Synthesis， Structure and Characterization of a 3d-4f Hetero-metallic Complex Based on Imidazole-dicarboxylate①

CHEN Jun-LingaFENG Xunb②XIE Shi-YubWANG Cai-LuaWANG Li-Yaa， b②LANG Xue-Mingba（School of Life Science and Technology， Nanyang Normal University， Nanyang 473061， China）

b（College of Chemistry and Chemical Engineering， Luoyang Normal University， Luoyang 471022， China）

A new 3d-4f polymer formulated as ｛［SmCo2（μ2-Hmimda）3（μ2-H2mimda）·4H2O］· 3H2O｝n（H3mimda = 1-H-2-methyl-4，5-imidazole-dicarboxylic acid） has been hydrothermally synthesized and structurally characterized by elemental analysis， X-ray single-crystal diffraction and magnetic susceptibility measurement. It crystallizes in monoclinic system， space group C2/c， with a = 17.251（6）， b = 16.965（6）， c = 14.668（5） Å， β = 118.565（5）°， Z = 4， V = 3770（2） Å3， F（000） = 1984，the final R = 0.0597 and wR = 0.1820. In the crystal， the dicarboxylate ligand links the Sm（III） and Co（II） ions to give rise to a heterometallic ［SmCo2（μ2-Hmimda）3（μ2-H2mimda）］ aggregate which acts as the second building unit. Then， the SBUs further extended a 2D corrugated layer with a （4， 4）topology. The title complex displays strong photoluminescence in the solid state.

H3mimda， X-ray single crystal， magnetic susceptibility， photoluminescence；

1 INTRODUCTION

Recently， the explorations of heterometal-organic frameworks （HMOF） comprised of both d and f block metal ions are attracting significant attentions，and the direct driving forces originate not only from their fascinating topological structures［1-4］but also from their versatile applications in adsorption，optoelectronic materials， luminescent probes and so on［5-8］. A variety of transition-lanthanide metal heterometallic complexes have been successfully achieved from self-assembly of mixed-metal ions and ligands containing different donor atoms［9］. The assembly of 3d-4f heterometallic complexes， however， is still a challenging task because of the variable and versatile coordination numbers of lanthanide ions， their low stereochemical preference and competitive reactions between lanthanide and transition metals for coordination to the same organic ligand［10］. Therefore， the selection of suitable ligands plays an important role in determining the topologies and properties of coordination networks［11］. According to the hard-soft acid base theory， lanthanide ions behave as hard acids preferring O-donors， while d-block metal ions are borderline acids， inclined to coordinate to both N-and O-donors， which provides a route to synthesize and design 3d-4f heterometallic complexes. In thiscontribution， a rigid 1-H-2-methyl-4，5-imidazoledicarboxylic acid was selected as an organic ligand to obtain a new 3d-4f heterometalic complex.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All chemicals and solvents purchased were of reagent grade and used without further purification. Elemental analyses （C， H and N） were performed on a Perkin-Elmer 2400 elemental analyzer. IR spectra were recorded in the range of 400～4000 cm-1using a VECTOR-22 spectrometer with KBr discs. Magnetic data were obtained using a Quantum Design MPMS SQUID 7S magnetometer at an applied field of 2000 Oe using multi-crystalline samples of the title complex in the temperature range of 2～300 K. The magnetic susceptibilities were corrected using Pascal’s constant and the diamagnetism of the holder. Solid state luminescent spectra in the visible ranges were measured at room temperature with an Edinburgh instrument F4500 fluorescence spectrometer.

2. 2 Synthesis of the title complex

Cobalt（II） acetate tetrahydrate （0.025 g， 0.1 mmol）and samarium（III） chloride hexahydrate （0.036 g，0.1 mmol） were added to the ethanol-water solution（10 mL） of H3mimda （0.079 g， 0.3 mmol）. After stirring for 30 min in air， the aqueous mixture was placed into a 25 mL Teflon-lined autoclave under autogenous pressure being heated at 155 ℃ for 96 h，and then the autoclave was cooled over a period of 24 h at a rate of 5 ℃/h. Red crystals of the title complex were obtained suitable for X-ray diffraction analysis. Yield: 0.0187 g （37%） based on the cobalt element. Elemental analysis （%）: calcd. for C24H31Co2N8O23Sm: C， 26.99； H， 2.93； N， 10.49. Found （%）: C， 27.02； H， 2.89； N， 10.45. IR （KBr pellet， cm-1）: 3243br， 1580s， 1406s， 1260m.

2. 3 X-ray structure determination

A single crystal of the title complex （0.28mm × 0.25mm × 0.14mm） was mounted on a Bruker SMART APEX II CCD diffractometer equipped with a graphite-monochromatized MoΚα radiation （λ = 0.71073 Å） by using a φ/ω scan mode at room temperature in the range of 1.80≤θ≤26.37° with index ranges of -21≤h≤18， 0≤k≤21 and 0≤l≤18. A total of 3819 reflections were collected， of which 3422 were independent （Rint= 0.0000）. Corrections for Lp factors were applied and all nonhydrogen atoms were refined with anisotropic thermal parameters. The structure was solved by direct methods with SHELXS-97［12］. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by use of geometrical restrains. A full-matrix leastsquares refinement on F2was carried out using SHELXL-97［13］. For the title complex， the disordered solvate molecules are difficult to identify， so the solvate molecules were accounted for by using the program PLATON/SQUEEZE （Spek， 2009） in order to remove the contributions of disordered solvent. Elemental analyses and IR spectra were employed to further confirm the structure. The final R = 0.0597， wR = 0.1820 （w = 1/［σ2（Fo2） + （0.1202P）2+ 41.5920P］， where P = （Fo2+ 2Fc2）/3）， S = 1.090， （Δρ）max= 4.026 and （Δρ）mix= -1.974 e/Å3. The selected bond lengths and bond angles and the hydrogen bond parameters are listed in Tables 1 and 2.

Table 1. Selected Bond Lengths （Å） and Bond Angles （°） for Title Complex

Symmetry transformations used to generate the equivalent atoms: #1: -x， y， -z+1/2； #2: -x+1/2， y-1/2， -z+1/2； #3: x-1/2， y-1/2， z； #4: x+1/2， y+1/2， z

Table 2. Hydrogen Bonds Lengths （Å） and Bond Angles （°） for the Title Complex

3 RESULTS AND DISCUSSION

3. 1 Infrared spectra

In the IR spectrum of the title complex， the wide absorption at about 3243 cm-1is characteristic of OH-group in the coordinated water. The observation of strong band at 1580 cm-1is attributed to characteristic stretching vibration of C=N of H3mimda. Besides， the presence of sharp characteristic peak in the frequency of 1406 cm-1is assigned to symmetric stretching vibrations of the carboxylic group［14， 15］.

3. 2 Structure description of the title complex

The principal perspective view of the title complex with atomic labeling scheme is illustrated in Fig. 1. The title complex displayed a butterfly-like configuration， which contains one Sm（Ⅲ） ion， two Co（II） ions， three Hmimda2-ligands， one H2mimdaligand and four coordinated water molecules. Each Sm（Ⅲ） ion is eight-coordinated being bound by an O8 donor set， all of which are from imidazolyl dicarboxylate， yielding a distorted square antiprism geometry with the top and bottom planes defined by O（6）， O（6）#1， O（2）#2， O（2）#3 and O（7）， O（7）#1，O（3）#2， O（3）#3 atoms， respectively. In the structure，the average Sm-O length is 2.395（3） Å， which is comparable with the literature values［16］. Besides，each Co（II） ion is six-coordinated with a distorted octahedron geometry and defined by two carboxylic O atoms （Co（1）-O（4）， Co（1）-O（5） bond lengths are 2.080（5） and 2.100（5） Å， respectively） and two imidazolyl N atoms （Co（1）-N（3）， Co（1）-N（1） bond lengths are 2.077（6） and 2.090（7） Å， respectively）from the H3mimda ligands， as well as two O atoms from two water molecules （Co（1）-O（9）， Co（1）-O（10）bond lengths are found to be 2.092（6） and 2.166（6）Å， respectively）. Both Co-O and Co-N bond lengths are well-matched to those observed in similar complexes［17］.

Fig. 1. Illustration of coordination environments in the title complex （color code: Sm: purple； Co: yellow； O: red； N: blue； C: black. Hydrogen atoms are omitted for charity） （Symmetry transformations used to generate the equivalent atoms: #1: -x， y， -z+1/2； #2: -x+1/2， y-1/2， -z+1/2； #3: x-1/2， y-1/2， z； #4: x+1/2， y+1/2， z）

In the complex， the imidazole dicarboxylate ligand acts as tetradentate ligands， and adopt a μ2-kO，O’:kN， O mode connecting Sm（III） and Co（II）centers into a heterometallic ［SmCo2（μ2-Hmimda）3（μ2-H2mimda）］ aggregate as the secondary building unit （SBU） adopting bis-（bridging） and chelating modes. The shortest separation of Sm-Co is 6.155 Å， which is slightly larger than the literature values［16］. The SBU aggregates further linked with each other in a head-to-tail mode to form a onedimensional （1D） robust polymeric chain approximately along the ab plane， as described in Fig. 2.

Fig. 2. View of 1-D heterometallic chain of the title complex

The carboxylic oxygen and imidazolyl nitrogen atoms crosslink these 1D chains into a 2D corrugated grid-like Sm-Co layer approximately along the bc plane， in which the Sm（III） and Co（II） ions are linked in an alternating fashion and the shortest separations of Sm-Sm and Co-Co are 12.098 and 8.868 Å， respectively. An alternate description: These SBU structures alternately linked with each other， adopting head-to-end and head-to-head modes into a square configuration via the carboxylic oxygen and imidazolyl nitrogen atoms. Finally， the square units extended in all directions to form a 2D layer， as described in Fig. 3.

Fig. 3. Polyhedral view of corrugated-shaped 2D layer constructed from ［SmCo2（μ2-Hmimda）3（μ2-H2mimda）］ SBUs

From the topological view， if each Sm（III） ion is considered as a 4-connected node， the Co（II） ions and H3mimda ligands can be simplified as linear connectors， then an uninodal 4-connected grid net with （4，4） topology was generated in this structure. Finally， two layers are bridged by the coordination water molecules and carboxylic group through OH··O hydrogen bonds into double-layers， which stack parallel to each other along the c direction to form a 3-D supramolecular hydrogen-bonding network， as described in Fig. 4.

Fig. 4. Schematic diagram of the 3D layer in the title complex

3. 3 Photoluminescence property

The solid state photo luminescent spectra of the Sm-Co complex in multi-crystal samples were measured at room temperature. In our previous work，we have measured the luminescent property of free H3mimda ligand， which shows an emission band with the maximum at 361 nm upon 278 nm excitation， which may be assigned to the π* → nelectron transition （ILCT） character［18-20］. In this Sm-Co complex， there exists an emission band with the maximum at 376 nm upon the excitation at 305 nm， as displayed in Fig. 5. It bears large similarity with that for free ligand， albeit with much larger intensity， also assigned to the ligand-centered （LC）fluorescence based on the emission spectrum of free H3mimda ligand.

3. 4 Magnetic property

As displayed in Fig. 6， the χMT value is 3.76 cm3K mol-1at room temperature （300 K）， which is close to the theoretical value expected for one free Sm（III）ion［21， 22］and two isolated Co（II） ions［23］. As the temperature is lowered， the χMT value displays a sharp decrease to the lowest value of 0.66 cm3K mol-1at 2.0 K. Lowering the temperature caused a decrease in χMT， which could arise from a selective depopulation of the excited crystal-field state， and such a behavior is also possible to be the result of a weak antiferromagnetic interaction within the complex.

Fig. 5. Photoemission spectra of the title complex （excited at 305 nm） in the solid state at room temperature

Fig. 6. Temperature dependence of χM and χMT products under 2000 Oe for the title complex

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Received 18 December 2014， accepted 5 February 2015 （CCDC 1038054）

① This work was supported by the National Natural Science Foundation of China （No. 21273101）， Program for Science & Technology Innovation Talents in Universities of Henan （2014HASTIT014）， Tackle Key Problem of Science and Technology Project of Henan Province （No. 142102310483）， Key program of Foundation of Education Committee of Henan Province （No. 14B150033）， and Graduate Innovation Foundation Project of Nanyang Normal University （No. 20141110）

② Corresponding author. Majoring in coordination chemistry. E-mail: hnlyfx@163.com and wlya@lynu.edu.cn

10.14102/j.cnki.0254-5861.2011-0606