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

Abstract Four ternary solid complexes, Eu(Et₂dtc)₃(phen) (d), Gd(Et₂dtc)₃(phen) (e), Tb(Et₂dtc)₃(phen) (f) and Dy(Et₂dtc)₃(phen) (g), were synthesized with sodium diethyldithiocarbamate (NaEt₂dtc), 1,10-phenanthroline (o-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 NaEt₂dtc and nitrogen atoms of o-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.
Keywords RE(Et₂dtc)₃(phen), thermochemistry, combustion energies, standard enthalpies of combustion, standard enthalpies of formation

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)₂dtc], o-phen·H₂O 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(Et₂dtc)₃(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 of determination of the constant-volume energies combustion of complexes. The final results would provide theoretical basis for enlarging their application range.

Table 1 Analytical Results Related to the composition for d, e, f and g

    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(Et₂dtc)₃(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, NaEt₂dtc·3H₂O and o-phen·H₂O (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 o-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 o-phen coordinate to Eu³⁺. Contrasting with that of 1477 cm^-1 in the ligand of NaEt₂dtc·3H₂O, n_CN of the complex shifts to higher wave number, and presents a double-bond character in the complex, which can be attributed to that NCS₂^- group has two main forms of vibration^[13]: (1) and (2)
                         
             (1)                        (2)                             (3)

the vibration intensity of the latter one is enhanced when the two sulfur atoms of ligand coordinated to Eu³⁺ to form the new cycle (3), thus n_CN moves to the higher wave number. On the other hand, increase of wave number of n_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 n_CN^[12]. The changes of n_CN and n_CSS indicate 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(Et₂dtc)₃(phen), the detailed data are listed in Table 3.

Fig. 2 IR spectra of the ligands and the complex
(a)EuCl₃·3.94H₂O (b) NaEt₂dtc·3H₂O (c) o-phen·H₂O (d) Eu(Et₂dtc)₃(phen)

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