Article pubs.acs.org/crystal
Cu- and Ag-Based Metal−Organic Frameworks with 4‑Pyranone-2,6dicarboxylic Acid: Syntheses, Crystal Structures, and Dielectric Properties Bo-Tao Qu, Jian-Cheng Lai, Sheng Liu, Feng Liu, Yan-Dong Gao, and Xiao-Zeng You* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *
ABSTRACT: Three Cu- and Ag-based metal−organic frameworks (MOFs) with different crystal structures from the same ligand CA (CA = 4-pyranone-2,6-dicarboxylic acid), {[Cu(CA)]·CH3CN}n (1), {[Cu(CA)(H2O)2]·H2O}n (2), and [Ag2(CA)]n (3) have been synthesized under solovthermal conditions. Crystal structures of all the compounds possessing the same crystal nonpolar point group of 2/m were determined by single-crystal X-ray diffraction analysis. Compound 1 has a (3,6)-connected three-dimensional (3D) rtl framework with (4·6 2) 2(42·610·83) Schlafli topology based on binuclear secondary building units equivalent to a paddle wheel. Compound 2 features a (3,3)-connected two-dimensional (2D) fes (4·82) layer, which is further extended to a 3D supramolecular network by interlayer hydrogen bonds. Compound 3 is a rare 3D framework with a (46)2(49·618·8) topological symbol without solvent molecules, which contains three rings that are 14-member, 10-member, and 8-member, respectively. Moreover, the dielectric properties and mechanisms of these MOFs have been measured at various frequencies and temperatures, while compound 3 possesses a relatively large dielectric constant (ε′ = 321), which is considered to be one of the best among crystalline porous coordination polymers within the nonpolar point group.
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INTRODUCTION In the past two decades, metal−organic frameworks (MOFs) have received extensive attention for their structural diversity and potential applications, such as catalysis,1,2 sensors,3−5 gas adsorption and storage,6−8 luminescent materials,9−11 and magnetism.12−14 Organic ligands play an important role in the design and synthesis of versatile MOFs. Among them, multicarboxylate compounds have been vastly used as building blocks to construct diverse frameworks. Up to now, many reports show that benzene dicarboxylic acid, benzene tricarboxylic acid, and benzene tetracarboxylic acid can be used to build intriguing architectures with multifunctional properties.15−17 On the other side, dielectric materials have always been a main research topic in material science in light of their attractive applications in capacitor and dielectric resonators. Ferroelectric and dielectric materials also largely focused on dense MOFs like [C(NH2)3M(HCOOH)3].18,19 Much attention has been drawn to the ferroelectric properties with polar point group, magnetic, catalytic and other properties of coordination polymers. However, the dielectric behavior of coordination polymers with centrosymmetric nature, such as low κ materials, still remain less studied. The different polar molecules encapsulated in MOFs show host−guest interaction through hydrogen bonds and interesting dielectric behavior from polar molecule motion and also provide important information to clarify the thermal behavior of solvent molecules.20−22 © 2015 American Chemical Society
Until now, heterocyclic diolefin dicarboxylate ligands as rigid linkers have not been thoroughly studied. Moreover, only a few of group IB in the Periodic Table of Elements Cu- and Agbased MOFs have been reported based on 4-pyranone-2,6dicarboxylic acid (abbreviation: CA) ligand, which have a mononuclear, one-dimensional (1D) chain, 1D ladder chain, and two-dimensional (2D) layer structure.23−28 Most coordination polymers based on the CA ligand have been obtained in the presence of metal ions and auxiliary neutral ligands. However, there are relatively few investigations on Cu and Ag MOFs with a single CA ligand. In this work, we attempted to design one new type of molecule channel combining both external closed pores and internal open pores in favor of improving dielectric performance by the suitable microstructures of the crystals. In this molecule, the CA ligand possesses two carboxyl and one ketonic group, which leads to various coordination capacities with metals by diverse modes. Meanwhile, CA ligand provides the pyranone heterocyclic ring, favoring supramolecular polymer assembly with hydrogen-bond or π−π stacking interaction. For the condition mentioned above, we report the synthesis, crystal structures, and dielectric properties of three new MOFs based on the CA ligand in the presence of Cu and Ag ions, {[Cu(CA)]·CH3CN}n (1), Received: November 22, 2014 Revised: January 28, 2015 Published: February 26, 2015 1707
DOI: 10.1021/cg501706t Cryst. Growth Des. 2015, 15, 1707−1713
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Crystal Growth & Design Scheme 1. Synthetic Route of Compounds 1−3
cm−1): 3296 (s), 3092 (w), 1627 (s), 1588 (s), 1424 (s), 1364 (s), 1125 (m), 969 (m), 891 (m), 805 (m), 726 (m). Preparation of [Ag2(CA)]n (3). A mixture of AgNO3 (34.4 mg, 0.2 mmol), CA (18.0 mg, 0.1 mmol), NaOH (0.1 M, 1 mL), H2O (1 mL), and CH3OH (4 mL) was sealed in Teflon-lined stainless steel autoclave and heated at 373 K under autogenous pressure for 72 h and then cooled to room temperature. Colorless needle-like crystals suitable for X-ray analysis were obtained and dried in air. Yield: 75% (based on Ag). Anal. Calcd for C7H2Ag2O6 (Mr: 397.83): C, 21.13; H, 0.51%. Found: C, 21.15; H, 0.46%. IR (KBr, cm−1): 3404 (m), 3056 (m), 1620 (s), 1555 (m), 1404 (m), 1347 (s), 1126 (w), 965 (w), 896 (m), 782 (m), 727 (m), 538 (w). X-ray Crystallography. Single-crystal X-ray diffraction analyses of 1−3 were carried out on a Bruker Smart APEX II CCD diffractometer at 296 K using the ω-scan technique, equipped with graphitemonochromated Mo Kα radiation (λ = 0.71073). All absorption corrections were applied using the SADABS programs.29 The structures were solved by direct methods and refined with full-matrix least-squares technique on F2 using the SHELXTL crystallographic software package.30 All the non-hydrogen atoms were refined anisotropically. All the hydrogen atoms on organic ligands were generated geometrically, and the solvent hydrogen atoms were located from difference Fourier maps and fixed isotropic displacement parameters. For 1, the guest CH3CN molecule is highly disordered and was treated using a split-atom model with isotropic displacement parameters. The SQUEEZE of PLATON methods was utilized to calculate the diffraction contribution from the solvent molecules and to obtain a set of solvent-free diffraction intensities. The details of the crystal parameters and data collection for the compounds are summarized in Table S1, Supporting Information, selected bond lengths and angles are listed in Table S2, Supporting Information ,and hydrogen bonding data are given in Table S3, Supporting Information.
{[Cu(CA)(H2O)2]·H2O}n (2), and [Ag2(CA)]n (3). The crystal structures of 1−3 have been investigated by singlecrystal X-ray diffraction analyses. Moreover, the dielectric properties and their mechanisms of these MOFs have been studied by the dependence of temperature and frequency.
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EXPERIMENTAL SECTION
Materials and Methods. All chemicals were purchased from commercial sources (Sigma-Aldrich) and were used without further purification. Elemental analyses of C, H, and N were carried out with a PerkinElmer 240C elemental analyzer. The Fourier transform infrared (FT-IR) spectra were recorded in the range of 400−4000 cm−1 on a Bruker Vector 22 FT-IR spectrophotometer by using KBr pellets. Thermogravimetric analyses (TGA) were performed on a simultaneous SDT 2960 thermal analyzer from 35 to 600 °C with a heating rate of 10 °C·min−1 under N2 atmosphere. Powder X-ray diffraction (PXRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation at 40 kV and 40 mA. A sorption experiment was carried out on a Micromeritics ASAP 2020 surface area porosimetry system. The ac dielectric constant and loss of compounds 1−3 were measured using HP4194A Impedance/GainPhase analyzer at various frequencies from 100 Hz to 1 MHz using powdered samples in pellets. The magnetic measurements in the temperature range of 2−300 K were carried out on a Quantum Design MPMS7 SQUID magnetometer in an applied field of 2000 Oe. Preparation of {[Cu(CA)]·CH3CN}n (1). A mixture of Cu(NO3)2· 3H2O (23.5 mg, 0.1 mmol) and CA (18.0 mg, 0.1 mmol) in 10 mL of DMF-CH3CN (v/v = 1/4) was sealed in Teflon-lined stainless steel autoclave and heated at 373 K under autogenous pressure for 72 h and then cooled to room temperature. Green block crystals suitable for Xray analysis were isolated and dried in air. Yield: 81% (based on Cu). Anal. Calcd for C9H5CuNO6 (Mr: 286.68): C, 37.71; H, 1.76; N, 4.89%. Found: C, 37.65; H, 1.80; N, 4.81%. IR (KBr, cm−1): 3269 (m), 1641 (s), 1408 (m), 1357 (m), 1113 (w), 958 (m), 924 (w), 809 (m), 731 (w), 544 (w). Preparation of {[Cu(CA)(H2O)2]·H2O}n (2). A mixture of Cu(NO3)2·3H2O (24.0 mg, 0.1 mmol), CA (36.5 mg, 0.2 mmol), NaOH (0.2 M, 1 mL), H2O (3 mL), and CH3CN (6 mL) was sealed in Teflon-lined stainless steel autoclave and heated at 373 K under autogenous pressure for 48 h and then cooled to room temperature. Green block crystals suitable for X-ray analysis were isolated and dried in air. Yield: 63% (based on Cu). Anal. Calcd for C7H8CuO9 (Mr: 299.67): C, 28.06; H, 2.69%. Found: C, 28.11; H, 2.60%. IR (KBr,
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RESULTS AND DISCUSSION
FT-IR Spectra, PXRD Patterns, Magnetism and Thermal Behaviors. As we know, the solvent and pH play a vital role in the construction of coordination polymers with the same ligand in the structure designing of MOFs. The compounds 1−3 were synthesized under solvothermal conditions by the aid of different mixed solvents (Scheme 1). Compound 1 was obtained using a mixture of DMF and CH3CN as the reaction solvent. By changing the mixture solvents and adding sodium hydroxide solution to the system, 1708
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Figure 1. (a) The coordination environment of Cu (II) ions in 1. The hydrogen atoms and CH3CN molecules are omitted for clarity. Symmetry code: #1 x, −y + 3/2, z + 1/2, #2 −x + 1, y − 1/2, −z + 1/2, #3 −x + 1, −y + 1, −z + 1, #4 −x + 2, −y + 1, −z + 1, #5 −x + 1, y + 1/2, −z + 1/2, #6 x, −y + 3/2, z − 1/2, #7 −x + 1, −y + 2, −z + 1. (b) The 3D framework of 1 constructed by 1D channels along the a axis, in which CH3CN molecular filled in the void are omitted.
Figure 2. (a) The coordination environment of Cu(II) ions in 2. The hydrogen atoms and free water molecules are omitted for clarity. Symmetry code: #1 x − 1, −y + 1/2, z + 1/2, #2 −x, −y + 1, −z + 2, #3 x + 1, −y + 1/2, z − 1/2. (b) The 2D undulating layer structure of 2. The 3D supramolecular framework of 2 stacked through hydrogen bonding interaction (orange line: hydrogen bond).
337−482 K corresponding to the release of the free guest CH3CN molecules (calc. 14.30%), and above 482 K, the framework began to collapse, which is ascribed to the removal of organic ligands. As for the compound 2, the weight loss of 17.96% was observed from 371 to 473 K, which could be attributed to the loss of the lattice and coordinated water molecules (calc. 18.02%). The decomposition of the framework occurred at 683 K due to the loss of the organic ligands. In the case of 3, there is one loss step above 546 K; the framework of compound 3 gradually decomposes originating from the release of the organic ligands. Crystal Structure of {[Cu(CA)]·CH3CN}n (1). Single-crystal X-ray diffraction reveals that compound 1 crystallizes in monoclinic space group P21/c and exhibits a three-dimensional (3D) framework. The asymmetric unit contains one crystallographically independent Cu (II) ion, one CA ligand, and one disordered lattice CH3CN molecule. As shown in Figure 1a, Cu1 is five-coordinated with square pyramid coordination geometry defined by four carboxylate oxygen atoms (O3, O4, O5, O6) from four CA ligands and one ketonic oxygen atom (O7) from one CA ligand. Thereinto, O3, O4, O5, and O6 located in equatorial plane and O7 was in axial sites. The Cu− O distance ranged from 1.963(2) to 2.131(2) Å. The coordination mode of CA ligand adopts a (κ1O-κ1O)-(κ1Oκ1O)-κ1O-μ5 bridging mode (Scheme S1a). Two Cu atoms are bridged by four carboxylate groups from four CA ligands to generate a binuclear secondary building unit (SBU) Cu(CO2)4O2 equivalent to a paddle wheel, producing a Cu···Cu distance of 2.6989(2) Å. Many SBUs are linked together by the carboxylate of CA ligands to a 2D layer motif, which are further joined via the bond of ketone to generate a 3D framework (Figure 1b). The total potential solvent accessible volumes of 1 after removal of the CH3CN molecules are 48.4%, as calculated
compounds 2 and 3 were obtained at different metal conditions, respectively. The IR spectra of 1−3 display a representative v(CO) stretching mode in the range of 1380−1704 cm−1, suggesting the presence of vas(COO) and vs(COO) vibrations, respectively. The presence of strong peaks in three compounds at 1641 cm−1 for 1, 1627 cm−1 for 2, and 1620 cm−1 for 3 indicates that all deprotonated carboxylate groups of the CA ligand coordinated to metal ions, which is also confirmed by the result of the X-ray diffraction analysis. Three compounds exhibit a slightly weak peak in the 1400−1430 cm−1 region, which might be ascribed to v(CC) stretching vibration. In order to investigate the phase purity of the compounds 1− 3, the powder X-ray diffraction (PXRD) experiments are carried out at room temperature. As shown in Figure S7, the experiment PXRD peaks are consistent with simulated PXRD results from the structure data demonstrating the bulk assynthesized samples are in single phase. The difference in intensity may come from the preferred orientation of the microcrystalline powder samples. The magnetic measurements were performed on Cu (II) compounds 1 and 2 in the temperature range of 2−300 K with an applied field of 2000 Oe. As shown in Figure S8, magnetic studies indicate that compounds 1 and 2 both exhibit weak antiferromagnetic behavior with Weiss constants θ = −0.53 K for 1 and θ = −0.21 K for 2 by the Faraday equation. We suggest the Weiss constants θ of compound 1 is stronger than that of compound 2 due to the coupling interaction of binuclear Cu ions.31−33 To investigate the thermal stability of compounds of 1−3, thermogravimetric analyses (TGA) were performed in N2 atmosphere. As shown in Figure S9, the TGA curve of compound of 1 displays a weight loss of 14.86% in the range of 1709
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Crystal Growth & Design using the PLATON/VOID routine. In consideration of the moderate void of the framework, the N2 adsorption of compound 1 exhibits low uptake capacity (Figure S6); this could be due to the decomposition of the framework in the heating process of the removal of CH3CN molecules. To get a better insight into the structure of 1, topological analysis was carried out. If the binuclear unit is treated as 6connector, the CA ligands can be regarded as 3-connector. On the basis of the simplification principle and the analysis of TOPOS, the structure of 1 can be simplified as a binodal (3,6)connected 3D rtl framework, and its point (Schläfli) symbol is (4·62)2(42·610·83) (Figure S3a). Crystal Structure of {[Cu(CA)(H2O)2]·H2O}n (2). To further investigate the effect of medium water, compound 2 was synthesized employing CH3CN/H2O/NaOH as a mixed solvent instead of the DMF/CH3CN system in 1. In comparison with 1, single-crystal X-ray diffraction reveals that compound 2 also crystallizes in monoclinic space group P21/c and has a 2D undulating layer with fes topological type. As shown in Figure 2a, there are one crystallographically independent Cu (II) ions, one CA ligand, two coordinated H2O molecule and one lattice H2O molecule. Each Cu(II) ion is coordinated with two carboxylate oxygen (O1, O3#1) and one ketonic oxygen (O5#2) atoms from two CA ligands, two oxygen atoms (O7W, O8W) from two coordinated H2O molecule, giving a square pyramid coordination geometry, where one oxygen atom locates at the top of the square pyramid and four other oxygen atoms construct the bottom plane. The length of the Cu−O bond is 1.922(0)−2.299(4) Å. The CA ligands adopt (κ1O-κ0O)-(κ1O-κ0O)-κ1O-μ3 coordination mode (Scheme S1b). Each Cu(II) ion is connected by CA ligands to a 2D undulating layer (Figure S4a), which can be described as the combination of two different characteristic building units including 30-membered rings and 14-membered rings units (Figure S1). The adjacent layers are stacked into a 3D supramolecular framework through hydrogen bonding interaction (Figure 2b). The diversity of MOFs from 3D (1) to 2D (2) could arise from the H2O molecules engaged in the reaction, which occupy the metal coordination positions, leading to the dimensionality reduction of framework. The typical hydrogen bonds are as follows: O(1W)···O(4)#4 2.821(3) Å, O(1W)···O(4)#1 2.867(2), O(7)···O(1W)#5 2.656(2) Å, O(8)···O(5)#6 2.723(2) Å, O(8)···O(2)#7 2.735(2) Å, O(7)···O(2) 2.713(2) Å (Table S2). If both Cu (II) ions and CA ligands are treated as three-connected node, the 2D undulating (3,3) layer can be simplified to a fes net with Schläfli symbol (4·82) (Figure S4b). Crystal Structure of [Ag2(CA)]n (3). The crystal structure determination reveals that compound 3 also crystallizes in monoclinic space group C2/c and displays a complicated 3D framework with a 4,8-connected net. The asymmetric unit is comprised of one crystallographically independent Ag(I) ion and one CA ligand. As depicted in Figure 3a, Ag1 is fourcoordinated by four oxygen atoms (O2, O3, O3#1, O4) from three carboxylate oxygen (O2, O3, O3#1) and one ketonic oxygen (O4) atoms, showing a distorted tetrahedral coordination geometry. The Ag−O distances vary in the range of 2.284(3)−2.455(3) Å. The distance of Ag1#8···Ag1#9 is 3.212(7) Å, smaller than twice the van der Waals radius of silver atoms (3.44 Å), indicating the existence of a Ag···Ag interaction.34 The CA ligand adopts (κ1O-κ2O)-(κ1O-κ2O)κ2O-μ8 bridging to link eight Ag (I) ions (Scheme 1c), which result in the complicated 3D framework (Figure S5a and 3b). It
Figure 3. (a) The coordination environment of Ag (I) ions in 3. The hydrogen atoms are omitted for clarity. Symmetry code: #1 x − 1, y, z, #2 −x + 3/2, −y + 3/2, −z + 1, #3 −x + 2, y, −z + 1/2, #4 −x + 1, −y + 2, −z + 1, #5 x − 1/2, y + 1/2, z, #6 x + 1/2, y − 1/2, z, #7 x + 1, y, z. (b) The 3D network in the structure of 3, in which the bimetallic 14-, 10-, 8-member rings are shown as three different channels.
should be mentioned that there are three kinds of different channels in the framework: the largest channel openings are 14membered-ring openings with a window size of 5.70 × 8.27 Å2, the moderate channel openings are 10-membered-ring openings with a window size of 4.44 × 4.62 Å2, and the smallest channel openings are 8-membered-ring openings with a window size of 3.11 × 4.83 Å2 (Figure S2). A better insight into this 3D framework can be provided by a topology analysis. The 3D structure of 3 can be clarified as a classical 4,8connected net with the vertex symbol of (46)2(49·618·8) by considering Ag (I) ions as the 4-connected nodes and the CA ligands as the 8-connected nodes (Figure S5b). The crystal structures of MOFs 1−3 depend on the number of composition, ion size, polarization, weak intermolecular interaction, and other reaction conditions in the experiment, as described previously. For 1, the new 3D rtl framework was constructed by utilizing binuclear SBU Cu(CO2)4O2 equivalent to a paddle wheel as linkers, in which the coordination mode of CA ligand adopts a (κ1O-κ1O)-(κ1O-κ1O)-κ1O-μ5 bridging mode (Scheme 1a). However, when Cu(NO3)2·3H2O was replaced by AgNO3, a novel 3D structure of 3 was obtained. Compound 3 displays a complicated 3D framework with the vertex symbol of (46)2(49· 618·8). Interestingly, there are three kinds of different channels in the framework, the CA ligand links eight metals, and the coordination mode can be described as (κ1O-κ2O)-(κ1O-κ2O)κ2O-μ8 (Scheme 1c). By virtue of the fact that the mixed solvents do not act in coordination but neutralize, the difference in the structures between 1 and 3 demonstrates that metals ions play a vital role in the construction of coordination compounds and MOFs, probably due to the different ion radius, polarizabilities, and coordination numbers (CN) between copper and silver ions (rCu2+ = 0.79 Å, αCu2+ = 0.294 Å3, CNCu2+ = 5 and rAg+ = 1.14 Å, αAg+ = 1.72 Å3, CNAg+ = 4). In comparison with 1, compound 2 reveals a 2D undulating layer with fes topological type. Thereinto, the CA ligands adopts (κ1O-κ0O)-(κ1O-κ0O)-κ1O-μ3 coordination mode (Scheme 1b). The H2O molecules engaged in the reaction occupy the coordination position of metal, leading to the dimensionality reduction of framework from 3D (1) to 2D (2). Dielectric Properties of MOFs. Dielectric behavior in MOFs is mainly caused by polarization, which can be attributed to the order−disorder orientation arrangement and hydrogenbond interaction of polar guest molecules inside host frameworks or the displacement and shift caused by atom 1710
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Figure 4. Temperature dependence of the dielectric constant ε′ (a−c left) and dielectric loss ε″/ε′ (a−c right) of 1−3 at various frequencies (102− 106 Hz) corresponding to compounds 1−3. The insets show the corresponding magnified images.
vibration in the crystal lattices.35−43 The complex dielectric behaviors ε (ε = ε′ − iε″, where the ε′ and ε″ are the corresponding real part and imaginary part, respectively) is dependent on temperature and electric field at various frequencies. Because of the difficulty in obtaining enough large single crystals, herein we explored dielectric properties of the three MOFs 1−3 at different temperatures with frequencies 102 Hz to 106 Hz based on pellets from their powdered samples. The temperature dependence of the real dielectric constant ε′ and the dielectric loss ε″/ε′ (usually noted by tan δ) for 1−3 were measured in the temperature range of 273− 573 K. For a start, we discuss the dielectric properties of MOFs of 1 and 2 containing different guest molecules in the host frames. As for 1, as shown in Figure 4a, left, the dielectric constant ε′ rapidly increases and reaches a maximum at 482 K in the
temperature range from 450 to 485 K. Then the dielectric constant slowly decreases as the temperature further increases. The dielectric constant peak of ε′ about 44 at 100 Hz appears up to 482 K, implying that guest CH3CN molecules or its small cluster in the void of 3D structure bring about order−disorder orientation along with the release of the guest CH3CN molecules, which is consistent with its heating process of TGA at the temperature range of 337−482 K in Figure S7. To further investigate the dielectric behavior caused by the guest molecule, we have measured its temperature dependence of the dielectric constant ε′ from 293 to 495 K. The dielectric constant reaches maximum as the temperatures increasing, with no peak appearing at the reversed process (Figure S10). It means that the guest molecules in the channel of the framework are transferred to a paraelectric state by a phase change. When 1711
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values of Curie constants C within the range of 300−10000 K, belonging to order−disorder-type ferroelectrics-like. Their phase transition behaviors are induced by an order−disorder orientation arrangement for 1 and hydrogen-bond interaction for 2 at 486−495 K. For 3, the larger Curie constant C of 14285 K indicates that compound 3 is in the transition stage from order−disorder-type ferroelectrics-like to displacive-type ferroelectrics-like, as confirmed by the vibration of the large polarizability of Ag+ ion.44,45 It is clear that, with the rise of temperature, the positional freedom of guest molecules will be increased inside the channel of framework, leading to a larger polarizability in the host− guest MOF materials. From the macroscopic perspective in 1 and 2, the appearance of dielectric behavior (ε1′ ≈ 44 at 482 K for 1, ε2′ ≈ 35 at 486 K for 2) can be ascribed to the thermal motion of CH3CN and water molecules in the void of framework. Further, the difference of peak positions in 1 and 2 comes from the interaction of H2O molecules with a framework that makes the peak slightly shift to high temperature. From structural analysis and the information in hand, we attribute the dielectric behavior of compounds 1 and 2 to the existence of dipolar guest molecules allocated in the void along with their orientation polarization. Meanwhile, the weak interactions in 1 and 2 have been proven by magnetic performance. In comparison, the crystal of 3 without guest molecules located in the void of the framework shows a large enhancement of the dielectric constant (ε3′ ≈ 321 at 498 K), which is approximately 7 times larger than in the case of 1. Temperature rising contributes to the polarization enhancement of silver ions under an applied field due to the polarizability of Ag ions. Compared to nonintrinsic super dielectric materials (ε′ > 500), this kind of intrinsic dielectric material containing a carboxylic acid structure like 3, to our knowledge, may be the first sample for of a 3D framework with the nonpolar point group of 2/m. To a large extent, the polarization of inorganic−organic hybrid materials often involves several mechanisms operating simultaneously.
the temperature rises continually, guest molecules are released along with the framework decomposition.35−37 The dielectric behavior of 2 is similar to that of 1 (Figure 4b, left), but the temperature dependence of the dielectric constant shows two characteristic peaks around 486 K. The peak intensity of dielectric constant ε′ appears at 35. It may be ascribe to the release of free water and coordinated water molecules, respectively. It is well-known that water molecule is a typical polarized solvent as both proton donors and acceptors. On account of the existence of guest water molecules in the 2D layer structure of 2, the coordinated water molecules within the layers and the free water molecules between the layers involve hydrogen-bond interactions, both of which affect the dielectric constant, leading to the two sharp peaks in Figure 4b.38−40 As for 3, it is worth emphasizing that there are no solvents inside where we take it as a “vacant” here in the channel, but it displays a considerable large dielectric constant as high as 321 at 498 K (Figure 4c, left). Applying the polarization theory for ionic crystals as a simplified model, α=
12πε0a3 A(n − 1)
where a is the nearest neighbor distance (which here is the distance between metal cation and other anions from the ligand), A is Madelung constant and n is an undecided parameter. Here in our model, because all three compounds share the same point group, A and n should be the same, and hence a larger nearest neighbor distance a would lead to the higher polarizability α. In compound 3, as shown in Tables S1 and S2, the distances of Ag···Ag and Ag−O are significantly larger than the corresponding Cu···Cu and Cu−O distances in compounds 1 and 2. As a consequence, compound 3 has a stronger polarized interaction between the Ag+ and other anions from the ligand (Figure S5) comparing with the Cu2+ ion in 1 and 2. As we know, dielectric loss performance is dependent on the frequency of the electric field, as shown in Figure 4, right. The peak height declines with the continuous increasement of frequency due to the dipoles inversing as the frequency changes. When the frequency becomes very high, there exists a relaxation behavior as the change of the frequency of electric field exceeds the dipole inversions because of a certain space hindrance throughout the crystal frameworks. At high frequencies, some dipoles even stop rotating, leading the dielectric constant to be negligible practically.41 Figure 4a, right, 4b-right, and 4c-right shows the dielectric loss of these compounds 1−3, displaying a small dielectric loss. It would be reasonable to expect the development of highly polarizable materials like 3 or even the designing of novel dielectric devices.42,43 Additionally, the temperature dependence of the dielectric constant of 1−3 in the form of 1/ε′ versus T by the Curie− Weiss law of ε′ = C/(T − θ) are shown in Figure S11. All the 1/ε′ versus T curves of 1−3 are linear and obey the Curie− Weiss law with C = 654 K and C/TC = 1.4 for 1, C = 1176 K and C/TC = 2.6 for 2, and C = 14285 K and C/TC = 29.7 for 3 with the same frequency of 102 Hz at the temperature range of 486−495 K for 1, 486−495 K for 2, and 498−518 K for 3, respectively. It is well-known that ferroelectric compounds can be classified into different groups according to the Curie constant C. For comparing our intrinsic dielectric compounds with the nonintrinsic ones, we suggest compounds 1 and 2, with the
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CONCLUSIONS In summary, three MOFs [M(CA)](guest) (M: Cu and Ag; guest: CH3CN, H2O, and vacant) with different guestcontaining crystal structures from the same ligand CA have been synthesized. They possess various topology frameworks despite having the same crystal point groups of 2/m at room temperature. Compound 1 shows a 3D rtl framework with Schläfli symbol (4·62)2(42·610·83) topology, and compound 2 displays a 2D fes net with Schläfli symbol (4·82). While compound 3 is a rarely 3D framework with (46)2(49·618·8) topological symbol. The diversities of structures and intrinsic dielectric constants from compounds 1−3 imply that the coordination mode of metals and different reaction condition have an important effect on the assembly process of MOFs. In addition, the intrinsic dielectric properties of three compounds have been discussed in detail by their crystal structure and ionic polarizability, in which the stable compound 3 may be one of the best crystal porous MOF structures exhibiting a large intrinsic dielectric constant ε′ and small dielectric loss ε″/ε′.
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ASSOCIATED CONTENT
S Supporting Information *
Crystallographic data in CIF format, selected bond lengths and angles, hydrogen bonds, additional pictures, TGA, PXRD, gas adsorption isotherm of compound 1, magnetic plots of 1712
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Crystal Growth & Design compounds 1 and 2, ε′ versus T for compound 1 at the temperature range of 293−495 K, and compound 2 at the temperature range of 290−526 K. 1/ε′ versus T curves of compounds 1−3. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of China (91022031, 21021062, 21271099) and the National Basic Research Program of China (2013CB922102, 2011CB808704). We also thank Prof. Junfeng Bai and Prof. You Song for valuable discussions.
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DOI: 10.1021/cg501706t Cryst. Growth Des. 2015, 15, 1707−1713
