Auxiliary Ligands Mediated One- and Two-dimensional Cd(II) Coordination Polymers Incorporating Methyl-3-hydroxy-5-carboxy-2-Thiophenecarboxylate Ligand①

LI Zhao-Hao LI Zhen-Yuan CHEN Yun LU Yuan-Yuan ZHAO Bang-Tun

(College of Chemistry and Chemical Engineering, and Henan Key Laboratory of Function-Oriented Porous Materials, Luoyang Normal University, Luoyang 471934, China)

1 INTRODUCTION

Recently, a great deal of attention has been focused on the assembly of coordination polymers (CPs)owing to their structural richness and potential applications as functional materials[1-4].The rational design of functional ligand and the choice of associated metal-ion are very critical to the target novel CPs.In this regard, nitrogen-containing ligands, such as 1,10-phenanthroline, 2,2?-bipyridine, 4,4?-bipyridine,and related species, have been successfully used as additional metal linkers with carboxylic acids in crystal engineering[5-9].However, the presently known examples of CPs with carboxylic acids and simple 3,5-dimethylpyrazole (dmpz) or 4-amino-1,2,4-triazole (atr) organic ligand are limited[10-14].In crystal engineering, terminal dmpz ligand may coordinate with central metal ions via a similar pyridine-type nitrogen atom, and the other pyrazoletype nitrogen atom (N–H) is often involved in the hydrogen bond formation (scheme 1).In many cases,molecular packing and arrangements are mainly dependent on these hydrogen bonds.Compared with dmpz, the atr ligand may take the bidentate bridging coordination mode using two nitrogen atoms,yielding intriguing polynuclear structures.

Scheme 1. Schematic drawing of the ligands used in this work

On the other hand, thiophene-2,5-dicarboxylic acid and its younger derivatives containing rich coordination sites have been proved to be excellent ligands, which can exhibit versatile coordination modes in the assembly of CPs[15-17].The methyl-3-hydroxy-5-carboxy-2-thiophenecarboxylate ligand(H2L) bears carboxyl, hydroxyl and carbomethoxyl groups in its structure, suggesting its potential use as a bridging ligand within a CP architecture[18].In this contribution, we report two auxiliary N-donor ligand-mediated Cd(II)-containing CPs incorporating H2L, [Cd(L)(dmpz)2]n(1) with a 1D chain structure and {[Cd(L)(atr)0.5(H2O)](H2O)}n(2) with a 2D layer structure based on dinuclear Cd(II) clusters.Their single-crystal structures, spectral properties and thermal stabilities are investigated.Moreover,luminescence properties of compounds 1 and 2 have been studied and discussed.

2 EXPERIMENTAL

All analytical grade chemicals and solvents were purchased and used as received without further purification.The IR spectra were recorded as KBr pellets on a Nicolet Avatar-360 spectrometer in the range of 4000 to 400 cm?1.Elemental analyses for C, H and N were carried out on a Flash 2000 elemental analyzer.Thermogravimetric analyses(TGA) were carried out on a SDTQ600 thermogravimetric analyzer.A platinum pan was used for heating the sample at a heating rate of 10 °C/min under air atmosphere.Fluorescence measurements were recorded with a Hitachi F4500 fluorescence spectrophotometer.

2.1 Synthesis of complex [Cd(L)(dmpz)2]n (1)

Cd(OAc)2·2H2O (26.7 mg, 0.1 mmol), H2L (20.2 mg, 0.1 mmol), 3,5-dimethylpyrazole (19.23 mg,0.2 mmol) and NaOH (2.00 mg, 0.05 mmol) were dissolved in distilled water (4 mL).The resulting solution was sealed in a 20 mL Pyrex glass tube and heated at 80 oC for 72 h, followed by cooling to room temperature at a rate of 5oC·h-1.Block yellow crystals were collected (yield: 56% based on Cd).Elemental analysis calcd.(%) for C17H20CdN4O5S:C, 40.41; H, 3.96; N, 11.09.Found (%): C, 40.43; H,3.99; N, 11.05.IR data (KBr, cm-1): 3516 (s), 2639(m), 1595 (s), 1537 (s), 1460 (s), 1432 (s), 1376 (s),1341 (s), 1167 (m), 921 (m), 834 (m), 750 (m), 732(m), 714 (m), 686 (m).

2.2 Synthesis of complex{[Cd(L)(atr)0.5(H2O)](H2O)}n (2)

An identical procedure with 2 was followed to prepare 1 except that 3,5-dimethylpyrazole was replaced by 4-amino-1,2,4-triazole (16.82 mg, 0.2 mmol).Block yellow crystals were collected (yield:62% based on Cd).Elemental analysis calcd.(%)for C8H10O7N2SCd: C, 24.57; H, 2.56; N, 7.17.Found: C, 24.55; H, 2.58; N, 7.19.Selected IR peaks (in cm?1): 3516 (s), 3355 (m), 3294 (m), 2638(m), 1590 (s), 1533 (s), 1463 (s), 1431 (s), 1374 (s),1341 (s), 1166 (m), 922 (m), 835 (m), 752 (m), 733(m), 715 (m), 686 (m).

2.3 X-ray structure determination

The structures of 1 and 2 were determined by single-crystal X-ray diffraction technique.Diffraction data were collected on an Oxford Diffraction Gemini with MoKα radiation (λ = 0.71073 ?) at 293 K.The structures were solved by direct methods using the Olex2 program as an interface together with the SHELXT and SHELXL programs, in order to solve and refine the structure respectively[19-21].All non-hydrogen atoms were refined anisotropically.The hydrogen atoms on water molecules were located from difference Fourier maps and were refined using a riding model.Other hydrogen atoms were placed at the calculation positions.For complex 1: orthorhombic system, space group Pbca, with a =15.4808(8), b = 14.8407(10), c = 17.4612(8) ?, V =4011.6(4) ?3, Z = 8, Mr= 504.83, Dc= 1.672 Mg/m3,μ = 1.229 mm-1, F(000) = 2032, the final R = 0.0436 and wR = 0.0827 (w = 1/[σ2(Fo2) + (0.0290P)2+3.38P], where P = (Fo2+ 2Fc2)/3), S = 1.073, (Δρ)max= 0.47 and (Δρ)min= ?0.39 e/?3; for 2: monoclinic system, space group C2/c, with a = 17.1867(8), b =9.1342(3), c = 17.3208(8) ?, β = 116.938(6)°, V =2424.1(2) ?3, Z = 8, Mr= 390.64, Dc= 2.141 Mg/m3,μ = 2.006 mm-1, F(000) = 1536, the final R = 0.0233 and wR = 0.0598 (w = 1/[σ2(Fo2) + (0.0280P)2+6.96P], where P = (Fo2+ 2Fc2)/3), S = 1.076, (Δρ)max= 0.38 and (Δρ)min= ?0.67 e/?3.Selected bond and angle parameters and hydrogen bond parameters are listed in Tables 1, 2, 3 and 4, respectively.

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

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

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (°) in 1

Table 4. Hydrogen Bond Lengths (?) and Bond Angles (°) in 2

3 RESULTS AND DISCUSSION

Compounds 1 and 2 were obtained as block yellow crystalline materials by the reaction of H2L and cadmium acetate with dmpz/atr in aqueous medium in a molar ratio of 1:1:2.It should be noted that the weak basic pH values in the starting reaction solutions are essential to successfully prepare these compounds.Attempts to synthesize 1 and 2 in an acidic or neutral pH value failed.The reasons may be because the weak basic environments could facilitate H2L deprotonation and make it easy to coordinate with Cd(II) ions, which was also consistent with the single-crystal X-ray analysis.

Compound 1 is a 1D chain structure crystallizing in the orthorhombic crystal system with space group Pbca.There are one Cd2+ion, two dmpz ligands,and one L2-anion in the asymmetric unit.As shown in Fig.1a, the coordination sphere around the Cd(II)center adopts a distorted square pyramidal environment and the coordination sphere consists of two nitrogen atoms from two different dmpz ligands with Cd?N bond lengths of 2.258(4) and 2.243(4) ?,three oxygen atoms from two L2-anions (Cd(1)?O(3) = 2.184(3), Cd(1)?O(4) = 2.375(3), Cd(1)?O(2)#1 = 2.247(3) ?; symmetry code: (#1) 0.5?x,1?y, z–0.5) (Table 1).The bond angles around Cd(II)ion range from 79.24(12) to 172.13(10)° and the dihedral angle between two dmpz ring planes is 29.285°.

In 1, L2-anions exhibit the μ2-(η1,η0)-(η1)-(η1)coordination fashion to connect adjacent Cd2+ions,thereby generating a 1D chain structure, which is further decorated by terminal dmpz ligands (Fig.1b).Compound 1 is stabilized by significantly strong hydrogen bonding interactions involving carbonyl oxygen atoms and pyrazolyl protons, generating a 2D supramolecular hydrogen-bonded layer structure.Moreover, a 3D supramolecular structure is further constructed by intermolecular C?H···O interactions(C(7)?H(7B)···O(2)#4 and C(10)?H(10)···O(4)#5;symme try code: (#4): 1 – x, 1 – y, 1 – z; (#5): 1 – x,y – 0.5, 0.5 – z) (Table 3, Fig.1c).

Fig.1. (a) Coordination environment of the Cd(II) center.Hydrogen atoms are omitted for clarity.Symmetry codes: (#1) 0.5 ? x, 0.5 ? y, z – 0.5.(b) 1D chain structure.(c) Compound 1 showing N?H···O and C?H···O hydrogen bonding interactions

Compound 2 crystallizes in the monoclinic C2/c space group and exhibits a 2D framework with a sql topology, which is constructed from dinuclear Cd(II)clusters as nodes.In each asymmetric unit, there exist one independent Cd2+ion, one L2?anion, one half a neutral atr linker, one coordinated water molecule and one free water molecule (Fig.2a).Each Cd(II) center is surrounded by one triazole N atom (N1) from one atr linker, six O atoms from three L2?anions and one coordinated water molecule, leaving a pentagonal bipyramidal geometry.The Cd?O bond distances are in the range of 2.281(2)～2.477(2) ? and the Cd?N bond length is 2.356(3) ?, which are in normal range.The O?Cd?O angles range between 34.71(8)° and 154.33(8)°, and the O?Cd?N angles can be found in the 84.86(9)～178.48(10)° range (Table 2).

In compound 2, each L2-anion takes a tridentate μ3-(η2)-(η1)-(η1) bridging fashion to link three Cd2+ions.Two crystallographically equivalent Cd2+ions are bridged by two hydroxyl oxygen atoms form two L2-anions in this fashion to give a basic dinuclear [Cd2O2] cluster with a shorter separation of Cd···Cd (3.5543 ?).The binuclear clusters are connected together through carboxylate/carbonyl oxygen atoms from L2-anions to form a 2D network along the ab plane, which are further decorated by μ2-atr ancillary ligands (Fig.2b).In topology, each[Cd2O2] cluster could be viewed as a 4-connected node to link four other equivalent ones through four L2-anions, thereby forming a 4-connected sql-type network with 44·62topology (Fig.2c).The 2D layers are held together and assembled into a 3D supramolecular architecture through the rich hydrogen bonding interactions involving the carboxylate oxygen atoms, amino nitrogen atoms,thiophene sulfur atoms, and the coordinated and free water molecules (Fig.2d, Table 4).As calculated by the PLATON program, the structure also contains C?H···π interactions involving the C(7)?H(7B)donor group and the triazole ring of atr, O?H···π interactions between O(7)?H(3W) and the thiophene ring of H2L, further consolidating the 3D supramolecular network (d = 2.60 and 2.97 ?, A =122, and 139°; d and A stand for H···π separations and C/O?H···π angles in the C/O?H···π patterns,respectively).

Fig.2. (a) Coordination environment of Cd(II) centers and the dinuclear [Cd2(O)2] cluster unit.Hydrogen atoms are omitted for clarity.Symmetry codes: #1: 1 ? x, y, 0.5 – z; #2: 0.5 + x,0.5 + y, z; #3: 0.5 ? x, 0.5 + y, 0.5 ? z.(b) 2D layer decorated by μ2-atr ancillary ligands.(c) 4-connected 2D sql topology.(d) 3D supramolecular architecture of compound 2

To examine the thermal stabilities of 1 and 2,thermal behaviors were examined by thermal gravimetric analysis (TGA) in a dry air atmosphere from 30 to 700 °C.As seen in Fig.3, compound 1 shows no obvious weight loss until the compound decomposes rapidly at 253 oC, indicating that there are no lattice solvents in 1, as determined by single- crystal crystallographic analysis.Compound 2 undergoes two steps of weight loss, a weight loss of 9.28%from 95 to 195 oC corresponding to the release of water molecules (calc.9.22%) and then another intense weight loss from 195 oC, which is attributed to the decomposition and collapse of the structure.

CPs with d10metal centers usually present good photoluminescent properties with potential applications such as chemical sensors and photochemistry fields.In this work, the luminescent properties of solid 1 and 2 were explored at room temperature and the according results are shown in Fig.4.The main emission peaks are observed at 449 nm (λex=300 nm) for 1 and 441 nm (λex= 273 nm) for 2,

Fig.3. TGA curves for 1 and 2

4 CONCLUSION

In summary, two new CPs based on methyl-3-hydroxy-5-carboxy-2-thiophenecarboxylate and dmpz/atr ligands have been prepared and characterized.Compound 1 is a 1D chain structure, which is further extended into a 3D supramolecular structure by hydrogen bonds.Compound 2 features respectively.According to the previous report, free dmpz and atr ligands do not emit any luminescence[22,23].We have analyzed the photoluminescence property of the free H2L ligand which shows a broad peak at 448 nm (λex= 358 nm) in the solid state at room temperature.The emission peaks of compounds 1 and 2 are slightly shifted and much similar to the free H2L ligand, which may be tentatively attributed to the intraligand emission charge transfer of H2L ligand[24].Compared with the H2L ligand, the enhanced luminescence efficiency is therefore assigned to the complexation of Cd(II) ion and H2L ligand that effectively increase the rigidity of the ligand and reduces the loss of energy via radiation-less decay. The photoluminescent properties of compounds 1 and 2 indicate that they may act as luminescent materials.a 2D network with a 4-connected sql topology,which is assembled into a 3D supramolecular architecture by hydrogen bonds.The results indicate dmpz/atr ligand may act as additional metal linkers to mediate the structures of CPs with carboxylic acids in crystal engineering.Moreover, 1 and 2 exhibit strong photoluminescence and may be good candidates for potential luminescence materials.

Fig.4. Solid-state emission spectra of 1 and 2

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