A Three-Dimensional Zn(Ⅱ) Metal-Organic Framework with Fluorescent Properties

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(College of Chemistry and Chemical Engineering, Mu Danjiang Normal University, Mu Danjiang 157012, China)

Abstract:The complex {[Zn(BTC)2/3]·[HCON(CH3)2]·[C2H5OH]}n(1) was prepared from 1,3,5-benzenetricarboxylic acid (H3 BTC) and Zn(Ⅱ) cation via a solvothermal technique. The complex 1 was characterized by single crystal X-ray diffraction(SXRD), Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD) and fluorescence spectroscopy. Single crystal X-ray diffraction analysis indicates that complex 1 belongs to space group P21/n of the monoclinic system. In complex 1, the Zn(Ⅱ) ion is coordinated with BTC3- anion through the oxygen atom on the carboxyl group, forming a three-dimensional Zn(Ⅱ) metal-organic framework (MOF) structure with channels. And through intermolecular hydrogen bond interaction, ethanol and N, N′-dimethylformamide (DMF) molecules are encapsulated into parallelogram channels. Fluorescence spectrum analysis at room temperature shows that when the excitation wavelength is 297 nm, complex 1 has the strongest emission peak at 601 nm.

Key words:Zn(Ⅱ) complex; solvothermal synthesis; 1,3,5-benzenetricarboxylic acid; metal-organic framework; crystal structure; fluorescent property

0 Introduction

Metal-organic frameworks (MOFs) are crystalline porous materials with inorganic metals (metal ions or metal clusters) as the center, which connect organic ligands through self-assembly to form a periodic network structure. MOFs materials are an attractive platform for responsive material design[1-4]. In the research of modern materials, MOFs have shown great development potential and attractive development prospects[5-26]. MOFs have the advantages of diverse synthesis methods, large specific surface area, adjustable channel, etc. They have broader application prospects than other porous materials, mainly used in the fields of sensing[27], catalysis[28-29], drug release[30], and separation analysis[31-32].

The organic ligand 1,3,5-benzenetricarboxylic acid (H3BTC) can be coordinated in three directions due to its binding sites, and is one of the most widely used ligands to construct an organic fluorescent material[33-35]. H3BTC has good coordination ability, and the synthesized complexes are easy to form three-dimensional (3D) structure with large porosity. It has a very wide range of applications in the fields of adsorption[36], detection[37], and medicine[38].

This article reports a new MOFs material {[Zn(BTC)2/3]·[HCON(CH3)2]·[C2H5OH]}n(H3BTC=1,3,5-Brnzenetricarboxylic acid), in which N, N’-dimethylformamide (DMF) and ethanol molecules supported by hydrogen bonds are encapsulated in parallelogram channels. Complex1was characterized by single crystal X-ray diffraction (SXRD), FT-IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction (PXRD) and fluorescence spectroscopy. This work has a certain enlightening effect on the synthesis and application of MOFs.

1 Experimental

1.1 Material and physical measurement

Zn(NO3)2·6H2O, H3BTC, N, N’-dimethylformamide, and ethanol were purchased commercially and used without further purification. Elemental analysis (C, H, and N) adopted a PerkinElmer 2400 CHN elemental analyzer. Fourier transform-infrared spectrum was measured on a Mattson Alpha Centauri spectrometer with KBr pellets, and the test range is 400 cm-1to 4 000 cm-1. The powder X-ray diffraction (PXRD) was measured with Cu-Kαradiation (λ=0.154 056 nm) on a Bruker D8 Advance X-ray diffractometer in the range of 2θ=5° to 50°. The thermogravimetric analysis (TGA) was performed on the PerkinElmer thermal analyzer, and the heating rate was 10 ℃·min-1. LS 55 fluorescence spectrophotometer was used to record the fluorescence spectrum.

1.2 Synthesis of complex {[Zn(BTC)2/3]·[HCON(CH3)2]·[C2H5OH]}n (1)

Zn(NO3)2·6H2O (0.029 7 g, 0.1 mmol) and H3BTC(0.021 0 g, 0.1 mmol) were added to a mixed solvent of H2O (1 mL), DMF (5 mL), and ethanol (4 mL). The mixture was stirred until completely dissolved, then sealed in a 25 mL Teflon reactor autoclave and heated at 100 ℃ for 72 h. After cooling down to room temperature at a rate of 2.5 ℃·h-1, the colorless acicular crystal was collected by filtration (62% yield was based on H3BTC).

1.3 Single-crystal structure determination

The single crystal X-ray diffraction data of complex1was obtained using an Oxford Diffraction Gemini R Ultra diffractometer at 296 K, and the detector used graphite monochromatic Mo-Kα(λ=0.071 073 nm) to perform multiple scanning absorption corrections. Use OLEX2 and SHELXTL 2014 programs to solve the structure by direct method and differential Fourier method onF2, then use the full matrix least square method to refine. All non-hydrogen atoms in the complex were refined by using anisotropic shift parameters. Table 1 summarizes the crystal data and structure modification of complex1. The main bond lengths and bond angles of complex1are shown in Table 2. The hydrogen bond interactions in complex1are listed in Table 3. The crystallographic data is stored in the Cambridge Crystallographic Data Center (CCDC) as supplementary publication numbers CCDC 1977742.

Table 1 Crystal data and structure refinement for 1

Table 2 Selected bond lengths (nm) and bond angles (°)

Table 3 C—H…O interactions geometry for 1

2 Results and discussion

2.1 Crystal structure

Single crystal X-ray diffraction analysis indicates that complex1crystallizes as monoclinic system with space group ofP21/n(Table 1). The asymmetric unit consists of a Zn(Ⅱ) cation, a BTC3-anion, an ethanol molecule and a N, N-dimethylformamide (DMF) molecule. As shown in
Fig.1, the Zn(Ⅱ) cation is coordinated with the four O atoms (O5, O21, O32, O43) from four separate BTC3-anions, forming four coordinated tetrahedral configuration. The Zn-O bond ranges from 0.193 4(2) nm to 0.19 70(2) nm, which are all within the normal range (Table 2). The four BTC3-ligands form a planar and spread to form a two-dimensional (2D) layer (Fig.2). The other two BTC3-ligands act as pillar-ligands to coordinate with the external Zn atoms, and then rise into a 3D framework (Fig.3). The connection between the binuclear zinc unit and the ligands creates a three-dimensional (3D) framework that was characterized by parallelogram channels encapsulating ethanol and DMF molecules by hydrogen bonds (Fig.3). One of the uncoordinated carboxylate O1 atoms forms a hydrogen bond with ethanol solvent molecule (C1—H1A…O1=0.269 7(6) nm). There are also hydrogen bonds between ethanol solvent molecules and DMF solvent molecules (C1—H1B…O7=0.274 1(9) nm) (Table 3). It can be clearly seen from
Fig.3 that the estimated size of the channel of complex1is approximately 1.209 3 nm×1.261 3 nm.

Fig.1 Coordination environment around the BTC3- ligand in 1

Fig.2 2D layer diagram of 1

Fig.3 3D framework structure and estimated channel size of 1

2.2 FT-IR, TGA and PXRD analysis

The crystal structure of complex1was further characterized by FT-IR spectroscopy (Fig.4). The characteristic peak range of the benzene ring C=C bond stretching vibration is 1 670 cm-1to 1 400 cm-1. There is an absorption peak at 1 619 cm-1in complex1, which can be regarded as the characteristic absorption peak of the benzene ring. The double peak at 1 372 cm-1is related to the stretching vibration of the carboxylate. The absorption in the fingerprint region can be attributed to the substitution of the benzene ring. In complex1, a double peak appears at 718 cm-1, which can be attributed to the substitution of Zn ions on the benzene ring groups.

Fig.4 FT-IR spectrum of complex 1

The thermal decomposition of complex1between room temperature and 600 ℃ was tested, and the thermogravimetric analysis chart as shown in
Fig.5 was obtained. In the range of 20 ℃ to 346 ℃, the weight loss proportion of complex1is 12.73% (calcd.11.76%), which is equivalent to the loss of ethanol molecules in the channels. In the range of 346 ℃ to 457 ℃, the weight loss proportion reaches 32.35% (calcd.30.42%), which is equivalent to the loss of DMF molecules in the channels. After further heating, it was found that the weight dropped again, which meant that the frame began to collapse.

Fig.5 TGA curve of complex 1

To detect phase purity, the PXRD data of complex1(Fig.6) was measured at room temperature. The results show that the powder X-ray diffraction pattern is basically consistent with the simulated pattern, indicating that complex1’s purity is high. The different peak intensities may be caused by the different preferred orientations of the crystal sample and the powder sample.

Fig.6 PXRD patterns of complex 1

2.3 Fluorescence properties

The room temperature solid-state luminescence properties of complex1were studied.
Fig.7 shows the solid-state fluorescence spectrum of complex1and H3BTC. As shown in
Fig.7, H3BTC is excited at 304 nm and has the strongest emission peak at 618 nm. Compared with H3BTC ligand, complex1exhibits an emission peak at 601 nm when the excitation wavelength is 297 nm. The blue shift of complex1is 17 nm, probably caused by the electronic transition between ligand and metal. That is to say, when the complex is excited, the electron of BTC3-anion will transfer to the d*orbital of Zn(Ⅱ) ion with the energy loss process, and finally reach the lowest excited state where the radiative transition occurs[39-41]. When BTC3-anions form a complex with Zn(Ⅱ)ions of d10configuration, the energy loss during the transition increases and the fluorescence intensity of the complex decreases.

Fig.7 Solid-state fluorescence spectrum of complex 1 and H3BTC at room temperature

3 Conclusion

In conclusion, a complex with MOF structure was synthesized from as-obtained 1,3,5-benzenetricarboxylic acid (H3BTC) and Zn(Ⅱ) cations by a solvothermal method. Complex1encapsulates ethanol and N, N′-dimethylformamide (DMF) molecules into parallelogram channels through intermolecular hydrogen bond interactions. Fluorescence spectrum measurement shows that the maximum emission wavelength of the complex1is 601 nm. Complex1has fluorescent properties and can be used as a potential fluorescent optical material. These results provide a new strategy for the construction of new MOFs materials.