Solvothermal Synthesis, Structural Diversity, and Properties of Alkali

Publication Date (Web): July 18, 2013. Copyright © 2013 American Chemical Society ... ParmarEringathodi Suresh. Crystal Growth & Design 2018 Article ...
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Solvothermal Synthesis, Structural Diversity, and Properties of Alkali Metal−Organic Frameworks Based on V‑shaped Ligand Duraisamy Senthil Raja,† Jheng-Hong Luo,‡ Cheng-You Wu,† Yu-Jung Cheng,† Chun-Ting Yeh,† Ya-Ting Chen,† Sheng-Han Lo,† Yu-Lun Lai,§ and Chia-Her Lin*,†,‡ †

Department of Chemistry, Chung Yuan Christian University, Chung-Li 32023, Taiwan Master Program in Nanotechnology, Chung Yuan Christian University, 200 Chung-Pei Road, Chung-Li 32023, Taiwan § Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan ‡

S Supporting Information *

ABSTRACT: A series of four new metal−organic frameworks, [Na2(SBA)] (1 or CYCU-6), [K8(H2O)2(SBA)4(DMF)] (2), [Rb2(SBA)] (3), and [Cs(H2O)(HSBA)]·DMF (4), have been constructed under solvothermal conditions by using 4,4′-sulfonyldibenzoic acid (H2SBA) as ligand. The structure of the complexes has been determined by single-crystal X-ray diffraction analysis and further characterized by elemental analyses, reflectance UV−vis, and IR spectra, powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). The single crystal X-ray structural studies showed that all the complexes display threedimensional (3D) structures containing inorganic motifs with one-dimensional chains connected through organic linkers and forming 3D networks. Among the four complexes, the Na(I) complex (1) displays very high thermal stability, which was inferred from TGA and PXRD results. Moreover, the solid state luminescent properties of the new complexes have been investigated at room temperature. In addition, the gas sorption properties of 1 toward nitrogen, hydrogen, carbon dioxide, and methane are reported.

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INTRODUCTION Porous metal−organic frameworks (MOFs) have attracted intensive attention in recent years because of their intriguing structural topologies and their potential applications in various fields, such as catalysis, gas separation, gas storage, ion exchange, luminescence, magnetism, and so on.1 Though a variety of MOFs with interesting properties have been synthesized to date, the rational control of the construction of MOFs remains a great challenge. In this regard, one of the finest ways to construct MOFs is to employ a multifunctional ligand to link metal ions into an infinite framework. In particular, flexible di- and polycarboxylic acids are good candidates for the construction of novel MOFs2 because the carboxyl groups can form four-membered rings with central metal ions, thereby improving the stability of MOFs. Furthermore, di- and polycarboxylic acids have two or more carboxyl groups that can be completely or partially deprotonated, which results in a rich variety of coordination modes and many interesting structures with higher dimensions. In addition, these ligands can act as both hydrogen-bond acceptors and hydrogen-bond donors to generate supramolecular topologies. Hence, more attention has been paid to the use of flexible di- and polycarboxylates as ligands.3 However, the studies on semirigid V-shaped dicarboxylates as ligands are relatively limited.4 In this regard, 4,4′-sulfonyldibenzoic acid (H2SBA) is a typical example of a semirigid V-shaped dicarboxylate ligand and is a versatile ligand for the © XXXX American Chemical Society

construction of novel MOFs, as it has six potential donor atoms that allow the formation of variable structures with different topologies.4 On the other hand, MOFs based on s-block metal centers are studied relatively less. But, our group is actively involved in the construction of MOFs of s-block metals. Recently, we reported that V-shaped dicarboxylate multidentate O-donor ligands such as 4,4′-sulfonyldiebnzoate and 4,4′-oxybisbnzoate can be used to construct a variety of MOFs, which contain robust structure.5 And, the coordination polymers which were constructed by using 4,4′-sulfonyldiebnzoate and alkaline earth metal ions have displayed promising gas adsorption properties.5a Until now, only few articles have been found in the literature regarding alkali MOFs capable of gas sorption.6 Moreover, the coordination chemistry and structural properties of MOFs containing the H2SBA ligand have seldom been documented to date.7 With all this above background in mind, herein, we report four new alkali MOFs, [Na 2(SBA)] (1 or CYCU-6), [K8(H2O)2(SBA)4(DMF)] (2), [Rb2(SBA)] (3), and [Cs(H2O)(HSBA)]·DMF (4), which are synthesized by using H2SBA as a ligand under solvothermal reactions. Among the four complexes, the complex 1 has rhombuslike interlace Received: May 23, 2013 Revised: July 4, 2013

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Table 1. Crystallographic Data for 1−4 complex

1

2

3

4

formula formula weight T (K) λ (Å) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z Dcalc (g cm−3) μ (mm−1) reflections collected independent reflections R (int) R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data) goodness-of-fit on F2 CCDC no.

C14H8Na2O6S 350.24 295 (2) 0.71073 triclinic P1̅ 6.0616 (4) 10.9133 (7) 12.8782 (7) 72.017 (3) 79.418 (3) 78.143 (3) 786.37 (8) 2 1.479 0.286 14243 3898 0.0254 0.0703 0.1541 0.0765 0.1564 1.189 939009

C59H43K8NO27S4 1639.02 293 (2) 0.71073 monoclinic C2/c 25.8796 (5) 13.3553 (3) 23.3400 (5) 90 110.287 (1) 90 7566.6 (3) 4 1.439 0.641 36161 9353 0.0374 0.0663 0.2446 0.1246 0.2921 1.055 939010

C14H8O6RbS 389.73 293 (2) 0.71073 monoclinic P21/c 12.1526 (4) 4.8183 (2) 12.3055 (4) 90 109.222 (2) 90 680.38 (4) 2 1.902 3.819 6399 1693 0.0457 0.0461 0.1171 0.0676 0.1264 1.075 939011

C17H18CsNO8S 529.29 293 (2) 0.71073 triclinic P1̅ 6.0544 (2) 12.4991 (3) 14.7539 (5) 114.462 (1) 93.213 (1) 100.177 (1) 989.83 (5) 2 1.773 2.020 17248 4823 0.0234 0.0213 0.0568 0.0227 0.0578 1.081 939012

(0.0612 g, 0.2 mmol), NaOH (0.04 g, 1 mmol), DMF (7.0 mL), EtOH (2.0 mL), and H2O (1.0 mL). The mixture was heated at 120 °C for 2 days. The colorless crystals of 1 were filtered off, washed with EtOH, dried in room temperature atmosphere, and collected with the yield of 0.0375 g (53.57%, based on the H2SBA). Anal. found/calcd ([Na2(SBA)]·0.7EOH): C, 48.0/48.1; H, 2.3/2.3% for 1. IR (KBr, cm−1): 3426(br), 2930(w), 1934(w), 1814(w), 1674(s), 1611(s), 1560(s), 1480(w), 1400(s), 1293(s), 1162(s), 1100(m), 1011(s), 850(m), 787(m), 740(s), 688(m), 615(s), 578(w), 536(w), 458(s). Synthesis of [K8(H2O)2(SBA)4(DMF)] (2). It was obtained from a reaction mixture of H2SBA (0.0612 g, 0.2 mmol), KOH (0.0561 g, 1 mmol), DMF (7 mL), EtOH (2.0 mL), and H2O (1.0 mL). The mixture was heated at 120 °C for 2 days. The colorless crystals of 2 were filtered off, washed with EtOH, dried at room temperature, and collected with the yield of 0.0725 g (41.26%, based on the H2SBA). Anal. found/calcd.: N, 0.8/1.3; C, 43.2/43.3; H, 2.6/3.5% for 2. IR (KBr, cm−1): 3426(br), 1668(w), 1560(m), 1375(w), 1293(m), 1163(s), 1100(s), 1010(w), 870(w), 833(w), 780(w), 682(w), 609(s), 572(w), 458(w), 411(w). Synthesis of [Rb2(SBA)] (3). Reaction mixture of H2SBA (0.153 g, 0.5 mmol), RbCl (0.06 g, 0.5 mmol), DMF (7.0 mL), EtOH (2.0 mL), and H2O (1.0 mL) was heated at 90 °C for 2 days. The colorless crystals of 3 were filtered off, washed with EtOH, dried at room temperature, and collected with the yield of 0.0725 g (41.26%, based on the H2SBA). Anal. found/calcd.: C, 43.1/42.8; H, 2.1/2.5% for 3. IR (KBr, cm−1): 3446(br), 3087(m), 2920(w), 1705(br), 1574(s), 1475(w), 1386(s), 1324(m), 1204(w), 1157(s), 1000(w), 953(w), 865(w), 693(br), 505(s). Synthesis of [Cs(H2O)(HSBA)]·(DMF) (4). The complex 4 was obtained from a reaction mixture of H2SBA (0.186 g, 0.6 mmol), CsCl (0.1684 g, 1 mmol), DMF (7.0 mL), EtOH (2.0 mL), and H2O (1.0 mL). The mixture was heated at 90 °C for 2 days. The colorless crystals of 4 were filtered off, washed with EtOH, dried at room temperature, and collected with the yield of 0.0202 g (38.17%, based on the H2SBA reagent). Anal. found/calcd.: N, 2.6/2.0; C, 38.6/38.2; H, 3.4/3.4% for 4. IR (KBr, cm−1): 3504(w), 3442(w), 3364 (br), 3092(w), 2920(w), 1940(w), 1668(s), 1596(w), 1543(w), 1392(s),

channels that can exhibit high nitrogen sorption and robust porous holes. It is similar to the case that we reported in CYCU-1 and CYCU-2,5a but the difference here is that we are unable to offer any reference concerning about whether or not Na(I) MOFs have gas sorption. To this end, this work is a rare example of permanent accessible and porous Na(I)−MOFs, hence, we are presenting a profound investigation on this unique character in this article.

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EXPERIMENTAL SECTION

Materials and General Methods. All chemicals were reagent grade and used without purification. The solvothermal reactions have been carried out in Teflon-lined digestion bombs (internal volume of 23 mL) by heating the reaction mixtures at a rate of 60 °C h−1 to reach the target temperature and followed by slow cooling at the rate of 6 °C h−1 to room temperature under autogenous pressure. The phase purity of the synthesized complexes has been examined by powder X-ray diffraction (PXRD). Elemental analyses were carried out using a PE2400 CHN elemental analyzer instrument. Thermogravimetric analyses (TGA) (using a DuPont TA Q50 analyzer) have been performed on powder samples under flowing N2 gas with a heating rate of 10 °C min−1. The reflectance UV−vis spectra were measured on a Varian Cary 100 UV−vis spectrophotometer equipped with an integrating sphere attachment and a standard BaSO4 plate used as reference. Emission spectra were measured by a Hitachi F-4500 fluorescence spectrophotometer. FT-IR spectra have been recorded in the range of 400−4000 cm−1 on a JASCO FT/IR-460 spectrophotometer using KBr pellets. The gas sorption isotherms have been measured at 77 K for N2 and H2, for CH4 at 298 K, and for CO2 at 273 and 298 K by using Micromeritics ASAP 2020 adsorption apparatus. One-hundred milligrams of adsorbent has been used for the gas sorption studies. The initial gas removal process was performed under vacuum at 423 K for 12 h. The free space of the system was determined with the use of He gas. Ultrahigh pure CO2, N2, CH4, H2, and He gases were used as received in all the gas sorption experiments. Synthesis of MOFs. Synthesis of [Na2(SBA)] (1 or CYCU-6). The complex 1 has been synthesized by the reaction mixture of H2SBA B

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Figure 1. (a) Coordination spheres of the sodium atom in compound 1. (b) Edged-sharing 1D inorganic chain with the channels along the a axis (left) and the 3D structure view (right) of 1 with the channels along the a axis. (c) Pore diameters of 1 with the cross section (left) and the diagonal section (right).

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1209(w), 1157(s), 1100(s), 1000(m), 959(w), 802(w), 715(s), 620(s), 573(s), 520(s), 453(m). Single-Crystal X-ray Diffraction Studies. The diffraction measurements have been performed on Bruker AXS KAPPA APEX II diffractometer (Mo K radiation, graphite monochromator, λ = 0.71073 Å). The collected data were corrected for Lorentz and polarization effects, and then, program SADABS in APEX2 was used for absorption correction.8 The solvent molecules in 1 were highly disordered and were impossible to refine using conventional discreteatom models, thus the contribution of solvent electron density was removed by the SQUEEZE routine in PLATON.9 All the calculations were performed using APEX2 programs.10 Relevant data concerning data collection and details of the structure refinements are summarized in Table 1. Selected bond lengths are given in Table S1 of the (Supporting Information). The corresponding H-bonding data are listed in Table S2 of the (Supporting Information).

RESULTS AND DISCUSSION

Synthesis and Characterization. All four complexes have been synthesized as described in Experimental Section, using similar solvothermal reaction conditions. In each synthetic case, a particular H2SBA and metal source molar ratio has been used to prepare the complexes, which were 1:5, 1:5, 1:1, and 3:5 for 1, 2, 3, and 4, respectively. The complexes 1 and 2 have been prepared under identical solvent and reaction conditions by using NaOH and KOH as a metal source, respectively, whereas RbCl and CsCl have been used to synthesize 3 and 4, respectively. All the complexes were stable in air and insoluble in common organic solvents such as acetone, methanol, ethanol, dichloromethane, acetonitrile, chloroform, DMF, and DMSO. The elemental analysis results of the complexes are in C

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Figure 2. (a) Coordination spheres of potassium atom and coordinated SBA ligands in compound 2. (b) Edged-sharing 1D inorganic chain with the channels along the c axis (left) and the 3D structure view (right) of 2 with the channels along the c axis.

coordinated through five oxygen atoms from the carboxylate groups belonging to four SBA ligands and one oxygen atom from the SO2 group of another SBA ligand, whereas the Na(2) site is five-coordinated through four oxygen atoms from the carboxylate groups belonging to four SBA ligands and one oxygen from the SO2 group of the other SBA ligand (Figure 1a). The coordination behavior of the ligand is described as dianionic undeca-dentate and coordinated to ten different Na+ ions (Figure S7a of the Supporting Information). All of the Na−O bonds distances range from 2.31(4) to 2.69(5) Å (Table S1 of the Supporting Information).11 As shown in Figure 1b, it is interesting to note that the inorganic motif displays a 1D chain which was formed by the corner-sharing NaO6 and NaO5 polyhedra. These chains are further linked together by bridging ligands in two directions and generated the 3D metal−organic framework of 1. As shown in Figure 1c, the rhomboidal channels with a free diameter of 7.6 × 9.8 Å2 (between benzene ring centers and considering the van der Waals radii of atoms). The solvent accessible volume (SAV) calculated by PLATON9 analysis is 152.3 Å3, which is 19% of the total unit cell volume.

accord with the theoretical values. The diffuse reflectance UV− vis spectra at room temperature for the complexes, 1−4 (see Figure S1 of the Supporting Information), indicated that the observed peaks are mainly due to the intraligand charge transfer transitions. The IR peak shifts of the complexes (see Figure S2 of the Supporting Information) gave an idea about its structure. The exact polymeric structure of the new complexes was finally confirmed by single crystal X-ray crystallographic studies. The purity and homogeneity of the bulk products of 1, 2, 3, and 4 have been determined by the comparison of simulated and experimental PXRD patterns. The peak positions of the experimental patterns for 1, 2, 3, and 4 nearly matched those of the simulated ones generated from single-crystal X-ray diffraction data, as depicted in Figures S3−S6 of the Supporting Information. The differences in intensity may be due to the preferred orientation of the powder samples. Single-Crystal Structural Analysis. Structural Description of [Na2(SBA)] (1). Single crystal X-ray diffraction studies showed that the structure of the 1 possesses an extended threedimensional (3D) framework. The asymmetric unit consists of two Na atoms and one SBA ligand. The Na(1) site is sixD

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Figure 3. (a) Coordination spheres of the rubidium atom in compound 3. (b) 3D structure view of 3 with the channels along the c axis. (c) 3D structure view of 3 with the channels along the b axis.

Structural Description of [K8(H2O)2(SBA)4(DMF)] (2). Single crystal structural analysis of 2 showed that the structure of 2 possesses an extended 3D framework and crystallizes in the space group of C2/c. Each asymmetric unit of 2 consists of four K+ ions, two ligand molecules (SBA2−), one coordinated water molecule and half of a DMF molecule. The K(1) and K(4) sites are ten-coordinated and bonding with eight oxygen atoms of the carboxylate groups belonging to four SBA ligands and two oxygen atoms of the water molecules, whereas K(2) and K(3) sites are seven-coordinated, which are bonding with four oxygen atoms from the carboxylate groups belonging to four SBA ligands and two oxygen atoms of the SO2 group belonging to the other two SBA ligands and one oxygen atom of the water molecule; in the case of the six-coordinated K(5) site, K(5) is coordinated to four oxygen atoms of the carboxylate groups belonging to four SBA ligands, one oxygen atom of the water molecule, and one oxygen atom of the DMF molecules (Figure 2a). Both ligands in the asymmetric unit have different coordination behavior; one has a dianionic dodeca-dentate nature and coordinated to ten different K+ ions, and the other one acted as dianionic deca-dentate and coordinated to eight different K+ ions (Figure S7b of the Supporting Information). The K−O bond distances ranged from 2.61(3) to 3.18(4) Å (Table S1 of the Supporting Information), which agreed well with those reported earlier.12 As shown in Figure 2b, the inorganic motif displaying a 1D chain were formed by the edged-sharing KO10, KO7, and KO6 polyhedra. These chains are further linked together by SBA ligands in two directions and generated the 3D metal−organic framework of 2. Intermolecular O−H···H hydrogen bonds have been found between the carboxylate oxygen atoms and coordinated water molecules

(O1w···O2 and O1w···08 of Table S2 of the Supporting Information). Structural Description of [Rb2(SBA)] (3). Single crystal X-ray diffraction results showed that the structure of the complex 3 possesses a 3D framework and crystallizes in the space group of P21/c. The asymmetric unit of 3 consists of two Rb+ ions and one SBA2− ligand unit. The sulfur atom (Figure S1 of the Supporting Information) of the ligand acted as the center of the asymmetric unit; the asymmetric unit of 3 actually consisted of only one Rb atom, and half of the ligand molecule. The independent six-coordinated Rb+ ion is coordinated by six oxygen atoms of the carboxylate groups belonging to six SBA ligands (Figure 3a). The ligand in the asymmetric unit has acted as a dianionic hexa-dentate and coordinated to six Rb+ ions (Figure S7c of the Supporting Information). The Rb−O bonds distances range from 2.83(3) to 3.08(3) Å (Table S1 of the Supporting Information).13 As shown in Figure 3b, the inorganic motif displaying a 1D chain were formed by the edged-sharing RbO6. These chains are linked together by the μ6-links of H2SBA ligands in two directions and constructed with the 3D metal−organic framework of 3 (Figure 3c). Structural Description of [Cs(H2O)(HSBA)]·(DMF) (4). Single crystal X-ray diffraction shows that the structure of the 4 possesses an extended 3D framework. The asymmetric unit of 4 consist one Cs atom, one ligand molecule, one coordinated water and lattice DMF molecule. The nine-coordinated Cs center is coordinated with seven oxygen atoms of the carboxylate groups belonging to seven ligands and two oxygen atoms from the coordinated water (Figure 4a). The ligand in the asymmetric unit has acted as monoanionic hepta-dentate and coordinated to seven Cs+ ions (Figure S7d of the E

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Figure 4. (a) Coordination spheres of the cesium atom in compound 4. (b) Edge-sharing 1D inorganic chain with the channels along the a axis (left) and 3D structure view (right) of 4 with the channels along the a axis. (d) Pore diameters of 4 with the cross section (left) and the diagonal section (right).

channels have a free diameter of 9.1 × 11.4 Å2 (between benzene ring centers and considering the van der Waals radii of atoms). The SAV calculated without DMF and water molecules by PLATON11 analysis is 258.2 Å3, which is 26.2% of the unit cell volume. So, overall, the above structural investigation revealed that the ligand exhibited interesting and varying coordination modes in the new alkali MOFs. In particular, the ligand, H2SBA exhibited two different modes of coordination in 2. It is interesting to note that, unlike in 1, 2, and 4, the oxygen atoms of the SO2 group of H2SBA ligand in 3 did not involve coordination. Furthermore, it is worth noting that all the

Supporting Information). All of the Cs−O bond distances range from 3.07(16) to 3.75(15) Å (Table S1 of the Supporting Information).14 As shown in Figure 4b, the inorganic motif displays 1D chains, which were formed by the corner-sharing and edged-sharing of CsO9 polyhedra. These chains are further linked together by the μ7-links of SBA ligands in two directions to generate the 3D metal−organic framework of 4. Intermolecular hydrogen bonds have been found between the carboxylate oxygen atoms and coordinated water (O1w···O3, O1w···O4, and O1w···O5, Table S2 of the Supporting Information). The lattice water molecules are located in the rhomboidal channels. As shown in Figure 4c, the rhomboidal F

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Gas Sorption Properties. Though, all the complexes have 3D structures, only 1 and 4 have porous channels in it. Due to low thermal stability of 4, the gas sorption properties of 1 have only been investigated. The gas sorption isotherms of 1 were measured for N2, CO2, CH4, and H2 gases. The low-pressure N2 adsorption measurements performed on 1 at 77 K showed that it is closely related to type I isotherms which are characteristic of microporous materials (Figure 6). The surface

complexes showed some interesting pores in their framework architecture. Thermal Stability. In order to investigate the thermal stabilities of the new MOFs, the TGA and PXRD analyses have been performed. The TGA curves of the complexes 1−4 are shown in Figure 5. TGA curve of 1 showed that the initial

Figure 5. Thermogravimetric analysis of 1−4.

weight loss observed at 90−200 °C may be attributed to the release of lattice unresolved solvent molecules; after that, the framework looks stable up to 450 °C. The PRXD patterns of 1 at various temperatures have also supported the better thermal stability of 1 but only up to 350 °C because the PRXD pattern of 1 at 400 °C indicated that the framework may be transformed from its original form to the other form. In the TGA curve of 2, the first weight loss of about 11% at around 60−130 °C mainly corresponded to the partial loss of the coordinated water and DMF molecule. Then, the second weight loss started only at 430 °C. But the PXRD studies suggested that the 2 have its original framework only up to 100 °C. The TGA curve and the PXRD pattern of 3 revealed that the framework of 3 is thermally stable up to 300 °C. In the case of 4, the TGA curve showed that 4 has first weight loss at 80− 150 °C, corresponding to the release of coordinated water and lattice DMF molecules. The dehydrated phase is thermally stable up to 300 °C. But, the PXRD studies suggested that the framework of 4 is stable up to 100 °C and transferred to a new phase above 100 °C. Further heating of all the compounds up to 800 °C results in decomposition of all structures (Figure 5). Photoluminescence Properties. The solid-state excitation−emission spectra of the new alkali MOFs complexes (1− 4) have been studied at room temperature and their corresponding spectra are shown in Figure S8 of the Supporting Information. The reported emission peak for the free H2SBA is at 329 nm.15 When compared with the free ligand, the strongest emission peaks for 1, 2, 3, and 4 are at 397, 406, 418, and 398 nm, and their excitation spectra mainly show strong peaks at 361, 351, 353, and 365 nm, respectively. The emissions of the new complexes are similar to the free ligand transitions and may be ligand-centered electronic transitions perturbed by the coordination to metal ions rather than to protons. The difference in their emissions is mainly due to the differences in metal ions and the coordination environment around them.16 The enhancement of luminescence may be attributed to the chelation of the ligand with metal center, which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay.17

Figure 6. The N2 gas sorption isotherms of 1 at 77 K.

area and pore volume, which were fitted from the data, gave a Brunauer−Emmett−Teller (BET) surface area of 187.3 m2/g and a Langmuir surface area of 264.1 m2/g. It is to be noted that the nitrogen isotherms of adsorption and desorption are not coincidental. It may be due to the weak interaction between frameworks and N2 molecules at low pressure. The maximum CO2 adsorptions of 1 (Figure 7) at 1 atm is 1.73/1.33 mmol/g at 273/298 K. These values are better than that of some results on USO-3-In-A (1.59 mmol/g), SNU-9 (1.25 mmol/g), MOF-5 (microwave synthesis, 1.12 mmol/g), and ZIF-8/ZIF-100 (1.02/0.96 mmol/g). The unique structural factors of zigzag pores in 1 are similar to CYCU-1 and CYCU2, but the amount of uptakes on N2 and CO2 are more than CYCU-1 and CYCU-2, due to their smaller atoms which are less occupied on the pores. Further, the ability of gas adsorption to methane and hydrogen has been carried out for 1 to access its selectivity toward gas sorption. The methane gas sorption measurement results at 298 K showed that 1 has no methane gas sorption ability at 298 K (Figure S9 of the Supporting Information). Whereas, the H2 sorption measurement results at 77 K demonstrated that compound 1 has a 1.69 wt % hydrogen uptake at 1 atm (Figure 8). Finally, the comparison of PXRD patterns of complex 1 after gas sorption studies with its original patterns (Figure S9 of the Supporting Information) clearly indicated that the framework of 1 is stable after gas sorption experiments also.

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CONCLUSIONS In summary, we have successfully synthesized and characterized four novel alkali metal−organic frameworks with 3D networks. The isolation of the new MOFs not only provides unprecedented examples of chemical topology but also confirms the diversity of coordinative network chemistry. G

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Figure 7. The CO2 gas sorption isotherms of 1 at (A) 273 K and (B) 298 K.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +886-3-2653399. Tel: +886-3-2653315. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial assistance received from the National Science Council, Taiwan (Grants NSC101-2113-M-033-007-MY3, 101-2811-M-033-011, and NSC100-2632-M-033-001-MY3) is gratefully acknowledged. Acknowledgment is also made to Mrs. C.-W. Lu, Instrumentation Center, National Taiwan University, Taiwan for her help in elemental analyses of the new complexes. We would like to extent our sincere thanks to the Bureau of Energy, Ministry of Economic Affairs of Taiwan for their valuable help.

Figure 8. H2 gas uptake of 1 at 77 K.

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Among the four complexes, the Na-MOF (1) demonstrated high thermal stability. The BET surface area of 1 was measured to be 187.3 m2/g. Whereas, the CO2 adsorptions of 1 at 1 atm was 1.73 and 1.33 mmol/g at 273 and 298 K, respectively. The compound was also able to sorb hydrogen with 1.69 wt % uptake at the liquid nitrogen temperature.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data (CIF) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication numbers CCDC-939009, 939010, 939011, and 939012 for the complexes 1−4. Copies of the data can be obtained free of charge from the CCDC (12 Union Road, Cambridge CB2 1EZ, U.K.; Tel: +44-1223-336408, Fax: +44-1223-336003, and email: [email protected]; Web site http://www.ccdc.cam. ac.uk). Further, the Supporting Information file contains figures showing UV−vis absorption and FT-IR spectra of 1−4, PXRD patterns with varied temperatures for 1−4, coordination modes of SBA ligand in 1−4, and photoluminescence emission spectra for 1−4, PXRD patterns of 1 before and after gas sorption studies, and tables showing selected bond lengths and H-bond lengths. This material is available free of charge via the Internet at http://pubs.acs.org. H

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Crystal Growth & Design

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