彭艷芬 劉天寶 吳秋艷 姚國建 徐冬冬

（池州學(xué)院化學(xué)與材料工程學(xué)院，池州 247000）

0 Introduction

In recent years,the synthesis and characterization of coordination polymers （CPs）or metal-organic frameworks （MOFs）have attracted widely attention not only due to their structural diversity but also many physical properties with potential applications in many areas,such as ion exchange or absorption[1-2],cation/small molecule solvent recognition[3-5],catalysis[6-7],gas absorption,storage and separation[8-9],luminescence[10].Although more and more coordination polymers have been synthesized, the controlled synthesis of complexes is still a great challenge,since many factors affect the structures and properties of coor-dination polymers.Of all factors,a judicious selection of organic bridging ligands is very fruitful in the crystal engineering[11-12].Because of the diversity of the coordination modes and high-connected nods,polycarboxylate ligandsare used to construct MOFsor CPs[13-16].On the other hand,N-donor ligands also play an important role in synthesizing CPs or MOFs.More and more new flexible or rigid N-donor ligands have been synthesized and used in the synthesis of metal-organic frameworks.Especially,in recent years,the 4-substituted-bis（1,2,4-triazole）derivatives ligands have attracted much more attention of many researchers.We havesynthesized somecoordination polymers based on rigid 4-substituted-bis（1,2,4-triazole）ligand 1,4-bis-（1,2,4-triazol-4-yl）benzene （btx）[17-18],flexible ligand 1,4-bis（1,2,4-triazol-4-ylmethyl）benzene （btrb）[19]and 1,2-bis（1,2,4-triazol-4-yl）ethane （btre）[20-21]. With four possible coordination sites,btre has different coordination modes,such as bis-monodentate,tridentate and quadridentate mode.Ligand btre can adopt cis or trans conformation according to different coordination environments or different metal ions because of its flexibility.Meanwhile,the weak supramolecular interactions such as hydrogen bond,π-π stacking are as important as coordination bond in crystal engineering.These covalent interactions can improve the stability of the network and increase dimensionality[22-24].

Herein,three new coordination polymers{[Zn（μ2-btre）（μ2-adc）]·H2O}n（1·H2O）,[Zn2（μ2-btre）（μ2-1,4-bdc）2（H2O）2]n（2）and[Zn（μ2-btre）（μ2-1,2-bdc）]n（3）were synthesized based on three kinds of dicarboxylate （btre=1,2-bis （1,2,4-triazol-4-yl）ethane,adc=1,3-adamantanedicarboxylate,1,4-bdc=1,4-benzenedicarboxylate,1,2-bdc=1,2-benzenedicarboxylate）（Scheme 1）.1 and 3 show two-dimensional （4,4）networks.2 exhibits 3D network.The thermal stabilities of complexs 1～3 were investigated.

Scheme 1 Structures of ligands H2adc,1,4-H2bdc,1,2-H2bdc and btre

1 Experimental

1.1 Materials and measurement

Ligand btre was synthesized according to the literature method[25].All other reagents are of analytical grade and used without further purification.Elemental analyses for C,H and N were performed on a Perkin-Elmer 240C analyzer.The IR spectra were obtained using KBr pellets on a Nicolet iS10 spectrophotometer in the 4000～400 cm-1region.Powder X-ray diffraction（PXRD）were performed on a D/MAX-3C diffractometer with the Cu Kα radiation （λ=0.154 06 nm,U=40 kV,I=40 mA）over the 2θrange of 5°～50°at room temperature.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 carried out using a Thermal Analyst 2100 TA Instrument and SDT 2960 Simultaneous TGA-DTA Instrument in flowing dinitrogen at a heating rate of 10℃·min-1.

1.2 Syntheses of the complexes

1.2.1 Synthesis of{[Zn（μ2-btre）（μ2-adc）]·H2O}n（1·H2O）

A solution of H2adc （0.2 mmol）in 10 mL of H2O was adjusted to pH=6 with 1.0 mol·L-1NaOH solution and Zn（NO3）2·6H2O （0.2 mmol）was added with stirring.Then 5 mL methanol solution of btre （0.2 mmol）was added and continuously stirred for 10 min.The mixture solution was filtered and stood for one week to give colorless single crystals of 1·H2O （0.033 g,Yield:35%based on btre）.Anal.Calcd.for C18H24N6O5Zn（%）:C,46.02;H,5.15;N,17.89;Found（%）:C,46.05;H,5.11;N,17.80.IR （cm-1）:3 434s,3 113s,2 937s,2 906s,2 855m,1 578s,1 458m,1 383s,1 308s,1 207m,1 201m,1 075m,1 031w,974w,892w,810w,697m,641s.

1.2.2 Synthesis of[Zn2（μ2-btre）（μ2-1,4-bdc）2（H2O）2]n（2）

The synthesis of 2 was similar with that of 1 using 1,4-H2bdc （0.2 mmol）in stead of H2adc.Yield：0.045 g,28.43%based on Zn.Anal.Calcd.for C22H20N6O10Zn2（%）:C,40.08;H,3.06;N,12.75;Found（%）:C,40.03;H,3.10;N,12.79.IR （cm-1）:3 233s,3 101s,1 609s,1 603s,1 565m,1 383s,1 352s,1 270s,1 201 m,1 144w,1 075m,1 081m,892m,842s,748s,635s,540m,489w.

1.2.3 Synthesis of[Zn（μ2-btre）（μ2-1,2-bdc）]n（3）

The synthesis of 3 was similar with that of 1 using 1,2-H2bdc （0.2 mmol）in stead of H2adc.Yield：0.033 g,42%yield based on Zn.Anal.Calcd.for C14H12N6O4Zn（%）:C,42.71;H,3.07;N,21.35;Found（%）:C,42.73;H,3.10;N,21.39.IR （cm-1）:3 436w,3 109s,3 012w,1 627s,1 530s,1 390s,1 360s,1 214m,1 087m,1 032s,898m,832m,771m,716m,643s,582w,491w.

1.2.4 X-ray crystallography

The diffraction data of 1 and 2 were collected on the Xcalibur,Atlas,Gemini,3 on Rigaku Mercury CCD diffractometer with graphite monochromated Mo Kα （λ=0.071 073 nm for 1 and 2,0.071 070 nm for 3）radiation.Intensities were collected by the ω scan technique.The structures were solved and refined by the SHELXTL package[26-27].All non-hydrogen atoms were refined anisotropically and hydrogen atoms were determined with theoretical calculations and refined isotropically.The lattice water of 1·H2O is highly disordered and could not be modeled properly and was removed by the SQUEEZE routine in PLATON[28].The number of lattice water molecules for 1·H2O was deduced from the TGA and elemental analysis.The parameters of the crystal data collection and refinement of 1～3 are given in Tables 1.Selected bond lengths and bond angles of 1～3 are listed in Table 2.

CCDC:1848223,1;1848224,2;1848225,3.

Table 1 Crystallographic collection and refinement parameters of 1～3

Table 2 Selected bond lengths(nm）and angles(°）for 1～3

2 Results and discussion

2.1 Crystal structures of the complexes

2.1.1 Crystal structure of{[Zn（μ2-btre）（μ2-adc）]·H2O}n（1·H2O）

Single-crystal X-ray analysis shows that 1 crystallizes in the triclinic system with P1 space group.The asymmetric unit of 1 consists of one Znギion,two halfμ2-btre ligands and one μ2-adc ligand.Two carboxyl oxygen atoms （O（1）,O（3）A）from two μ2-adc ligands,two nitrogen atoms （N（1）,N（4）from two μ2-btre ligands are coordinated to Znギforming a distorted tetrahedron geometry [ZnO2N2].The bond lengths of Zn-O/N are in the range of 0.195 0～0.209 5 nm （Fig.1a）.In 1,ligand μ2-adc shows bis-monodentate coordination mode and connects two Znギwith the distance of 0.960 3 nm.Ligand btre adopts transconformation and coordinated to two Znギwith the Zn…Zn distance of 1.186 02 nm.Each Znギ links two adc and two btre ligands and expands to a waved 2D（4,4）network （Fig.1b）.The adjacent 2D （4,4）nets were connected to give a 3D net by strong π-π interactions between triazole rings[29].The centroid-tocentroid,perpendicular distances and dihedral angle between two Cg（3）are 0.375 1 nm,0.361 3 nm and 0°,where Cg（3）represents the triazole ring N1-N2-C2-N3-C1.The centroid-to-centroid,perpendicular distances and dihedral angle between two Cg（4）are 0.361 6 nm,0.339 42 nmand 0°,where Cg（4）representsthe triazole ring N4-N5-C5-N6-C4.In addition,the p-πinteractions are found between C9-（H9B）of adc ligand and Cg（3）.The distances of C9 and H9B to centroid of Cg（3）are 0.393 7 nm and 0.299 nm （Fig.1c）.

2.1.2 Crystal structure of[Zn2（μ2-btre）（μ2-1,4-bdc）2（H2O）2]n（2）

Fig.1 （a）Coordination environment of 1;（b）2D （4,4）network of 1;（c）3D network of 1

Fig.2 （a）Coordination environment of 2;（b）Two zigzag chains of 2;（c）3D network of 2

Complex 2 crystallizes in the triclinic system with P1 space group.The asymmetric unit of 2 consists of two Znギions,four half 1,4-bdc ligands,one btre ligand and two coordinated water molecules.There are two crystallographically independent Znギ（Zn（1）and Zn（2）in 2.The Zn（1）/Zn（2）are surrounded by two carboxyl oxygen atoms （O（1）,O（3）/（O（5）,O（7）from two 1,4-bdc ligands,one nitrogen atom（N（1）/（N（4）from btre ligand and one coordinated water molecule （O（9）/O（10）to give a distorted tetrahedron geometry [ZnO3N],respectively （Fig.2a）.The bond lengths of Zn-O/N are in the range of 0.191 2～0.200 3 nm,and the bond angles around Znギa(chǎn)re in the range of 97.46°～122.88°.Ligand 1,4-bdc shows bismonodentate coordination mode and connects two Znギ.Zn（1）atoms and Zn（2）atoms are connected by 1,4-bdc to form two interwoven zigzag chains with the distances Zn（1）…Zn（1）and Zn（2）…Zn（2）of 1.100 24,1.101 04 and 0.110 93,1.090 083 nm,respectively（Fig.2b）.Ligand btre connects Zn（1）and Zn（2）of interwoven zigzag chains in bis-monodentate coordination mode to form a three-dimensional network and the distance of Zn（1）…Zn（2）is 0.613 80 nm （Fig.2c）.Habib and co-worker synthesized 3D network[Zn2（μ2-1,4-bdc）2（μ4-btre）]nby hydrothermal reaction based on the same ligands and same ion.But,btre ligands show quadridentate,and there is no coordinated water molecule in[Zn2（μ2-1,4-bdc）2（μ4-btre）]n[30].

2.1.3 Crystal structure of[Zn（μ2-btre）（μ2-1,2-bdc）]n（3）

Single-crystal X-ray analysis shows that 3 crystallizes in the monoclinic system with P21space group.The asymmetric unit of 3 consists of one Znギion,one btre ligand and one 1,2-bdc ligand.The Zn（1）atom is four-coordinated and surrounded by two oxygen atoms （O（1）,O（4）A）from two 1,2-bdc ligands,two triazole nitrogen atom （N（1）,N（4）B）from two btre ligands in a distorted tetrahedron geometry[ZnO2N2].The bond length of Zn-O/N is in the range of 0.193 5～0.203 5 nm （Fig.3a）.In 3,each btre ligand acts as a gauche-conformation and connects two Znギwith the distance of 0.808 02 nm.Each 1,2-bdc ligand adopts bis-monodentate coordination mode and bridges two Znギ and forms a[Zn（1,2-bdc）]nleft-hand helix with a Zn…Zn distance of 0.739 37 nm （Fig.3b）.Each Znギconnects two btre and two 1,2-bdc ligands and expands to a waved 2D （4,4）network （Fig.3c）.

Ligand btre shows trans-conformation in 1,and gauche-conformation in 2 and 3.The dihedral angle between two triazole rings of btre is 0.00°in 1,45.713°and 61.370°in 2 and 3.The torsion angles of N3-C3-C3C-N3C/N6-C6-C6D-N6D,N3-C3-C4-N6,N3-C1-C2-N6 of btre are 180.00°/-180.00°,-54.083°and 53.758°in 1,2 and 3,respectively.In the three complexes,Znギa(chǎn)toms are all four-coordinated and located at center of distorted tetrahedron geometry.The bond lengths of Zn-O/N and bond angles of O-Zn-N,O-Zn-O,N-Zn-N are in the normal range,which are comparable to the values in others zincギpolymers based on btre ligand[20-21].

Fig.3 （a）Coordination environment of 3;（b）Left-hand helix of 3;（c）2D （4,4）network of 3

2.2 PXRD patterns and thermogravimetric analyses of 1～3

In order to characterize the purity of complexes 1～3,and the as-synthesized samples were measured by X-ray powder diffraction at room temperature.As shown in Fig.4,the peak position of the measured patterns are in good agreement with the simulated patterns,indicating the purity of the samples.The thermal stabilities of 1～3 were examined by thermogravimetric analyses.As shown in Fig.5,1 is stable before 134℃,and the lattice water lost from 134 to 167 ℃ （Calcd.3.83%,Obsd.3.91%）.The framework of 1 was thermally stable upon 297℃.Then the decomposition happened quickly until 540℃.The main residue should be ZnO （Calcd.17.32%,Obsd.18.13%）.The TG curve of 2 showed that the coordinated water lost from 173 to 213 ℃ （Calcd.5.46%,Obsd.5.40%）.The framework of 2 is very stable from 213 to 400℃.Then the weight decreased quickly and did not end until 528℃.The main residue should be ZnO （Calcd.24.69%,Obsd.25.09%）.Before 302 ℃,the weight of 3 decreased very slowly.Then the weight decreased quickly until 800℃,The main residue should be ZnO （Calcd.20.67%,Obsd.20.92%）.

Fig.4 Measured and simulated PXRD patterns of complexes 1～3

Fig.5 TG curves of 1～3

2.3 Photoluminescence properties of 1～3

There is continuous interest in the study of photoluminescence coordination polymers with d10electronic configuration because of their ability to shift,quench or enhance luminescent emission of organic ligands in coordination polymers.So,the solid state luminescence spectra of 1～3 and btre were investigated at room temperature （Fig.6）.The free btre ligand does not show luminescence at room temperature which is the same as the literature[31].The btre luminescence is possibly quenched by the thermal intra-ligand rotations around the C-C and C-N bonds[32-33].Complex 1 exhibits the emission at 442 and 471 nm （λex=370 nm）.Complexes 2 and 3 show strong emission band maxima at 436 and 384 nm upon excitation at 320 and 309 nm,respectively.The strong emission of 2 is blue-shift for 9 nm compared with[Zn2（μ2-1,4-bdc）2（μ4-btre）]n[30].The free H2adc has no obvious emission at the same condition[34].The free 1,4-H2bdc and 1,2-H2bdc exhibit the emissions at 390 and 375 nm,respectively[35].Due to the d10configuration,Znギion is difficult to oxidize or reduce,the emissions are neither metal-to-ligand charge transfer（MLCT）nor ligand-to-metal charge transfer （LMCT）.The emissions of 1～3 may be attributed to the intraligand and ligand-to-ligand charge transition（LLCT）[30,36].

Fig.6 Solid-state emission spectra of the complexes and btre

3 Conclusions

Three zincギcoordination polymers based on different dicarboxylate ligands and flexible 4-substituted bis（1,2,4-triazole）ligand were successfully synthesized and characterized.Complex 1 is a 2D（4,4）network,and the adjacent 2D networks are connected by strong π-π interactions to form a 3D supramolecular architecture.The structure of 2 and 3 are 3D and 2D （4,4）network,respectively.Thermogravimetric analysis showed that the framework of 2 and 3 have better thermal stability.In addition,the photoluminescence investigation shows that three complexes may have potential application as luminescent materials.