Synthesis, Crystal Structure and Properties of a Copper(I) Coordination Polymer Based on Benzimidazole Derivative①

JIA Qiang-Qiang LI Jun-Yi ZHANG Hao-Xi CHEN Li-Zhuang WANG Guo-Xi

(School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China)

ABSTRACT A novel copper(I) coordination polymer [C30H18Cu3Cl3N4]n was solvethermally synthesized with 1-(prop-2-en-1-yl)-2-{4-[1-(prop-2-en-1-yl)-1H-1,3-benzodiazol-2-yl]phenyl}-1H-1,3-benzodiazole as the flexible ligand. It crystalizes in orthorhombic system, space group Pbcn with a = 22.0890(7), b = 6.3626(2), c = 18.2480(6) Å, V = 2564.64(14) Å3 and Mr = 365.73. The compound characterized by elemental analysis, infrared spectroscopy and powder X-ray diffraction analysis displays a 2D structure. The complex shows strong green luminescence at 506 nm and better property of electrochemical oxygen evolution.

Keywords: bisbenzimidazole ligand, copper(I) coordination polymer, crystal structure, electrocatalytic property; DOI: 10.14102/j.cnki.0254–5861.2011–2739

1 INTRODUCTION

With the development of heterocyclic chemistry[1-7], benzimidazole complexes have attracted more attention, which are widely applied in drug synthesis, coordination chemistry[8,9]and bioinorganic chemistry[10]fields. Benzimi- dazole is a heterocyclic compound[11,12]with two nitrogen atoms and a non-centrosymmetric structure, which can be used as a monodentate/bidentate[13,14]ligand to form a topology structure with metallic atoms. Due to the special structure, it is easier to form hydrogen bonds between molecules. Therefore, there are abundant hydrogen bonds and several stacking modes in the complexes based on benzi- midazole. The complexes, which are formed by benzimi- dazole and metallic atoms, show some significant properties, such as biological activity, thermodynamics and kinetic stability, outstanding optical properties and coordination ability and other fields[15-23].

Based on the conjugated structure, bisbenzimidazole can be applied in the field of LED. The ligand of benzimidazole was suitably modified and reacted with CuCl to form coordination polymers under solve-thermal reaction conditions. Unfortuna- tely, it didn’t adopt the olefin-copper(I)[24,25]structure. The complex was characterized by IR spectrum, powder X-ray diffraction, fluorescence spectrum and thermal stability analysis. Besides, the electrochemical properties of the complex were also studied in this article. The luminescent test shows that it has a good luminous property and can be further applied in the field of luminescence. Electrochemical property[26-28]test shows that it has potential applications in catalytic oxygen evolution.

2 EXPERIMENTAL

2. 1 Materials and instruments

All reagents were purchased from Sinopharm Reagents, and used without furhter purification. Thermo-gravimetric (TGA) and derivative thermo-gravimetric (DTG) analyses were carried out in the environment of nitrogen in the range of 20～800 °C with a heating rate of 10 °C·min–1. Powder X-ray diffraction data were gathered in the 2θ range of 5～50º at room temperature. Infrared spectrum test was recorded by infrared spectrometer (KBr tablet) with wave number of 500～4000 cm–1. Powder X-ray diffraction test conditions: tube voltage 40 kV, tube current 10 mA, CuKα radiation, wavelength 1.5406 Å, test angle range 5～50°, step 0.02° and scan speed 6 °/min. The sample was ground into powder in an agate mortar and pressed into a tablet press, and the 7 × 2 × 0.4 mm3(length, width and height) sample was cut and tested using a conductive silver paste fixed on a six-claw electrode. The test frequency range was 500 Hz ～ 1 MHz. Fluore- scence properties of the ligand and complex were also studied at room temperature.

A catalyst dispersion was prepared (Fig.1) using a mixture of water (0.2 mL), ethanol (0.3 mL), 5 wt% nafion (0.3 mL) solution and the complex (20 mg), and was ultrasonically shacked for 2 hours. The dispersion (10 µL) was evenly loaded on a freshly polished glassy carbon electrode (area = 0.5 cm2). This electrode was used as a working electrode with a loading of 0.2 mg/cm2catalyst[26].

Fig.1. Fragmentation of coordination polymers

Electrochemical test was performed in a three-electrode electrochemical cell (pine instruments), using Pt wire and Ag/AgCl as the counter and reference electrodes, respectively. To make the test more stringently, the electrolyte solution (1 mol/L) was prepared in a polypropylene bottle using high-purity KOH (99.98%). Prior to detecting the O2species, the high-purity N2gas was bubbled through the solution for 30 min to remove O2from the electrolyte. All measured potentials were calibrated to a reversible hydrogen electrode (RHE) using the following formula:

ERHE = EAg/AgCl+ 0.197V + 0.059pH.

For the oxygen evolution reaction (OER) test, the electrochemical accessibility of the working electrode was optimized by performing a potential cycle between 1.3 and 2.2V at 50 mVs–1in 1 mol/L KOH until a stable voltammetric curve was obtained.

2. 2 Single-crystal X-ray diffraction

A light yellow block single crystal with a moderate size and complete appearance was selected for single-crystal X-ray diffraction analysis. Data collections were performed with an ω scan mode at 296(2) K in the range of 2.4<θ<24.3° on a Bruker SMART APEX-II CCD diffractometer equipped with MoKα radiation (λ = 0.71073 Å). The data were corrected by empirical absorption correction using SADABS. A total of 20294 reflections were collected and 2255 were unique (Rint= 0.040). The final refinement gave R = 0.0860 and wR = 0.2724 for 1636 observed reflections with I > 2σ(I) (w = 1/[σ2(Fo2) + (0.194P)2+ 12.1611P], where P = (Fo2+ 2Fc2)/3), S = 1.073, (∆/σ)max= 0.001, (∆ρ)max= 2.284 and (∆ρ)min= –1.382e/Å3. The crystal structure was solved by direct methods and refined by full-matrix least-squares method on F2by means of SHELXL software package. All non-hydrogen atoms were refined anisotropically and all hydrogen atoms were located and refined geometrically. Furthermore, calcu- lations of distances and angles between some stoms were performed by DIAMOND or SHELXL.

2. 3 Synthsis of the ligand

The ligand was synthesized as described in Scheme 1. PPA (5 mL) and phosphoric acid (20 mL) were added to a round-bottomed flask and heated to 60 °C. Terephthalic acid (1.64 g, 10 mmol) and o-phenylenediamine (2.37 g, 22 mmol) were added to the flask. The mixture was slowly heated to 200 °C and maintained the uniform temperature for 6 h. When reaching 140 °C, the system changed from colorlessness to dark green, which indicated the beginning of the reaction. After cooling to room temperature, the reaction mixture was poured into ice water (400 ml) and stirred for 6 h. When the pH was adjusted to 10 with 30% NaOH solution, a large number of light green crystals were precipitated. The solid component was collected by suction filtration, and recrystallized from DMF, and dried to output the product (3.17 g) with a yield of 79%. ESI-MS (m/z): [M-H]-308.8; [M+H]+309.9.

The above intermediate was added to dried DMF (30 mL) in a flask. The solution of allyl bromide (2.66 g 22 mmol) in DMF (10 mL) was added dropwise to the system in 30 minutes. The mixture was heated to 30 °C and monitored by TLC. After the reaction was completed, the reaction solution was poured into distilled water (200 mL) and stirred for 30 min. The precipitate was collected by suction filtration and eluted by water. The filtrate cake was dried and recrystallized to output the ligand (4.26 g) with a yield of 74%. ESI-MS (m/z): [M-H]-391.2; [M+H]+392.2.

Scheme 1. Synthetic route of the ligand

2. 4 Preparation of the title complex

The ligand (1 mmol), CuCl (1 mmol), methanol (1 mL) and pure water (0.2 mL) were placed in a Pyrex tube. The tube was frozen with liquid nitrogen, evacuated under vacuum, and then sealed by a torch. The tube was kept at 65 °C for 3 days under hydrothermal reaction conditions to obtain high-quality yellow block crystal [C30H18Cu3Cl3N4]nexisting on the tube wall. The yield was about 29%. Anal. Calcd. for C30H18Cu3Cl3N4: C, 49.36; H, 2.41; N, 7.86%. Found: C, 51.13; H, 2.56; N, 7.95%. IR (KBr, cm–1): 3436(b, w), 3047(b), 2923(s), 2850(w), 1621(b, w), 1461(s), 1411(s), 1326(s), 925(w), 856(s), 736(s).

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description

A properly sized single crystal was selected for structure characterization. The crystal data were collected by an X-ray diffractometer at room temperature, showing that the complex crystalizes in orthorhombic crystal system. In particular, twisted hexagons [Cu3Cl3] formed by cross-connection of Cu and Cl are formed (Cl(3)–Cu(1)–Cl(3), 130.57(2)o; Cl(3)– Cu(1)–Cu(2A), 41.21(1)o, Table 1) in the crystal, and the copper chains are formed by copper clusters (Fig.2). Some specific atoms are disordered because of the severe thermal motion. All copper atoms in the crystal are three-coordinated, in which Cu(2) is connected to N(8) in the ligand, and the bond length is 1.968(7) Å, which is within the normal range, and Cu(2A) is connected to two Cl(3) to form a triple coordination environment. Another nitrogen atom in the benzimidazole is occupied by an allyl group, leading to its large steric hindrance that could not participate in coordination, and the crystal adopts a two-dimensional structure. As a bridge atom, Cl(4) connects to the cluster of Cu3Cl3to form a two-dimension structure, which is also a three-coordinated model.

Fig.2. Smallest asymmetric unit (Hydrogen atoms are omitted for clarity). Symmetry codes: A = –x + 1/2, –y + 1/2, z + 1/2; B = x + 1/2, –y + 1/2, –z; C = –x, y, –z + 1/2

Table 1. Parts of Lengths (Å) and Angles (o) for the Title Complex

Fig.3 shows a part of the long chain in the crystal. It can be seen that a Cl atom from the copper cluster connects a H atom from the allyl group to form a hydrogen bond (Cl(3)–H(9A)– C(9), 2.851(3) Å), which makes the structure more stable. As shown in Fig.4 (left), continuous long chains are tiled into a plane on the bc plane. As can be seen, the ligand directions of each chain are exactly opposite, showing a staggered structure. For clarity, Fig.4 (right) is a simplified view of the plane along the a-axis, where the long line represents the ligand.

Fig.3. Chains along the a-axis

Fig.4. Stacking diagram of the complex (left) and simplified diagrams (right, with the long line representing the ligand)

3. 2 Powder X-ray diffraction

To verify the phase purity, the complex was characterized by powder X-ray diffraction (PXRD). After comparison and calculation, the crystal powder X-ray diffraction matches the calculation simulation of the synthetic complex, and can be determined as a pure product (Fig.S4).

3. 3 Fluorescent properties

The luminescent spectrum of the complex is quite distinc- tive. Fluorescence efficiency can be correlated with many structural features of chemicals including π-π* and n-π* transitions, structural rigidity, noncovalent interactions (e.g. hydrogen bonds, π-π interactions[29], and hydrophilic and hydrophobic interactions), interior intermolecular energy transfers, and photo induced electron transfers. As shown in Fig.5, the solid-state fluorescent spectrum of the complex measured at room temperature displays maximal emission peaks at 506 nm. The result suggests that the sample may be promising for green-light emitted materials.

Fig.5. Fluorescence of the ligand and the complex

3. 4 Thermal stability analysis

In order to further understand the thermal stability of the complex, thermogravimetric (black curve) analysis was also performed. Fig.S5 shows that the chemical structure begins to collapse at 400 °C. At the same time, it can be seen from the derivative thermogravimetric (red curve) that the weight loss rate of the complex reaches to the maximum at 400 °C, proving that the structure begins to collapse.

3. 5 Electrochemical properties

Dielectric curves (Fig.6) show that the dielectric constant of the complex increases in a heating run with a temperature range of 223～373 K, and the dielectric constant increases stepwise with increasing frequency at different frequencies (500 Hz ～ 1 MHz). It can be concluded that the complex is a positive dielectric substance.

By testing the oxygen evolution reaction of electrochemical properties, it can be seen from the voltammetric characteristic curve that the voltage at a current density of 10 mA is 1.8164 V and the overpotential is 526 mV (Fig.7). Unfortunately, it is difficult to generate electron transfer in the yoke system, which results in a larger voltage required to maintain the current density. Some improvements to the structure of the ligand will be continued in the later work, and good electro- chemical performance is expected.

Fig.6. Dielectric of the complex

Fig.7. Curve of the volt-ampere characteristic

4 CONCLUSION

By constructing benzimidazole-copper(I) coordination polymer, a novel 2D copper(I) coordination polymer [C30H18Cu3Cl3N4]nwas synthesized via solvethermal me- thod with 1-(prop-2-en-1-yl)-2-{4-[1-(prop-2-en-1-yl)-1H- 1,3- benzodiazol-2-yl]phenyl}-1H-1,3-benzodiazole as flexible ligand. Through the characterization of fluorescence and electrochemical properties show that the coordination poly- mer has potential applications in the field of LED and electro- chemical oxygen evolution.