YANG Ying-Qun LI Wei YANG Pin-Fang KUANG Yun-Fei

a (Key Laboratory of Functional Metal-organic Compounds of Hunan Proνince & Key Laboratory of Functional Organometallic Materials , Uniνersity of Hunan Proνince, Hengyang421008, China)

b (Department of Chemistry and Materials Science, Hengyang Normal Uniνersity, Hengyang421008, China)

ABSTRACT A new terbium(III) complex Tb2(C15H11O3)6(phen)2 has been synthesized with 2-(4-methylben- zoyl)benzoic acid and 1,10-phenanthroline as ligands.Crystal data for the complex are as follows: monoclinic, space group P21/n, a= 15.0786(6), b= 13.5762(5), c= 22.9683(9) Å, β= 104.130(4)°, V= 4559.6(3) Å3, Dc= 1.540 g/cm3, Z= 2, μ(MoKα) = 1.615 mm-1, F(000) = 2136, the final R= 0.0528 and wR= 0.0740.The Tb(III) ion is coordinated by eight atoms to give a distorted square antiprism coordination geometry.The complex shows four fluorescence emission bands arising from the transitions of Tb3+:5D4 → 7F6 (490 nm), 5D4 → 7F5 (546 nm), 5D4 → 7F4 (588 nm) and 5D4 → 7F3 (622 nm).The complex is an antiferromagnet in the range of 300～2 K.Also reported in the paper is the thermal stability property of the title complex.

Keywords: terbium(III) complex, crystal structure, fluorescence and magnetic properties;

1 INTRODUCTION

Lanthanide(III) complexes have captured the extensive interest of researchers due to their well-defined luminescence property, including hypersensitivity to the coordination environment, high efficiency of light conversion, wide emission spectrum wavelength distribution, narrow bandwidth and millisecond luminescence decay time[1-4].As a result, in this field, much attention has been focused on the terbium(III) complexes because of their interesting physicochemical behaviors and potential applications in various areas including nonlinear optics, light emitting diodes (LEDs), luminescent probes, single-molecule magnets and so forth[5-8].It is well-known that the fascinating physicochemical properties of terbium complexes mainly result from the rich 4felectrons of the terbium element.However, absorption coefficients of the optical transitions for terbium(III) ions are usually very low, which limits the transition of 4felectrons.In order to obtain ideal luminescent materials, researchers have designed and prepared many new terbium complexes with aromatic carboxylic acid ligands, for those ligands can probably absorb the ultraviolet light and then transfer the absorbed energy to the terbium(III) ion[9-13].Among aromatic carboxylic acid ligands, the 2-(4-methylbenzoyl)benzoic acid and its derivatives have been shown to be efficient sensitizers for terbium(III) luminescence[14-16].With the aim of proceeding previous jobs[17-19]and constructing new terbium(III) complexes possesing efficient photoluminescence perfor- mance, we report herein a new terbium(III) complex Tb2(C15H11O3)6(phen)2based on 2-(4-methylbenzoyl)benzoic acid.The complex shows four fluorescence emission bands arising from the transitions of Tb3+:5D4→7F6(490 nm),5D4→7F5(546 nm),5D4→7F4(588 nm) and5D4→7F3(622 nm), and it is an antiferromagnetic system in the range of 300～2 K.Thermal stability property of the complex is also reported.

2 EXPERIMENTAL

2.1 Reagents and instruments

The reagents were obtained from commercial sources and used without further purification.FT-IR spectrum was obtained for KBr pellets on a Shimadzu Prestige-21 spectrophotometer in the 4000～400 cm-1range.C, H and N analyses were conducted with a PE-2400(II) apparatus.A fluorescence spectrum was obtained at room temperature on an RF-5301 fluorescence spectrophotometer.Magnetic measurements in the range of 300～2 K were performed on a MPMS-SQUID magnetometer at a field of 2 kOe on a crystalline sample in the temperature settle mode (1 kOe = 7.96 × 104A·m-1).Thermogravimetric analyses were recorded on a NETZSCH TG 209 F3 instrument at a heating rate of 20 ℃/min from room temperature to 850 ℃ under air.

2.2 Synthesis of the complex

A mixture of 2-(4-methylbenzoyl)benzoic acid (0.28 mmol) and terbium(III) nitrate hexahydrate (0.17 mmol) was dissolved in 8 mL mixed solvent of methanol and water (volume ratio 1:1).The pH value of the solution was adjusted to 5～6 by adding 0.25 mL sodium hydroxide solution (0.17 mol·L-1).The resultant solution was poured into 20 mL glass test-tube.Then 4 mL mixed solvent of water and methanol (volume ratio 1:2) and 1,10-phenanthroline (0.38 mmol) dissolved in 4 mL ethanol were added to this test-tube which, afterwards, was covered with plastic film.The mixture was put at room temperature for slow diffusion.Colorless single crystals suitable for X-ray diffraction analysis were obtained after six weeks.Yield: 37.4%.Anal.Calcd.(%) for Tb2(C15H11O3)6(phen)2: C, 64.78; H, 3.91; N, 2.65.Found (%): C, 64.72; H, 3.90; N, 2.64.IR(ν/cm-1): 1682(s), 1668(s), 1630(vs), 1605(s), 1547(s), 1408(vs), 1294(m), 1148(m), 854(m), 837(m), 743(m), 723 (s), 474 (w), 430 (w).

2.3 Structure determination and refinement

The X-ray diffraction measurement for the complex was carried out on a Bruker SMART APEX CCD area detector at 293(2) K by using graphite-monochromatized MoKα(λ= 0.71073 Å) radiation.The structure was solved by direct methods and refined by full-matrix least-squares with the SHELXL-2015 program package[20].Corrections forLpfactors and empirical adsorption adjustment were applied and all non-hydrogen atoms were refined with anisotropic thermal parameters.The final refinement including hydrogen atoms converged toR= 0.0528 andwR= 0.0740 (w= 1/[σ2(Fo2) + (0.0176P)2], whereP= (Fo2+ 2Fc2)/3), (Δ/σ)max= 0.002 andS= 0.922.

3 RESULTS AND DISCUSSION

3.1 IR spectra

Two strong peaks at 1668 and 1408 cm-1could be assigned to theνas(coo) andνs(coo) of 2-(4-methylbenzoyl)benzoic acid ligand, respectively.In addition, the characteristic peaks of 1,10-phenanthroline ligand appear at 858 and 725 cm-1in the complex.

3.2 Structural description

Fig.1 shows the molecular structure of the complex and the coordination polyhedron for the central Tb(III) ion.Selected bond lengths and bond angles are listed in Table 1.

As shown in Fig.1, the complex consists of two Tb(III) ions, six 2-(4-methylbenzoyl)benzoic acid anions and two 1,10-phenanthroline molecules.Two Tb(III) ions are linked by four 2-(4-methylbenzoyl)benzoic acid groups.The whole molecule of the complex displays a symmetric dimeric structure.The distance betweenTb(1) and Tb(1A) is 4.1477(5) Å.In the complex, 2-(4-methylbenzoyl)benzoic acid anions are coordinated with Tb(III) ions in two ways: bidentate chelation and bidentate bridging, respectively.Tb(III) ion is coordinated by six oxygen atoms from five 2-(4-methylbenzoyl)benzoic acid anions, and two nitrogen atoms from one 1,10-phenanthroline molecule.The central Tb(III) ion adopts a distorted square antiprism coordination geometry.In the coordination polyhedron (TbN2O6), atoms N(1), N(2), O(2) and O(1) give the upper plane of the square antiprism, and atoms O(8A), O(5A), O(7) and O(4) determine the plane below.Their plane equations are 14.082x- 4.612y- 2.751z= -0.3216 and 14.087x- 4.008y- 0.785z= 3.6658, respectively.The dihedral angle of the two planes is 5.7°.The bond lengths Tb(1)-O(1), Tb(1)-O(2), Tb(1)-O(4), Tb(1)-O(5A), Tb(1)-O(7) and Tb(1)-O(8A) are 2.496(4), 2.392(4), 2.312(4), 2.349(4), 2.289(4) and 2.381(4) Å, respectively and their average is 2.370 Å, which falls in the normal range.The bond angles of O-Tb-O vary from 54.26(14)° to 145.19(14)°.The N-Tb-O bond angles range from 69.05(14)° to 144.72(15)°.

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

Fig.1.Molecular structure of the title complex and the coordination polyhedron for the Tb(III) ion at 30% displacement ellipsoids

3.3 Fluorescent properties

The fluorescent properties of the complex in the solid state were measured at room temperature in the range of 220～640 nm.Both excitation and emission spectra are shown in Fig.2.As shown in Fig.2, the complex has a broad excitation peak near 330 nm.When the excitation wavelength is 330 nm, the complex shows four fluorescence emission bands at 490, 546, 588 and 622 nm, respectively, and the fluorescence is the strongest at 546 nm.They correspond to the transitions of Tb3+:5D4→7F6,5D4→7F5,5D4→7F4and5D4→7F3, respectively.

3.4 Magnetic properties

The magnetic susceptibility of the complex was investigated from 2 to 300 K with an applied magnetic field of 2 kOe.The temperature dependence of the molar magnetic susceptibility of the title complex is revealed in Fig.3 in the forms ofXmTand 1/Xm νs.T.As shown in Fig.3, the product ofXmTis 14.07966 cm3·K·mol-1at 300 K.Upon cooling, theXmTgradually decreases until 22 K to reach a product of 12.80582 cm3· K·mol-1and then quickly decreases to the corresponding product of 8.46034 cm3·K·mol-1at 2 K.In addition, in the temperature range of 300～2 K, the data are in linear relationship in the form of 1/Xm νs.T.The linear regression equation is 1/Xm= 0.0701T+ 0.1356, and the correlation coefficient is 1.According to the Curie-Weiss law,Xm=C/(T−θ), the Weiss constant (θ) can be obtained to be -1.934 K as a negative value.These magnetic behaviors show that the title complex exhibits antiferromagnetism.

Fig.2.Fluorescence spectrum of the title complex in the solid state

Fig.3.Temperature dependence of the magnetic susceptibility of the complex in the form of XmT and 1/Xm vs.T

3.5 Thermal analysis

The thermogravimetry/differential thermal gravity (TG-DTG) analysis was performed in air atmosphere at a heating rate of 20 ℃/min.TG-DTG curves are shown in Fig.4.As shown in the DTG curves of Fig.4, the title complex began to decompose at 287 ℃, and there is the first maximum weight loss rate temperature at 371 ℃.The second maximum weight loss rate temperature appears at 561 ℃.There are three weight-loss stages from 287 to 826.3 ℃.The first stage takes place from 287 to 446 ℃ with the weight loss of 22.63% (theoretical value: 22.71%), corresponding to the release of two 1,10-phenanthroline molecules and 1/2 2-(4-methylbenzoyl)benzoic acid anion.From stages 2 to 3, the total weight loss of 60.12% (theoretical value: 60.00%) occurs at 457 ～826.3 ℃ due to the loss of 5.5 2-(4-methylbenzoyl)benzoic acid anions excluding three oxygen atoms.The final products are Tb2O3, with the final residual rate being 17.25% (theoretical value: 17.30%).

Fig.4.TG-DTG curve of the title complex