Synthesis,Structure and Luminescent Property of a 2D Cd(II) Coordination Polymer Based on Mixed-ligands of 1,3-Bis(imidazol)propane and Pyridine-3,5-dicarboxylic Acid①

LU Jiu-Fu　LIU Ju　JIANG Min　GE Hong-Gung（Shnxi University of Technology，Chemicl Engineering College，Hnzhong 723001，Chin）

b（College of Pharmacy，Liaoning University，Shenyang，Liaoning110036，China）

Synthesis,Structure and Luminescent Property of a 2D Cd(II) Coordination Polymer Based on Mixed-ligands of 1,3-Bis(imidazol)propane and Pyridine-3,5-dicarboxylic Acid①

LU Jiu-Fua②LIU JubJIANG MinaGE Hong-Guangaa（Shaanxi University of Technology，Chemical Engineering College，Hanzhong 723001，China）

b（College of Pharmacy，Liaoning University，Shenyang，Liaoning110036，China）

A new Cd（）Ⅱ coordination polymer，namely，［Cd（1，3-bip）（3，5-pdc）］n（1，3-bip = 1，3-bis（imidazol）propane and3，5-pdc = pyridine-3，5-dicarboxylic acid）has been synthesized under hydrothermal conditions.Compound 1 was characterized by infrared spectrum，elemental analysis，powder X-ray diffraction （PXRD）and single-crystal X-ray diffraction analysis.It crystallizes in monoclinic，space group P21/c with a = 1.40178（7），b = 1.72502（12），c = 1.41635（6）nm，β = 92.653（4）º，V =3.4212（3）nm3，Z = 4，C16H15CdN5O4，Mr= 453.73，Dc= 1.762 g/cm3，F（000）= 1808，μ = 1.310 mm-1，R = 0.0899 and wR = 0.1945.In compound 1，each3，5-pdc ligand links three Cd（）Ⅱ ions and each Cd（）Ⅱ attaches to bip ligands to form a complicated 2D double-layer structure.In addition，the thermal stability and luminescent property of 1 have been studied in the solid state at room temperature.

Cd()Ⅱ coordination polymer,mixed-ligands,crystal structure,luminescent property

1　INTRODUCTION

Construction of new coordination polymers has attracted attention due to their diverse structural topologies and potential applications as functional materials such as gas storage，magnetism，catalysis，and luminescence［1，2］.The diversity in the framework structures of such materials greatly depends on the metal centers and structures of spacer ligands.Up to now，the selection of suitable organic ligands is crucial for constructing extended coordination frameworks.To the best of our knowledge，the bi bearing alkyl spacers is a good choice of N-imidazole ligands with -CH2- spacers to be good bridging ligands.The flexible nature of -CH2- spacers allows the ligands to bend and rotate freely when coordinating to metal centers to conform to the coordination geometries of metal ions［3-8］.Meanwhile，the mixture of carboxylates with N-donor auxiliary ligands is good for the construction of new coordination polymers［9，10］.In particular，carboxylate ligands have been frequently employed in that their ability to engage in diverse bonding modes allows wide structural diversity and the tailoring of physicalproperties［11-14］.In this case，3，5-pdc has been employed in the preparation of metal-organic complexes in possession of multi-dimensional networks and interesting properties.

In this work，a new Cd（Ⅱ）coordination polymer 1 has been synthesized under hydrothermal conditions，and its structure has been determined by single-crystal X-ray diffraction analysis.In addition，the luminescent property of the complex has also been investigated in the solid state at room temperature.

2　EXPERIMENTAL

2.1Materials and methods

All starting materials obtained from commercial sources were of reagent quality and used without further purification.Elemental analysis for C and H was performed on a PE 240C elemental analyzer.IR spectra were recorded in the range of 400～4000 cm-1on a Nicolet iS10 spectrometer using KBr pellets.Thermogravimetric analyses were performed with a Perkin Elmer Pyris 1 TGA instrument in the range of 400～800 ℃under a nitrogen flow at a heating rate of 10 ℃·min-1.Powder X-ray diffraction （PXRD）data were collected on a Rigaku Ultima ІV X-ray diffractometer with CuKα radiation （λ = 0.154056 nm）.The luminescent spectra for polycrystalline samples were measured at room temperature on a Perkin Elmer LS 55 fluorescence spectrometer with a xenon arc lamp as the light source.

2.2Synthesis of [Cd(1,3-bip)(3,5-pdc)]n(1)

A mixture of3，5-pdc （0.10 mmol，16.7 mg），1，3-bip （0.10 mmol，17.6 mg）and Cd（NO3）2.4H2O（0.10 mmol，30.8 mg）in distilled water （15 mL）was placed in a Teflon-lined stainless steel vessel，heated to 120 ℃ for3 d，and then cooled to room temperature over 24 h.Colorless block crystals of 1 were obtained.Yield:75%.Elemental analysis （%）:calcd.for C16H15CdN5O4（Mr= 453.74）:C，42.31；H，3.30；N，15.43.Found:C，42.61；H，3.04；N，15.23.IR （cm-1）:3111 （s），2945 （m），1629 （s），1565 （m），1451 （m），1424 （w），1113 （m），813 （m），655 （m）.

2.3Structure determination

A single crystal of the title compound with dimensions of 0.22mm × 0.16mm × 0.17mm for 1 was used for data collection on a Bruker SMART APEX CCD diffractometer equipped with a graphite-monochromated MoKα （λ = 0.071073 nm）radiation at 296（2）K，using a narrow-frame method with scan width of 0.30 in ω and exposure time of 10 s per frame.In the ranges of 0.98≤θ≤25º，-14≤h≤16，-20≤k≤19 and -16≤l≤15，a total of 14742 reflections with 5914 independent ones were collected，of which 5099 were observed with I ＞2σ（I）（Rint= 0.0237）and used in the structure solution and refinement.The data were integrated using the SAINT processing program［15］.Empirical absorption correction was applied.The structure was solved by direct methods using SHELXS-97［16］and refined on F2by full-matrix least-squares with SHELXL-97［17］.All non-hydrogen atoms were refined anisotropically.All hydrogen atoms were put in the calculated positions.All hydrogen atoms were refined isotropically with isotropic vibration parameters related to the non-hydrogen atoms to which they are bonded.Selected bond lengths and bond angles of 1 are given in Table 1.

Table 1.Selected Bond Lengths (nm) and Bond Angles (°) of Compound 1

N（1）-Cd（1）-N（6）94.3（4）N（6）-Cd（1）-N（5）　175.2（4）O（6）-Cd（1）-N（5）　90.5（3）O（2）-Cd（1）-N（5）92.1（4）N（1）-Cd（1）-O（5）　86.5（4）N（1）-Cd（1）-O（1）83.4（3）N（6）-Cd（1）-O（5）　92.7（4）N（6）-Cd（1）-O（1）91.7（4）O（2）-Cd（1）-O（5）137.7（3）O（2）-Cd（1）-O（1）52.5（3）O（6）-Cd（1）-O（5）　52.9（3）O（6）-Cd（1）-O（1）　137.4（3）N（5）-Cd（1）-O（5）　86.8（3）N（5）-Cd（1）-O（1）89.6（3）O（5）-Cd（1）-O（1）　169.3（3）

3　RESULTS AND DISCUSSION

3.1Description of structure 1

Single-crystal X-ray diffraction analysis showed that the asymmetrical unit of complex 1 contains one Cd atom，one3，5-pdc anion and one 1，3-bip ligand.The seven-coordinate Cd ions are ligated to four O atoms from two3，5-pdc anions，one N atom from one3，5-pdc anion and two N atoms from two 1，3-bip ligands resulting in the distorted decahedral geometry （Fig.1）.The Cd-O bond lengths vary from 0.2323（9）to 0.2600（9）nm，and the Cd-N distances fall in the 0.2267（11）～0.2410（10）nm.Each3，5-pdc anion coordinates to three Cd ions to form a 2D infinite undulated sheet （Fig.2）.The 1，3-bip ligands display a trans-configuration with a dihedral angle of32.17° between the two imidazole rings in 1.The bip ligand acts as a bis-monodentate bridging ligand to link Cd（II）centers to form a 1D right-handed ［Cd（1，3-bip）］n2+helical chain along the a axis （Fig.3）.Furthermore，these 1D right-handed helical chains are connected by the pdc ligand to generate a complicated 2D double-layer framework （Fig.4）.analysis (TGA)

Fig.1.View of the coordination environments of Cd(II) atoms in complex 1

Fig.2.View of 2D infinite undulated sheet constructed by Cd(II) and3,5-pdc

Fig.3.1D right-handed [Cd(1,3-bip)]n2+helical chain along the a axis

Fig.4.Schematic representation of the 2D framework in [Cd(1,3-bip)(3,5-pdc)]n

3.2PXRD and thermogravimetric

Powder X-ray diffraction （PXRD）has been used to check the phase purity of the bulky samples in the solid state.For 1，the measured PXRD patterns closely match the simulated patterns generated from the results of single-crystal diffraction data （Fig.5）.

As shown in Fig.6，thermogravimetric analysis （TGA）of complex 1 was conducted to investigate its thermal stability.The crushed single-crystal sample of 1 was heated up to 800 ℃ under N2gas at a heating rate of 10 ℃·min-1.From the TG curve of Fig.6，it can be seen that the first weight loss of36.5% from375 to390 ℃ corresponds to the loss of3，5-pdc ligand （calcd.36.8%）.The weight loss of 1，3-bip ligand occurs from390 ℃ and ends at 600 ℃ （weight loss:38.4%；calcd.38.9%）.The final residual weight can be assigned as CdO （obsd.28.5%；calcd.28.2%）.The TGA results basically agree with the formula of the title compound.

Fig.5.Powder XRD patterns of complex 1(a) as experimental and (b) simulated(b)　from the single-crystal X-ray data

Fig.6.TG curve of the title compound

Fig.7.Solid-state emission spectrum of 1,3-bip ligand and 1

3.3Luminescent property

The photoluminescence spectra of complex 1 are shown in Fig.7.The free ligand bip displays photoluminescence with emission maximum at 462 nm （λex=330 nm）.It can be presumed that this peak originates from the π*→π transition.To the best of our knowledge，the emission of dicarboxylate belongs to π*→n transition which is very weak compared to that of the π*→π transition of the bip，so the dicarboxylates almost have no contribution to the fluorescent emission of the as-synthesized MOFs［18］.Upon complexation of these ligands withCd（）Ⅱ ions，intense emissions are observed at 436 nm for 1 under330 nm excitation.The emission peak of 1 is blue-shifted by about 26 nm compared to pure 1，3-bip ligand.According to the literature［19］，the luminescent property of 1 can be attributed to ligand-to-metal charge transfer （LMCT）transition.

4　CONCLUSION

A new Cd（）Ⅱ coordination polymer，［Cd（1，3-bip）（3，5-pdc）］n1，has been synthesized using the reaction of 1，3-bip，3，5-pdc and corresponding metal salt.The compound features a 2D double layered structure.In addition，the thermal stability and luminescent properties of the compound were investigated in detail.Further work is in progress in order to obtain other high thermally stable new transition metal coordination polymers with interesting physical and chemical properties.

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10.14102/j.cnki.0254-5861.2011-0449

10 July 2014；accepted 15 October 2014 （CCDC 963674）

① Supported by the National Natural Science Foundation of China （No.21373132）and the Science Foundation of Education Department of Shaanxi Provincial Government （No.12JK0637）

② Corresponding author.Lu Jiu-Fu （1981-），instructor，master，majoring in coordination chemistry.E-mail:jiufulu@163.com