Ionothermal Synthesis, Crystal Structure and Photocatalytic Property of a New Cobalt Coordination Polymer①

ZHANG Tian XUE Li-Ping

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Ionothermal Synthesis, Crystal Structure and Photocatalytic Property of a New Cobalt Coordination Polymer①

ZHANG Tian XUE Li-Ping②

(471022)

The reaction of flexible bis(imidazole) ligand and 1,2-bis(imidazol-1?-yl)methane (bimm) with Co(II) salt under ionothermal method resulted in the formation of a new coordination polymer {[Co(bimm)3]·(PF6)2}n(1). X-ray single-crystal diffraction determination reveals that 1 crystallizes in the triclinicspace group, with= 8.647(6),= 12.092(9),= 14.967(11) ?,= 88.912(8),= 81.199(8),= 89.395(8)°,= 1546 (2) ?3,= 2,M= 793.39,D= 1.704 Mg/m3,= 0.768 mm-1,(000) = 798, the final= 0.0626 and= 0.1634 for 4319 observed reflections with> 2(). In compound 1, the Co(II) ion is connected to another Co(II) by two bimm ligands to form 1double chains which are further linked by bimm ligands to form a 2wavelike layer. Topologically, the structure of 1 represents a uninodal 24-connected sql/Shubnikov tetragonal plane net. Moreover, thermogravimetric analyses and photocatalytic property for 1 have also been investigated.

cobalt coordination polymer, crystal structure, photocatalytic property;

1 INTRODUCTION

To date, considerable attention given to coordina- tion polymers (CPs) stems from not only their intrinsic aesthetics beauty but also their potential applications[1-3]. Generally, these CPs are prepared under hydro(solvo)thermal synthesis, often in sealed autoclaves under autogenous pressure. By far the most common solvent used is water, but many other organic solvents such as N,N-dimethyl-formamide, acetonitrile, methanol and ethanol have been tried[4-7]. However, the precursors of CPs usually exhibit low solubility in traditional hydro(solvo)the- rmal synthesis system, which is intrinsic shortco- mings. Up to now, ionothermal synthesis,where ionic liquids serve as solvents instead of water or organic solvents,have provided opportunities to develop new functional CPs in the context of crystal engineering[8-12]. As is known, ionic liquidscan not produce autogenous pressure, even at elevated temperature, owing to little or no volatility.So CPs can be synthesized at elevated temperature yet ambient pressure in the ionothermal synthesis. Moreover, ionic liquids may act as structure-direc- ting templates and result in completely different framework structures in the CPs self-assembly. In this regard, common “old” ligands also have chance to construct new CPs in the field of crystal en- gineering.

At present, the flexible ligand bimm has been justified as an efficient and versatile linker for the construction of CPs in hydro(solvo)thermal syn- thesis[13-17]. However, CPs based on the ligand with varied structures and topologies could be achieved by changing the assembly environments. On the other hand, the 2,5-thiophenedicarboxylic acid (H2tdc) as multicarboxylate ligand with monodentate, bridgingbidentate and bridging tridentate is a useful building block in the construction of CPs[18-20]. Here, the reaction of cobalt salt, CPs and bimm ligand under ionothermal conditions yielded a new coordination polymer of {[Co(bimm)3]·(PF6)2}(1). The compound features a 2wavelike layer, which can be simpli- fied as a uninodal 24-connected sql/Shubnikov tetragonal plane netwith {44.62} topology. Moreover, photocatalytic property for 1 has been investigated.

2 EXPERIMENTAL

All reagents were obtained commercially and used without further purification. The hydrothermal reaction was performed in a 25 mL Teflon-lined autoclave under autogenous pressure. Elemental analyses for C, H, and N were carried out on a Flash 2000 elemental analyzer. IR spectra were recorded as KBr pellets on a Nicolet Avatar-360 spectrometer in the range of 4000～400 cm-1. Thermogravimetric analyses (TGA) were carried out on a SDTQ600 thermogravimetric analyzer. A platinum pan was used to heat the sample at a rate of 10 ℃/min under a N2atmosphere.

2. 1 Synthesis of compound {[Co(bimm)3]·(PF6)2}n(1)

A mixture of H2tdc (34.4 mg, 0.2 mmol), bimm (29.6 mg, 0.20 mmol), Co(NO3)2·6H2O (58.2 mg, 0.2 mmol) and 500.0 mg 1-butyl-3-methylimidazo- lium hexafluorophosphate was sealed in a glass tube and heated to 120 ℃for 3 days, and then the mixture was cooled naturally to form purple block crystals of 1 (yield: 12% based on Co). Elemental analysis calcd. (%) for C21H24CoF12N12P2: C, 31.76; H, 3.03; N, 21.18%. Found: C, 31.72; H, 3.05; N, 21.36%. IR data (KBr, cm-1): 3139(w), 1658(w), 1572(w), 1528(w), 1403(m), 1340(w), 1289 (w), 1228(m), 1171(m), 1084(m), 1033(m), 934(w), 839(s), 750(m), 716(w), 678(w).

2. 2 X-ray structure determination

The structure of 1 was determined by single- crystal X-ray diffraction technique. Diffraction data were collected on a Bruker SMART Apex CCD diffractometer with Mo-radiation (= 0.71073 ?) at 293(2) K using a phi andscan mode. Data reduction and absorption correction were made with SADABS software[21]. The structure was solved by direct methods and re?ned by full-matrix least- quares techniques using SHELXL-97[22]. All non- hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were placed at the calculation positions. Selected bond distances and bond angles of 1 are summarized in Table 1. Crystal data for 1: triclinic, space group,= 8.647(6),= 12.092(9),= 14.967(11) ?,= 88.912(8),= 81.199(8),= 89.395(8)°,= 1546(2) ?3,= 2,M= 793.39,D= 1.704 Mg/m3,= 0.768 mm-1,(000) = 798, the final= 0.0626 and= 0.1634 for 4319 observed reflections with> 2()and= 0.0828 and= 0.1787 for all data. (Δ)max= 0.785, (Δ)min= –0.738 e/?3, and= 1.058.

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

Symmetry transformation:#1: –, –+2, –+2; #2: –, –+2, –+1; #3:–1,,

3 RESULTS AND DISCUSSION

X-ray crystallographic data show that compound 1 crystallizes in triclinic space group. The structure of 1 consists of a 2[Co(bimm)3]ncationic layer and PF6-counterions. There are one indepen- dent Co(II) ion, threebimm ligands, and two uncoordinated PF6-anionsin the asymmetric unit. As shown in Fig. 1, each Co(II)center is coordinated to six imidazole nitrogen atoms from six bimm ligands andthe geometry around the Co(II)center can be best described as a slightly distorted octahe- dronwith theN–Co–N angles ranging from 86.23(13) to 93.55(14)°. The Co–N bond lengths range from 2.147(4) to 2.218(4) ?, with an average value of about 2.182 ?, which are within the normal ranges found in other Co(II)complexes.It should be noted that the coordination mode of bimm ligand may adopt two kinds of configurations (- or-). In complex 1, the bimm1 ligand adopts a-configuration (Scheme 1a) to coordinate to two Co atoms into a 1chain structure with an adjacent Co···Co distance of 8.647(6) ? along the-axis (Fig. 2a). Then the 1double ladder-like chain is further formed through the bimm2 ligands and Co(II) atoms (Fig. 2b), in which the bimm2 ligand also exhibits a-configuration (Scheme 1b). The corresponding dihedral angles between imidazole rings are 89.785° for bimm1 and 82.108° for bimm2, respectively. Such 1ladder-like double chains are further arranged parallel to each other and are further linked by bimm3 ligands to form a 2wavelike layered network along the crystallographicplane (Fig. 2c). It is noted that bimm3 ligand exhibits a- configuration (Scheme 1c) and the corresponding dihedral angle between imidazole rings is 78.526°. The hexafluorophosphate counterions are encapsula- ted in the rhombohedral grid, which mayeither neutralize the overall charge in the solid or serve as a template. Topologically, Co(II) ions could be re- gardedas 4-connected nodes, and the bimm ligands as linear linkers. As a result,1 represents a uninodal 24-connected sql/Shubnikov tetragonal plane net with {44.62} topology (Fig. 3).

Scheme 1. (a) cis-bimm1, (b) cis-bimm2 and (c)trans-bimm3

Fig. 2 . (a) 1D chain structure connected by bimm-1 ligands (b) A 1D double ladder-like chain (c) 2D Co-bimm layer structure

Fig. 3 . Schematic representation of the 2D 4-connected sql topology

During the assembly of 1, it is noteworthy that the addition of H2tdc is very important, although H2tdc has not incorporated in the structure. When H2tdc was removed from the synthesis system, attempts to obtain 1 were unsuccessful.Similar results related H2tdc have not been reported, but the results for others were reported in literature. Moreover, only precipitate unsuitable for the study of X-ray dif- fraction was obtained under hydrothermal con- ditions. Complex 1 was insoluble in water and com- mon organic solvents such as methanol, ethanol, toluene, acetonitrile and N,N-dimethylformamide. Thermogravimetric (TG) analyses of complex 1 were performed on polycrystalline samples under a N2atmosphere (Fig. 4). No weight loss is observed in the temperature ranges of 30～155 ℃. It is in good agreement with the crystal structure of 1, in which no solvents are included. Above 155 ℃, the sample suffered an abrupt weight loss, indicating the decomposition of organic components and the collapse of the network.

Recently, using CPs as catalysts has attracted a great deal of interest[23, 24]. Under UV irradiation,the photocatalytic experiment of 1 was carried out with photodegradationof the methylene blue through a representative route: 50 mg crystalline powder of 1 was placed in 90 mL (5.0 mg·L?1) methylene blue solution in the dark, magnetically stirred in order to ensure the adsorption-desorption equilibrium. Then the whole mixture was exposed to UV irradiation and kept continuously stirring. 1.5 mL of sample was successively taken for analysis every 10 min. The photocatalytic performance of 1 and control experiments on the photodegradation of methylene blue are shown in Fig. 5. No significant change in the degradation of methylene blue was observed in the dark or without 1. It should be noted that 1 is a photocatalytic material to degrade methylene blue with a conversion of more than 75% within 9 h in the distinctly shortened degradation time.

Fig. 4 . TGA curve for 1

Fig. 5 . Degradation profiles of methylene blue under UV irradiation in the presence of (a) 1, (b) without 1 and (c) in the dark

4 CONCLUSION

In conclusion, a new 2cobalt coordination polymer has been synthesized and characterized. The complex is the first example of coordination polymer constructed only from simple organic bimm ligand. Furthermore, thermogravimetric analysis and photocatalytic properties of the com- plex are also investigated.

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9 September 2014; accepted17 November 2014 (CCDC 1022901)

①This work was supported by the Natural Science Foundation of Henan Province (132300410326) and the Foundation of the Education Department of Henan Province (13A150801 and 14A150040)

. Doctor, majoring in material chemistry. E-mail: lpxue@163.com

10.14102/j.cnki.0254-5861.2011-0050