CHEN Shun-Yu　YANG E　XIE Chun-LingLIU Yun-Zhen　XIAO Xiu-Feng②（College of Chemistry &Chemicl Engineering，Fujin Norml University，Fuzhou350007，Chin）（College of Mterils Science &Engineering，Fujin Norml University，Fuzhou350007，Chin）

Synthesis，Structure and Luminescent Property of a New Zn（Ⅱ）Coordination Polymer Based on 1，3，5-Benzenetricarboxylate and 4，4΄-Bipyridine Ligands①

CHEN Shun-YuaYANG EbXIE Chun-Linga
LIU Yun-ZhenaXIAO Xiu-Fenga②a（College of Chemistry &Chemical Engineering，Fujian Normal University，Fuzhou350007，China）
b（College of Materials Science &Engineering，Fujian Normal University，Fuzhou350007，China）

A new Zn（II）coordination polymer ［Zn3（BTC）2（4，4΄-bpy）（CO）2］n（1，H3BTC = 1，3，5-benzenetricarboxylic acid，4，4΄-bpy = 4，4΄-bipyridine）has been prepared through urothermal reaction of zinc nitrate hexahydrate with 1，3，5-benzenetricarboxylic acid and 4，4΄-bipyridine，and characterized by elemental analysis，IR spectroscopy，TGA and powder XRD，and its crystal structure was determined by single-crystal X-ray diffraction analysis.The title compound crystallizes in the monoclinic system，space group C2/c，with a = 10.262（3），b = 18.838（5），c = 15.083（4）Å，β = 99.203（4）º，V = 2878.3（14）Å3，C30H14Zn3N2O14，Mr= 822.54，Z = 4，Dc= 1.898 g/cm3，μ = 2.561 mm-1，F（000）= 1640，R = 0.0363 and wR = 0.0932 for 2828 observed reflections （I ＞ 2σ（I））.X-ray analysis shows that the asymmetric unit of the title compound contains two crystallographically unique Zn（II）atoms which are connected by BTC3-ligands and 4，4΄-bpy coligands to form a3D framework with 1D channels.The solid luminescence of ligand and the title complex was also studied at room temperature.

zinc coordination polymer，1，3，5-benzenetricarboxylic acid，4，4΄-bipyridine，urothermal synthesis，photoluminescence；

1　INTRODUCTION

Metallic-organic frameworks （MOFs）have attracted increasing attention not only for their intriguing structural and topological diversities，but also for their potential applications as functional materials such as gas storage，molecular separations，ion exchange，etc［1-9］.To choose suitable ligands with certain features and metal precursors is the key in the rational design and construction of such functional MOFs.It is well known that the rigid aromatic organic multi-carboxylate ligands are good for the construction of coordination polymers because of their diversity of coordination modes and high structural stability［10-13］.The 1，3，5-benzenetricarboxylic acid is a desired organic ligand for the construction of high-dimensional metal-organic hybrid compounds for it can produce multiple products ofH2BTC-，HBTC2-and BTC3-with the number of potential donor oxygen atoms varying from one to six through complete or partial deprotonation of its three carboxyl groups［14］.In addition，N-containing mixed ligands，especially neutral bipyridyl-type coligands like 4，4΄-bipyridine can connect multicarboxylate coordination polymer motifs into higher dimensions.So，the employment of 4，4΄-bipyridine in the self-assembly process is another effective approach to obtain high dimensional frameworks of coordination polymers［15，16］.Based upon such considerations，we choose 1，3，5-benzenetricarboxylic acid as a multi-dentate O-donor ligand and 4，4΄-bipyridine as a N-donor coligand to coordinate with the d10block metal Zn（II）to generate new coordination polymers.Several examples of coordination networks constructed by using 1，3，5-benzenetricarboxylic acid and 4，4΄-bipyridine as mixed ligands under hydrothermal or solvothermal conditions have been reported［17，18］.However，the final structures of MOFs are also greatly influenced by solvent.Changing solvent will result in a new structure.Recently，urothermal synthetic method that uses urea derivatives as solvents is emerging and opens a new approach toward the creation of framework materials with fascinating architecture and promising applications.2-Imidazolidinone （e-urea）is the solvent most frequently utilized in urothermal reaction，and a variety of fascinating coordination polymers constructed by using e-urea as solvent have been reported.However，so far，compounds based on 1，3，5-benzenetricarboxylic acid and 4，4΄-bipyridine mixed ligands have not been constructed under the action of e-urea.In this paper，we report the structural characterization and luminescent property of a new3D Zn（II）coordination polymer based on 1，3，5-benzenetricarboxylic acid and 4，4΄-bipyridine mixed ligands synthesized under urothermal conditions by using urea derivative e-urea as solvent.

2　EXPERIMENTAL

2.1Materials and instruments

All reagents were purchased commercially and used without further purification.Elemental analyses for C，H and N were carried out on a Perkin-Elmer 240C elemental analyzer.The powder X-ray diffraction （PXRD）experiment was performed on a D/MAX-3CX diffractometer with Cu-Kα radiation （λ = 1.5406 Å）at room temperature.Thermogravimetric analysis （TGA）experiment was carried out using a simultaneous SDT 2960 thermal analyzer in the temperature range of 25～800℃under a flow of nitrogen with a heating rate of 10/min℃.Solid state emission spectrum of the title complex was recorded using 48000DSCF fluorescence spectrometer.Crystal structure determination was carried out on a Bruker SMART CCD diffractometer.The IR spectrum was recorded in the range of 4000～400 cm-1on a Nicolet Avatar360 FTIR spectrometer using KBr pellets.

2.2Synthesis of ［Zn3（BTC）2（4，4΄-bpy）（CO）2］n（1）

A mixture of Zn（NO3）2·6H2O （90.0 mg，0.303 mmol），H3BTC （40.0 mg，0.190 mmol），4，4΄-bipyridine （30.0 mg，0.190 mmol）and e-urea·0.5H2O （1840 mg，21.4 mmol）was sealed in a 25 mL Teflon-lined stainless steel autoclave，which was heated at 140 ℃ for 72 h.After the reaction was cooled to room temperature，the product was washed by ethanol，then orange single crystals were obtained with the yield of 63%.Anal.Calcd.for C30H14Zn3N2O14（%）:C，43.80；H，1.72；N，3.41.Found （%）:C，43.63；H，1.80；N，3.35.IR （KBr pallet，cm-1）:3420m，1618s，1562s，1436s，1354s，1219m，1074w，1015w，821m，759m，642w，572m，458m.

2.3Crystal structure determination

X-ray crystallography suitable single crystal of 1 with dimensions of 0.35mm × 0.20 mm × 0.18mm was carefully selected under an optical microscope and glued to thin glass fiber.The diffraction data were collected at 293（2）K on a Bruker SMART CCD diffractometer equipped with a graphite-monochromatic Mo-Kα radiation （λ = 0.71073 Å）.Intensities were collected by using an ω scan mode in the range of3.31≤θ≤27.48°.A total of 11012 reflections were collected，of which3197 were indepen-dent （Rint= 0.0234）and 2828 were observed with I ＞2σ（I）.The structure was solved by direct methods using SHELXS-97 program and refined with SHELXL-97 by full-matrix least-squares technique on F2［19，20］.All non-hydrogen atoms were refined anisotropically and the hydrogen atoms were located according to geometrical calculations.The final R = 0.0363，wR = 0.0932 （w = 1/［σ2（Fo2）+ （0.0504P）2+ 7.1398P］，where P = （Fo2+ 2Fc2）/3），S = 1.060 and （Δ/σ）max= 0.000.The highest and lowest residual peaks in the final difference Fourier map are 0.633 and -0.483 e/Å3，respectively.The selected bond lengths and bond angles are listed in Table 1.

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

3　RESULTS AND DISCUSSION

3.1Description of the structure

As shown in Fig.1，the asymmetric unit of complex 1 contains one BTC3-anion，one carbonyl group，a half 4，4΄-bipyridine ligand and two crystallographically unique Zn（II）atoms.The two Zn（II）atoms show different coordination environments.The Zn（1）atom is four-coordinated by O（1），O（4）and symmetry-related O（1）i，O（4）icarboxylate oxygen atoms from four different BTC3-ligands，forming a tetrahedral coordination geometry.In turn，the Zn（2）atom also adopts the four-coordinated tetrahedral geometry completed by N（1）atom of 4，4΄-bpy molecule，one carbonyl group and two carboxylate oxygen atoms （O（5），O（3））from two different BTC3-ligands.The Zn-O/N bond length spans the range of 1.907（2）～2.063（3）Å.In complex 1，all carboxylate groups are deprotonated and the three carboxylate groups of each BTC3-ligand anion show different coordination modes.One carboxylate group adopts a bidentate-bridging coordination mode，while the other two adopt a monodentate terminal mode，so the completely deprotonated BTC3-ligand plays a four-connected role in linking four Zn atoms.On the other hand，the 4，4΄-bpy ligand shows the didentate coordination mode to bridge two Zn（2）atoms.As such，Zn（1）and Zn（2）atoms are connected by BTC3-ligands and 4，4΄-bpy coligands to form a3D framework with 1D rhomboid channels along the c-axial direction （Fig.2）.

3.2IR spectra

IR spectra of complex 1 show broad bands in3420 cm-1due to the absorption of water molecules.The absence of strong bands from 1690 to 1710 cm-1indicates the complete deprotonation of carboxylic groups.IR spectra of the title compound also show the characteristic antisymmetric stretching bands of carboxylate groups （1618 and 1562 cm-1）and symmetric stretching bands （1436 and 1354 cm-1）with respective values of ∆ = va（COO-）- vs（COO-）126 and 264 cm-1，which clearly indicates the presence of bidentate （∆ ＜ 200 cm-1）and monodentate （∆ ＞ 200 cm-1）coordination modes of the carboxylate groups［21］.All these agree well with the result of X-ray single-crystal analysis.

Fig.1.Asymmetric unit of 1 showing the coordination environment of Zn（II）atom and the coordination modes of BTC3-and 4，4΄-bpy ligands.Hydrogen atoms are omitted for clarity （Symmetry codes:i:1 - x，y，1/2 - z；ii:3/2 - x，1/2 - y，1 - z；iii:3/2 - x，1/2 + y，1/2 - z；iv:1/2 + x，1/2 + y，z；v:1/2 + x，1/2 - y，1/2 + z）

Fig.2.3D framework of 1

3.3Thermal analysis

To measure the thermal stability of complex 1，the thermogravimetric analysis （TGA）was carried out by heating the sample to 800 ℃ at a rate of 10 ℃/min in a N2atmosphere and the weight loss curve is shown in Fig.3.It can be seen that complex 1 has high thermal stability，and its framework can remain stable up to 425 ℃.Above that temperature，the framework begins to decompose.The remaining product may be ZnO （Obsd.:30.85%，Calcd.:29.68%）.

3.4Powder X-ray diffraction analysis

To confirm whether the crystal structure is truly representative of the bulk materials，samples of complex 1 were measured by X-ray powder diffraction at room temperature.As shown in Fig.4，the peak positions of the experimental pattern are in good agreement with the simulated one based on the single-crystal structure analysis，which indicates that the bulk synthesized materials and the measured single crystal are the same.

3.5Fluorescence property

The photoluminescence properties of complex 1 and the free H3BTC ligand were studied in the solid state at room temperature.From Fig.5，we can see that the emission maximum of complex 1 is almost equal to the free H3BTC ligand.Considering that the Zn（II）ion is difficult to oxidize or reduce due to its d10configuration，the emission of 1 can probably be assigned to intraligand fluorescent emissions or a mixture of intraligand and ligand-to-ligand charge transitions.The enhanced fluorescence emission of complex 1 can be attributed to that the coordination of H3BTC ligand with metal center increases the rigidity of the ligand，and thus reduces the loss of energy by thermal vibrational decay.

Fig.3.TGA diagram of 1

Fig.4.Experimental and simulated XRD spectra of 1

Fig.5.Solid-state emission spectra of complex 1 and free H3BTC ligand at room temperature

4　CONCLUSION

In summary，a new coordination polymer has been successfully synthesized through urothermal reaction of zinc salt with polydentate O-donor H3BTC ligand and coligand of rigid bidentate N-donor 4，4΄-bipyridine.The synthesized complex features a3D structure with 1D channels and shows obvious photoluminescent emission at room temperature in the solid state.The results suggest that complex 1 will be useful in the optical material.

REFERENCES

（1）Lin，J.B.；Zhang，J.P.；Chen，X.M.Nonclassical active site for enhanced gas sorption in porous coordination polymer.J.Am.Chem.Soc.2010，132，6654-6656.

（2）Seo，J.S.；Whang，D.；Lee，H.；Jun，S.I.；Oh，J.；Jeon，Y.J.；Kim，K.A homochiral metal-organic porous material for enantio-selective separation and catalysis.Nature 2000，404，982-986.

（3）Zhang，J.P.；Zhang，Y.B.；Lin，J.B.；Chen，X.M.Metal azolate frameworks:from crystal engineering to functional materials.Chem.Rev.2012，112，1001-1033.

（4）Li，J.R.；Sculley，J.；Zhou，H.C.Metal-organic frameworks for separations.Chem.Rev.2012，112，869-932.

（5）Eddaoud，M.；Kim，J.；Rosi，N.；Vodak，D.；Wachter，J.；O΄keeffe，M.；Yaghi，O.M.Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage.Science 2002，295，469-472.

（6）Yang，W.B.；Lin，X.；Blake，A.J.；Wilson，C.；Hubberstey，P.；Champness，N.R.；Schroder，M.Self-assembly of metal-organic coordination polymers constructed from a bent dicarboxylate ligand:diversity of coordination modes，structures，and gas adsorption.Inorg.Chem.2009，48，11067-11078.

（7）Fang，Q.R.；Zhu，G.S.；Xue，M.；Sun，J.Y.；Sun，F.X.；Qiu，S.L.Structure，luminescence and adsorption properties of two chiral microporous metal-organic frameworks.Inorg.Chem.2006，45，3582-3587.

（8）Zhang，J.；Chen，S.M.；Wu，T.；Zheng，S.T.；Chen，Y.；Nieto，R.A.；Feng，P.；Bu，X.Urothermal synthesis of crystalline porous materials.Angew.Chem.Int.Ed.2010，49，8876-8879.

（9）Zhong，D.C.；Lu，W.G.；Jiang，L.；Feng，X.L.；Lu，T.B.Three coordination polymers based on 1H-tetrazole （HTz）generated via in situ decarboxylation:synthesis，structures，and selective gas adsorption properties.Cryst.Growth Des.2010，10，739-746.

（10）Chen，S.Y.；Yang，E.；Liu，Z.S.Synthesis，crystal structure and luminescent property of a Cd（II）coordination polymer constructed by2-amino-1，4-benzene-dicarboxylate and 2-imidazolidinone.Chin.J.Inorg.Chem.2013，29，2485-2490.

（11）Sun，Y.Q.；Liu，S.Y.；Wang，J.Heterometallic trinuclear Ni2Zn and Ni2Mn complexes of macrocyclic oxamide:synthesis and crystal structures.Chin.J.Struct.Chem.2013，32，39-44.

（12）Chen，S.Y.；Yang，E.；Zhang，J.Synthesis and crystal structure of a new Zn（II）coordination polymer constructed by 1，2，4，5-benzenetetracarboxylate.Chin.J.Struct.Chem.2013，32，149-153.

（13）Feng，G.D.；Jiang，L.；Chen，Q.；Yang，D.S.；Wang，M.J.；Li，Z.X.；Luo，X.L.A 2D wavy parquet architecture based on the biphenyl-3，3΄，4，4΄-tetracarboxylic acid and bis（2-benzimidazole）alkanes ligands.Chin.J.Struct.Chem.2013，32，45-50.

（14）Yaghi，O.M.；Li，H.L.；Groy，T.L.Construction of porous solids from hydrogen-bonded metal complexes of 1，3，5-benzenetricarboxylic acid.J.Am.Chem.Soc.1996，118，9096-9101.

（15）Zhang，X.；Li，Z.J.；Qin，Y.Y.；Yao，Y.G.Polycatenated bilayer motif constructed from flexible N，N΄-ipyridyl and aromatic dicarboxylate ligands.Inorg.Chem.Comm.2012，15，1-4.

（16）Yang，E.；Zhang，J.；Li，Z.J.；Gao，S.；Kang，Y.；Chen，Y.B.；Wen，Y.H.；Yao，Y.G.Interweaving3D network with double helical tubes filled by 1D coordination polymer chains.Inorg.Chem.2004，43，6525-6527.

（17）Luo，F.；Che，Y.X.；Zheng，J.M.One3D noninterpenetrating metallic-organic framework with the rare （3，4）-connected （8.102）2（4.84.10）2（42.82.102）topology.Inorg.Chem.Commun.2006，9，1045-1048.

（18）Xue，M.；Zhu，G.S.Zhang，Y.J.；Fang，Q.R.；Hewitt，I.J.；Qiu，S.L.Rational design and control of the dimensions of channels in a series of3D pillared metal-organic frameworks:synthesis，structures，adsorption，and luminescence properties.Cryst.Growth Des.2008，8，427-434.

（19）Sheldrick，G.M.SHELXS-97:Program for the Solution of Crystal Structures.University of Göttingen，Germany 1997.

（20）Sheldrick，G.M.SHELXL-97:Program for the Refinement of Crystal Structures.University of Göttingen，Germany 1997.

（21）Nakamoto，K.Infrared and Raman Spectra of Inorganic and Coordination Compounds.Chemistry Industry Press，Beijing 1986.

10.14102/j.cnki.0254-5861.2011-462

24 July 2014；accepted 1 December 2014 （CCDC 940190）

① This work was supported by the National Natural Science Foundation of China （60976019）and the Foundation of Education Committee of Fujian Province （No.JB13007）

② Corresponding author.E-mail:xfxiao@fjnu.edu.cn