1. Introduction

The design and synthesis of new open-framework mixed-ligands coordination polymers have been the focus of current chemical and new material research. Of particular importance is the ability of the organic linkers to influence profoundly the structures of synthesized products, and to direct their formation with specific structures and potential applications [1,2,3,4,5], such as magnetism, host–guest chemistry, shape specificity and catalysis. Squarate dianions (C₄O₄²⁻), the de-protonated form of squaric acid (3, 4-dihydroxycyclobut-3-ene-1, 2-dione, H₂C₄O₄) has been paid much attention to, from the crystal engineering point of view, as the building units with template capacity to set up the assembly procedure of stable one-, two-, and three-dimensional (1D, 2D and 3D) crystalline materials. In the relevant approach, the squarate serves as a unique ligand, which possesses a four-membered cyclic aromatic structure characterized by extensive π electron delocalization all over carbon and oxygen atoms which has been widely used as a polyfunctional ligand, such as hydrogen bonding or π–π interactions, for the construction of extended supramolecular architecture and also used as a bridging ligand with various coordination modes (μ₂ to μ₈ bridges) to build up many coordination polymers with novel extended networks, including 1D chain, 2D layer, 3D cube-and cage-like frameworks [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47]. The planar D_4h structures of squarate have been well established on their metal salts or complexes. In our previous study, we have reported several metal-squarate MOFs associated with N, N-donor bridging/chelating type co-ligands, like 4, 4'-bipyridine (4, 4'-bipy), and 1, 2-bis(4-pyridyl)ethane (dpe) [43,44,45,46,47]. With our continuous effort on the study of metal-squarate coordination polymers, the structural topology of metal ions with the used co-ligands, 1, 2-bis(2-pyridyl)ethylene (2, 2'-bpe), seems to be interesting and worth to be further investigated. Focusing on this approach, we report here the synthesis, structural characteristics and thermal stability of a coordination polymer, [Cd(2, 2'-bpe)(C₄O₄)(H₂O)₂]_n (1), in which the squarate and 2, 2'-bpe both act as the bridging ligands with μ_{1, 3}-coordination and bis-monodentate mode, respectively, to build up its 2D extended layered MOF. The intra-and inter-layer hydrogen boning interaction plays an important role on the construction of its 3D supramolecular architecture.

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2. Materials and method

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2.1. Materials and physical techniques

All chemicals were of reagent grade and were used as commercially obtained without further purification. Elementary analyses (carbon, hydrogen and nitrogen) were performed using a Perkin-Elmer 2400 elemental analyzer. The infrared spectra was recorded on a Nicolet Fourier Transform IR, MAGNA-IR 500 spectrometer in the range of 500–4000 cm⁻¹ using the KBr disc technique. Thermogravimetric analysis (TGA) was performed on a computer-controlled Perkin-Elmer 7 Series/UNIX TGA7 analyzer. Single-phased powder samples were loaded into alumina pans and heated with a ramp rate of 5 ℃/min from room temperature to 800 ℃ under nitrogen flux.

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2.2. Synthesis of [Cd(C₄O₄)(2, 2-bpe)(H₂O)₂]_∞ (1)

An ethanol/H₂O solution (1:1, 3 mL) of H₂C₄O₄ (5.8 mg, 0.05 mmol) was added to an ethanol/water (1:1, 6 mL) solution of Cd(NO₃)₂·4H₂O (15.5 mg, 0.05 mmol) and 1, 2-bis(2-pyridyl)ethylene (18.2 mg, 0.05 mmol) at room temperature. After standing for one week, light-brown block crystals of 1 (yield, 12.1 mg; 54.6%) were obtained which are suitable for X-ray diffraction analysis. Anal. Calc. for C₁₄H₁₂N₂O₆Cd₁ (1): C 43.41, N 6.33, H 3.19; Found: C 43.69, N 5.95, H 2.91. IR (KBr pellet): ν = 3278 (s), 3083 (m), 1711 (w), 1601 (s), 1520 (vs), 1438 (m), 1341 (m), 1222 (m), 1159(m), 1096 (m), 1008 (m), 966 (m), 780 (m), 694 (m), 637 (m) cm⁻¹.

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2.3. Crystallographic data collection and refinements

Single-crystal structural analysis was performed on a Siemens SMART diffractomer with a CCD detector with Mo Kα radiation (λ = 0.71073 Å) at room temperature. A preliminary orientation matrix and unit cell parameters were determined from 3 runs of 15 frames each, each frame corresponds to a 0.3° scan in 10 s, following by spot integration and least-squares refinement. For each structure, data were measured using ω scans of 0.3° per frame for 20 s until a complete hemisphere had been collected. Cell parameters were retrived using SMART [48] software and refined with SAINT [49] on all observed reflections. Data reduction was performed with the SAINT [49] software and corrected for Lorentz and polarization effects. Absorption corrections were applied with the program SADABS [50]. Direct phase determination and subsequent difference Fourier map synthesis yielded the positions of all non-hydrogen atoms, which were subjected to anisotropic refinements. For compound 1, all hydrogen atoms were generated geometrically (C–H_sp2 = 0.93 Å) with the exception of the hydrogen atoms of the coordinated water molecules, which were located in the difference Fourier map with the corresponding positions and isotropic displacement parameters being refined. The final full-matrix, least-squares refinement on F² was applied for all observed reflections [I > 2σ(I)]. All calculations were performed using the SHELXTL-PC V 5.03 software package [51]. Crystallographic data and details of data collections and structure refinements of compound 1 are listed in Table 1.

CCDC1819496 for 1 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; Fax: (internet.) +44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].

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2.4. In situ X-ray powder diffraction

Variable temperature synchrotron powder X-ray diffraction data were collected at BL01C2 powder diffraction beamline in National Synchrotron Radiation Research Center. The powder sample was packed into a glass capillary and heated by a heating gun with ramp rate in 10 degree/minute. The X-ray energy was used at 12 keV (wavelength = 1.03321 Å). The in-situ powder diffraction data were recorded on a Mar3450 image plate with 72 seconds exposure time. The data calibration and integration were performed by GSAS-Ⅱ program [52], where diffraction angle is calibrated based on the standard powder sample of LaB₆.

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3. Results and discussion

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3.1. Synthesis and IR spectroscopy

Compound 1 was synthesized by direct mixing of Cd(Ⅱ) salts, 2, 2'-bpe ligands and squaric acid (H₂C₄O₄) with stoichiometric molar ratio of 1:1:1. The infrared spectrum shows a broad band in the region 3000–3400 cm⁻¹, which can be assigned to the stretching vibration, ν(O–H) of the water molecules. The most relevant IR features are those associated with the chelating squarate ligands. The peak at 1711 cm⁻¹ is assigned to the uncoordinated carbonyl groups, which exhibits a double bond character. The coordinated C–O groups of squarate are characterized by medium absorptions at 1601 cm⁻¹. A very strong and broad band centered at around 1520 cm⁻¹, which is attributed to vibrational modes representing mixtures of C–O and C–C stretching motions and is in agreement with the characteristic of the (CO)_n²⁻ salts [53].

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3.2. Structural description of [Cd(C₄O₄)(2, 2'-bpe)(H₂O)₂]_n (1)

The crystal structure of 1 is determined with a 3D supramolecular network being constructed from 2D-layered MOFs. The Cd(1) lies in a distorted octahedral environment and locates at the inversion center, consisting of two nitrogen atoms from 2, 2'-bpe ligand, with the bond distance of Cd(1)–N(1) = 2.400(1) Å and four oxygen atoms from two bridging squarate ligands and two water molecules with the bond distances of Cd(1)–O(1) = 2.265(1), Cd(1)–O(3) = 2.324(1) Å as shown in Figure 1a. The related bond lengths and angles around the Cd(Ⅱ) ion are listed in Table 2. The squarate and 2, 2'-bpe both act as a bridging ligand with μ_{1, 3}-bis-monodentate and bis-monodentate coordination modes, respectively, leading the formation of a 2D layered MOF (Figure 1b) with the quadrilateral grid as the basic fundamental building unit. The pore size of the quadrilateral grids based on the Cd$\cdots$Cd separations are approximately 8.396(1) × 8.398(1) Å via the bridges of squarate and 2, 2'-bpe, respectively. The 2D MOF of 1 can be viewed in a simplified way using TOPOS [54,55] as a four-connected uninodal net with the point symmetry (Schläfli symbol) {4⁴.6²}. Adjacent 2D layers are arranged in a parallel ABAB manners to complete its 3D supramolecular network (Figure 1c). Hydrogen bonding interaction plays an important role on the stabilization of the 3D supramolecular network (yellow dashed lines shown in Figure 2c). In the crystal packing, the coordinated water molecules (O(3)) are held together with the uncoordinated oxygen atoms (O(2)) of squarate (C₄O₄²⁻) by means of one intra-layer O–H$\cdots$O hydrogen bond with O$\cdots$O distances of 2.708(5) Å and the other inter-layer O–H$\cdots$O hydrogen bond with O$\cdots$O distances of 2.803(5) Å, respectively, on the further stabilization of its 3D supramolecular architecture. Related bond distances and angles of O–H$\cdots$O hydrogen bonds are listed in Table 3.

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3.3. Thermal-stability of 1 by thermogravimetric analysis (TGA) and in-situ powder X-ray diffraction measurement (PXRD)

To assess the thermal stability and structural variation as a function of the temperature, thermogravimetric analysis (TGA) and in-situ temperature dependent XRD measurements of 1 were performed on single-phase polycrystalline samples. TGA associated with in-situ temperature dependent XRD measurements agree well with the crystallographic observation. During the heating process, the TG analysis (Figure 2a) revealed that compound 1 underwent a two-step weight loss and thermally stable up to 83.5 ℃. The first weight loss of 8.8% (calc 8.2%) corresponded to the loss of coordinated water molecules was occurred in the range of approximate 83.5–119.1 ℃. In the temperature range of approximate 119.1–228.2 ℃, 1 was stable without any weight lost. On further heating, these samples slowly decrease from 228 ℃ to 360 ℃, but decrease rapidly from 360 ℃ to 450 ℃. The first mass drop may be attributed to the structure transformation after removing the coordination water molecules from Cd(Ⅱ) center. We expect the un-coordinated oxygen atom of squarate ligand in 1 will coordinate to the Cd(Ⅱ) and create a rigid 3D structure after the removal of coordinated water molecules in the de-hydrated form. The slow mass drop between 228 to 360 ℃ might come from a small part of sample decomposition in the 3D structure. Above 360 ℃, the structure cannot sustain anymore and a rapid mass-loss is attributed to the sample decomposition process. The thermal stability of the materials was further investigated by in-situ powder X-ray diffractometry (see Figure 2b). The powder crystalline structure at room temperature is matching well to its single crystal structure. From room temperature to 140 ℃, the crystal structure maintained well and no phase transition occurred. Above 170 ℃, a phase transition was observed and the structure is stable even at 380 ℃. The cell parameters from the temperature-variable PXRD patterns were obtained by using indexing program, Dicvol, and listed in the Table 4. During the heating process from room temperature to 170 ℃, the cell volume increases step by step. The a, c axis lengths and beta angles are changed slightly. However, the b axis is stretched obviously. The b axis is correlated the 2D layer distance. We strongly suspected the extremely stable structure is still a framework system. Based on the TGA result, the two coordinated water molecules were removed. We propose a preliminary model that the non-coordinated oxygen atoms of squarate ligands were chelated on the open active site of Cd(Ⅱ) ions after the removal of coordinated water molecules. The O(2) of 1 replace the O(3) position and form a five-member ring to stabilize the 3D de-hydrated framework. We try to index the cell and solve the structure through simulation annealing method. Unfortunately, the diffraction data sets above 170 ℃ cannot be indexed due to the broad diffraction peaks and cannot afford enough information to obtain accurate structure.

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4. Conclusions

A mixed-ligands 3D supramolecular network, [Cd(C₄O₄)(2, 2'-bpe)(H₂O)₂] (1), built up by 2D-layered MOFs via the bridges of squarate and 2, 2-bpe ligands with bis-monodentate coordination modes has been successfully synthesized and structurally characterized. Adjacent 2D layers were then self-assembled via the parallel arrangement to form a 3D supramolecular network. Intra-and inter-layer hydrogen bonding interactions between the uncoordinated oxygen atoms of squarate and coordinated water molecules in the framework provide extra-energy on the stabilization of the 3D supramolecular architecture. TGA and in-situ powder X-ray diffraction (PXRD) measurements are in accordance with the crystallographic observations, and the result provide evidence that 1 has quite high thermo-stability that keep their crystalline forms up to 228 ℃.

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Acknowledgments

The authors wish to thank the Ministry of Science and Technology and Soochow University, Taiwan for financial support.

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Conflict of interest

The authors declare that there is no conflict of interest regarding the publication of this manuscript.

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