Abstract Rare earth(III) complex with salicylic acid Eu(H₂Sal)(HSal)(Sal)·H₂O（H₂Sal = C₇H₆O₃，HSal = C₇H₅O₃，Sal = C₇H₄O₃）was synthesized, and it was charaterized by IR and CV (Cyclic Voltammetry). The constant-volume combustion energy of complex, , was determined as (-9029.50 ± 3.96 ) kJ·mol^-1 by a precise rotating-bomb calorimeter at 298.15K. Its standard molar enthalpy of combustion, , and standard molar enthalpy of formation, , were calculated as (-9029.50 ± 3.96 ) kJ·mol^-1 and (-2488.96 ± 43.82) kJ·mol^-1 , respectively.
Keywords rare earth, salicylic acid; cyclic voltammetry; constant-volume combustion energy; standard molar enthalpy of formation

Nowadays great attention has been paid to the lanthanide aromatic carboxylates owing to its novel structures and potential applications in material sciences such as superconductor, magnetic materials, and luminescent probes ^[1-5]. Moreover, they are a kind of potential luminescent materials for further application. The luminescence, mechanism of thermal decomposition, stability constants and other properties of some lanthanide complexes with salicylic acid have been reported in literature ^[6-9]. However, thermochemistry of lanthanide complexes with salicylic acid has not been investigated. In this paper, the constant–volume combustion energy of the complex was determined by a precise rotating-bomb calorimeter at 298.15K, and its standard molar enthalpy of combustion and standard molar enthalpy of formation were calculated on the basis of the constant-volume combustion energy of complex. The final results would provide theoretical basis for enlarging their application fields.

1 EXPERIMENTAL
1.1 Reagents and apparatus    
Europium chloride hydrate EuCl₃·3.53H₂O was prepared according to Ref. ^[10]. Salicylic acid and other reagents are of A. R. grade.
    The C and H contents were measured by a Vario EL III CHNOS elemental analyzer made in Germany; the rare earth ion was determined by EDTA volumetric analysis; IR spectra was recorded with a Model EQUINOX55 FTIR spectrophometer ( KBr pellet); Cyclic voltammograms were made with Potentiostat/Galvanostat Model 283 (EG&G company, America), using with a three-electrode system with glass-carbon electrode as a working electrode, SCE (saturated calomel electrode) as a reference electrode and a platinum wire counter electrode; the constant-volume combustion energy of complex was carried out by a RBC-type II precise rotating-bomb calorimeter; melting point of the complex was measured with WRS-1A digital melting-point apparatus.
1.2 Preparation of complex   
The complex was prepared according to Ref. [11], 8.2873g salicylic acid was dissolved in NaOH solution with molar ratio of H₂Sal: NaOH=1: 1, and the salicylate sodium formed at 60℃. 6.4409g EuCl₃·3.53H₂O was dissolved in distilled water (the molar ratio of EuCl₃: NaHSal is 1:3), the solution of EuCl₃·3.53H₂O was added dropwise into the solution of salicylate sodium at the condition of electromagnetic stirring, pH was adjusted to 6.0, then the yellow precipitate was formed. After cooling the solution, filtered, the solid product was washed by distilled water repeatedly, then it was kept in a vacuum desiccator and dried by P₄O₁₀ until constant weight. The product was 10.4238g, and the yield was 89.7%.

2 RESULTS AND DISCUSSION
2.1 The analytical result of Eu(H₂Sal)(HSal)(Sal)·H₂O Composition 
Found (%): Eu 26.19, C 43.75, H 2.75; Calc. (%): Eu 26.15, C 43.38, H 2.95.
2.2 IR spectra of the complex           
The IR spectra of salicylic acid and the complex were recorded over the range 4000-400cm^-1. It can be seen from the IR spectra that salicylic acid has five characteristic absorption peaks: n_O-H (COOH, intramolecular hydrogen bonds, 3237.45cm^-1), n_O-H (COOH, intermolecular hydrogen bonds, 2857.57 cm^-1), n_O-H (COOH, 2596.33 cm^-1), n_C=O (COOH, 1660.76cm^-1), d_O-H ( phenolic hydroxyl, 1480.43 cm^-1). IR spectra of complex has a hydroxyl characteristic peak at 3289.31 cm^-1, in which it appeared the symmetrical stretching vibration absorption peak (n_sCOO^-,1387.32 cm^-1) and dissymmetrical stretching vibration absorption peak (n_asCOO^-,1560.93 cm^-1) of carboxyl. It is assumed that carboxyl disengaging the proton as acid radical coordinates to Eu³⁺. Three modes which carboxyl coordinated with Eu³⁺ were obtained from the crystal structure of the complex ^[12], some is monodentate coordination and others are bidentate coordination (Fig.1), only parts of phenolic oxygen participate in coordination. So there still are hydroxyl characteristic peaks in IR spectra of the complex, the molecular formula for the complex is Eu(H₂Sal)(HSal)(Sal)·H₂O.

Fig.1 Three coordination modes of salicylate with Eu³⁺

Fig.2 The cyclic voltammetry curves of Eu(H₂Sal)(HSal)(Sal)·H₂O (curve 1) and EuCl₃·3.53H₂O(curve 2) on glass-carbon electrode

2.3 Electrochemical behavior                      
The test bottom liquid of electrochemistry was Tris-HCl buffer solution (pH=6.0). The complex was dissolved with a small quantity of DMF and the solution was diluted to 1.6×10^-4mol·L^-1 with the bottom liquid. The oxygen was removed by aerating nitrogen 10 min. The scan scope was 0.0 - -1.0V, the scan speed was 100mV/s. Then the cyclic voltammetry behavior of Eu(H₂Sal)(HSal)(Sal)·H₂O was showed as Fig.2.
    At potential scope of 0.0 - -1.0V (vs. SCE), the complex Eu(H₂Sal)(HSal)(Sal)·H₂O (curve 1) has only a pair of redox peaks in the buffer solution of Tris-HCl (pH=6.0), potential of reduction peak E_pc=-0.698V, potential of oxidation peak E_pa=-0.59V, DE=108mV. This result is similar to Eu(III)-tryptophane-benzimidazole ternary complex^[13], so it can be concluded that the electrochemical active center of Eu(H₂Sal)(HSal)(Sal)·H₂O was Eu³⁺, and the electrochemical process of the title complex was single electron redox process.

    At the same time, the cyclic voltammetry behavior of EuCl₃·3.53H₂O (curve 2) was also tested in the buffer solution of Tris-HCl (pH=6.0). It also can be seen from the curve 2 that there was a pair of redox peaks, the potential of reduction peak E_pc=-0.658V, the potential of oxidation peak E_pa=-0.60V, DE=58mV. Moreover, we can find in the figure that the redox peaks of dissociating Eu³⁺ was not very clear-cut, which indicates the electrochemical reversibility of dissociating Eu³⁺ is stronger than the complex. It also can be noticed that the reduction peak of complex is more negative than dissociating Eu³⁺, which shows that the Eu³⁺ become steadier as the center of the complex, and the electrochemical reversibility of complex is less than dissociating Eu³⁺.
2.4 Combustion energy of the complex
2.4.1 Experimental conditions
The constant-volume combustion energy of the complex was determined by a precise rotating-bomb calorimeter (RBC-type II)^[14]. The analytical methods and main experimental procedures were described previously^[14].
    The initial temperature was regulated to (25.0000 ± 0.0005) ℃, and the initial oxygen pressure was 2.5 MPa. The correct value of the heat exchange was calculated according to Linio-Pyfengdelel-Wsava formula^[15]. The calorimeter was calibrated with benzoic acid of 99.999 % purity. (Chengdu Chemical Reagent Company), which has an isothermal heat of combustion of -26434 J·g^-1 at 25℃, the calibrated experimental results with an uncertainty 4.18×10^-4 were (17775.09±7.43) J·K^-1. The analytical methods of final products (gas, liquid and solid) were the same as these in Ref. ^[14], the analytical results of the final products showed that the combustion reaction was complete. The calibrated experimental results were summarized in Table 1.

Table 1 Result for Calibration of Energy Equivalent of the Rotating-bomb Calorimeter

2.4.2 Constant–volume combustion energy of the complex
The methods of determination and calculation of the constant-volume combustion energy for complex are the same as for the calibration of the calorimeter with benzoic acid. The values are calculated by means of the following equation:
(1)
    where (complex, s) denotes the constant-volume combustion energy of the complex (in J·g^-1), W is the energy equivalent of the rotating-bomb calorimeter (in J·K^-1), a is the length of the actual Ni-Cr wire consumed (in cm). G is the combustion enthalpy of Ni-Cr wire for ignition (0.9 J·cm^-1), 5.97 is the formation enthalpy and solution enthalpy of acid corresponding to 1 mL of 0.1000 mol·L^-1 solution of NaOH (in J·mL^-1), b is the volume (in mL) of consumed 0.1000 mol·L^-1 solution of NaOH and DT is the correct value of the temperature rise. m is the mass (in g) of the complex. The experimental results of the combustion energy of the complex were given in Table 2.

Table 2 Experimental Results for the Combustion Energy of the Complex

2.4.3 Standard molar combustion enthalpy of the complex
The standard molar combustion enthalpy of the complex, [Eu(C₇H₅O₃)₃·H₂O, s, 298.15K], refers to the combustion enthalpy change of the following ideal combustion reaction at 298.15K and 100kPa.
Eu(C₇H₅O₃)₃·H₂O(s)+ 21O₂(g)=Eu₂O₃(s)+21CO₂(g)+H₂O(l) (2)
    The standard molar combustion enthalpy of the complex is calculated by the following equations:
[Eu(C₇H₅O₃)₃·H₂O, s]= +ΔnRT (3)
= n_g (products)-n_g (reactants) (4)
    where n_g is the total amount in mole of gases present as products or as reactants, R=8.314 J·K^-1·mol^-1, T=298.15K. The result of the calculation of [Eu(C₇H₅O₃)₃·H₂O, s, 298.15K] is (-9029.50 ± 3.96 ) kJ·mol^-1.
2.4.4 Standard molar enthalpy of formation for the complex
The standard molar enthalpy of formation of the compound, [Eu(C₇H₅O₃)₃·H₂O, s, 298. 15K], is calculated by Hess's law according to the above thermochemical equation (2)
[Eu(C₇H₅O₃)₃·H₂O, s]= [(Eu₂O₃, s)+21(CO₂, g)+(H₂O, l)] -[Eu(C₇H₅O₃)₃·H₂O, s] (5)
    where (Eu₂O₃, s) = (-1650.4 ± 4.0) kJ·mol^-1; (CO₂, g) = (-393.51 ± 1.3) kJ·mol^-1; (H₂O, l) = (-285.830 ± 4.0) kJ·mol^{-1 [16,17]}.
   The result of the calculation of [Eu(C₇H₅O₃)₃·H₂O, s, 298. 15K] is (-2488.96 ±43.82 ) kJ·mol^-1.
    The standard molar enthalpy of formation of salicylic acid is -589.5kJ·mol^{-1 [17]}. Compared with the enthalpy of formation for the complex, the latter is more negative; on the other hand, compared with its melting point, the melting point of the complex (>300℃) is higher than that of salicylic acid (157-159℃) ^[18], all these indicate that the complex is steadier than the ligand.

配合物Eu(H₂Sal)(HSal)(Sal)·H₂O的热化学性质研究
孙妩娟 杨旭武* 张宏芳 张航国 李娜 朱丽 高胜利
（西北大学 化学系/陕西省物理无机化学重点实验室, 陕西 西安 710069）
摘要 合成了稀土水杨酸配合物Eu(H₂Sal)(HSal)(Sal)·H₂O（H₂Sal= C₇H₆O₃，HSal=C₇H₅O₃，Sal=C₇H₄O₃），通过红外，循环伏安法对配合物进行表征。在298.15K下，用精密转动弹热量计测定出配合物的恒容燃烧能为(-9029.50±3.96) kJ·mol^-1，计算出标准摩尔燃烧焓和标准摩尔生成焓分别为(-9029.50±3.96) kJ·mol^-1，（-2488.96±43.82）kJ·mol^-1。
关键词  稀土，水杨酸，循环伏安法，恒容燃烧能，标准摩尔生成焓