ZHANG Yong QIN Hong-Ni LI Bo-Long WU Bing

a (China Suzhou Industrial Park Institute of Services Outsourcing, Suzhou 215123, China)

b (State and Local Joint Engineering Laboratory for Functional Polymeric Materials, College of Chemistry,Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China)

ABSTRACT Two copper(II) coordination polymers {[Cu(bib)(nip)]·1.5H2O}n (1) and [Cu2(bib)(glu)2]n (2) (bib= 1,4-bis(2-methyl-imidazol-1-yl)butane, H2nip = 5-nitroisophthalic acid, H2glu = glutaric acid) were synthesized by the hydrothermal method and characterized by single-crystal X-ray diffraction, elemental analyses, IR and solid-state diffuse-reflection spectra. 1 forms a 2D network with the point symbol of (44·62) and 2D 2D polythreaded network. 2 constructs the 6-connected 3D network based on the [Cu2(COO)2] dimer with the point symbol of (44·610·8). The energy band gaps (Eg) of 1 and 2 were 2.453 and 2.162 eV, respectively. 1 and 2 present high photocatalytic activity for the degradation of methylene blue under visible light irradiation.

Keywords: coordination polymer, structure, 3D network, photocatalysis,1,4-bis(2-methylimidazol-1-yl)butane; DOI: 10.14102/j.cnki.0254–5861.2011–2989

1 INTRODUCTION

Water pollution issue is an important environmental problem with the modern industrial development and the population growth recently[1]. The organic dyes from textiles and dye industries are one of the kinds of important contaminations which result into water pollution and environmental problem[2]. The conventional biological treatment methods and physical methods, such as adsorption and coagulation, are the main techniques to remove organic dyes from the wastewater[3]. However, these conventional methods do not eliminate dyes and may produce secondary pollutants.Therefore, the development of new methods to remove organic dyes from wastewater with high efficiency and low cost is very helpful for the wastewater treatment and environment protection.

The inorganic semiconductors such as TiO2, ZnO, CuO,Ag2O, ZnS and CdS have been developed for photocatalytic degradation of organic pollutants under UV light irradiation[4-7]. Since MOF-5 was used as photocatalyst for the degradation of phenol[8], the MOF photocatalysts have been exploited by inorganic and materials scientists for the photocatalytic degradation of organic dyes[9-12]. However,there is still a great challenge to obtain new MOF photocatalysts with high efficiency and low cost under visible light irradiation.

Coordination polymers have gained great interest due to their intriguing topologies and potential applications as functional materials like gas separation and storage[13],nonlinear optics[14], magnetism[15], luminescence[16,17],chemical sensors[18,19]and photocatalysis[20]in recent two decades. The organic ligands and metal central atoms play the key role in the assembly of coordination polymers[21-23].

The combination ligands systems based on N-donor ligands(such as imidazole and triazole ligands) and O-donor ligands(multicarboxylate ligands) and metal central cations can construct coordination polymers with various topologies and multifunctional properties[24,25]. The flexible bis(imidazole)ligand 1,4-bis(2-methyl-imidazol-1-yl)butane is a good Ndonor ligand for the assembly of coordination polymers[26,27].In the present work, the bispodal N-donor imidazole ligand 1,4-bis(2-methyl-imidazol-1-yl)butane and biscarboxylate ligand system was selected for the synthesis of new coordination polymers. Therefore, two copper(II) coordination polymers {[Cu(bib)(nip)]·1.5H2O}n(1) and [Cu2(bib)(glu)2]n(2) were synthesized by the hydrothermal method (bib =1,4-bis(2-methyl-imidazol-1-yl)butane, H2nip = 5-nitroisophthalic acid, H2glu = glutaric acid). The syntheses, crystal structure, and photocatalytic properties were studied.

2 EXPERIMENTAL

2. 1 Materials and general methods

All other reagents were of analytical grade and used without purification. Elemental analyses for C, N, and H were performed on a Perkin-Elmer 240C analyzer. UV-vis diffuse-reflection spectra of the solid samples were collected with a Cary 500 spectrophotometer. IR spectra were obtained for KBr pellets on a Nicolet 170SX FT-IR spectrophotometer from 4000 to 400 cm-1.

2. 2 Synthesis of {[Cu(bib)(nip)]·1.5H2O}n (1)

The aqueous solution (8 mL) of H2nip (1.00 mmol) and bib(1.00 mmol) was adjusted to pH 6 with 1.0 M NaOH solution.Then the aqueous solution (5 mL) of Cu(NO3)2·2.5H2O (1.50 mmol) was added and stirred for ten minutes. The above solution was sealed in a Teflon-lined stainless-steel vessel (25 mL) and heated to 120 ℃ for 72 h. The blue crystals 1 were obtained (Yield: 0.252 g, 48.6% based on bib). Anal. Calcd.for C20H24CuN5O7.50(1): C, 46.38; H, 4.67; N, 13.52%. Found:C, 46.19; H, 4.64; N, 13.45%. IR data (cm-1): 3370m, 3223m,1623s, 1566m, 1534m, 1453m, 1424m, 1364s, 1282w, 1152w,1122w, 1101w, 1015w, 903w, 787m, 737s, 680w.

2. 3 Synthesis of [Cu2(bib)(glu)2]n (2)

The aqueous solution (10 mL) of H2glu (1.00 mmol) and bib (0.50 mmol) was adjusted to pH 6 with 1.0 M NaOH solution. Then the aqueous solution (5 mL) of Cu(NO3)2·2.5H2O (1.20 mmol) was added and stirred for ten minutes. The above solution was sealed in a Teflon-lined stainless-steel vessel (25 mL) and heated to 110 ℃ for 48 h.Block blue crystals 2 were obtained (Yield: 0.159 g, 52.5%based on bib). Anal. Calcd. for C22H30Cu2N4O8(2): C, 43.63;H, 4.99; N, 10.04%. Found: C, 43.54; H, 4.94; N, 9.98%. IR data (cm-1): 1616s, 1534m, 1449m, 1412s, 1314w, 1280m,1163w, 1070m, 805m, 760m, 670s, 651m, 632m.

2. 4 Photocatalytic experiment

The experiment was performed on a PCR-I multipurpose photoreactor equipped a CEL-HXF300 Xe lamp with UV cut-off filter (providing visible light with> 400 nm)(Beijing China Education Au-Light Company Limited, China).The crystalline samples of 1 or 2 (20 mg) and 0.50 mL 30%H2O2were added into the methylene blue (MB) (10 mg/L)solution (100 mL). The suspension solutions were stirred in the dark room for 30 min. Then, the suspension solutions were stirred continuously under visible light irradiation. At a given interval, the aliquots of the reaction solutions were taken and analyzed with a UV-vis spectrophotometer at 664 nm.

2. 5 Structure determination

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

Single-crystal X-ray diffraction analysis showed that 1 belongs to triclinic space group ofP1.The asymmetric unit of 1 consists of one Cu(II) cation (Cu(1), one bib, one nip2-anion ligand, and one and half lattice water. Each Cu(II)cation is coordinated by three carboxylate oxygen atoms(O(1), O(3B), O(4B)) from two nip2-ligands and two imidazole nitrogen atoms (N(2), N(4A)) from two bib ligands in the distorted square-pyramidal geometry (Fig. 1a). Two largest coordination angles are 166.3(2)ofor N(2)–Cu(1)–N(4A) and 160.2(2)ofor O(1)–Cu(1)–O(3B) (Table 1). The trigonal distort from square-pyramidal geometry indexis 0.102, indicating its square-pyramidal geometry. Each bib shows the gauche-anti-gauche conformation and behaves as a 2-nodal ligand with its two imidazole nitrogen atoms (N(2)and N(4)) and joins two Cu(II) cations with the Cu···Cu distance of 12.702(6) Å. One carboxylate group (O(1)O(2) of nip2-exhibits a monodentate coordination mode with O1 coordination atom. Other carboxylate group (O(3)O(4)) of nip2-presents a chelating coordination mode with O(3) and O(4) coordination atoms. Each nip2-also behaves as a 2-nodal ligand and connects two Cu(II) cations with the Cu···Cu distance of 10.154(6) Å.

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

Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for 2

Each Cu(II) cation is coordinated with two bib and two nip2-ligands and presents a 2Dnetwork (Fig. 1b) with the point symbol of (44·62). Two adjacent 2Dnetworks polythreaded each other with their nitro groups and show a 2D2Dpolythreaded network (Fig. 1c).

Fig. 1. (a) Coordination environment of the copper(II) cation in 1. Symmetry cods: A x, y, z–1; B x+1, y, z.(b) Two-dimensional network in 1. (c) 2D 2D polythreaded network in 1

3. 2 Crystal structure of 2

2 crystallizes in the monoclinic space group ofC2/c. The asymmetric unit of 2 consists of one copper(II) cation (Cu1),half bib and one glu2-ligand. The Cu(1) cation is coordinated with four carboxylate oxygen atoms (O(1), O(2A), O(3C),O(4B)) from four glu2-ligands in the square plane and one imidazole nitrogen atom (N(2)) from one bib ligand in the apex position with the square-pyramidal geometry (Fig. 2a).Two largest coordination angles are 167.05(19)ofor O(1)–Cu(1)– O(2A) and 166.6(2)ofor O(4B)–Cu(1)–O(3C)(Table 2). The trigonal distortion from square-pyramidal geometry indexis 0.0075, indicating its square-pyramidal geometry. Two carboxylate groups (O(1)O(2) and O(3)O(4))of one glu2-ligand both perform the bidentate coordination modes and coordinate two copper(II) cations with the Cu···Cu distance of 2.6795(5) Å. One glu2-ligand acts as a tetra-dentate coordination mode and connects four copper(II)cations (Fig. 2b). The copper(II) cations are joined by glu2-ligands and produce a [Cu2(glu)2]ntwo-dimensional network(Fig. 2c).

Each bib exhibits theanti-anti-anticonformation and bridges two copper(II) cations with the Cu···Cu distance of 12.112(2) Å. The [Cu2(glu)2]ntwo-dimensional networks are connected by bib ligands and form a three-dimensional network (Fig. 2d). According to the topological analysis[30],the [Cu2(COO)2] dimer can be simplified as a 6-connected node. The bib and glu2-ligands are 2-connected nodes. The point symbol of 3Dnetwork of 2 is (44·610·8).

Fig. 2. (a) Coordination environment of the copper(II) cation in 2. Symmetry cods: A: –x+1/2, –y+1/2, –z+2; B: –x+1/2, y+1/2,–z+3/2; C: x, –y, z+1/2; D: –x+1, –y+2, –z+2. (b) Coordination mode of glu2- ligand. (c) Two-dimensional network[Cu2(glu)2]n. (d) Three-dimensional network in 2. (e) 6-Connected 3D network in 2. The black red balls show 6-connected [Cu2(COO)2] dimer. Pink and bright green sticks exhibit glu and bib ligands, respectively

3. 3 Solid-state diffuse-reflection spectra and catalytic activity for the degradation of methylene blue

Solid-state diffuse-reflection spectra of crystalline solids 1 and 2 were investigated at room temperature. The absorption data were obtained according to the Kubelka-Munk function using the reflection data. The energy band gaps (Eg) of 1 and 2 were 2.453 and 2.162 eV, respectively, according to the extrapolation of the linear portion of the absorption edges(Fig. 3). The lowEgvalues exhibit that 1 and 2 should have semiconductor nature and photocatalytic activity.

Fig. 3. Solid-state optical diffuse-reflection spectra of crystalline solids 1 and 2

The photocatalytic degradation of methylene blue (MB)using 1 and 2 as catalysts was presented in Figs. 4, 5 and 6.The degradation efficiency was merely 17.7% in the blank experiment (only H2O2) after 105 min visible light irradiation.The degradation efficiencies reached 91.5% and 95.5% using catalysts 1 and 2, respectively, at the same experimental conditions and time (105 min). The photocatalytic experimental result exhibited that 1 and 2 are good photocatalysts for the degradation of MB.

Fig. 4. UV-vis absorption spectra of MB solution using catalyst 1 in the photocatalytic experiment

Fig. 5. UV-vis absorption spectra of MB solution using catalyst 2 in the photocatalytic experiment

In order to explore the catalytic mechanism, the catalytic experiments in the presence of scavenger benzoquinone (BQ),mannitol and ammonium oxalate (AO) were carried out (Figs.6 and 7). The efficiencies of degradation of MB using catalysts 1 and 2 were decreased to 81.6%, 72.8%, 21.7% and 83.3%, 74.9%, 19.2%, respectively in the presence of BQ,mannitol and AO. Because BQ, mannitol and AO are•O2-radical,•OH radical and the hole (h+) scavengers, respectively,the efficiencies of degradation of MB were greatly decreased from 91.5% to 21.7% and from 95.5% to 19.2%, respectively by using catalysts 1 and 2 in the presence of scavenger AO,while they were slightly dropped in the presence of scavenger BQ and mannitol. The results exhibited that the holes (h+) are mainly active matters for the degradation of MB, and the•O2-and•OH radicals have some roles for the degradation of MB.

Fig. 6. Concentration changes of MB solutions in the photocatalytic experiment using catalyst 1,2 and in the presence of scavenger BQ, mannitol and AO, and the blank experiment

Fig. 7. Catalytic degradation efficiencies for MB using catalysts 1 and 2 in the presence of scavenger BQ, mannitol and AO, respectively

The photocatalytic mechanism of degradation of MB was proposed[9-12]. The electrons of the valence band (VB) in the photocatalysts 1 and 2 can be transferred to the conduction band (CB), leaving the holes (h+) in the VB, when the photocatalysts were excited by the visible light with enough energy (equal to or greater than the energy band gaps: 2.453 eV for 1, 2.162 eV for 2). The generated electrons can interact with O2to generate the active species•O2-radicals or interact with H2O2to produce•OH radicals. The holes (h+),•OH radicals and•O2-radicals can oxidize organic dye MB to generate CO2, H2O or other small molecules.

4 CONCLUSION

Two copper(II) coordination polymers were successfully synthesized and characterized. 1 shows the 2Dnetwork with the point symbol of (44·62) and 2D2Dpolythreaded network. 2 exhibits the 6-connected 3Dnetwork based on the[Cu2(COO)2] dimer with the point symbol of (44·610·8). 1 and 2 are good photocatalysts for the degradation of methylene blue.