Synthesis, Structures and Properties of Two Metal-Organic Coordination Polymers Derived from Manganese(ΙΙ), Thiabendazole and Polydentate Carboxylic Acids

Two novel binuclear Mn(II) metal-organic coordination complexes [Mn2(TBZ)2(CDC)(C2O4)]n (1), {[Mn2(TBZ)2(BDC)0.5(BTC)(H2O)2]·ET}n (2), (where TBZ = thiabendazole, H2CDC = trans-1,4-cyclohexanedicarboxylic acid, H2C2O4 = oxalic acid, H3BTC = 1,3,5-benzenetricarboxylic acid, ET = ethanol, H2BDC = 1,4-benzenedicarboxylate) have been hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, electrochemical analysis and single crystal X-ray diffraction. The X-ray structure analysis reveals that 1 is s two-dimensional layer and 2 is s one-dimensional chain. In complex 1, it reveals 2-D layers composed of multi-(bidentate) bridging ligands (CDC and C2O4), and in 2, the coordinated BTC ligands adopt a monodentate mode and with BDC ligands adopt alternately chelating Mn1 and Mn2 bridges into 1-D chains. The 3-D structures of the two complexes are stabilized by π-π stacking interactions and hydrogen bonds.


Introduction
During the last decades, chemists have devoted themselves to the development of new crystalline materials with a variety of properties, functions, and potential applications such as gas sorption, luminescence, molecular magnetism, nonlinear optics, catalysis, and ion-exchange [1][2][3][4][5].
To the best of our knowledge, selecting appropriate ligands is the most effective strategy for obtaining coordination polymers. New network structures utilizing both covalent and hydrogen bonds have attracted much interest recently due to their flexible structural features. The auxiliary ligands containing N-donors, such as TBZ, were introduced into reaction systems so as to inhibit the expansion of polymeric frameworks to obtain the desired low dimensional coordination polymers. TBZ has aroused considerable interest in biology and medicine due to its antiproliferative activities [6][7][8][9]. It is an antimicrobial drug belonging to the benzimidazole derivative class, and has exhibited wide applications in human and veterinary medicine. The use of mixed ligands has been demonstrated to be a very effective approach for constructing diverse coordination frameworks [10][11][12][13]. However, the hybrid coordination polymers constructed by TBZ and aryl-acid combinations are rarely reported, although these two ligands are familiar to us. In this paper, we describe the syntheses of two novel binuclear Mn (2), and we report the crystal structures, elemental analyses, electrochemical analysis, IR spectroscopy and thermal properties of these two novel Mn(II) complexes with TBZ ligands [14,15].

Crystal Structure Descriptions
The single-crystal X-ray diffraction analysis reveals that the complexes 1 and 2 crystallize in the triclinic system with space group P-1 and P-1, respectively (Table 1).  Figure 1.

Crystal Structures of Complex 2
The ORTEP drawing of the fundamental building unit of 2 is shown in Figure 5. Single crystal X-ray diffraction analysis shows that the asymmetric unit of 2 consists of two Mn(II) ions, one completely deprotonated [BTC] 3− anion, half of a completely deprotonated [BDC] 2− anion, two coordinated water molecules and a free ethanol. In the structure, each Mn1 center is hexacoordinated by two nitrogen atoms (N4, N5) from one TBZ ligand, two oxygen atoms (O7, O8) from one chelating carboxylate group [BDC] 2− , one oxygen atom (O5) from another monodentate carboxylate group [BTC] 3− and one oxygen atom (O9) from a coordinated water.   (Figure 6b). As complex 2 is a 1-D chain, and the TBZ ligands are arranged alternately on both side of the chain, there will be a rich π-π stacking interaction between these chains. This will produce a kind of π-π stacking interactions, whose distances is 3.563 Å (Figure 7).
Each Mn2 center is five-coordinated by two nitrogen atoms (N1, N2) from one TBZ ligand, two oxygen atoms (O2, O4) from two different monodentate carboxylate groups [BTC] 3− and one oxygen atom (O10) from a coordinated water.   Topology symbol {6 3 } (TD10=166); extended point symbol: [6.6.6]; 3,3-c net; uninodal net; topological type: hcb (Figure 9). The H 2 BDC ligand connects two triangle metal frames, and the distance within the planes of two metal frames is 5.248 Å. It is noteworthy that the adjacent Mn2 metal centers lead to a straight chain and the distance between Mn2-Mn2 is 10.269 Å, and the distance between two chains is 11.696 Å (Figure 8b). The π-π stacking interactions make the 1-D chains to generate 2-D network, and the hydrogen bonds interlink make the 2-D layers to generate 3-D supramolecular architectures (Figure 10).

Thermal Analysis
Thermogravimetric analyses show that compounds 1 and 2 have a low thermal stability as illustrated in Figure 11. Complex 1 displays mainly three weight loss steps. The first starts from 172 to 228 °C with a mass loss of 10.56%, corresponding to loss of a [C 2 O 4 ] 2− group (calcd. 11.37%), then it is stable up to 286 °C. The second stage occurs in the 286-395 °C range with a mass loss of 21.72% (calcd. 22.15%) that correlates with elimination of [CDC] 2− . The third stage in the 516-594 °C range corresponds to release of TBZ molecules with a weight loss of 50.73% (calcd. 51.91%). There is no obvious weight loss before 158 °C in the TG curve of compound 2. Beyond this temperature, two indiscernible processes with a total weight loss of 10.15% corresponding to the combustion of water molecules (calcd. 5.36%) and free ethanol (calcd. 4.24%) was observed. Beyond 308 °C, two indiscernible processes with a total weight loss of 32.48% corresponding to the combustion of [BTC] 3− (calcd. 23.78%), [BDC] 2− (calcd. 9.42%) was observed. When the temperature continues rising, the product lost 44.72% of the total weight beyond 526 °C, which is related to the loss of two TBZs (calcd. 45.33%). Figure 11. The TG curves of the two complexes.

Electrochemical Properties
The electrochemical behavior of complex 1-2 in LiClO 4 (0.05 mol·L −1 ) and ethanol solution has been investigated by cyclic voltammetry in the potential range from −1.8 to 0.3 V. The resulting cyclic voltammogram (CV) of complex 1 is shown in Figure 12a. Only two reduction peaks (E pc = −1.034 V, E pc = −0.712 V), and no significant oxidation peak is seen in complex 1, which shows that Mn II can easily be restored continuously, and is not easily oxidized. Complex 2 displays two pairs of quasi-reversible oxidation and reduction waves with a reduction potential ranging from −0.66 to −0.77 V, −0.97 to −1.18 V and an oxidation potential ranging from −0.17 to −0.26 V, −0.79 to −0.94 V (Figure 12b). The reduction and the oxidation peaks were assigned to the Mn III /Mn II and Mn II /Mn I couples which correspond to two electron-transfer processes.

Materials and Instrumentation
All chemicals were commercial materials of analytical grade and used without purification. Elemental analysis for C, H, and N was carried out on a Perkin-Elmer 2400 II elemental analyzer (Waltham, MA, USA). The FT-IR spectrum was obtained on a PE Spectrum One FT-IR Spectrometer Fourier transform infrared spectrometer in the 4000-400 cm −1 region, using KBr pellets. Powder X-ray diffraction patterns were obtained using a pinhole camera (Bruker, Munich, Germany) operating with a point focused Ni-filtered Mo-Kα radiation in the 2θ range from 5 to 50° with a scan rate of 0.08° per second. Cyclic voltammetry were performed on a CHI 660C electrochemical workstation. Thermogravimetric analysis was performed on a Perkin-Elmer TG/DTA 6300 thermal analyzer under a N 2 atmosphere at a heating rate of 10 °C·min −1 .

Synthesis of [Mn 2 (TBZ) 2 (CDC)(C 2 O 4 )] n (1)
A solution of H 2 CDC (84.1 mg, 0.5 mmol) in DMF (3 mL) was added dropwise with stirring at room temperature to a solution of TBZ (100.3 mg, 0.5 mmol) and MnCl 2 ·4H 2 O (97.9 mg, 0.5 mmol) in the mixture of water (10 mL) and methanol (5 mL). Then an aqueous solution of oxalic acid was added dropwise with stirring to adjust the pH value of the solution to 5. The resulting mixture was sealed in a 25 mL Teflon-lined stainless reactor, and kept under autogenous pressure at 130 °C for 72 h, and then slowly cooled to room temperature at a rate of 5 °C per hour. Yellow block-shaped crystals suitable for X-ray diffraction were isolated directly, washed with carbinol and dried at room temperature (yield: 52%, based on Mn). Anal. Calcd. for C 15

X-ray Structure Determination
Diffraction experiments for 1-2 were carried out with Mo-Kα radiation using a BRUKER SMART APEX CCD diffractometer at 296K. The structures were solved by direct methods and refined with full-matrix least-squares on F 2 using SHELXS-97 and SHELXL-97 [16,17]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed at geometrically calculated positions by using a riding model. A summary of the crystallographic data and structure refinements was shown in Table 1, selected bond lengths and angles of the complexes were listed in Table 2, and hydrogen bond geometries were given in Table 3. Crystallographic data for the structures reported here have been deposited with CCDC (Deposition No. CCDC-953643 (1), No. CCDC-956333 (2). These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, E-mail: deposit@ccdc.cam.ac.uk. Table 2. Selected bond lengths (Å) and angles (°) for 1-2.

Conclusions
In summary, two novel binuclear Mn(II) coordination complexxes were obtained with the aid of thiabendazole and an aromatic carboxylic acid. X-ray structure analysis reveals that complex 1 forms a two-dimensional layer and 2 is a one-dimensional chain. Hydrogen bonds interlinking and π-π stacking interactions make them to generate 3D supramolecular architectures. In addition, electrochemical measurements reveal that the two complexes exhibit good redox potential at room temperature. The thermal decomposition process and powder X-ray diffraction of the complexes were also investigated.