HUANG Deng-Deng CHEN Li-Zhuang

(School of Biology and Chemical Engineering,Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, China)

1 INTRODUCTION

During the past few years, the preparation of metal-organic frameworks (MOFs) has been more and more popular not only because of their structural diversity but also due to their potential applications in the fields such as gas storage, microelectronics,ion exchange, catalysis, chemical sensors and nonlinear optical materials[1-8]. Recently , heterocyclic carboxylic acids containing nitrogen and oxygen atoms has been widely used for constructing various functional metal-organic frameworks (MOFs) because they can offer potential hydrogen bonding interaction sites and versatile coordination modes[9-10]. As an emerging bridging ligand among many other heterocyclic carboxylic acids, 2-(pyridin-3-yl)-1H-imidazole-4,5-dicarboxylate is supposed to be an terrific organic linker for it can be partly or fully deprotonated dependent on the pH level, and the deprotonated HPyIDCn-(n = 0, 1, 2)exerts flexible coordination modes to metal ions, and also has the ability to act as hydrogen-bond acceptors and donors to assemble various supramolecular structures[11]. To the best of our knowledge, the study on the coordination chemistry of 2-(pyridine-3-yl)-1H-imidazole-4,5-dicarboxylate is relatively rare[12-14]. In our continuing effort in this field[15-18],we choose H3PyIDC as an organic building block in an attempt to construct one novel metal-organic framework with new structural features. Herein, we report a novel metal-organic framework by choosing 2-(pyridin-3-yl)-1H-imidazole-4,5-dicarboxylate as an organic building block.

2 EXPERIMENTAL

2.1 Materials and instruments

All chemicals except the H3PyIDC ligand obtained from commercial sources were of reagent grade and used without further purification. The ligand H3PyIDC was prepared by the method reported in the literature[19]. Infrared spectra were recorded on a SHIMADZU IRprestige-21 FTIR-8400S spectrometer in the spectral range of 4000～500 cm-1. Dielectric constant was conducted using an automatic impedance TongHui2828 Analyzer with frequency of 5 to 1 MHz, and pellet sample was made through high-pressure of 8 MPa. Elemental analyses were taken on a Perkin-Elmer 240C elemental analyzer.

2.2 Preparation of complex 1

A mixture of Gd(NO3)3·6H2O (0.10 mmol, 45.1 mg), H3PyIDC (0.10 mmol, 23.3 mg), NaOH (0.05 mmol, 2 mg) and 4,4?-bipyridine (bpy, 0.10 mmol,15.6 mg) was placed in a thick Pyrex tube (ca. 20 cm in length). After the addition of water (2 mL), the tube was frozen with liquid N2, evacuated under vacuum, and sealed with a torch. The tube was heated at 160 ℃ for five days. After slowly cooling to room temperature, colorless block crystals were obtained in 52% yield. Some crystals suitable for X-ray diffraction analysis were collected manually,washed with water and dried in the air. Anal. Calcd.(%) for C30H34Gd2N10O22: C, 30.00; H, 2.85; N,11.66. Found (%): C, 30.12; H, 2.90; N, 11.75. IR data (KBr pellet, ν (cm-1)): 3438(s), 3253(m), 2955w,2853w, 2358s, 2340s, 1582s, 1445w, 1365w, 1358w,1331w, 1264s, 1104s, 1031m, 932w, 906w, 861w,805s, 722w, 683w, 667m, 653w, 615w, 534w, 505w.

2.3 X-ray structure determination

A single crystal of the title complex with approximate dimensions of 0.30mm × 0.30mm × 0.20mm was selected for data collection on a Bruker SMART APEX-II CCD diffractometer equipped with a graphite-monochromatic Mo-Kα radiation (λ =0.071073 nm) using an ω scan mode at 298(2) K.The structure was solved by direct methods with SHELXS-97 and refined by full-matrix least-squares on F2with SHELXL-97[20-21]with the θ range for data collection from 1.46 to 25.40°. Of the 20317 reflections collected, 8887 were unique (Rint=0.0773). All non-hydrogen atoms were refined with anisotropic thermal parameters. All hydrogen atoms attached to C, N and O atoms were added up theoretically and refined with a riding model and fixed isotropic thermal parameters. The final refinement converged at R = 0.0554, wR = 0.1181 (w =1/[σ2(Fo2) + (0.0387P)2+ 5.6351P], where P = (Fo2+ 2Fc2)/3), S = 0.986, (Δ/σ)max= 0.001, (Δρ)min=–1.196 and (Δρ)max= 0.918 e/?3. The selected inter atomic distances and bond angles are shown in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Symmetry transformation: #1: –x+2, –y+2, –z; #2: x+1, y, z; #3: –x+2, –y+1, –z+1; #4: x–1, y, z

3 RESULTS AND DISCUSSION

3.1 IR analysis

The absence of any strong bands around 1700 cm–1in the IR spectrum of 1 indicates that two-COOH have been completely deprotonated to generate COO–anions, which are in agreement with their X-ray single-crystal structures. The broad band around 3253 cmˉ1indicates the presence of v(O–H)stretching frequency of coordinated water molecules. vas(COO-) and vs(COO-) were observed at 1582 and 1365 cmˉ1for 1. The difference between asymmetric stretching and symmetric stretching bands of the carboxyl groups (Δν = νas(C=O)–νs(C=O))are at 217 cm-1, suggesting the strong coordination of carboxylate oxygen to the mental center[22].

3.2 Crystal structure description

Complex 1 crystallizes in the triclinic space group P, with the asymmetric unit consisting of two crystalo-graphically independent Gd ions, two HPyIDC2-ligands, one bpy molecule, two NO3-anions, five coordinated and three uncoordinated water molecules. As shown in Fig. 1, the Gd(1) ion is eightcoordinated by five oxygen atoms, one nitrogen atom from four different HPyIDC2-ligands, and two oxygen atoms from two water molecules to give the distorted dodecahedral geometry. The Gd(2) ion coordinates with five oxygen atoms and one nitrogen atom from four different HPyIDC2-anions and three oxygen atoms from three water molecules — nine donor atoms forming a distorted tricapped trigonal prism configuration. The imidazole dicarboxylate motif adopts the μ3-kO, O′: kO′, O′: kO′′ coordination modes to bridge three Gd ions and form a 1D infinite zigzag chain (Fig. 2). The shortest Gd··Gd distance is 4.2048(15) ?, whereas the others are 5.5342(19) and 5.8985(22) ?, respectively. Moreover, the pyridyl groups in the HPyIDC2-anions further link the Gd ions in the neighboring 1D chains to form a 2D network (Fig. 3). Therefore, each Gd ion coordinating to four HPyIDC2-ligands acts as a 4-connected node and each HPyIDC2-ligand can be regarded as a 4-connected linker, so that a (4,4) network is formed by the connection between the Gd centers and the HPyIDC2-anions. The whole 2D net of 1 cam be represented as a (3,6)-connected 2-nodal network with a kgd topology. The point (Schl?fli)symbol of complex 1 can be represented as (43)2(46.66.83) calculated by TOPOS (Fig. 4). The 2D layer structure is accumulated into a 3D framework via weak force (Fig. 5). In addition, these 2D layers are retained by diverse hydrogen bonds involving bpy molecules, NO3-anions and water molecules (Table 2). In complex 1, the pyridine nitrogen atom of the HPyIDC2-ligand is involved in coordination with the Gd ion, which leads to a completely new coordination network.

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Fig. 1. Coordination environment of Gd in 1

Fig. 2. View of the 1D zigzag structure of 1

Fig. 3. 2D sheets constructed by 1D zigzag chain in 1

Fig. 4. Schematic description of the(3,6)-connected 2D network of 1

Fig. 5. Supramolecular architecture via weak interactions of 1

Fig. 6. Dependence of the dielectric constants of complex 1 on temperature from –150 to 20 ℃at different frequency ranges (5 kHz, 10 kHz, 100 kHz, 1 MHz)

3.3 Dielectric property

The materials with dielectric constants that rapidly fluctuate depending on the temperature may be piezoelectric, phase transition and ferroelectric ones.Recently, our group has fabricated a series of hybrid inorganic-organic dielectric materials[23], and herein we study the permittivity properties of complex 1.The permittivity (ε = ε1– iε2, where ε1and ε2are the corresponding real and imaginary parts of the dielectric constant, dielectric dissipation factor D =tanδ = ε2/ε1) of powdered samples of 1 was measured in the form of pellets. The temperature dependence of the dielectric constant at different frequency ranges shows that the ε1values gradually increase with rising temperature (–150～20°C) (Fig. 6). In addition, the dielectric constant gradually decrea- ses when continuously increasing the frequency from 5×103→ 104→ 105→ 106Hz. At 1 MHz, ε1increases less evidently (2.56～9.12) than that at 5 kHz (3.17～154.55), which is consistent with the low dielectric dissipation (ε2/ε1) at higher frequencies. This behavior is due to the chain microvibration that leads to an overall chain-chain dipolar relaxation, being in good accordance with the 1D-ordered –N–Gd(1)–O–Gd(2)–O–C–O–Gd(2)–O–Gd(1)– zigzag chain. Dipolar chain relaxation at low frequency is consistent with the direction change of AEF (alternating electric field). However, at higher frequency, dipole reversal fails to keep up with the AEF frequency change, thus rending ε1to decrease with elevating frequency and to remain thereafter[24-25]. The change of dielectric constant at 106Hz of complex 1 is obviously more than that of the compounds reported by zhang et al.[26]which keeps a small change from 4.6 to 5.05 and 4.7 to 5.6 with rising temperature.

In summary, we used the multifunctional ligand H3PyIDC containing both imidazole and carboxylate groups for the construction of a new 2D lanthanide coordination polymer under hydrothermal conditions and structurally characterized it. The lanthanide contraction effect plays a crucial role in forming the complex. Moreover, H3PyIDC exhibits diverse coordination modes and is a good candidate for constructing novel frameworks.

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