WU Qi-Qi WEN Yi-Hang

(Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, China)

ABSTRACT The 2,2΄-((sulfonylbis(4,1-phenylene))bis(oxy))diacetic acid (H2L) and 4,4΄-bipy- ridine (4,4΄-bipy) are employed as organic ligands to react with metal salts to obtain two new metal-organic frameworks (MOFs), namely [M2L2(4,4΄-bipy)] (M = Zn(1), Cu(2)). They have been successfully synthesized under hydrothermal conditions. Single-crystal X-ray diffraction reveals that two complexes are isostructural and crystallize in orthorhombic system with space group Pbcn. All L2- ligands bridge metal ions to form an infinite 1D Z-shaped chain along the a axis. Then through 4,4΄-bipy, contiguous Z-shaped chains are linked to form a 2D layer structure. The luminescent studies of the H2L ligand and complex 1 in the solid state and thermogravimetric analyses (TGA) of two complexes are investigated.

Keywords: hydrothermal syntheses, 2,2΄-((sulfonylbis(4,1-phenylene))bis(oxy))diacetic acid, crystal structure, luminescence property; DOI: 10.14102/j.cnki.0254-5861.2011-2418

1 INTRODUCTION

Metal-organic frameworks (MOFs) have attracted tremendous interest on ultrahigh surface area, permanent porosity and fascinating topology structures[1-3], as well as functional properties and applications in the fields of gas storage and separation[4-6], heterogeneous catalysis[7-9], photolu- minescence[10,11], magnetic materials[12-14], electro- catalysis[15-17], and so on.

Nevertheless, there is a major challenge to construct suitable MOFs with desirable architecture owing to the factor in kinetics and thermodynamics. Researchers had put a mass of efforts on optimizing reaction conditions including temperature, solvent, pH,etc[18-23]. In addition, not only the reaction environment, but also coordination mode of metal ions and the selection of appropriate organic ligands play significant roles in the configurations of MOFs[24,25]. To the best of our knowledge, carboxyl group, which can afford diverse coordination modes, is the most commonly employed to bond metal ions[26,27]. Hence, we are mainly focused on using 2,2΄-((sulfonylbis(4,1-phenylene))bis(oxy))diacetic acid (H2L) to synthesize different structures of MOFs. This organic ligand with two carboxylate groups supports more coordination modes and can construct high-dimensional structures[28].

In this work, two isostructural MOFs, [Zn2L2(4,4΄-bipy)] (1) and [Cu2L2(4,4΄-bipy)] (2), have been obtained. All L2-ligands bridge metal ions to form an infinite 1D Z-shaped chain. The further linkages of these 1D chainsvia4,4΄-bipy ligands generate a 2D layer in the [010] plane. The TGA for 1 and 2 and the luminescent property of 1 have been also investigated.

2 EXPERIMENTAL

2. 1 Materials and measurements

The organic ligand H2L was prepared according to the literature[29]. All other commercially available chemical reagents and solvents were of reagent grade and used without further purification. The IR spectra were recorded on a FTIR-8700 spectrometer using KBr pellets in the range of 4000～400 cm-1. Elemental analysis (EA) was carried out on a Perkin-Elmer 2400(II) element analysis instrument. The crystal data collections were carried out on a Bruker SMART APEX-II CCD diffractometer. Thermogravimetric analyses (TGA) were taken on a Mettler-Toledo TGA/SDTA 851e in the range of 25～700 °C (the heating rate was 10 °C·min-1) under air atmosphere. The solid-state fluorescent spectra were performed on a HITACHI F-2500 fluorescence spectrometer at room temperature.

2. 2 Synthetic method

2. 2. 1 Synthesis of complex 1

A mixture of Zn(CH3COO)2·2H2O (0.1048 g, 0.5 mmol), 4,4΄-bipy (0.039 g, 0.25 mmol) and H2L (0.183 g, 0.5 mmol) was dissolved in 18 mL deionized water. Then the pH of the solution was adjusted to 8 by NaOH (2 mol/L). The whole mixture was heated at 160 °C for 3 days in a sealed 25 mL stainless-steel reactor. The mixture was then cooled to room temperature at 5 K/h, and colorless block crystals were obtained (yield 28%, based on H2L). Elemental Anal. Calcd. (%) for C21H16ZnNSO8: C, 49.63; H, 3.15; N, 2.76. Found (%): C, 48.97; H, 3.19; N, 2.66. IR (KBr, cm-1): 3032(m), 2923(m), 1596(s), 1501(s), 1401(s), 1248(m), 1211(s), 1058(m), 808(m), 626(m).

2. 2. 1 Synthesis of complex 2

A mixture of CuCl2·2H2O (0.171 g, 1 mmol), 4,4΄-bipy (0.039 g, 0.25 mmol) and H2L (0.183 g, 0.5 mmol) was dissolved in 18 mL deionized water. Then the pH of the solution was adjusted to 7 by NaOH (2 mol/L). The whole mixture was heated at 140 °C for 3 days in a sealed 25 mL stainless-steel reactor. The mixture was then cooled to room temperature at 5 K/h, and green block crystals were obtained (yield 65%, based on H2L). Elemental Anal. Calcd. (%) for C21H16CuNSO8: C, 49.80; H, 3.16; N, 2.70. Found (%): C, 49.82; H, 3.14; N, 2.8. IR (KBr, cm-1): 3432(w), 2941(m), 1601(s), 1421(s), 1215(s), 1083(s), 891(m), 636(m), 601(m).

2. 2. 3 X-ray crystallography

Single-crystal X-ray diffraction data were collected on a Bruker SMART APEXII CCD detector diffractometer equipped with MoKαradiation (λ= 0.71073 Å) at room temperature. The absorption correction was performed by using the SADABS program. Data intensity was corrected by Lorentz-polarization factors and empirical absorp- tion. The structures of the complexes were measured by direct methods and expanded with Fourier techniques. Anisotropic displacement parameters based onF2were applied to all non- hydrogen atoms in full-matrix least-squares refinements[30]. The hydrogen atoms were assigned with isotropic displacement factors and included in the final refinement cycles by the use of geometrical restrains. The details of all pertinent crystallo- graphic data and structure refinement for complexes 1 and 2 are summarized in Table 1. A1l calculations were performed with SHELX-97 package[31]. The select bond lengths and bond angles are listed in Table 2.

Table 1. Crystal Data and Structure Refinement for the Complexes

Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for 1 and 2

Symmetry transformations used to generate the equivalent atoms: i = x, -y+2, z-1/2; ii = -x, -y+2, -z+1; iii = -x, y, -z+1/2; iv = x, y+1, z

3 RESULTS AND DISCUSSION

3. 1 Crystal structures of the complexes

Two MOFs are determined by single-crystal X-ray diffraction, and the result reveals that complexes 1 and 2 are isostructural, and crystallize in the orthorhombic system withPbcnspace group. Only the structure of 1 is described in details.

The coordination environments of the Zn center as shown in Fig. 1, the asymmetric unit is composed of one Zn center, one fully deprotonated L2-anion, and half of a 4,4΄-bipy ligand. Each Zn center is five-coordinated by four O atoms from four different L2-ligand (Zn-O 2.0241(17) and 2.0602(18) Å) and one N atom from the 4,4΄-bipy ligand (Zn-N from 2.034(2) to 2.034(3) Å), exhibiting a trigonal bipyramidal geometry. As to L2-ligand, both carboxylate groups of the L2-ligands adopt the same bidentate chelating mode, neighboring Zn2+ions are linked by carboxylate oxygen atoms to form a double nuclear Zn2(COO)4secondary building unit (SBU). And the L2-ligands can be viewed asμ2-bridging linersto orderly bridge the SBUs to form an infinite one-dimen- sional Z-shaped chain along theaaxis. Furthermore, the 4,4΄-bipy ligands link two Z-shaped chains to make up a 2D layer in the [010] plane (Fig. 2).

To better understand the structure interior, the topology of complex 1 was analyzed by TOPOS 4.0 program[32]. Each Zn2(COO)4secondary building unit (SBU) serves as a 4-connected node, and thus the whole framework can be simplified to a (4,4)- connected network with (44.62) topology (Fig. 3).

3. 2 Thermogravimetric analysis

To check the thermal stability of complexes 1 and 2, the thermogravimetric analyses (TGA) of two complexes were performed. The complexes were heated to 700 °C at a rate of 10 °C/min under air atmosphere. The TGA curves of complexes 1 and 2 are given in Fig. 4. As regards complex 1, it displays the weight loss of 80.17% in the range of 341～578 °C, which may correspond to the removal of 4,4΄-bipy and L2-ligands (calcd. 80.21%). Then the framework of 1 is destroyed at 578 °C. The final product may be ZnO (calcd. 17.21%). In the case of complex 2, a weight loss of 72.46% is found in the temperature range of 236～438 °C, which may contribute to the departure of 4,4΄-bipy and L2-ligands (calcd. 80.49%), with the residue to be stable up to about 438 °C.

Fig. 1. Coordination environments of Zn(II) ions in compound 1 showing 30% probability ellipsoids and the atom-labeling scheme (Symmetry codes: i = x, -y+2, z-1/2; ii = -x, -y+2, -z+1; iii = -x, y, -z+1/2; iv = x, y+1, z)

Fig. 2. 2D layer in the [010] plane in complex 1

Fig. 3. Topology of complex 1

Fig. 4. TGA curves of complexes 1 and 2

3. 3 Luminescence property

MOFs withd10metal centers have been shown to exhibit some luminescent properties for luminescent materials[33]. Accordingly, the solid-state lumine- scence properties of the H2L ligand and complex 1 were investigated at room temperature. As indicated in Fig 5, for the free H2L ligand, an intense emission is observed at 388 nm (λex= 340 nm), while complex 1 exhibits a strong fluorescent emission peak at 428 nm (λex= 340 nm). It suggests a significant red-shift compared to the H2L ligand, which may be due to the emission of ligand-to- metal charge transfer (LMCT)[34]. A small peak appears at 450 nm belonging to the system peaks and has no effect on the result.

Fig. 5. Fluorescence emission spectra of the H2L ligand and complex 1 in the solid state at room temperature

4 CONCLUSION

In summary, we demonstrate two unusual metal- organic frameworks [M2L2(4,4΄-bipy)] (M = Zn, Cu) under hydrothermal conditions. All H2Ls bridge metal ions to form an infinite one-dimensionalZ-shaped chain. And it is shown to self-assemble into a 2D plane with 4,4΄-bipy ligand. The TGA for complexes 1 and 2 and the luminescent properties of the H2L ligand and 1 have also been investigated.