Syntheses,Structures,and Photoluminescent Properties of Two 2D Coordination Polymers Based on 1,3-Di(2,4-dicarboxyphenyl) Benzene and 1,3-Di(4-pyridyl)propane①

WANG Ji-Jiang SU Yong-Chao GAO Lou-Jun（Department of Chemistry and Chemical Engineering，Shaanxi Key Laboratory of Chemical Reaction Engineering，Yan’an University，Yan’an 716000，China）

Syntheses,Structures,and Photoluminescent Properties of Two 2D Coordination Polymers Based on 1,3-Di(2,4-dicarboxyphenyl) Benzene and 1,3-Di(4-pyridyl)propane①

WANG Ji-Jiang SU Yong-Chao GAO Lou-Jun②
（Department of Chemistry and Chemical Engineering，Shaanxi Key Laboratory of Chemical Reaction Engineering，Yan’an University，Yan’an 716000，China）

Two new 2D coordination polymers，［Zn（HL）（Hbpp）］n（1）and ［Cd（HL）（Hbpp）（H2O）］n（2）（H4L = 1，3-di（2，4-dicarboxyphenyl）benzene，bpp = 1，3-di（4-pyridyl）propane），have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis，elemental analysis，TGA and IR spectroscopy.Complex 1 crystallizes in monoclinic，space group P21/n，with a = 13.1031（6），b = 14.7315（7），c = 15.0041（7）Å，β = 91.202（1）°，V = 2895.6（2）Å3，Dc= 1.532 g/cm3，Mr= 667.97，F（000）= 1376.0，μ = 0.909 mm-1，Z = 4，the final R = 0.0481 and wR = 0.1304 for 7027 observed reflections with I ＞ 2σ（I）.Complex 2 also belongs to the monoclinic space group P21/n，with a = 13.6326（4），b = 14.6432（4），c = 14.9395（5）Å，β = 93.132（1）º，V = 2977.84（16）Å3，Dc= 1.613 g/cm3，Mr= 1445.98，F（000）= 733.0，μ = 0.794 mm-1，Z = 2，the final R = 0.0283 and wR = 0.0784 for 7230 observed reflections with I ＞ 2σ（I）.Both complexes 1 and 2 form 2D net structures，which are further linked to form3D supramolecular net structures via intermolecular hydrogen bonds and aromatic π-π stacking interactions.

1,3-di(2,4-dicarboxyphenyl) benzene,Zn(II),Cd(II),photoluminescent properties;

10.14102/j.cnki.0254-5861.2011-0416

1　INTRODUCTION

The rational design and construction of coordination polymers have been paid increasing attention due to their potential applications in catalysis［1，2］，magnetism［3-5］，fluorescence［6-8］，drug delivery［9］，gas storage and separation［10］and so on.As everyone knows，the structures of complexes depend on organic ligands，metal ions，synthetic methods［11-13］，etc.，so the choice of appropriate metal ions and organic ligands with suitable functional group is very important in the design and synthesis of complexes.Up to date，many interesting coordination polymers have been constructed by flexible aromatic carboxylic acid ligands.And the flexible polypyridyl ligands are also interesting linkers with strong coordination ability.It is widely recognized that the flexible 1，3-di（4-pyridyl）propane （bpp）ligand is an excellent candidate for constructing novel structures as auxiliary ligands and may act as a bidentate bridge or in a monodentate terminal mode，leading to the formation of kinds of 1D chain，2D layer and3D structure motifs［14-16］.In this paper we choose H4L as the organic ligand，considering that H4L hasmultiple bridging moieties and provides abundant structural motifs；as well as no coordination polymers constructed from the H4L ligand has been reported，and much work is still necessary to understand.Herein we report the syntheses and structures of one Zn（II）and one Cd（II）coordination polymers with H4L and bpp as co-ligands.

2　EXPERIMENTAL

2.1Materials and methods

All reagents and solvents employed were commercially available and used without further purification.Elemental analyses for carbon，hydrogen，and nitrogen were performed with a Vario EL III elemental analyzer.The FT-IR spectra were recorded from KBr pellets in the range of 4000～400 cm-1on a Bruker EQUINOX-55 spectrometer.Fluorescence spectra were performed on a Hitachi F-4500 fluorescence spectrophotometer at room temperature.Thermogravimetric analyses （TGA）were performed under nitrogen with a heating rate of 10 ℃/min using a NETZSCH STA 449F3 thermogravimetric analyzer.

2.2Syntheses

［Zn（HL）（Hbpp）］n（1）The mixture of Zn（NO3）2· 6H2O （0.10 mmol，29.7 mg），bpp （0.10 mmol，20.0 mg），H4L （0.10 mmol，40.6 mg）and 10 mL H2O was stirred for30 min.The mixture was then placed in a 23 mL Teflon-lined stainless-steel vessel and heated at 160 ℃ for3 d.Colorless block crystals were obtained when the mixture was cooled to room temperature.Yield:ca.62% based on Zn.Calcd.for C35H26ZnN2O8（%）:C，62.93；H，3.92；N，4.19.Found （%）:C，63.04；H，3.91；N，4.16.IR （KBr pellet，cm-1）:3446 s，3078 s，2941 s，2343 w，1716 s，1622 s，1543 w，1435 s，1367s ，1226 w，806w，694 w，472 w.

［Cd（HL）（Hbpp）（H2O）］n（2）The synthesis was similar to that described in 1 except using Cd（NO3）2·4H2O （0.10 mmol，30.8 mg）for 2 instead of Zn（NO3）2·6H2O （0.10 mmol，29.7 mg）.Colorless block crystals were obtained when the mixture was cooled to room temperature.Yield:ca.62% based on Cd.Calcd.for C35H28CdN2O9（%）:C，57.35；H，3.85；N，3.82.Found （%）:C，58.23；H，2.84；N，3.83.IR （KBr pellet，cm-1）:3446 s，3053 w，2924 w，2343s，1616 s，1589 w，1500 s，1419 s，1381 w，813 w，781w，690 s，468 w.

2.3Crystal structure determination

Single crystals of complexes 1 and 2 were mounted on glass fibers in a random orientation for single crystal diffraction.The data were collected at 296（2）K on a Bruker Smart APEX II CCD diffractometer equipped with a graphite-monochromated Mo-Kα radiation （λ = 0.71073 Å）using an ω-φ scan mode.A semi-empirical absorption correction was applied using the SADABS program［17，18］.The structure was solved by direct methods and refined by full-matrix least-squares on F2using the SHELXS-97 and SHELXL-97 programs，respectively［19，20］.Non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in the geometrically calculated positions.For complex 1，a total of 17887 reflections were collected in the range of 1.91＜θ〈28.32° （-16≤h≤17，-14≤k≤19，-19≤l ≤18）and 7027 were independent with Rint= 0.0530，of which 7027 with I ＞ 2σ（I）（refinement on F2）were observed and used in the succeeding structure calculation.The final R = 0.0481，wR = 0.1047 （w = 1/［σ2（Fo2）+ （0.0617P）2+ 0.0000P］，where P = （Fo2+ 2Fc2）/3），（Δρ）max= 0.421 and （Δρ）min= -0.513 e/Å3.For complex 2，18414 reflections were collected in the range of 2.04＜θ〈28.37° （-18≤h≤17，-19≤k≤19，-19≤l≤20）and 7230 were independent with Rint= 0.0174，of which 7230 with I ＞ 2σ（I）（refinement on F2）were observed and used in the succeeding structure calculation.The final R = 0.0283，wR = 0.0729 （w = 1/［σ2（Fo2）+ （0.0404P）2+ 105543P］，where P = （Fo2+ 2Fc2）/3），（Δρ）max= 0.421 and （Δρ）min= -0.513 e/Å3.The selected bond lengths and bond angles are listed in Tables 1 and 2.

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

Table 2.Selected Bond Lengths (Å) and Bond Angles (º) for Complex 2

3　RESULTS AND DISCUSSION

3.1Crystal structure

3.1.1[Zn(HL)(Hbpp)]n(1)

Fig.1.Coordination environment of the Zn(II) (a) in 1 and Cd(II) (b) in 2

Single-crystal X-ray diffraction analysis reveals that complex 1 is of monoclinic crystal system with P21/n space group.The asymmetric unit comprises a Zn ion，a HL3-anion and a bpp molecule.As shown in Fig.1a，the Zn（II）ion is four-coordinated and the coordinated atoms around it are three oxygen atoms supported by three different Hdtca anions and one nitrogen atom from one bpp ligand to give the distorted tetrahedral geometry.

Fig.2.Zn-carboxylate 1D zigzag chain of complex 1 (All bpp molecules are omitted for clarity)

In complex 1，the carboxyl groups of Hdtca ligands adopt a monodentate mode.Thus，the structure infinitely extends to be a zigzag 1D chain （Fig.2）by Hdtca anions.Then the 1D zigzag chains are further bridged by HL3-ligands to generate a 2D layer framework （Fig.3）.There exist O-H··O hydrogen bonding interactions between adjacent layers （Table3），with uncoordinated oxygen and nitrogen atoms from carboxylate and bpp ligand serving as acceptors［21］，which link the layers into a3D supramolecular structure.

Fig.3.View of the 2D wave-like layer of complex 1 (All bpp molecules are omitted for clarity)

What’s more，there are π-π stacking interactions between the bpp ligands in neighboring layers，with a centroid-to-centroid distance of3.708 Å to consolidate the final3D structure （Fig.4）.The bond lengths and bond angles of hydrogen bonds are listed in Table3.

Fig.4.View of the3D structure of complex 1

3.1.2[Cd(Hdtca)(Hbpp)(H2O)]n(2)

Single-crystal X-ray diffraction analysis shows that complex 2 is also of monoclinic crystal system with P21/n space group.The Cd（II）ion is six-coordinated，as shown in Fig.1b，and surrounded by three oxygen atoms from three different HL3-anions，one nitrogen atom from one bpp and one oxygen atom from one coordinated H2O molecule，forming a distorted octahedral configuration.

Complexes 1 and 2 belong to the isomorphism.In addition to the different coordination ways，their structures are basically the same.The bond lengths and bond angles of hydrogen bonds are listed in Table3.The aromatic π-π stacking interactions have a centroid-to-centroid distance of 4.103 Å.

Table3.Hydrogen Bond Lengths (Å) and Bond Angles (°)

3.2IR spectra

In complexes 1 and 2，the same strong and broad band centered at3446 cm-1is attributable to the H-O-H stretching vibration.IR spectra of these two complexes exhibit typical antisymmetric and symmetric stretching bands of carboxyl groups （1367，1622 cm-1in 1；1381，1419 and 1616 cm-1in 2）.The values Δv （vas （COO-）- vs（COO-））of 1 （255 cm-1）and （235 cm-1，197 cm-1），respectively indicate the coordination of H4L with Zn（II）in a monodentate mode in 1 and not only monodentate but also bidentate-chelating modes in 2［12-14］.Meanwhile，characteristic bands near 1530 cm-1（1543 cm-1in 1，1500 cm-1in 2）belong to the stretching vibration of -N=C- of bpp ligands.The results of IR spectra analyses of the complexes are in consistent with that of single-crystal X-ray analyses.

Fig.6.Thermogravimetric analysis for complexes 1 and 2

Fig.7.Emission spectra of H4L and complexes 1 and 2 in the solid state at room temperature

3.3Thermal analysis

The stability of complexes 1 and 2 was investigated by thermogravimetric analysis （Fig.6）.Complex 1 decomposes vigorously from375.℃ The weight loss of complex 2 in the temperature range of 153～155 ℃ （2.56%）corresponds to the release of one coordinate water molecule （calcd.2.48%）.And complex 2 decomposes vigorously from367 .℃

3.4Photoluminescent property

Taking into account the excellent photoluminescent properties of d10coordination polymers，the photoluminescent properties of complexes 1 and 2 were studied in the solid state at room temperature.As depicted in Fig.7，the free H4dtca displays photoluminescence with an emission maximum at 462 nm （λex=377 nm），while both 1 and 2 exhibit an intense broad emission at 438 nm （λex=377 nm），which is quite similar to that of free H4L.Therefore，the origin of emissions of complexes 1 and 2 may be attributed to the internal charge transfer （π → π* transitions）of the H4L ligands.The observation indicates that complexes 1 and 2 may be candidates for potential photoluminescent materials.

4　CONCLUSION

In summary，two new coordination polymers，1 and 2，have been synthesized by hydrothermal reactions，and both of them exhibit 2D layer frameworks，which are further linked by carboxyl groups of HL3-to form3D supramolecular structures via intermolecular hydrogen bonds and aromatic π-π stacking interactions.As expected，complexes 1 and 2 show photoluminescence in the visible region and high thermal stability.

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16 June 2014；accepted 12 November 2014 (CCDC 1007039 for 1 and 1007040 for 2）

① Supported by the National Natural Science Foundation of China （No.21373178），the Open Foundation of Key Laboratory of Synthetic and NaturalFunctional Molecule Chemistry of Ministry of Education （338080043），and the Natural Scientific Research Foundation of Shaanxi Provincial Education Office of China （No.13JS124）

② Corresponding author.Male，born in 1963，professor，majoring in functional coordination chemistry.E-mail:glj@yau.edu.cn