COORDINATION OF TWO URANIUM IONS TO THE [KAs 4 W 40 O 140 ] 27 CRYPTATE HETEROPOLYANION. UV- VIS AND IR INVESTIGATION

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1 COORDINATION OF TWO URANIUM IONS TO THE [KAs 4 W 40 O 140 ] 27 CRYPTATE HETEROPOLYANION. UV- VIS AND IR INVESTIGATION Cora Crăciun 1, L. David 1, T. Jurcuţ 2, O. Cozar 1, C. Vâju 1 1 "Babes-Bolyai" Univ., Dept. of Physics, Cluj-Napoca, 3400 Romania 2 University of Oradea, Dept. of Physics, 3700 Oradea, Romania Abstract The inclusion of the uranium(iv) ions in the lacunary regions of the [KAs 4 W 40 O 140 ] 27 cryptate was investigated by UV-VIS and IR methods. The IR spectrum of the K 19 [U 2 KAs 4 W 40 O 140 ] 42H 2 O heteropolyoxotungstate contains the ν as (U O) 1133 cm 1 vibration band and presents changes of the ν as (W=O d ), ν as (W O a ) and ν as (W O b,c W) bands compared to the IR spectrum of the cryptand. The visible absorption spectra of the complex contain the f f transitions of uranium ions with a 3 H 4 ground state, situated in quasicubic configuration. The low intensity and the shift of the p π (O b,c,d ) d π* (W) electronic bands with cm 1 are due to the displacement of W VI ions, caused by the U IV ions coordination. 1. Introduction Polyoxometalates (POM) are molecular blocks of metal oxide formed by WO 6 and/or MoO 6 octahedra sharing vertices or edges [1]. In the

2 last years, polyoxometalates and their metallic complexes have been intensely studied for the important applications in catalysis, medicine and materials science [2-6]. Different ions can coordinate the POM by occupying their lacunary sites. This is an advantage because of the possibility of stocking the radioactive waste into the POM complexes [7,8]. In this paper we are dealing with uranium(iv) complex of [KAs 4 W 40 O 140 ] 27 cryptate heteropolyanion (Fig. 1). The cryptand is one of the heaviest polyoxometalates and Leyrie prepared it in 1978 for the first time [9]. It contains four trilacunary {AsW 9 } Keggin units bound by four WO 6 octahedra. Each unit consists of three W 3 O 13 units forming a trigonal pyramidal cavity in which the As III central ion is placed. The metallic complexes of [KAs 4 W 40 O 140 ] 27 heteropolyanion present five sites [10], one central cryptate site (K) in which an alkaline ion is eight coordinated and four other lacunary sites (S 1 S 4 ) which can be coordinated by different cations. The flexibility of the structure allows the uranium cation to coordinate the active sites (S 1, S 2 or S 3, S 4 ), in spite of its important size and electrostatic charge [11]. Fig. 1. Structure of the [KAs 4 W 40 O 140 ] 27 cryptate polyoxometalate 110

3 The main goal of this paper is the report of the spectroscopic investigation of K 19 [U 2 KAs 4 W 40 O 140 ] 42H 2 O complex. 2. Experimental 2.1. Synthesis of K 19 [U 2 KAs 4 W 40 O 140 ] 42H 2 O polyoxometalate A solution containing 11.8 g (1 mmol) K 27 [KAs 4 W 40 O 140 ] 45H 2 O dissolved in 60 ml distilled water was added stepwise under continuous stirring to a solution containing 0.86 g (2 mmol) U(SO 4 ) 2 in 25 ml distilled water. The ph have been adjusted at 4.5 ph value, with 0.1 M NaOH and the mixture was heated at 70 C for 30 minutes. Finally, the aqueous solution was cooled to 5 C and filtered. The brown microcrystals were obtained after 48 hours at 5 C and were purified by recrystallization from distilled water having 4.5 ph. The chemical analysis was carried out by atomic absorption methods after the complex decomposition by boiling with 6M NaOH solution. Anal. Calcd. for H 84 K 20 O 182 As 4 W 40 U 2 (M = ): K 6.57, As 3.00, W 61.76, U 3.99, H 2 O 6.35%. Found K 6.70, As 2.32, W 62.65, U 3.96; H 2 O 6.84% Methods and Instrumentation UV and visible electronic spectra were performed in the wavelength range λ = nm in aqueous solutions using a Specord UV VIS spectrometer. IR spectra were obtained in KBr pellets in the range cm 1 with a Carl Zeiss Jena model UR-20 spectrophotometer. 111

4 3. Results and discussion 3.1. Infrared Spectra The representative frequency range of the IR spectra for the K 27 [KAs 4 W 40 O 140 ] 45H 2 O ligand and for the U IV POM complex is given in Fig. 2 and some vibration bands are presented in Table 1. The presence of an additional band at 1133 cm 1 in the U IV POM complex spectrum, which can be attributed to the ν as (U O) vibration 13,14, confirms the coordination of the uranium ions to the ligand by oxygen atoms. Vibrations corresponding to the polyoxoanion frame appear in the cm 1 region 15. The IR spectrum of the ligand presents two asymmetric vibration Fig. 2. IR spectra of the ligand (a) and of the U IV POM complex (b) 112

5 Table 1 Some IR bands (cm 1 ) of the ligand and U IV POM complex Band Ligand (cm 1 ) U IV POM (cm 1 ) ν as (W=O d ) 950 m,sp; 970 s,sp 960 s,b ν as (W O c W) 890 s,b 895 s,b ν as (W O b W) 805 sh,b; 785 vs,b 810 sh,b; 770 sh,b ν as (W O a ) + ν as (As O W) 765 sh,b 740 sh,b δ(h O H) 1630 m,b; 1648 sh,b 1633 m,b; 1655 sh,b 1668 sh,b ν(h 2 O) 3450 s,b; 3580 s,b 3500 s,b m medium, s strong, sh shoulder, b broad, sp sharp, vs very strong frequencies for the terminal W=O d bonds (at 950 and 970 cm 1 ), from the {AsW 9 } units and the four binding WO 6 octahedra, respectively 16,17. In the U IV POM spectrum, the ν as (W=O d ) vibration band appears as a single broad absorption at 960 cm 1. In the IR spectrum of the complex, the asymmetric vibrations of the tricentric W O c W bonds of the edge-sharing WO 6 octahedra are shifted with 5 cm 1 compared to the ligand spectrum. The ν as (As O a ) vibration band which appears as a shoulder into the ligand spectrum is missing into the complex spectrum. The frequencies corresponding to the W O b W bonds of the corner-sharing WO 6 octahedra of the ligand present a red-shift of 15 cm 1 into the IR spectrum of the complex, when O b binds one {AsW 9 } unit and one coupling WO 6 octahedron, and a small blue shift of 5 cm 1, when O b binds two octahedra of the same Keggin unit. The stretching and deformation bands of the water molecules (Table 1) are very broad and present some shoulders both in the ligand and complex spectra. This proves the existence of some hydrogen bonds 18 involving terminal oxygen atoms O d. The analysis of the IR spectra indicates the coordination of U IV ions in the lacunary regions of 113

6 the ligand (S 1 S 4 sites) by the O d oxygen atoms pointing to the vacant region of the Keggin units and WO 6 binding octahedra. Fig. 3. UV spectra of the ligand (a) and U IV POM complex (b) obtained in mol l 1 aqueous solution, at room temperature 3.2. Electronic Spectra The UV spectra of the U IV POM complex and of the ligand obtained in aqueous solutions are similar (Fig. 3). The absorption bands centered at cm 1 for the free ligand and at cm 1 for the U IV POM complex are assigned to p π (O d ) d π* (W) electronic transitions into the terminal bonds and the broad bands at cm 1 for the ligand and cm 1 for the uranium complex are assigned to p π (O b,c ) d π* (W) transitions in the tricentric bonds 22. The presence of the two shoulders indicates that the trilacunary Keggin anions have nonequivalent W O b,c W bonds. The low intensity and the observed shift 114

7 of these bands in the complex spectrum are due to the displacement of W VI ions, caused by the U IV ions coordination. Fig. 4. Visible electronic spectra of U IV POM complex in aqueous solution at different concentrations (a)10 3 mol l 1, (b) mol l 1, (c) mol l 1, (d) mol l 1, (e) 10 2 mol l 1 ) The visible absorption spectra of the complex, performed in aqueous solutions at different concentrations (Fig. 4) contain the f f transitions of the uranium ions situated in a cubic configuration (Table 2). As a consequence of the large spin-orbit 115

8 Table 2 Electronic transitions of the K 19 [U 2 KAs 4 W 40 O 140 ] 42H 2 O complex from the visible absorption spectra Band ν (cm 1 ) Assignment L H 4 3 P 0 O H 4 1 D 2 ( 1 G 4 ) R H 4 3 P 1 V H 4 1 I 6 W H 4 3 P 2 coupling 23, the electronic ground state is 3 H 4. The sharp L band is centered at cm 1 indicates that even if some distortions of the cubic environment are present, they are small. The band starting at cm 1 corresponds to the U IV W VI charge transfer. 4. Conclusions The IR spectrum of the U IV POM contains an additional band (ν as (U O)) compared to the ligand spectrum, a proof of the coordination of the uranium ions to the oxygen atoms of the polyanion. The vibration bands of the polyoxometalate are shifted in the complex spectrum compared to the ligand spectrum due to the coordination of the uranium ions into the S i (i = 1,,4) active sites of the ligand, at terminal oxygen atoms O d from {AsW 9 } units and binding WO 6 octahedra. Electronic spectra indicate the presence of cubic environments around the uranium ions with 3 H 4 electronic ground state. References 1. Y.P. Jeannin, Chem. Rev., 98 (1998) P. le Maguerés, L. Ouahab, S. Golhen, D. Grandjean, O. Peña, J.-C. Jegaden, C. J. Gómez-Garcia, P. Delhaés, Inorg. Chem., 33 (1994) 116

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