WANG Ji-Wu SU Yong-Cho WANG Ji-Jing②

a (Department of Chemical Engineering, Yulin Vocational and Technical College, Yulin 719300, China)

b (Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan’an University, Yan’an 716000, China)

1 INTRODUCTION

Crystal engineering of coordination polymers has attracted a great deal of interest in recent years due to their intriguing topological structures and potential functions[1-8]. It is well known that the rational design and synthesis of new coordination polymers with desired structures are still a great challenge considering several factors that could have influences on the resulting frameworks, such as organic ligands, metal ions, template, pH value, and so on. The suitable selection of organic ligands is one of the most important factors for constructing coordination polymers with excellent properties.Among numerous organic ligands, the carboxylate ligands are good spacers for the construction of functional coordination polymers, owing to their diverse coordination modes and orientations[9-11].Recently, the nitro-functionalized carboxylate ligands have attracted interest of scientists for a number of reasons[12-14].

As a continuation of our previous study[15], in this contribution, transition metal ion Cd(II) was selected as the central metals, and nitro-functionalized carboxylate ligand 6,6?-dinitro-2,2?,4,4?-biphenyltetracarboxylic acid (H4L) and 1,2-di(4-pyridyl)ethylene (dpe) have been used as the mixed ligands to explore new architectures formed by employing carboxylate with the nitro groups as well as to explore the effect of such groups on the resulting coordination framework. As a result, a new 3D pillared bilayer Cd (II) coordination polymer,[Cd2L(dpe)]n(1), has been obtained. The thermal stability and photoluminescent property of 1 are also discussed.

2 EXPERIMENTAL

2.1 Materials and methods

All reagents and solvents employed were commercially available and used without further purification. The C, H and N microanalyses were carried out with a Vario EL elemental analyzer. IR spectra were recorded with a Shimadzu Prestige-21 spectrometer using the KBr pellet technique. Thermogravimetric analyses (TGA) were performed under nitrogen at a heating rate of 10 ℃/min using a NETZSCH STA 449F3 thermogravimetric analyzer. Photoluminescence spectra were performed on a Hitachi F-4500 fluorescence spectrophotometer at room temperature.

2.2 Syntheses of complex 1

A mixture of Cd (NO3)2·4H2O (0.2 mmol), H4L(0.1mmol), dpe (0.1 mmol), NaOH (0.4 mmol), and 10 mL H2O was stirred for 30 min. The mixture was then placed in a 25 mL Teflon-lined stainless steel vessel and heated for 160 ℃ for 3 d. Colorless block crystals were obtained when the mixture was cooled to room temperature. Yield: ca. 62% based on Cd. Calcd. for C28H14Cd2N4O12(%): C, 40.85; H,1.71; N, 6.81. Found (%): C, 40.96; H, 1.80; N, 6.73.IR (KBr pellet, cm-1): 3430 m, 3087 w, 3100 m,3043 w, 1612 s, 1542 s, 1453 s, 1377 s, 1332 w,1071 s, 976 w, 925 w, 823 m, 728 s, 709 s, 550 m,417 m.

2.3 Crystal structure determination

Diffraction intensities for complex 1 were collected at 293(2) K on a Bruker Smart APEX II CCD diffractometer equipped with a graphite-monochromated MoKα radiation (λ = 0.71073 ?) using an ω-φ scan mode. A semi-empirical absorption correction was applied using the SADABS program[16]. The structure was solved by direct methods and refined by full-matrix least-squares on F2using the SHELXS-97 and SHELXL-97 programs, respectively[17,18]. Non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in the geometrically calculated positions. A total of 7351 reflections of complex 1 were collected in the range of 1.70<θ<25.50o (–9≤h≤9,–2≤k≤25, –17≤l≤17) and 2633 were independent with Rint= 0.0186, of which 2239 with I >2σ (I) (refinement on F2) were observed and used in the succeeding structure calculation. The final R=0.0231, wR = 0.0629 (w = 1/[σ2(Fo2) + (0.0472P)2+ 3.0000P], where P = (Fo2+ 2Fc2)/3), (Δρ)max=0.439 and (Δρ)min= –0.601 e/?3. The selected bond lengths and bond angles are listed in Table 1.

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

3 RESULTS AND DISCUSSION

3.1 Crystal structure

Complex 1 exhibits a new 3D pillared bilayer framework constructed from 2D Cd(II)-L4-bilayer networks and dpe pillars. There are two unique Cd(II)ions in the asymmetric unit of 1 (Fig. 1). The Cd(1)ion is coordinated by two nitrogen atoms from two dpe ligands and four oxygen atoms from three carboxylic groups of three L4-ligands, adopting a distorted octahedral geometry. The Cd(2) ion is also six-coordinated by two pairs of chelating oxygen atoms from two different L4-ligands, and two bridging oxygen atoms from two different L4-ligands. Its coordination geometry can be also described as a distorted octahedral geometry. The Cd–O and Cd–N bond lengths span a range of 2.157(3)～2.426(3) ?, which are comparable with those observed in the reported Cd(II)-2,2?-dinitro-4,4?-biphenyldicarboxylate coordination polymers[19].Adjacent Cd(1) and Cd(2) ions are bridged by 4(4?)-and 2(2?)-carboxyl groups of two different L4-ligands to form a dinuclearunit (Cd(1)···Cd (2) = 4.240(6) ?).

Fig. 1. Coordination environments of Cd(II) in 1

Fig. 2. Coordination modes of L4- in 1

In fact, the L4-ligand in 1 acts as an octadentate ligand; two 4,4?-carboxyl groups adopt a bidentate bridging mode, while the other two 2,2?-carboxyl groups exhibit a bidentate chelating/bridging mode(Fig. 2). On the basis of the connection mode, the dimeric units are linked by L4-ligands to form a 2D monolayer network (Fig. 3a),which is connected to another parallel 2D network via 2(2?)-carboxyl groups of L4-ligands, resulting in a special 2D bilayer structure (Fig. 3b). Interestingly,the bilayer networks are connected by dpe pillars to generate a 3D pillared bilayer structure (Fig. 4).From the viewpoint of topology, the final structure of complex 1 can be defined as an unprecedented(4,5,7)-connected net with the Schl?fli symbol of(414·67)(42·65·83)(44·62) by deno- ting the L2-ligands to 7-connected nodes, Cd(1) to 5-connected nodes,and Cd(2) to 4-connected nodes (Fig. 5).

Fig. 3. (a) 2D monolayer network architecture of 1 assembled by units.(b) 2D Cd (II)-L4- bilayer network in 1 (All H atoms and dpe ligands are omitted for clarity)

Fig. 4. 3D pillared bilayer structure in 1

Fig. 5. Schematic view of the unprecedented (4,5,7)-connected (414·67) (42·65·83) (44·62) net(green spheres: Cd(II) ions; teal spheres: L2- ligands)

Comparing the final structures of 1 and the previous [Cd2(btc)(dpe)1.5(H2O)]n(H4btc = biphenyl-2,2?,4,4?-tetracarboxylic acid)[15], the added NO2-groups cause the distinctness of coordination modes of the carboxylate ligands and finally result in the formation of different 3D frameworks.

3.2 IR analysis

The IR spectrum of 1 showed the typical antisymmetric (vas= 1612 cm-1) and symmetric (vs=1542 and 1453 cm-1) stretching bands of carboxylate groups. The respective values of (vas(COO)– vs(COO)) clearly indicate the presence of chelating (70 cm-1) and bridging coordination modes(159 cm-1) in 1. The absence of the expected 1730～1690 cm-1for the protonated carboxylate groups illustrates the complete deprotonation of H4L in the reaction with Cd (II) ion[20]. These IR results are consistent with the crystallographic structural analyses.

3.3 Thermal analysis

The stability of complex 1 was investigated by thermogravimetric analysis (Fig. 6). Complex 1 shows no weight loss from room temperature to 356 ℃, suggesting that the frameworks are thermally stable. Above 356 ℃, a rapid weight loss is obser- ved, which is attributed to the burning of L4-and dpe ligands from 356 to 800 ℃ (obsd.71.65%, calcd 72.69%).

3.4 Photoluminescent property

The photoluminescent property of complex 1 was investigated at room temperature in solid state. To better understand the photoluminescent mechanism of complex 1, the photoluminescence of the free H4L and dpe was also measured under the same conditions. As depicted in Fig. 7, the free dpe displays photoluminescence with an emission maximum at 395 nm (λex= 365 nm), whereas complex 1 and H4L do not show any luminescence in the range of 380～600 nm. In 1, the quenching of photoluminescence is perhaps caused by the strongly electron-withdrawing NO2-groups of H4L ligands.

Fig. 6. TG of complex 1

Fig. 7. Emission spectrum of dpe in the solid state at room temperature

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