LU Jiu-Fu JIN Ling-Xia SONG Juan ZHAO Cai-Bin YUE Si-Yu LI Li YANG Hai-Tao CAO Xiao-Yan GE Hong-Guang

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A Novel 3D Metal Coordination Polymer Based on Tetranuclear Cobalt Cluster Building Blocks: Synthesis, Crystal Structure and Photocatalytic Property①

LU Jiu-Fu②JIN Ling-Xia SONG Juan ZHAO Cai-Bin YUE Si-Yu LI Li YANG Hai-Tao CAO Xiao-Yan GE Hong-Guang②

(723001)

A novel 3D MCP, [Co2(μ3-OH)(btc)(bmip)]n(1, H3btc = 1,3,5-benzenetricar- boxylate acid, bmip = 1,3-bis(2-methylimidazolyl)propane), was synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, powder XRD, FT-IR, TGA and elemental analysis techniques. MCP 1features a3D framework based on tetranuclear Co(Ⅱ) clusterswhere the four cobalt ions are coplanar, and shows an unusual binodal (3,10)-connected topology. Furthermore, the photocatalytic experiment result indicates the degradation ratios of rhodamine B(RhB) reach 78.2% when MCP 1 acts as catalyst.

hydrothermal synthesis, structure, Co(Ⅱ) clusters, photocatalytic property;

1 INTRODUCTION

Construction of new metal coordination polymers (MCPs) has attracted attention due to their diverse structural topologies and potential applications as functional materials such as gas storage, magnetism, catalysis, anti-microbial and luminescence[1-5].Many efforts have been devoted to the construction of MCPs with tunable properties using metal clusters as secondary building units (SBUs) and organic ligands as linkers.The creation of anticipated cluster, espe- cially high-connected cluster SBUs, is still a challenge. Some compounds based on polynuclear CoIIn(n = 2～8) clusters and multicarboxylate have been reported[6, 7]. In recent years, the direct use of two types of organic ligands has been found to be an effective method for the synthesis of MCPs, because of their rich coordination modes, including mono- dentate, bridging and chelating. To date, many binodal MCPs have been prepared on the basis of carboxylate-type O-donors and amine- or N-donors. Thereinto, tri- and polycarboxylic acids are widely used as bridging ligands to construct coordination frameworks with versatile structures[8], such as 1,3,5-benzenetricarboxylate acid (H3btc) which is good candidate for the construction of metal-organic frameworks[9–12]. Meanwhile, the 1,3-bis(2-methyli- midazolyl)propane(bmip) bearing alkyl spacers is a good choice of N-donor ligands, in which the flexible nature of spacers allows the ligands to bend and rotate when it coordinates to metal centers, and this often causes structural diversity. In our previous work, a series of unusual 6-, 8-, and 10-connected MCPs have been constructed from the mixed-ligand systems of different aromatic polycarboxylic and nitrogen coligands, which have shown interesting photocatalytic activities and luminescent pro- perties[13, 14].

In this paper, we report the synthesis of a novel MCP, [Co2(μ3-OH)(btc)(bmip)]n(1) by using 1,3,5- benzenetricarboxylate acid and 1,3-bis(2-methyl- midazolyl)propane as the mixed ligands (Scheme 1). Its single crystal structure and photocatalytic pro- perty have been investigated.

Scheme 1. Structures of the organic ligands used in this work

2 EXPERIMENTAL

2.1 Materials and physical measurements

All reagents used in the syntheses were com- mercially available and used as purchased. Elemental analyses for carbon, hydrogen, and nitrogen atoms were performed on a Vario EL III elemental analyzer. The infrared spectra (4000～400 cm–1) were recorded by using KBr pellet on an Avatar 360 E.S.P. IR spectrometer. Powder X-ray diffraction (PXRD) data were collected on a Rigaku Ultima ?V X-ray diffractometer with Curadiation (= 0.154056 nm). UV-Vis and fluorescence emission spectroscopy was measured by F-4500 analytical instruments. Thermogravimetric analysis (TGA) was performed on a TA-SDT Q600 thermal analyzer under N2atmosphere with a heating rate of 10 oC·min–1in the range of 30～1000 oC. Topology analysis for MCP 1 was used Topos4.0 program package.

2.2 Synthesis of [Co2(μ3-OH)(btc)(bmip)]n (1)

A mixture of Co(NO3)2·6H2O (0.2 mmol, 58.2 mg), bmip (0.20 mmol, 40.8 mg), NaOH (0.2mmol, 8mg), H3btc (0.15 mmol, 31.5 mg) and 8 mL H2O was placed in a 25 mL Teflon-lined stainless-steel container, which was heated to 130 oC for 3 days, and then cooled to room temperature over 24 hours. Purple block crystals of 1 were collected. Yield: 63% based on cobalt. Elemental analysis (%): calcd. for C20H20Co2N4O7(M= 546.26): C, 43.94; H, 3.66; N, 10.25. Found: C, 43.23; H, 3.72; N, 10.81. IR(cm–1): 3442(s), 3127(w), 2919(w), 1618(s), 1559(m), 1501(w), 1422(m), 1351(m), 1271(w), 1152(w), 1090(w), 998(w), and 720(m).

2.3 Determination of crystal structures

Suitable single crystals were carefully selected under an optical microscope, and data collection was performed on a CrysAlisPro, Oxford Diffraction Ltd, Version 1.171.34.36 CCD automatic diffractometer with graphite-monochromatized Moradiation (= 0.71073 ?) by using the-scan mode at room temperature. The raw data frames were intergraded into SHELX-format reflection files and corrected using the SAINT program. Absorption corrections based on multi-scan were obtained using the SADABS program. All the structures were solved by direct methods and refined with a full-matrix least-squarest on2using SHELX 97[15, 16]. Hydro- gens were located by geometric calculations, and their positions and thermal parameters were fixed during the structure refinement. The data of relevant bond lengths and bond angles for MCP 1 are summarized in Table 1.

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

Symmetry transformations used to generate the equivalent atoms: #1)+1/2,, –+1/2; #2), –+3/2,–1/2;#3) –,–1/2, –+1/2; #4) –,+1/2, –+1/2; #5) –, –+2, –+1

3 RESULTS AND DISCUSSION

3.1 Description of structure 1

Single-crystal X-ray diffraction study revealed that the asymmetric unit of 1 contains two Co(II) cations, one btc3-anion, one bmip ligand, and one3-coor- dinated water molecule. The two crystallographically independent Cobalt(II) ions have different coordina- tion geometries. As depicted in Fig. 1. Co(1) is six-coordinated by three carboxylate oxygen atoms from three btc3-ligands, one nitrogen atom of bmip ligand, and two coordinated water molecules.Co(2) ion is four-coordinated by two carboxylate oxygen atoms from two btc3-ligands, one water molecule, and one nitrogen atom in a distorted tetrahedral geometry. In 1, Co(1)#1 and Co(2)#1 are related to Co(1) and Co(2), generated by the symmetry.Main bond distances and bond angles around the Co(Ⅱ) ions are listed in Table 2. The Co(1) and Co(2) ions are bridged by a water molecule and two2-car- boxylate groups to form a dinuclear [Co2(3- OH)(2-COO)3] cluster, as shown in Fig. 2.Two dinuclear [Co2(3-OH)(2-COO)3] units are bridged by one pair of carboxylate groups to give a tetra- nuclear [Co4(3-OH)2(2-COO)6] secondary building cluster unit occupying an inversion center. It is noted that the four cobalt atoms lie on a plane forming a Co(4) parallelogram. The Co(1)···Co(2) and Co(1)···Co(2)#1 (symmetry code: #1+1/2,, –+1/2) separations are 3.3 and 3.8 ?, respectively.

Fig. 1. Coordination environment of Co(II) ion in 1, in which the hydrogen atoms are omitted for clarity

Fig. 2. A view of the tetranuclear cobalt cluster

As depicted in Fig. 3, a 2layer was constructed by2-bmip ligands and [Co4(3-OH)2(2-COO)6] SBUs, which joins six neighboring btc3-ligands with sexadentate coordination mode to form a com- plicated 3framework.Thus, the 3D architecture is detected based on [Co4(3-OH)2(2-COO)6] SBUs with the TOPOS 4.0 program, in which the bmip ligands can be simplified to be linear connectors; btc3-ligands are simplified as topologically 3-con- nected nodes; Each [Co4(3-OH)2(2-COO)6] SBUs is bound by six btc3-ligands and four bmip ligands to act as a 10-connected node, forming an unprece- dented binodal (3,10)-connected network with a point symbol of (3.52)2(34.44.520.614.72.8) (Fig. 4).

Fig. 3. A 2layer constructed by bmip ligands and tetranuclear cobalt cluster

Fig. 4. A view of the unusual 3D binodal (3,10)-connected net of (3.52)2(34.44.520.614.72.8)topology in MCP 1

3.2 FT-IR spectrum

As shown in Fig. 5, the middle band at 3442 cm–1for 1 is the stretching vibration of O-H group. The strong absorptions at 1618 and 1559 cm–1, and 1422 and 1351 cm–1, respectively are characteristic of asymmetric and symmetric stretching bands of carboxylate groups (COO-), and the difference in the value between them is less than 200 cm–1, which indicates that the carboxylate groups behave as the chelate coordination modes.

3.3 X-ray diffraction

The phase purity of MCP 1 was confirmed by powder X-ray diffraction measurements. As shown in Fig. 6, the experimental patterns of 1 are con- sistent with the simulated ones based on the single- crystal X-ray diffraction data.

Fig. 5. IR spectra of MCP 1

Fig. 6. Powder XRD patterns of MCP 1

3.4 Thermal behaviors

To have insight into the changes occurring during heat treatment of the prepared MCP, thermal gravi- metric analysis (TGA) and differential thermal analysis (DTA) of the sample were carried out from 30 to 1000 °C at a heating rate of 10 °C/min. Based on the TGA curve depicted in Fig. 7, MCP 1 shows a slight weight loss of 3.29% in the temperature range of 30～400 °C. It can be ascribed to the loss of one coordinated water molecule per unit cell (Calcd.: 3.30%), which is indicated by an exothermal peak at 220 °C in the DTA curve. The second obvious weight loss from 400 to 600 °C can be attributed to the decomposition of the compound, which is indicated by two endothermal peaks at 440 and 510 °C in the DTA curve. After decomposition, the final residue is 33.35%, which can be attributed to Co2O3and its intermediate product.

Fig. 7. TG-DTA curves for MCP 1

3.5 Photocatalytic property of MCP 1

Inspired by the previous reported studies about Co(II)/Cu(II) coordination polymers for photoca- talytic degradation of organic dyes[17], herein the photocatalytic activity of MCP 1 for degradation of RhB under xenon arc lamp irradiation was also explored. The characteristic adsorption peak of RhB at about 554 nm was selected to monitor the photocatalytic degradation process. As shown in Fig. 8, with the irradiation time increasing from 0 to 60 min, the intensities of the characteristic adsorption peaks at about 554 nm decreased gradually, indicating that the photocatalytic activity of MCP 1 gradually enhanced. As shown in Fig. 9, when the irradiation time reached 60 min, almost 78.2% RhB was degraded. The mechanism for this photocatalytic degradation process was similar with that of the reported coordination polymers[18]. These results indicated that MCP 1 can be used as a visible-light photocatalyst for the degradation of RhB.

Fig. 8. UV-vis absorption spectra of the RhB solution during the decomposition reaction in the presence of MCP 1

Fig. 9. Photocatalytic degradation of RhB solution under visible-light irradiation with the use of MCP 1. The black curve is the control experiment without any catalyst

4 CONCLUSION

In conclusion, we presented a 3D coordination polymer constructed from tricarboxylate and flexible bis-imidazole mixed ligands. MCP 1 is a 3D framework with a rare high-connected (3.52)2(34.44.520.614.72.8) topology based on tetranuclear Co4SBU.Photocatalytic property inves- tigation shows that MCP 1 can be used as a visible-light photocatalyst for the degradation of RhB. Further experiments exploring the structural effects of the spacer length of bis(imidazole) ligands on coordination polymersand any resulting changes in physicochemical propertiesare underway in our laboratory.

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16 March 2018;

8 June 2018 (CCDC 997337)

the National Natural Science Foundation of China (No.21373132, 21603133), Key scientific research project of education department of Shaanxi province (17JS027) and the Science Foundation of Shaanxi University of Technology (No. SLGQD2017-14)

E-mails: jiufulu@163.com and gehg@snut.edu.cn

10.14102/j.cnki.0254-5861.2011-2006