Synthesis and Characterization of a New Metal-organic Framework with Nou Topology①

YANG ELIN Li-XingDING Qing-RongLIN Zheng-HunLING Qi-Dn②(College of Mterils Siene & Engineering, Fujin Norml University, Fuzhou 350007, Chin)(College of Chemistry & Chemistry Engineering, Fujin Norml University, Fuzhou 350007, Chin)(Stte Key Lortory of Struturl Chemistry, Fujin Institute of Reserh on the

Structure of Matter, the Chinese Academy of Sciences, Fuzhou 350002, China)

Synthesis and Characterization of a New Metal-organic Framework with Nou Topology①

YANG Ea,cLIN Li-XiangbDING Qing-RongcLIN Zheng-HuanaLING Qi-Dana②a(College of Materials Science & Engineering, Fujian Normal University, Fuzhou 350007, China)b(College of Chemistry & Chemistry Engineering, Fujian Normal University, Fuzhou 350007, China)c(State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, the Chinese Academy of Sciences, Fuzhou 350002, China)

A new compound, [H2N(CH3)2]2[Zn(btec)].H2O (1, H4btec = 1,2,4,5-benzenetetracarboxylic acid) has been solvothermally synthesized and structurally characterized by singlecrystal X-ray diffraction.The crystal structure is of monoclinic, space group P21/c with a = 9.5741(3), b = 14.5044(4), c = 13.7329(5) ?, β = 100.582(4)°, V = 1874.61(10) ?3, C14H20N2O9Zn, Mr= 425.7, Z = 4, Dc= 1.387 g/cm3, μ = 1.349 mm-1, F(000) = 776, R = 0.0560 and wR = 0.1684 for 2834 observed reflections (I > 2σ(I)).Compound 1 based on μ4-btec and tetrahedral building unit features a three-dimensional (3D) anionic framework with mixed connected nou topology.The luminescent property and thermogravimetric analysis of compound 1 are investigated.

solvothermal synthesis, crystal structure, topology, metal organic framework;

1 INTRODUCTION

Porous coordination polymers (PCPs) or metalorganic frameworks (MOFs) are infinite systems built up with metal ions and organic ligands as main elementary units linked via coordination bonds and other weak chemical bonds.In the past decades, the interest in the construction of PCPs and MOFs have attracted much attention not only for their potential applications in luminescence, gas storage, separation and heterogeneous catalysis, but also for their fascinating architectures and diverse structural topologies[1-5].As we know, the secondary building units (SBUs) are essential to the design of directionality for the construction of MOFs and to the achievement of robust frameworks[6].SBUs also play an important role in the structural design and synthesis of metal-carboxylate frameworks.To date, some MOFs with mixed connected topologies have also been reported, such as Pt3O4[7], boracite[8], and rutile[9].However, owing to severe hampering by the number of available coordination sites at the metal centers and the sterically demanding nature of organic ligands, there is a disappointing lack of systematic investigations on binodal highly connected structures, such as (3, 6)-, (4, 6)-, and (4, 8)-connected frameworks[10-15].It is also a great challenge to rationally prepare and control the structures and composition of target products based on binodal highly connected nets in crystal engineering.

Herein we report a three-dimensional (3D) anionic framework [H2N(CH3)2]2[Zn(btec)] (1, H4btec = 1,2,4,5-benzenetetracarboxylic acid) with rarely reported mixed connected nou topology, which is solvothermally synthesized and structurally characterized.In addition, the luminescent property of compound 1 is investigated in the solid state at room temperature.

2 EXPERIMENTAL

The compound 1,4-benzeneditetrazol-5-yl (H2bdt) was prepared as described previously[16].All other reagents were purchased commercially and used without further purification.All Powder X-ray diffraction (PXRD) analyses were recorded on a Rigaku Dmax2500 diffractometer with CuKa radiation (λ = 1.54056 ?) with a scan speed of 0.5 ° min-1.Thermogravimetric analysis (TGA) was carried out on a Netzsch STA449C thermal analyzer under a nitrogen atmosphere at a heating rate of 10 °C min-1.Fluorescence spectra were measured with a HORIBA Jobin-Yvon FluoroMax-4 spectrometer.

2.1 Synthesis of 1

2.2 Structure determination

A suitable single crystal of the synthesized compound was carefully selected under an optical microscope and glued to a thin glass fiber with epoxy resin.Diffraction data were collected at room temperature on an 'Xcalibur, Eos' diffractometer equipped with a graphite-monochromated MoKα radiation (λ = 0.71073 ?).A total of 6924 reflections were collected in the range of 2.78≤θ≤25.00o by using an ω scan mode, of which 3301 were unique with Rint= 0.0228 and used in the succeeding structure calculations.All structures were solved by direct methods with SHELXS-97[17]and refined by full-matrix least-squares methods on F2using the program SHELXL-97[18].The hydrogen atoms attached to carbon were located by geometrical calculations, and their positions and thermal parameters were placed during the structure refinement.The electron density of the disordered molecules was modeled using the SQUEEZE subroutine of PLATON[19].The final R = 0.0560 and wR = 0.1684+ (0.1065P)2+ 4.3322P], where P =/3), S = 1.072, (Δ/σ)max= 0.001, (Δρ)max= 2.186 and (Δρ)min= –0.754 e/?3.The selected bond lengths and bond angles are listed in Table 1.

Table 1.Selected Bond Lengths (?) and Bond Angles (o) for Compound 1

3 RESULTS AND DISCUSSION

3.1 Structure description

Compound 1 was solvothermally synthesized and has a new 3D framework structure.It crystallizes in the monoclinic P21/c space group.The asymmetric unit of 1 contains one Zn center, one btec ligand, two [H2N(CH3)2]+cations and one free hydrone.TheZn cation has the distorted tetrahedral coordination geometry and is coordinated by four O atoms from four carboxylate groups of four btec ligands (Fig.1a).The Zn–O distances are ranging from 1.956(3) to 1.982 ?3(Table 1).In compound 1, there are two independent btec ligands, both adopting the μ4-bridging mode with four carboxylate groups being a unidentate fashion.The Zn atoms are linked by μ4-btec ligands to generate a 3D anionic porous framework (Fig.1c), and these open channels are filled by [H2N(CH3)2]+cations and free water molecules.Interestingly, compounds 1 and [H2N(CH3)2]2[Zn(btec)]·DMF[20]which has been reported by us are supramolecular isomers.

From the viewpoint of structural topology, each Zn atom and each μ4-btec ligand can be viewed as a 4-connected node, respectively (Fig.1b); thus, the whole framework of 1 can be simplified as a trinodal (4, 4, 4)-connected network topology with the Point (Schl?fli) symbol of (4.65)2(42.84)(64.82) (Fig.1d).When further analyzed by TOPOS package, the vertex symbols give (4.62.6.6.6.6), (4.4.82.82.88.88), and (62.62.62.62.82.82) for these four-connected nodes.By the knowledge of us, the resulting (4,4,4)-connected topology is a rare case of nou topological net.

Fig.1.(a) Coordination environment in 1 (All guest and hydrogen atoms are omitted for clarity); (b) Defined 4-connected nodes; (c) 3D framework of 1; (d) Nou net of 1

3.2 Powder X-ray diffraction and thermal analysis

Powder X-ray diffraction (PXRD) was measured to confirm the phase purity and to examine the crystallinity of bulk sample (Fig.2a).The simulative (black line) and experimental (red line) X-ray powder diffraction patterns for compound 1 are compared, respectively.The diffraction (XRD) peaks correspond well in position, indicating the phase purity of the experimental compound.Thermogravimetric analysis (TGA) in N2atmosphere with a heating rate of 10 ℃/min was performed on polycrystalline sample to determine their thermal stability from 28 to 800 ℃.TGA result of 1 indicates that the host framework was stable up to ca.297 ℃ (Fig.2b).

Fig.2.(a) Powder XRD patterns of 1; (b) Thermogravimetric analysis curves of compound

3.3 Luminescent properties of 1

The luminescent property of compound 1 was investigated in the solid state at room temperature.The emission peaks of the compound are shown in Fig.3.Compound 1 displays an intense emission band at about 448 nm (λex= 350 nm).In comparison to the free 1,2,4,5-btec ligand which shows the emission band at 510 nm (λex= 350 nm), the emission peaks of compound 1 are obviously blue-shifted, and we speculate that the 1,2,4,5-btec ligand makes a significant contribution to the fluorescent emission of 1 and the blue shifts may be due to the ligandto-metal charge transfer (LMCT) since the formation of Zn–O coordination bonds[20-22].

Fig.3.Solid-state emission spectra of 1 and H4btec ligand at room temperature

4 CONCLUSION

In summary, we have prepared a new threedimensional MOF based on aromatic polycarboxylic 1,2,4,5-H4btec ligand, which possesses very rare nou topology.Furthermore, the luminescent property and thermogravimetric analysis of compound 1 are investigated.

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10.14102/j.cnki.0254-5861.2011-0572

17 November 2014; accepted 14 March 2015 (CCDC 1015179)

① This work was supported by the National Natural Science Foundation of China (21401023 and 21374017), Cultivating Fund for Excellent Young Scholar of Fujian Normal University (FJSDJK2012063), and Program for Innovative Research Team in Science and Technology in Fujian Province University (IRTSTFJ)

② Corresponding author.Ling Qi-Dan.E-mail: qdling@fjnu.edu.cn