三個不同穿插的鋅配合物的合成、結構和熱動力學分析

何甜　岳可芬，*　陳三平　周春生　晏妮

（1西北大學化學與材料科學學院，教育部合成與天然功能分子化學重點實驗室，西安710069；2商洛學院化工與現代材料科學學院，陜西省尾礦資源利用重點實驗室，陜西商洛726000）

三個不同穿插的鋅配合物的合成、結構和熱動力學分析

何甜1岳可芬1，*陳三平1周春生2，*晏妮1

（1西北大學化學與材料科學學院，教育部合成與天然功能分子化學重點實驗室，西安710069；2商洛學院化工與現代材料科學學院，陜西省尾礦資源利用重點實驗室，陜西商洛726000）

使用醋酸鋅，柔性的1，4-二甲基咪唑丁烷（bib）和三個剛性直鏈型羧酸混合配體，在溶劑熱條件下合成了三個具有不同穿插結構的配合物。并通過元素分析，紅外，X射線單晶衍射進行了表征。配合物1是一個具有三種Z字鏈的四重穿插結構，配合物2是一個特殊的［2+2］型四重穿插結構，配合物3是一個具有雙核結構單元的三重穿插結構。通過使用熱重分析/微分熱重和差示掃描量熱（TG/DTG-DSC）技術研究了它們的熱分解過程，由熱重分析得出特殊的［2+2］型四重穿插結構穩定性最好，四重穿插結構比三重穿插結構穩定。使用Kissinger和Ozawa-Doyle法對配合物骨架坍塌過程進行了計算，得出配合物1－3的表觀活化能分別為276.887、318.515、149.310 kJ?mol－1，可以得出配合物1－3的反應速率關系為3>1>2。從熱力學和動力學的角度來說明配合物的結構穩定性。其次，還對配合物1－3的熒光性質進行了表征。

配合物；穿插；熱力學；動力學

1　lntroduction

Very young children are fascinated with building blocks because elaborate structures may be created from far simpler objects. It is with the same enthusiasm that chemists have much interest in coordination chemistry due to the versatile coordination modes of complexes1－4.Coordination polymers（CPs）with an entangled structure have been given great attention in recent years owing to their intriguing features and topologies5－7as well as their unique properties，such as in catalysis8，chiral separation9，luminescence10，nanoscale magnetism materials11，etc.It has long been considered that entangled systems are common in nature and a major theme of CPs while interpenetrating net is one of the most investigated types of entanglement with a major advantage in the achievement of stable crystalline structures.

Thermal analyses of solid materials are very important in understanding their thermal behavior，which could be useful in estimating upper temperature limits for devices operating at elevated temperatures，in predicting the ageing of concretes，in determining the conditions under which explosive materials can be handled in safety or when solid catalysts will work most efficiently and so on12.

As reported previously，the ligand 1，4-bis（2-methylimidazol-1-yl）butane（bib）with a flexible―（CH2）4―backbone can adopt mutable conformations and geometries via bending，rotating，or twisting to produce diverse structures，such as interpenetrating，helical and catenating network13－16.In addition，we find that rigid line-shaped carboxylate ligand would be a powerful precursor for the construction of interpenetrating structure15.In this work，three different interpenetrating CPs have been obtained.Their thermal decompositions were studied using the TG/DTG-DSC coupling techniques.The thermodynamics and kinetics of skeleton collapse for the complexes were calculated by Kissinger?s method and Ozawa-Doyle?s method.In light of thermodynamics and kinetics，the structural characteristics of the complexes have been expounded.

2　Experimental

2.1Materials and general methods

1，4-Bis（2-methyl-imidazol-1-yl）butane was prepared by published procedures（w=98.57%）16.All other chemicals for synthesis were commercially available.Elemental analyses for C，H，and N were performed on a Perkin-Elmer 2400C CHNS/O elemental analyzer made by America.The IR spectra were recorded using KBr pellets on a Nicolet Avatar 360 FTIR spectrometer made by America.Powder X-ray diffraction（PXRD）data were collected on a Bruker D8-ADVANCE made by Germany using Cu Kαradiation（λ=0.15418 nm）.Luminescent spectra were performed on a Perkin-Elmer LS55 luminescence spectrometer made by America.The thermogravimetry（TG）and differential scanning calorimetry（DSC）were performed on a Q600 SDT thermal analyzer instrument made byAmerica.

2.2Synthesis of complexes 1－3

2.2.1｛［Zn2（bib）2（1，4-ndc）2?H2O］｝n（1）

A mixture of Zn（Ac）2?2H2O（0.051 g，0.1 mmol），1，4-H2ndc （w=99.99%，0.022 g，0.1 mmol）and bib（0.044 g，0.1 mmol）in ethanol（EtOH）（w=99.80%，8 mL）and water（2 mL）was heated in a stainless steel reactor with Teflon liner（25 mL）at 150°C for 36 h and cooled to ambient temperature at a rate of 10°C?h－1. Colorless block crystals of 1 were isolated by washing with mother liquor and dried in air.The yield was 81.34%based on Zn. Anal.Calc.（%）for C48H46Zn2N8O8（993.71）:C，57.96；H，4.63；N，11.27.Found（%）:C，60.41；H，5.33；N，13.58.IR date（KBr，cm－1）: 3439（m），2939（m），1602（m），1401（m），1355（s），1003（m），828 （m），762（m），587（m）.

2.2.2｛［Zn0.5（bib）0.5（bdc-Br2）0.5］?0.5H2O｝n（2）

The same synthetic method as that for 1 was used except that 1，4-H2ndc was replaced by H2bdc-Br2（w=99.99%，0.032 g，0.1 mmol）.Colorless block crystals of 2 were isolated by washing with mother liquor and dried in air.The yield was 79.60%based on Zn.Anal.Calc.（%）for C20H20ZnBr2N4O7（653.59）:C，36.72；H，3.06；N，8.57.Found（%）:C，36.47；H，4.19；N，8.66.IR（KBr，cm－1）:3489（s），3129（w），1640（s），1467（m），1377（s），1054（m），825（m），676（m），549（m）.

2.2.3｛［Zn2（bib）（4，4′-bpdc）2］?H2O｝n（3）

The same synthetic method as that for 1 was used except that 1，4-H2ndc was replaced by 4，4?-H2bpdc（w=99.99%，0.029 g，0.1 mmol）.Colorless block crystals of 3 were isolated by washing with mother liquor and dried in air.The yield was 57.24%based on Zn.Anal.Calc.（%）for C40H34N4O9Zn2（845.45）:C，56.77；H，4.02；N，6.62.Found（%）:C，58.69；H，3.88；N，7.51.IR（KBr，cm－1）:3396（m），3124（s），2939（m），2864（m），1872（m），1820 （w），1623（s），1559（m），1303（s），1280（s），1123（m），1003（m），901（m），757（s），608（m），458（m）.

2.3Crystallography

Suitable single crystals of 1－3 were carefully selected under an optical microscope and glued to thin glass fibers.The single crystal X-ray data were collected on a Bruker APEX CCD diffractometer using graphite monochromated Mo-Kαradiation（λ=0.07107 nm）at 296 K.The structures were solved by the direct methods with SHELXS-97 and refined by full matrix least squares refinements based on F2 17.All non-hydrogen atoms were refined anisotropically with the hydrogen atoms added to their geometrically ideal positions and refined isotropically.The final chemical formulas of 1－3 were confirmed by the elemental microanalyses，infrared spectroscope data，and thermogravimetry analysis（TGA）data.Selected crystallographic data as well as structure refinement results of compounds 1－3 are listed in Table S1（Supporting information），and selected bond lengths and angles are given in Table S2（Supporting information）.

3　Results and discussion

Fig.1　（a）Coordination environment of Zn（II）ion in complex 1；（b）schematic representation of the 4-fold interpenetrating network

Fig.2（a）Coordination environment of Zn（II）ion in complex 2；（b）schematic representation of the 3D［2+2］interpenetrating network

3.1Structure description

Complex 1 crystallizes monoclinic space group C2/c whose asymmetric unit contains two independent Zn（II）ions，two bib linkers，two deprotonated 1，4-ndc2－anions，and one lattice water. The central Zn（II）ion is four-coordinated by two O atoms from two different 1，4-ndc2－anions，and two different N atoms from two bib ligands to generate a distorted tetrahedral coordination geometry（Fig.1（a））.The Zn―O and Zn―N bond lengths are in the range of 0.192（1）－0.200（1）and 0.202（1）－0.204（1）nm，respectively.The adjacent Zn（II）ions are linked by the 1，4-ndc2－ligand to shape two kinds of zigzag chains along the b direction with a pitch of 1.702 nm，where the distance between two adjacent Zn（II）ions is 1.112 nm，and the Zn…Zn…Zn angle is 99.870°. Each bib ligand adopts the trans-conformation（Scheme S1（a）and S1（b））to bridge the Zn（II）ions into 1D zigzag chains with Zn…Zn…Zn angle of 76.068（1）°and Zn…Zn distances of 1.278 and 1.395 nm，respectively（Fig.S1（a））.The combination of three kinds of 1D zigzag chains generates a dia network by sharing the Zn（II）ions with the point symbol of 66（Fig.S1（b），Supporting information）.Finally，the potential voids are large enough to allow three independent equivalent frameworks to generate a 4-fold interpenetrating architecture（Fig.1（b））.

Complex 2 crystallizes in the orthorhombic space group Pnna whose asymmetric unit is composed of a half Zn（II）ion，a half bib ligand，a half H2bdc-Br2ligand，and a half lattice water.Zn（II）is four-coordinated by two N atoms from two different bib ligands and two O atoms from two different H2bdc-Br2ligands，forming a distorted tetrahedral coordination geometry（Fig.2（a））.The bond lengths of Zn―O and Zn―N are 0.193（7）and 0.201（7）nm，respectively.Each bib adopts the trans-conformation（Scheme S1 （c））to bridge the Zn（II）ions into 1D zigzag chains with Zn…Zn…Zn angle of 75.859°and Zn…Zn distance of 1.396 nm.Zn（II）ions are bridged by the carboxylate groups of rigid bdc-Br22－anions to generate a meso-helical chain［Zn（bdc-Br2）］n（Fig.S2（a）），which is linked to each other through bibligands to form a threedimensional（3D）dia network with the point symbol of 66.The 3D dia network consists of［Zn10（bib）6（bdc-Br2）6］subunits which possesses maximum dimensions（the longest intracage distances across the unit along the directions）of 3.572 nm×2.778 nm× 2.017 nm（2a×2b×2c）.Such a large cavity causes the unusual 4-fold interpenetration networks，which can be best described as two sets of 2-fold net（Fig.S2（b），Supporting Information）. Namely，complex 2 shows a 4-fold interpenetration dia network with an unusual［2+2］mode（Fig.2（b））.

Complex 3 crystallizes monoclinic space group C2/c.As shown in Fig.3（a），two equivalent Zn（II）are bridged by two carboxylate groups from different 4，4?-bpdc2－to form a dinuclearZn（II）-dicarboxylate unit.The Zn…Zn distance in the dinuclear metal unit is 0.386 nm.Each Zn（II）is in a distorted tetrahedral geometry and is coordinated by three oxygen atoms from different 4，4?-bpdc2－ligands［Zn―O，0.195（4）－0.200（3）nm］and one nitrogen atom from bib ligand［Zn―N，0.199（4）nm］.Each 4，4?-bpdc2－links two adjacent dinuclear metal units into a 3D pores structure（Fig.S3（a））.However，each bib adopts the transconformation（Scheme S1（d））and bridges two Zn（II）ions filling in the pores to allow the structure becomes a microporous structure（Fig.S3（b），Supporting information）.The potential voids are also enough to permit two independent equivalent frameworks to interpenetrate it，resulting in a 3-fold interpenetrating net. Topology analysis indicates that the structure can be regarded as a（3，4）-connected network，by denoting the Zn（II）ions to fourconnected nodes and 4，4?-bpdc2－simplified as three-connected nodes，with the point symbol of（4?82）（4?85）（Fig.3（b））.

Fig.3（a）Coordination environments of Zn（II）ions in complex 3；（b）schematic representation of 3-fold interpenetrating network

Fig.4TG（black）/DTG（red）-DSC（blue）curves ofthe complexes 1（a），2（b），and 3（c）color online，Φ:heat flow

3.2PXRD analysis

PXRD experiment was performed for complexes 1－3 at 298 and 423 K.The diffraction profiles are almost the same and basically identical to the respective simulated patterns，indicating the high phase purity of the bulk products as well as the skeleton did not collapse after the solvent molecules had been removed.（Fig. S4，Supporting information）.

3.3Thermal decomposition

TG/DTG-DSC curves，under a linear heating rate of 10 K?min－1with a nitrogen atmosphere，are shown in Fig.4.They explore the melting decomposition behavior of complexes 1－3.For complex 1，the DSC curve shows that one intense endothermic process starts at 598 K and ends at 663 K，with the peak at a temperature of 628 K.The TG/DTG curve shows that complex 1 undergoes two weight loss stages before 638 K.The first weight loss stage starts at 348 K and ends at 378 K，accompanied by about 1.77% weight loss，which is attributed to the loss of one free water molecule（1.81%），and the endothermic peaks of this stage did not appear obviously in DSC curves.An abrupt weight loss is observed from 608 to 638 K，considered as the collapse of the main framework and corresponding to the intense endothermic process in the DSC curve.For complex 2，the DSC curve shows that one intense exothermic process starts at 573 K and ends at 668 K，withthe peak at a temperature of 633 K.The TG/DTG curve shows that complex 2 undergoes three weight loss stages before 788 K.The first weight loss stage starts at 323 K and ends at 372 K，accompanied by about 1.50%weight loss，which is attributed to the loss of one free water molecule（1.46%），and the endothermic peaks of this stage did not appear obviously in DSC curves.An abrupt weight loss is observed from 608 to 658 K，considered as the collapse of the main framework and corresponding to the intense exothermic process in the DSC curve.The third weight loss stage starts at 658 K and ends at 788 K，which is attributed to the decomposition of the residue and corresponding to the gentle endothermic process in the DSC curve.For complex 3，it undergoes three weight loss stages before 873 K.The first stage shows loss of lattice water molecule from 323 to 373 K（obsd. 2.41%，calc.2.13%）and this small heat absorption is observed in DSC.The rest two stages are similar to complex 2.In short，the ultimate frameworks of 1－3 can be stabilized at more than 573 K，which is larger than the usual non interpenetrating structure18.

3.4Non-isothermal kinetics analysis

In order to investigate the thermodynamics and kinetics for the melt decomposition of 1－3，we employed Kissinger?s method19and Ozawa-Doyle?s method20，21to obtain the apparent activation energy（E）and the pre-exponential factor（A）in the current work，which are universally applied in this field，using the peak temperatures measured at four different heating rates of 2，5，8，10 K?min－1.The Kissinger and Ozawa-Doyle equations are as follows，respectively:

where Tpis the peak temperature（K）；A is the pre-exponential factor（s－1）；E is the apparent activation energy（J?mol－1）；R is the gas constant（J?mol－1?K－1）；β is the linear heating rate（K?min－1）and C is a constant.

Based on the peak temperatures measured with four different heating rates（2，5，8，and 10 K?min－1），the thermodynamics and kinetics parameters are obtained and are listed in Table 1.The calculated results using the two methods are similar.By substituting the activation energy E and pre-exponential factor A into Eqs.（3－5）22，23，thermodynamic parameters of the complexes at the peak temperatures are evaluated.

Table 1Kinetic parameters of the complexes 1－3

where ν is the Einstein vibration frequency，ν=kBT/h（kBand h are Boltzmann and Planck constants，respectively），ΔG≠the Gibbs energy of activation，ΔH≠the enthalpy of activation，and ΔS≠the entropy of activation.The values of entropy，enthalpy，and the Gibbs free energy of activation at the considered peak temperature are shown in Table 2.

From Table 2，it can be seen that ΔG≠>0 of the complexes indicates that the skeleton collapse is not spontaneous.In addition，ΔH1=3448 J?mol－1，ΔH2=－2886.3 J?mol－1，and ΔH3=2355.7 J?mol－1，suggests that the reaction of complexes 1 and 3 are endothermic and 2 is exothermic，which is in agreement with the intense peak of the DSC curve respectively.Moreover，the activation energies E，276.887，318.515，149.310 kJ?mol－1in Table 1 demonstrate that the reaction rate of the thermal decomposition reaction is slow，which can be explained by the influence of activation energy on the reaction rate:the greater activation energy，the slower reaction rate24.The Arrhenius equation is expressed as follows:lnk1=21.102－276.887×103/RT for 1，lnk2=24.760－318.515×103/RT for 2，lnk3=10.028－149.310×103/RT for 3，which can be applied to estimate the rate constants of the initial melting decomposition processes of 1－3.Obviously，the unusual ［2+2］interpenetrating framework exhibits the highest thermal stability in three interpenetrating frameworks，and 4-fold interpenetrating framework exhibits higher thermal stability than 3-fold interpenetrating framework.

Table 2Thermodynamic parameters of the complexes 1－3

3.5Photoluminescence properties

The emission spectra of 1－3 were examined in the solid state at room temperature，shown in Fig.S5（Supporting information）. The free ligand bib displays photoluminescence with emission maxima at 400 nm（λex=330 nm），assigned to π*→π transition of bib25.The carboxylate ligands display photoluminescence withemission maxima 466 nm（λex=370 nm）for 1，4-H2ndc，480 nm （λex=330 nm）for H2bdc-Br2，and 421 nm（λex=300 nm）for 4，4?-H2bpdc26.As previously reported，solid-state benzene-dicarboxylate ligands can also exhibit fluorescence at room temperature，and the emission bands of these ligands can be assigned to the π*→n transition.Fluorescent emission of benzene-dicarboxylate ligands resulting from the π*→n transition is very weak compared with that of the π*→π transition of the bib ligand，so benzene-dicarboxylate ligands almost have no contribution to the fluorescent emission of as-synthesized CPs27.Therefore，the emission bands would be assigned to π*→π transition of coordinated bib ligands.The emission spectra have broad peaks with maxima 375 nm（λex=325 nm）for 1，460 nm（λex=385 nm）for 2，and 455 nm（λex=300 nm）for 3.In comparison to the free bib ligand，the emission bands for 1 is blue-shifted by 55 nm，which may be attributed to the chelating of the bib ligand to the metal ion （LMCT），which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay of the intraligand emission excited state28，while 2，3 are red-shifted by 20 and 44 nm，which probably are related to the intraligand fluorescent emission，and similar red shifts have been observed before29.

4　Conclusions

In this work，we selected the flexibility bib and three rigid lineshape dicarboxylic acid mix-ligands to build three interpenetrating CPs.1 presents a 4-fold interpenetrating framework.2 exhibits an unusual［2+2］interpenetrating framework，and 3 features a 3-fold interpenetrating network.Moreover，the fluorescent property of complexes 1－3 was also studied.The emission bands for 1 is blueshifted by 55 nm，which may be attributed to the chelating of the bib ligand to the metal ion（LMCT）and 2，3 are red-shift by 20 and 44 nm，which probably are related to the intraligand fluorescent emission.The melting decomposition of 1－3 was investigated by simultaneous TG/DTG-DSC techniques.TG curves exhibit that complexes 1－3 possess good thermostability（T1=598 K，T2=608 K，T3=573 K）.The thermodynamic parameters（ΔH≠，ΔG≠，and ΔS≠）at the intensest peak temperatures of the DTG curves were also calculated.ΔG≠>0 of the complexes indicates that the skeleton collapse is not spontaneous.In addition，in the DSC curve，ΔH1=3448 J?mol－1，ΔH2=－2886.3 J?mol－1，and ΔH3=2355.7 J?mol－1，suggest that the reaction of complexes 1 and 3 are endothermic and 2 is exothermic.Moreover，the activation energies E（E=276.887 kJ?mol－1，E=318.515 kJ?mol－1，

12E3=149.310 kJ?mol－1）demonstrate that the reaction rate of the melting decomposition reaction is slow.The unusual［2+2］interpenetrating framework exhibits the highest thermal stability in three interpenetrating frameworks，and 4-fold interpenetrating framework exhibits higher thermostability than 3-fold interpenetrating framework.

Supporting lnformation:The crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre，CCDC reference numbers 1405656 for 1，1405654 for 2，and 1416220 for 3.Copies of the data can be obtained free of charge from the Cambridge Crystallographic Data Centre，12 Union Road，Cambridge CB2 1EZ，UK（Fax:+44-1223-336-033；Email:deposit@ccdc.cam.ac.uk）.Partial structural description，powder XRD patterns，luminescences of free ligand bib and complexes，Crystal data and structure refinements，selected bond lengths and angles，a variety of spatial configuration of bib in 1－3 are given in Supporting Information.This information is available free of charge via the internet at http://www. whxb.pku.edu.cn.

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Synthesis，Structure and Thermodynamics/Kinetics Analysis of Three Different lnterpenetrating Zinc（ll）Coordination Architectures

HE Tian1YUE Ke-Fen1，*CHEN San-Ping1ZHOU Chun-Sheng2，*YAN Ni1
（1Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education，College of Chemistry and Materials Science，Northwest University，Xi′an 710069，P.R.China；2Shaanxi Key Laboratory of Comprehensive Utilization of Tailings Resources，College of Chemical Engineering and Modern Materials，Shangluo University，Shangluo 726000，Shaanxi Province，P.R.China）

Based on flexible 1，4-bis（2-methyl-imidazol-1-yl）butane（bib）and three rigid line-shaped carboxylate mix-ligands，three Zn（II）coordination polymers，｛［Zn2（bib）2（1，4-ndc）2］?H2O｝n（1），｛［Zn0.5（bib）0.5（bdc-Br2）0.5］?0.5H2O｝n（2），｛［Zn2（bib）（4，4′-bpdc）2］?H2O｝n（3）（1，4-H2ndc=1，4-naphthalenedicarboxylic acid，H2bdc-Br2=2，5-dibromoterephthalic acid，4，4′-H2bpdc=4，4′-biphenyldicarboxylic acid）have been synthesized under solvothermal conditions and characterized by elemental analysis，infrared（IR）spectrometry，and single crystal X-ray diffraction.1 presents a 4-fold interpenetrating framework including three kinds of zigzag chains.2 exhibits an unusual［2+2］interpenetrating framework.3 features a 3-fold interpenetrating network.Their thermal decomposition behaviors were investigated by simultaneous thermogravimetry/differential thermal gravity and differential scanning calorimetry（TG/DTG-DSC）techniques.The TG curves indicate that the unusual［2+2］interpenetrating framework exhibits the highest thermal stability of the three frameworks，and the 4-fold interpenetrating framework exhibits higher thermal stability than the 3-fold interpenetrating framework.ThethermodynamicsandkineticsofskeletoncollapseforthecomplexeswerecalculatedbytheintegralKissinger′s method and Ozawa-Doyle′s method.The activation energies（E）of 276.887，318.515，and 149.310 kJ?mol－1illustrate the relationship of the reaction rates of complexes 1－3:3＞1＞2.The structural characteristics could be elucidated from the thermodynamics and kinetics.Moreover，the fluorescent properties of complexes 1－3 were also studied.

Coordination polymer；Interpenetration；Thermodynamics；Kinetics

January 25，2016；Revised:March 9，2016；Published on Web:March 10，2016.

［Article］10.3866/PKU.WHXB201603102www.whxb.pku.edu.cn

O641；O642；O643

*Corresponding authors.YUE Ke-Fen，Email:ykflyy@nwu.edu.cn；Tel:+86-29-81535026.ZHOU Chun-Sheng，Email:slzhoucs@126.com.cn.

The project was supported by the National Natural Science Foundation of China（21543001，21273137），Open Foundation of Key Laboratory of

Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education，China（338080041），and Open Foundation of the Shaanxi Key Laboratory of Comprehensive Utilization of Tailings Resources，China（2014SKY-WK002）.

國家自然科學基金（21543001，21273137），教育部合成與天然功能分子化學重點實驗室開放基金（338080041）和陜西省尾礦資源綜合利用實驗室開放基金（2014SKY-WK002）資助項目

?Editorial office ofActa Physico-Chimica Sinica