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Period ate and Tellurate Complexes of Terbium(IV) Yu. M. Kiselev and A. S. Baranov 1

Moscow State University, Vomb'evy gory, Moscow, / /9899 Russia

Received July II , 2001

Abstract-Periodate and tellurate complexes of terbium(IV) were obtained as solutions and solids. The indi­ viduality oCthe terbiull1(IV) complexes was established by X-ray powder diffraction and electronic absorption spectroscopy. Based on the data on the energies of charge-transfer transitions in these complexes and literature data, the following complex stability row has been proposed: heteropolytungstates are the most stable; then, the stability successively decreases in going Lo the periodate, tellurate, and carbonate complexes,

Terbium(lV) derivatives are highly unstable in aqueous solutions due to a high redox potential of the Tblllrrb iv couple (-3.0 V) [I]. Therefore, a search for systems capable of stabilizing this high oxidation state is of interest. In addition, terbium(IV) compounds are good models for complexes of transition metals in very high oxidation states, first of all, due to the extremely large £0 value for Tblll{fbIV, It is the potential value that primarily determines the stability of an oxidation state, whereas the size factors (including ionic radii) have a lesser effect on the stability. From this standpoint, of great importance are the Tb(lV) derivatives in which the coordination sphere of the metal ion is composed of the most hard-to-oxidize and undeformable ligands, as well as the complexes in which the electron anion-to­ cation density transfer is insignificant, as in iodate, anion), and tellurate period ate ([Hs _ ,,106],,([H6 _"Te06]'r- anion) complexes [2].

time required for separation of terbium from a lan­ thanide mixture. In [6], alkaline solutions of ter­ bium(IV) periodate were obtained by the oxidation of Tb(lll) periodate complexes. The anions stabilizing Ln(1V) have not been determined. In [7], Tb(lII) was oxidized by potassium persulfate in an alkaline solution in the presence of potassium periodate. The stability of the complexes obtained was shown to decrease with decreasing pH. As noted, the reactant ratios and the ,nature of anion of the starting terbium salt are of great importance in formation of the oxidized complex.

Similar Tb(IV) complexes have been poorly stud­ ied, Barnes [3] prepared Tb(IV) iodate, which was for­ mulated based on X-ray single-crystal diffraction data as [Tb1v OH(l03h] . H 20. The crystal hydrates MI[Tb IV I06] • nH 20 (MI = Li, Na, K) were isolated from an ozonized alkaline solu­ tion of potassium periodate and Tb(N0 3h [4]. Elec­ tronic absorption spectra of solutions of these com­ pounds were studied, and their magnetic properties were measured (J.l- 8 J.lB) (note that, in light of this lat­ ter fact, one can judge the occurrence of the Tb(IV) oxi­ dation state only on the basis of the electronic absorp­ tion spectra). In [5], Tb(lV) periodate was obtained by liquid extraction of periodate complexes of Th(lV, III) with quaternary amines. According to [5], the Tb(N0 3h-KIO c KOH system is the most favorable for stabilizing Tb(IV) in aqueous solutions; the conversion of Tb(III) into Tb(IV) can reach more than 90% under appropriate conditions. The complexes can retain Tb(IV) for a long time, which is much longer than the I

E-mail: [email protected]

688

Tellurate complexes of Tb(lV) in an aqueous solu­ tion were studied in [8]. The isolated red-brown precip­ itate was assumed to be a complex of Tb(IV). A Tb(IV) tellurate complex was obtained in [9] by oxidation of the corresponding Th(lII) complex with ozone in an alkaline solution. In both works, the crucial evidence for the occurrence of Tb(IV) in these tellurate com­ plexes (as in the periodate complexes described above) was the presence of a charge-transfer band in their elec­ tronic absorption spectra. The aim of this work was to develop the procedures for obtaining Tb(lV) tellurate and periodate complexes, to study their properties, and establish the influence of the nature of the second ligand sphere on the stability of an oxidized form. EXPERIMENTAL The following reagents were used in the work: K2Te04 . 5H 20 of analytical grade, HI04 . nH20 of reagent grade, and Tb40 7 of high purity grade, as well as solutions with concentrations CTb - 0.05 molll and CHIO 4 -

0.2 molll.

Electronic absorption spectra were recorded on a Specord UVNIS spectrophotometer in a range of 200­ 800 nm (quartz cuvettes with l = 2 or \0 mm). X-ray powder diffraction analysis was performed on a Stoe diffractometer (CuKu radiation, a Ni filter) and an

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PERIOllATE AND lELLURA TE COMPLEXES OF lERBIUM(IV)

689

Table 1. X-ray powder diffraction data for Th(IV) complexes Tellurate complex

Periodate complex

d,A

1/10

d,A

4.1284 3.7801

100

2.4274

I

18 45

2.3407 2.1960

77 15

2.0662

3 18 18

1.7731

14

3.0466 2.6551 2.6016

1/10

Enraf-Nonius FR-552 monochromator camera (CuK 1 radiation, Ge standard). a Solid periodate complexes of Tb(IV) were synthe­ sized by two procedures. Procedure 1. Equivalent volumes of Tb(N0 3)3 and periodic acid solutions were poured together. The resulting solution was adjusted to pH> 14. An ozonized oxygen flow (hereafter, with an ozone content of up to 10%) was bubbled through this colorless solution with a precipitate until the solution turned a dark reddish brown. Then, the colored solutions were centrifuged, and the resulting colored precipitates were dried over P4010 in a vacuum desiccator. Procedure 2. Crystalline KOH was added to a peri­ odic acid solution until potassium periodate precipi­ tated (the solution must remain clear). To 2 ml of this solution, a solution ofTb(N0 3h (1 ml) was added drop­ wise. The resulting mixture was ozonized (if required, an alkali was added to the solution). After the solution turned a. dark reddish brown, it was concentrated by evaporatton under vacuum. Then, the mother liquor was separated on a centrifuge (-10000 rpm), and the resulting crystalline residue was dried over KOH in a vacuum desiccator. When the periodate complexes were synthesized by the first procedure, the solutions (pH> 14) became dark brown upon ozonization for several hours, whereas the solutions obtained by the second procedure turned this color after only 10-15 min. The electronic absorption spectra of the colored solutions of terbium periodate display a strong band at 400 nm. Its position is nearly independent of the condi­ tions for obtaining colored Tb(IV) solutions. The colored alkaline Tb(lV) periodate solutions are slowly reduced on long storage in air (in about a week). Addition of water accelerates this process. Acidifica­ tion rapidly makes the solutions colorless. Solid Tb(lV) tellurate complexes were obtained by the following procedure. A required amount ofK2Te04' 5H20 (powder) and a small amount of KOH (solid) were added to a concenRUSSIAN JOURNAL OF INORGANIC CHEMISTRY

d,A

1Il0

d,A

4.1379 3.9845 2.8375

100 37 34

2.0668 2.0271 1.9449

14 10 23

2.6527

47

1.7763

2.6048 2.5823

39 30

1.7308 1.6854

7 14

2.1826

17

1Il0

34

trated aqueous solution of Tb(N03h, and the resulting solution was ozonized. In this process, the solution must be in equilibrium with ice. The solutions to be ozonized must be supercooled (semifrozen).This was reached by al~ernation of cooling and heating of the system to main­ tam the lowest temperature of the reaction. After the pro­ cess was over (the criterion was the maximum color strength), the precipitates were separated from the mother liquor on a centrifuge (-10000 rpm) and dried over P40 lO in a vacuum desiccator at a temperature of around ODe(!). . Terbium(III) is oxidized in the presence of potas­ SlUm tellurate only in concentrated alkaline media and, more importantly, when vigorously cooled. First, the colored precipitates are formed . .Then, their amount decreases and the solution becomes colored. The oxida­ tion of terbium in this system proceeds much slower than that in the presence of potassium periodate. The residues obtained by centrifuging are light brown in color. A yellow-orange color of the solutions is likely caused by a rapid reduction of Tb(lV) at room temperature. We succeeded in measuring the absorption spectrum of the slightly colored solution. The spectrum displays a broad band with a maximum at 365 nm, which is in good agree­ ment with the literature data. The low band intensity is due to a low concentration of Tb(IV) [10]. The color of terbium tellurate precipitates depends on the mode of drying (dry solids consist of a colorless bulk and brown crystals indicating that terbium tellu­ rate is considerably reduced). The precipitate does not become colorless when kept at room temperature for more than 3 days, even after adding water. The solid terbium(IV) tellurate and periodate com­ pounds (more than 20 solid samples) were character­ Ized by X-ray powder diffraction and shown to be free of admixtures of corresponding Tb(III) derivatives. The single-phase compounds, characterized by individual sets of interplanar distances, were isolated as well (Table 1). The analysis of X-ray diffraction patterns showed also that more than one form of periodate Tb(III) complex could exist as was observed for lan-

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690

KISELEV, BARANOV

Table 2. Maxima of the charge-transfer bands in the spectra of solutions of Tb(IV) complexes with various ligands Ligand [TbW II 0 39 H 20]7­ [TbzW 1706 IH20] 10­ [IO~]

6­

[Te0 6

[CO;-]

]

A,nm

v,cm- I

440 460 420 400 360 377 370 365 385*

22730 21740 23810 25000 27780 26520 27030 27400 25970

V, cm~1

} } } }

(average)

Reference [13],[14] [ 13] [6], [7], l15] This work [8], [9] This work [ 15] [16] [17]

22240 24405 27150 27220

* The absorption band maximum of Tb(IV) in the film of the solid solution of TbO z in Sn02 obtained from carbonate solutions. than ide periodate complexes whose structure was stud­ ied in [2]. The electronic absorption spectra of the periodate complex solutions show broad bands with maxima at 25000 em-I (400 nm). The tellurate complex gives rise to an absorption band at 26500 cm- I (377 nm). The width at half-height of these bands is more than 5 x 103 em-I. This fact, together with high intensities of these bands (E > 2000 Ilmol em), allows us to assign them to the ligand-to-metal charge-transfer (CT) transi­ tions [0 2- ___ Tb IV ] 2 As is known, properties of lanthanide complexes should correlate with the same properties of actinide complexes. For actinide chloro complexes, the correla­ tion between the energies of first CT transitions and corresponding redox potentials was discussed in detail in [11]. It was established that the energies of the first CT transitions depended primarily on the difference in the optical electronegativities (X) of the central ion (M) and ligand (X). In chloro complexes, an electron charge transfers from the t 1)1 nonbonding n orbital of the ligand (chlorine) to the antibonding (j-n) molecular orbital of f2// symmetry. In M(lV) compounds, this transition cor­ responds to the intramolecular reduction of M(IV) to M(III), and the energy of this transition correlates with the EO MIII/MIV potential. This correlation exists not only for potentially one-electron transfers but also for two-electron transitions [12], as well as not only for actinides and lanthanides but also for numerous d tran­ sition metal compounds; that is, this relation is of gen­ eral character and can be used in our case. When the maxima of absorption bands are shifted toward larger wavelengths, it means that the corre­ sponding redox potentials increase. Therefore, the 2 Our

assignment to the Tb(lV) oxidation state in the period ate and tellurate complexes on the basis of their CT transitions is conven­ tional and substantiated. because the electronic adsorption spec­ tra of Tb(lII) compounds do not display charge-transfer bands in the visible region independent of the ligand nature.

complexes that give rise to charge-transfer absorption bands at the longest wavelengths are the most stable to a redox transition. Based on this and using available literature and experimental data (Table 2), we constructed a stability row for Tb(IV) complexes with various ligands. We invoked also the data on Tb(IV) complexes existing in heteropolytungstate and carbonate media. The absorp­ tion spectra of these compounds, as those of the perio­ dates and tellurates, also show the charge-transfer tran­ sitions (ligand-to-metal [02- - - - Tb 1V]) [13-17]. Indeed, the most stable (towards reduction) com­ pounds are likely Tb(IV) heteropolytungstates (A. = 440-460 nm) (according to [18], the complexation with these ligands decreases the redox potential of the sys­ tem with Tb(IV) by -1.3 V). Then, the stability decreases in successively going to the periodate (A. = 400-420 nm), tellurate, and carbonate complexes (A. = 350-380 nm). These conclusions agree with chemical practice, because the obtainment of solutions of Tb(IV) tellurate and carbonate complexes is the most challeng­ ing task. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 99-03-32488. REFERENCES l. Kiselev, Yu.M., Zh. Neorg. Khim., 1994, vol. 39, no. 5, p. 1266. 2. Levason, W., Coord. Chem. Rev., 1997, vol. 161, no. I, p.33. 3. Barnes, J.c., Abstracts of Papers, 13th Rare Earth Res. Conf, New York: Pergamon, 1978, p. 291. 4. Ying, Y and Ru-Dong, Y, Polyhedron, 1992, vol. 11, no. 7, p. 963. 5. Su Qiang, Wu Zhijian, and Gao Chong1i, Solvent Extr. /on Exch., 1995, vol. 13, no. 2, p. 275.

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PERIODATE AND 1ELLURATE COMPLEXES OF 1ERBIUM(IV) 6. Lanping, M ., Ying, Y., and Ru-Dong, Y., Gaodellg Xu ex­ iao Huexue XlIebao, 1993, vol. 14, no. 2, p. 233. 7. Ning, T. and Minyu, T., Zhongguo Xitu Xuebao, 1985, vol. 3, no. 1, p. 7. 8. Suming, H. and Rudong, Y., Gaodeng Xuexiao Huaxue XlIebao, 1982, vol. 3, no. 4, p. 573. 9. Ying, Y., Lanping, M., Lan, Y., et aI., Zhongguo Xitll Xuebao, 1995, vol. 13, no. 2, p. 268. 10. Kiselev, Yu.M. and Baranov, AS., Abstracts of Papers, 4th Int. Conf on f Elements, Madrid, 2000, p. BP26. 11. lonova, G.V, Pershina, VG., and Spitsyn, VI., Elektron­ noe stroenie aktinidov (Electronic Structure of Actini­ des), Moscow:, Nauka, 1986. 12. Kopelev, N.S . and Kiselev, Yu.M., Abstracts of Papers, XVI Vsesoyuznoe Chugaevskoe soveshchanie po khirnii koordinatsionnykh soedinenii (XVI AIl-Union Chugaev

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Conf. on Chemistry of Coordination Compounds), Minsk, 1990, vol. 2, p. 244. 13. Saprykin, AS., Shilov, VP:, Spitsyn, V.I., and Krot, N.N. , Dokl. Akad. Nauk SSSR, 1976, vol. 226, no. 5, p. 853. 14. Fedoseeva, AM., Krot, N.N., and Spitsyn, VI., Radiokhimiya, 1986, vol. 28, no. 1, p. 89. 15. Frenkel, VYa., Kulyako, Yu.M., Chistyakov, VM., etat., J. Radioanal. Nuc/. Chem. Lett., 1986, vol. 104, no. 112, p. 191. 16. Hobart, DE, Samhoun, K., Young, J.P., et at., Inorg. Nucl. Chem. Lett. , 1986, vol. 16, no. 2, p. 321. 17. Propst, R.S., J. Inorg. Nucl. Chern., 1974, vol. 36, no. 4, p. 1085. 18. Song ina, O.A, Keme1eva, N.G., and Dubonosova, E.I., Vestn. Akad. Nauk Kaz. SSR, 1981, no. 2, p. 67.

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2002