated the phase purity of the assynthesized product (see Supporting Information Figure S2). Elemental analysis: found (wt%) C 41.25, H 3.09, N 2.26; calcd (wt%) C 41.57, H 3.49, N 2.11. IR spectrum (KBr, cm-1): 3065br, 2923br, 2212w, 1849w, 1625s, 1564s, 1501w, 1433s, 1366s, 1262 m, 1106s, 1056 m, 910 m, 860w, 767s, 722s, 673 m, 555w, 488 m. To investigate the thermal stability of the framework and structural integrity after guest removal, powder X-ray diffraction studies (PXRD) and thermogravimetric analyses (TGA) were carried out. The sample was exchanged with acetone and fully evacuated at 100 °C for 8 h. As proved by PXRD studies (see Supporting Information Figure S2), the material maintains its crystallinity upon solvent removal and thermal treatment, which is indicative of a fairly rigid and robust framework. Thermogravimetric analysis for the compound (as shown in Supporting Information Figure S4) showed the first mass loss of 34.38% between 35 and 240 °C because of the release of three guest water molecules (calcd 8.13%), one guest DMF molecule (calcd 11.00%), and two coordinated water molecules (calcd 5.42%) and partial release of the organic ligand (calcd 9.83%), the further weight loss occurring between 240 and 420 °C (41.19%) should be attributed to the decompose of the remaining part of the organic ligand (two parts amount to 51.02%, calcd 51.50%), the postresidue of 24.43% is CuO, which has been tested by the XRD analysis. Crystal Structure Determinations. A suitable single crystal of the compound (0.29  0.27  0.23 mm3) was selected for single-crystal X-ray diffraction analyses. The intensity data was collected on a Bruker Apex II diffractometer using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). To prevent crystal decomposition during data collection, the crystal was put in a thin glass tube. The number of collected reflections and independent reflections were 25548 and 2930. Data processing was accomplished with the SAINT processing program The structure was solved by direct methods and refined by full-matrix least-squares on F2 using SHELXTL, version 5.1.9 All the metal atoms were located first, then the oxygen and carbon of the compound were subsequently found in difference Fourier maps. The hydrogen atoms of the ligand were placed geometrically, and the hydrogen atoms of the coordinated water molecules could not be located but are included in the formula. All non-hydrogen atoms were refined anisotropically. Approximately 73.6% of the unit cell volume comprises a large region of the disordered solvent which could not be modeled as discrete atomic sites. The final formula was derived from crystallographic data combined with elemental and thermogravimetric analysis data. The crystallographic data is given in Table 1.

Results and Discussion Crystal Structure of {[Cu2(BDDC)(H2O)2] 3 DMF 3 3H2O}n. Crystal structure determination confirms that the compound

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Figure 1. Single-crystal structure of the compound showing (a) Cu2(CO2)4 square MBBs (green) and (b) rectangular BDDC4- MBBs (red) to form a 3-periodic crystal structure (c, d) having two different types of pores and a basic NbO net. Cu, green; C, gray; O, red. Guest molecules and hydrogen atoms are omitted for clarity.

crystallizes in the hexagonal space group R3m with one Cu2þ ion, half of a molecule of BDDC4-, and one water molecule in the asymmetric unit. Each Cu2þ ion is coordinated by five O atoms; four O atoms come from the organic ligand and the surplus is from the water molecule. Four carboxylates groups bridge two Cu2þ cations to form a Cu2(CO2)4 paddle-wheel unit with the four carboxylate groups surrounding the Cu-Cu axis, thus forming a square-planar 4-connected node (see Figure 1a). The organic ligand BDDC4- can also be viewed as a 4-connected node having rectangular-planar geometry since the four carboxylate groups on the two isophthalate moieties (and the two alkyne functionalities) are in the same plane (see Figure 1b). These two types of planar 4-connected nodes, alternately connected, as expected, result in a MOF having an NbO-type network of 64 3 82 topology in which there exists one-dimensional hexagonal channels with a pore diameter of approximately 10.4 A˚ viewed along the c axis. The distance of Cu 3 3 3 Cu in the paddle-wheel cluster is 2.659 A˚; the bond of Cu-Owater is 2.129 A˚, and Cu-OBDDC4- bond distance averages 1.960 A˚. The NbO network of the compound can also be viewed as the packing of a combination of two types of metal-ligand cages. These two types of cage have different forms due to the constituent of MBBs. The first, larger one is defined by six organic ligands and twelve inorganic MBBs, it is ellipsoidal, and the diameter along the c axis is about 26 A˚ (see Figure 1c), while the second, smaller cage consists of six inorganic MBBs and six organic ligands, it is spherical, and the diameter is about 14 A˚ (see Figure 1d). The cages are arranged with the large cage surrounded by eight small cages, each of which is also surrounded by eight large cages. It is very interesting to find that there are two different window shapes at the interconnection of the two cages, which can be described in terms of the connection of Cu2(CO2)4 paddle-wheel units. The first type consists of three Cu2(CO2)4 paddle-wheel units, with a diameter of about 10 A˚, while the other type is based on six dimeric copper units bridged by six carboxylate ligands, and the diameter is around 20 A˚.

Our MOF, MOF-505, and another tetracarboxylate-based compound, [Cu2(EBTC)(H2O)2] 3 8H2O 3 DMF 3 DMSO,10 can all be simplified as having the NbO net topology, but they differ because of the lengths and nature of the bridging ligands employed. It can easily be seen that by elongating the ligand, the size, and the shape of the cages in the structure are dramatically influenced. The structures of these compounds reveal hexagonal channels along the c axes; by modifying the amount of alkyne functionalities, the length of the ligands varies, and the size expands along the b axes (see Supporting Information Figure S5). Gas Sorption Studies. By calculating from single crystal structure with PLATON/SOLV, the accessible void in the desolvated structure of our compound was estimated to correspond to 73.6% of the total volume. In order to explore the gas sorption properties of the compound, about 120 mg of acetone-exchanged material (washing 8-10 times) was activated at 100 °C for 8 h. Gas sorption experiments were performed at 77K and 0-760 Torr on a Micromeritics ASAP 2020 instrument; the fully evacuated framework exhibits permanent microporosity, as evident by the reversible type I N2 sorption isotherms (see Figure 2) and takes up a significant amount of N2 at 77 K (719 cm3 g-1 at 1 bar). The estimated Langmuir surface area and BET surface area of the compound are 3111 m2/g and 2357 m2/g, respectively. The total pore volume measured with N2 is 1.113 cm3/g. The nitrogen uptake of 719 cm3 g-1 in the compound at 77 K and 1 bar is much higher than that of 448 cm3 g-1 in Cu-BPTC 4 and 652 cm3 g-1 in Cu-EBTC.10 We expect that this is, likely, because of the incorporation of two additional CtC bonds in the original BPTC organic backbone. The low-pressure H2 adsorption isotherms for the compound at 77 and 87 K are shown in Figure 3. The total H2 uptakes of 1.64 wt % (equivalent to 15 g/L, the volumetric storage density of the adsorbed hydrogen was calculated on the basis of the pore volume from N2 isotherms) and 0.86 wt % (8 g/L) at 77 and 87 K, respectively, and 0.95 bar. Both

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Figure 2. N2 adsorption isotherm for the compound at 77 K. Filled and open circles represent adsorption and desorption, respectively.

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Figure 4. High-pressure hydrogen adsorption isotherm for the compound at 77 K. Squares and circles represent excess and total H2 uptake, respectively.

work), Cu-BPTC,4e and Cu-EBTC.10 All of them are constructed from dicopper paddlewheel MBBs and tetracarboxylate ligands (see Supporting Information Figure S5). For hydrogen adsorption at 77 K, Cu-BDDC is saturated at 17 bar with hydrogen uptake of about 3.98 wt %, Cu-BPTC is saturated with hydrogen uptake of about 4.02 wt %, while Cu-EBTC is saturated with hydrogen uptake of about 5.01 wt %. Though the hydrogen adsorption of our compound is lower than these two compounds, to the best of our knowledge, it is comparable with many other MOFs.11 Conclusions

Figure 3. H2 sorption isotherms of the compound at 77 (red) and 87 K (blue), with filled and open symbols representing adsorption and desorption data, respectively.

adsorption isotherms are completely reversible. The isosteric heat of adsorption is calculated to be ∼6.9 kJ/mol, which is within the range of values for adsorption enthalpy of hydrogen reported in many MOFs with high surface areas and open metal sites.5c,11 Considering its high porosity and large cages, we evaluated high-pressure hydrogen adsorption on our compound at 77 K; the compound was activated by the same procedure as the N2 adsorption experiments. Consistent with its high surface area, the compound shows good performance as a hydrogen storage material at higher pressures (see Figure 4). At 77 K, the excess H2 uptake reaches a maximum of 3.98 wt % (36 g/L) at 17 bar, while the total uptake is 4.60 wt % (41 g/L). At 31 bar, a total uptake of 4.82 wt % is attained, corresponding to a volumetric storage density of 43 g/L. The lower value of density of adsorbed hydrogen for the compound in comparison to liquid hydrogen (70.8 g/L) and MOF-5 (66 g/L)11b is because of the larger pores of the compound, wherein a significant amount of H2 gas can reside far from the influence of the framework walls. To better understand the elongated ligand affects the hydrogen uptake capacity, it is useful to compare the hydrogen uptake of the three compounds with open metal sites: Cu-BDDC (this

In this contribution, we presented the design of a novel tetracarboxylate ligand with an extended π-system based on isophthalate moieties connected by alkyne functionalities, and we successfully synthesized and characterized a novel 3D MOF with extended NbO topology by using our unique, rigid linker as a rectangular-planar node to bridge square paddle-wheel Cu2(CO2)4 units. Its N2 and H2 adsorption properties have also been investigated. The compound is highly porous and N2 sorptions reveal that the Langmuir surface area of the compound is 3111 m2/g. High-pressure hydrogen sorption studies show excess and total H2 uptake as high as 3.98 and 4.60 wt %, respectively, at 77 K and 17 bar. The novel ligand can potentially be utilized to combine with other established inorganic SBUs to construct targeted MOFs with large pores and corresponding applications, and such work is currently underway in our laboratory. Acknowledgment. We gratefully acknowledge the financial support of the Natural Science Foundation of China (Grants No. 20671041, No. 20701015, No. 20788101). Supporting Information Available: The CCDC-760800 containing the supplementary crystallographic data for this paper can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 1H NMR spectra, Powder XRD patterns, IR spectra, and TGA-DTA curve, as well as some structure views of the compound. This material is available free of charge via the Internet at http://pubs.acs.org.

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