Insight into Understanding Dielectric Behavior of a Zn-MOF Using

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Insight into Understanding Dielectric Behavior of a Zn-MOF Using Variable-Temperature Crystal Structures, Electrical Conductance, and Solid-State 13C NMR Spectra Yuan-Bo Tong,† Shao-Xian Liu,† Yang Zou,*,† Chen Xue,† Hai-Bao Duan,*,†,‡ Jian-Lan Liu,† and Xiao-Ming Ren*,†,§,∥ †

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 210009, P. R. China ‡ School of Environmental Science, Nanjing Xiao Zhuang University, Nanjing 211171, P. R. China § College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China ∥ State Key Lab & Coordination Chemistry Institute, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: A Zn-based metal-organic framework (MOF)/ porous coordination polymer (PCP), (EMIM)[Zn(SIP)] (1) (SIP3− = 5-sulfoisophthalate, EMIM+ = 1-ethyl-3-methylimidazolium), was synthesized using the ionothermal reaction. The Zn2+ ion adopts distorted square pyramid coordination geometry with five oxygen atoms from three carboxylates and one sulfo group. One of two carboxylates in SIP3− serves as a μ2-bridge ligand to link two Zn2+ ions and form the dinuclear SBU, and such SBUs are connected by SIP3− ligands to build the three-dimensional framework with rutile (rtl) topology. The cations from the ion-liquid fill the channels. This MOF/PCP shows two-step dielectric anomalies together with two-step dielectric relaxations; the variable-temperature single-crystal structure analyses disclosed the dielectric anomaly occurring at ca. 280 K is caused by an isostructural phase transition. Another dielectric anomaly is related to the dynamic disorder of the cations in the channels. Electric modulus, conductance, and variable-temperature solid-state 13C CP/MAS NMR spectra analyses revealed that two-step dielectric relaxations result from the dynamic motion of the cations as well as the direct-current conduction and electrode effect, respectively.

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INTRODUCTION

sitions upon thermal stimulus to give novel switchable physical properties.16 Some of metal−formate frameworks with the general formula [cat][M(HCOO)3] exhibit ferroelectric/ antiferroelectric19,20 and multiferroic21−24 properties, where M = Mg2+, Zn2+, Mn2+, Ni2+, Co2+, and Fe2+; cat = ammonium/ alkylammonium cations, and these novel functionalities are related to the rotation or motion of ammonium/alkylammonium cations in the framework. Besides metal−formate frameworks, another family of perovskite-type MOFs/PCPs, which has been extensively investigated, is the metal−cyano frameworks. The cyanide ions usually bridge monovalent alkali ions (B′) and trivalent 3d transition metallic ions (B″) to construct double-perovskite-type complexes with a general formula of [cat]2[B′B″(CN)6], where cat = ammonium, alkylammonium, or N-heterocyclic cations.16 It is similar to the family of metal−formate frameworks, most of members reported in the family of metal−cyano frameworks show switchable dielectric and optical functionalities, which are

Metal−organic frameworks (MOFs) or porous coordination polymers (PCPs) are becoming one of the most rapidly developing fields in chemistry and material sciences,1 because these open-structure materials have exploitable properties for applications in the areas of gas adsorption and separation,2 catalysis,3 luminescence,4 sensing,5 proton conduction,6 SHG response,7 piezoelectrics,7 ferroelectrics,8−11 etc. The unique structure of open framework provides an opportunity for guest molecules or the charge-compensated ions residual in the lattice of MOFs/PCPs. If the rotatable or motionable guest components possess the sensitive and controllable responses to external stimuli, such as thermal, light, and electricity, this type of MOF/PCP has promising applications in the optoelectronic devices.12−15 In this context, a subset of perovskite-type MOFs/PCPs based on diatomic or multiatomic bridges (such as CN−, N3−, HCOO−, SCN−, and N(CN)2−), recently, have been widely investigated14,16−18 and systematically and excellently reviewed by Xu and co-workers.16 Interestingly, most of the members in this family showed one or more displacive-type or order−disorder-type phase tran© XXXX American Chemical Society

Received: July 21, 2016

A

DOI: 10.1021/acs.inorgchem.6b01759 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) The asymmetric unit of 1 at 293 K with 50% thermal ellipsoid probability; (b) the dinuclear SBU; (c) the top structure of anion framework; (d) the charge-assisted H-bonding interactions between the EMIM+ cations and SIP3− ligands.

dielectric) functionalities have been scarcely found in other MOFs/PCPs.30 To explore such new MOFs/PCPs systems, it is probably an efficient strategy to achieve such a desirable MOF/PCP, where the rotatable or motionable cations residue in the lattice as charge-compensated ions via ionothermal synthesis method. In this paper, we present the exploratory study of this issue. We achieved an MOF/PCP, (EMIM)[Zn(SIP)] (1), where SIP3− = 5-sulfoisophthalate and EMIM+ = 1-ethyl-3-methylimidazolium, via ionothermal reaction. In the crystal structure of 1, the Zn2+ ions are connected by the SIP3− ligands to form anionic framework, and the EMIM+ cations from the ion-liquid with low rotation energy barrier fill the cavities and interact with the framework via weak H-bonds and van der Waals forces. This MOF/PCP exhibits multistep dielectric anomalies together with dielectric relaxations.

associated with the diverse dynamic motions of the cations, in “A” sites of double-perovskite structure.16,25 With respect to the metal−formate and metal−cyano frameworks, there are only a few examples of the perovskitetype MOFs/PCPs based on other multiatomic bridges, such as N3−, SCN−, and N(CN)2−; however, the relevant studies have been attracting much attention in this area, and some significant achievements have been accomplished.16,26−29 The perovskitetype MOFs/PCPs, [(CH3)nNH4−n][Mn(N3)3] (n = 1−4), show a phase transition with thermal magnetic hysteresis, and such magnetic bistability originates from modification of the magnetic coupling induced by the phase transition of the flexible framework;26 interestingly, the member with n = 4 has been suggested as a single-phase multiferroic candidate, which exhibits the coexistence of antiferroelectricity, ferroelasticity, and magnetic bistabilities above room temperature,27 and its analogue [(CH3)4N][Cd(N3)3] undergoes three-step reversible phase transitions,28 including an above-room-temperature ferroelastic phase transition; all of these novel functionalities are related to the dynamic motion of (CH3)4N+ in the framework. The above-mentioned perovskite-type MOFs/PCPs possess the commonly and importantly structural characters (1) the guest components residual in the cavities or channels of framework have the rotatable or the motionable freedom and (2) the MOF is flexible, and the small lattice energy leads to the lattice being easy deformation to match the dynamic motion of cations in the lattice. These distinctively structural features give rise to such types of MOFs/PCPs showing double minima or multiminima in the potential landscape of lattice energy. If the magnitude of the energy barrier between the neighboring minima is thermally accessible, the structural phase transition would occur upon thermal stimulus, and the switchable magnetic, optical, and electrical (or dielectric) properties would be achievable. It is notable that, except the perovskite-type MOFs/PCPs based on diatomic or multiatomic bridges, the structural phase transitions and switchable magnetic, optical, and electrical (or

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RESULTS AND DISCUSSION Crystal Structure at 293 K. MOF/PCP 1 crystallizes in monoclinic space group P21/c at 293 K. The asymmetric unit contains one Zn2+ ion, one SIP3− ligand, and one EMIM+ cation (Figure 1a). The Zn2+ ion adopts slightly distorted square pyramid coordination geometry with five oxygen atoms; four of them come from three carboxylates, and one of them is from the sulfo group. The carboxylate containing O4 atom serves as a μ2-bridge ligand to link two Zn2+ ions, and the carboxylate containing O6 atom adopts the binding manner of η2-COO− (ref Figure 1b), and the sulfo group acts as an η1SO3− ligand. The Zn−O lengths range from 1.958 to 2.221 Å within the square pyramid coordination sphere, and these coordination bond lengths are normal. Two ZnO5 square pyramids, which share two μ2-bridging carboxylates to form a dinuclear secondary building unit (SBU; ref Figure 1b), are related to each other through an inversion center with dZn···Zn = 3.704 Å. The three-dimensional anionic framework is built from the SUBs connected by SIP3− ligands, and the EMIM+ cations fill in the channels for charge balance. B


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Inorganic Chemistry

Figure 2. Plots of (a) ε′ vs T and (b) ε″ vs T at selected frequencies for 1.

Figure 3. Imaginary part of (a) the electric modulus vs temperature at selected frequencies (b−d) the electric modulus vs frequency at the selected temperatures and (e, f) plots of ln τ vs 1000/T of 1 for the relaxations in the different frequency regions.

The SIP3− ligand acts as an organic trinodal building block, while the dinuclear Zn cluster Zn2(SO3)(COO)4 serves as a hexatopic node. The combination of the hexanodes and the trigonal trinodes gives rise to a three-dimensional network with a binodal (3, 6)-connected network topology. From the

topological point of view, the single (3, 6)-connected net can be viewed as a rutile (rtl) net (ref Figure 1c). Although assembling trigonal building blocks with octahedral building blocks can lead to a framework of pyrite (pyr) topology as the highest possible symmetry network, a framework of an rtl C


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Inorganic Chemistry

displayed in Figure 3b−d, the plots of M″−f of 1 at the selected temperatures significantly show two steps of dielectric relaxations in the selected temperature and frequency regimes. One of them starts to appear at the temperature of T = 233 K, and the maximum of relaxation peak shifts toward higher frequency with the increasing temperature in the range of 233− 278 K. The maximum of relaxation peak is almost unchanged between 278 and 298 K (ref Figure 3c) and then shifts toward higher frequency, as the temperature is elevated. It was noted that another dielectric relaxation process occurs in the lowerfrequency regime (f < 1 × 102 Hz) when the temperature is over ca. 373 K (ref the inset of Figure 3d). The Arrhenius equation, τ = τ0 exp(Ea/kBT), was used to estimate the activation energy of the dielectric relaxation processes, and the best fit gave Ea = 0.85 eV and τ0 = 9.41 × 10−20 s in the temperature range of 233−278 K and 0.50 eV and τ0 = 5.82 × 10−12 s in the temperature range of 373−473 K for the relaxation occurring at higher-frequency side (Figure 3e), while Ea = 1.23 eV and τ0 = 2.0 × 10−17 s for the relaxation in the low frequency side. The dielectric relaxation activation energies in the higher-frequency region are comparable to that observed in other organic−inorganic hybrid solids, [C2H10N2][SnCl(SCN) 2 ] 2 , 31 and [1-hexyl-3-methylimidazolium][PbBr3],34 where the relaxation processes correspond to the dynamic motion of organic cations in the lattice. The fitted prefactors also fall within the range of τ0 values of dielectric relaxation arising from dynamic motion of organic cations in the framework.31 The derivatives of imidazolium generally show lower outplane rotation energy barrier for the imidazole ring; such a dynamic motion is related to the dipole orientation change that results in the dielectric relaxation behavior. The dielectric relaxation arising from the swing motion of alkyl chain and the out-plane rotation of the imidazole ring has been observed in ionic liquid containing derivatives of imidazolium, such as HmimBr (Hmim = 1-hexyl-3-methylimidazolium)35 and BminBMSF (Bmin = 1-butyl-3-methylimidazolium and BMSF = bis(trifluoromethanesulfonate)imide).36 Therefore, the relaxation in the high frequency and at low temperature is assigned to the swing motion of alkyl chain and the libration/out-plane rotation motion of the imidazole ring for 1. According to Debye relaxation process, all dipoles in the system relax with the same relation time, and thus, the Debye relaxation process is called a single relaxation time approximation, corresponding to the relaxation process with α = 0 in eq 2. This process is usually found in rigid molecule systems. Generally, the so-called non-Debye relaxation process is observed in all kinds of dielectric materials. The non-Debye relaxation stems from a number of reasons, and one of the most important reasons is the differences in the local environments that each of the relaxing units (molecular dipoles in organic/ macromolecular substances, impurity-vacancy dipoles in ionic crystals, etc.) experiences. This produces a range of relaxation times for the units participating in the phenomenon, which leads to experimental signals featuring broadening and peculiar asymmetries. Furthermore, after a rotational or translational process, the “relaxation” of the local structure around each relaxing unit complicates further the experimental signals. The nature of dielectric relaxations occurring in 1 is also investigated using the analysis for Cole−Cole plots, which is expressed using eq 2

topology was obtained as the next-highest possible symmetry network. Ligand SIP3− is a trinodal SBU; however, in the reduced symmetry environment seen here, it leads to a network with an rtl topology rather than a pyr topology. As shown in Figure 1d, the charge-assisted H-bonding interactions are observed between the carbon atoms of EMIM+ cations and oxygen atoms of anionic framework with the distances of dO2···C13 of 3.17 Å, dO2#...C9 of 3.20 Å, and dO6···C10 of 3.05 Å (symmetric code # = 1 + x, 2 − y, 1 − z) in the crystal structure at 293 K. Dielectric Properties. The plots of ε′ and ε″ against temperature are shown in Figure 2a,b at the selected frequencies for 1, respectively, to display temperatureindependent dielectric permittivity with ε′ ≈ 6 and dielectric loss with ε″ ≈ 0.03 at the temperature below 230 K, and this observation indicates that the thermally activatable dipole motion is suppressed at lower temperature. As the temperature is elevated, both ε′ and ε″ sharply increase and depend strongly on the alternating-current (ac) frequency. In both plots of ε′ versus T and ε″ versus T, there are two clear dielectric anomalies. One round maximum peak appears at ca. 280 K; moreover, the temperature of peak is weakly dependent on ac frequency. Another dielectric anomaly occurring at higher temperature shows strongly ac frequency-dependent character; for instance, the temperature of peak is ca. 360 K at 10 Hz, and shifts toward 425 K at 1000 Hz in plots of ε′ versus T (ref Figure 2a). The ac frequency-dependent dielectric permittivity and loss demonstrate the existence of dielectric relaxations at elevated temperature. The dielectric relaxations arise probably from the processes of ion migrations, molecular dipole orientations, electrode polarization, and space charge injection effects. The ion migrations and molecular dipole orientations are the intrinsic nature of a dielectric material; however, the electrode polarization and space charge injection belong to the extrinsic effects. A typical electrode polarization process is caused by the double-layer capacity of the nonhomogenous surface.31 In ac case, most carriers injected at electrodes during a half of cycle are ejected during the next half cycle, and thus, the net balance of charge on a cycle is practically zero. However, a small fraction of the carriers can be trapped at levels deep enough to retain them when the field is inverted. The amount of charge in ac should increase slower than in dc and become observable after longer periods of time, and this is so-called space charge injection. Since the electrode polarization and space charge injection effect undergo in the case at the low-frequency and the high-temperature and can be significantly reduced using the dielectric modulus analysis,32,33 the dielectric modulus analysis could make relaxation progress more clearly. The electric modulus (M*) is calculated by eq 1.32,33 M *(ω)

ε ′ + jε ″ 1 = 2 = M′ + jM″ ε*(ω) ε′ + ε″2

(1)

where M′ and M″ are the real and imaginary parts of the complex modulus M*, respectively. The plots of M″−T are shown in Figure 3a for 1, to disclose the existence of two steps of dielectric anomalies. The peak temperature is almost independent of the ac frequency for the dielectric anomaly undergoing at ca. 280 K, whereas it is strongly related to the ac frequency for that occurring in the higher temperature side. These results are in good agreement with the observation from the plots of ε′ versus T and ε″ versus T. In addition, as D


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Inorganic Chemistry ε = ε∞ +

ε0 − ε∞ 1 + (iωt )1 − α

fitted using the equivalent circuit method, where each impedance semicircle can be represent by a resistor R and capacitor C in parallel. The resistance and conductivity are obtained through fitting the impedance spectra at the selected temperatures, and the corresponding values are listed in Table S3. The σdc values against temperatures are plotted in the form of log σdc versus 1000/T in the temperature range of 373−473 K, which shows linear relationship (ref Figure 5b), and the activation energy (Ea) was estimated to be 1.17 eV; this value is approximately equal to that (1.23 eV) obtained from the analysis for the dielectric relaxation in the lower-frequency region, demonstrating that the dielectric relaxation in the low frequency and at elevated temperatures is caused by dc conduction (including the contribution from both electrons and ions transport) and electrode polarization. Solid-State Variable-Temperature 13C CPMAS NMR Spectra. High-resolution solid-state NMR spectrum is the simplest and powerful tool to document molecular thermal motion. On the basis of the dielectric relaxation analysis, the dynamic motion of the cations is suppressed in the lowertemperature region; as a result, the solid-state 13C CP/MAS NMR spectra of 1 were measured in the temperature range of 298−463 K to elucidate the dynamic behavior of the EMIM+ cations. The 13C CP/MAS NMR spectrum of 1 at 298 K is displayed in Figure S7 together with the assignments for the chemical shifts of carbon atoms in the SIP3− ligand, and the temperature-dependent solid-state 13C CP/MAS NMR spectra are show in Figure 6. The chemical shifts of C(11) and C(12) in the imidazole ring are located at 118.9 and 123.1 ppm, respectively. The chemical shift of C(10) is sited at 137 ppm. As the temperature is elevated, the chemical shifts show no sizable change for all carbon atoms in both SIP3− ligand and the EMIM+ cation; however, the significant changes are observed for the peak intensity of some chemical shifts in this temperature range. For example, the signals arising from the C(10), C(11), and C(12) atoms are weakened and broadened, especially, when the temperature is over 433 K. The intensity variations of C(11) and C(12) signals really reflect the dynamic processes related to out-plane rotation motion.37,38 As shown in Figure 6b, the chemical shifts of carbon atoms in the methyl and methylene are located at 14.8, 37.2, and 42.8 ppm at 298 K, corresponding to the methyl of ethyl chain C(14), methylene carbon C(13), and the methyl of imidazole ring C(9), respectively. The intensity of the signal at 14.8 and 42.8 ppm reduces obviously from 298 to 463 K, suggesting the existence of rapid motion for the C(14) and C(9) atoms. As demonstrated in the section of temperature-dependent singlecrystal structure (ref next section), the anisotropic atom

(2)

where ε0 and ε∞ are the low- and high-frequency limits of the dielectric permittivity, respectively, ω is the angular frequency, τ is the macroscopic relaxation time, and α is the distribution of the relaxation times parameter. The Cole−Cole plots are shown in Figure 4 for 1 at the selected temperatures, illustrating two typical characters,

Figure 4. Cole−Cole plots (imaginary (ε″) parts vs real (ε′) part).

namely, a spur appears in the lower-frequency region (especially, in the case at the elevated temperature), which is related to the conductance, and an asymmetrical semicircle occurs in the higher-frequency regime at all selected temperatures, indicating that the relaxation process in 1 seriously deviates from the Debye dielectric response model. Each Cole− Cole plot at the selected temperature was fitted using eq 2; the experimental and fitted plots are shown in Figure S6, and the ε0, ε∞, and α parameters are summarized in Table S2. The α parameters span from 0.37 to 0.5; these values are far away from the zero, indicating that the relaxation occurring in 1 is a process with multiple relaxation times. This is due to the existence of heavily structural disorder for the EMIM+ cations, which results in the existence of differences in the local environment of EMIM+ cations. The Nyquist plots at the selected temperatures are shown in Figure 5a and Figure S7. A single semicircle appears at lower temperature; with increasing temperature, the typical Nyquist plot shows an arc in the high-frequency region together with a spur in the low-frequency range at higher temperature. The arc and the spur in a Nyquist plot are due to the bulk resistance and electrode contribution (a typical electrode polarization effect arises from the blocking of ions at either the electrode or the grain boundaries), respectively. The impedance plots were

Figure 5. (a) Typical Nyquist plots at the selected temperatures. (b) The fitting result for log(σdcT) versus 1000/T. E


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Inorganic Chemistry

Figure 6. Variable-temperature solid-state 13C CP/MAS NMR spectra in (a) imidazole ring and 5-sulfoisophthalate carbon response region and (b) the methyl and methylene response regions; signals corresponding to C11, C12, and C13 are indicated with stars.

Table 1. Crystallographic Data and Refinement Parameters of 1 temp/K wavelength/Å formula FW CCDC No. crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3)/Z ρ (g·cm−1) F(000) abs. coeff. (mm−1) θ ranges index ranges

Rint independ ref/ rest/para refinement method GOF F2 R1, wR2a [I > 2σ(I)] R1, wR2a [all data] residual (e·nm−3) a

150 0.710 73 C14H14N2O7SZn 419.7 1431326 monoclinic P21/c 11.2208(6) 11.7575(6) 16.2014(7) 90.00 129.170(2) 90.00 1657.1(14)/4 1.682 856 1.648 2.91−24.98 −13 ≤ h ≤ 13 −13 ≤ k ≤ 13 −19 ≤ l ≤ 19 0.0206 2906/0/228

200

250

293

353

413

1431327

1431328

1414022

1414029

1507839

11.2499(6) 11.7888(6) 16.2271(6) 90.00 129.399(2) 90.00 1663.0(14)/4 1.676

11.2582(6) 11.8812(6) 16.2122(7) 90.00 129.572(2) 90.00 1671.6(14)/4 1.668

11.2574(9) 12.0034(11) 16.1430(10) 90.00 129.603(4) 90.00 1680.7(2)/4 1.659

11.2896(14) 12.1049(15) 16.1579(15) 90.00 129.914(6) 90.00 1693.7(3)/4 1.646

11.3504(13) 12.1923(13) 16.1827(13) 90.00 130.491(5) 90.00 1703.1(3)/4 1.637

1.642 2.91−25.50 −13 ≤ h ≤ 13 −14 ≤ k ≤ 14 −19 ≤ l ≤ 19 0.0230 3100/0/228

1.634 2.91−25.49 −13 ≤ h ≤ 13 −14 ≤ k ≤ 14 −19 ≤ l ≤ 19 0.0240 3109/0/228

1.625 2.35−27.57 −14 ≤ h ≤ 12 −15 ≤ k ≤ 15 −19 ≤ l ≤ 21 0.0273 3838/0/228

1.613 2.35−27.54 −14 ≤ h ≤ 13 −13 ≤ k ≤ 15 −21 ≤ l ≤ 20 0.0259 3780/0/228

1.604 2.35−27.50 −12 ≤ h ≤ 14 −15 ≤ k ≤ 15 −21 ≤ l ≤ 17 0.0263 3889/0/228

1.073 0.0237, 0.0610 0.0305, 0.0637 0.325/−0.311

1.043 0.0304, 0.0746 0.0407, 0.0784 0.527/−0.365

1.173 0.0322, 0.0940 0.0461, 0.1072 0.720/−0.882

1.073 0.0329, 0.0915 0.0515, 0.1084 0.550/−0.680

full-matrix least-squares method on F2 1.016 1.069 0.0225, 0.0226, 0.0609 0.0586 0.0267, 0.0286, 0.0631 0.0613 0.649/−0.393 0.324/−0.324

R1 = ∑∥Fo| − |Fc∥/|Fo|, wR2 = [∑w(∑Fo2 − Fc2)2/∑w(Fo2)2]1/2.

packing structures show high similarity for 1 in the whole temperature range as well. When cooled from 413 to 150 K, as shown in Figure 7, the length of a, b axes slightly shorten by 1.2% and 3.6%, respectively, and the β angle also shrinks by 1.0%. It is worth noting that the a- and b-axes, β angle, and cell volume inconsistently reduce with temperature decreasing and that the reflection point is located at ∼280 K; this temperature is close to the critical temperature that one of two dielectric anomalies undergoes. Most interestingly, as the temperature decreases, the c-axis length first shrinks and follows by expansion, and the reflection point of the c-axis length change is also located at ca. 280 K (ref Figure 7a). The abnormality changes of temperature-dependent unit cell parameters indicate

displacement parameters and the thermal ellipsoid of the C(9) and C(14) are increased with temperature increasing even when the temperature is below the room temperature; this observation suggests that the rapid swing motion of ethyl group occurs at the lower temperature. Thus, variable-temperature solid-state 13C CP/MAS NMR spectra indicated that out-plane rotation motion of imidazole ring and swing motion of ethyl group induce the dielectric relaxation at higher-frequency regime. Variable-Temperature Crystal Structures. The temperature-dependent single-crystal structures were inspected for 1 in the range of 150−413 K, revealing that the space group and the asymmetric unit are not changed (ref Table 1), and the F


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Inorganic Chemistry

Figure 7. (a) Temperature-dependent axis length of unit cell, (b) β angle, (c) cell volume, and (d) the interatomic distances Zn(1)···S(1) and Zn(1)···Zn(1)#1 (symmetric code #1 = −x, 2 − y, 2 − z) at selected temperatures.

elevated temperature. For the imidazole ring, the Ueq and Uii (i = 1−3) values rapidly increase for C(11) and C(12), whereas they gradually increase for C(10), N(1), and N(2) with increasing temperature; this finding demonstrates that the libration motion probably occurs in the imidazole ring at elevated temperature (ref Figure S5).

an isostructural phase transition occurring at ca. 280 K. It is worth mentioning that the continuous changes, but with a reflection point, of temperature-dependent unit cell parameters at ∼280 K demonstrate the structure phase transition having the character of second-order phase transition, and this finding is in agreement with the observation of absence of thermal anomaly in differenctial scanning calorimetry (DSC) plot in the temperature region around 280 K (ref Figure S2). The analogous phenomenon was previously reported in other inorganic−organic hybrid solid, [(CH3)4N][(C2H5)4N]ZnCl4, which showed a dielectric anomaly at ca. 268 K but absence of the thermal event in the corresponding temperature regime.39 An isostructural phase transition, accompanied by change in neither the crystallographic space group nor the occupied Wyckoff positions, has been categorized as “isostructural phase transition” (IPT), which is also known as “Cowley’s type zero” transitions40,41 after Cowley postulated the possibility of such a transition, based on the behavior of acoustic phonons within the framework of displacively structural phase transitions. Consequently, the intermolecular distances and the relative orientation were carefully inspected. As illustrated in Figure 7c (Figures S3 and S4), the interatomic distances, such as the Zn(1)···S(1) distance and the Zn(1)···Zn(1)#1 (symmetric code #1 = −x, 2 − y, 2 − z) distance in an SBU, as well as the interatomic distances between the cation and the framework, show abnormality changes around 280 K, which are in good agreement with the change trend of the unit cell parameters. To explore the dynamic motion of EMIM+ cation in the channel, the atom displacement parameters are listed in Table 2 for the cation. The Ueq and Uii (i = 1−3) values show rapid increase for C(14) as the temperature is increased from 150 to 413 K, indicating the existence of thermally activating dynamic motion. With respect to C(14), the Ueq and Uii (i = 1−3) values show gradual increase for C(13) upon heating, and these results suggest that the swing motion undergoes in the ethyl group at

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CONCLUSION In summary, we have achieved a three-dimensional MOF/PCP using an ionothermal reaction, and the cations with rotatable or the motionable freedom (ion-liquid) were successfully incorporated into the MOF. This MOF/PCP exhibits fascinating dielectric natures, two-step dielectric anomalies together with two-step dielectric relaxations. The dielectric anomaly occurring at ca. 280 K is associated with an isostructural phase transition, and another one undergoes in the higher temperature regime (>353 K) and is strongly dependent on the ac frequency, which probably arises from the dynamic disorder of the cations in the channels. The dielectric relaxation in the lower-temperature and higher-frequency regimes results from the out-plane rotation motion of imidazole ring and swing motion of ethyl group in the cation, which is the inherent nature of the MOF/PCP, whereas the dielectric relaxation in the higher-temperature and lower-frequency regimes is caused by dc conduction and electrode polarization, which results from the extrinsic factors. Our study has shed new light on design and construction of new types of MOFs/PCPs with switchable dielectric nature.

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EXPERIMENTAL SECTION

Chemicals and Materials. All reagents and chemicals were purchased from commercial sources and used without further purification. Preparation of 1. A mixture of Zn(NO3)2·6H2O (0.6 mmol) and 5-sulfoisophthalic acid monosodium salt (0.6 mmol) was placed in a G


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Inorganic Chemistry Table 2. Atom Displacement Parameters (Ueq, U11, U22, and U33) in EMIM+ at the Selected Temperatures

3.26; N, 6.52%. The purity of 1 was examined with elemental analyses of C, H, and N and powder X-ray diffraction (Figure S1). Physical Measurements. Elemental analyses (C, H, and N) were performed with an Elementar Vario EL III analyzer. IR spectra at room temperature were recorded on a Bruker VERTEX80 V Fourier

25 mL Parr Teflon-lined stainless steel vessel with 1.2 g of 1-ethyl-3methylimidazolium tetrafluoroborate. The vessel was sealed and heated at 160 °C for 5 d and then naturally cooled to ambient temperature to give colorless crystals of 1.Yield ∼55%. Anal. Calcd For (EMIM)[Zn(SIP)]: C, 40.02; H, 3.33; N, 6.60%. Found: C, 39.46; H, H


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Inorganic Chemistry transform infrared spectrometer (KBr disc; Figure S1). Thermogravimetric (TG) analysis was performed on an SDT Q600 V20.9 Build 20, in the temperature range of 303−873 K under nitrogen atmosphere, and the heating rate was 5 K min−1 (Figure S2). DSC measurement was performed on a NETZSCH DSC 204F1 Phoenix for powder sample between 123 and 473 K, with a temperature scanning rate of 5 K min−1(Figure S2). Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance powder diffractometer operating at 40 kV and 40 mA using Cu Kα radiation with λ = 1.5418 Å. Samples were scanned 2θ from 5 to 50°. The measurements of temperature and frequency-dependent dielectric permittivity were performed using a concept 80 system (Novocontrol, Germany) in the temperature range of 123−473 K; the sample was prepared in the form of a disk that was 10 mm in diameter and ca.0.89 mm in thickness, and the disk was sandwiched between two parallel copper electrodes; the ac frequencies span from 1 to 1 × 107 Hz. The 13C cross-polarization/magic-angle spinning (CP/MAS) spectra were acquired at UCLA on a Bruker Avance 300 solid-state spectrometer operating at a 13C frequency of 75 MHz for the finely powdered crystalline sample. The experiment includes simultaneous high-power 1H decoupling and fast sample spinning (5−20 kHz) at the magic angle (54.7°) to remove the line broadening that comes from static anisotropic interactions mediated by the external magnetic field. X-ray Single Crystallography. Single-crystal X-ray diffraction data were collected at 150, 200, 250, 293, 353, and 413 K for 1 using graphite monochromated Mo Ka (λ = 0.710 73 Å) radiation on a CCD area detector (Bruker-SMART). Data reduction and absorption corrections were performed with the SAINT and SADABS software packages,42 respectively. Structures were solved by the direct method using the SHELXL-97 software package.43 The non-hydrogen atoms were anisotropically refined using the full-matrix least-squares method on F2. All hydrogen atoms were placed at the calculated positions and refined as riding on the parent atoms. The details of data collection, structure refinement, and crystallography are summarized in Table 1.

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National Nature Science Foundation of China (Grant Nos. 20123221110013, 21671100, and 21271103) for financial support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01759. PXRD plot and IR spectroscopy at room temperature, TG and DSC plots, H-bond diagrams, Zn···Zn and Zn··· S distance versus temperature, Cole−Cole plots for electric permittivity and Z″−Z′ plots at selected temperatures, solid-state 13C CPMAS NMR spectrum at 298 K and atom labeling, typical interatomic distances between the cation and the framework at selected temperatures, the parameters of ε0, ε∞ and α obtained from fits, the resistance and conductivity at selected temperatures (PDF) X-ray crystallographic information (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: +86 25 58139476. Fax: +86 25 58139481. E-mail: [email protected] (RXM). *E-mail: [email protected]. (Y.Z.) *E-mail: [email protected]. (H.-B.D.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors thank the Priority Academic Program Development of Jiangsu Higher Education Institutions, Special Research Fund for the Doctoral Program of Higher Education, and the I


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