Solvothermal Synthesis, Crystal Structure and Luminescent Property of a New Zn(II) Coordination Polymer With a Six-fold Interpenetrating Diamondoid Net①

DU Jie

(College of Chemistry and Materials Engineering, Guiyang University, Guiyang 550005, China)

1 INTRODUCTION

Assembly of coordination polymers (CPs) has attracted great attention, and CPs with various network topologies have been prepared from the metal ions and organic ligands[1-4]. They have the potential applications to gas storage, catalysis, magnetism, luminescence and so on[5-12]. The topological matrix of diamondoid net is well-defined and realized in the field of CPs. It is of interest to build diamondoid networks with big cavities. Interpenetrating diamondoid networks with open structures including counterions and guest molecules have been demonstrated[13-16]. Generally speaking, the degree of interpenetration mainly relied on the distance of metal ions, which is strongly related to the length of organic spacers. To some extent, the bigger separation of metal ions will bring higher interpenetrating number. And yet, in some cases this verdict is void, because so many factors can influence the degree of interpenetration, such as the configuration of organic spacers, the size of counterions, and so on[18-20]. Thereby, based on the current state of arts, it is still very difficult to rationally design, predict, and fully interpret the interpenetrating degree of diamondoid net.

Considering the above mention, herein, we chose a long flexible dicarboxylate ligand and rigid 1,4-bis(1-imidazolyl) benzene to assemble with Zn(II) ion under solvothermal conditions. As a result,we successfully obtained [(CH3)2NH]2[Zn(bpea)(bib)]·2H2O (1) which features a six-fold diamondoid topology.

2 EXPERIMENTAL

2.1 General materials and methods

All reagents and solvents were commercially available and used as received. C, H and N analyses were carried out with a Perkin-Elmer 240C elemental analyzer. FT-IR spectra were recorded with a Bruker Equinox 55 FT-IR spectrometer (KBr pellet)from 400 to 4000 cm-1. Solid-state fluorescence spectra were recorded with a Hitachi F-4600 equipped with a xenon lamp and a quartz carrier at room temperature. The powder X-ray diffraction(XRPD) measurements were performed with a Bruker D8 diffractometer operating at 40 kV and 40 mA using CuKa radiation (k = 0.15418 nm).

2.2 Synthesis of complex

[(CH3)2NH]2[Zn(bpea)(bib)]·2H2O (1)A mixture of Zn(NO3)2·6H2O (14.9 mg, 0.05 mmol), H2bpea (13.4 mg, 0.05 mmol), bib (10.9 mg,0.05 mmol) and DMF/H2O (4 mL, 3:1) was added to a 10 mL vial which was sealed, held at 120 ℃ for 48 h, and then cooled to room temperature. Colorless crystals of 1 were obtained in 68% yield based on Zn. Anal. Calcd. for 1: C, 57.53; H, 5.73; N, 12.58%.Found: C, 57.49; H, 5.72; N, 12.56%. IR(cm-1/KBr):3406 w, 3026 w, 2865w, 2218 w, 1638 s, 1608 m,1533 s, 1493 m, 1259 m, 1087 m, 1024 m, 938 m,806 m, 759 m, 654m.

2.3 X-ray crystallography

A suitable colorless block crystal was mounted on a glass fiber and the data were collected on a Bruker APEX2 CCD area-detector diffractometer with a Mo-Kα radiation (λ = 0.71073 ?) at 293(2) K by using an ω-scan mode in the range of 2.17<θ<27.51°(–22≤h≤22, –7≤k≤7, –22≤l≤24). For complex 1, a total of 11759 reflections with 4029 unique ones(Rint= 0.030) were measured, of which 3266 were observed with I > 2σ(I) to the final R = 0.0519 and wR = 0.0667 (w = 1/[σ2(Fo)2+ (0.0936P)2+ 0.772P],where P = (Fo2+ 2Fc2)/3), S = 1.081 and (Δ/σ)max=0.000. All non-hydrogen atoms were located by successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. The hydrogen atoms were located by geometrical calculations, and their positions and thermal parameters were fixed during the structure refinement. The highest and lowest residual peaks in the final difference Fourier map are 0.546 and -0.397 e/?3,respectively. The structure was solved by direct methods with SHELXS 97 and Fourier techniques and refined by full-matrix least-squares techniques on F2with SHELXL 97 program[19,20]. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

3 RESULTS AND DISCUSSION

3.1 Structure description of

[(CH3)2NH]2[Zn(bpea)(bib)]·2H2O Single-crystal X-ray diffraction analysis shows that complex 1 crystallizes with monoclinic system,space group P2/c. In the structure of complex 1, the asymmetric unit consists of one crystallographically independent Zn(II) atom, one bpea2-anion, a half of bib ligand, one dimethylamine molecule and one free water molecule. As shown in Fig. 1, the Zn(II)atom adopts a tetrahedral geometry defined by two carboxylate oxygen atoms (Zn(1)–O(2) = 1.970(2) ?)and two nitrogen atoms (Zn(1)–N(1) = 2.004(2) ?).Each Zn(II) is connected to four adjacent Zn(II)atoms through two bpea2-and two bib ligands to result in a three-dimensional diamondoid framework with Schl?fli symbol 66and the long symbol 62.62.62.62.62. 62. with large adamantanoid cages, which is a typical of class Ia dia topology (Figs. 2 and 3). The Zn···Zn distances separated by bpea2-and bib ligands are 17.5822 and 13.5599 ?, respectively. The Zn in the adamantanoid cage exhibits maximum dimensions (the longest intracage distances) of 36.293? ×34.936? × 18.814?. Such a large cavity causes six independent equivalent cages to inter- penetrate with each other (Fig. 4). Despite the six- interpenetration and the free molecules filled in the cavity, the total void value is estimated to be 118.7 ?3,approximately 6.8% ?3of the total crystal volume of 1753.9 ?3by platon.

Fig. 1. A view of the local coordination of the ZnII ion, showing the atom-numbering scheme

Fig. 2. View of the three-dimensional structure of 1

Fig. 3. A single dia unit cage in 1

3.2 Luminescence property

In order to substantiate the phase purity of the as-synthesized complex 1, powder XRD was performed before their photoluminescent property was measured (Fig. 5). The experimental powder XRD patterns are in good agreement with the corresponding simulated ones.

Fig. 5. PXRD for complex 1

Fig. 4. Schematic representation of six-fold interpenetrating dia topology for 1

The photoluminescent property of complex 1 was investigated at room temperature. As shown in Fig. 6,complex 1 shows the emission maximum at 412 nm(λex= 309 nm). The free bib displays fluorescent emission at 456 nm in the solid state at room temperature. The emission maximum of H2bpea is at 466 nm[21]. The emission bands of free ligands may be assigned to the π*-π or π*-n transitions[22,23].Compared to the emission of organic ligands, the significant blue-shit of the emission band of complex 1 is observed. These situations may be attributed to the coordination of organic ligand to the Zn(II) center, which significantly increases the rigidity and asymmetry of the ligand and reduces the loss of energy by radiationless decay.

4 CONCLUSION

In this paper, we obtained one new Zn(II) coordination polymer based on flexible dicarboxylate and imidazole-containing ligands. Complex 1 features a 4-connected six-fold interpenetrated 3D dia topology with the point symbol 66. Furthermore,complex 1 exhibits emission in the solid state at room temperature.

Fig. 6. Luminescent spectra for bib ligand and complex 1 in the solid state at room temperature

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