Syntheses, structures and properties of two zinc coordination polymers based on bis(triazole) and sulfoisophthalate

Min Li, Yan-Fen Peng, Shan Zhao, Bao-Long Li* and Hai-Yan Li
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China. E-mail: libaolong@suda.edu.cn

Received 18th January 2014 , Accepted 5th March 2014

First published on 7th March 2014


Abstract

Two zinc(II) coordination polymers {[Zn(tmtz)(H2O)4][Zn2(tmtz)2(sip)2]·3H2O}n (1) and [Zn3(tmtz)2(sip)2(H2O)4]·2H2O}n (2) (tmtz = 1,4-bis(1,2,4-triazol-1-ylmethyl)-2,3,4,5-tetramethylbenzene, sip = 5-sulfoisophthalate) were synthesized and structurally characterized. X-ray structural analysis shows that 1 is comprised of two distinct and crystallographically independent polymeric motifs polythreading together. The first motif of 1 is the (3,5)-connected 3D anionic network [Zn2(tmtz)2(sip)2]n2n with point symbol of (42·6)(42·65·83). The second motif in 1 is the [Zn(tmtz)(H2O)4]n2n+ 1D cationic chain. A polythreading array was formed by a (3,5)-connected 3D anionic network and 1D cationic chains in 1. 2 exhibits a (3,4)-connected 2D network with point symbol of (42·8)(42·6·82·10) and constructs a 3D hydrogen bonding architecture. 1 and 2 are effective catalysts for the degradation of methyl orange in the presence of H2O2. The luminescence and thermal analyses are also performed.


Introduction

Current interest in coordination polymers on the basis of the assembly of metal ions and multifunctional organic ligands is rapidly expanding owing to their intriguing architectures and their potential application in the fields of luminescence, gas storage, molecular adsorption, molecular recognition and catalysis.1,2 The assembly of coordination polymer frameworks is mainly affected by the combination of a few factors including the metal ion, organic ligand, metal-to-ligand ratio and auxiliary ligand, solvent and the reaction temperature.1–3 However, it still remains a great challenge to rationally design and construct the desired coordination polymers through controlling the factors that affect their structures. The flexible bis(triazole) ligands are widely used to construct coordination polymers because they can adopt different conformations according to the geometric needs of the different metal ions.4,5

At the same time, the versatile multicarboxylate ligands have been extensively used as multifunctional organic linkers to construct coordination polymers, because they usually have good ligating ability to metal ions and readily adjustable geometry and length.6–8 The strong coordinating ability of the carboxylate groups can result in materials with good thermal stabilities and the functionalization. The coordination polymers based on flexible bis(triazole) N-donor ligands and aromatic multicarboxylate ligands can result in novel topologies and intriguing properties.

On the other hand, the increasing number of coordination polymers reported in the literature has led to new and more complex types of entanglement being recognized in polythreaded species.9 Polythreaded structures are characterized by the presence of closed loops, as well as of elements that can thread through the loops, and can be considered as extended periodic analogues of molecular rotaxanes and pseudorotaxanes, depending on the “ideal” possibility of being extricated.9–15 Polythreaded structures based on 3D components are really rare.15

Our synthetic approach starts by focusing on the construction of new topological frameworks using flexible ligands such as 1,2-bis(1,2,4-triazol-1-yl)ethane (bte),16 1,3-bis(1,2,4-triazol-1-yl)propane (btp),17 1,4-bis(1,2,4-triazol-1-yl)butane (btb)18 and 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene (bbtz).19 The organic ligand 1,4-bis(1,2,4-triazol-1-ylmethyl)-2,3,4,5-tetramethylbenzene (tmtz), a derivative of bbtz, has four extra methyl groups with the steric hindrance and electron pushing ability and can play the key role in the constructing coordination polymers.20

In the present work, we synthesized two zinc(II) coordination polymers {[Zn(tmtz)(H2O)4][Zn2(tmtz)2(sip)2]·3H2O}n (1) and [Zn3(tmtz)2(sip)2(H2O)4]·2H2O}n (2) with same ligands 1,4-bis(1,2,4-triazol-1-ylmethyl)-2,3,4,5-tetramethylbenzene (tmtz) and 5-sulfoisophthalate (sip) by different synthetic methods. 1 exhibits a polythreading array formed by a (3,5)-connected 3D anionic network and 1D cationic chains. 2 exhibits a (3,4)-connected 2D network. The finding of different polymeric topologies in the same crystal is unusual.17,18a,19a,21

Experimental section

Materials and physical measurements

All reagents were of analytical grade and used without further purification. Elemental analyses for C, H and N were performed on a Perkin-Elmer 240C analyser. IR spectra were obtained for KBr pellets on a Nicolet 170SX FT-IR spectrophotometer in the 4000–400 cm−1 region. The luminescence measurements were carried out in the solid state at room temperature and the spectra were collected with a Perkin-Elmer LS50B spectrofluorimeter. TGA was performed on a Thermal Analyst 2100 TA Instrument and SDT 2960 Simultaneous TGA-DTA Instrument under the N2 atmosphere at a heating rate of 10 °C min−1.

Synthesis of {[Zn(tmtz)(H2O)4][Zn2(tmtz)2(sip)2]·3H2O}n (1)

A 10 mL solution of CH3OH–H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1, v/v) was carefully layered over an aqueous solution (10 mL) of Zn(NO3)2·6H2O (0.20 mmol) in a test tube. Then 10 mL of CH3OH solution of tmtz (0.20 mmol) and NaH2sip (0.20 mmol) was adjusted to pH 6 with dilute NaOH solution and was carefully layered above the CH3OH–H2O solution. Colorless single crystals of 1 were obtained in 52% yield (based on tmtz) after the mixture was allowed to stand at room temperature for two weeks. Anal. calcd for C64H80N18O21S2Zn3: C, 45.28; H, 4.75; N, 14.85. Found: C, 45.21; H, 4.67; N, 14.77. IR (cm−1, KBr): 3414m, 3135m, 1622s, 1570m, 1527m, 1441m, 1375m, 1285m, 1249w, 1166w, 1131m, 1032m, 994m, 882w, 836w, 783w, 716w, 676m, 625w, 558w, 458w.

Synthesis of {[Zn3(tmtz)2(sip)2(H2O)4]·2H2O}n (2)

The solution of tmtz (0.10 mmol), NaH2sip (0.15 mmol) and Zn(NO3)2·6H2O (0.15 mmol) in CH3OH (8 mL) and H2O (12 mL) was adjusted to pH 6 with dilute NaOH solution. The mixture was placed in a Teflon-lined stainless steel vessel and was sealed and heated to 120 °C for 24 h, followed by cooling to room temperature at 5 °C h−1. Colorless crystals 2 were obtained in yield 57% (based on tmtz). Anal. calcd for C48H58N12O20S2Zn3: C, 41.68; H, 4.23; N, 12.15. Found: C, 41.58; H, 4.17; N, 12.17. IR (cm−1, KBr): 3421m, 3143m, 1609s, 1568m, 1524m, 1438m, 1373s, 1279w, 1249w, 1164m, 1131s, 1108w, 1033s, 994m, 880w, 781m, 714w, 675m, 627m.

X-ray crystallography

Suitable single crystals of 1 and 2 were carefully selected under an optical microscope and glued to thin glass fibers. The diffraction data were collected on the Rigaku Mercury CCD diffractometer with graphite monochromated MoKα radiation. Intensities were collected by the ω scan technique. The structures were solved by direct methods and refined with full-matrix least-squares technique (SHELXTL-97).22 The parameters of the crystal data collection and refinement of 1 and 2 are given in Table 1. Selected bond lengths and bond angles are listed in Table S1 in ESI.
Table 1 Crystallographic data for 1 and 2
  1 2
Formula C64H80N18O21S2Zn3 C48H58N12O20S2Zn3
Fw 1697.69 1383.29
T/K 293(2) 293(2)
Crystal system Triclinic Triclinic
Space group P[1 with combining macron] P[1 with combining macron]
a 9.691(2) 9.8033(17)
b 14.157(4) 12.462(2)
c 16.055(4) 13.8706(14)
α (°) 107.408(4) 65.881(14)
β (°) 99.937(3) 74.327(16)
γ (°) 104.264(3) 86.14(2)
V3 1963.4(9) 1487.3(4)
F(000) 880 712
Z 1 1
ρcalcd (g cm−3) 1.436 1.544
μ (mm−1) 1.042 1.353
Reflections collected 18976 14181
Unique reflections 7139 [R (int) = 0.0341] 5398 [R (int) = 0.0545]
Parameter 517 404
Goodness of fit 1.098 1.054
R1 [I > 2σ(I)] 0.0624 0.0679
wR2 (all data) 0.1837 0.1701


Catalytic activity of 1 and 2 for the degradation of methyl orange

The catalytic activity is studied according to the literature method.23 1 or 2 (0.10 mmol) and 2 mL of 30% H2O2 were added into a 200 mL of methyl orange (MO) solution (10 mg L−1), of which the pH value was adjusted to 3 with sulfuric acid (0.5 mol L−1), and the temperature was maintained at 318 K using a thermostat. At a given interval, aliquots of the reaction mixture were periodically taken and analyzed with a UV-Vis spectrophotometer at an absorption wavelength of 506 nm. This procedure was repeated in the absence of catalysis 1 or 2 as a blank comparison experiment.

Results and discussion

Crystal structure of {[Zn(tmtz)(H2O)4][Zn2(tmtz)2(sip)2]·3H2O}n (1)

As far as we know, it is quite rare to observe distinct coordination polymers, especially with different chemistry, within the same crystal, affording an unusual packing lattice. X-ray single-crystal diffraction analysis reveals that 1 consists of two distinct and crystallographically independent polymeric motifs polythreading together. The first motif [Zn2(tmtz)2(sip)2]n of 1 is an unusual (3,5)-connected 3D anionic network. The asymmetry unit consists of one Zn(II) atom, one sip3− anion and two halves tmtz. Each Zn(II) atom displays a distorted trigonal bipyramidal coordination geometry (ZnN2O3), coordinated by three oxygen atoms from three sip3− anions and two nitrogen atoms from two tmtz (Fig. S1 in ESI). One carboxylate group (O1O2) of one sip3− anion exhibits the bidentate bridging mode and joins two Zn(II) atoms. The other carboxylate group (O3O4) acts as a monodentate mode and links one Zn(II) atom. The sulfo group is uncoordinated. Each sip3− anion acts as a tridentate ligand to connect three Zn(II) atoms. The Zn(II) atoms are joined by the 3-connected sip3− ligands and extend to form the [Zn2(sip)2]n 1D ladder (Fig. 1a).
image file: c4ra00531g-f1.tif
Fig. 1 (a) The [Zn2(sip)2]n 1D ladder in 1. (b) The [Zn2(tmtz)2(sip)2]n 3D anionic network in 1. (c) Schematic represent the [Zn2(tmtz)2(sip)2]n 3D anionic network in 1. The red balls at the benzene ring center of sip3− ligands exhibit the 3-connected sip3− ligands. The blue long sticks represent the tmtz ligands. (d) The [Zn(tmtz)(H2O)4]n 1D cationic chain in 1. (e) The [Zn(tmtz)(H2O)4]n 1D cationic chains polythread the [Zn2(tmtz)2(sip)2]n 3D anionic network. The blue dash lines show the hydrogen bond interactions.

Two kinds of tmtz ligands all show the trans-conformation and bis-monodentate mode with the Zn⋯Zn separations of 13.559(3) and 15.555(3) Å for N3 and N6 tmtz ligands. Each tmtz ligand bridges two [Zn2(sip)2]n 1D ladders. Each [Zn2(sip)2]n 1D ladder connects four identical [Zn2(sip)2]n 1D ladders by tmtz bridges and extend to form an unusual 3D anionic network (Fig. 1b).

Each Zn(II) atom connects three sip3− anions and two tmtz ligands and is 5-connected. The sip3− ligands are 3-connected and tmtz ligands are 2-connected nodes. According to the simplification principle, the structure of one motif in 1 is binodal with 5-connected (Zn(II) units) and 3-connected (ligands sip3−) nodes and exhibits a fascinating 3D structure (Fig. 1c). The point symbol for the first motif in 1 is (42·6)(42·65·83).24

The second motif in 1 is the [Zn(tmtz)(H2O)4]n 1D cationic chain. The asymmetry unit consists of half Zn(II) atom, half tmtz and two coordination water molecules. Each Zn(II) atom displays a distorted octahedral coordination geometry (ZnN2O4), coordinated by four oxygen atoms from four water and two nitrogen atoms from two tmtz ligands (Fig. 1d). Each tmtz ligand shows the trans-conformation and links two Zn(II) atoms to form a 1D chain with the Zn⋯Zn distance of 15.058(3) Å.

The interesting structural feature of 1 is that there are two chemically and structurally distinct coordination frameworks, the [Zn2(tmtz)2(sip)2]n 3D anionic network and the [Zn(tmtz)(H2O)4]n 1D cationic chains stacking in the lattice. The finding of different polymeric topologies in the same crystal is unusual.17,18a,19a,21 The entanglement of two motifs in 1 can be described as polythreading of a 3D network with 1D chains (Fig. 1e). The hydrogen bond interactions between the coordination water molecules from the 1D cationic chains and the oxygen atoms from uncoordinated sulfo groups of 3D anionic network stabilize the polythreading array (Table S2 in the ESI).

Polythreaded structures with finite components are unusual.9–15 The few species known include polythreaded 0D rings with side arms that give 1D or 2D arrays,10 1D chains of alternating rings and rods (1D → 1D),11 molecular ladders with dangling arms, resulting in (1D → 2D) or (1D → 3D) polythreaded arrays,12 1D chains and 2D sheets (1D + 2D → 2D) or (1D + 2D → 3D)13 and 2D sheets (2D → 3D).14 Polythreaded structures based on 3D components are really rare.15

Crystal structure of [Zn3(tmtz)2(sip)2(H2O)4]·2H2O}n (2)

2 exhibits a (3,4)-connected 2D network and constructs a 3D hydrogen bonding architecture. The asymmetry unit consists of one and half Zn(II) atoms, one sip3− anion, one tmtz, two coordination water and disordered lattice water. Zn1 atom displays a distorted octahedral coordination geometry (ZnN2O4), coordinated by four oxygen atoms from four water and two nitrogen atoms from two tmtz ligands (Fig. S2 in ESI). Zn2 atom shows a distorted trigonal bipyramidal coordination geometry (ZnNO4), coordinated by four carboxylate oxygen atoms from three sip3− anions and one tmtz nitrogen atom (Fig. S2 in ESI). One carboxylate group (O1O2) of one sip3− anion exhibits the bidentate bridging mode and joins two Zn(II) atoms. The other carboxylate group (O3O4) acts as a chelating mode and links one Zn(II) atom. The sulfo group is uncoordinated. Each sip3− anion acts as a tetradentate ligand to connect three Zn(II) atoms. The Zn(II) atoms are joined by the 3-connected sip3− ligands and extend to form the [Zn2(sip)2]n 1D ladder (Fig. 2a).
image file: c4ra00531g-f2.tif
Fig. 2 (a) The [Zn2(sip)2]n 1D ladder in 2. (b) The 2D network in 2. (c) Schematic represent the 2D network in 2. The red balls at the benzene ring center of sip3− ligands exhibit the 3-connected sip3− ligands. The bright green long sticks represent the tmtz ligands. (d) The 3D hydrogen bond network in 2. The blue dash lines show the hydrogen bond interactions.

The tmtz ligands show the trans-conformation, bis-monodentate mode and bridge two Zn(II) atoms (Zn1 and Zn2) with the Zn⋯Zn separation of 13.132(4) Å. Adjacent two [Zn2(sip)2]n 1D ladders are connected by [Zn(H2O)4(tmtz)2] units and extend to generate an unusual 2D network (Fig. 2b).

Each Zn1 atom connects two tmtz ligands and is 2-connected. Each Zn2 atom bonds three sip3− anions and one tmtz ligands and is 4-connected. The sip3− ligands are 3-connected and tmtz ligands are 2-connected nodes. According to the simplification principle, the structure of 2 is binodal with 4-connected (Zn2 units) and 3-connected (ligands sip3−) nodes and exhibits a (3,4)-connected 2D network (Fig. 2c). The point symbol of the 2D network is (42·8)(42·6·82·10).24

There are hydrogen bond interaction between the coordination water and the sulfo group (O8⋯O6, O8⋯O5) and carboxylate groups (O(9)⋯O(4)) of the adjacent 2D layers to result a 3D hydrogen bond architecture (Fig. 2d). There are also hydrogen bonding interactions between coordination and lattice water (O9⋯O4) (Table S3 in the ESI).

The comparison of the syntheses and structures

1 and 2 were synthesized by the diffusional and hydrothermal methods, respectively, using the same ligands tmtz, sip and Zn(NO3)2 which exhibits that the assembly of coordination polymers is affected by the reaction conditions. Analysis of the structures of 1 and 2, one can see that 1 and 2 form similar [Zn2(sip)2]n 1D ladder. The zinc(II) atoms in the ladder all coordinate three sip ligands and show similar distorted trigonal bipyramidal coordination geometry in 1 and 2. The sip ligands all exhibit 3-connected and link three zinc(II) atoms in 1 and 2. Each [Zn2(sip)2]n 1D ladder connects four identical ladders to forms an unusual 3D anionic network in 1. However adjacent two [Zn2(sip)2]n 1D ladders are connected by [Zn(H2O)4(tmtz)2] units to generates an unusual 2D network in 2. The different structures of 1 and 2 are mainly tuned by one small difference between two coordination modes of one carboxylate group of sip ligands in 1 and 2 (monodentate in 1 and chelating in 2) (Scheme 1).
image file: c4ra00531g-s1.tif
Scheme 1 The coordination modes of sip ligands in 1 and 2.

PXRD and catalytic activity of 1 and 2 for the degradation of methyl orange

The measured and simulated PXRDs confirm the purity of 1 and 2 (Fig. S3 and S4 in ESI). To date, most attention has been focused on the Fenton and Fenton-like reaction to oxidize contaminants of concern such azo dyes, and is largely dependent on transition metal based catalysts.23,25,26 The degradation experiment of methyl orange (MO) was tracked by visible spectroscopy and the results are depicted in Fig. 3. It was found that the degradation rates of MO are 70.4% and 50.2% at 360 min in the present of 1 or 2. However, when the research was conducted in the blank experiment, the degradation efficiency of the reaction was reduced to 31.9% in 360 min. Clearly, 1 and 2 are effective catalysts for the degradation of methyl orange in the present of H2O2.
image file: c4ra00531g-f3.tif
Fig. 3 The experiments of the degradation of methyl orange using 1, 2 and blank experiment.

H2O2 is the precursor of hydroxyl radicals, which are effective and highly active oxidizing species. The catalytic reaction mechanism for 1 and 2 are complicate and not very clear because the Zn(II) ion is difficult to oxidize or to reduce due to the d10 configuration. During the catalytic process, the MOFs 1 and 2 may induce H2O2 to generate ˙OH radicals. The ˙OH radical is known to have high activity to destroy the organic dyes. Transition metal-based catalysts for the Fenton and Fenton-like reaction to oxidize contaminants are main the metal–organic frameworks based the metal centers which are easy to oxidize or reduce. The d10 metal-based catalysts for the Fenton and Fenton-like reaction to oxidize contaminants are rare and usual low effective.23,25,26 After the catalytic degradation of the MO solution, the catalysts 1 and 2 can be separated by simple centrifugation for their insolution in water. After catalysis, the PXRD patterns of 1 and 2 are in good agreement with these of the original compounds implying that 1 and 2 maintains their structural integrity after the catalysis reaction, which confirmed that their stability towards catalysis is good (Fig. S3 and S4 in ESI).

Photoluminescence properties

Due to the excellent fluorescent properties of d10 metal complexes, the solid state photoluminescence properties of 1, 2 and the free tmtz ligand were investigated at room temperature, as depicted in Fig. 4. The free tmtz ligand displays the emission band at 310 nm upon excitation at 270 nm,20 which can probably be assigned to the π–π* transitions.26 1 and 2 exhibit the emission bands maximum at 349 and 353 nm, respectively, upon excitation at 290 nm. Because the Zn(II) ion is difficult to oxidize or to reduce due to the d10 configuration, the emissions are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT). The emissions can be tentatively attributed to the intra-ligand charge transition.27,28
image file: c4ra00531g-f4.tif
Fig. 4 The emission spectra of 1, 2 and the free tmtz ligand in the solid state at room temperature.

Thermal analysis

To characterize the coordination polymers more fully in terms of thermal stability, the thermal behaviors of 1 and 2 were examined by TG (Fig. S5 in the ESI). In the TG curve of 1, the lattice and coordination water molecules were lost from 50 to 150 °C (calcd: 7.43%, found: 7.68%). The anhydrous substance was stable upon heating to 280 °C. Then the gradual and rapid weight decrease happened and did not stop until 500 °C in 1. 2 showed that the first weight loss of 7.87% appeared from 40 to 130 °C, which corresponded to the loss of the lattice and coordination water molecules (calcd: 7.81%). The anhydrous substance was stable up to 300 °C. The weight began to decompose quickly above 330 °C and did not stop until 500 °C.

Conclusions

In summary, we synthesized two zinc coordination polymers with same bis(triazole) and sulfoisophthalate ligands using the diffusional and hydrothermal methods. 1 exhibits a polythreading array formed by a (3,5)-connected 3D anionic network and 1D cationic chains. 2 exhibits a (3,4)-connected 2D network. 1 and 2 are effective catalysts for the degradation of methyl orange. The successful syntheses of 1 and 2 show that the structures of coordination polymers can be adjusted by the synthetic methods.

Acknowledgements

This work is supported by the Natural Science Foundation of China (no. 21171126), Natural Science Foundation of An Hui Province (no. KJ2013Z238), the Priority Academic Program Development of Jiangsu Higher Education Institutions and Key laboratory of Organic Synthesis of Jiangsu Province.

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Footnote

Electronic supplementary information (ESI) available: Selected bond lengths and angles, hydrogen bonding, additional figures for the crystal structures and TG curve. CCDC 976274 and 976275. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra00531g

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