LI WeiLI Chang-HongLI Heng-FengTAN Xiong-Wen（Shool of Materials Siene and Engineering， Central South University， Changsha 410083， China）（Department of Chemistry and Materials Siene， Hengyang Normal University， Hengyang 421008， China）（Department of Chemial Engineering， Hunan Institute of Tehnology， Hengyang 421002， China）

Synthesis, Crystal Structure, Spectroscopic Characterization, Thermal Stability and Magnetic Properties of a Dinuclear Copper(II) Complex①

LI Weia， bLI Chang-HongcLI Heng-Fenga②TAN Xiong-Wenba（School of Materials Science and Engineering， Central South University， Changsha 410083， China）
b（Department of Chemistry and Materials Science， Hengyang Normal University， Hengyang 421008， China）c（Department of Chemical Engineering， Hunan Institute of Technology， Hengyang 421002， China）

A new copper（II） complex ［Cu2（MNA）2（2，2΄-bipy）2］·2.5H2O with methy-5-norbornene-2，3- dicarboxylic acid （MNA） and 2，2΄-bipyridine as ligands has been synthesized in the mixed solvents of DMF and water. It crystallizes in monoclinic， space group P 1， with a = 10.4191（11）， b = 12.8883（13）， c = 16.1114（16） Å， α = 70.8090（10）， β = 80.568（2）， γ = 77.440（2）º， V = 1984.3（4） Å3， Dc= 1.551 g/cm3， Z = 2， F（000） = 962， the final GOOF = 1.051， R = 0.0431 and wR = 0.0980. The crystal structure shows that the whole molecule consists of two independent dinuclear units， in which two copper ions are bridged by two μ2-η1:η03-carboxylate groups of MNA2-. The coordination environment of Cu（II） ion is CuO3N2， giving a distorted square pyramidal geometry. The spectroscopic characterization， thermal stability and magnetic properties of the complex were investigated.

copper(II) complex, spectroscopic characterization, thermal stability, magnetic properties;

1 INTRODUCTION

Design and synthesis of metal-organic complexes have attracted much attention not only for their novel structures， but also for their potential applications in catalysis［1］， photochemistry［2］， magnetism［3］， materials science［4］， sorption properties［5］and medicine［6］. The aromatic carboxylic acids with a variety of coordinating modes are often used as ligands to construct metal-organic complexes because their complexes can exhibit high thermal stability and good physical and chemical properties in practical use［7］. Methy-5-norbornene-2，3-dicarboxylic acid （MNA） is an important aromatic carboxylic acid， which is a main raw material of curing agent in the setting system of epoxy resin， so it has been applied widely in electroinsulating materials home and overseas， especially those for large electric appliances［8］. In order to obtain knowledge about the structure and function of metal organic complexes constructed with flexible cyclic carboxylic ligands， we synthesized a new copper complex ［Cu2（MNA）2（2，2΄-bipy）2］·2.5H2O using MNA as the ligand. We also report our preliminary results of the spectroscopic characterization， thermal stability and magnetic properties of the complex.

2 EXPERIMENTAL

2. 1 Materials and instruments

All reagents were of analytical grade and used as obtained from commercial sources without further purification， while MNA was prepared by ourselves［9］. Crystal structure determination was carried out on a Bruker SMART APEX CCD single-crystal diffractometer. Elemental analyses were performed on a Perkin-Elmer 2400 elemental analyzer. The IR spectra were recorded on a Bruker Vector22 FT-IR spectrophotometer using KBr discs. The luminescence spectra for the powdered solid samples were measured on an RF-5301PC spectrofluorometer with a xenon arc lamp as the light source. Thermogravimetric analyses were performed on a simultaneous SPRT-2 pyris1 thermal analyzer at a heating rate of 10 ℃ min-1. Magnetic measurements in the range of 2.8～300 K were performed on a MPMS -SQUID magnetometer at a field of 2 kOe on a crystalline sample in the temperature settle mode.

2. 2 Synthesis of the complex

0.48 mmol of MNA （about 0.0752 g） and 0.4 mmol of copper acetate （about 0.0798 g） were added to the mixed solvents （20 mL） of DMF and water （7:2 in volume）， stirred at 60～65 ℃ for about 2.0 h，and then 0.52 mmol of 2，2΄-bipyridine （about 0.1187 g） was added， followed by adjusting the pH value to 6.0～7.0 with dilute sodium hydroxide， and stirring this mixture at 65 ℃ for about 14 h. Afterwards， the resultant solution was filtrated， and the filtrate was kept untouched and evaporated slowly at room temperature. Blue block-shaped single crystals suitable for X-ray diffraction analysis were obtained after three weeks. Yield: 30.5%. m.p.: 212～214 ℃. Anal. Calcd. for C40H47Cu2N4O13.50: C， 51.83； H， 5.11； N，6.04%. Found: C， 51.68； H， 5.10； N， 6.07%. Main IR （KBr， cm-1）: 3422（s）， 3051（s）， 1691（vs）， 1598（vs），1508（vs）， 1372（vs）， 1311（m）， 1283（m）， 1161（m），1032（m）， 760（vs）， 702（m）， 666（w）， 665（w）， 546（w），426（w）.

2. 3 Crystal structure determination

A single crystal with dimensions of 0.18mm × 0.17mm × 0.15mm was put on a Bruker SMART APEX CCD diffractometer equipped with a graphite-monochromatic MoKα radiation （λ = 0.71073Å） using a φ-ω scan mode at 173（2） K. A total of 11，442 reflections were collected in the range of 1.60≤θ≤25.01°， of which 3，546 were independent （Rint= 0.0513） and 2，722 were observed （I ＞ 2σ（I））. All data were corrected by Lp factors and empirical absorption. The crystal structure was solved directly by program SHELXS-97， and refined by program SHELXL-97［10］. The hydrogen and non-hydrogen atoms were corrected by isotropic and anisotropic temperature factors respectively through full-matrix least-squares method. The final R = 0.0457， wR = 0.0974 （w = 1/［σ2（Fo2） + （0.0389P）2+ 2.6432P］，where P = （Fo2+ 2Fc2）/3）； （∆/σ）max= 0.00， S = 1.051，（∆ρ）max= 0.781 and （∆ρ）min= -0.280 e·Å-3.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of the complex

The molecular structure of the complex is shown in Fig. 1. The selected bond lengths and bond angles are shown in Table 1， and hydrogen bonds in Table 2.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°) of the Complex

Symmetry transformations used to generate the equivalent atoms: A: -x + 1， -y， -z + 1； B: -x + 1， -y ， -z

Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°)

Fig. 1. Molecular structure of the title complex with water molecules omitted for clarity

The crystal structure in Fig. 1 shows that the whole molecule of the complex consists of two independent dinuclear units which contain two copper ions， two MNA molecules， two 2，2΄-bipyridine molecules， and two and a half water molecules. The copper ions are coordinated with three oxygen atoms from two MNA molecules and two nitrogen atoms from the 2，2΄-bipyridine molecule， forming a distorted square pyramidal geometry. In CuN2O3， N（1），N（2）， O（1） and O（3） are located at the equator plane，and O（3A） lies in the axial direction. The Cu··Cu distance through the MNA bridge is 3.206～3.199 Å. The Cu-N bond lengths range from 1.983 to 2.001 Å，and Cu-O vary from 1.935 to 2.229 Å， and Cu（2）-O（5） is 2.233 Å. The bond angle O/NCu-N/O is 80.71（9）～174.23（10）º. Another noticeable characteristic of the title complex is the strong hydrogen bonding interactions， which exist between water molecules such as O（11）-H（11B）··O（13）（2.784（3） Å， 170°） and O（13）-H（13B）··O（14）（2.826（3） Å， 176°） as well as between water molecules and MNA like O（11）-H（11A）··O（8）（2.793（3） Å， 174°） and O（13）-H（13A）··O（2）（2.757（3） Å， 169°）. These hydrogen bonding interactions benefit the stability of the complex structure.

Fig. 2. Spectra of the title complex A: Elicitation spectrum (λmax= 320 nm); B: Emission spectrum (λmax= 431 nm)

3. 2 Spectroscopic characterization

Infrared spectrum of the complex demonstrates anabsorption peak at 3，422 cm-1ascribed to the characteristic absorption peak of OH in H2O. The anti-symmetric and symmetric stretching vibration absorption peaks of carboxylic group in the ligand appear at 1，691 and 1506 cm-1， which are in contrast with the corresponding peaks of free ligands （at 1，712 and 1，224 cm-1， respectively） and shift remarkably. The Δv （vas（coo-）- vs（coo-）） of 185 cm-1is less than 200 cm-1. The IR spectra of strong peaks are observed at 1598， 1508 and 760 cm-1due to the characteristic absorption peaks of ligand 2，2΄-bipy.

The fluorescent properties of the title complex were studied in the solid state at room temperature （Fig. 2）. As illustrated in Fig. 2， complex shows strong blue fluorescence， with an emission maximum at 431 nm upon excitation at 318 nm. According to Cu（II） clusters with MNA ligands and simple Cu（II） complex of mixed-ligand systems［11］， the emission is tentatively assigned to be dominated by a ligand-to-metal charge transfer （LMCT） （L→Cu） character.

3. 3 Magnetic properties

The temperature dependence of magnetic susceptibility of the title complex was investigated from 300 to 2.8 K with an applied magnetic field of 2 kOe. The XMT vs. T and 1/XMvs. T curves are shown in Fig. 3. The product of XMT decreases gradually from 0.778 cm3·K·mol-1at 300 K to 0.854 cm3·K·mol-1at 2.8 K， which exists a maximum 0.878 cm3·K·mol-1at 43 K， and then declines sharply. In addition， according to the Curie-Weiss law， XM= C/（T - θ）， the Curie constant （C = 0.108 cm3K·mol1）and Weiss constant （θ = 0.190 K） are obtained from a linear fit of the 1/χmdata between 2.8 and 300 K. Such magnetic behavior indicates that the copper complex is a paramagnetic system， which shows weak ferromagnetic properties. The XMT drops suddenly at low temperature， which can be attributed to the antiferromagnetic interactions in chains.

Fig. 3. Temperature dependence of the magnetic susceptibility of the title complex in the forms of XmT vs. T and 1/Xmvs. T

3. 4 Thermal stability

The thermogravimetric analysis （Fig. 4） of the title complex demonstrated that the weight loss of the complex in the air from room temperature to 600 ℃ occurred mainly in 3 stages. The first stage takes place from 144 to 188 ℃with the weight loss of 4.90%， corresponding to the release of two and a half free water molecules （calcd. 4.86%）. The second stage occurs at 188～270.7 ℃ with the weight loss of 33.70%， resulting from the removal of two 2，2΄-bipy molecules （calcd. 33.70%）. There is a strong endothermic peak near 212.2 ℃ attributed to the melting point of the complex. The third stage is observed from 270.7 to 392.5 ℃ with the weight loss of 44.20% due to the departure of two MNA molecules （calcd. 44.28%）， confirming the crystal structure. In air， the final product is copper oxide with the residual weight being about 17.20% （calcd. 17.16%）. Based on the above judgment， the pyrolytic process of the complex may be shown as follows:

Fig. 4. TG and DTG curves of the title complex

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Received 15 January 2015； accepted 28 April 2015 （CCDC 1001513）

① Supported by the Natural Science Foundation of Hunan Province （No.11JJ9006）， Key Project of Science and Technology Plan of Hunan Province （2012FJ2002） and the Construct Program of the Key Discipline in Hunan Province

② Corresponding author. E-mail: lihengfeng@gmail.com

10.14102/j.cnki.0254-5861.2011-0780