A New Three-dimensional Zn(II) Coordination Polymer Based on Trinuclear Zn(II) Units with a Three-fold Hex Topology①

DU Jie

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

1 INTRODUCTION

Recently, the design and construction of coordination polymers (CPs) have attracted much attention because of their intriguing structural diversities and potential applications as function materials[1-7].Chemists have devoted considerable efforts to the investigation of controllable assemblies of CPs,hoping to obtain CPs with unique structural features and certain functions. As is well known, the synthetic strategies play the key role in the formation of CPs. Several effective strategies for synthesizing the CPs have been certified by chemists which are effective methods such as the post-synthetic modification, the reticular approach and pillaring strategy[8,9]. Among the strategies to construct CPs, the ‘pillar-layer’ method is one of the most effective ways to synthesize new materials with predictable structures[10,11]. In ‘pillar-layer’method, the two-dimensional layers contain the unsaturated metal sites. It is important to introduce a rigid nitrogen-containing bridging ligand as a pillar in the preparation of 3D pillar-layer CPs, such as 1,4-diazabicyclo [2.2.2] octane and 4,4?-bipyridine[12,13]. N,N?-di(4-pyridyl)-1,4,5,8-naphthalenetetra carboxydiimide (DPNDI) is an important bridging ligand in construction of pillar-layered CPs[14,15].

Based on the above mentions, we adopt the‘pillar-layer’ method and use the biphenyl-4,4?-dicarboxylate and N,N?-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide to construct a new Zn(II) pillar-layer 3D structure which displays a three-fold hex topology based on a 362D layer.

2 EXPERIMENTAL

2.1 Materials and physical measurements

All the reagents were purchased commercially and used as received without further purification.Elemental analysis was carried out on a Carlo Erba 1106 full-automatic trace organic elemental analyzer.FT-IR spectra were recorded with a Bruker Equinox 55 FT-IR spectrometer as a dry KBr pellet in the 400～44000 cm-1range. X-ray powder diffraction(XRPD) measurements were performed on a Bruker D8 diffractometer operated at 40 kV and 40 mA using Cu-Kα radiation (λ = 0.15418 nm). Solid-state fluorescence spectra were recorded on a Hitachi F-4600 spectrometer equipped with a xenon lamp and a quartz carrier at room temperature.

2.2 Syntheses of complex

[Zn3(bpdc)3(DPNDI)]·3NMF (1)

A mixture of Zn(NO3)2·6H2O (14.9 mg, 0.05 mmol), H2bpdc (12.1 mg, 0.05 mmol), DPNDI (20.6 mg, 0.05 mmol) and NMF (3 mL) was added to 10 mL vial. The vial was sealed, held at 90 ℃ f or 48 h,and then cooled to room temperature. Colorless crystals of 1 were obtained in 72% yield based on Zn. Anal. Calcd. for 1: C, 57.77; H, 3.19; N, 5.78.Found: C, 57.74; H, 3.21; N, 5.75. IR (KBr pellet/cm-1): 3406 w, 3165 w, 2808 w, 1635 s, 1539 s, 1428 s, 1269 m, 1184 m, 1064 m, 935 m, 862 m,766m, 657 m.

2.3 X-ray crystallography

A suitable colorless block crystal with dimensions of 0.30mm × 0.25mm × 0.20mm 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 1.74＜θ＜27.52°(–12≤h≤14, –19≤k≤17, –22≤l≤25). For the title complex 1, a total of 21934 reflections with 7255 unique ones (Rint= 0.018) were measured, of which 6209 were observed with I ＞ 2σ(I) to the final R = 0.0343 and wR = 0.0419 (w = 1/[σ2(Fo)2+(0.0578P)2+ 1.6809P], where P = (Fo2+ 2Fc2)/3), S= 1.051 and (Δ/σ)max= 0.001. All non-hydrogen atoms were located by place in 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 0.585 and -0.380 e/?3, respectively. The structure was solved by direct methods with SHELXS 97 and Fourier techniques and refined by full-matrix least-squares method on F2with SHELXL 97 program[16,17]. 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 Description of the crystal structure

Single-crystal X-ray structural analysis reveals that complex 1 is a 3D pillar-layered framework with a three-fold hex topological net. The structure analysis reveals that there exists a trinuclear Zn(II)secondary building unit (SBU), which is composed of one inequivalent central Zn(1) (II) ion lying on the crystallographic inversion center and two symmetry-related Zn(2)(II) ions occupying the crystallographic positions, as depicted in Fig. 1. Zn(1) (II)is hexa-coordinated by six carboxylate oxygen atoms to form a distorted octahedral geometry(Zn–O, ranging from 2.0716(17) to 2.0994(17) ?).Zn(2) (II) is defined by three carboxylate oxygen atoms (Zn(2)–O(1) = 1.9351(17) ?, Zn(2)–O(3) =1.9490(17) ? and Zn(2)–O(6) = 1.9324(17) ?) and one nitrogen atom (Zn(2)–N(1) = 2.063(2) ?) in a tetrahedral geometry. The Zn–O/Zn–N distances are in the normal range for Zn(II) coordination polymers[18,19]. The adjacent Zn···Zn distance is 3.583 ?.In 1, the carboxylate groups adopt a μ2-η1:η1coordination mode. Each trinuclear Zn(II) unit for the 2D layer is connected to six trinuclear Zn(II)units to form a triangle-tessellated layer structure.

Fig. 1. Coordination environment for Zn(II) ions in 1. Hydrogen atoms are omitted for clarity

If the tirnuclear SBUs and organic carboxylate ligands are regarded as 6-connected nodes and linkers, respectively, the 2D layer can be simplified as a 36net. The 2D layers are further pillared by DPNDI ligands to form a 3D framework (Fig. 3). In the final pillar-layered framework, the trinuclear SBUs act as eight-connected nodes to generate the 3D hex topology with point symbol (36·418·53·6) (Fig.4a). Due to the long feature of DPNDI linker, the three-interpenetrated hex topological net is formed(Fig. 4b). To the best of our knowledge, the pillared 2D 36layer in MOFs is rarely reported[20,21], and complex 1 is the first of DPNDI-based MOFs with a three-folded hex topology.

Fig. 2. (a) 2D layer structure formed by the carboxylate ligands and Zn(II)ion;(b) 36 topology with smaller trigonal windows

Fig. 3. 3D framework for complex 1

Fig. 4. (a) Hex topology for 3D framework in 1;(b) Three-fold interpenetrated hex for complex 1

3.2 Photoluminescent properties

Powder X-ray diffraction (PXRD) experiments on the bulk material of 1 showed that all major peaks match well with the simulated PXRD, indicating its crystalline phase purity (Fig. 5).

As shown in Fig. 6, complex 1 exhibits an emission peak at 415 nm (λex= 330 nm), which shows a large red shift compared with the H2bpdc ligand emission (λem= 396 nm) and blue shift compared with the DPNDI ligand emission (λem=430 nm)[23,24]. These emissions for complex 1 could be assigned to the ligand-to-metal charge transfer(LMCT)[25].

Fig. 5. XRD for complex 1

4 CONCLUSION

We successfully synthesized a new Zn(II) coordination polymer by solvothermal method. Complex 1 features a 6-connected 3-fold interpenetration hex topology and shows the blue luminescent emission at room temperature.

Fig. 6. Solid-state photoluminescent spectrum of complex 1 at room temperature

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