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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.52 n.4 Concepción  2007

http://dx.doi.org/10.4067/S0717-97072007000400004 

 

J. Chil. Chem. Soc, 52, N° 4 (2007), págs: 1291-1293

 

SYNTHESIS AND THERMAL DECOMPOSITION KINETICS OF LANTHANUM(III) COMPLEX WITH UNSYMMETRICAL SCHIFF BASE

 

BI CAIFENG*1, AI XIAOKANG1, FAN YUHUA1, BI SHUANGYU2 AND XIE SITAN1

1Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao , 266100, P. R. China
2
Division of Life Science and Technology, Ocean University of China, Qingdao, China


ABSTRACT

A new unsymmetrical solid Schiff base (LLi) was synthesized using L-lysine, salicylaldehyde and furfural. Solid lanthanum(III) complex of this ligand [LaL(NO3)]NO3-2H20 have been prepared and characterized by elemental analyses, IR , UV and molar conductance .The thermal decomposition kinetics of the complex for the second stage was studied under non-isothermal condition by TG and DTG methods. The kinetic equation may be expressed as : da/dt = A · e-ElRT. (1-a)2 y The kinetic parameters(Δ, A), activation entropy ΔS* and activation free-energy ΔG* were also gained, E = 212.7 kJ/molΔlnLA =44.12, ℘S*=116.8 J/mol-KΔΔG*=148.1 kJ/mol.

Keywords: Unsymmetrical Schiff base, La(III) complex, Thermal decomposition, Non-isothermal kinetics.


INTRODUCTION

Some Schiff base complexes derived from amino acid are particularly active in biology. Recently, studies of such metal complexes of mono-Schiff bases have been reported[1-4]. To continent the investigation in this area, a new unsymmetrical Schiff base ligand has been synthesized starting from L-lysine, salicylaldehyde and furfural by a new method. Since this ligand dose not exist in literature , this paper deals with the preparation and characterization of the complex formed from this Schiff base ligand

with La(III). As thermal aspects are essential in the complex , the thermal decomposition process of [LaL(NO3)]NO3-2FLO by TG-DTG is described in this

paper and the corresponding non-isothermal kinetics are discussed .The kinetic equation

of thermal decomposition for the complex and the corresponding kinetic parameters were gained. This paper offered a new method preparing this kind of unsymmetrical Schiff base and its complexes. It is of important significance in the fields of biology and catalysis.

EXPERIMENTAL

Chemicals

All reagents used in this work were of analytical grade. Hydrated lanthanum(III)nitrate was prepared by the reaction of lanthanum (III) oxide with nitric acid.

Preparation of the ligand

Mono-Schiff base (HR): L-lysine (1.462 g, 10 mmol) was dissolved in 130 mL anhydrous ethanol and methanol in the proportion of 1:1(v/v) and heated for 1.5 hours at 55-50°C, and filtered. Salicylaldehyde (1.2 mL, 10 mmol) was added drop-wise to the above filtered solution and stirred for 2 hours at 50-55°C to obtain a yellow precipitate. The precipitate was collected by filtration, washed with ethanol and dried in vacuum. Yield, 1.902 g (76%); mp: 223~224°C.

Unsymmetrical Schiff base (LLi): HR(1.251 g, 5.0 mmol) and lithium hydroxide(0.120 g, 5.0 mmol) were dissolved in 60 mL anhydrous methanol and isopropanol in the proportion of l:5(v/v) and stirred for lhour at 50-55°. Furfural ( 0.481 g, 5.0 mmol) dissolved in 10 ml isopropanol was added drop-wise to the above solution and stirred for 4hour at 50-55°C to give a yellow precipitate. The precipitate was collected by filtration, washed with ethanol and dried in vacuum. The yield of the Schiff base (LLi) was 1.221 g (73%) and the purity was higher than 99%. Calculated for C18H19N204Li (334.3): C, 64.67; H, 5.73; N, 8.38, found: C, 65.45; H, 5.78; N, 8.32%.

Preparation of the complex

The unsymmetrical Schiff base(l.003 g, 3.0 mmol) dissolved in 60 mL anhydrous methanol and isopropanol in the proportion of 1:5 (v/v) was mixed with

La (NO3)3-6H20 (1.300 g, 3.0 mmol) dissolved in 15 mL anhydrous ethanol and stirred for 3 hours at 50-55°C to give yellow precipitate. The precipitate was filtered, recrystallized with anhydrous methanol and isopropanol in the proportion of 1:5 (v/v), and dried in vacuum. The yield of the complex was 1.315 g (70%) and the purity was higher than 99%. Calculated for C18H23N4012La (626.3): C, 34.52; H, 3.70; N, 8.95; La, 22.19, found: C, 34.77; H, 3.74; N, 8.94; La, 23.03%.

Physical measurement

Elemental analyses were carried out with a model 2400 Perkin-Elmer analyzer. The metal content was determined gravimetrically. The ultraviolet spectra were recorded on a Shimadzu UV-3000 spctrophotometer in DMSO. The molar conductance was measured with a Shanghai DDS-11A conductivity meter. Infrared spectra of the ligand and complex were recorded in KBr pellets using a Bio-Rad FTS 165 spctrophotometer. Thermogravimetric measurements were made using a Perkin-Elmer TGA7 instrument. The heating rate was programmed to be 10°C/min with a protecting stream of N2 flowing at a rate of 40 ml /min. The mass spectrogram of the ligand was recorded on a Finnegan MAT-212 mass spectrometer.

RESULTS AND DISCUSSION

The reaction activity and steric hindrance of the two -NH2 in L-lysine is different and the -NH2 in a seat have higher activity than the -NH2 in e seat because of the induced effect of-COO- in L-lysine. When the molar ratio of L-lysine and salicylidene was 1:1, the salicylidene reacted with the -NH2 in a seat first forming the mono-Schiff base. Then the mono-Schiff base reacted with furfural forming the unsymmetrical di-Schiff base. The synthesis reactions of the ligand are shown in Fig 1.The synthesis of the complex may be represented as:

La(NO3)3·xH20 + LLi = [LaL(NO3)]NO3-2H20 + LiNO3 +( x-2)H20

The molar conductance value of the complex determined in DMSO is 75.9 S- cm2-.mol-1, which is expected for 1:1 electrolytes[5]. This suggests that one nitrate ion is within the coordination sphere and the second is ionic and not coordinated. The complexes are stable in air and soluble in DMSO and DMF; however they are insoluble in diethyl ether, benzene, acetone .


Mass Spectrum

The mass spectrum of LLi is shown in Fig. 2. The molecular weight of LLi is 334, which indicates that the reaction product of L-lysine with salicylidene and furfural is an unsymmetrical di-Schiff base.


IR Sprctra

The shift of vC=N from 1634.1cm-1 in the ligand to 1620.9 cm-1 in the complex, suggests the formation of a C=N-La bond system. The vibrationv(Ar-O) of LLi occurs at 1247.5 cm-1. The shift to lower frequency about 48 cm-1 in the complex indicates the coordination of hydroxyl oxygen to metal ion .The shift o fv(C-O-C) from 1018.0 cm-1 in the ligand to 1125.5 cm-1 in the complex, which indicates the coordination of the oxygen in the methoxyl to metal ion. In the spectrum of the complex, five additional bands, which are not present in the spectrum of the ligand, were observed. Of these, the bond of 1033.3 cm-1 is assigned to the v2 mode of the nitrate group. The bands of 1537.7 and 1281.5 cm-1 in the complex are the two split bands of v4 andvt, respectively, of the coordinated nitrate group .The magnitude ofv -v is more thanl80 cm-1 for the complex, which indicates that the nitrate group in coordinated to the metal ion in a bidentate fashion. The bands at 1402.7 and 818.3 cm-1 are assigned to the non-coordinated nitrate group[6]. The shift of vas (coo-) and vs(coo-) from 1634.1 and 1402.3 cm-1 in the ligand to 1609.9 and 1402.7 cm-1 in the complex, respectively, suggests the coordination of the oxygen in the carboxylate group to the metal ion. The magnitude of vas (coo-) and vs(coo-) is more than 200 cm-1 in the complex, which indicates that the -coo- group is coordinated to the metal ion in a monodentate fashion[7]. The broad bands at 3160.4 cm-1 in the complex is attributed to v of phenol and water molecules.

Electronic spectra

The electronic spectrum of the complex in DMSO exhibits two spectral bands at 266 and 360nm, having the molar extinction coefficients ε=3.87x10-5, 5.13x10-3 L·mol-1cm-1, respectively. These bands occur at 270, 365nm (ε=5.43x104℘6.45x103 L·mol-1cm-1) in the spectrum of the ligand. The change of the molar absorptivity suggests that the ligand is coordinated to the metal.

Thermal decomposition studies

The TG and DTG curves of the complex are shown in Fig. 3, which indicates that the complex decomposes in four steps. The first weight loss stage has a decomposition temperature range of60-130℘, with a weight loss of 5.79% , which corresponds to the loss of two molecules of water (caled. 5.75%). The fact that the water molecule was lost at a low temperature suggests that the water is crystal water. The second weight loss stage showed a continuous weight loss between 130 and 370D, with a weight loss of 33.79%, which corresponds to the loss of unsymmetrical Schiff base ligand (caled. 34.20%). The third and fourth stage showed a continuous weight loss between 370 and 770℘, and 25.92% of the original sample remained. With its calculated weight percentage of 26.00%, La2O3 is the final product.


On the basis of 30 kinetic functions in both differential and integral forms commonly used in recent reviews[8], the non-isothermal kinetics of the steps were investigated using the Achar differential method[9] and the Coats-Redfern integral method[10].

The original kinetic data for the second step obtained form the TG and DTG curves are listed in Table 1, in which T. is the temperature at any point; on the TG and DTG curves, a1. is the corresponding decomposition rate, (da/dt)i= ß/(W0-W1] x (dWldT). in which (dWldT). is the height of the peak in the DTG curve, ß is the heating rate, and W0 and W1 are the initial and final weight at that stage, respectively. The calculated kinetic parameters (E, A) and correlation coefficients (r) of steps (2) are listed in Table 2.



The kinetic parameters (E, A)obtained from Achar differential method and the Coats-Redfern integral method are approximately the same when based on function No. 18 and the correlation coefficients (r) are larger than 0.98, the kinetic equation may be expressed as: da / dt = A · e-E/BST ·(1-a)2, E = 212.7 kJ/mol, 1n/A =44.12, r=0.9833.

The activation entropy ΔS* and activation free-energy ΔG* are calculated by the following equations[11-12]: A=kTsexp(ΔS*/R)/h, Ae-E/BST = kTexp(ΔS*/R) exp(-ΔH*/RT)/h, ΔG* = ΔH*-TΔS*, in which T is the temperature at the top of peak(2), k is Boltzmann constant, if is gas constant, h is Plank constant. The activation entropy ΔS* and activation free-energy ΔG* for second thermal decomposition stage were gained, ΔS*=116.8 J/mol·KΔΔG*=148.1 kJ/mol.

CONCLUSION

The results presented here indicate that L-lysine can react with salicylaldehyde and furfural forming unsymmetrical Schiff base LLi and lanthanum nitrate can form stable solid complex with this ligand. The composition of the complex is confirmed to be [LaL(NO3)]NO3-2H20. The kinetic equation for second decomposition step may be expressed as: da / dt=A · e-E/RT-(1-a)2, E = 212.7 kJ/molΔln/A =44.12 Δr=0.9833, ΔS*=116.8 J/ molΔKΔΔG*=148.1 kJ/mol.

 

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