ZHAO Wei-XingWEN Pu-Hong WANG Yan JIANG Luan FENG Guo-Dong LI Zong-Xiao（College of Chemistry and Chemical Engineering，Baoji University of Arts and Sciences， Shaanxi 721013， China）

Synthesis， Crystal Structure and Physical Properties of a Novel Cadmium（II） Supramolecular 3D Framework①

ZHAO Wei-Xing②WEN Pu-Hong WANG Yan JIANG Luan FENG Guo-Dong LI Zong-Xiao
（College of Chemistry and Chemical Engineering，Baoji University of Arts and Sciences， Shaanxi 721013， China）

A new inorganic-organic hybrid cadmium（II） complex ［Cd2（H1/2C4BIm）2（CH3COO）］nhas been prepared by a hydrothermal reaction， and characterized by fluorescence， IR， TGA， and single-crystal X-ray diffraction analysis. In its crystal structure， there are two kinds of coordinating forms for those Cd（II） ions. One is that the Cd（II） ion is coordinated by one carboxylate oxygen atom from one acetate oxygen ligand and three benzimidazole nitrogen atoms from three 2，2΄-（1，4-butanediyl）bis （1H-benzimidazole） （H2C4BIm） ligands， and the other is that the Cd（II） ion is coordinated by four benzimidazole nitrogen atoms from four H2C4BIm ligands to furnish two distorted tetrahedral coordination geometries. It exhibits a beautiful structure for a 2D layer and a 3D network， with its four-connecting nodes provided by the H2C4BIm ligands. Furthermore， this complex shows intense luminescent property at room temperature. In ［Cd2（H2C2BIm）2（C9H6O4）2］n，the H2C2BIm ligand lost the role as a bridging ligand because of the shorter alkyl chain compared with that of H2C4BIm ligand.

cadmium， hydrothermal synthesis， crystal structure， fluorescence；

1 INTRODUCTION

In recent years， there has been an upsurge of research in supramolecular chemistry from their potential applications in microelectronics， fluorescence， non-linear optics， porous materials and catalysis［1， 2］. Most coordination polymers are constructed by using the appropriate organic ligand particularly that containing multidentate oxygen，nitrogen or sulfur donor to coordinate to transition metal centres［3， 4］. The designed synthesis and characterization of metal-organic frameworks containing channels and cavities with various sizes and shapes are of great interest， most motivated by their intriguing structural features and enormous range of application in catalysis， adsorption， separation， and electrical conductivity and magnetism.

Bis（2-benzimidazoles） and some substituted bis（2-benzimidazolyl）alkanes are attractive choices for their multifunctional linking groups. Many complexes based on them have been reported because of their wide-ranging antivirus activity［5， 6］，importance in selective ionexchange resins［7， 8］and coordination chemistry in the context of mimicking biological systems［9］. In addition， Cd（II）-containing coordination polymers have attracted extensiveinterests of us due to the amiability to bond with different donors simultaneously， the large radius，various coordination modes and potential applications in catalysis， fluorescent materials， non-linear optics materials and so on［10］. Hence， we intend to chose H2C4BIm and carboxylate group as organic ligands combining Cd（II） ion to construct a novel cadmium（II） supramolecular 3D framework containing asymmetric bridging benzimidazole nitrogen atoms and acetate oxygen atoms by a hydrothermal reaction. These ligands lead to 2D layer and 3D network structures and produce a new coordination polymer. Since this polymer has a structure similar to the above， it indicates that this compound has potential applications in microelectronics， fluorescence， non-linear optics， porous materials and catalysis.

2 EXPERIMENTAL

2. 1 Materials and methods

All chemicals for syntheses and analyses were commercially purchased and used without further purification. The H2C4BIm ligand was prepared according to the reported method［11］. And H2C2BIm was prepared according to the reported method［12］. Elemental analyses （C， H， and N） were performed on a Perkin-Elmer 2400CHN Elemental Analyzer. The emission/excitation spectra were recorded on a Varian Cary Eclipse spectrometer. FT-IR spectra were recorded in the range of 400～4000 cm-1on an Alpha Centaurt FT/IR Spectrophotometer as KBr pellets. TG analyses were performed on a Perkin-Elmer TGA instrument in flowing N2at a heating rate of 10 ℃·min-1. Powder X-ray diffraction （XRD）analysis of the sample was carried out on a SHIMADZU XRD-6100 X-ray diffractometer with CuKα （λ = 0.15418 nm） radiation.

2. 2 Synthesis of complex ［Cd2（H1/2C4BIm）2（CH3COO）］n（1）

A mixture of Cd（CH3COO）2（69.1 mg， 0.3 mmol），H2C4BIm （59.0 mg， 0.3 mmol）， NaOH （24.0 mg， 0.6 mmol）， and H2O （14.0 mL） was sealed in a 25 mL Teflon-lined stainless steel container， which was heated at 423 K for 72 h and then cooled down to room temperature at a rate of 5 K·h−1. Hereafter，some colourless needle-like crystals were obtained with the yield of 68% （based on Cd（II） salts）. Anal. Calcd. （%） for C38H36Cd2N8O2（formula weight 861.57）: C， 52.98； H， 4.21； N， 13.01. Found （%）: C，53.05； H， 4.11； N， 13.01. Selected IR data （KBr，cm−1）: 3030， 2947， 2754， 1605， 1572， 1543， 1449，1410， 1273， 749，730， 447.

2. 3 Synthesis of complex ［Cd2（H2C2BIm）2（C9H6O4）2］n（2）

A mixture of Cd（CH3COO）2（69.1 mg， 0.3 mmol），H2C2BIm （78.7 mg， 0.3 mmol）， 2-carboxymethylbenzoic acid （C9H6O4， 54.0 mg， 0.3 mmol）， NaOH （24.0 mg， 0.6 mmol）， and H2O （14 mL） was sealed in a 25 mL Teflon-lined stainless steel container，which was heated at 423 K for 72 h and then cooled down to room temperature at a rate of 5 K·h−1. After that， some colourless needle-like crystals were obtained with the yield of 42.0%［13］. Anal. Calcd. （%）for C50H40Cd2N8O8（formula weight 1105.72）: C，54.31； H， 3.65； N， 10.13. Found （%）: C， 54.37； H，3.58； N， 10.12.

2. 4 X-ray single-crystal structural determination

X-ray diffraction measurements were performed using a Bruker APXII CCD diffractometer with graphite monochromated MoKα （λ = 0.71073 Å）radiation at room temperature. An empirical absorption correction was performed by using the SADABS computer program. The structural model was solved using SIR-97 and completed using difference Fourier maps calculated with SHELXL-97， which was also used for the final refinement. All programs were run under the WinGX system with atomic coordinates and anisotropical thermal parameters for all non-hydrogen atoms. The hydrogen atoms of aromatic rings were included in the structure factor calculation at the idealized positions by using a riding model. The detailed crystallographic data and structure refinement parameters for complex 1 are as follows. This compound crys-tallizes in triclinic， P 1 space group， with a = 11.290（5）， b = 12.597（5）， c = 14.315（5） Å， α = 91.628（5）， β = 101.747（5）， γ = 107.978（5）°， V = 1886.8（13） Å3， Z = 2， Dc= 1.516 g·cm-3， F（000） = 864.0， Rint= 0.0609 and wRref（F2） = 0.1058. The selected bond lengths and bond angles are listed in Table 1. Those data for complex 2 are given in the supplementary material.

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

3 RESULTS AND DISCUSSION

3. 1 Description of the crystal structures

Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the triclinic space group Pand the asymmetric unit of complex 1 consists of two Cd（II） ions and three ligands. In the crystal structure， there are two kinds of coordinating forms for those Cd（II） ions. One is that the Cd（II） ion is coordinated by one carboxylate oxygen atom from an acetate oxygen ligand and three benzimidazole nitrogen atoms from three H2C4BIm ligands， as shown in Fig. 1； and other is that the Cd（II） ion is coordinated by four benzimidazole nitrogen atoms from four H2C4BIm ligands to furnish two distorted tetrahedral coordination geometries （Fig. 2）.

Fig. 1. （a） Ball-rod structural models of 2，2΄-（1，4-butanediyl）bis（1H-benzimidazole）. （b） 2，2΄-（1，2-ethanediyl）bis（1H-benzimidazole）. Color codes: C， black； N， dark blue； H， dark red

Fig. 2. Coordination environment of Cd（II） ion in complex 1 with the ellipsoids drawn at 30% probability level. Hydrogen atoms were omitted for clarity

This complex crystallizes in the triclinic space group Pwith all unique atoms located on the general positions. Selected interatomic distances and angles are listed in Table 1. There are one Cd-O and seven Cd-N bonds in the crystal structure. The Cd-O （Cd（2）-O（2） 2.179（3） Å） and Cd-N （Min: Cd（2）-N（7） 2.191（4） Å， max: Cd（2）-N（5） 2.231（4）Å） bond distances are all in the normal ranges［14-17］. Meanwhile， each Cd（II） ion is attached to four ligands， which can be considered as a fourconnected node， and each benzimidazole nitrogen ligand connects with two Cd（II） ions. Thus， the ligand can be considered as raise cross-curve-linker. Four benzimidazole nitrogen ligands possessing the same coordination mode connect the Cd（1）. Three benzimidazole nitrogen ligands with the same coordination mode and one acetate oxygen ligand connect the Cd（2）. The acetate oxygen ligand connects with one benzimidazole nitrogen ligand by a hydrogen bond N（3）-H（11）··O（2） （N（3）··O（2） = 2.800 Å， N（3）-H（11）··O（2） = 149.33°）. The carboxyl plays the role of a bridging ligand. Those alkyl chains linking benzimidazole rings from H2C4BIm ligands form three kinds of different stretched conformations， with their distances to be 5.096 Å （C（7）··C（12））， 6.377 Å （C（34）··C（34A））and 6.380 Å （C（25）··C（25A））， respectively. Besides，it can be observed from Fig. 2 that the partial superposition between the aromatic ring （C（1）-C（6））and another aromatic ring （C（19）-C（24）） produces the π-π interaction of offset edge-to-face.

Based on this simplification， four Cd centers are connected by sixteen bridging ligands， generating a diamondoid cage. This structure extends into a 1D chain， as shown in Fig. 3（a）. One benzimidazole nitrogen ligand connects two Cd（1） and one Cd（2），and the structure extends into a 2D layer， as shown in Fig. 3（b）. The 2D layer is parallel to the （0 1 0）plane， which is a 5-nodal net with point symbol of ｛42.6｝4｛42.82.102｝｛43.62.8｝2｛44.62｝. The C（1）-H（1）bond in one layer connects the neighbourhood C（34）atom in another layer， forming a hydrogen bond （bond length: 3.740 Å and bond angle: 156.70°）. The hydrogen-bonding interactions also contribute to the formation of a 3D network， as shown in Fig. 3（c）. It is accomplished by connecting four curve ligands to the tetrahedral building.

Fig. 3. Extension of the structure for complex 1. （a） 1D chain； （b） 2D layer； （c） 3D network. Color codes: Cd， red； N， dark blue； O， dark green

The topological analysis of complex 1 reveals that it is a typically diamondoid framework containing large adamantanoid cages with the topology “Schläfli” symbol， as shown in Fig. 4.

Fig. 4. Diamondoid framework contained in complex 1

Fig. 5 reveals the extensions of the structure for complex 2 viewed along the a-， b-， and c-axes. In the crystal structure of 2 （triclinic， P1）， the Cd（II） center is six-coordinated by two nitrogen atoms from one H2C2BIm ligand， two carboxylic oxygen atoms and two oxygen ions from two homophthalates，respectively. There are two Cd-N and four Cd-O bonds in the crystal structure. The Cd-N （Cd（1）-N（2）2.296（2） Å and Cd（1）-N（4） 2.244（2） Å） and Cd-O （Min: Cd（1）-O（3） 2.289（2） Å， max: Cd（1）-O（4）2.480（2） Å） bond distances are all in normal ranges［14-17］. The two homophthalate ions bridge two Cd（II） ions to form a dimeric complex molecule. These latter are linked by N-H··O hydrogen bonds between an H2C2BIm ligand and a neighbourhood homophthalate ligand， forming a one-dimensional chain structure. The N-H··O hydrogen bonds formed by other N-H bonds from the H2C2BIm ligand link the chains to form a layered structure， as shown in Fig. 5 （a and b）. And this layered structure has approximately rectangular apertures along ［001］（Fig. 5（c））.

Fig. 5. Extensions of the structure for complex 2 viewed down the a-axis （a）， b-axis （b）， and c-axis （c）. Color codes: Cd， red； O， dark green. Dashed lines indicate the hydrogen bonding interactions

Comparing the structures of the two complexes，we found that different roles of bis（2-benzimidazolyl）alkanes ligand are a main factor causing the structural difference. In complex 1， the H2C4BIm ligand plays a role in bridging the Cd（II） ions， and establishes a 13-membered and an 18-membered large rings. Different from that of complex 1， the homophthalate ligand， which substituted the H2C2BIm ligand， plays a role in connecting the Cd（II） ions in complex 2. The H2C2BIm ligand lost the role as a bridging ligand because of its shorter alkyl chain compared with that of H2C4BIm ligand，and it establishes a 7-membered large ring by forming two Cd-N bonds.

3. 2 Luminescent properties

The photoluminescent properties of free H2C4BIm ligand and complex 1 were examined in the solid state at room temperature. As shown in Fig. 6， the main emission peak of free H2C4BIm ligand is at 308 nm （λex= 272 nm）， which can be assigned to π* → n and π* → π transitions of the intraligands. Complex 1 shows the maximum emission at 421 nm（λex= 411 nm）. In comparison with that of free ligand， the maximum emission wavelength of complex 1 shifted slightly to red， probably due to the intraligand charge transitions［18， 19］. The fluorescence emission of complex 1 might be attributed to the ligand-to-metal charge transfer （LMCT）［20， 21］. This observation indicates that complex 1 may be a good candidate for potential photoactive materials.

Fig. 6. Solid-state excitation spectra of free 2，2΄-（1，4-butanediyl）bis（1H-benzimidazole） ligand （A） and complex 1 （B）

3. 3 Thermal properties

The TGA analysis of complex 1 was carried out under N2atmosphere from 44 to 800 ℃ with a heating rate of 10 ℃·min−1. The TG curve of the complex is shown in Fig. 7. It is found that the framework collapsed in the temperature range of 360 ～535 ℃， corresponding to the release of organic components. The remaining weight of 29.91% corresponds to the percentage （calcd. 29.81 %） of Cd and O components， indicating that the final product may be CdO.

Fig. 7. TG curve of complex 1

3. 4 Powder X-ray diffraction properties

The simulated and experimental powder XRD patterns of complex 1 are shown in Fig. 8. The simulated and experimental powder XRD patterns are in good agreement with each other， indicating the phase purity of the product. The differences in intensity may be due to the preferred orientation of the powder sample.

3. 5 IR spectrum properties

A typical FT-IR spectrum of complex 1 sample is shown in Fig. 9. In this spectrum， the sharp bands located at 1543 and 1273 cm−1can be assigned to theν（C=N） and ν（C-N） respectively， the bands located at 1572 and 1449 cm−1to the framework vibration of benzimidazole ring， the broad band at 3030 cm−1toν（C-H） of the ring， the two bands at 2947 and 2754 cm−1to ν（C-H） of methylene， and that at 730 cm−1to the framework vibration of -（CH2）4-， respectively［22］. The two bands at 1605 and 1410 cm−1are assigned to the asymmetric （-COO-as） and symmetric （-COO-s） stretches of the carboxylate group， respectively. The wavenumber difference betweenν（-COO-as） and ν（-COO-s） bands （1950 cm−1） indicates that the complex shows a typical coordinationmode for the carboxyl group， which is consistent with the single-crystal X-ray analysis［5， 23］.

Fig. 8. Powder XRD patterns of complex 1 （A: simulated； B: experimental）

Fig. 9. IR spectrum of complex 1

4 CONCLUSION

In conclusion， by selecting H2C4BIm ligand and synthetic strategy， we have hydrothermally synthesized a novel 3D network interpenetrating diamondoid array coordination polymer and it contains large adamantanoid cages. Complex 1 exhibits intense fluorescence at about 421 nm in the solid state. Therefore， the photoluminescent analysis re- sult indicates this complex may be a good can- didate for fluorescence material. It can be found that the H2C2BIm ligand in complex 2 lost the role as a bridging ligand because of its shorter alkyl chain compared with that of the H2C4BIm ligand.

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Received 17 November 2014； accepted 11 February 2015 （CCDC 801634）

① This work was supported by the Natural Science Foundation of China （No. 21173003）， the Scientific Research Project （No. 13JS007） from Shaanxi Key Laboratory of Phytochemistry and Key Research Project （No. ZK12016） from Baoji University of Arts and Sciences

② Corresponding author. E-mail: weixingzhao@163.com

10.14102/j.cnki.0254-5861.2011-0576