LI Wei PENG Gng LAI Hu HUANG Geng LI Chng-Hong

a (School of Chemistry and Materials Science,

Hengyang Normal University, Hengyang, Hunan 421008, China)

b (School of Materials and Chemical Engineering,

Hunan Institute of Technology, Hengyang, Hunan 421002, China) c (Key Laboratory of Functional Metal-organic Compounds of Hunan

Province Hengyang, Hengyang, Hunan 421008, China)

ABSTRACT A new binuclear cage-like samarium(III) complex Sm2(C7H4ClCOO)6(C12H8N2)2(H2O)2 (1) with samarium(III) nitrate, m-chlorobenzoic acid and 1,10-phenanthroline(phen) was synthe- sized. It crystallizes in the triclinic space group P1, with a = 8.0217(2), b = 12.9037(4), c = 15.3764(5) ?, α = 85.373(3)°, β = 84.396(3)°, γ = 80.443(3)°, V = 1558.64(8) ?3, Dc = 1.636 g/cm3, Z = 2, F(000) = 806, the final GOOF = 1.051, R = 0.0754 and wR = 0.1388. The whole molecule consists of two samarium ions bridged by four m-chlorobenzoic acid anions. The Sm(III) ion is coordinated by eight atoms to give a distorted square antiprism coordination geometry. The TG analysis and fluorescent properties of 1 were studied.

Keywords: binuclear samarium(III) complex, thermal stability, fluorescent property;

1 INTRODUCTION

In recent years, lots of people devote themselves to explore and synthesize 3d-4fheteronuclear coor- dination polymers, because this kind of polymer has not only various crystal structures but also potential applications in the fields of adsorption, catalysis, magnetism, etc., especially in the field of lumine- scence[1-3]. Lanthanide coordination complexes have gained much interest due to their broad applications in light emitting diodes, fluorescent probes, sensors, bioimaging utilities, time-resolved luminescent immunoassays, and anion sensing[4-6]. In particular, the photoluminescent properties of lanthanide com- plexes are notable due to their unique properties such as sharp emission bands, long emission lifetime and large stokes shift[7-9]. On the other hand, the assembly of lanthanide(III) complexes has attracted much attention because of their interesting lumine- scent and magnetic properties[10-12]. In order to obtain strongly luminescent lanthanide complexes, the selection and design of organic ligands are very important, and it should be able to efficiently absorb and transfer energy to the central metal ion and encapsulate and protect the lanthanide ion from the coordination of solvent molecules[13,14]. In this study, we have presented the synthesis, crystal structure and photoluminescent properties of a binuclear cage-like Sm(III) complex Sm2(C7H4ClCOO)6(C12H8N2)2(H2O)2withm-chlorobenzoic acid and 1,10-phenanthroline (phen). The TG analysis and fluorescent properties of 1 were also reported.

2 EXPERIMENTAL

2. 1 Materials and instruments

All reagents were of analytical grade and used as obtained from commercial sources without further purification. 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 analy- zer. XT4 binocular microscope melting point apparatus was used to measure the melting point with thermometer unadjusted. IR spectra were recorded on a Bruker Vector22 FT-IR spectrophotometer using KBr discs. The luminescence spectra for the powdered samples were measured on a RF-5301PC spectrofluorometer with a xenon arc lamp as the light source. In the measurement of emission and excita- tion spectra, the pass width is 5 nm, and all the measurements were carried out in the solid state at room temperature. Thermogravimetric analyses were performed on a simultaneous SPRT-2 pyris1 thermal analyzer at a heating rate of 10 K/min.

2. 2 Synthesis of 1

A mixture ofm-chlorobenzoic acid (46.8 mg, 0.3 mmol), phen (18.0 mg, 0.1 mmol), and Sm(NO3)3·5H2O (85.3 mg, 0.2 mmol) was dissolved in 25 mL of mixed solvent (the volume ratio of ethanol and water: 1:1). The pH value of the resultant mixture was adjusted to 6.5 by adding sodium hydroxide solution. The reaction was kept stirring at 150 ℃ for 54 h, and it was cooled to room temperature at the speed of 10 ℃/ h, obtaining colourless crystals of 1 suitable for X-ray diffraction analysis in 45.36% yield. m.p.: 285～287 ℃. Anal. Calcd. (%) for C32H40Cu2N8O14: C, 48.62; H, 2.72; N, 3.44. Found (%): C, 48.47; H, 2.71; N, 3.45. Main IR (KBr, cm-1): IR (v/cm-1): 1678(vs), 1595(vs), 1484(vs), 1423(vs), 1290(m), 1234(w), 1105(w), 1072(m), 841(m), 806(w), 727(vs), 694(s), 601(s), 470(s).

2. 3 Determination of the crystal structure

A single crystal with dimensions of 0.25mm × 0.22mm × 0.20mm was put on a Bruker SMART APEX CCD diffractometer equipped with a graphite- monochromatic MoKαradiation (λ= 0.71073 ?) using aφ-ωscan mode at 153.72(10) K. A total of 12507 reflections were collected in the range of 3.25≤θ≤25.01°, of which 5496 were independent (Rint= 0.0555) and 4813 were observed (I＞ 2σ(I)). All data were corrected byLpfactors and empirical absorption. The crystal structure was solved directly by program SHELXS-97, and refined by program SHELXL-97[15]. The hydrogen and non-hydrogen atoms were corrected by isotropic and anisotropic temperature factors respectively through full-matrix least-squares method. The finalR= 0.0754,wR= 0.1388 (w= 1/[σ2(Fo2) + (0.0261P)2+ 0.5359P], whereP= (Fo2+ 2Fc2)/3), (Δ/σ)max= 0.002,S= 1.149, (Δρ)max= 0.985 and (Δρ)min= -0.884 e·?-3.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

The coordination structure of 1 is revealed in Fig. 1, and square antiprism coordination geometry of the central Sm(III) ion of 1 is shown in Fig. 2. Selected bond lengths and bond angles are listed in Table 1.

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

Fig. 1. Coordination structure of complex 1 (i: 1 - x, 1 - y, 1 - z)

Fig. 2. Square antiprism coordination geometry of the central Sm(III) ion of 1

Sm2(C7H5ClCOO)6(C12H8N2)2(H2O)2(1) is a binuclear cage-like structure complex in space groupP1. As illustrated in Fig. 1, the complex is inclusive of two Sm(III) ions, two phen molecules, sixm-chlorobenzoic acid anions and two water mole- cules. As depicted in Fig. 2, the coordination polyhedron of the samarium cations is formed by two equivalent nitrogen atoms from phen molecule, five oxygen atoms of them-chlorobenzoic acid anions and one oxygen atom from the water molecule, giving a distorted square antiprism coordination geometry. Atoms O(3), O(5), O(4i) and O(6i) give a upper plane of the square antiprism, and O(1), O(7), N(1) and N(2) determine the plane below, with the dihedral angle between them to be 7.5°. The Sm-N bond length range is 2.616(4)～2.639(4) ? and the Sm-O scope is 2.353(5)～2.441(4) ?. Sm(1)-O(4i) of 2.441(4) ? is longer than the other Sm-O, and the bond angle O(6i)-Sm(1)-N(1) (149.71(11)°) is greater than the other angles. All the above bond features are characteristic of this type of polyhedron. The coordination mode of the carboxylate groups are bidentate chelate or monodentate with the average Sm-O(carboxyl)distance to be 2.3988 ?, slightly shorter than the average Sm-O(water)bond length of 2.428(3) ?. The Sm···Sm distance is 4.268(3) ?, which is slightly longer than that in the similar Sm(III) complexes ([Sm(2,5-pydc) (NO3)(H2O)]- (H2O), Sm···Sm = 4.0294 ?)[16]and ([Sm- (btcH2)0.5(btc)0.5((H2O)]n·2n(H2O), Sm···Sm = 3.951 ?)[17], but slightly shorter than that of [Sm(pic)2- (pydc)4(H2O)]n(Sm···Sm = 4.52 ?)[18], which falls in the normal range.

There are hydrogen bonding interactions with D···A separations in the 2.635(4)～2.774(4) ? region and the D-H-A angels vary from 132.7(1)° to 159.8(3)°. The hydrogen bonding interactions are linked by oxygen atoms between water molecules andm-chlorobenzoic acid anions (O(7)-H(7A)···O(6) and O(7)-H(7B)···O(2)), extending the mononuclear compound into a 3Dsupramolecular network (Fig. 3). From Fig. 3, there are lots ofπ-πaccumulations between adjacent molecules due to a large number of aromatic rings. The results suggest that the reaction conditions have remarkable influence on the structure of the complex.

Fig. 3. Hydrogen bonding and π-π role between the benzene ring

3. 2 Spectrum analysis of 1

Solid-state luminescent properties of complex 1 were measured at room temperature. As can seen from Fig. 4, when excited at 356 nm, 1 presents samarium-centered luminescence with its emission peak at 561 nm (4G5/2→6H5/2), 601 nm (4G5/2→6H7/2) and 643 nm (4G5/2→6H9/2). The most intense emission peak appears at 643 nm, which is in good agreement with the previously reported Sm3+complexes[19-21]. The intense broad band corresponds to the excitation of organic chromophore and other weaker peaks due to intra-configurational transitions from the ground state of Sm3+[13,14].

Fig. 4. Emission spectra of complex 1 in the solid state

3. 3 Thermal stability of 1

The thermogravimetric analysis (Fig. 5) of 1 demonstrated that its weight loss in air occurs mainly in 3 stages from room temperature to 600 ℃. The first one occurs from 110 to 140 ℃ with the weight loss of 2.20%, corresponding to the release of two free water molecules (calcd.: 2.21%). The second one is observed from 140 to 260 ℃ with the weight loss of 22.20% resulting from the release of two phen molecules (calcd.: 22.10%). The third stage takes place at 260 to 390 ℃ with the weight loss of 40.73% due to the departure of sixm-chlorobenzoic acid anions (calcd.: 40.81%). In air, the final product is samarium oxide with the final residual rate to be about 34.87% (calcd.: 34.88%).

Fig. 5. TG of complex 1