LV Cong CHEN Lin LIU Bo ZHOU Shi

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Two Interpenetrated and Polycatenated Zn(II) Coordination Polymers: Syntheses, Structures and Luminescent Properties①

LV Conga, bCHEN Lina,bLIU Boa, b②ZHOU Shia, b②

a(()130103)b(136000)

Two fascinating Zn(II) entangled coordination polymers, [Zn(eoba)(bbi)]n·nH2O (1) and [Zn(boba)(bbi)1/2]n(2), (bbi = 1,1?-(1,4-butanediyl)-bis(imidazole), H2eoba = 4,4?- (ethane-1,2-diyldioxy)-dibenzoic acid, H2boba = 4,4?-(butane-1,4-diyldioxy)-dibenzoic acid), were obtained by hydrothermal technology and characterized by elemental analysis, infrared spectrum, thermogravimetric analysis and single-crystal X-ray diffraction. 1 is a rare 2D → 3D example with a 3-fold parallel interpenetration and polycatenaned architecture. 2 features a scarce 2D → 2D example with a 3-fold parallel interpenetrating network which possesses polyrotaxane and polycatenane characters. Moreover, the luminescent properties of 1 and 2 have been discussed.

3-fold interpenetration, luminescent property, crystal structure;

1 INTRODUCTION

The rational design and synthesis of coordination polymers (CPs) have been diffusely investigated because of their diversity of fascinating structures and potential applications as new materials[1]. Chemists and material scientists have been devoting themselves to explore the relationship between structures and functions of crystalline materials, and great efforts to understand compelling topologies and structural diversities associated with CPs have promoted many meaningful results[2-5]. Among them, a number of structures were reported due to the formation of independent motifs entangled in different ways, including interpenetration and polycatenation, which can be attributed to the existence of large free voids in a network[6-11]. Although this type of CPs has been appeared, adjustment and control of the structure are still a great challenge.

Thus far, mixed ligand assembly system has been widely used in the building of new CPs[12-14]. And it is very important to the choice of spacer ligand, because altering the structure of ligands can control and adjust the topology of CPs[15-17]. In numerous organic ligands, multi-carboxylates have attracted a great deal of attention on account of their multiple coordination modes with metal centers[18-20]. As we know, the employment of flexible long carboxylic acid ligands usually generate entangled frameworks. Therefore, we design and synthesize two different bridging multi-carboxylic acids which are 4,4?- (ethane-1,2-diyldioxy)-dibenzoic acid (H2eoba) and 4,4?-(butane-1,4-diyldioxy)-dibenzoic acid (H2boba). In H2eoba and H2boba, the chains of -O-X-O- segment are different with respect to the length of CH2linkages. Furthermore, the flexible 1,1?-(1,4-butanediyl)-bis(imidazole) (bbi) ligand as a secondary N-donor bridging ligand, owing to the presence of a flexible -CH2- spacer, can exhibit more flexible conformations. However, the work based on H2eoba/H2boba and bbi has not been carried out widely. In addition, Zn CPs have gained significant attention due to their diversity structural features and applications in luminescence, photoca- talysis, sensing and so on[13, 21-23]. Therefore, we synthesize and report two Zn(II) CPs: [Zn(eoba)(bbi)]n·nH2O (1) and [Zn(boba)(bbi)1/2]n(2), which exhibit 3-fold interpenetrated and polycatenated nets. Thermal and photoluminescent properties of the compounds were investigated.

2 EXPERIMENTAL

2.1 Materials and physical measurements

H2eoba, H2boba and bbi were synthesized by a procedure reported earlier[24, 25], and other reagents and solvents employed were commercially available and used as received without further purification.Elemental analyses (C, H, N) were carried out with a EuroVector EA3000 CHN elemental analyzer. Infrared (IR) spectrum was obtained on a Perkin- Elmer Frontier FT/IR instrument as KBr pellets (4000~400 cm-1). Thermogravimetric (TG) analysis is conducted on a Rigaku Thermo plus EVO II TG8120 thermal analyzer. The photoluminescent spectra were recorded on a Hitachi F-4600 Fluore- scence Spectrometer at room temperature. The powder X-ray diffraction (PXRD) data were collec- ted on a Rigaku RINT2000 diffractometer at room temperature with Curadiation.

2.2 Synthesis

2. 2. 1 Synthesis of [Zn(eoba)(bbi)]n·nH2O (1)

A mixture of Zn(OAc)2·2H2O (0.3 mmol), bbi (0.3 mmol), H2eoba (0.3 mmol), NaOH (0.4mmol), water (5 mL) and CH3OH (5 mL) was placed in a Teflon reactor (20 mL) which was then sealed and heated to 160 °C for 4 days. The reaction system was slowly cooled to room temperature. Colorless crystals suitable for single-crystal X-ray diffraction analysis were collected from the final reaction system by filtration, washed several times with distilled water and dried in air at ambient temperature. Yield: 29% based on Zn.IR (KBr, cm-1): 3448s, 1610s, 1542s, 1508s, 1409s, 1303s, 1174s, 1104w, 1052w, 939m, 848m, 783m, 661w. Anal. Calcd. for C26H28ZnN4O7(%): C, 54.41; H, 4.92; N, 9.76. Found (%): C, 54.38; H, 4.97; N, 9.73.

2. 2. 2 Synthesis of [Zn(boba)(bbi)1/2]n(2)

A mixture of Zn(NO3)2·2H2O (0.3 mmol), bbi (0.3 mmol), H2boba (0.3 mmol), NaOH (0.12mmol), water (5 mL) and CH3OH(5 mL) was placed in a Teflon reactor (20 mL) which was then sealed and heated to 160 °C for 4 days. The others are similar to 1.Yield: 26% based on Zn. IR (KBr, cm-1): 3431s, 3129s, 2953s, 1605s, 1566s, 1508s, 1471s, 1367w, 1303w, 1169m, 1108m, 1044m, 950w, 856s, 785s, 700s, 655s, 511s. Anal. Calcd. for C23H23ZnN2O6(%): C, 56.51; H, 4.74; N, 5.73. Found (%): C, 56.48; H, 4.81; N, 5.69.

Notably, when the cobalt or cadmium saltswere employed as the starting materials,similar results can not be obtained.

2.3 X-ray data collection and structure refinement

Single-crystal diffraction data of 1 and 2 were respectively collected on a Bruker CCD diffrac- tometer equipped with a graphite-monochroma- tized Mo(= 0.71073 ?) radiation by using anscan method at 293(2) K. The structure was solved by direct methods with SHELXS-97 program[26]and refined with SHELXL-97[27]by full-matrix least- squares techniques on2. All non-hydrogen atoms were refined anisotropically and hydrogen atoms of the ligands were refined as rigid groups. The hydrogen atoms associated with free water mole- cules were located from difference Fourier maps. Details of the crystal parameters, data collection, and refinement of two compounds are summarized in Table 1. Selected bond lengths and bond angles are listed in Table 2.

Table 1. Crystal Data and Structure Refinement for 1 and 2

Table 2. Selected Bond Distances (?) and Bond Angles (°) for 1 and 2

Symmetry transformations: compound 1:#A:,–1,–1; compound 2:#A: –, –+2, –; #B: –+1, –+1, –; #C:–1,+1,

3 RESULTS AND DISCUSSION

3.1 Description of the crystal structure

Compound 1 It is a rare 2D→3D example with 3-fold parallel interpenetration architecture of wavelike sql nets. As shown in Fig.1, the asym- metric unit of 1 contains one Zn2+cation, one eoba2-anion, two half bbi ligands and one freewater molecule. The crystallographically independent Zn2+cation is four-coordinated in a [ZnO2N2]tetrahedral geometry structure coordinated by two carboxylate oxygen atoms from two different eoba2-anions and two nitrogen atoms from two different bbi ligands. Further extension of [ZnO2N2] through the-bbi ligands and-eoba2-anion adopting a bis-monodentatecoordinated fashion forms a 4-connected highly pleated layer structure with 44-sql net topology (Fig. 2). Notably, the pleated sql net within each layer has two different rhomboid windows with dimensions of 12.6364×16.7076 ?2and 12.8482×16.7076 ?2, and corrugated angle of 117.657° (defined by Zn···Zn distances and Zn···Zn···Zn angles). The Zn2+cations are not all coplanar in the layer. Rather, half falls in one plane, and half in the other parallel plane. The pleated nature of the layers with large rhomboid windows may offer a good chance to form an interpenetration network,thusallowing 3-fold parallel interpene- tration between the adjacent sheets to occur (Fig. 3).The 44-sql layers parallel stack with an ABAB sequence and each of these equivalent layers is polycatenated with the adjacent two parallel layers (Fig. 3). Finally, 1 shows a very unusual 2D→3D architecture.

Fig. 1. Coordination environment of Zn2+cations in 1.H atoms were omitted for clarity. Symmetry code: #A:,–1,–1; Displacement ellipsoids are drawn at 40% probability

Fig. 2. Zn2+cations are interlinked by two eoba2-anions and two bbi ligands to give 2D pleated layers, and two different rectangles of windows in 1

Fig. 3. A schematic view of the 3-fold parallel interpenetrating and 2D → 3D polycatenated framework in 1

Compound 2 In order to investigate the influence of multicarboxylate ligands with different lengths on ultimate structures, the longer H2boba is selected instead of H2eoba, and 2 is obtained. It features a scarce 2D → 2D example with a 3-fold parallel interpenetrating network which possesses polyrotaxane and polycatenane characters. In the asymmetric unit,2 consists of one Zn2+cation, one boba2-anion, and half bbi ligand (Fig. 4). The metal center is five-coordinated by four O atoms from four different boba2-anions and one N atom from one bbi ligand. Each pair of Zn2+cations is bound by four carboxy groups to yield a binuclear Zn2+unit. The binuclear units joint boba2-anions to form a 1D beaded chain. The neighboring beaded chains are further connected together by bbi ligands to generate a 2D sheet (Fig. 5). There are two different size windows of [Zn4(boba)2] and [Zn8(boba)4(bbi)2] with large void space (the dimensions of 10.60? × 11.07? and 15.45? × 16.92?) in the sheet, which can allow 3-fold interpenetration to occur in a parallel fashion (Figs. 5 and 6). Moreover, both polyrotaxane and polycatenane characters present in this network. Compared to our previous work, 2is isostructural with different metal center[28].

Fig. 4. Coordination environment of Zn2+cations in 2.H atoms were omitted for clarity. Symmetry codes: #A: –, –+2, –; #B: –+1, –+1, –; #C:–1,+1,. Displacement ellipsoids are drawn at 40% probability

Fig. 5. View of the 1D beaded chain (top) and 2D sheetin 2 (bottom)

Fig. 6. 3-Fold interpenetration structure (left) and schematic diagram of polyrotaxane and polycatenane (right) in 2

When theZn2+cation and bbi ligand are selected to react respectively with H2eoba and H2boba, 1 and 2 with different structural types have been obtained. Based on the different lengths and coordination modes for H2eoba and H2boba, they link Zn2+cations to generate a single chain in 1 and a beaded- type chain in 2, which are connected by bbi ligands to form two distinct parallel interpenetrating networks.

3.2 IR analysis

Compound 1 The strong absorption bands observed at 1610 and 1409 cm-1can be attributed to the asymmetric and symmetric stretching vibrations of carboxylate groups in H2eoba[28], which has a separation △((OCO)assym–(OCO)sym) of 201 (＞200), indicating the monodentate coordination of COO-to the Zn2+cation[29].

Compound 2 The strong absorption bands observed at 1566 and 1367 cm-1can be assigned to the asymmetric and symmetric stretching vibrations of the carboxylate groups in H2boba[28], which has a separation △((OCO)assym–(OCO)sym) of 199 cm-1(＜200), indicating the bidentate mode ofCOO-to the Zn2+cation[29].

The IR spectral results are consistent with the X-ray single-crystal structure analysis.

3.3 TG analysis and PXRD results

As shown in Fig. 7,the crushed single-crystal samples were heated up to 750 ?Cunder N2gas at a heating rate of 10 ?C?min-1. For 1, the first weight loss of 2.87% occurs between room temperature and 120 ℃, corresponding to the removal of water molecules (calcd. 3.13%). In the range of 120～700 ℃, the second weight loss of 82.05% can be ascribed to the release of organic ligands (calcd. 82.68%). The residual weight 15.08% (calcd. 14.19%) corresponds to ZnO. For 2, the weight loss of 83.4% from room temperature to 548 ?C ascribes to the loss of organic ligands (calcd. 83.35%). Upon further heating, above 548 oC no weight loss is observed, indicating the complete decomposition of 2. The residual weight 16.60% (calcd. 16.65%) also belongs to ZnO.

Fig. 7. TG curves of compounds 1 and 2

To confirm whether the crystal structures are truly representative of the bulk materials, PXRD experiments were carried out for 1 and 2. The experimental PXRD patterns are in good agreement with the corresponding simulated ones (Fig. 8) except for the relative intensity variation because of preferred orientations of the crystals.

3.4 Photoluminescent properties

Recently, CPs with10metal centers have attracted much attention due to the potential applica- tions such as in chemical sensors, photochemistry, and electroluminescent display[30]. Hence, the photoluminescent properties of the ligands, 1 and 2 have been investigated at room temperature (Table 3 and Fig. 9). 1 and 2 reveal the fluorescence with main emission peaks at 428 nm (ex= 332 nm) and 395 nm (ex= 329 nm), respectively. Because the Zn2+cation is difficult to oxidize or reduce for the10configuration, the emission is neither metal-to- ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT)[31]. As a result, the emission may be assigned to intraligand transitions.

Table 3. Wavelengths of the Emission Maximums and Excitation (nm)

Fig. 8. PXRD of compounds 1 and 2

Fig. 9. Solid-state emission spectra of 1, 2 and ligands at room temperature

4 CONCLUSION

In summary, we have synthesized two mixed- ligand CPs under hydrothermal conditions. Both of them display 3-fold parallel interpenetrating struc- tures and show polycatenane character. By careful inspection of these structures, we find that different features have been achieved by increasing the carboxylate ligand length. It is believed that multi-carboxylates and N-donor ligands with different coordination modes and conformations are important for the formation of different structures. These results suggest that novel extended entan- glements containing both polyrotaxane and polyca- tenane characters can be realized by combining metal nodes with flexible N-donor ligands and carboxylate ligands. Investigations in this direction are currently under way.

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28 February 2018;

7 June 2018 (CCDC 1820456 (1) and 1820455 (2))

the Science and Technology Development plan of Jilin Province (20150520006JH), Science and Technology Research Project of Education Department of Jilin province (2016219, JJKH20180776KJ), and Science and Technology Development plan of Siping City (2013055)

Liu Bo, majoring in coordination chemistry. E-mail: 1872176575@qq.com; Zhou Shi, E-mail: emily_zs@qq.com

10.14102/j.cnki.0254-5861.2011-1993