Synthesis, Crystal Structure and Thermal Decomposition Mechanism of a Samarium o-Chlorobenzoate Complex with 1,10-Phenanthroline

O complexo [Sm(o–ClBA) 3 phen] 2 (o–ClBA= o-clorobenzoato; phen= 1,10-fenantrolina) foi preparado pela reação de SmCl 3 ·6H 2 O com ácido o-clorobenzóico e 1,10-fenantrolina. O complexo foi caracterizado por um estudo de difração de raios-X, análise elementar, espectroscopia no infravermelho e técnicas TG-DTG. O cristal é triclínico, grupo espacial P I com a = 13,157(3) Å, b = 14,139(4) Å, c = 17,397(4) Å, α = 83,755(3)o, β = 84,064(3)o, γ = 73,434(3)o, Z=2, Dc=1,722 Mg/m, F(000)=1572. Contém duas unidades cristalograficamente distintas de Sm(o-ClC 6 H 4 COO) 3 (C 12 H 8 N 2 ), formando dois tipos de moléculas binucleares. Cada Sm encontra-se coordenado a dois átomos de oxigênio de um grupo carboxilato bidentado quelante, cinco átomos de oxigênio de dois grupos carboxilatos bidentados ligados em ponte e de três grupos carboxilatos tridentados quelantes e ligados em ponte, e a dois átomos de nitrogênio de uma molécula de 1,10-fenantrolina, completando uma geometria monoencampuzada antiprismática quadrada. A decomposição térmica deste composto foi estudada por técnicas TG-DTG, e os parâmetros cinéticos da primeira etapa da decomposição foram determinados pelos métodos de Kissinger e Ozawa-Doyle. A equação de meia vida após 10% de perda de peso, lnτ= –26,8166+20319,94/T, foi deduzida por análise termogravimétrica da isoterma.


Introduction
The coordination chemistry of rare-earth cations with various carboxylic acids has become increasingly significant in the recent years, due to the potential applications in many areas, such as extraction, separation, germicide preparation, catalysis, luminescence and functional material preparation. 1Perhaps, the most important feature of the rare-earth carboxylate complexes is the large number of coordination types exhibited by these ligands, e.g, bidentate chelating, bidentate bridging or tridentate chelatingbridging. 2 For this reason there has been continuing interest in the study of such complexes.In previous work, [3][4][5][6][7][8][9][10][11][12][13][14][15] we reported the synthesis, crystal structure and thermal decomposition behavior of a number of europium or terbium complexes with benzoic acid or its derivatives and 1,10phenanthroline.In this work, we describe the synthesis and crystal structure, of the title complex, determined by a single crystal X-ray diffraction study.

Infrared spectra
The free acid COOH group band at 1693 cm -1 , completely disappears in the spectra of the complex.However, the bands arising from asymmetric and symmetric vibrations of the COO -group occur at 1591 cm -1 , 1545 cm -1 , 1519 cm -1 and 1477 cm -1 , 1403 cm -1 , 1350 cm -1 , respectively.This indicated that the carboxyl groups are bonded to Sm(III) ion in three coordination modes, 19 which was confirmed by X-ray diffraction analysis.In addition, the bands of ν C=N (1646 cm -1 ) and δ C-H (854cm -1 , 740cm -1 ) attributed to 1,10phenanthroline are observed to move lower wave numbers at 1617 cm -1 , 852 cm -1 and 730 cm -1 , respectively.This indicated that the Sm 3+ ions were coordinated by the nitrogen atoms of the 1,10-phenanthroline. 20

Crystal structure
Selected bond lengths and angles are listed in Table 2.The crystal is composed of two types of binuclear molecules, which are identical in composition with similar structures, as shown in Figure 1 13 in that they both consist of two similar types of binuclear molecules.However the average Sm-O and Sm-N distances in the later complex are slightly longer than those in [Sm(o-ClBA) 3 phen] 2 .The reason may be that the methyl is electron-donation group, while the chlorin is electron-withdraw group, which result in the enhancement of electron cloud density and the strength of the chemical bond.
In both binuclear molecules (a) and (b), the Sm-O bond distances of the chelating carboxylate are longer than those of the bridging carboxylates groups, which clearly shows that the coordination of Sm 3+ ion with the chelating carboxylate is weaker than that of Sm 3+ ion with the bridging carboxylates groups, or rather that the formation of fourmembered chelate ring with Sm 3+ ion is unstable. 21

Thermal decomposition mechanism
The TG-DTG curves of the title complex at a heating rate of 5 °C min -1 under a nitrogen atmosphere are shown in Figure 2. The DTG curve reveals three decomposition stages.The first stage starts at 279.55 °C and completes at 428.62 °C with a loss of 22.16% (theoretical loss is 22.60%), corresponding to the remove of 2 mol phen.The bond distance of the structure of the complex can demonstrate the degradation.Average bond length of Sm-N is longer than that of Sm-O, theoretically, Sm-N bond is less stable and easy to be broken down.The IR spectra of the residue at 428.62 °C also shows the disappearance of the absorption band of C=N at 1617 cm -1 .
The loss of the o-chlorobenzoate ligands occurs from 428.62 to 888.50 °C in two steps.The total weight loss is 54.80% (theoretical loss is 55.50%).Up to 888.50 °C, the title complex was completely degraded into Sm 2 O 3 (observed, 22.09%; calculated, 21.87%), which was demonstrated by the IR spectra of the residue at 888.50 °C.The bands of ν as(COO) (1580 -1546 cm -1 ) and ν s(COO) (1479 -1408cm -1 ) disappeared, and the IR spectra of the residue were same to standard spectra of Sm 2 O 3 .

Kinetic parameters of [Sm(o-ClBA) 3 phen] 2
The kinetic parameters for the first-step thermal decomposition of [Sm(o-ClBA) 3 phen] 2 could be determined by using Kissinger's method 16 and Ozawa- Doyle's method. 17,18The calculated results are listed in Table 3, which are consistent with each other.
Therefore, the Arrhenius equation can be expressed with the average of E and ln A for the first-step decomposition as follows: ln k = 39.26-229.1×10 3 / RT.

Lifetime
The general lifetime formula of materials is, 10 where τ is the lifetime at temperature T(K), a and b are constants.In this paper, the mass-loss of 10% lifetime was measured by isothermal temperature TG at 563.15, 573.15, 583.15 and 593.15K and listed in Table 4.By substituting the values in Table 4 into equation ( 1), the constants a, b and linear correlation coefficients r were obtained by the linear least squares method.The lifetime equation is lnτ= -26.8166+20319.94/T.Linear correlation coefficient r is 0.9967.

Materials
All reagents used were Analar grade and were used without further purification.

Preparation of complex [Sm( o-ClBA) 3 phen] 2
SmCl 3 •6H 2 O was obtained from the reaction of Sm 2 O 3 (99.95%)and HCl (6.0 mol L -1 ).A stoichiometric amount of o-chlorobenzoic acid and a quantitative amount 1,10phenanthroline were mixed and then dissolved in 95% ethanol solution.The pH value of the mixture was adjusted to 6-7 by adding 1.0 mol L -1 NaOH solution, and then added dropwise to the ethanolic SmCl 3 solution, while a white precipitate formed.The solution mixture was stirred for 8h at room temperature and then deposited for a day.The precipitate was filtered out and washed with deionized water and 95% ethanol.Yield: 89%.Colorless cubic crystals for X-ray diffraction analysis were obtained by the slow evaporation of the filtrate.IR (KBr) ν max /cm -

Experimental equipment and conditions
Elemental analysis was performed on a Carlo-Erba model 1106 elemental analyzer.The content of samarium was assayed using EDTA titration method.Infrared spectra were recorded over the range 4000-400 cm -1 using Bio-Rad FTS-135 spectrometer (KBr discs).X-ray diffraction data collection for a crystal with dimension 0.38 × 0.30 × 0.22 mm was performed on Bruker Apex IICCD diffractometer with graphite-monochromated Mo Kα radiation (λ=0.71073Å) at 293K, using Φ-ω scan mode.A semi-empirical absorption correction based on SADABS was applied.10701 Unique data (R int =0.0172) were used to solve the structure by direct methods using SHELXS-97 program, and refined on F 2 by full-matrix least squares methods using SHELXL-97 program.All non-H atoms were refined anisotropically, and the H atoms were included in the structure-factor calculations.The crystal data and refinement details of the complex are summarized in Table 1.The TG and DTG experiments for the title compound were determined using a Perkin-Elmer's TGA7 Thermogravimetric analyzer under a nitrogen atmosphere, at a flow rate of 25 mL min -1 .The heating rate used was 3, 5 and 7 °C min -1 from ambient to 925 °C, respectively.The sample weight was 2.6±0.2mg.

Conclusions
The title complex [Sm(o-ClBA) 3 phen] 2 were successfully synthesized in ethanol solution.Its crystal structure was determined by single crystal X-ray diffraction.The results show that the coordination number is nine and the coordination behavior of carboxylate groups with Sm 3+ ion appears to three different coordination modes.Thermal decomposition mechanism of the complex was proposed.The values of E and lnA for the first-step thermal decomposition were also determined.The Arrhenius equation can be expressed as ln k = 39.26-229.1×10 3 / RT.The lifetime equation at mass-loss of 10% was deduced as lnτ= -26.8166+20319.94/T.
(a) and (b).In each binuclear molecule, two Sm(o-ClC 6 H 4 COO) 3 (C 12 H 8 N 2 ) units are combined through crystallographic inversion center.Each Sm 3+ ion is coordinated to nine atoms, of which five atoms are from the bidentate bridging and tridentate chelating-bridging carboxylate groups, two oxygen atoms are from bidentate chelating carboxylate groups and two nitrogen atoms are from a 1,10phenanthroline molecule.The coordination polyhedron of Sm(III) ion adopts a distorted mono-capped square antiprism geometry.The oxygen atom from the tridentate chelating-bridging carboxylate group is at the capped position.The Sm-Sm distance in the molecule (a)(4.047

Table 2 .
Selected bond lengths (Å) and angles ( o ) for the molecules (a) and (b)