Zhu Li, Yang Xuwu, Chen Sanping, Gao Shengli, Shi Qizhen
(Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi’an Shaanxi 710069, China)

Received May 00, 2004; Supported by the National Natural Science Foundation of China (No. 20171036), Education Department of Shaanxi Province (No. 01JK229) and Northwest University (No.02NW02) for financial support

AbstractFour ternary solid complexes, Eu(Etdtc)(phen) (d), Gd(Etdtc)(phen) (e), Tb(Etdtc)(phen) (f) and Dy(Etdtc)(phen) (g), were synthesized with sodium diethyldithiocarbamate (NaEtdtc), 1,10-phenanthroline (-phen) and low hydrated lanthanide chlorides in absolute ethanol by improved method of reference. IR spectra of the complexes showed that the RE³⁺ (RE=Eu, Gd, Tb, Dy) is coordinated with sulfur atoms of NaEtdtc and nitrogen atoms of -phen. The constant-volume combustion energies of complexes,  were determined by a precision rotating-bomb calorimeter at 298.15 K. The standard enthalpies of combustion,and standard enthalpies of formation, , were calculated for these complexes, respectively.

The series of complexes lanthanide sulfide have been largely used for the precursors of ceramics and thin film materials ^[1-4]. For instance, the complexes synthesized with [(alkyl)dtc], -phen·HO and lanthanide salts had been acted as the volatile precursors for preparing lanthanide sulfide. Their friction properties in lubricant was investigated in literature ^[5], and the preparation and properties of these complexes were also documented in literature ^[6]. In addition, the crystal structure and spectroscopic properties of Eu(Etdtc)(phen) had been reported ^[7]
To our best knowledge, thermodynamic data could offer better interpretation to the essence of lanthanide-sulfide bonds and stability of this series of complexes, and no investigation has been carried out concerning the thermochemical properties for these complexes. In this paper, their standard enthalpies of combustion and standard enthalpies of formation have been calculated on the basis ofdetermination of the constant-volume energies combustion of complexes. The final results would provide theoretical basis for enlarging their application range.

1 EXPERIMENTAL
1.1 Reagents
Lanthanide chloride hydrate, RECl·O (RE=Eu, Gd, Tb, Dy; =3 – 4) were prepared according to Ref. ^[8]. Sodium diethyldithiocarbamate (NaEtdtc·3HO) (b) are of A. R. grade from Shanghai Reagent Company, 1,10-phenanthroline (-phen·HO) (c), absolute ethanol and CHCl are of A. R. grade from Xi’an Chemical Reagent Company.
1.2 Preparation and composition of the complexes
The complexes were synthesized by the following chemical equation:
RECl·O + 3NaEtdtc·3HO + -phen·HO  —–>        RE(Etdtc)(phen) + 3NaCl + (10+x)HO        (1)
(RE=Eu, Gd, Tb, Dy )

8mmol RECl·O, 24mmol NaEtdtc·3HO and 8 mmol -phen ·HO were dissolved in a minimal amount of anhydrous ethanol, respectively, then alcoholic solution of -phen and NaEtdtc were mixed together, and to it the salt alcoholic solution was dropped slowly when keeping electromagnetic stirring.After the addition, the mixture was allowed to stand 30 min and filtered. The crude product was rinsed by three a small amount of absolute ethanol portions, followed by purifying with CHCl. The fine crystal was obtained and kept in vacuum over P₁₀ to dryness.
RE³⁺ were determined with EDTA by complexometric titration; C, H, N and S analyses were carried out by an instrument of Vario EL III CHNOS of German. the final results were showed in Table 1, which are identified as the general formula of RE(Etdtc)(phen).

Table 1Analytical Results Related to the composition for , , and

Complexes	RE%	S%	C%	N%	H%
	19.60 (19.56)	24.77 (24.76)	41.72 (41.74)	9.04 (9.01)	4.89 (4.93)
	20.05 (20.10)	24.51 (24.59)	41.44 (41.46)	8.97 (8.95)	4.91 (4.90)
	20.28 (20.27)	24.52 (24.54)	41.29 (41.37)	8.99 (8.93)	4.92 (4.89)
	20.85 (20.63)	24.89 (24.43)	40.77 (41.18)	8.80 (8.89)	4.68 (4.86)

^{The data in brackets are calculated values.}

1.3 Apparatus and experimental conditions
The constant-volume combustion energies of the complexes were determined by a precision rotating-bomb calorimeter (RBC-type II)^[9]. The main experimental procedures were described previously ^[9]. The bicyclic support as showed in Fig.1 was used as the holder of the crucible in theoxygen bomb, which facilitates in the crucible stable relatively to the bomb when the bomb was rotated crosswisely and vertically, assuring that combustion reaction is going completely.

 

Fig.1 Bicyclic structure of the crucible support in the oxygen bomb
1.support; 2.-axle; 3.outside ring; 4.-axle; 5.inside ring

The initial temperature was regulated to (25.0000 ± 0.0005)ºC, and the initial oxygen pressure was 2.5 Mpa. The digital indicator for temperature measurement was used to promote the precision and accuracy of the experiment. The correct value of the heat exchange was calculated according to Linio-Pyfengdelel-Wsava formula:
) = nV +-n)              (2)
where) denotes the correct value of the heat exchange; is the number of readings for the main (or reaction) period; and are the rate of temperature change at the initial and final stages, respectively ( is positive when temperature decreased); is the average temperature of calorimeter at the initial and final stages, respectively (average temperature for first and last reading); is the last reading of the initial stage; is the first reading of the final stage;  is the sum of all the readings, except for the last one of the main period;is the constant.
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 25ºC. The calibrated experimental results with an uncertainty of 4.18×10^-4 were summarized in Table 2. The energy equivalent of the rotating-bomb calorimeter was calculated according to the following equation:

Where is the energy equivalent of the rotating-bomb calorimeter (in J·K^-1), is the combustion enthalpy of benzoic acid (in J·g^-1), is the mass of determined benzoic acid (in g), is the combustion enthalpy of Ni-Cr wire for ignition (0.9 J·cm^-1), is the length of the actual Ni-Crwire consumed (in cm), 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), is the volume (in mL) of consumed 0.1000 mol·L^-1 solution of NaOH and  is the correct value of the temperature rise.

Table 2 Result for Calibration of Energy Equivalent of the Rotating-bomb Calorimeter

No.	Mass of complex /g	Calibrated heat of combustion wire /J	Calibrated heat of acid /J	Calibrated /K	Energy equivalent /J·K-1
	0.99702	10.35	24.78	1.4834	17790.45
	0.78940	8.10	20.89	1.1746	17789.88
	0.83060	12.60	20.43	1.2382	17758.93
	0.96869	12.60	17.43	1.4418	17780.82
	0.99485	12.60	20.80	1.4800	17798.18
	1.12328	9.09	21.85	1.6735	17761.41
	0.90036	9.28	21.67	1.3429	17745.97
mean					17775.09±7.43

The analytical methods of final products (gas, liquid and solid) were the same as these in Ref^[10], the analytical results of the final products showed that the combustion reactions were complete.

2 RESULTS AND DISCUSSION
2.1 IR spectra of the complexes
IR spectra of the complexes are similar because of their similar structure. Taking Eu(Etdtc)(phen) for example, and referring to the literature ^[11,12], IR spectra of salt, ligands and the complex depicted in Fig. 2.are assigned as follows: Compared with the spectra of salt, NaEtdtc·3HO and -phen·HO (3390, 3366 and 3388 cm^-1), the characteristic absorption of hydroxyl group is not present in the complex, showing that the complex do not consist of water. As those in the ligand of -phen, the peaks of 1624, 1589, 1572, and 1516cm^-1 are assigned to the skeleton vibration of benzene ring and the peaks of 852, and 730 cm^-1 are assigned to the bend vibration of C–H in the complex, which display certain shifts in contrast with those of (1617, 1587, 1561, 1504 cm^-1) and (854, 739 cm^-1) in the ligand. It is thus assumed that two nitrogen atoms in the ligand of -phen coordinate to Eu³⁺. Contrasting with that of 1477 cm^-1 in the ligand of NaEtdtc·3HO,_CNof the complex shifts to higher wave number, and presents a double-bond character in the complex, which can be attributed to that NCS group has two main forms of vibration^[13]: (1) and (2)

the vibration intensity of the latter one is enhanced when the two sulfur atoms of ligandcoordinated to Eu³⁺ to form the new cycle (3), thus_CN moves to the higher wave number. On the other hand, increase of wave number of_css stretching vibration is observed compared with that of ligand. Obviously, this can be due to the new formed cycle and its formation increases the vibration the intensity of_CN^[12]. The changes of_CNand_CSSindicate that the two sulfur atoms of ligand coordinate to Eu³⁺ in a bidentate manner. The final results demonstrate that it is an octa-coordinated complex, and one pentaatomatomic ring and three tetraatomatomic rings are formed. As for the other complexes, showing the similarity with the complex Eu(Etdtc)(phen), the detailed data are listed in Table 3.

Fig. 2 IR spectra of the ligands and the complex
(a)EuCl·3.94HO (b) NaEtdtc·3HO (c) -phen·HO (d) Eu(Etdtc)(phen)

Table 3Data of IR Absorption for Main Groups of Ligands and Complexes (cm^-1

Complexes	(OH-1	(C═C)	(C─H)	(CN)	(CSS)
RECl·	3437-3441
	3366			1477	986
	3388	1617,1587, 1561, 1504	854, 739
	—–	1624, 1589, 1572, 1516	852, 730	1482-1516	1001
	—–	1624, 1589, 1572, 1516	852, 730	1481-1516	1000
	—–	1624, 1589, 1572, 1516	852, 730	1482-1516	1002
	—–	1624, 1589, 1572, 1517	853, 730	1482-1517	1001

2.2 Combustion energies of the complexes
The methods of determination and calculation of the constant-volume combustion energies for complexes are the same as for the calibration of the calorimeter with benzoic acid. the values are calculated by means of the following equation:

where (complex, s) denotes the constant-volume combustion energies of the complexes, is the calibrated heat of acids is the mass in g of the complexes, the other symbols are as in equation (3). The results of experiment were given in Table 4.

Table 4Experimental Results for the Combustion Energies for , , and

Complexes	No.	Mass ofsample m/	Calibratedheat of combustion wire /J	Calibrated heat of acid /J	Calibrated /K	Combustion energy of sample /J·g-1
		0.76470	12.60	1578.86	1.0548	22437.17
		0.78265	12.60	1615.92	1.0773	22386.23
		0.74369	12.60	1535.48	1.0247	22409.95
		0.75006	12.60	1548.63	1.0322	22379.83
		0.74235	12.60	1532.71	1.0219	22387.09
		0.74536	12.60	1538.93	1.0285	22445.73
	mean					22407.67±11.52
		0.80140	12.60	1633.22	1.1686	23865.92
		0.81258	12.60	1656.00	1.1857	23883.59
		0.80035	12.60	1631.08	1.1664	23851.05
		0.79857	12.60	1627.45	1.1666	23913.21
		0.80820	12.60	1647.07	1.1787	23870.12
		0.81357	12.60	1658.02	1.1852	23814.12
	mean					23870.84±10.42
		0.72684	11.70	1477.31	1.0037	22497.18
		0.73013	11.70	1484.06	1.0093	22522.89
		0.72755	12.60	1478.84	1.0069	22550.06
		0.73002	12.60	1483.86	1.0070	22469.32
		0.72545	12.60	1474.57	1.0026	22515.87
		0.72589	12.60	1475.46	1.0028	22505.89
	mean					22510.20±11.02
		0.82838	12.60	1683.59	1.0842	21216.79
		0.83020	12.60	1687.29	1.0891	21270.73
		0.83479	11.70	1696.62	1.0936	21239.50
		0.82655	12.60	1679.87	1.0833	21248.91
		0.82850	12.60	1683.83	1.0873	21279.93
		0.82937	11.70	1685.60	1.0850	21207.27
	mean					21243.86±11.75

2.3 Standard combustion enthalpies of complexes
The standard combustion enthalpies of the complexes,  (complexes, s, 298.15K), refer to the combustion enthalpy changes of the following ideal combustion reaction at 298.15K and 100kPa.
RE(Etdtc)(phen) (s) + (g) = RE (s) + 27CO (g) + 19 HO+ 6 SO (g) + (g) (5)
(RE=Eu, Gd, Tb, Dy)
The standard combustion enthalpies of the complexes are calculated by the following equations:
(complex, s, 298.15K)=(complex, s, 298.15K)+RT
(products)-(reactants)
where is the total amount in mole of gases present as products or as reactants, =8.314 J·K^-1·mol^-1, =298.15K. The results of the calculations are given in Table 5 for comparison.

Table 5 Combustion Energies, Standard Combustion Enthalpies and Standard Enthalpies of Formation for d, e, f and g at 298.15 K

Complexes	( kJ·mol-1	( kJ·mol-1	( kJ·mol-1
	-17410.63 ± 8.95	-17429.84 ± 8.95	-1238.06 ± 9.75
	-18673.71 ± 8.15	-18692.92 ± 8.15	-51.28 ± 9.17
	-17646.95 ± 8.64	-17666.16 ± 8.64	-1084.04 ± 9.49
	-16730.21 ± 9.25	-16749.42 ± 9.25	-2019.68 ± 10.19

2.4 Standard enthalpies of formation of the complexes
The standard enthalpies of formation of the compounds, (complex, s, 298. 15K), are calculated by Hess’s law according to the above thermochemical equation (5)
(RE(Etdtc)(phen), s) = [(RE_3, s) + 27(CO, g) + 19(HO,l)+
+ 6 (SO_2, g) + (N , g)] –  (RE(Etdtc)(phen), s) (8)
where (Eu, s) = (-1663.00 ± 1.62) kJ·mol^-1;(Gd, s) = (-1815.60 ± 3.60) kJ·mol^-1; (Tb, s) = (-1827.6 ± 2.0) kJ·mol^-1; (Dy, s) = (-1865.39 ± 3.89) kJ·mol^-1, (CO, g) = (-393.51 ± 0.13) kJ·mol^-1, (HO, l) = (-285.830 ± 0.042) kJ·mol^-1, (SO, g) = (-296.81 ± 0.20) kJ·mol^-1[14,15]，The results of calculation are also listed in Table 5.

Fig.3 Plot of  against the atomic numbers (Z_RE) of middle rare-earth for the complexes
●. ■.

In Fig.3,values of the complexes are plotted against the atomic numbers of middle rare-earth. obviously, they appears as curve relationship not linear, suggesting a certain amount of covalence is present in the chemical bond between the RE³⁺ and ligands, which is in agreement with Nephelauxetic effect of 4f electrons of rare earth ions.