Syntheses，Crystal Structures and Luminescent and Magnetic Properties of Cerium（III）and Zinc（II）Coordination Compounds Constructed from3，5-Dibromobenzoic Acid①

GAO Zhu-Qing　LI Hong-Jin　GU Jin-Zhong（School of Chemicl nd Biologicl Engineering，Tiyun University of Science nd Technology，Tiyun 030021，Chin）（College of Chemistry nd Chemicl Engineering，Lnzhou University，Lnzhou 730000，Chin）

Syntheses，Crystal Structures and Luminescent and Magnetic Properties of Cerium（III）and Zinc（II）Coordination Compounds Constructed from3，5-Dibromobenzoic Acid①

GAO Zhu-Qinga②LI Hong-JinaGU Jin-Zhongba（School of Chemical and Biological Engineering，Taiyuan University of Science and Technology，Taiyuan 030021，China）
b（College of Chemistry and Chemical Engineering，Lanzhou University，Lanzhou 730000，China）

Two coordination compounds，namely ［Ce（3，5-Br2BC）3（3，5-HBr2BC）］n（1）and ［Zn（3，5-Br2BC）2（phen）］ （2，3，5-HBr2BC =3，5-dibromobenzoic acid，phen = 1，10-phenanthroline），were assembled.Single-crystal X-ray diffraction studies show that compound 1 possesses a 1D chain coordination network，which is further extended into a3D supramolecular architecture via Br··Br and Br··O halogen bonding.Compound 2 consists of a mononuclear molecule，which is assembled to a3D supramolecular framework through C-H··O hydrogen bond，π-π stacking interactions and Br··π halogen bonding.Luminescent and magnetic properties of both compounds have also been studied.

coordination compound，3，5-dibromobenzoic acid，luminescent properties，magnetic properties

1　INTRODUCTION

In the past decades，considerable attention has been focused on the crystal engineering of supramolecular architecture assembled by covalent bonds and the non-covalent forces，including hydrogen bonds，π-π interactions and halogen bonding，because of their intriguing variety of architectures and potential applications in gas absorption，luminescence，catalysis，and magnetism［1-12］.The selection of ligand is very important in the construction of coordination compounds.A lot of ligands have been used for this purpose.Among them，organic aromatic carboxylate ligands are important multidentate O-donor ligands due to their multidentated and excellent coordinated ability［8，11，13，14］.In order to extend our research in this field，we chose3，5-HBr2BC as a functional ligand based on the following considerations:（a）3，5-HBr2BC possesses one carboxyl group for coordination to the metal center，and its two Br atoms are potentially capable of taking part in oxygen （or nitrogen）··Br halogen bonding and related Br··π and Br··Br interactions；（b）to our knowledge，3，5-HBr2BC ligand has not been adequately explored in the construction of metal coordination polymers.

Taking into account these factors，we herein report the synthesis，crystal structures and luminescent and magnetic properties of Ce（III）and Zn（II）coordination complexes constructed from3，5-Br2BC-ligands.

2　EXPERIMENTAL

2.1General procedures

All chemicals and solvents were of AR grade and used without further purification.Carbon，hydrogen and nitrogen were determined using an Elementar Vario EL elemental analyzer.IR spectra were recorded on a Bruker EQUINOX 55 spectrometer with KBr pellets.Thermogravimetric analysis （TGA）was performed under N2atmosphere at a heating rate of 10 K/min on a LINSEIS STA PT1600 thermal analyzer.Excitation and emission spectra were recorded for the solid samples on an Edinburgh FLS920 fluorescence spectrometer at room temperature.Magnetic susceptibility data were collected in the 2～300 K temperature range with a Quantum Design SQUID Magnetometer MPMS XL-7 in a field of 0.1 T.A correction was made for the diamagnetic contribution prior to data analysis.

2.2Synthesis of compound 1

A mixture of Ce（NO3）3·6H2O （0.065 g，0.15 mmol），3，5-HBr2BC （0.083 g，0.3 mmol），NaOH （0.012 g，0.3 mmol），and H2O （10 mL）was stirred at room temperature for 15 min，and then sealed in a 25 mL Teflon-lined stainless-steel vessel，and heated at 433 K for3 days，followed by cooling to room temperature at a rate of 10 K·h-1.Colourless needleshaped crystals of 1 were isolated manually，and washed with distilled water.Yield:50% （based on3，5-HBr2BC）.Anal.Calcd.（%）for C28H13Br8CeO8:C，26.76；H，1.04.Found （%）:C，27.08；H，1.08.IR （KBr，cm-1）:1681m，1609m，1578m，1544s，1521s，1437s，1387s，1288w，1262w，1239w，1145w，1102w，992w，874w，820w，788m，743m，727m，710w，661w，584w，470w，422m.

2.3Synthesis of compound 2

A mixture of Zn（NO3）2·6H2O （0.045 g，0.15 mmol），3，5-HBr2BC （0.083 g，0.3 mmol），phen（0.030，0.15 mmol），NaOH （0.012 g，0.3 mmol），and H2O （10 mL）was stirred at room temperature for 15 min，and then sealed in a 25 mL Teflon-lined stainless steel vessel，and heated at 433 K for3 days，followed by cooling to room temperature at a rate of 10 K·h-1.Colourless block-shaped crystals of 2 were isolated manually，and washed with distilled water.Yield 60% （based on Zn）.Anal.Calcd.（%）for C26H14Br4ZnN2O4:C，38.87；H，1.76；N，3.49.Found （%）:C，38.49；H，1.83；N，3.37.IR （KBr，cm-1）:1599w，1546s，1518w，1427m，1407w，1366s，1227w，1148w，1106w，1049w，993w，903w，887w，862s，783s，732s，659w，645w，433w.

2.4Structure determination

Two single crystals of the title compounds with dimensions of 0.32mm × 0.20mm × 0.18mm （1）and 0.22mm × 0.15mm × 0.12mm （2）were mounted on a Bruker CCD diffractometer equipped with a graphite-monochromatic MoKα （λ = 0.71073 Å）radiation using a φ-ω scan mode at 293（2）K in the ranges of 2.96＜θ＜25.05º and3.01＜θ＜25.04º，respectively.The structures were solved by direct methods with SHELXS-97［15］and refined by full-matrix least-squares techniques on F2with SHELXL-97［16］.All non-hydrogen atoms were refined anisotropically.All hydrogen atoms （except those bound to water molecules）were placed in the calculated positions with fixed isotropic thermal parameters and included in structure factor calculations in the final stage of full-matrix least-squares refinement.The hydrogen atoms of water molecules were located by difference Fourier maps and constrained to ride on their parent O atoms.The final R = 0.0387 in 1 for 407 parameters and 4806 observed reflections with I ＞2σ（I）and wR = 0.0780 （w = 1/［σ2（Fo2）+ （0.000P）2+ 0.000P］，where P = （Fo2+ 2Fc2）/3）for all 6611 reflections.S = 1.072，（Δρ）max= 0.822，（Δρ）min= -0.928 e/Å3and （Δ/σ）max= 0.002.And in 2，the final R = 0.0482 for 168 parameters and 2273 observed reflections with I ＞ 2σ（I）and wR = 0.1100 （w = 1/［σ2（Fo2）+ （0.0532P）2+ 4.1202P］，where P = （Fo2+ 2Fc2）/3）for all 4710 reflections.S = 1.042，（Δρ）max= 1.072，（Δρ）min= -1.035 e/Å3and （Δ/σ）max= 0.001.The selected important bond parameters are given in Tables 1 and 2.

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

Table 2.Selected Bond Lengths （Å）and Bond Angles （°）for Compound 2

3　RESULTS AND DISCUSSION

3.1Crystal structure of 1

Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinic space group P21.The asymmetric unit of compound 1 contains one crystallographically unique Ce（III）atom，three3，5-Br2BC-ligands，and one3，5-HBr2BC moiety.As depicted in Fig.1，each Ce（III）atom is nine-coordinated by eight carboxylate O atoms of six individual3，5-Br2BC-ligands and one carboxylate O atom from one3，5-HBr2BC moiety，which resembles a distorted tricapped trigonal geometry.The Ce-O （2.401（6）～2.745（7）Å）bond lengths are in good agreement with those observed in some other Ce（III）compounds［17，18］.In 1，the3，5-Br2BC-ligands adopt two different coordination modes （Scheme 1，modes I and II），in which the carboxylate groups show μ2-η1:η1bidentate and μ2-η1:η2tridentate modes.The3，5-HBr2BC ligand takes a monodentate coordination mode （Scheme 1，mode III）.The carboxylate groups of3，5-Br2BC-ligands bridge alternately the neighboring Ce（III）ions to form a chain with the Ce··Ce separation of 4.230（2）Å （Fig.2）.Adjacent chains are interlinked by Br··Br （Br（4）··Br（5i）=3.466（2）Å，i = x，y，z + 1）halogen bonding to generate 2D sheet motifs （Fig.3），which are further assembled into a3D supramolecular framework via Br··O （Br（2i）··O（4）=3.344（3）Å，i = x - 1，y，z）halogen bonding （Fig.4）.

Scheme 1.Coordination modes of the3，5-Br2BC-/3，5-HBr2BC ligands in compounds 1 and 2

Fig.1.Coordination environment of the Ce（III）atom in compound 1.The hydrogen atoms are omitted for clarity except those bound to carboxylate oxygen atoms （Symmetry code:i:-x+1，y-1/2，-z+1）

Fig.2.A perspective of the 1D chain along the ab plane in compound 1.The benzene rings of3，5-Br2BC-ligands were omitted for clarity （Symmetry codes:i:-x+1，y+1/2，-z+1；ii:-x+1，y-1/2，-z+1；iii:x，y+1，z；iv:x，y-1，z）

Fig.3.A perspective of 2D supramolecular structure along the bc plane.Blue dashed lines present the Br··Br halogen bonding （Symmetry codes:i:x，y，z+1；ii:x，y，z -1）

Fig.4.A perspective view of the3D supramolecular structure along the ab plane.Blue dashed lines present the Br··O halogen bonding （Symmetry code:i:x-1，y，z）

3.3Crystal structure of 2

Compound 2 possesses a monomeric molecular structure.As shown in Fig.5，the six-coordinate Zn（II）center binds four carboxylate O atoms from two different3，5-Br2BC-ligands and two N atoms from one phen moiety，thus forming a distorted octahedral geometry.The Zn-O bonds vary from 1.998（4）to 2.463（5）Å，while the Zn-N distances are 2.087（5）Å，which are comparable to those in the reported Zn（II）compounds［19，20］.In 2，the3，5-Br2BC-ligand adopts the coordination mode IV （Scheme 1），in which the carboxylate group shows a μ2-η1:η1bidentate mode.The discrete monomeric units of 2 are interlinked by C-H··O hydrogen bonds （C（13i）-H（13i）··O（1），C（13i）··O（1）=3.325（3）Å，i = -x + 1，y - 1，-z + 1/2）and π-π packing interactions （the centroid-centroid separation of adjacent phenyl planes of the3，5-Br2BC-ligands is3.596（2）Å）to generate 2D sheet motifs （Fig.6），which are further assembled into a3D supramolecular framework via the Br··π （3.377（2）Å）interactions between the Br atoms and the adjacent phenyl planes of phen ligands （Fig.7）.

Fig.5.Coordination environment of the Zn（II）atom in compound 2.The hydrogen atoms are omitted for clarity （Symmetry code:i:-x+1，y，-z+1/2）

Fig.6.A perspective view of the 2D supramolecular structure along the ab plane.Blue and green dashed lines present the C-H··O H-bond and π-π packing interactions，respectively （Symmetry codes:i:-x+1，y-1，-z+1/2；ii:-x+1，y，-z+1/2；iii:x，y-1，z）

Fig.7.A perspective view of the3D supramolecular structure along the ab plane in compound 2.Green dashed lines present the Br··π interactions

3.4Thermal analysis

To determine the thermal stability of compounds 1 and 2，their thermal behaviors were investigated under nitrogen atmosphere by thermogravimetric analysis （TGA）.As shown in Fig.8，the TGA curves of the compounds indicate that they are stable up to 705 and 546 K，respectively.Further heating leads to their decomposition.

3.5Luminescent properties

Fig.8.TGA curves of compounds 1 and 2 spectra of the ligand and compound 2

Fig.9.Solid state excitation and emission

The emission spectra of3，5-HBr2BC and compound 2 were measured in the solid state at room temperature，as depicted in Fig.9.The “free”3，5-HBr2BC ligand displays a weak photoluminescence with an emission maximum at 480 nm if excited at330 nm excitation.For compound 2，the significantly more intense emission band is observed with a maximum at 510 nm （λex=335 nm），which can be assigned to the intraligand （π* → n or π* → π）emission［21］.The luminescence enhancement in compound 2 can be attributed to the binding of ligands to the metal centers，which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay［22］.The observed strong photoluminescence of 2 indicates that this compound can be an excellent candidate for potential applica- tions as a photoactive material.

3.6Magnetic properties

The magnetic behaviors of 1 are shown in Fig.10.The χMT value of 0.850 cm3·mol-1·K at room temperature is close to the expected value of 0.806 cm3·mol-1·K for one magnetic isolated Ce（III）ion （S = 1/2，L =3，2F5/2，g = 6/7）.Upon lowering the temperature，the χMT value decreases continuously.Between 185 and300 K，the magnetic susceptibi-lities can be fitted to the Curie-Weiss law，χM= CM/（T - θ），with CM= 1.23 cm3·mol-1·K and θ = -127.5 K.These results indicate stronger magnetic interactions between the neighboring Ce（III）ions.

Fig.10.Temperature dependence of χMT （О）and 1/χM（□）vs.T for complex 1.The black line shows the Curie-Weiss fitting

4　CONCLUSION

In this work，two new coordination polymers derived from3，5-dibromobenzoic acid and 1，10-phenanthroline ancillary ligand were synthesized and fully characterized.This work shows that3，5-dibromobenzoic acid is an excellent bridging ligand for the construction of coordination compounds.

REFERENCES

（1）Mahmudov，K.T.；Kopylovich，M.N.；Maharramov，A.M.Barbituric acids as a useful tool for the construction of coordination and supramolecular compounds.Coord.Chem.Soc.2014，265，1-37.

（2）Holliday，B.J.；Mirkin，C.A.Strategies for the construction of supramolecular compounds through coordination chemistry.Angew.Chem.Int.Ed.2001，40，2022-2043.

（3）Kreno，L.E.；Leong，K.；Farha，O.K.；Allendorf，M.；Van Duyne，R.P.；Hupp，J.T.Metal-organic framework materials as chemical sensors.Chem.Rev.2012，112，1105-1125.

（4）Stck，N.；Biawas，S.Synthesis of metal-organic frameworks （MOFs）:routes to various MOF topologies，morphologies，and composites.Chem.Rev.2012，112，933-969.

（5）Cheon，Y.E.；Suh，M.P.Enhanced hydrogen storage by palladium nanoparticles fabricated in a redox-active metal-organic framework.Angew.Chem.Int.Ed.2009，48，2899-2903.

（6）Chen，B.；Xiang，S.；Qian，G.Metal-organic frameworks with functional pores for recognition of small molecules.Acc.Chem.Res.2010，43，1115 -1124.

（7）Muller，I.M.；Robson，R.；Separovic，F.A Metallosupramolecular capsule with the topology of the tetrahedron，33，assembled from four guanidine-based ligands and twelve cadmium centers.Angew.Chem.Int.Ed.2001，40，4385-4386.

（8）Eddaoudi，M.；Moler，D.B.；Li，H.L.；Chen，B.L.；Reineke，T.M.；Keeffe，M.O.；Yaghi，O.M.Modular chemistry:secondary building units as a basis for the design of highly porous and robust metal-organic carboxylate frameworks.Acc.Chem.Res.2001，34，319-330.

（9）Wang，J.L.；Jiao，Y.；Yang，B.S.Supramolecule assembled from sulfonyl hydrazone Ni（Ⅱ）complex via π··π and C-H··π interactions.Chin.J.Inorg.2014，30，411-418.

（10）Zhang，S.F.；Lin，J.G.；Lin，J.；Xie，S.B.；Liu，J.B.；Li，H.H.；Chen，Z.R.A new bismuth（III）iodide inorganic/organic hybrid solid containing large heterocyclic conjugated organic ligand:synthesis，properties and calculations.Chin.J.Struct.Chem.2014，33，549-556.

（11）Tothadi，S.；Joseph，S.；Desiraju，G.R.Synthon modularity in cocrystals of 4-bromobenzamide with n-alkanedicarboxylic acids:type I and type II halogen··halogen interactions.Cryst.Growth Des.2013，13，3242-3254.

（12）Zhao，Y.；He，L.L.；Xu，H.；Li，X.Y.；Zang，S.Q.Two metal-organic coordination polymers assembled from a flexible iodo-dicarboxylate ligand and N-donor ancillary ligands.Inorg.Chim.Acta 2013，404，201-205.

（13）Gu，J.Z.；Kirillov，A.M.；Wu，J.；Lv，D.Y.；Tang，Y.；Wu，J.C.Synthesis，structural versatility，luminescent and magnetic properties of a series of coordination polymers constructed from biphenyl-2，4，4΄-tricarboxylate and different N-donor ligands.CrystEngComm.2013，15，10287-10303.

（14）Gu，J.Z.；Gao，Z.Q.；Tang，Y.pH and auxiliary ligand influence on the structural variations of 5（2΄-carboxylphenyl）nicotate coordination polymers.Cryst.Growth Des.2012，12，3312-3323.

（15）Sheldrick，G.M.SHELXS 97，Program for Solution of Crystal Structure.University of Göttingen，Germany 1997.

（16）Sheldrick，G.M.SHELXL 97，Program for Refinement of Crystal Structure.University of Göttingen，Germany 1997.

（17）Almeida，L.D.；Grandjean，S.；Rivenet，M.；Patisson，F.；Abraham，F.Hydrazinium lanthanide oxalates:synthesis，structure and thermal reactivity of N2H5［Ln2（C2O4）4（N2H5）］·4H2O，Ln = Ce，Nd.J.Chem.Soc.，Dalton Trans.2014，43，4680-4688.

（18）Chen，X.Y.；Goff，G.S.；Scott，B.L.；Runde，W.Comparison of structural variations of Ln（III）compounds with （pyrazol-1-yl）acetic acid.Polyhedron 2014，65，80-86.

（19）Chen，L.；Gou，S.H.；Wang，J.Q.Three new3D coordination polymers constructed by biphenyl-2，2΄，4，4΄-teracarboxylic acid:effect of metal ions and the second ligands.J.Mol.Struct.2011，991，149-157.

（20）Gao，Z.Q.；Li，H.J.；Gu，J.Z.Synthesis，crystal structure and magnetic properties of a Zn-Nd heterometallic coordination polymer constructed by 2-propyl-1H-imidazole-4，5-dicarboxylic acid.Chin.J.Inorg.Chem.2013，29，2460-2464.

（21）Sun，D.；Han，L.L.；Yuan，S.；Deng，Y.K.；Xu，M.Z.；Sun，D.F.Four new Cd（II）coordination polymers with mixed multidentate N-donors and biphenyl-based polycarboxylate ligands:syntheses，structures，and photoluminescent properties.Cryst.Growth Des.2013，13，377-385.

（22）Zhang，Y.；Guo，B.B.；Li，L.；Liu，S.F.；Li，G.Construction and properties of six metal-organic frameworks based on the newly designed 2-（p-bromophenyl）-imidazole dicarboxylate ligand.Cryst.Growth Des.2013，13，367-376.

10.14102/j.cnki.0254-5861.2011-0455

18 July 2014；accepted 23 September 2014 （CCDC 1013867 for 1 and 1013868 for 2）

① This project was supported by the Natural Science Foundation of Shanxi Province （No.2013011011-6）and the Subject Team of Taiyuan University of Science and Technology

② Corresponding author.Doctor，mainly engaged in coordination chemistry.E-mail:zqgao2008@163.com and gujzh@lzu.edu.cn