Construction of a New(4,8)-Connected Coordination Polymer Based on the Zinc Clus

AN Yn-Yn LU Li-Ping ZHU Mio-Li, b



Construction of a New(4,8)-Connected Coordination Polymer Based on the Zinc Cluster: Structure, Topology, and Luminescence①

AN Yan-YanaLU Li-Pinga②ZHU Miao-Lia, b②

a(030006)b(030006)

The title complex, [Zn3(L)2(H2O)2]n(1, H3L = 5-((3-formylphenoxy)methyl)iso- phthalic acid), has been synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction, IR spectroscopy and thermogravimetric analysis.Compound 1 exhibits a 3D binodal (4,8)-connected net based on trinuclear [Zn3(COO)4] clusters with the topology symbol of (416·612)(44·62)2.It crystallizes in monoclinic system, space group21/, with= 13.548(3),= 13.291(3),= 8.2750(1) Å,= 97.08(3)°,= 1478.7(6) Å3,= 2,M= 858.6 g/mol,D= 1.928 mg/m3,= 2.49 mm-1,(000) = 864,= 1.12, the final= 0.0457 and= 0.1329 for 4334 observed reflections with> 2().Additionally, the photoluminescent behaviours of 1 and H3L have also been investigated in the solid state at room temperature.

5-((3-formylphenoxy)methyl)isophthalic acid, zinc(II) compound, crystal structure, luminescent property;

1 INTRODUCTION

The synthesis, structural characterization, and potential applications in luminescence, catalysis, gas storage and separations of metal-organic frameworks (MOFs) have attracted a great deal of attention from chemists during the past decades[1-4].Metal-organic frameworks (MOFs), as a series of crystal-line coor- dination polymers, can be constructed by self-as- sembling from metal ions or clusters and organic ligands[5].Up to now, vast compounds with different structures and properties have been obtained from various inorganic metal units and organic ligands, as well as the connection modes between them.However, the analysis of topology for MOFs indica- tes that most of the reported MOFs are based on low-connected nodes[6].And utilizing polynuclear clusters as SBUs can serve as high-connected nodes due to metal clusters bearing different sizes and connectivity[7].Therefore, intense interests have been paid to such high-connected MOFs by employing polynuclear clusters as SBUs[5].So far, although few examples of high-connected (≥8) nodes have been obtained, it is still a great challenge for chemists because of the limited coordination numbers of metal centers and the steric hindrance of most organic ligands[8, 9].

5-((3-Formylphenoxy)methyl)isophthalic acid (H3L) as flexible tricarboxylate ligand possesses the rigid isophthalic acid and benzoic acid moieties, which can exhibit a great variety of coordination modes.As a typical flexible linker, the ligand can bend freely to meet the requirement of the coor- dination conformation in the assembly process.At the same time, unprecedented attention has been paid to10metal coordination compounds because of their outstanding luminescent properties[10-12].In this text, we report a new high-connected MOF based on semirigid tricarboxylic acid and10metal center Zn(II).The formula can be expressed as [Zn3(L)2(H2O)2]n.It exhibits a 3D (4,8)-connected network.In addition, the solid-state luminescent properties have also been studied at room tempera- ture.

2 EXPERIMENTAL

2.1 Materials and measurements

5-((3-Formylphenoxy)methyl)isophthalic acid (H3L) was purchased from Jinan Henghua Sci.& Tec.Co.Ltd.of China.All reagents in the present work were purchased from commercial sources and used without further purification.The FT-IR spectra were obtained on a BRUKER TENSOR27 spectrometer in the range of 4000～400 cm−1using KBr pellet technique.Thermogravimetric experiment was per- formed on a Dupont thermal analyzer from room temperature to 800 °C under a N2flow at a heating rate of 5 °C·min−1.Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance X-ray diffractometer equipped with Cuat a scan speed of 5 °·min−1in the 2range of 5～50°.The solid- state luminescence spectra were measured on a Varian Cary Eclipse Fluorescence spectrophotometer equipped with a stablespec-xenon lamp (450 w) as the light source.

2.2 Synthesis of [Zn3(L)2(H2O)2]n (1)

A mixture of Zn(NO3)2·6H2O (0.20 mmol, 0.059 g), H3L (0.10 mmol, 0.032 g), KOH (1.0 mL, 0.20 M) and H2O (7.0 mL) was placed in a 25 mL Teflon-lined stainless-steel vessel, which was heated at 120 °C for 3 days.Followed by slow cooling to room temperature, colourless block crystals of 1 were obtained in 32.3% yield (based on Zn(II)).FT-IR (KBr pellet, cm−1) selected bands: 3449 (s), 1628 (s), 1584(m), 1539 (m), 1451 (m), 1377 (s), 1237 (m), 1123 (w), 1071 (w), 1005 (m), 942 (w), 902 (w), 769 (s), 717 (m), 677 (w), 640 (w), 574 (w), 530 (w).

2.3 Crystal structure determination

Single crystals were prepared by the method described in the synthetic procedure.Single-crystal X-ray diffraction data were collected in the Beijing Synchrotron Radiation Facility (BSRF) beamline 3W1A, which were mounted on a MARCCD-165 detector (= 0.7100 Å) with the storage rings working at 2.5 GeV.In the process, the crystals were protected by liquid nitrogen at 100(2) K.Data reduction and correction were processed with HKL2000software[13].The structure was solved by direct methods with SHELXS-2016, and refined on2with full-matrix least-square technique using the program SHELXL-2016[14].Anisotropic thermal parameters were used to refine all non-hydrogen atoms of compound 1.Hydrogen atoms attached to carbon atoms were generated geometrically and refined using a riding mode.Hydrogen atoms attached to O(water) were located in difference Fourier maps and refined as riding in their as-found positions.A total of 4769 reflections were collected and 4769 were independent (int= 0.053), of which 4334 were observed with> 2().The final refinement gave= 0.0457,= 0.1329 (= 1/[2(F2) + (0.0642)2+ 0.0100], where= (F2+ 2F2)/3) for 4334 observed reflections with= 1.124, (∆)max= 1.348 and (∆)min= –1.124 e/Å3.Selected bond lengths, bond angles and H-bonds for 1 are listed in Tables 1 and 2, respectively.

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

Symmetry transformation: i: –+1,–1/2, –+1/2; ii:–+2, –, –+1; iii: –+2,–1/2, –+3/2; iv:, –+1/2,–1/2; v: –+1, –, –

3 RESULTS AND DISCUSSION

3.1 Synthesis and characterization

The crystal samples of complex 1 were prepared by a reaction of H3L and zinc(II) salts under hydrothermal condition.A rational approach to the design and synthesis of complex 1 is critical.The ligand and compound 1 display certain IR spectra in the range of 4000～400 cm−1, as shown in Fig.S1.The infrared absorption spectra show that the broad band at 3449 cm−1is assigned to the O–H stretching of the coordinated water molecules[15].The charac- teristic strong absorption bands in the ranges of 1628～1539 and 1451～1377 cm−1are attributed to the asymmetric and symmetric stretching vibrations of the coordinated carboxylate groups for complex 1, respectively.Compared with the free ligand, red-shift occurs relative to the carboxylate groups of H3L, which can further suggest the form of organic-metal compound.

Table 2. Hydrogen Bond Lengths (Å)

Symmetry transformation: ii: –+2, –, –+1; vi:,,−1

3.2 Description of the crystal structure

Single-crystal X-ray analysis shows that com- pound 1 crystallizes in the monoclinic space group21/with a 3D supramolecula framework.There are one and a half Zn(II) cations, one L3-anion and one bridged coordination water molecule in the asymme- tric unit.As displayed in Fig.1, the Zn(1) ion is four-coordinated in a slightly distorted tetrahedral geometry, where four coordinated oxygen atoms come from four L3-ligands.While, Zn(2) dispays a common six coordination sphere with an octahedron geometry.The basal positions are occupied by four oxygen atoms from four L3-anions.Other two oxygen atoms derived from two coordination water molecules occupy the apical positions.The Zn–O distances range from 1.907(7)to2.130(3) Å, which is comparable with reported Zn(II) complexes[16].As shown in Figs.2a and 2b, Zn(1), Zn(2) and symme- try-related Zn(1A) are briged by four carboxylate groups of four different L3-to give a trinuclear unit [Zn3(COO)4].The carboxylate groups derived from L3-ligands bridge adjacent trinuclear units to generate 1D metal-chains and a 2D sheet.Further, these {Zn3}-based SBUs propagate into a 3D networkeghit L3−linkers (Fig.2c).In this 3D structrue, the L3-anion can be defined as a 4-connected node, where every L3-ligand streches into four directions to connect four {Zn3} SBUs by using a5-1:1:1:1:0:1coordination mode.Using the topological method, each trinuclear metal unit is regarded as an 8-connected node through linking eight L3-ligands.Thus, one new high-connected network with a unique 3D (4,8-connected) net was well obtained (Fig.3).The topology symbol is defined as (416·612)(44·62)2from ToposPro pro- gram[17].

3.3 PXRD and TG analyses

To check the phase purity, the PXRD patterns of complex 1 were recorded at room temperature.As shown in Fig.4, the experimental PXRD patterns are similar with the simulated spectra based on single-crystal diffraction results, indicating the phase purity of the bulk sample.The differences in intensity could be attributed to different orientations of the powder samples.

Fig.1. ORTEP view with 30% probability level ofthe coordination environments of the Zn(II) ions in complex 1

Thermogravimetric analysis (TGA) was carried out to check the thermal stability of the as-synthe- sized compound.The experiment was performed under N2atmosphere in the temperature range of 25～800 °C at a heating rate of 5 °C per minute.As displayed in Fig.5, compound 1 was found to be stable up to 230 °C.Then the first step weight loss of 4.8% (calcd.4.3%) from 230 to 280 °C is due to the loss of two coordination water molecules.Afterwards, the compound still exhibited a stable plateau until ending at 335 °C, where the framework starts to decompose.The remaining weight corresponds to the formation of ZnO (obsd.27.7%, calcd.28.4%).

Fig.2. (a) View of the 1D chain structure of 1.(b) 2D network built from L3-ligands linking the Zn3metal clusters.(c) View of the 3D framework built from L3-ligands linking the adjacent 2D layers

Fig.3. (a) Topologically 4- and 8-connected nodes in 1.(b) Schematic illustrating the 3D (4,8)-connected network

Fig.4. Powder X-ray diffraction patterns of complex 1

Fig.5. TGA curve for complex1

3.4 Luminescent property

Taking into account the excellent luminescent properties of10electron configuration MOFs, the luminescent spectra of H3L and complex 1 were determined in the solid state at room temperature, as shown in Fig.6.When excited at 315 nm, free H3L ligand and complex 1 exhibit similar luminescent spectra with the same main emission peak at 405 nm.Due to the special10configuration, the Zn(II) ion is difficult to oxidize or reduce.So, the emission peak located in 1 may be attributed to the ligand-centered emission, which is neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature[18, 19].However, compared with the case of free H3L ligand, complex 1 shows obvious enhancement in intensity.It may result from that the coordination framework can effectively increase the conformational rigidity of the ligand and reduce the loss of energy by radiationless decay[20].

4 CONCLUSION

One novel Zn(II) compound based on semirigid tricarboxylic acid ligand was synthesized success- fully.The ligand H3L exhibits a unique5-1:1:1:1:0:1coordination mode to connect three Zn(II) ions to form a 3D trinuclear framework.Topologically, compound 1 displays a (4,8-connected) net with (416·612)(44·62)2schläfli symbol.In addition, compound 1 possesses enhanced photoluminescence compared with the uncoordinated H3L.

Fig.6. Solid-state luminescence spectra of H3L and complex 1at room temperature

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4 January 2018;

17 April 2018 (CCDC 1814590)

①This project was supported by the National Natural Science Foundation of China (Nos.21571118 & 21671124) and single-crystal X-ray diffraction data were collected in the beamline 3W1A of Beijing Synchrotron Radiation Facility

Lu Li-Ping, professor, majoring inbiology and molecular engineering.E-mail: luliping@sxu.edu.cn; Zhu Miao-Li, professor, majoring in inorganic chemistry.E-mail: miaoli@sxu.edu.cn

10.14102/j.cnki.0254-5861.2011-1945