張雁紅 Adhikari Shiba Prasad Day Cynthia Lachgar Abdou＊,

(1內(nèi)蒙古師范大學(xué)化學(xué)與環(huán)境科學(xué)學(xué)院，內(nèi)蒙古自治區(qū)綠色催化重點(diǎn)實(shí)驗(yàn)室，呼和浩特010022) (2維克森林大學(xué)化學(xué)系，美國北卡羅萊納州，NC 27109)

張雁紅＊,1Adhikari Shiba Prasad2Day Cynthia2Lachgar Abdou＊,2

(1內(nèi)蒙古師范大學(xué)化學(xué)與環(huán)境科學(xué)學(xué)院，內(nèi)蒙古自治區(qū)綠色催化重點(diǎn)實(shí)驗(yàn)室，呼和浩特010022) (2維克森林大學(xué)化學(xué)系，美國北卡羅萊納州，NC 27109)

在溶劑熱條件下，合成了2個(gè)堿土金屬配位聚合物[Ca(tdc)(DMF)2]n(1)和[Ba(tdc)]n(2)(H2tdc=2，5-噻吩二甲酸)，分別用元素分析、紅外光譜、X射線單晶衍射、粉末衍射、熱重分析和熒光光譜對它們進(jìn)行了表征。結(jié)構(gòu)分析表明，配合物1具有4,4連接的二維層狀結(jié)構(gòu)，拓?fù)浞枮?44·62)，而配合物2呈現(xiàn)三維網(wǎng)絡(luò)結(jié)構(gòu)。固體熒光測試表明配合物1比配合物2具有更顯著的熒光性能。

配位聚合物；堿土金屬；晶體結(jié)構(gòu)；熒光

0 Introduction

The design and synthesis of metal-organic coordination polymers(MOCPs)have attracted considerable attention due to their appealing structural,topological novelty and potential applications in functional materials such as storage,separation,catalysis, luminescence,ion exchange,drug delivery,electrical conductivity,and molecular magnetism[1-13].Although many efforts and great progress have been made in the construction of diverse architectures,the rational design and controllable preparation of the coordinationpolymers still remain a great challenge in the field of crystal engineering.It iswell known that the assembly process of MOCPs is deeply influenced by many factors,such as connectivity of organic building blocks ormetal ions,solvents,molar ratio of reactants,temperature,the counter anions,pH value and so on[14-17]. Of all these,the judicious selection of organic linkers, such as flexibility,shape,length,steric effects,and substituent group is crucial to the structural architectures and functionalities of MOCPs.Polycarboxylic ligands are considered to be good candidates because of their versatile coordination modes[18-22].Although a substantial number of coordination polymers incorporating various kinds of aromatic polycarboxylic acids and even including N-heterocyclic derivatives have been reported,there have been far less focus on the investigation of S-containing polycarboxylic acid ligands.Outof the different carboxylate based ligands, 2,5-thiophene dicarboxylate(H2tdc)has two carboxyl groups that may be completely or partially deprotonated,leading to abundant coordination modes and readily adjustable geometries to different metal centers[23-28].The unique V-shaped arrangement of this ligand between the two carboxyl groups enables noncentrosymmetric structures,which is a prerequisite for preparing nonlinear optical materials.Furthermore, the lone electron pair of the hetero-S atom in the H2tdc ligand is more likely delocalized within the thiophene ring and can easily promote charge transfer associated with the target coordination polymers[29].It is expected to be an important intermediate in the developmentof photoluminescentmaterials.

On the other hand,in contrast to abundant research in d-block transition metal or f-block rare earth metal polymers,the coordination behavior and potential applications of alkaline earth metal coordination polymers have remained largely an unexplored area due to their relatively weak complexing ability[30].However,alkaline earth metal ions have considerable advantage for constructing MOCPs,such as low molecular weight,high charge density plus the relative abundance with low cost.Out of the different alkaline earth metals,magnesium and calcium perform numerous biological functions in all life forms and some Mg and Ca-based MOFs exhibit exceptional gas adsorption properties and photoluminescence properties[31].Barium and strontium metals have been known as antagonists for potassium and calcium, respectively[32].

Taking into account all these contexts,we have strategically combined 2,5-thiophenedicarboxylate (H2tdc)organic linkerwith alkaline earthmetal ions to build new metal-organic coordination polymers.In the present paper,two new polymers,namely,[Ca(tdc) (DMF)2]n(1)and[Ba(tdc)]n(2)have been successfully synthesized.Their single crystal structures,solid state thermal studies and fluorescent properties have also been investigated.

1 Experimental

1.1 M aterials and physicalmeasurements

All reagents used in the syntheses were commercially available and used as purchased.Elemental analyses for C,H,N,S were performed on a LECO CHNS-932 Elemental Analyzer.FT-IR spectra were recorded using KBr pellet on a Perkin Elmer Spectrum 100 Spectrometer in the 4 000～600 cm-1region.The powder X-ray diffraction(PXRD)datawere collected on a Bruker D2 PHASER equipped with a graphite monochromator using Cu Kαradiation(λ= 0.154 060 nm)in 2θrange of 7°～50°at room temperature,operated at 30 kV and 10 mA.Thermogravimetric analyses(TGA)were performed on a SDTQ600 V20.9 Build 20 analyzer under nitrogen atmosphere with a heating rate of 10℃·min-1in the range of 30～800℃.The fluorescence spectra weremeasured on a FLS920 fluorescence spectrophotometer.

1.2 Synthesisof the com plex[Ca(tdc)(DM F)2]n(1)

A mixture of CaCl2·4H2O(18.3 mg,0.10 mmol) and H2tdc(17.2 mg,0.10 mmol)was dissolved in 5 mL of DMF/H2O(4∶1,V/V)in a 10 mL glass vial. After ultrasonication for about 30 min,the resulting solution was placed in an autoclave and heated at 90℃for 5 days.After the mixture was slowly cooled to room temperature,a large amount of colorless block crystals of 1 were collected.(Yield:69%based onCa).Elemental analysis Calcd.for C12H16CaN2O6S(%): C,40.44;H,4.52;N,7.86;S,8.99.Found(%):C, 40.39;H,4.60;N,7.80;S;8.94.FT-IR(KBr pellet, cm-1)selected bands:1 597(s),1 564(s),1 524(m), 1 457(w),1 355(s),1 332(s),1 251(w),1217(w),1 121 (m),1 035(m),847(w),776(s),683(m).

1.3 Synthesis of the complex[Ba(tdc)]n(2)

Complex 2 was synthesized with the same procedure as that of 1,except that CaCl2·4H2O was substituted with BaBr2·2H2O.Light yellow crystals of 2 were obtained after filtration,washed with DMF, and dried in air.The yield was 60%based on Ba. Elementalanalysis Calcd.for C6H2BaO4S(%):C,23.44; H,0.66;S,10.43.Found(%):C,23.54;H,0.75;S,10.38. FT-IR(KBrpellet,cm-1)selected bands:1 517(s),1 457 (w),1 360(s),1 334(m),1 316(m),1 248(m),1 215(w), 1 126(w),1 043(m),849(w),805(s),772(s),683(m).

1.4 X-ray crystallographic studies

The single crystals of complexes 1 and 2 weremounted on a Bruker SMARTAPEXCCDwith graphite -monochromatized Mo Kαradiation(λ=0.071 073 nm) by using theωscan technique at 193 K.Empirical absorption corrections were applied by using the SADABS program[33].The structures were solved by direct methods and refined by the full-matrix leastsquares on F2with anisotropic thermal parameters for all non-hydrogen atoms[34].All hydrogen atoms were added in idealized positions and refined isotropically. Further details of the structure analysis were summarized in Table 1.Selected bond lengths and bond angles are listed in Table 2.In complex 2,the sulfur atom of the thiophene ring shows serious disorder over two positions and was refined in two complementary positions with 0.94 and 0.06 occupancies.Regardless of the disorder problems,the results are clearly sufficient to establish the connectivity of themoleculewithout any ambiguity.

Table 1 Crystal data and structure refinement for the com plexes

CCDC:1509673,1;1509674,2.

Table 2 Selected bond lengths(nm)and angles(°)for comp lexes 1 and 2

2 Results and discussion

2.1 Crystal structure of[Ca(tdc)(DM F)2]n(1)

Single-crystal X-ray analysis reveals that the complex 1 crystallizes in a monoclinic system with space group C2/c.As shown in Fig.1a,the asymmetric unit of 1 contains one Caion,one tdc2-anion and two coordinated DMF molecules.Each Caion acquires a distorted octahedral geometry,which is provided by four carboxylate oxygen atoms from four different tdc2-moieties which are occupied in the equatorial position and other two oxygen atoms from the coordinated DMF molecules in the axial position. The bond lengths of equatorial plane are 0.235 77(7) nm(Ca1-O1)and 0.236 22(8)nm(Ca1-O2)respectively.The axial Ca1-O3 bond length is 0.234 19(9) nm.The bond angles around the Cacenter are lying in the range of 81.43(3)°～180.00(3)°.In 1,each tdc2-anion adoptsμ4-η1∶η1∶η1∶η1bis-bidentate coordinationmode(Scheme 1,modeⅠ),connecting four Caions to form an infinite one dimensional chain along the c axis,and the Ca…Ca distance across the bridging tdc2-is 0.896 00(9)nm.Further the 1D chains are linked together by carboxylate groups from two individual tdc2-ligands down the b direction generating a two-dimensional(2D)layer(Fig.1b).The neighboring Ca ions are separated by 0.569 50(6)nm. If the Caion is simplified as a 4-connected node and tdc2-ligand is considered as linear linker,thus the structure of 1 can be described as a 4-connected uninodal netwith Schl?fli symbol of(44·62)(Fig.1c).

Scheme 1 Coordinationmodes of the tdc2-ligand in complexes 1 and 2

Fig.1(a)Coordination environmentof Cacenter in complex 1;(b)View of the 2D layer structure along the a axis;(c)Schematic view of the 4-connected net in 1(teal nodes for tdc2-ligand and purple nodes for Cacenters

2.2 Crystal structure of[Ba(tdc)]n(2)

Complex 2 crystallizes in the monoclinic system, with C2/c space group.As illustrated in Fig.2a,the asymmetric unit of 2 is composed of one Baion and one tdc2-anion.Each Bacenter is hexa-coordinated with the contribution of the six carboxylate oxygen atoms from six different tdc2-ligands which results a distorted octahedral geometry.The Ba-O bond lengths aremeasured in the range of 0.264 59(2)～0.274 78(2) nm,and the bond angles around the Bacenter are lying in the range of 73.84(9)°～180.0(1)°.In 2,The tdc2-ligand displays coordination modesμ6-η1∶η2∶η1∶η2to bridge six Baions through its four oxygen atoms (Scheme 1,modeⅡ),which are different from those in 1.The identical Baions are bridged by theμ3-η1∶η2-tdc2-ligands to form a 2D layer on the bc plane, and the neighboring Ba…Ba distance are 0.456 88(3) nm for Ba1-Ba1iiand 0.595 57(6)nm for Ba1ii-Ba1iv(Fig.2b).Furthermore,the adjacent layers are interco-nnected through the tdc2-ligands to generate a 3D extended network consisting of elongated hexagonal channels with approximate dimensions of 1.013 52 nm×0.792 17 nm(diagonal distances).

Fig.2 (a)Coordination environmentof Bacenter in complex 2;(b)View of the 2D layer structure along the a axis;(c)3D framework of complex 2 along the b axis

2.3 Thermal stabilities and powder X-ray diffraction

To investigate the stability of the complexes 1 and 2,thermogravimetric analysis(TGA)experiments have been performed on single crystal samples under N2atmosphere.As shown in Fig.3,the TGA curve of complex 1 exhibits an initialweight loss(19.2%) between 30 and 450℃,corresponding to the release of coordinated DMF solvent molecules.Then the significant weight loss occurred from 450 to 500℃, which could be attributable to the decomposition of the organic ligand tdc2-.Unlike 1,Complex 2 is stable up to 450℃and undergoes an abrupt weight loss (30.2%)in the temperature range of 450～550℃, which corresponds to the collapse of themain framework.The final residues are detected with the weight of 48.67%for 1 and 69.8%for 2.

Fig.3 TGA curves for the complexes 1 and 2

In order to confirm the phase purity of the bulk materials,powder X-ray diffraction(PXRD)experiments were carried out on complexes 1 and 2 at room temperature.The experimental PXRD patterns(Fig.4) are in good agreement with the corresponding simulated ones except for the relative intensity because of the preferred orientations of the crystals.

Fig.4 PXRD patterns of the complexes 1 and 2

2.4 Photolum inescent properties

Photoluminescent coordination polymers have aroused great interest with their various applications in chemical sensors,photochemistry and electroluminescent display[34-35].Therefore,the luminescent properties of complexes 1 and 2,as well as free H2tdc ligand were investigated in solid state at room temperature.As shown in Fig.5,the free H2tdc ligand shows a wide emission band in the range of 325～375 nm with amaximum at 344 nm(λex=302 nm),which maybeattributed to theπ*→n orπ*→πtransition[36-37]. Upon an excitation band at 302 nm,intense emissions are observed at 350 nm for 1 and 354 nm for 2, respectively.The emissions for the two complexes are quite similar to that of the free H2tdc ligand in terms of position and band shape.The enhancement of luminescence intensity compared to the free ligand perhaps result from the coordination interactions of the ligand to the metal center,which effectively increases the rigidity of the ligand and reduces theloss of energy by radiationless decay[38-39].Moreover,it can be seen that the calcium complex exhibits stronger emission intensity than the barium complex even the same ligand has been used,which might be resulted from the coordination mode diversities of the ligand and difference ofmetal ion radii.

Fig.5 Solid-state emission spectra of H2tdc ligand and complexes 1 and 2 at room temperature

3 Conclusions

In summary,two new alkaline earth metal coordination polymers based on 2,5-thiophene dicarboxylate ligand have been successfully synthesized and characterized.The crystal structural analyses indicate that complex 1 possesses a two-dimensional (2D)layer structure because the solvent DMF participate in the coordination and restricts the extension of the polymer to form 3D network.In complex 2,the metal ions organized by tdc2-ligand giving rise to a fascinating 3D framework.Thermogravimetric analysis reveals that complex 2 shows better thermal stability than complex 1.In addition,Complex 1 exhibits stronger emission energy than complex 2 which may be due to the coordination diversities of the ligand and the rigid differences of the two coordination frameworks.The results demonstrate that coordination mode,solvents,and metal centers are critical to the assemblies of MOCPs in some systems.Further experiments exploring the effect of selectivity of the auxiliary ligand,temperature,pH value and other subtle reaction condition changes on coordination polymers,are underway in our laboratory.

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Syntheses,Crystal Structures and Characterization of Caand BaCoordination Polymers Derived from Thiophene-2,5-dicarboxylate

ZHANG Yan-Hong＊,1Adhikari Shiba Prasad2Day Cynthia2Lachgar Abdou＊,2
(1College of Chemistry and Environment Science,Inner Mongolia Key Laboratory of Green Catalysis, Inner Mongolia Normal University,Hohhot 010022,China)
(2Departmentof Chemistry,Wake Forest University,Winston-Salem,NC 27109,USA)

Two new alkaline earthmetal coordination polymers[Ca(tdc)(DMF)2]n(1)and[Ba(tdc)]n(2)(H2tdc=2,5-thiophene dicarboxylate)have been synthesized under solvothermal condition.They were characterized by elemental analysis,IR spectroscopy,single crystal X-ray diffraction,powder X-ray diffraction,thermogravimetric analysis,and fluorescent analysis.Complex 1 exhibits a two-dimensional(2D)layer structure with a uninodal (4,4)-connected(44·62)topological type;whereas,complex 2 features a 3D framework consisting of elongated hexagonal channels.Solid-state photoluminescent properties indicate that complex 1 shows noticeable fluorescent emissions upon excitation in comparison to that of complex 2.CCDC:1509673,1;1509674,2.

coordination polymer;alkaline earthmetal;crystal structure;fluorescence

O614.23+1；O614.23+3

A

1001-4861(2017)07-1305-08

10.11862/CJIC.2017.158

2017-03-08。收修改稿日期：2017-05-27。

內(nèi)蒙古自治區(qū)自然科學(xué)基金(No.2014BS0206)和國家留學(xué)基金委(No.201408155050)資助項(xiàng)目。＊

。E-mail：zhangyh@imnu.edu.cn，lachgar@wfu.edu