Two Cu(II) coordination polymers based on a flexible bis(pyridyl-tetrazole): Solvent-ratio induced various structures and distinct adsorption performance for organic dyes

Inorganica Chimica Acta 464 (2017) 114–118

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Research paper

Two Cu(II) coordination polymers based on a flexible bis(pyridyltetrazole): Solvent-ratio induced various structures and distinct adsorption performance for organic dyes Jing Zhao, Hong-Yan Lin ⇑, Guo-Cheng Liu, Xiang Wang, Xiu-Li Wang ⇑ Department of Chemistry, Bohai University, Jinzhou 121000, PR China

a r t i c l e

i n f o

Article history: Received 31 March 2017 Received in revised form 6 May 2017 Accepted 6 May 2017 Available online 8 May 2017 Keywords: Coordination polymers Bis(pyridyl-tetrazole) Solvent ratio Dye adsorption

a b s t r a c t Two new coordination polymers (CPs), namely, [Cu(4-bptzp)(BDC)] (1) and [Cu2(4-bptzp)2(BDC)Cl2] (2) (4-bptzp = 1,4-bis(5-(4-pyridyl)tetrazolyl)propane, H2BDC = 1,3-benzenedicarboxylic acid) were synthesized by changing the proportion of the solvents under the solvothermal conditions, and structurally characterized by single crystal X-ray diffraction, infrared spectroscopy (IR), powder X-ray diffraction (PXRD), elemental analyses and thermogravimetric analyses (TGA). The 4-bptzp was firstly introduced into the CPs. Complexes 1 and 2 show two kinds of 1D chain structures. Complex 1 contains the 1D ladder-like [Cu(BDC)]n chains and Cu2(4-bptzp)2 rings, while complex 2 consists of the 1D linear [Cu2(BDC) Cl2]n chain containing the bridging Cl ions and Cu2(4-bptzp)2 rings. The adjacent 1D chains in 1 and 2 are extended to 2D supramolecular layers by the hydrogen bonding interactions, respectively. In addition, the electrochemical properties and dye adsorption properties of complexes 1 and 2 have been investigated. The solvent ratio plays a key role in adjusting their structures and adsorption property. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction With rapid industrial development, environment pollution from organic dyes, a kind of frequently observed and abundant pollutants of water, is becoming serious all over the world. Therefore, it is pressing to find an effective way to deal with the organic dyes of the wastewater before letting it into the environment [1]. Until now, several approaches, such as membrane filtration, photocatalysis, coagulation, advanced oxidation and adsorption have been developed to remove organic dye wastes [2]. Among these methods, adsorption is considered to be one of the most effective, feasible, low-cost and simple operation way. In this regard, a number of adsorbents, such as activated carbons, zeolites and polymeric materials have often been utilized to adsorb organic dye pollutants [3]. Hence, finding a stable and highly efficient adsorbent material is the key factor for practical application of adsorption technology. Recently, the design and synthesis of coordination polymers (CPs) have attracted more and more concern due to their potential applications as the adsorption material [4]. For example, Bu’s group has reported two anionic metal-organic frameworks, which showed distinct adsorption performance for organic dyes [5]. ⇑ Corresponding authors. E-mail addresses: [email protected] (H.-Y. Lin), [email protected] (X.-L. Wang). http://dx.doi.org/10.1016/j.ica.2017.05.008 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

As is well known, the formation and final structures of CPs may be influenced by several factors, such as organic ligands, reaction time, temperature, solvent system, intensity of pressure, the ratio of solvents and so on [6]. Among these factors, the design of appropriate organic ligands with specific geometry, coordination ability, length, and relative orientation of donor groups is vital to the construction of CPs [7]. In our previous work, we successfully designed a series of flexible bis(pyridyl-tetrazole) ligands, and prepared some novel polyoxometalate-based multinuclear clusters by introducing them into POM-CuI/AgI systems [8]. These flexible bis(pyridyl-tetrazole) ligands possess the following advantages: (i) they contain two pyridyls and two tetrozole groups and can provide multiple coordination sites and show versatile coordination modes; (ii) they contain flexible -(CH2)n- backbones that can freely rotate to fill the need of coordination environment around metal ions. As far as we know, examples of the CPs derived from the flexible bis(pyridyl-tetrazole) have not been reported so far. In this work, we try to introduce the flexible bis(pyridyl-tetrazole) 1,4bis(5-(4-pyridyl)tetrazolyl)propane (4-bptzp) into the Cu(II)-dicarboxylate system. Fortunately, two new Cu(II) CPs, namely, [Cu(4bptzp)(BDC)] (1) and [Cu2(4-bptzp)2(BDC)Cl2] (2) were obtained under solvothermal conditions. Complexes 1 and 2 show two different 1D chain structures. The effects of proportion of solvents on the assembly and structures of the title complexes are dis-

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cussed. In addition, the dyes adsorption and electrochemical properties of complexes 1 and 2 were investigated. 2. Experimental 2.1. Materials and measurement All reagents and solvents for syntheses were purchased from commercial sources and used as received without further purification. FT-IR spectra (KBr pellets) were taken on a Scimitar 2000 Near FT-IR Spectrometer. Powder X-ray diffraction (PXRD) patterns were measured on an Ultima IV with D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Ka (k = 1.5406 Å) radiation. Thermogravimetric (TG) analyses were performed on a PyrisDiamond TG instrument under a flowing N2 atmosphere with a heating rate of 10 °C
min 1. A CHI 760 electrochemical workstation was used for control of the electrochemical measurements. A conventional three-electrode cell was used at room temperature. The carbon paste electrodes modified with complexes 1–2 (1-CPE, 2-CPE) were used as the working electrode. An SCE (saturated calomel electrode) and a platinum wire were used as reference and auxiliary electrodes, respectively. UV–Vis absorption spectra were obtained using a SP-1901 UV–Vis spectrophotometer. 2.2. Syntheses of title complexes 2.2.1. Synthesis of complex 1 A mixture of CuCl2
2H2O (0.034 g, 0.2 mmol), 4-bptzb (0.033 g, 0.1 mmol), 1,3-H2BDC (0.025 g, 0.15 mmol), H2O (4 mL) and acetonitrile (6 mL) was added in a Teflon lined autoclave (25 mL), then

Fig. 1. (a) The photograph of complex 1 crystal. (b) View of the coordination environment of Cu(II) ion in 1. (c) The 1D [Cu(BDC)]n ladder-like chain of 1. (d) The 1D cross-like chain of 1.

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heated at 120 °C for 4 days. After slow cooling to room temperature, light blue block crystals of 1 were obtained (Fig 1a). Yield 37% based on Cu. Elemental anal. (%) calc. for C23H18N10O4Cu: C 49.16, H 3.23, N 24.92. Found: C 48.97, H 3.16, N 24.84. IR (KBr pellet, cm-1): 3452s, 2345w, 1839w, 1627s, 1558s, 1461m, 1427m, 1361s, 1211m, 1041w, 952w, 841m, 717s, 632w, 532m. 2.2.2. Synthesis of complex 2 Complex 2 was prepared in the same way as 1 except that H2O (5 mL) and acetonitrile (5 mL) was used instead of H2O (4 mL) and acetonitrile (6 mL), blue block crystals of 2 were obtained (Fig 2a). Yield 40% based on Cu. Elemental anal. (%) calc. for C38H32Cu2N20O4Cl2: C 44.28, H 3.13, N 27.18. Found: C 44.16, H 3.01, N 27.33. IR (KBr pellet, cm 1): 3413s, 3012w, 2942w, 2356w, 1620s, 1547s, 1461m, 1384s, 1207m, 1030m, 956m, 837s, 721s, 640w, 536m. 2.3. Preparation of complexes 1 and 2 bulk-modified carbon paste electrodes (1/2-CPEs) The complexes 1 and 2 bulk-modified CPEs (1/2-CPEs) were fabricated by mixing 0.11 g graphite powder and 0.022 g complex 1 or 2 in an agate mortar for approximately 30 min to achieve a uniform mixture; then 0.10 mL paraffin oil was added and stirred with a glass rod [9]. The homogenized mixture was packed into a 3 mm inner diameter glass tube and the tube surface was wiped with weighing paper. The electrical contact was established with the copper wire through the back of electrode. 2.4. X-ray crystallography Crystallographic data for compounds 1–2 were collected on a Bruker Smart APEX II diffractometer with Mo-Ka radiation (k = 0.71073) by x and h. The SQUEEZE routine of PLATON was applied to remove the contributions to the scattering from the solvent molecules. The reported refinements are of the guest-free

Fig. 2. (a) The photograph of complex 2 crystal. (b) The coordination environment of Cu(II) ions in 2. (c) The 1D [Cu2(BDC)Cl2]n linear chain of 2. (d) The 1D bunch-like chain of 2.

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structures using the ⁄.hkp files produced by using the SQUEEZE routine. For complexes 1–2, the crystal parameters, data collection, and refinement results are summarized in Table 1. Selected bond distances and bond angles are listed in Table S1. 3. Result and discussion 3.1. The influence of the solvent ratios on the synthesis of complexes 1 and 2 In this work, we obtained complexes 1 and 2 under solvothermal conditions using a mixture of acetonitrile and water. In order to obtain the target compounds, we performed a series of parallel experiments by utilizing mixed solvents of CH3CN/H2O or DMF/ H2O with various ratios (v/v 1:1, 3:2, 2:3, 4:1, 1:4). As a consequence, only the single crystals of complexes 1 and 2 were obtained at 1:1 and 3:2 (v/v) of CH3CN/H2O, respectively. Other solvent or different solvent ratios only resulted in some irregular diminutive unknown crystals or precipitates or mixed products. That is, the pure crystalline product can only be obtained at the specific solvent and solvent ratio. The results indicated that the solvent ratios play an important role in the formation of the title complexes. 3.2. Structural description 3.2.1. Structural description of complex 1 Single-crystal X-ray diffraction measurement reveals that com space group. plex 1 crystallizes in the triclinic system with P1 Complex 1 consists of one crystallographically independent Cu(II) ion, one 4-bptzp ligand, and one BDC anion. A view of 1 with atom labeling is shown in Fig 1b and the coordination geometry around the Cu(II) center could be described as a distorted octahedral environment. The coordination sphere around the six-coordinated Cu (II) center is composed of two N atoms from two 4-bptzp ligands [Cu(1)-N(1) = 2.039(7) Å and Cu(1)-N(2) = 2.031(7) Å], and four carboxylic O atoms from three BDC anions [Cu(1)-O(1) = 2.035 (5) Å, Cu(1)-O(2) = 2.504(2) Å, Cu(1)-O(3) = 2.242(6) Å, and Cu(1)O(4) = 1.960(5) Å]. The BDC anion acts as a l3-bridge linking three Cu(II) ions, in which one carboxylic group adopts a l2-g1:g1 bridging mode to connect two Cu(II) ions, the other carboxylic group

Table 1 Crystal data and structure refinement for complexes 1 and 2.

a b

Complex

1

2

Formula Formula weight Cryst. syst Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å 3) Z D/g cm 3 l/mm 1 F (0 0 0) Reflection collected Unique Reflections Rint R1a [I > 2r(I)] wR2b (all data) GOF

C23H18CuN10O4 562.01 Triclinic P-1 8.8740(16) 10.1413(18) 13.767(2) 72.769 78.590 80.606 1152.7(4) 2 1.619 1.003 574 6731 4143 0.0431 0.0923 0.2709 1.053

C38H32Cu2N20O4Cl2 1030.82 Monoclinic C 2/c 14.4062(9) 26.9225(18) 22.8104(14) 90.00 98.2930 90.00 8754.5(10) 8 1.564 1.160 4192 27,218 8677 0.0515 0.0488 0.1110 1.040

R1 = R(||Fo| |Fc||)/R|Fo|. wR2 = [w(|Fo|2 |Fc|2)2/(w|Fo|2)2]1/2.

adopts a chelating l1-g1:g1 mode to coordinate with one Cu(II) ion. The adjacent Cu(II) ions are bridged by BDC anions to form a 1D [Cu(BDC)]n ladder-like chain (Fig 1c). As shown in Fig 1d, two Cu(II) ions are connected by two 4-bptzp ligands to form a [Cu2(4-bptzp)2] ring, which hang at two sides of the 1D [Cu(BDC)]n chain. Furthermore, the adjacent 1D chains are further linked together via hydrogen bonding interactions to generate a 2D supramolecular network (Fig. S1). The distance of weak hydrogen bonding interactions between the carboxylic O atom of the BDC anion and the C atom from 4-bptzp ligand (O2


C11) is 3.077 Å. 3.2.2. Structural description of complex 2 Single-crystal X-ray diffraction analysis indicates that complex 2 crystallizes in the monoclinic system with C2/c space group. Complex 2 consists of two crystallographically independent Cu (II) ions [Cu(1) and Cu(2) ], two 4-bptzp ligands, one BDC anion and two Cl- ions. As shown in Fig. 2b, the two Cu(II) centers exhibit a distorted octahedral environment, which are coordinated by two N atoms from two 4-bptzp ligands [Cu(1)-N(2) = 2.018(3) Å and Cu (1)-N(12) = 2.025(3) Å, Cu(2)-N(1) = 2.041(3) Å, and Cu(2)-N(11) = 2.020(3) Å ], two carboxylic O atoms from one BDC anion [Cu (1)-O(3)#1 = 1.987(2) Å, and Cu(1)-O(4) = 2.591(5) Å, Cu(2)-O(2) = 1.978(2) Å, and Cu(2)-O(1) = 2.557(5) Å, Symmetry codes: #1, x 1, y, z], and two Cl ions [Cu(1)-Cl(1) = 2.2957(9) Å, Cu(1)-Cl (2) = 2.7149(9) Å, Cl(1)-Cu(2) = 2.7157(9) Å, and Cu(2)-Cl(2) = 2.3020(9) Å]. The BDC acts as a l2-bridge linking two Cu(II) ions, in which the carboxylic groups adopt a l1-g1:g1 mode to chelate one Cu(II) ion. The adjacent Cu(II) ions are bridged by BDC and Cl anions to form a 1D [Cu2(BDC)Cl2]n linear chain (Fig. 2c). Similar to that in 1, the [Cu2(4-bptzp)2] rings hung on two sides of the 1D [Cu2(BDC)Cl2]n chain (Fig. 2d). Furthermore, the adjacent 1D chains are further linked together through the hydrogen bonding interactions to build a 2D supramolecular structure (Fig. S2). The distance of the weak interactions (O4


C22) is 3.505 Å. 3.3. FT-IR spectra The IR spectra of complexes 1 and 2 are determined in the frequency range of 500–4000 cm 1, as shown in Fig. S3. The strong peaks at 1627 and 1211 cm 1 for 1, 1620 and 1207 cm 1 for 2, may be attributed to the asymmetric and symmetric vibrations of carboxyl groups [10a]. The strong peaks at 1361 and 1041 cm 1 for 1, 1384 and 1030 cm 1 for 2, may be attributed to the mC-N stretching vibrations of the pyridyl ring of the 4-bptzp ligands [10b]. The strong peaks at 1461, 1427 cm 1 for 1, 1461, 1384 cm 1 for 2, may be attributed to m(N@N) of the tetrazolyl ring from the 4-bptzp ligand [10c]. 3.4. Powder X-ray diffraction and thermal stability analyses The powder X-ray diffraction (PXRD) patterns of complexes 1 and 2 are presented in Fig. S4. The as-synthesized patterns are in good agreement with the corresponding simulated ones, indicating the phase purities of the samples. In order to characterize the complexes more fully in terms of thermal stability, the thermal behaviors of 1 and 2 were examined by thermogravimetric analyses (TGA). The experiments were performed on samples consisting of numerous single crystals under N2 atmosphere with a heating rate of 10 °C/min (Fig. S5). Complexes 1 and 2 have one step of weight loss, respectively. The weight loss of 82.40% (for 1) and 80.17% (for 2) (calcd. 83.21% for 1, 80.48% for 2) in the range of 250–510 °C (for 1) and 270–580 °C (for 2) is consistent with the release of organic ligands. The remaining weight corresponds to the formation of CuO (obsd. 17.60, 19.83%, calcd. 17.79, 19.52%).

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3.5. Electrochemical behaviors of complexes 1 and 2 The complexes 1 and 2 are insoluble in water and general organic solvents. Therefore, the bulk-modified carbon paste electrodes (CPE) with complexes 1 and 2 (1-CPE and 2-CPE) were fabricated as the working electrodes, which become a best choice to explore the electrochemical behavior of these complexes [11]. Fig. 3 shows the typical cyclic voltammetric behaviors of 1/2-CPEs in 0.01 M H2SO4 + 0.5 M Na2SO4 aqueous solution. It can be noticed clearly that in the potential ranges of 400 to 400 mV and 600 to 400 mV, a pair of reversible redox peaks are observed at the 1CPE and 2-CPE, respectively, which could be attributed to the redox of Cu(II)/Cu(I) [12]. The mean peak potentials E1/2 = (Epa + Epc)/2 are 1.8 mV for 1-CPE and 18.2 mV for 2-CPE (100 mV s 1), respectively. The results suggested that the copper complexes 1 and 2 showed stable redox behaviors in aqueous solution. In addition, the influences of scan rates on the electrochemical behaviors of 1/2-CPEs are studied in 0.01 M H2SO4 + 0.5 M Na2SO4 aqueous solution in the potential range of 400 to 400 mV and 600 to 400 mV, respectively. As shown in Fig. 3, with the increase of scan rates from 20 to 200 mVs 1, the peak potentials of the 1/2CPEs vary gradually: the cathodic peak potentials shift to the negative direction and the corresponding anodic peak potentials shift to the positive direction. The insets of Fig. 3 show the plots of peak

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currents versus scan rates. It can be clearly seen that the peak currents are proportional to the scan rates, which suggests that redox process of 1/2-CPE is surface-controlled. 3.6. Adsorption property of organic dyes Different kinds of organic dyes are usually observed in wastewater from industries such as printing, textile, food, paper, pharmaceutical and so on [13]. Nevertheless, many organic dyes are supposed to be noxious and even carcinogenic to human beings, and most noxious organic dyes are stable under the general condition such as light, oxidants and so on, which make them difficult to be removed. Therefore, how to get rid of the organic dye molecules in the wastewater becomes especially significant. Recently, Kartik’s group utilized a CP, {[Cd(L)2(H2O)2]
(H2O)2(ClO4)2}n (L = N-pyridin3-yl-2-[4-(pyridin-3-ylcarbamoylmethyl)phenyl]acetamide), to remove methylene orange (MO), in which the complex possessed efficient adsorption activities towards the removal of MO [14]. Currently, the CPs have become a sort of potential materials that can remove the organic dye molecules through adsorption. In this work, the adsorption behaviors of complexes 1 and 2 toward organic dyes were evaluated by utilizing MO as a typical example. In addition, three other organic dyes including rhodamine B (RhB), methylene blue (MB) and congo red (CR) were also used for evaluating the adsorption selectivity of complexes 1 and 2. In a typical dye adsorption experiment, the crystal samples of 1 and 2 (20 mg) were immersed in 100 mL aqueous solution of the organic dyes (40 mg L 1) and the mixture was kept in the dark. Within 30 min, the light orange solution of MO turned to colorless, while the blue crystals of 2 turned to yellow. Within 60 min, the red CR solution turned to slightly red, while the blue crystals also turned to yellow. The change in the dye concentration in the aqueous solution was monitored by UV–vis absorption spectroscopy at different time intervals after immersing the crystal samples of 1 and 2 (Fig. 4). The adsorption amount qt (mg g 1) of dye was calculated by the following equation, qt = (C0 Ct)V/W. Where, C0 and Ct (mg L 1) are the beginning and balancing concentrations of MO in solution (mg/L), respectively; V is the volume of solution (L); and W (g) is the mass of the adsorbent. From this equation, the calculated amount of MO and CR that can be adsorbed by 1 g of 2 is 200 mg and 167.21 mg, respectively. These results proved that complex 2 showed good adsorption capacity for the MO and CR in dark, while showed poor adsorption capacity for the MB and RhB (Fig. S7). Complex 1 showed poor adsorption capacity of the above organic dyes (Fig. S6). Moreover, after adsorption experiments, the samples were dipped into CH3CN solution to desorb MO/CR. Then the colorless solution gradually changes to yellow/red. Considering that the CPs contain aromatic organic ligands and numerous hydrogen atoms, while there are conjugate rings in MO/CR, hence, there may exit some weak interactions (such as hydrogen bonding, p– p stacking and weak static interactions) between complex 2 and MO/CR. From the above experiments, we can further conclude that complex 2 could serve as a good adsorbent to remove MO/CR efficiently. 4. Conclusion

Fig. 3. Cyclic voltammograms of the 1-CPE and 2-CPE in 0.01 M H2SO4 + 0.5 M Na2SO4 aqueous solution at different scan rates (from inner to outer: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200 mV
s 1); Inset: The plots of peak currents vs. scan rates for 1/2-CPEs.

Two new coordination polymers have been successfully synthesized under solvothermal conditions by the reaction of 1,3-H2BDC and 4-bptzp together with Cu(II) salts. The title complexes represent the first examples of coordination polymers derived from flexible bis(pyridyl-tetrazole) ligand. Two complexes show different 1D chain structures. The structural differences of these complexes can be attributed to the different solvent ratios in the reaction sys-

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Fig. 4. UV–vis spectra of MO (left) and CR (right) solutions with complex 2 in dark.

tems. Complex 2 shows good dye adsorption property for MO and CR. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21671025, 21501013 and 21401010) and Program for Distinguished Professor of Liaoning Province (No. 2015399). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2017.05.008. References [1] F.Y. Yi, W. Zhu, S. Dang, J.P. Li, D. Wu, Y.H. Li, Z.M. Sun, Chem. Commun. 51 (2015) 3336. [2] (a) B.Y. Shi, G.H. Li, D.S. Wang, C.H. Feng, H.X. Tang, J. Hazard. Mater. 143 (2007) 567; (b) J.W. Lee, S.P. Choi, R. Thiruvenkatachari, W.G. Shim, H. Moon, Water Res. 40 (2006) 435; (c) D. Mahanta, G. Madras, S. Radhakrishnan, S. Patil, J. Phys. Chem. B 112 (2008) 10153; (d) J. Fernández, J. Kiwi, C. Lizama, J. Freer, J. Baeza, H.D. Mansilla, J. Photochem. Photobiol., A 151 (2002) 213; (e) W.X. Chen, W.Y. Lu, Y.Y. Yao, M.H. Xu, Environ. Sci. Technol. 41 (2007) 6240. [3] (a) Y.A. Degs, M.A.M. Khraisheh, S.J. Allen, M.N. Ahmad, Water Res. 34 (2000) 927; (b) A.G. Espantaleón, J.A. Nieto, M. Fernández, A. Marsal, Appl. Clay Sci. 24 (2003) 105; (c) C.K. Lee, S.S. Liu, L.C. Juang, C.C. Wang, K.S. Lin, M.D. Lyu, J. Hazard. Mater. 147 (2007) 997; (d) Y. Yu, Y.Y. Zhuang, Z.H. Wang, M.Q. Qiu, Ind. Eng. Chem. Res. 42 (2003) 6898.

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