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2D→3D polycatenated and 3D→3D interpenetrated metal–organic frameworks constructed from thiophene-2,5-dicarboxylate and rigid bis(imidazole) ligands
Journal of Solid State Chemistry 210 (2014) 261–266
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
2D-3D polycatenated and 3D-3D interpenetrated metal–organic frameworks constructed from thiophene-2,5-dicarboxylate and rigid bis(imidazole) ligands$ Hakan Erer a, Okan Zafer Yeşilel a,n, Mürsel Arıcı a, Seda Keskin b, Orhan Büyükgüngör c a
Department of Chemistry, Faculty of Arts and Sciences, Eskişehir Osmangazi University, 26480 Eskişehir, Turkey Department of Chemical and Biological Engineering, Koç University, İstanbul, Turkey c Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Samsun, Turkey b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 July 2013 Received in revised form 16 November 2013 Accepted 24 November 2013 Available online 4 December 2013
Hydrothermal reactions of rigid 1,4-bis(imidazol-1-yl)benzene (dib) and 1,4-bis(imidazol-1-yl)-2, 5-dimethylbenzene (dimb) with deprotonated thiophene-2,5-dicarboxylic acid (H2tdc) in the presence of Zn(II) and Cd(II) salts in H2O produced three new metal–organic frameworks, namely, [Zn(m-tdc)(H2O) (m-dib)]n (1), [Cd(m-tdc)(H2O)(m-dib)]n (2), and {[Cd2(m3-tdc)2(m-dimb)2] (H2O)}n (3). These MOFs were characterized by FT-IR spectroscopy, elemental, thermal (TG, DTA, DTG and DSC), and single-crystal X-ray diffraction analyses. Isomorphous complexes 1 and 2 reveal polycatenated 2Dþ2D-3D framework based on an undulated (4,4)-sql layer. Complex 3 exhibits a new 4-fold interpenetrating 3D framework with the point symbol of 66. Molecular simulations were used to assess the potentials of the complexes for H2 storage application. Moreover, these coordination polymers exhibit blue fluorescent emission bands in the solid state at room temperature. & 2013 Elsevier Inc. All rights reserved.
Keywords: 2D-3D Polycatenated 3D-3D Interpenetrated Metal–organic frameworks Thiophene-2,5-dicarboxylate complexes Bis(imidazole) ligands
1. Introduction In the recent years, metal organic frameworks (MOFs) with entangled architectures have gained a great deal of attention not only due to their potential applications in fields such as gas adsorption and separation [1–4], drug delivery [5] and catalysis [3,6] but also due to their diverse structural topologies. It is well known that several factors, such as the organic ligands, metal centers, pH values, reaction temperatures and solvents have great influence in the final structures [7,8]. Among these factors, the organic ligands play a crucial role in construction of MOFs. Polycarboxylic acids are frequently used for the construction of MOFs because of their variety of coordination modes and structural stability. Interpenetrated networks are one of the most investigated type of MOFs [9,10]. Various interesting interpenetrating structures were reviewed by Batten, Robson and Ciani et al. [11–13]. Formation of interpenetrated networks generally gives rise to small pores or no pores in the structure and reduces adsorption capacities [4,14] but they do not prevent the formation
of open porous materials [15]. In this work, thiophene-2,5-dicarboxylic acid (H2tdc) was used as an anionic ligand because of its thermal stability and symmetry and ability to connect to metals in diverse coordination modes [16]. Recently, incorporation with polycarboxylates and rigid or flexible N-donors auxiliary ligands has become an effective approach for construction of new frameworks with the intriguing versatile architectures [17]. Among the N-donor ligands, bis(imidazole) ligands have been utilized to construct novel frameworks owing to their coordination ability [18]. In this study, we selected two rigid bis(imidazole) bridging ligands which are 1,4-bis(imidazol-1-yl)benzene (dib) and 1,4-bis (imidazol-1-yl)-2,5-dimethylbenzene (dimb) and three new metal organic frameworks with H2tdc were synthesized and characterized by elemental analysis, IR spectra and single crystal X-ray diffraction. Photoluminescence and thermal properties of synthesized compounds were investigated in detail. In addition, molecular simulation studies of complexes 1-3 for H2 storage applications were performed. 2. Materials and physical measurements
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This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. n Corresponding author. Tel.: þ 90 222 2393750; fax: þ 90 2222393578. E-mail addresses: [email protected], [email protected] (O.Z. Yeşilel). 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.11.036
All reagents and solvents were commercially available except for dib and dimb, which were synthesized according to the literature [19]. The IR spectra were recorded in the range of 400–4000 cm 1 by means of a Bruker Tensor 27 FT-IR spectrometer with KBr pellets.
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Elemental analyses for C, H, and N were carried out at the TÜBİTAK Marmara Research Centre. A Perkin Elmer Diamond TG/DTA Thermal Analyzer was used to record simultaneous TG, DTG and DTA curves in the static atmosphere at a heating rate of 10 1C min 1 in the temperature range 30–700 1C using platinum crucibles. The decomposition enthalpies (ΔH, kJ/mol) of each stage were examined by differential scanning calorimetry (DSC) at a heating rate of 10 1C min 1 in a Seiko DSC 6200 (Exstar 6000, Seiko Instruments Inc.). Powder X-ray diffraction patterns (PXRD) were recorded on a Rikagu Smartlab X-ray diffractometer operating at 40 kV and 30 mA with Cu Kα radiation (λ ¼ 1.5406 nm). The photoluminescence spectra for the solid complex samples were determined with a Perkin-Elmer LS-55 spectrophotometer. Diffraction measurements were performed at 293 K on a STOE IPDS 2 diffractometer using Mo-Kα radiation (λ ¼0.71073 Å). The structures were solved by direct methods using the program SHELXS97 [20] with anisotropic thermal parameters for all non-hydrogen atoms. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares methods SHELXL-97 [20]. Topological analyses were performed using TOPOS40 software [21]. Details of the refinement are presented in Table 1; the crystallographic information files of 1-3 are deposited with the CCDC 918342, 918343 and 948566. Gas adsorption properties of these three MOFs using molecular simulations were studied. Grand Canonical Monte Carlo (GCMC) simulations were used to compute the single component adsorption of H2 at 298 K in 1, 2 and 3. Rigid structures were used in all simulations and the solvent molecules coordinated to metal atoms in structures 1 and 2 were removed in simulations. The universal force field (UFF) [22] was used for the framework atoms. Previous studies have shown that adsorption simulation results based on UFF agree well with the experimental measurements for MOFs [23]. Spherical Lennard-Jones (LJ) 12-6 potentials were used to model H2 [24]. The Lorentz–Berthelot mixing rules were employed to calculate the adsorbate–adsorbent and adsorbate–adsorbate LJ cross interaction parameters. Interactions between adsorbate molecules and the atoms of 1-3 were modeled using pair–wise interactions between adsorbates and each atom in the framework. We computed single component adsorption isotherms of H2 using conventional GCMC simulations by specifying the temperature and fugacity of the adsorbing gases and calculating the number of adsorbed molecules at equilibrium [25]. Periodic boundary conditions were applied in all simulations and the size of the simulation box was set to 2 2 2 crystallographic unit cells. Simulations at the lowest fugacity for each system were started from an empty matrix and each subsequent simulation at higher fugacity was started from the final configuration of the previous run. The intermolecular potentials were truncated at 13 Å. Simulations consisted of a total of 1.5 107 steps to guarantee the equilibration, followed by 1.5 107 steps to sample the desired properties.
2.1. Preparation of the complexes 2.1.1. [Zn(m-tdc)(H2O)(m-dib)]n (1) A mixture of Zn(NO3)2 6H2O (0.47 g; 1.6 mmol), H2tdc (0.27 g, 1.6 mmol), 1,4-bis(imidazol-1-yl)benzene (dib) (0.33 g; 1.6 mmol) and water (30 mL) was stirred at 90 1C for half an hour. Then the mixture was sealed in a 45 mL Parr brand teflon-lined acid digestion bomb and heated at 170 1C for 4 days, and then cooled to room temperature at a rate of 5 1C/h. Pale yellow crystals of 1 were obtained (yield: 0.386 g, 52% based on Zn(NO3)2 6H2O). Anal. Calcd. for C18H14N4O5SZn: C, 46.61; H, 3.04; N, 12.08; S, 6.91%. Found: C, 46.26; H, 3.23; N, 12.11; S, 6.99%. IR data (KBr, cm 1): 3406m, 3105m, 3018w, 1599s, 1573s, 1531s, 1492m, 1365s, 1317s, 1269m, 1249m, 1132m, 1107w, 1072m, 962m, 941w, 875w, 831m, 765m, 657m, 534m, 493w and 468w.
Table 1 Crystallographic data and structural refinement summary for 1–3.
Chemical formula FW (g mol 1) Crystal system Space group a (Å) b (Å) c (Å) α (1) β (1) γ (1) V/Å3 Z dc (g cm 3) μ(Mo-Kα) (mm 1) F(0 0 0)
1
2
3
C18H14N4O5SZn 463.76 Triclinic P-1 10.1698 (5) 10.1653 (5) 10.7015 (5) 108.382 (4) 109.154 (4) 104.374 (4) 912.22 (8) 2 1.688 1.50 472
C18H14N4O5SCd 510.79 Triclinic P-1 10.4339 (9) 10.4424 (9) 10.8423 (9) 107.888 (7) 108.978 (6) 104.610 (7) 978.34 (14) 2 1.734 1.26 508
C40H34N8O9S2Cd2 1059.67 Orthorhombic Pnn2 15.380 17.156 7.906 90.000 90.000 90.000 2086.1 2 1.687 1.18 1060
2.2, 26.5 13-13; 13-13; 13-13 11842, 3994, 0.048 3492
1.8, 27.6 –20-20; –21-22; –10-10 30899, 4781, 0.031 4671
3994, 270 0.028, 0.060 1.03 0.36, 0.59
4781, 282 0.016, 0.043 1.08 –0.64, 0.20
Data collection Radiation [Å] ThetaMin–Max [1] Dataset
Mo-Kα ¼0.71073 2.2, 26.5 12-12; 12-12; 13-13 Tot., uniq.data, R(int) 11964, 3797, 0.023 Observed data [I 42s(I)] 3602 Refinement Nref, Npar R, wR2 S Min. and max. resd. dens. [e/Å3]
3797, 270 0.025, 0.065 1.16 0.30, 0.34
2.1.2. [Cd(m-tdc)(H2O)(m-dib)]n (2) Complex 2 was obtained in a similar method to that of 1, but Zn (NO3)2 6H2O was replaced by Cd(NO3)2 4H2O (0.49 g; 1.6 mmol). Pale orange crystals of 2 were obtained (yield: 0.542 g, 66% based on Cd(NO3)2 4H2O). Anal. Calcd. for C18H14N4O5SCd: C, 42.32; H, 2.76; N, 10.97; S, 6.28%. Found: C, 42.07; H, 2.93; N, 11.09; S, 6.11%. IR data (KBr, cm 1): 3408m, 3105m, 3041w, 1577s, 1564s, 1525s, 1494m, 1384s, 1375s, 1317m, 1265m, 1245m, 1132m, 1105w, 1068m, 962w, 935w, 866w, 833m, 765m, 653m, 534m, 491w and 464w. 2.1.3. {[Cd2(m3-tdc)2(m-dimb)2] (H2O)}n (3) Complex 3 was obtained in a similar method to that of 1, but Zn (NO3)2 6H2O and dib were replaced by Cd(NO3)2 4H2O (0.49 g; 1.6 mmol) and dimb (0.38 g; 1.6 mmol). Colorless crystals of 3 were obtained (yield: 0.486 g, 57% based on Cd(NO3)2 4H2O). Anal. Calcd. for C40H34N8O9S2Cd2: C, 45.34; H, 3.23; N, 10.57; S, 6.05%. Found: C, 45.71; H, 3.21; N, 10.32; S, 6.18%. IR data (KBr, cm 1): 3491m, 3464m, 3122m, 2954w, 2922w, 1579s, 1550s, 1521s, 1398s, 1367s, 1332s, 1232m, 1124m, 1074m, 1031w, 962w, 935w, 900w, 840w, 815m, 773m, 655m, 514m and 468w.
3. Result and discussion 3.1. Crystal structures 3.1.1. [Zn(m-tdc)(H2O)(m-dib)]n (1) The complex 1 crystallizes in triclinic space group of P-1. The results of single crystal X-ray diffraction analysis revealed that the asymmetric unit of 1 consists of one Zn(II) ion, one dib, one tdc and one aqua ligands. As shown in Fig. 1, the Zn(II) ion is five coordinated with distorted square pyramidal geometry (τ ¼0.73)
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Fig. 2. A view of 2D undulated sql networks in 1.
Fig. 1. The crystal structure of 1 and the coordination environment of Zn(II) ion.
O
O S
Zn
O
Zn
O
O S
O
Cd
Complex 1
O
O
Cd
Complex 2 and Complex 3
Scheme 1. The coordination modes of tdc ligand in complexes 1-3.
[26] by two N atoms (N1 and N3), two O atoms (O2, O5i) from two different tdc ligands and one O atom (O1) from aqua ligand ((i) x, 1 þy,z) (Scheme 1(a)). Each tdc ligand connects two Zn(II) ions to form an infinite one dimensional chain, and then the chains are further linked together by dib ligands to form a two-dimensional (2D) (4,4)-sql undulated network with the window dimensions of 10.17 13.56 Å2 (defined by Zn Zn distances, Fig. 2). The adjacent wave-like layers interlock each other to form the resulting 3D framework (Fig. 3). PLATON [27] analysis indicated that the 3D structure was composed of voids of 912.2 Å3 that represent 2.9% per unit cell volume. As shown in Fig. 4(a), the 2D-3D polycatenated framework is further stabilized by hydrogen bonding interactions between carboxyl oxygen atoms (O1 O2vi ¼2.829 (2), O1 O4v ¼2.728(2), (vi) 2 x, 2 y, 2 z and (v) 2 x, 1 y, 2 z) of tdc and aqua ligands to generate the R22 ð8Þmotif (Table S1). The interpenetration layers interact with each other through π⋯π stacking between thiophene and phenyl rings, with a separation of 3.553 and 3.665 Å. Furthermore, there are the C–H⋯π interactions between C11–H11 and imidazole ring [Cg1 ¼N3–C13–C14–N4–C15, H11 Cg1iii ¼2.837 Å, C11–H11 Cg1iii ¼142.33 1 (iii)1 x, 1 y, 1 z] (Fig. 4 (b)).
3.1.2. [Cd(m-tdc)(H2O)(m-dib)]n (2) The crystal structure of complex 2 is isomorphous with that of 1. Single-crystal X-ray diffraction analysis reveals that complex 2 is also a polycatenated 2Dþ 2D-3D framework based on an
Fig. 3. (a) Schematic representation of the (4,4)-connected topological network with the point symbol of 44.62, (b) catenation motif of 1.
undulated (4,4) sql layer. It crystallizes in a triclinic space group P-1. The asymmetric unit of 2 contains one Cd(II) ion, one dib, one tdc and one aqua ligands. As shown in Fig. 5, each Cd(II) ion is coordinated by three carboxyl oxygen atoms from two tdc ligands, two nitrogen atoms from two dib ligands and one aqua ligand to constitute a distorted octahedral geometry (Scheme 1(b)). The tdc ligand acts as μ2-bridge linking two Cd(II) ions to generate a 1D chain. In 2, two carboxylate groups of tdc2 anion adopt two kinds of coordination modes which are different from those in 1: one carboxylate group adopts bidentate chelating mode and the other
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Fig. 5. The crystal structure of 2 and the coordination environment of Cd(II) ion.
Fig. 4. (a) The interlayer hydrogen-bonding interactions and view of R22 ð8Þmotif of 1 (b) The C–H π and π π interactions of 1.
exhibits monodentate mode. Adjacent 1D chains further are linked by dib ligand to produce undulated 2D networks. Within each layer, rectangle void with dimensions of 10.43 14.19 Å and angles of 84.63 and 95.37 1 (defined by Cd Cd distances and Cd Cd Cd angles). Therefore, the undulated layer is further catenated to the adjacent layers to give a 2Dþ2D-3D polycatenated network. The most important structural feature of 2 is the 2D-3D parallel polycatenation with 44.62-sql topology. To the best of our knowledge, only limited numbers of examples of 2D-3D parallel polycatenation have been reported to date. PLATON [27] analysis indicated that the 3D structure was composed of voids of 978.3 Å3 that represent 3.9% per unit cell volume. The total potential solvent volume of 2 is also 38.2 Å3. The crystal structure of 2 is further stabilized by both π⋯π interactions between thiophene and phenyl rings (center-to-center distances of 3.7510 and 3.5712 Å) and hydrogen bonding interactions (O1 O3v ¼ 2.734 (5) and O1 O5vi ¼2.773(2) Å; (v) 1 x, 1 y, 1 z, (vi) x, 1 y, 1 z) (Table S2).
3.1.3. {[Cd2(m3-tdc)2(m-dimb)2] (H2O)}n (3) When the dimb ligand was replaced by the dib ligand, a new 4fold interpenetrating 3D framework was obtained (Fig. 7). The complex 3 crystallizes in the orthorhombic crystal system with Pnn2 space group. The asymmetric unit of 3 contains one Cd(II), one dimb and one tdc ligands and one crystal water. As shown in Fig. 6, each Cd(II) is coordinated by three carboxylate oxygen atoms from two different tdc ligands and one nitrogen atom from dimb ligand at the basal positions and one nitrogen atom from one dimb ligand at the axial position to constitute a distorted square pyramidal geometry
(τ ¼0.174) [26]. The tdc ligand exhibits a tridentate-bridging coordination using three carboxylate oxygen atoms. The complex 3 displays dia topology with the point symbol of 66 (Fig. 7). A single framework in 3 possesses a large void space, which is filled via 4-fold interpenetration and quest water molecules. The total void value of the channel is estimated to be 273.5 Å, approximately 12.6% of the total crystal volume of 2164.9 Å. The total potential solvent volume of the 1D channel is 50.2 Å3, which account for 2.4% of the total cell volume (2086 Å3) as calculated by PLATON [27]. There are hydrogen bonding interactions between water molecule and uncoordinated carboxyl oxygen atoms, which further stabilized the 3D framework [O5 O2i ¼2.879(2) Å, (i) x ½, yþ ½, z ½]. 3.2. Photoluminescent and thermal stabilities As shown in Fig. 8, solid-state photoluminescence spectra of complexes 1-3 and free ligands (H2tdc, dib, dmib) were investigated at room temperature. The free ligand H2tdc displays intense emission bands at 487 and 537 nm upon excitation at ca. 380 nm. Moreover, free ligands dib, dmib exhibit intense emission bands at 373 and 395 nm upon excitation at ca. 340 nm. The emission bands of the free ligands may be assigned to the πn-n or πn-π transitions [28]. Emission spectra of complexes 1-3 are similar but complex 3 exhibits very strong emission while complex 2 shows weak emission. Emission maxima at 441 and 520 nm for 1, 437 and 537 nm for 2 and 447 and 523 nm for 3 are observed upon excitation at ca. 380 nm. These emissions can probably be assigned to ligand-to-metal charge-transfer (LMCT) and/or metal-to-ligand charge-transfer (MLCT). Compared to the emission of tdc, dib and dmib, the significant blue-shift and red-shift of the emission bands of complexes 1-3 are observed, respectively. These situations may be attributed to coordination of the tdc ligand to the Zn(II) and Cd(II) centers, which significantly increases the rigidity and asymmetry of the ligand and reduces the loss of energy by radiationless decay [29]. The PXRD patterns of the as-synthesized complexes 1-3 are wellmatched with simulated patterns from their single-crystal structures to confirm the phase purity of the complexes (Figs. S1–S3). Moreover, complexes 1-3 were activated at 200 1C, 200 1C and 150 1C, respectively according to thermal analysis results. As shown in Figs. S1-S3, the PXRD patterns of the activated complexes 1-3 are
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Fig. 6. The crystal structure of 3 and the coordination environment of Cd(II) ion (i) ½ x, ½ þy, ½ þ z; (ii) ½ x, ½ þ y, ½ þz.
Fig. 7. (a) A view of 3D framework in 3 (b) Schematic representation of 4-fold interpenetrating topological structure in 3.
1.E+00
H2adsorption (mg/g)
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1
0
20
40
60
2
80
3
100
Fugacity (bar) Fig. 9. Theoretical H2 adsorption isotherms of 1-3 at 298 K.
Fig. 8. Photoluminescence emission spectra of 1-3 and H2tdc, dib and dmib.
similar with the as-synthesized complexes 1-3, respectively. This result indicates that complexes 1-3 retain their structural integrities. Thermal analyses (TG/DTA) were performed to verify the thermal stability of the complexes (Figs. S4–S6). The TG curves show that thermograms of 1 and 2 are similar. The anhydrous complexes are stable up to 285 1C and 258 1C, respectively. Thermal decomposition of complexes proceeds in three stages. The first thermal decomposition stage of the complexes containing aqua ligands are endothermic dehydration. The second stage results in a successive decomposition of the dib ligands. Last stage involves the highly exothermic decomposition of tdc ligands. The complex 3 shows two stages decomposition process. The anhydrous complex is stable up to 356 1C. In the first stage, complex 3 starts to lose crystal water molecule. The last stage is related to the decomposition of tdc and dimb ligands by exothermic effect. The final products of the thermal decomposition were also identified by IR spectroscopy (ZnO for 1, CdO for 2 and 3). The thermo analytical results are summarized in Table S4.
3.3. Molecular simulations for H2 gas adsorption Fig. 9 shows the single component adsorption isotherms of H2 at 298 K as a function of fugacity. Adsorption of H2 increases with pressure at constant temperature as expected. The complex 1 is the material which adsorbs the highest amount of H2 at 100 bar and 298 K among the three materials. The adsorption capacity of the synthesized complexes are less than the H2 adsorption capacity of MOF-5 (0.28 wt%) and CuBTC (0.35 wt%) under the same conditions [30]. This can be attributed to the smaller pore sizes of the complexes 1-3 compared to MOFs with large pores, MOF-5 and CuBTC.
4. Conclusions In summary, three new Zn(II) and Cd(II) 3D metal–organic frameworks were synthesized by the combination of rigid tdc, rigid dib and dimb ligands. The complexes 1 and 2 are unusual
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2D-3D frameworks based on the polycatenated 2D layers. The complex 3 shows four-fold 3D-3D framework. In complex 1, the tdc ligand exhibits a m-bridging coordination mode with bis-monodentate carboxylate groups. In complex 2, the tdc ligand adopts a m-bridging coordination mode with one monodentate and one chelating carboxylate group. In complex 3, the tdc ligand adopts a m3-bridging coordination mode with using three carboxylate oxygen atoms. Complex 3 may be a candidate for light emitting device because of intense photoluminescence property. Moreover, among the three materials, the complex 1 adsorbs the highest amount of H2 at 100 bar and 298 K. Acknowledgments This work was supported by the Research Fund of Eskişehir Osmangazi University by project Number: 201219C101. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2013.11.036. References [1] J.R. Li, Y.G. Ma, M.C. McCarthy, J. Sculley, J.M. Yu, H.K. Jeong, P.B. Balbuena, H.C. Zhou, Coord. Chem. Rev. 255 (2011) 1791–1823. [2] Y.S. Bae, R.Q. Snurr, Angew. Chem. Int. Ed. 50 (2011) 11586–11596. [3] L. Ma, W. Lin, Functional Metal–Organic Frameworks: Gas Storage, Separation and Catalysis, in: M. Schröder (Ed.), Springer, Nottingham, 2010, pp. 175–205. [4] Y.L. Liu, K.F. Yue, B.H. Shan, L.L. Xu, C.J. Wang, Y.Y. Wang, Inorg. Chem. Commun. 17 (2012) 30–33. [5] C.Y. Sun, C. Qin, X.L. Wang, Z.M. Su, Expert Opin. Drug Discovery 10 (2013) 89–101.
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2D-3D polycatenated and 3D-3D interpenetrated metal–organic frameworks constructed from thiophene-2,5-dicarboxylate and rigid bis(imidazole) ligands$ Hakan Erer a, Okan Zafer Yeşilel a,n, Mürsel Arıcı a, Seda Keskin b, Orhan Büyükgüngör c a
Department of Chemistry, Faculty of Arts and Sciences, Eskişehir Osmangazi University, 26480 Eskişehir, Turkey Department of Chemical and Biological Engineering, Koç University, İstanbul, Turkey c Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Samsun, Turkey b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 July 2013 Received in revised form 16 November 2013 Accepted 24 November 2013 Available online 4 December 2013
Hydrothermal reactions of rigid 1,4-bis(imidazol-1-yl)benzene (dib) and 1,4-bis(imidazol-1-yl)-2, 5-dimethylbenzene (dimb) with deprotonated thiophene-2,5-dicarboxylic acid (H2tdc) in the presence of Zn(II) and Cd(II) salts in H2O produced three new metal–organic frameworks, namely, [Zn(m-tdc)(H2O) (m-dib)]n (1), [Cd(m-tdc)(H2O)(m-dib)]n (2), and {[Cd2(m3-tdc)2(m-dimb)2] (H2O)}n (3). These MOFs were characterized by FT-IR spectroscopy, elemental, thermal (TG, DTA, DTG and DSC), and single-crystal X-ray diffraction analyses. Isomorphous complexes 1 and 2 reveal polycatenated 2Dþ2D-3D framework based on an undulated (4,4)-sql layer. Complex 3 exhibits a new 4-fold interpenetrating 3D framework with the point symbol of 66. Molecular simulations were used to assess the potentials of the complexes for H2 storage application. Moreover, these coordination polymers exhibit blue fluorescent emission bands in the solid state at room temperature. & 2013 Elsevier Inc. All rights reserved.
Keywords: 2D-3D Polycatenated 3D-3D Interpenetrated Metal–organic frameworks Thiophene-2,5-dicarboxylate complexes Bis(imidazole) ligands
1. Introduction In the recent years, metal organic frameworks (MOFs) with entangled architectures have gained a great deal of attention not only due to their potential applications in fields such as gas adsorption and separation [1–4], drug delivery [5] and catalysis [3,6] but also due to their diverse structural topologies. It is well known that several factors, such as the organic ligands, metal centers, pH values, reaction temperatures and solvents have great influence in the final structures [7,8]. Among these factors, the organic ligands play a crucial role in construction of MOFs. Polycarboxylic acids are frequently used for the construction of MOFs because of their variety of coordination modes and structural stability. Interpenetrated networks are one of the most investigated type of MOFs [9,10]. Various interesting interpenetrating structures were reviewed by Batten, Robson and Ciani et al. [11–13]. Formation of interpenetrated networks generally gives rise to small pores or no pores in the structure and reduces adsorption capacities [4,14] but they do not prevent the formation
of open porous materials [15]. In this work, thiophene-2,5-dicarboxylic acid (H2tdc) was used as an anionic ligand because of its thermal stability and symmetry and ability to connect to metals in diverse coordination modes [16]. Recently, incorporation with polycarboxylates and rigid or flexible N-donors auxiliary ligands has become an effective approach for construction of new frameworks with the intriguing versatile architectures [17]. Among the N-donor ligands, bis(imidazole) ligands have been utilized to construct novel frameworks owing to their coordination ability [18]. In this study, we selected two rigid bis(imidazole) bridging ligands which are 1,4-bis(imidazol-1-yl)benzene (dib) and 1,4-bis (imidazol-1-yl)-2,5-dimethylbenzene (dimb) and three new metal organic frameworks with H2tdc were synthesized and characterized by elemental analysis, IR spectra and single crystal X-ray diffraction. Photoluminescence and thermal properties of synthesized compounds were investigated in detail. In addition, molecular simulation studies of complexes 1-3 for H2 storage applications were performed. 2. Materials and physical measurements
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This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. n Corresponding author. Tel.: þ 90 222 2393750; fax: þ 90 2222393578. E-mail addresses: [email protected], [email protected] (O.Z. Yeşilel). 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.11.036
All reagents and solvents were commercially available except for dib and dimb, which were synthesized according to the literature [19]. The IR spectra were recorded in the range of 400–4000 cm 1 by means of a Bruker Tensor 27 FT-IR spectrometer with KBr pellets.
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Elemental analyses for C, H, and N were carried out at the TÜBİTAK Marmara Research Centre. A Perkin Elmer Diamond TG/DTA Thermal Analyzer was used to record simultaneous TG, DTG and DTA curves in the static atmosphere at a heating rate of 10 1C min 1 in the temperature range 30–700 1C using platinum crucibles. The decomposition enthalpies (ΔH, kJ/mol) of each stage were examined by differential scanning calorimetry (DSC) at a heating rate of 10 1C min 1 in a Seiko DSC 6200 (Exstar 6000, Seiko Instruments Inc.). Powder X-ray diffraction patterns (PXRD) were recorded on a Rikagu Smartlab X-ray diffractometer operating at 40 kV and 30 mA with Cu Kα radiation (λ ¼ 1.5406 nm). The photoluminescence spectra for the solid complex samples were determined with a Perkin-Elmer LS-55 spectrophotometer. Diffraction measurements were performed at 293 K on a STOE IPDS 2 diffractometer using Mo-Kα radiation (λ ¼0.71073 Å). The structures were solved by direct methods using the program SHELXS97 [20] with anisotropic thermal parameters for all non-hydrogen atoms. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares methods SHELXL-97 [20]. Topological analyses were performed using TOPOS40 software [21]. Details of the refinement are presented in Table 1; the crystallographic information files of 1-3 are deposited with the CCDC 918342, 918343 and 948566. Gas adsorption properties of these three MOFs using molecular simulations were studied. Grand Canonical Monte Carlo (GCMC) simulations were used to compute the single component adsorption of H2 at 298 K in 1, 2 and 3. Rigid structures were used in all simulations and the solvent molecules coordinated to metal atoms in structures 1 and 2 were removed in simulations. The universal force field (UFF) [22] was used for the framework atoms. Previous studies have shown that adsorption simulation results based on UFF agree well with the experimental measurements for MOFs [23]. Spherical Lennard-Jones (LJ) 12-6 potentials were used to model H2 [24]. The Lorentz–Berthelot mixing rules were employed to calculate the adsorbate–adsorbent and adsorbate–adsorbate LJ cross interaction parameters. Interactions between adsorbate molecules and the atoms of 1-3 were modeled using pair–wise interactions between adsorbates and each atom in the framework. We computed single component adsorption isotherms of H2 using conventional GCMC simulations by specifying the temperature and fugacity of the adsorbing gases and calculating the number of adsorbed molecules at equilibrium [25]. Periodic boundary conditions were applied in all simulations and the size of the simulation box was set to 2 2 2 crystallographic unit cells. Simulations at the lowest fugacity for each system were started from an empty matrix and each subsequent simulation at higher fugacity was started from the final configuration of the previous run. The intermolecular potentials were truncated at 13 Å. Simulations consisted of a total of 1.5 107 steps to guarantee the equilibration, followed by 1.5 107 steps to sample the desired properties.
2.1. Preparation of the complexes 2.1.1. [Zn(m-tdc)(H2O)(m-dib)]n (1) A mixture of Zn(NO3)2 6H2O (0.47 g; 1.6 mmol), H2tdc (0.27 g, 1.6 mmol), 1,4-bis(imidazol-1-yl)benzene (dib) (0.33 g; 1.6 mmol) and water (30 mL) was stirred at 90 1C for half an hour. Then the mixture was sealed in a 45 mL Parr brand teflon-lined acid digestion bomb and heated at 170 1C for 4 days, and then cooled to room temperature at a rate of 5 1C/h. Pale yellow crystals of 1 were obtained (yield: 0.386 g, 52% based on Zn(NO3)2 6H2O). Anal. Calcd. for C18H14N4O5SZn: C, 46.61; H, 3.04; N, 12.08; S, 6.91%. Found: C, 46.26; H, 3.23; N, 12.11; S, 6.99%. IR data (KBr, cm 1): 3406m, 3105m, 3018w, 1599s, 1573s, 1531s, 1492m, 1365s, 1317s, 1269m, 1249m, 1132m, 1107w, 1072m, 962m, 941w, 875w, 831m, 765m, 657m, 534m, 493w and 468w.
Table 1 Crystallographic data and structural refinement summary for 1–3.
Chemical formula FW (g mol 1) Crystal system Space group a (Å) b (Å) c (Å) α (1) β (1) γ (1) V/Å3 Z dc (g cm 3) μ(Mo-Kα) (mm 1) F(0 0 0)
1
2
3
C18H14N4O5SZn 463.76 Triclinic P-1 10.1698 (5) 10.1653 (5) 10.7015 (5) 108.382 (4) 109.154 (4) 104.374 (4) 912.22 (8) 2 1.688 1.50 472
C18H14N4O5SCd 510.79 Triclinic P-1 10.4339 (9) 10.4424 (9) 10.8423 (9) 107.888 (7) 108.978 (6) 104.610 (7) 978.34 (14) 2 1.734 1.26 508
C40H34N8O9S2Cd2 1059.67 Orthorhombic Pnn2 15.380 17.156 7.906 90.000 90.000 90.000 2086.1 2 1.687 1.18 1060
2.2, 26.5 13-13; 13-13; 13-13 11842, 3994, 0.048 3492
1.8, 27.6 –20-20; –21-22; –10-10 30899, 4781, 0.031 4671
3994, 270 0.028, 0.060 1.03 0.36, 0.59
4781, 282 0.016, 0.043 1.08 –0.64, 0.20
Data collection Radiation [Å] ThetaMin–Max [1] Dataset
Mo-Kα ¼0.71073 2.2, 26.5 12-12; 12-12; 13-13 Tot., uniq.data, R(int) 11964, 3797, 0.023 Observed data [I 42s(I)] 3602 Refinement Nref, Npar R, wR2 S Min. and max. resd. dens. [e/Å3]
3797, 270 0.025, 0.065 1.16 0.30, 0.34
2.1.2. [Cd(m-tdc)(H2O)(m-dib)]n (2) Complex 2 was obtained in a similar method to that of 1, but Zn (NO3)2 6H2O was replaced by Cd(NO3)2 4H2O (0.49 g; 1.6 mmol). Pale orange crystals of 2 were obtained (yield: 0.542 g, 66% based on Cd(NO3)2 4H2O). Anal. Calcd. for C18H14N4O5SCd: C, 42.32; H, 2.76; N, 10.97; S, 6.28%. Found: C, 42.07; H, 2.93; N, 11.09; S, 6.11%. IR data (KBr, cm 1): 3408m, 3105m, 3041w, 1577s, 1564s, 1525s, 1494m, 1384s, 1375s, 1317m, 1265m, 1245m, 1132m, 1105w, 1068m, 962w, 935w, 866w, 833m, 765m, 653m, 534m, 491w and 464w. 2.1.3. {[Cd2(m3-tdc)2(m-dimb)2] (H2O)}n (3) Complex 3 was obtained in a similar method to that of 1, but Zn (NO3)2 6H2O and dib were replaced by Cd(NO3)2 4H2O (0.49 g; 1.6 mmol) and dimb (0.38 g; 1.6 mmol). Colorless crystals of 3 were obtained (yield: 0.486 g, 57% based on Cd(NO3)2 4H2O). Anal. Calcd. for C40H34N8O9S2Cd2: C, 45.34; H, 3.23; N, 10.57; S, 6.05%. Found: C, 45.71; H, 3.21; N, 10.32; S, 6.18%. IR data (KBr, cm 1): 3491m, 3464m, 3122m, 2954w, 2922w, 1579s, 1550s, 1521s, 1398s, 1367s, 1332s, 1232m, 1124m, 1074m, 1031w, 962w, 935w, 900w, 840w, 815m, 773m, 655m, 514m and 468w.
3. Result and discussion 3.1. Crystal structures 3.1.1. [Zn(m-tdc)(H2O)(m-dib)]n (1) The complex 1 crystallizes in triclinic space group of P-1. The results of single crystal X-ray diffraction analysis revealed that the asymmetric unit of 1 consists of one Zn(II) ion, one dib, one tdc and one aqua ligands. As shown in Fig. 1, the Zn(II) ion is five coordinated with distorted square pyramidal geometry (τ ¼0.73)
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Fig. 2. A view of 2D undulated sql networks in 1.
Fig. 1. The crystal structure of 1 and the coordination environment of Zn(II) ion.
O
O S
Zn
O
Zn
O
O S
O
Cd
Complex 1
O
O
Cd
Complex 2 and Complex 3
Scheme 1. The coordination modes of tdc ligand in complexes 1-3.
[26] by two N atoms (N1 and N3), two O atoms (O2, O5i) from two different tdc ligands and one O atom (O1) from aqua ligand ((i) x, 1 þy,z) (Scheme 1(a)). Each tdc ligand connects two Zn(II) ions to form an infinite one dimensional chain, and then the chains are further linked together by dib ligands to form a two-dimensional (2D) (4,4)-sql undulated network with the window dimensions of 10.17 13.56 Å2 (defined by Zn Zn distances, Fig. 2). The adjacent wave-like layers interlock each other to form the resulting 3D framework (Fig. 3). PLATON [27] analysis indicated that the 3D structure was composed of voids of 912.2 Å3 that represent 2.9% per unit cell volume. As shown in Fig. 4(a), the 2D-3D polycatenated framework is further stabilized by hydrogen bonding interactions between carboxyl oxygen atoms (O1 O2vi ¼2.829 (2), O1 O4v ¼2.728(2), (vi) 2 x, 2 y, 2 z and (v) 2 x, 1 y, 2 z) of tdc and aqua ligands to generate the R22 ð8Þmotif (Table S1). The interpenetration layers interact with each other through π⋯π stacking between thiophene and phenyl rings, with a separation of 3.553 and 3.665 Å. Furthermore, there are the C–H⋯π interactions between C11–H11 and imidazole ring [Cg1 ¼N3–C13–C14–N4–C15, H11 Cg1iii ¼2.837 Å, C11–H11 Cg1iii ¼142.33 1 (iii)1 x, 1 y, 1 z] (Fig. 4 (b)).
3.1.2. [Cd(m-tdc)(H2O)(m-dib)]n (2) The crystal structure of complex 2 is isomorphous with that of 1. Single-crystal X-ray diffraction analysis reveals that complex 2 is also a polycatenated 2Dþ 2D-3D framework based on an
Fig. 3. (a) Schematic representation of the (4,4)-connected topological network with the point symbol of 44.62, (b) catenation motif of 1.
undulated (4,4) sql layer. It crystallizes in a triclinic space group P-1. The asymmetric unit of 2 contains one Cd(II) ion, one dib, one tdc and one aqua ligands. As shown in Fig. 5, each Cd(II) ion is coordinated by three carboxyl oxygen atoms from two tdc ligands, two nitrogen atoms from two dib ligands and one aqua ligand to constitute a distorted octahedral geometry (Scheme 1(b)). The tdc ligand acts as μ2-bridge linking two Cd(II) ions to generate a 1D chain. In 2, two carboxylate groups of tdc2 anion adopt two kinds of coordination modes which are different from those in 1: one carboxylate group adopts bidentate chelating mode and the other
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Fig. 5. The crystal structure of 2 and the coordination environment of Cd(II) ion.
Fig. 4. (a) The interlayer hydrogen-bonding interactions and view of R22 ð8Þmotif of 1 (b) The C–H π and π π interactions of 1.
exhibits monodentate mode. Adjacent 1D chains further are linked by dib ligand to produce undulated 2D networks. Within each layer, rectangle void with dimensions of 10.43 14.19 Å and angles of 84.63 and 95.37 1 (defined by Cd Cd distances and Cd Cd Cd angles). Therefore, the undulated layer is further catenated to the adjacent layers to give a 2Dþ2D-3D polycatenated network. The most important structural feature of 2 is the 2D-3D parallel polycatenation with 44.62-sql topology. To the best of our knowledge, only limited numbers of examples of 2D-3D parallel polycatenation have been reported to date. PLATON [27] analysis indicated that the 3D structure was composed of voids of 978.3 Å3 that represent 3.9% per unit cell volume. The total potential solvent volume of 2 is also 38.2 Å3. The crystal structure of 2 is further stabilized by both π⋯π interactions between thiophene and phenyl rings (center-to-center distances of 3.7510 and 3.5712 Å) and hydrogen bonding interactions (O1 O3v ¼ 2.734 (5) and O1 O5vi ¼2.773(2) Å; (v) 1 x, 1 y, 1 z, (vi) x, 1 y, 1 z) (Table S2).
3.1.3. {[Cd2(m3-tdc)2(m-dimb)2] (H2O)}n (3) When the dimb ligand was replaced by the dib ligand, a new 4fold interpenetrating 3D framework was obtained (Fig. 7). The complex 3 crystallizes in the orthorhombic crystal system with Pnn2 space group. The asymmetric unit of 3 contains one Cd(II), one dimb and one tdc ligands and one crystal water. As shown in Fig. 6, each Cd(II) is coordinated by three carboxylate oxygen atoms from two different tdc ligands and one nitrogen atom from dimb ligand at the basal positions and one nitrogen atom from one dimb ligand at the axial position to constitute a distorted square pyramidal geometry
(τ ¼0.174) [26]. The tdc ligand exhibits a tridentate-bridging coordination using three carboxylate oxygen atoms. The complex 3 displays dia topology with the point symbol of 66 (Fig. 7). A single framework in 3 possesses a large void space, which is filled via 4-fold interpenetration and quest water molecules. The total void value of the channel is estimated to be 273.5 Å, approximately 12.6% of the total crystal volume of 2164.9 Å. The total potential solvent volume of the 1D channel is 50.2 Å3, which account for 2.4% of the total cell volume (2086 Å3) as calculated by PLATON [27]. There are hydrogen bonding interactions between water molecule and uncoordinated carboxyl oxygen atoms, which further stabilized the 3D framework [O5 O2i ¼2.879(2) Å, (i) x ½, yþ ½, z ½]. 3.2. Photoluminescent and thermal stabilities As shown in Fig. 8, solid-state photoluminescence spectra of complexes 1-3 and free ligands (H2tdc, dib, dmib) were investigated at room temperature. The free ligand H2tdc displays intense emission bands at 487 and 537 nm upon excitation at ca. 380 nm. Moreover, free ligands dib, dmib exhibit intense emission bands at 373 and 395 nm upon excitation at ca. 340 nm. The emission bands of the free ligands may be assigned to the πn-n or πn-π transitions [28]. Emission spectra of complexes 1-3 are similar but complex 3 exhibits very strong emission while complex 2 shows weak emission. Emission maxima at 441 and 520 nm for 1, 437 and 537 nm for 2 and 447 and 523 nm for 3 are observed upon excitation at ca. 380 nm. These emissions can probably be assigned to ligand-to-metal charge-transfer (LMCT) and/or metal-to-ligand charge-transfer (MLCT). Compared to the emission of tdc, dib and dmib, the significant blue-shift and red-shift of the emission bands of complexes 1-3 are observed, respectively. These situations may be attributed to coordination of the tdc ligand to the Zn(II) and Cd(II) centers, which significantly increases the rigidity and asymmetry of the ligand and reduces the loss of energy by radiationless decay [29]. The PXRD patterns of the as-synthesized complexes 1-3 are wellmatched with simulated patterns from their single-crystal structures to confirm the phase purity of the complexes (Figs. S1–S3). Moreover, complexes 1-3 were activated at 200 1C, 200 1C and 150 1C, respectively according to thermal analysis results. As shown in Figs. S1-S3, the PXRD patterns of the activated complexes 1-3 are
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Fig. 6. The crystal structure of 3 and the coordination environment of Cd(II) ion (i) ½ x, ½ þy, ½ þ z; (ii) ½ x, ½ þ y, ½ þz.
Fig. 7. (a) A view of 3D framework in 3 (b) Schematic representation of 4-fold interpenetrating topological structure in 3.
1.E+00
H2adsorption (mg/g)
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1
0
20
40
60
2
80
3
100
Fugacity (bar) Fig. 9. Theoretical H2 adsorption isotherms of 1-3 at 298 K.
Fig. 8. Photoluminescence emission spectra of 1-3 and H2tdc, dib and dmib.
similar with the as-synthesized complexes 1-3, respectively. This result indicates that complexes 1-3 retain their structural integrities. Thermal analyses (TG/DTA) were performed to verify the thermal stability of the complexes (Figs. S4–S6). The TG curves show that thermograms of 1 and 2 are similar. The anhydrous complexes are stable up to 285 1C and 258 1C, respectively. Thermal decomposition of complexes proceeds in three stages. The first thermal decomposition stage of the complexes containing aqua ligands are endothermic dehydration. The second stage results in a successive decomposition of the dib ligands. Last stage involves the highly exothermic decomposition of tdc ligands. The complex 3 shows two stages decomposition process. The anhydrous complex is stable up to 356 1C. In the first stage, complex 3 starts to lose crystal water molecule. The last stage is related to the decomposition of tdc and dimb ligands by exothermic effect. The final products of the thermal decomposition were also identified by IR spectroscopy (ZnO for 1, CdO for 2 and 3). The thermo analytical results are summarized in Table S4.
3.3. Molecular simulations for H2 gas adsorption Fig. 9 shows the single component adsorption isotherms of H2 at 298 K as a function of fugacity. Adsorption of H2 increases with pressure at constant temperature as expected. The complex 1 is the material which adsorbs the highest amount of H2 at 100 bar and 298 K among the three materials. The adsorption capacity of the synthesized complexes are less than the H2 adsorption capacity of MOF-5 (0.28 wt%) and CuBTC (0.35 wt%) under the same conditions [30]. This can be attributed to the smaller pore sizes of the complexes 1-3 compared to MOFs with large pores, MOF-5 and CuBTC.
4. Conclusions In summary, three new Zn(II) and Cd(II) 3D metal–organic frameworks were synthesized by the combination of rigid tdc, rigid dib and dimb ligands. The complexes 1 and 2 are unusual
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2D-3D frameworks based on the polycatenated 2D layers. The complex 3 shows four-fold 3D-3D framework. In complex 1, the tdc ligand exhibits a m-bridging coordination mode with bis-monodentate carboxylate groups. In complex 2, the tdc ligand adopts a m-bridging coordination mode with one monodentate and one chelating carboxylate group. In complex 3, the tdc ligand adopts a m3-bridging coordination mode with using three carboxylate oxygen atoms. Complex 3 may be a candidate for light emitting device because of intense photoluminescence property. Moreover, among the three materials, the complex 1 adsorbs the highest amount of H2 at 100 bar and 298 K. Acknowledgments This work was supported by the Research Fund of Eskişehir Osmangazi University by project Number: 201219C101. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2013.11.036. References [1] J.R. Li, Y.G. Ma, M.C. McCarthy, J. Sculley, J.M. Yu, H.K. Jeong, P.B. Balbuena, H.C. Zhou, Coord. Chem. Rev. 255 (2011) 1791–1823. [2] Y.S. Bae, R.Q. Snurr, Angew. Chem. Int. Ed. 50 (2011) 11586–11596. [3] L. Ma, W. Lin, Functional Metal–Organic Frameworks: Gas Storage, Separation and Catalysis, in: M. Schröder (Ed.), Springer, Nottingham, 2010, pp. 175–205. [4] Y.L. Liu, K.F. Yue, B.H. Shan, L.L. Xu, C.J. Wang, Y.Y. Wang, Inorg. Chem. Commun. 17 (2012) 30–33. [5] C.Y. Sun, C. Qin, X.L. Wang, Z.M. Su, Expert Opin. Drug Discovery 10 (2013) 89–101.
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