Synthesis, Crystal Structure and Properties of a New Cd(II)Complex Based on Mixed 5-Hydroxy-isophthalic Acid and 1-(1H-imidazol-4-yl)-3-(4H-tetrazol-5-yl)benzene Ligands①

HU Zhi-Yong ZHU Juan-Juan CHEN Shui-Sheng QIAO Rui

(College of Chemistry & Chemical Engineering, Fuyang Normal College, Fuyang 236041, China)

1 INTRODUCTION

Much progress has been achieved in the rational design and synthesis of metal-organic frameworks(MOFs) due to their potential applications in the areas of catalysis, gas storage, magnetism and optics[1-6]. The choice of suitably-tailored ligands is undoubtedly the key in order to get the desirable coordination polymers although the nature of coordination sites, conformation and flexibility of organic ligands as well as the geometric preference and potential properties of metal centers all affect the structure and property of the final MOFs greatly[7-9]. Among these ligands, imidazole-containing organic ligands are proved to be effective units in construction of different structures including individual cages, one- (1D) tubes, two-dimensional(2D) networks, three-dimensional (3D) non-interpenetrating and interpenetrating frameworks in our previous studies[10-13]. On the other hand, proper organic bridging linkers are critical in constructing coordination polymers. The polycarboxylates, such as benzenedicarboxylate, benzenetricarboxylate, and biphenyldicarboxylate, which have advantages due to their rigid multicarboxylic groups, may act as bridging ligands with various coordination modes after being deprotonated[14,15]. On the basis of the advantages of nitrogen donor and carboxylate ligands, the strategy using mixed carboxylate and nitrogen donor ligands has come into playing in the construction of MOFs[16,17]. Taking the diverse coordinated modes into account, the series of designable ligands with imidazole and carboxylate groups or mixed ligands strategy of imidazole containing organic with carboxylates are also successfully utilized in the synthesis of MOFs by us[18-20].As the extension of our studies, we design a novel multi-N-donor ligand-1-(1H-imidazol-4-yl)-3-(4H-tetrazol-5-yl)benzene (H2L) which is employed to build new MOFs together with O-donor carboxylate ligand as a extension of our study. Herein, we report the syntheses, X-ray crystal structure and luminescent property of a new complex, [Cd2(5-IPA)-(HL)2(H2O)2]·4H2O (1).

2 EXPERIMENTAL

2.1 Materials and measurements

Ligand 1-(1H-imidazol-4-yl)-3-(4H-tetrazol-5-yl)-benzene was prepared according to the literature method[21]. The regents were used as commercial sources without further purification. Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer. IR spectra were recorded on a Bruker Vector22 FT-IR spectrophotometer using KBr discs. The luminescent spectra for the powdered solid samples were recorded at room temperature on an Aminco Bowman Series 2 spectrofluorometer with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra,the pass width is 5.0 nm. All the measurements were carried out under the same experiment conditions.

2.2 Synthesis of compound

[Cd2(5-IPA)(HL)2(H2O)2]·4H2O (1)

Reaction mixture of H2L (21.2 mg, 0.1 mmol),Cd(NO3)2·4H2O (23.6 mg, 0.1 mmol), 5-H2IPA (16.6 mg, 0.1 mmol) and H2O (8 mL) was adjusted to pH= 7 with 0.5 mol L-1NaOH solution. The mixture was then sealed into a 16 mL Teflon-lined stainless steel container and heated at 160 ℃ for 3 days. After cooling to room temperature, colorless block crystals of 1 were collected by filtration and washed by water and ethanol for several times with a yield of 75%. Anal. Calcd. for C28H30N12O11Cd2(%): C,35.95; H, 3.23; N, 17.97. Found: C, 35.83; H, 3.15;N, 17.82 (%). IR (KBr pellet, cm-1): 3727～2691 (s,br), 1605 (s), 1536 (s), 1458 (m), 1401(s), 1371 (m),1270 (w), 1235 (s), 1169 (w), 1133 (m), 1078 (w),1046 (w), 965 (w), 894 (w), 832 (w), 750 (m), 689(w), 645 (m), 497 (w).

2.3 Crystal structure determination

The colorless crystals of complex 1 were selected for diffraction data collection at 296(2) K on a Bruker Smart Apex II CCD diffractometer equipped with a graphite-monochromatic Mo-Kα radiation (λ= 0.71073 ?). A total of 9066 reflections were collected for 1, of which 2985 (Rint= 0.0189) were independent in the range of 2.09≤θ≤25.01o for 1 by using a φ-ω scan mode. The structure of complex 1 was solved by direct methods with SHELXS-97 program, and the non-hydrogen atoms were located from the trial structure and then refined anisotropically with SHELXL-97 using a full-matrix leastsquares procedure based on F2values[22,23]. All non-hydrogen atoms were refined anisotropically.All the hydrogen atoms except for those of water molecules were generated geometrically and refined isotropically using the riding model. The final R =0.049, wR = 0.1315 (w = 1/[σ2(Fo2) + (0.1538P)2+0.2105P], where P = (Fo2+ 2Fc2)/3), Rint=0.0189, (Δ/σ)max= 0.001, S = 1.055, (Δρ)max= 1.671 and (Δρ)min= –3.051 e/?3for 1. The selected bond distances and bond angles for complex 1 are listed in Table 1.

3 RESULTS AND DISCUSSION

3.1 Crystal structure of 1

The result of X-ray diffraction analysis revealed that complex 1 crystallizes in monoclinic, space group C2/c. The asymmetric unit consists of one Cd(II) atom, one partially deprotonated (HL)-ligand,a half of completely deprotonated 5-IPA2-, one free and two coordinated water molecules. As depicted in Fig. 1, the central Cd(II) atom is six-coordinated with distorted octahedral coordination geometry surrounded by two carboxylate oxygen (O(1) and O(2)) atoms from chelating carboxylate groups of 5-IPA2-and two imidazole nitrogen (N(1) and N(5B))atoms from two different (HL)-ligands in the equatorial plane, and one water molecule (O(4)) as well as N(4A) from (HL)-in the axial positions. The Cd–O bond lengths are 2.274(2) to 2.535(4) ?,while the Cd–N ones fall in the range of 2.217(3)～2.453(4) ? (Table 1), which are consistent with the reported six-coordinated Cd(II) complexes with O and N donors[24]. It should be mentioned that, the mixed tetrazolyl and 4-imidazol-containing H2L ligands are of polydentate aromatic nitrogen-donor ligand compound, which can act as multidentate bridging ligands to exhibit different coordination modes. Significantly, the 4-imidazolyl and tetrazolyl groups can easily deprotonate to give the imidazolate or tetrazolate anions participating in the construction of coordination polymers. The partially deprotonated (HL)-ligand in 1 also employs aμ3-bridge to connect three Cd(II) atoms by means of the heterocyclic nitrogen atoms to form a 2D double-layer fes network in the bc plane (Fig. 2a).On the other hand, the 5-IPA2-ligands are employed as two-connector pillars to link Cd(II) atoms of adjacent 2D nets to form 3D frameworks using its two terminal carboxylate groups with a μ1-η1∶η1-chelating coordination mode. From a topological point of view, this 2D Cd(II)-(HL)-net is also described by the fes 4·82net (Fig. 2b)[25], where both of the Cd(II) atoms and (HL)-linkers act as 3-connected nodes. The 3D framework can also be viewed to be a layer-pillared net, where the layer is a Cd(II)-(HL)-fes 4·82sheet and the pillar is a 5-IPA2-ligand (Fig. 3). As discussed above, the 5-IPA2-ligands ligate to Cd(II)-(HL)-fes 4·82sheets via both Cd–O contacts, so the 3-connected Cd node in the fes 4·82net is extended to be a 4-connected node in the layer-pillared framework. Thereby, this layer-pillared framework is ascribed to be a binodal(3, 4)-connected net with a Point (Schl?fli) symbol(4·6·8)(4·62·83)[26], which has been referred to as the fsc-3,4-C2/c topology (Fig. 4).

Fig. 1. View of the coordination environment of Cd(II) atom with thermal ellipsoids drawn at 30%probability level for complex 1 (Symmetry codes: (A) 0.5-x, 0.5+y, 1.5-z; (B) x, 1-y, -0.5+z; (C) -x, y, 1.5-z)

Fig. 2. (a) 2D double-layer framework formed by partially deprotonated (HL)- ligands and Cd(II) atoms. (b) Simplified 2D fes net; green balls and red ones represent Cd(II) atoms and the centers of benzene rings of (HL)- ligands

Fig. 3. 3D structure of 1 built from 2D networks (bright green) pillared by 5-IPA2- ligands (pink)

Fig. 4. Schematic representation of the fsc-3,4-C2/c topology of 1, where green balls represent the Cd(II) atoms and red ones represent the centers of benzene rings of (HL)- ligands

Table 1. Selected Bond Lengths (?) and Bond Angles (°) of [Cd2(5-IPA)(HL)2(H2O)2]·4H2O (1)

3.2 IR spectrum

The infrared spectrum of the complex has been recorded between 4000 and 450 cm-1and some important assignments are shown in the experimental section. The IR spectra exhibit strong absorption centered at 2691～3727 cm-1for 1, corresponding to the N-H/O-H stretching vibration of ligand or water molecule (see experimental section)[27]. Strong characteristic bands of carboxylate group are observed in the range of 1605～1536 cm-1for asymmetric vibrations and 1458～1401 cm-1for symmetric vibrations, respectively. Therefore, the complete deprotonation of the carboxylic acid to give the corresponding carboxylate ligand in 1 was confirmed by crystal structural analysis (vide post)as well as the IR spectral data since no vibrational bands in the range of 1760～1680 cm-1were observed in the IR spectra of 1, and the strong absorption bands between 1605 and 1371 cm-1in the IR spectra of the complex can be assigned to the coordinated carboxylate groups[27].

3.3 Thermal analysis and powder X-ray diffraction analysis

Complex 1 was subjected to thermogravimetric analysis (TGA) to ascertain the stability of supramolecular architecture, and the result is shown in Fig. 5.The first weight loss of 11.33% in 55～175 ℃indicates the exclusion of free and coordinated water molecules (calcd. 11.51%), and the decomposition of the residue occurred at 355 ℃. Powder XRD experiment was carried out to confirm the phase purity of bulk sample, and the experimental pattern of the as-synthesized sample can be considered comparable to the corresponding simulated one,indicating the phase purity of the sample (Fig. 6).

Fig. 5. Thermal analysis curve of complex 1

Fig. 6. Powder X-ray diffraction patterns of complex 1

Fig. 7. Solid-state photoluminescentspectra of 1 at room temperature

Fig. 8. N2 and H2O sorption isotherms at 77 and 298 K for 1

3.4 Photoluminescent property

Inorganic-organic hybrid complexes, especially those comprising the d10closed-shell metal center and aromatic-containing system, have been intensively investigated for attractive fluorescence properties and potential applications, such as chemical sensors and photochemistry[28]. In this paper, the solid-state photoluminescent property of complex 1 has been investigated as well as free H2L ligand in the solid state at room temperature. The free H2L ligand shows blue photoluminescence emission at 440 nm upon excitation at 372 nm, which is probably attributable to the π* → n or π* → π transitions, respectively[29], while complex 1 exhibits light blue emission with maximum at 456 nm upon 385 nm excitation, as depicted in Fig. 7. In contrast to the case for free ligand H2L, the emission bands of 1 are 16 nm red-shifted and the emission bands of 1 may be tentatively assigned to intraligand fluorescence since the free ligand exhibits a similar emission under the same condition[30].

3.5 Sorption property

The results of structural analyses show that the total void value of the channel accommodated by water guests is estimated to be 754.0 ?3, approximately 21.9% of the total crystal volume of 3441.4 ?3. And TGA and PXRD measurements have been carried out to ascertain the thermal stability of the complex for sorption property investigation. It was found that the water molecules in 1 can be removed completely by heating to give dehydrated samples of 1′, without destroying the structure (Fig. 6). As shown in Fig. 8, the sorption curves of N2at 77 K for samples 1′ suggests only surface adsorption.However, the H2O vapor sorption of 1′ was observed.The final values of 134.96 cm3g-1(108.45 mg g-1)for 1′ at P = 0.99 atm correspond to 5.63 H2O molecules per formula unit for 1, which are close to 6 H2O molecules in 1 indicated by crystallographic analysis. The large hysteresis and incomplete desorption imply the strong adsorbate-adsorbent interactions[31].

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