Physicochemical Properties and Theoretical Modeling of Actinide

10976

J. Phys. Chem. B 2008, 112, 10976–10988

Physicochemical Properties and Theoretical Modeling of Actinide Complexes with a para-tert-Butylcalix[6]arene Bearing Phosphinoyl Pendants. Extraction Capability of the Calixarene toward f Elements Flor de Marıá Ramı´rez,*,† Sabi Varbanov,‡ Juan Padilla,§ and Jean-Claude G. Bu¨nzli*,⊥ Departamento de Quı´mica, Instituto Nacional de InVestigaciones Nucleares, Carretera Me´xico-Toluca S/N. La Marquesa, Ocoyoacac. C.P. 52750, Me´xico, Institute of Polymers, Bulgarian Academy of Sciences, BG-1113 Sofia, Bulgaria, Departamento de Quı´mica, R-116. UniVersidad Autońoma Metropolitana-Iztapalapa, P. O. BOX 55-534, Me´xico, D.F., 09820, Me´xico, and Laboratory of Lanthanide Supramolecular Chemistry, Ećole Polytechnique Fe´de´rale de Lausanne (EPFL), BCH 1402. CH-1015 Lausanne, Switzerland ReceiVed: NoVember 13, 2007; ReVised Manuscript ReceiVed: May 14, 2008

The coordination ability of the hexaphosphinoylated p-tert-butylcalix[6]arene B6bL6 toward actinides is established, as well as its good separation ability of the actinide ions UO22+ and Th(IV) over trivalent rare earths such as La(III), Eu(III), and Y(III). Spectrophotometric titration of uranyl with B6bL6 in CH3CN yields log β11 ) 7.1 and log β12 ) 12.5 for the 1:1 and 1:2 (UO22+/B6bL6) species, respectively. Actinide complexes with 1:1 and 1:2 (M/L) stoichiometries are isolated and characterized by elemental analysis, IR, and UV-vis. Compounds 1 and 3 fulfill their CN ) 8 just with B6bL6, while compounds 2 and 4 require coordinated nitrates and/or water molecules. The luminescence spectra of the uranyl complexes and the parameters such as FWMH, vibronic spacing (υsp), and the U-O bond length, as well as the luminescence lifetimes, permit the understanding of the coordination chemistry of these actinide calixarene complexes. Energy transfer from the B6bL6 ligand to the uranyl ion is demonstrated to be relevant in compound 1 with Qabs ) 2.0%. The uranyl complex emission reveals a biexponential decay with τs from 210 to 220 µs and τL from 490 to 650 µs for compounds 1 and 3, respectively. The liquid-liquid extraction results demonstrate the good extraction capability of B6bL6 toward actinides but not for rare earths at room temperature. The extracted species keeps the 1(cation)/1(calixarene) ratio for the UO22+, Th4+, and Eu3+ ions. A good capacity of B6bL6 toward Th4+ ions using aqueous phase 2 containing even up to 0.3 M thorium nitrate and an organic phase of 2.47 × 10-4 M B6bL6 in chloroform is found. The spectroscopic properties of the isolated uranyl complexes and the extraction studies reveal a uranophilic nature of B6bL6. The molecular modeling results are in good agreement with the experimental findings. I. Introduction Suitable confinement of nuclear wastes in a geological repository has to prevent the escape of radioactive products into the environment. The spent nuclear fuel and/or fission products contain several families of metal ions, alkaline, alkaline-earth, d-transition metal, lanthanide, and actinide cations. As these cations migrate easily, the declassification of the wastes before confinement is imperative, and great efforts have been made along this line by developing efficient extraction and separation processes.1,2 Among these, the PUREX and TRUEX processes are the most important.1 In the latter, transuranic (TRU) elements are recovered and separated from solutions containing nitric acid, fission products, and high nitrate concentration. After treatment with TRUEX, the wastes containing uranium/plutonium compounds, americium, and rare earth fission products along with some transition metals and non-TRU materials with low-level radioactivity are processed, packaged, and disposed of separately in deep geological repositories, as required by current regulations. For this purpose, many simple and sophisticated iono* To whom correspondence should be addressed. † Instituto Nacional de Investigaciones Nucleares. Phone: (+) 53297200. Fax: (+) 53297301. ‡ Bulgarian Academy of Sciences. § Universidad Auto ´ noma Metropolitana-Iztapalapa. ⊥E ´ cole Polytechnique Fe´de´rale de Lausanne.

phoric organic ligands have been tested, and several of them proved to be excellent extractants.2–4 However, so far, no extractant possesses unique selectivity and high loading capacity toward a particular cation, therefore, the interest for testing new extracting agents. It is well-known that slight modifications in the nature and arrangement of the ligating groups affect enormously the interaction of an organic extractant with selected cations.2–4 In general, it has been found that ligands bearing carboxylic groups are good extractant agents for lanthanides; amide functions are better suited for the extraction of alkaline ions, while phosphine oxide groups perform best for actinide cations. In the last decades, the development of macrocyclic chemistry has contributed widely to the separation science.2–4b The grafting of ligating groups onto macrocyclic scaffolds changes dramatically the selectivity.2,3,4b,5 Here, we focus our attention on a class of three-dimensional macrocyclic ligands with attractive and flexible architectures, calixarenes,5 and on their interaction with rare earths and actinides.2a,b,d,e,3,5–10,11a,12,16,24 The current interest in properly functionalized calixarenes with pendant arms containing groups such as phosphoryl and/or amide stems from their powerful selectivity as extractants. In particular, they play an important role in the treatment of radioactive wastes containing lanthanides and actinides, alkalines, and alkaline earths, among others.2a,c,d,3,4b,5,7a–d,8–10 So far, calixarenes substituted with carboxamidophosphine oxide (CMPO)

10.1021/jp710848m CCC: $40.75  2008 American Chemical Society Published on Web 08/08/2008

Actinide Complexes with para-tert-Butylcalix[6]arene CHART 1: Calixarenes with Ether Amide and Phosphinoyl Pendant Arms in the Lower Rims

pendant arms proved to be the most selective,2a,b,d,3b,c,8a,9,10b but other calixarenes bearing phosphoric acid,8b,c phosphonates,10a phosphine oxide,3b,7c,d,16 amide,7d or hydroxamic7a functions are quite good extractants as well. Radiotoxicity of the actinides is a limitation in the development of their coordination chemistry. Therefore, analogue cations are often studied, for example, Eu(III) for Pu(III), Am(III), and Cm(III); Th(IV) for U(IV) and Pu(IV); and UO22+ for NpO22+ and PuO22+.1b,13d The most explored complexes are those of uranium and thorium.2e,13–15,17–21 Coordination numbers (CN) ranging from 6 to 12 have been found for thorium(IV) complexes with neutral and charged organic ligands,2a,13d,14,15d,19 CN ) 8 and 9 with square antiprismatic and trigonal prismatic geometries being the most common. Th(IV) complexes with macrocycles such as porphyrins15d and calixarenes are also known. Among the latter, the most important are those formed with carboxylated derivatives,7b partially ionized calixarenes,6 CMPO-derivatized calixarenes,10b and calixarene phosphine oxides,16 with CN being usually 8. With respect to uranyl, a large number of complexes have been studied in the solid state,13,15,17a,b,18,20,21 with the aim of establishing a relationship between their chemical structure and absorption and emission spectra, as well as studying their solution behavior.22a,23a Semiempirical and first-principles studies21b,c,e have also been essential to decipher the electronic and vibronic states, the nature of the chemical bonds in uranyl complexes,21 as well as to model extraction mechanisms.11 Uranyl has been mostly studied in aqueous solution, in which concentration,22a,b,d,e pH,22b–d and temperature22f affect the stability and concentration of the species in equilibrium. Both quenching and enhancing effects of organic substances on uranyl fluorescence have been evaluated by potentiometry22 and spectroscopy,23 but the photophysical properties of uranyl complexes with organic ligands have only been elucidated for a few of them.20a,f,23e Uranyl complexes with macrocyclic ligands such as Schiff bases, crown ethers, and expanded porphyrins usually possess pentagonal or hexagonal bipyramidal structures with CN ) 7 or 8, respectively.13,15,20,23e Calixarene complexes with uranyl revealed to be quite stable,6,15c,24 but to our knowledge, no phosphorylated calixarenes have been tested for uranyl extraction despite the known extraction potential of resorcinarenes functionalized with phosphine oxide or CMPO groups.2b During the last years, we have synthesized two series of calix[n]arenes (n ) 4, 6, 8; see Chart 1) fitted with ether amide12b,d and phosphinoyl pendant arms12a,c,e on the narrow

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10977 rim and investigated the structural and photophysical properties of their lanthanide complexes.12 Our interest extends now to the coordination chemistry of actinide cations, in particular UO22+ and Th(IV), with phosphinoylated p-tert-butylcalix[6]arene and to its extraction capability toward these actinide cations and representative trivalent rare earths (La, Eu, Y). We also devote special attention to the physicochemical properties of the uranyl complexes and to the molecular modeling of the isolated actinide complexes by MM3/CONFLEX procedures. II. Experimental Methods II.1. Materials. UO2(NO3)26H2O, Th(NO3)45H2O, ethanol, and di-isopropyl ether were purchased from Merck. HNO3 and NaNO3 were from Baker. Deionized water was kindly supplied by the staff of the Nuclear Reactor TRIGA Mark III from the Nuclear Center of Mexico. The narrow-rimsubstituted p-tert-butylcalix[6]arene (B6bL6 ) 5, 11, 17, 23, 29, 35-hexa-tert-butyl-37, 38, 39, 40, 41, 42-hexakis(dimethylphosphinoylmethoxy)calix[6]arene) was obtained as previously reported.12c Anhydrous ( 8.3 × 10-4 M, an emulsified interphase formed during thorium extraction (incipient only in uranyl extraction), which prevented adequate separation of the phases. The extractions were carried out as described above but with a more vigorous shaking (600 rpm for 3 h). II.3.4. Loading Capacity of B6bL6 toward Th(IV).3c Aqueous phases (5 cm3) of 3.26 × 10-4-0.3 M thorium nitrate in 1 M HNO3/3.5 M NaNO3 were prepared, mixed with 5 cm3 of 2.47 × 10-4 M B6bL6 in chloroform, and shaken for 15 h at 300 rpm. Once the phases were separated, 1 cm3 of the organic phase was removed, and drops of concentrated HNO3 (65%) were added. The mixture was heated at 90 °C until complete evaporation and then calcinated at 790 °C for 3 h. The white residue was dissolved in 2 cm3 of 1 M HNO3/3.5 M NaNO3, and an aliquot was treated as described in section II.3.2 for the determination of Th(IV) in the aqueous phase.3b II.4. Theoretical Modeling of the Molecules. The structures were built and their minimum energies calculated using the CAChe Pro 5.02 program package for windows (Fujitsu Ltd., 2000-2001). Sequential application of Augmented MM3/ CONFLEX procedures yielded the most stable conformers for compounds 1-4 and the free calixarene. Additionally, compound 1 and B6bL6 were simulated at 300 K by dynamics using Augmented MM3 parameters. The calixarene structure has also been calculated by MOPAC/PM5 and MOPAC/PM5/COSMO procedures. COSMO evaluates the solvent effect in the stabilization of a structure, and therefore, the resulting calixarene structure was the base for the simulation of the actinide complexes. The MOPAC procedure could not be applied to the actinide complex molecules because it lacks parameters for f elements. II.5. Instrumental Methods. IR spectra were measured on a Perkin-Elmer IR series 1600 spectrometer using KBr pellets. UV-vis spectra were recorded on a Perkin-Elmer Lambda 10 spectrophotometer (ININ-Mexico) using 1 cm quartz cells. The uranium content in the samples for/from extraction was measured on a Thermo Jarrell Ash-Atoms Advantage ICP-AES spectrometer (ININ-Mexico). Elemental analyses for C, H, and N were performed on a Perkin-Elmer 2400 series II (UAM-I, Mexico) instrument. Low-resolution luminescence spectra of solution samples at 291 K and frozen solutions at 77 K were recorded on a Perkin-Elmer LS-55 spectrofluorimeter (ININMexico) from 200 to 900 nm. A 290 nm filter was used in recording excitation and emission fluorescence and phosphorescence spectra to minimize Raleigh and Raman scatterings. Emission and excitation slits were set at 5 nm for frozen solutions at 77 K and at 5, 7, or 10 nm for 291 K measurements. Solutions of the uranyl complexes and free calixarene (>2.2 × 10-4 M) were prepared in spectroscopic-grade acetonitrile inside of a glovebox. Lifetimes of their frozen solutions (data reported are averages of at least five determinations) were measured on the same instrument. The emission spectra of uranyl nitrate in acetonitrile (5.08 × 10-4 and 5.10 × 10-3 M) and deuterium oxide (5.24 × 10-4 and 6.1 × 10-3 M) were recorded at 291 K and 77 K under the same experimental conditions as the uranyl complexes for demonstrating energy transfer from the calixarene to the uranyl ion. The quantum yield of the metal-centered luminescence was determined in the phosphorescence mode

Actinide Complexes with para-tert-Butylcalix[6]arene (time delay of 0.05 ms) using published procedures in degassed and dried CH3CN at 291 K with respect to quinine sulfate (QS, Qabs ) 54.6%) in 0.5 M aqueous H2SO4. The refractive indices were 1.334 for QS solution and 1.342 for compound 1 solution in CH3CN. The excitation wavelength was 270 nm (absorbance 0.30), and a filter (430 nm) was inserted to eliminate the Raleigh diffusion band and a second-order spectrum. Spectrophotometric titrations were performed at 291 K on the UV-vis spectrophotometer mentioned above. In a typical titration of the salt by the ligand, 3 cm3 of a 5 × 10-4 M uranyl nitrate solution was titrated by a solution of 5.55 × 10-4 M B6bL6; 22 spectra were recorded for [L]/[uranyl salt] ratios ranging from 0 to 4.4:1. Alternatively, 5 cm3 of a 1.7 × 10-4 M solution of B6bL6 was titrated by a solution of 5.2 × 10-4 M uranyl nitrate; 21 spectra were recorded for [uranyl salt]/[L] ratios from 0 to 6.7:1. Both experiments were carried out in the glovebox, and the solutions were prepared in dry spectrophotometric grade CH3CN; the titrant was delivered by a 1 cm3 micropipette. The recorded spectra for the M/L or L/M ratios were fitted using the SPECFIT/32 program, v.3.0.39 for Windows. III. Results and Discussion III.1. Complexation of B6bL6 with Actinide Ions. The interaction between B6bL6 and uranyl ions has been probed by spectrophotometric titrations of the ligand (1.7 × 10-4 M) with UO2(NO3)2 · 6H2O (5.2 × 10-4 M) and of the uranyl salt (5 × 10-4 M) with the ligand (5.55 × 10-4 M); both were carried out in degassed and anhydrous acetonitrile and up to ratios of [uranyl]t/[B6bL6]t ) 6.7:1 and [B6bL6]t/[uranyl]t ) 4.4:1, respectively. Factor analysis of the spectra from the titration of B6bL6 with uranyl nitrate indicated the potential presence of nine absorbing species, but a model taking into account only five species, M, L, 1M/1L, 1M/2L, and 2M/1L, could be fitted to the data with stability constants of log β11 ) 6.4 ( 0.7, log β12 ) 10.3 ( 1, and log β21 ) 12.6 ( 1, respectively. The percentage of the 1:2 species was very low, while the combined proportions of the 1:1 and 2:1 species was up to 92%. In the other titration, the uranyl and ligand solutions had similar concentrations, and the factor analysis suggested six absorbing species. Again, only a model taking into account a reduced number of species, namely, M, L, 1M/1L, and 1M/2L could be fitted to the data, with stability constants of log β11 ) 7.1 ( 0.9 and log β12 ) 12.5 ( 1.1, respectively. Partial shielding of uranyl by the calixarene because of its conformation in the 1:1 species and partial coordination of the two ligands to the uranyl in the second species could be the reason why their stability is smaller than that found for lanthanum complexes with the same ligand (log β11 ) 9.8; log β12 ) 19.6).12c III.2. Isolation and Characterization of the Actinide Complexes. The actinide complexes with the reported calixarene, UO2(NO3)2(B6bL6)n · xH2O (n ) 1, x ) 3, 1; n ) 2, x ) 12, 3) and Th(NO3)4(B6bL6)n · xH2O (n ) 1, x ) 3, 2; n ) 2, x ) 8, 4), denoted An(B6bL6)n below, were synthesized by a procedure similar to the one reported for the corresponding lanthanide complexes.12c Uranyl compounds are greenish yellow, while thorium compounds are white. The reaction yields (54-75%) are influenced by the nature of the cation and the stoichiometry, the largest yield being obtained for UO2(B6bL6)2. It seems that the host-guest complementarity in size and the peculiar conformational behavior of the derivatized p-tertbutylcalix[6]arene govern the formation of the complexes. According to elemental analyses, the An(B6bL6)2 complexes have larger hydration numbers (8-12) than the 1:1 complexes

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10979 (3). In fact, the high water content of the 1:2 complexes may be more favored by the affinity of the phosphinoyl groups for water12 than by the ability of the actinide cations to coordinate water. Indeed, water molecules play a fundamental role in the previously reported structure of B6bL6 12c in that they are responsible for the peculiar positioning of the phosphinoyl pendant arms, which is unusual for a calix[6]arene.2b,5 To get an insight into the composition of the inner coordination sphere of the complexes, vibrational spectra were recorded (see Experimental Methods). Upon formation of the 1:1 complexes, one component of the PdO vibrations of the free calixarene, at 1159 cm-1, disappears completely. The other component is blue-shifted (13 cm-1) with a decrease in intensity. Bands attributable to coordinated nitrate and water molecule(s) occur in the spectrum of the Th(IV) complex 2 but not for UO2B6bL6. After drying, the spectrum does not change in the region of coordinated water vibrations, in which the band at 362 cm-1 is apparent; this implies a direct coordination of the water molecule(s) to the thorium ion. In the case of the 1:2 complexes, fewer changes occur upon complexation; in particular, the two PdO components are still seen, albeit with smaller intensity. The shift of the aromatic ether bands is not significant (3-6 cm-1), and vibrations corresponding to coordinated water molecules are seen only for the Th(IV) complex 4. Moreover, the PdO shifts are larger for the uranyl complex than those for the thorium compound, which is indicative of a stronger uranyl-calixarene interaction. The spectra of the 1:2 complexes also suggest the presence of uncoordinated PdO moieties. This was expected since coordination numbers larger than 8 are rarely seen for uranyl, which usually accommodates four, five, or six donor atoms in the equatorial plane.18,21 When six donors are available in a macrocycle,15a–c they form a puckered ring in the plane perpendicular to the linear uranyl moiety, resulting in CN ) 8. In fact, calix[6]arenes, which are sometimes referred to as “uranophiles”, are ideally suited for the binding to UO22+ since coordination results in a preferred hexacoordinate planar geometry and in complexes with large stability.24 The asymmetrical stretching frequency of the uranyl ion (νasym U-O), which is usually observed between 910 and 980 cm-1,13,20,21e is found between 914 and 925 cm-1 in the spectra of the complexes with B6bL6, confirming the interaction of uranyl with the calixarene. In summary, the vibrational study points to 1:1 complexes in which all of the phosphinoyl groups are coordinated to the actinide cations, which, on the other hand, is not the case for 1:2 complexes; in the latter, noncoordinated phosphinoyl groups interact with lattice water molecules. The absence of coordinated nitrate and water in the uranyl complexes reveals a preferred coordination ability of the phosphinoyl moiety over nitrate and water, enhanced by their grafting on the calixarene platform. The coordination ability of calix[4-8]arene derivatives bearing phosphinoyl and/or CMPO pendant groups toward Th(IV) has been widely studied, with special emphasis on their extraction capability.2,3,8a,9a,10b,16 However, to the best of our knowledge, isolated thorium complexes with such calixarenes have not yet been reported. Similarly to the uranyl complexes, the IR spectra suggest that all of the phosphinoyl groups are coordinated in the 1:1 Th(IV) complex but not in the 1:2 complex, 4. Since the latter contains coordinated water, we tentatively propose CN ) 9 for this thorium complex. III.3. Solution Studies of the Isolated Complexes. III.3.1. Absorption Spectra. The UV-vis spectra of the B6bL6 complexes in acetonitrile (Figure S1, Supporting Information) display the main ligand band at around 270-272 nm, hardly displaced with respect to the free ligand (271 nm). The molar

10980 J. Phys. Chem. B, Vol. 112, No. 35, 2008

Ramı´rez et al.

TABLE 1: Main Luminescence Parameters of the Uranyl-Calixarene Complexes in Acetonitrile intensity ratio, In/I1b

E(0-0)/cm-1a cmpd

mean vibronic spacing (νUO)c cm-1

rU-O/pmd

mean fwhh/nm

lifetimes τ /ms (% emission) 77 K

mode

77 K

77 K

291 K

77 K

291 K

77 K

77 K

[UO2]

P

20779

1.40/0.91/0.47/0.25

875(24)

840(47)

173.9

10.5

1g

P

20161

0.99/0.45/0.15/0.05

34/0.88/0.4 3/0.20a 0.98/0.64

825(31)

857(27)

178.6

6.5

0.21 ( 0.01(86) 0.49 ( 0.07(14)

3g

F P

20185

1/0.60/0.31/0.16

1/1.01/0.60

840(73)

837(33)

177.1

9.1h

0.22 ( 0.01(76) 0.65 ( 0.11(24)

F

20185

1.03/0.63/0.21

1.01/0.62

855(7)

853(4)

175.7

6.9

2+f

0.002 ( 0.0001e

F ) fluorescence, P ) phosphorescence. n ) 2-5 at 77 K and 2-3 at 291 K. Standard deviation (2σ) in parentheses. d Determined with eq 1. e At room temperature solvated in dry acetone, from ref 22d. f Solvated in dry acetonitrile, this work. g λexc ) 270 nm, λemi ) 495 nm. h Overlap with triplet state emission of B6bL6. a

b

Figure 1. Phosphorescence spectra of (a) 0.308 mM B6bL6, (b) 0.508 mM UO2(NO3)2, and (c) 0.229 mM [UO2B6bL6](NO3)2 in frozen CH3CN solution at 77 K; λexc ) 270 nm; slit widths: 5 nm (excitation and emission); filter: 290 nm.

absorption coefficients (ε) of the 1:1 complexes are alike and about 16-18% larger compared to the free ligand (see Experimental Methods). For the 1:2 compounds, the increase amounts to 86 and 95% for uranyl and thorium, respectively. This is consistent with a partial coordination of the phosphinoyl groups only, leading to an overall smaller ligand-to-metal interaction. In addition, the number of water molecules seems to influence the molar absorption coefficient since complexes 1 and 2 with the same number of water molecules have comparable ε values, while complex 3 containing four water molecules more than complex 4 has a smaller ε value. The complexes were not soluble at concentrations larger than 3 × 10-4 M; therefore, it was not possible to measure the absorption spectrum of the uranyl complexes in the uranyl region (330-500 nm). III.3.2. Luminescence of the Uranyl Complexes. We have taken advantage of the luminescence properties of uranyl17,21–23 and those of the ligand 3ππ* emission12c to get more insight into uranyl-calixarene interaction in acetonitrile solutions and frozen acetonitrile solutions (2.20-2.29 × 10-4 M). Spectra were recorded in both fluorescence and phosphorescence modes at 291 and 77 K, and relevant data are listed in Table 1. Upon ligand excitation at 270 nm, uranyl luminescence was observed for the 1:1 and 1:2 complexes in both modes, but 3ππ* emission at 436 nm was only detectable in the phosphorescence mode (Figure 1 and Figures S2-S4, Supporting Information). The triplet state of the ligand is located at around 23 000 cm-1, and the lowest excited state of uranyl is at around 20 200 cm-1, henceforth an energy gap equal to ∼2800 cm-1, which indeed

c

appears to be ideally suited for transferring energy onto the metal ion. This transfer is ascertained by comparing the excitation and emission spectra of 1 and 3 with those of uranyl nitrate in acetonitrile and deuterium oxide at concentrations larger than the concentration of the macrocyclic complexes (up to 27-fold); enhancement of the uranyl luminescence is clearly evidenced in presence of B6bL6. The transfer is also proved by the excitation spectra (Figure 2), which display bands at 260 and 274 nm attributable to ligand levels; an additional weak and structured feature between 289 and 346 nm and centered at 333 nm seems to result from the first electron transfer from the uranyl oxygens to the uranium atom in the uranyl ion (LMCT).21c The absolute quantum yield (Qabs) found for the 1:1 uranyl complex in CH3CN is 2.0 ( 0.3%. The latter is high in comparison to that of the unhydrolyzed (UO22+)aq in acidic solutions with Qabs ) 0.1%21b and lifetime τ ) 1.37 ( 0.04 µs.22a Therefore, it is established that the noticeable quantum yield revealed by compound 1 results from a sizable sensitization by the calixarene, which is highly favored by the uranophyilic nature of B6bL6. Uranyl emission is characterized by an extended vibronic structure mainly reflecting the OdUdO symmetric stretching (870 ( 20 cm-1 for [UO2(H2O)5]2+),22b although other contributions, for example, from nitrate, are also documented. This structure depends strongly on the coordination environment around the uranyl ion, and changes in intensity, energy, and width of the vibronic components yield valuable information about the local symmetry around uranyl.17a,21–23 The phosphorescence spectrum of 1 at 77 K (Figure 1c) reveals five vibronic components at 496 (20 161), 517 (19 342), 541 (18 484), 565 (17 699), and (possibly) 593 nm (16 863 cm-1), labeled 1-5, arising from the lowest excited state of uranyl. The vibronic components are narrow, with a mean full width at half-height (fwhh) equal to 6.5 nm. This reveals little contribution from the lattice to the low-energy vibrational modes of uranyl, much as seen for uranyl nitrate solutions in acetonitrile (fwhh ≈ 10 nm). In water, bands are usually broader; for example, free UO22+ in acidic solution at pH e 2 has fwhh ≈ 13 nm.22c Vibronic component 1 corresponds to the 0-phonon transition, which is magnetic dipole allowed when U(VI) lies on an inversion center but may acquire appreciable electric dipole character for noncentrosymmetric environments;17a,b,21–23 it is red-shifted by 14 nm with respect to uranyl nitrate in acetonitrile (Figure 1), pointing to a strong uranyl-calixarene interaction. We note that a similar red shift has been reported for uranyl in 1 M phosphoric acid, in which strong interaction exists with hydrogenophosphate.22,23b The intensity ratios In/I1 (n ) 2-5)

Actinide Complexes with para-tert-Butylcalix[6]arene

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10981

Figure 2. (A) Fluorescence excitation spectra of UO2(NO3)2 in D2O at 291 K (a) 0.524 mM (λem ) 496 nm), (b) 6.1 mM (λem ) 493 nm), and of (c) [UO2B6bL6](NO3)2 0.229 mM in CH3CN (λem ) 496 nm). (B) Phosphorescence excitation spectra of (a) 0.308 mM B6bL6 (λem ) 436 nm), (b) 5.1 mM UO2(NO3)2 (λem ) 485 nm), and (c) 0.229 mM [UO2B6bL6](NO3)2 (λem ) 496 nm) in frozen CH3CN solution at 77 K. Slit widths: 9nm (excitation and emission); filter: 290 nm.

of the vibronic bands (see Table 1) are greatly affected by coordination to the calixarene; they differ considerably from those of uranyl nitrate in acetonitrile. The observed average vibrational splitting for 1 (825 cm-1 at 77 K) is very similar to the one reported for [UO2(NO3)2(dpepa)], where dpepa stands for diphenyl-N,N-diethylphosphine amide, at the same temperature (828 cm-1).20a The reported emission data of a hexacoordinate [UO2(18-crown-6)]2+ complex recorded at 298 K points to an average vibrational splitting of about 883 cm-1 for a compound with CN ) 8.23e On the other hand, it is smaller in the heptacoordinated TPPO (triphenylphosphine oxide) adduct of uranyl benzoylacetonate (818 cm-1).20f This comparison leads us to propose CN ) 8 for compound 1, which, again, implies coordination of the six phosphinoyl pendant arms. Other information that can be extracted from uranyl luminescence spectra is the length of the OdUdO bond, rUO. Indeed, a correlation has been established between the OdUdO symmetric stretching frequency νUO (in cm-1) and rUO (in pm) determined by diffraction methods for about 27 uranyl compounds21c,25

rUO ) 10 650(νUO)-2/3 + 57.5 [pm]

(1)

For complex 1, rU-O ) 178.6 pm is longer than for uranyl nitrate in acetonitrile (rU-O ) 173.9 pm) or for [UO2(H2O)5]2+ in water

at pH 2 (174.4 ppm). In uranyl complexes, a longer OdUdO bond length is often induced by the equatorial ligands. For complex 1, rU-O is in the range found experimentally by diffraction techniques21a for uranyl compounds with sixfold equatorial coordination. This again is in line with the proposed CN ) 8 for this macrocyclic complex. To complement the study, lifetimes (Table 1) have been determined since they also strongly depend on the uranium microenvironment,21,22f,23,25 much as for lanthanide compounds.17c,26 The luminescence decay for 1 in frozen acetonitrile (77 K) is biexponential. The component with the shorter lifetime (τS ) 0.21 ms) accounts for 86% of the luminescence, which means that the solutions contain a minor species with a longer lifetime (τL ) 0.49 ms). Simple uranyl compounds or salts have much shorter lifetimes, for example, 2 µs for [UO2(H2O)5]2+ at pH 2 or for solvated UO22+ in acetone;22d therefore, the biexponential decay does not reflects dissociation of the 1:1 complex but, rather, equilibrium between two species complexed to B6bL6. The presence of hydrolyzed species can also be rejected since fwhh ) 14-23 nm has been reported for the UO2OH+ and [(UO2)2(OH)2]2+ in water. This interpretation is backed by the other photophysical properties recorded at 291 and 77 K, a red shift of the 0-phonon transition, the fwhh, vibrational splitting, and the similarity between fluorescence and phosphorescence spectra (see Figures 1, 2, and S3c, Supporting Information). Therefore, we propose that the minor species with a longer lifetime is probably a macrocyclic complex with either another conformation of the macrocycle inducing a more rigid environment for the uranyl cation or, possibly, some of the second-sphere water molecules replaced by acetonitrile molecules. The luminescence spectra of UO2(B6bL6)2(NO3)2 · 12H2O (3) at 77 K are reported in Figure S4 (Supporting Information). The energy of the emission bands for 3 differs little from those of compound 1 (∼10 cm-1), suggesting a similar chemical environment around the uranyl cation in both complexes. Moreover, in fluorescence mode, the vibronic spacing shows little variation with a mean energy value of 855 cm-1. As a consequence, the calculated U-O bond length (Table 1) is 2.9 pm shorter in 3 compared to 1. This indicates a slightly higher symmetry around the uranium cation in complex 3 compared to 1.20a,f,21a,e In addition to the uranyl emission, the phosphorescence spectrum reveals an underlying poorly structured broad band arising from the ligand triplet state, which is usually seen in the range of 430-450 nm.12c This points to incomplete energy transfer from the two partially coordinated calixarenes. In addition, the relatively weak emission in frozen solution can be traced back to nonradiative lattice-induced deactivation processes, a consequence of the high content of water in this complex generating hydrogen bonds with nonbonded phosphinoyl groups. At 291 K, most of the luminescence parameters (energy, intensity ratios, vibronic spacing, fwhh) are similar to those found at 77 K. However, the hydrogen bonding network of the water molecules is probably broken, which results in an overall larger emission intensity. The larger phosphorescence intensity of 3 compared to 1 can be understood in terms of a better steric protection of the uranyl cation provided by the two macrocyclic ligands. The luminescence decay is also biexponential, though with the longer lifetime much larger than that of the 1:1 complex, a fact which substantiates the previous reasoning. The conclusion that we can draw from this luminescence study and from the comparison with the emission spectra of uranyl nitrate recorded under various experimental conditions is that coordination of uranyl to the calixarene results in a

10982 J. Phys. Chem. B, Vol. 112, No. 35, 2008 TABLE 2: Calculated Structures for the Free Calixarene and Their Thorium Complexes cmpnd

E/kcal mol-1a

proposed formula and geometryb

B6bL6

55, 55, -661, -727,

2

261, 242

4

222, 222

distorted alternate in-out cone conformer, C6V12c [Th(NO3)(B6bL6)(H2O)]3+ CN ) 8; flattened square antiprism mean Th r OdP: 2.45(2) Å [Th(B6bL6)2(H2O)]4+ CN ) 9; distorted monocapped dodecahedron mean Th r OdP: 2.44(2) Å

a In sequence: MM3, CONFLEX, MOPAC/PM5, and MOPAC/ PM5/COSMO. b From the optimum geometries found by MM3 and CONFLEX (CAChe System Pro 5.02).

stabilization of its triplet state (heavy atom effect), favoring an efficient energy transfer, although not complete in the case of 3. Additionally, the macrocyclic molecule(s) provide(s) a protective environment, minimizing nonradiative deactivation processes. III.4. Molecular Modeling. Attempts to get suitable crystals for X-ray analysis of the synthesized actinide calixarene complexes failed, perhaps as a consequence of their hygroscopic nature. Therefore, we resorted to molecular modeling to have an idea on the metal ion environment as well as on the conformation of the calixarene. In previous work, molecular mechanics and dynamics calculations using the Dreiding force field Cerius procedure pointed to an alternate in-out cone conformation for B6bL6 (1045 kJ mol-1), in line with the X-ray diffraction data.12c For this work, the calixarene has been recalculated with the MM3 and CONFLEX procedures, and the same conformation was found, albeit with a much lower energy, 230 kJ mol-1. The molecule of each complex was built taking into account the experimental data reported above and in the literature.14,18,19,20b,c The structures refined by performing optimized geometry calculations as well as their associated thermodynamic parameters are collected in Table 2. The main bond lengths and bond angles of the coordination polyhedra of the thorium complexes are collected in Table S1 (Supporting Information). III.4.1. Calculated Structures. By providing the coordination of six phosphinoyl arms, the macrocyclic ligand fulfills CN ) 8 required by the uranyl ion in the 1:1 complex, and experimental data point to the absence of coordinated nitrate and/or water molecules. Nevertheless, molecules with partial binding of the ligand and nitrate and/or water coordination were evaluated. Their energy minima were always larger than for the molecule where the uranyl was coordinated to the calixarene only. Usually, the coordination polyhedron of eight-coordinate U(VI) complexes is a hexagonal bipyramid with the uranyl OdUdO atoms occupying axial positions. Such an arrangement is made possible because of the very stable chemical bonds of the uranyl cation and the large size of the uranium atom which allows six ligands to form a hexagonal girdle in a puckered fashion. The steric effects in the latter are reduced by the alternation of the donor atoms above and below the equatorial plane.14c,e The structures of eight-coordinate uranyl complexes with macrocycles containing pyrrole and Schiff bases have revealed distortion of the equatorial planarity associated with the extent of the ligand conformational mobility.15b,e Uranyl complexes with crown ethers have structures in which the coplanar arrangement suggests distortion of the hexagonal bipyramidal geometry probably due to the distorted conformation adopted by the macrocycle in the complex.15a,c Size,

Ramı´rez et al. mobility, and steric bulkiness of the ligand affect substantially the arrangement of the donors around the equatorial plane. Recently, geometries far from bipyramidal geometries were found by X-ray crystallography for UO22+ complexes; for instance, bidentate benzaminato ligands can force four, five, or six donors out of the equatorial plane so that distorted tetragonal, pentagonal, and hexagonal bipyramidal geometries are no longer considered. In the case of the heptacoordinate uranyl complex, the geometry of the molecule was visualized as a quadrilateralfaced monocapped trigonal prism (0.14-0.62 Å out of the equatorial plane) rather than a distorted pentagonal bipyramid. In the case of the corresponding eight-coordinate uranyl complex, the three bidentate ligands are displaced out of the equatorial plane (0.42-0.61 Å) in a propeller arrangement.14d A long time ago, the structure of the polymeric eight-coordinate [UO2(ox)2]n2dianion was found to have a highly puckered equatorial arrangement of the oxygen donors (0.32-0.58 Å). Such a hexagonal ring puckering distorts the hexagonal bipyramid toward a cube, which, in turn, can transform into a dodecahedron.14c,e More recently, a distorted dodecahedral geometry was proposed for an eightcoordinate uranyl Schiff base complex, on the basis of spectroscopic evidence.15f Uranyl complexes formed with parent calixarenes in which the cation is surrounded by the phenolic oxygen atoms do not exhibit significant deviation from the free ligand conformation. In effect, the calixarenes stabilize equatorial four- or fivecoordination.14f We have demonstrated both experimentally and theoretically (see above)12c that B6bL6 bearing phosphinoyl arms adopts an alternate in-out cone conformation (Figure 3, bottom). This suggests a significant distortion in the six-coordinate equatorial geometry of the eight-coordinate U(VI) complexes 1 and 3. Considering the particular molecular geometry evidence discussed for eight-coordinate uranyl complexes,14c–e,15a–c,e it is probable that the conformational and steric effects of the calixarene are sufficiently large for the modeled molecules 1 and 3 being stabilized by a typical (although highly distorted) puckered six-coordinate arrangement of the PdO donors in the equatorial plane of the uranyl ion. However, despite that the calculated U r OdP bond lengths (2.311-2.332 Å) were close to those obtained by crystallography for ligands with OdP groups (2.340-2.359 Å),20a,b the model calculations yielded OdUdO bond angles differing widely from the usually observed one (176-180°); therefore, these calculations are not presented here. Molecules with CN ) 8 (effective ionic radius ) 1.19 Å) and 9 (1.23 Å) were modeled for the 1:1 thorium complex. The most stable is [Th(NO3)(B6bL6)(H2O)]3+, with CN ) 8, in which the nitrate is monodentate. Molecules with a bidentate nitrate, two monodentate nitrates, or two water molecules to reach CN ) 9 proved to be highly unstable. The optimized structure reveals that the nitrate and water molecules are oriented toward the hydrophobic cavity of the calixarene and form a H2O-Th-ONO2 bond angle of 131.6° while the phosphinoyl pendant arms drive the thorium cation toward the lower rim of the macrocycle (Figure 3, top), thanks to the adequate size of B6bL6. This structure, which is less stable than that modeled for the 1:2 complex (Table 2), features a square antiprismatic, slightly flattened coordination polyhedron compared to those found in the crystal structures of thorium complexes with CN ) 8.14a,c For the 1:2 complex, the most stable structure, with CN ) 9, contains a thorium ion coordinated to four phosphinoyl groups from each calixarene and to a water molecule, [Th(B6bL6)2-



Figure 3. Lowest-energy calculated structures of [Th(B6bL6)(NO3)(H2O)]3+ (top) and B6bL6 (bottom) in two views. Hydrogen atoms are not shown for clarity.

H2O]4+. The Th r OdP bond length is similar to the one found for the 1:1 complex while its minimum energy is lower, probably in view of the cage-like environment generated by the methyl substituents of the coordinated phosphinoyl pendants. The coordination polyhedron was a distorted monocapped dodecahedron (Figure S5, Supporting Information), in agreement with structure geometry for thorium complexes with CN ) 9.20a III.4.2. Discussion: Correlation with Experimental Data. The [UO2(NO3)2(H2O)2] unit, which features bidentate nitrates and two water molecules located in the trans position, is a typical example of eight-coordinate uranyl with a hexagonal bipyramidal arrangement of the oxygen atoms.18a,b According to theoretical calculations which led to U-ONO2 (2.43 Å) and U-OH2 (2.39 Å) distances in line with experimental data,18b this unit is affected by the presence of four water molecules in the second coordination sphere linked by hydrogen bonding to the inner water molecules. The replacement of nitrate and water by other donor molecules does not change this arrangement much, for example, in complexes with TPPO or dpepa.20a,b In our case, the peculiar conformation of the macrocyclic ligand and the spatial positioning of the phosphinoyl arms lead to a coordination polyhedron far from the typical hexagonal bipyramidal arrangement. For thorium complexes, coordination numbers larger than 8 are known, up to 12 when four bidentate nitrates are coordinated.14a,b,19 However, monodentate coordination of nitrates has been demonstrated by vibrational studies for complexes with ligands fitted with phosphine oxide derivatives.14a,b Thus, nitrates partly dissociate upon formation of complexes with multidonor organic ligands of adequate size and with a larger coordination ability than nitrate, a fact that was revealed

by the IR spectra of the 1:1 and 1:2 An/B6bL6 complexes presented here. The experimental data gathered for the isolated complexes can be interpreted on the basis of the modeled structures. For instance, the largest shift of the OdP vibrational frequency found for compound 3 with respect to that of compound 1 is in line with the shorter U-OdP bond length predicted by semiempirical calculations (average values: 2.331 Å for 3 and 2.333 Å for 1). Compound 1 has the largest shift of the dC-O-CH2 vibrational mode, a consequence of the location of the uranyl ion in the lower rim cavity. The thorium complexes 2 and 4 show trends in the shift of their vibrational frequencies similar to those of uranyl complexes, despite longer An r OdP bond lengths. The energy of the asymmetric vibration of the free uranyl ion (910 cm-1)13e is affected by the arrangement of the ligands around the ion; it is blue-shifted 5 and 15 cm-1 for compounds 1 and 3, respectively. The intensity ratios of the main bands of the luminescence spectra of compound 3 are also indicative of a more symmetric environment for the uranyl ion in 3. The less efficient energy transfer to uranyl ion observed in 3 is probably due to the uncoordinated pendant arms. The modeled structures of thorium complexes correctly reflect the composition of the inner coordination sphere of the metal ion inferred from the vibrational spectra. III.5. Liquid-Liquid Extraction. Ln and An have similar extraction properties, as hard acids coordinating to hard donor ligands to form complexes with the same stoichiometry, but the extent of the metal-acceptor binding varies significantly since the 5f orbitals of An interact more strongly with the donor ligands than 4f orbitals. This difference has a noticeable effect in the extraction capability of a particular ligand and the stability


Ramı´rez et al.

TABLE 3: Separation of Actinides and Rare Earths Using Three Liquid-Liquid Extraction Systems at 18 ( 2 oC organic phases: chloroform-calixarene aqueous phases

1.06 × 10-4 M; B6BL6 (4)

3.08 × 10-4 M B6bL6 (5)

percentages of the cations extracted Aqueous Phase 1: 1 M HNO3-0.5 M NaNO3 1.25 × 10-4 1.30 × 10-4 1.29 × 10-4 1.08 × 10-4 1.15 × 10-4

M M M M M

UO2(NO3)2 6H2O Th(NO3)4 5H2O La(NO3)3 6H2O Eu(NO3)3 6H2O Y(NO3)3 6H2O

48.89 ( 0.55 58.26 ( 0.9 5 ( 1.01 16 ( 1.04 7 ( 0.09 Aqueous Phase 2: 1 M HNO3-3.5 M NaNO3

82.64 ( 0.32 98.87 ( 2.5 8 ( 1.0 19 ( 1.07 9 ( 0.12

1.19 × 10-4 1.17 × 10-4 1.11 × 10-4 1.19 × 10-4 1.19 × 10-4

M M M M M


55.92 ( 0.50 80.21 ( 0.99 43 ( 1.03 1 ( 0.60 6 ( 0.05 Aqueous Phase 3: 3 M HNO3-0.5 M NaNO3

88.52 ( 0.83 97.62 ( 1.93 45 ( 1.06 3 ( 1.10 21 ( 1.01

1.12 × 10-4 1.09 × 10-4 1.16 × 10-4 1.28 × 10-4 1.24 × 10-4

M M M M M


37.26 ( 0.55 50.07 ( 0.9 7 ( 0.85 3 ( 0.07 no extraction

of the extracted species. For instance, the extraction affinity of phosphoryl groups toward An is greater than toward Ln.2a,b In this work, Th4+ and UO22+ were chosen for two main reasons. Firstly, thorium and uranium are of long-term environmental concern. Secondly, these cations are good models for other actinides, for instance, Th(IV) for Pu(IV) and UO22+ for NpO22+ and PuO22+. Transuranium elements are difficult to handle because of their high radioactivity.1b,13d Finally, it is common to find actinides together with rare earths in radioactive wastes; therefore, we are interested in investigating the extraction ability of the titled calixarene toward rare earths too. In this study, three main aspects of extraction were found: (1) the best system for the extraction of UO22+, Th(IV), La(III), Eu(III), and Y(III) from aqueous media using B6bL6 in chloroform, (2) the number of organic ligand molecules per cation in the UO22+, Th (IV), and Eu(III) extracted species, and (3) the loading capacity of the organic phase with thorium nitrate. III.5.1. Optimization of the Procedure. Neutral organophosphorous compounds are the most useful extractants for the extraction of actinides from strongly acidic media containing other metal ions.1,3,5c To this effect, our purpose was to assess the extraction capability of B6bL6 calixarene toward actinides, lanthanides, and yttrium ions from an aqueous medium; various experimental conditions were imposed to the aqueous solutions containing the actinide and rare earth nitrate salts. Thus, preliminary studies indicated that the calixarene in chloroform extracts 67% of uranyl (shaking for 3 h) from nitric acid solution in the absence of sodium salt even though an incipient aqueous-organic emulsion is noticed when an organic phase like 5 (Table 3) is used while thorium ions are not extracted and a “thick emulsified interphase” is formed even with a phase like 4. That emulsification after addition of CH3CN turned into a transparent liquid (turbid with CHCl3), and its UV-vis spectrum suggested that the emulsified phase contains ligand and water only, while the thorium concentration in the aqueous phase was kept after extraction. Such an “interphase” was also formed during the water or 1 M HCl aqueous solution work up of the hexaphosphinoylated p-tert-butylcalix[6]arene in CH2Cl2 as a result of the intermolecular hydrogen bonding between water molecules and the phosphinoyl groups of the calixarene.12c

84.88 ( 0.66 84.14 ( 0.95 10 ( 1.03 19 ( 1.03 no extraction

Moreover, the UV-vis spectra of the organic extracts (using organic phase 5) after contact with the acidic aqueous phases containing the actinide ions reveal bands corresponding to the calixarene and a weak structured broad band (310-390 nm) centered at 360 nm assignable to ionized nitric acid. Apparently, the lack of extraction of thorium under the same experimental conditions as uranyl could result from the highly hydrated nature and slow dehydration of the Th(IV) ion (small and highly charged) before the ion reaches the interface11a of the aqueous phase-organic phase and interacts with the calixarene. A great part of the latter interacts first with water molecules, forming the thick emulsified “interphase”; thus, the remaining calixarene in the organic phase is not enough concentrated for extracting the metal ion. The addition of sodium nitrate (0.5 M) to the aqueous acidic solution, 1 M HNO3 (aqueous phase 1), precludes the formation of the incipient interphase during uranyl extraction with organic phase 5, and its extraction is significantly increased (Table 3) despite the fact that little nitric acid is also extracted (revealed by UV-vis spectroscopy). Under this condition, thorium is substantially extracted, though a very thin emulsified interphase still remains (removed as indicated in section II.3.2). UV-vis spectrum of the extract shows an intense broad band at 340 nm assigned to ionized nitric acid (Figure 4). At 3.5 M NaNO3/1 M HNO3 (aqueous phase 2), no HNO3 is revealed in the extracts of both actinide ions, and there is no emulsion in the thorium extraction (Figure 4). Such a NaNO3 concentration in the acidic aqueous solution provokes the breaking of the intermolecular hydrogen bonding between PdO groups and water molecules; thus, the calixarene is free to interact with thorium ions. The 3 M HNO3 and a low concentration of sodium salt (aqueous phase 3) avoid the “interphase” formation in the thorium extraction also, but its extraction decreases substantially. It seems that high acidity promotes the formation of OONOH · · · OP- intermolecular hydrogen bonding since the extract shows a well-defined band at 243 nm (see Figure 4); we have already reported12c that in CH3CN, the free calixarene displays one large absorption band at around 205 nm assigned to a transition mainly located on the OP groups. It has to be this absorption maximum which suffers a bathochromic effect (38 nm) by interaction of the phosphoryl groups with the nondissociated nitric acid. La(III),


Figure 4. Influence of the aqueous medium conditions in the feature of the UV-vis spectrum of the extracts of Th4+ using organic phase 5 (see text).

Eu(III), and Y(III) ions are not extracted in the absence of NaNO3, but 0.25 M NaNO3 in 1 M HNO3 permits the extraction of Eu(III) up to 20%, while that of the others is negligible (37% larger than that found using aqueous phase 1. In contrast, a negative effect in the extraction of the Eu(III) ion is noticed. Y(III) extraction is favored but at [B6bL6] > 1.1 x10-4 M. Aqueous phase 3 was tested in order to find out the effect of more concentrated acid and solutions richer in nitrates upon the extraction of RE and An, that is, if HNO3 molecules and/ or the [NO3-]/[Na+] ratio affect the extraction of those ions. The results compared with those using aqueous phase 1 and organic phase 5 (Table 3) indicate a substantial decrease in thorium extraction; Y(III) ions are not extracted while uranyl, La(III), and Eu(III) ion extractions do not change. So far, our results indicate that uranyl extraction is little favored by the salting out effect27c (sodium nitrate acts as an electrolyte27a), while this effect is noticeable in the thorium extraction in two aspects: its extraction increases at low ligand concentration and the emulsified interphase disappears. The La(III) extraction also benefits from the salting out effect, with little influence of the calixarene concentration. In general, the characteristic of aqueous phase 3 exerts a negative or no effect in the extraction of the metal ions studied. The uranyl ion extraction is little affected by the aqueous medium conditions at >3 × 10-4 M calixarene concentration, while at a 1 × 10-4 M ligand concentration, it is. We believe that the particular coordination properties of the uranyl cation (total CN ) 8, with six sites to be filled by ligands in the equatorial plane), its charge density (assuming a formal charge of +2 or an effective charge of +3.318c for the U atom, r ) 1.779 Å for a CN ) 621a and Z2/r ) 2.25 or 6.12 e2 Å-1, respectively), and the intrinsic uranophile ability of the phosphinoylated p-tert-butylcalix[6]arene toward this cation permits the formation of a stable uranyl


Figure 5. Extraction of Th(IV), UO22+, and Eu(III) ions by B6bL6 in chloroform at variable concentration from aqueous phase 2 (An and RE nitrate solution (∼1 × 10 -4 M)/1 M HNO3/3.5 M NaNO3)) at 291 ( 2 K; n ) slope and R ) 0.9997 for Th(IV) and UO22+ and 0.9988 for Eu(III) ions.

complex species that is little affected by the nature of the aqueous medium. The experimental and calculated structural parameter results corroborate these assumptions. Unlike uranyl, the studied spherical metal ions are small and highly charged; for a CN ) 8, Z2/r (e2 Å–1) ) 6.92, 7.46, 7.76, and 13.44 for La3+, Eu3+,Y3+, and Th4+ ions, respectively; they are more affected by the nature of the aqueous medium since they can be more hydrolyzed and have to fulfill at least a CN ) 8, despite the fact that thorium has an affinity to ligand with OP donors, and this permits its good extraction. Wipff et al.11 have successfully applied molecular dynamic procedures for simulating experimental results concerning the interfacial phenomena in liquid-liquid extraction systems, in particular, for organic phases containing macrocycles. According to them, the calixarene adsorbs at the interface in an uncomplexed or complexed form, being intrinsically more polar, amphiphilic, and surface active. Despite its amphiphilic orientation, it cannot spontaneously lead to the extraction of the complex because the cation can remain highly hydrated and attracted by the water phase; then, its extraction toward the organic phase might be facilitated by saturation of the liquid-liquid interface and the attraction by the counterions in the organic phase as well as the electrostatic repulsion between polar (charged) species at the interface.11 In the present work, this explains, to some extent, the extraction behavior of thorium and rare earth ions (in the studied aqueous media at 291 K) by the phosphinoylated p-tert-butylcalix[6]arene extractant in chloroform. III.5.3. Influence of the Calixarene Concentration. The number of B6bL6 molecules coordinated to UO22+, Th(IV), and Eu(III) ions was investigated as established in section II.3.3. Figure 5 shows the graphs of log D versus the log of the B6bL6 concentrations for each metal nitrate. These reveal a slope equal to 1 for the actinides and 0.9 for europium, pointing to one calixarene for one cation. Therefore, CN ) 8 fulfilled with nitrates is assumed for the extracted thorium and europium complexes, while in the uranyl complex, the coordination sphere is fulfilled with the calixarene only. In Table 4 are collected the distribution coefficients, which are on the order Th(IV) > UO22+ . Eu(III). The latter has a very low D, confirming the low capability of the B6bL6 calixarene for extracting this lanthanide under the established conditions, even at higher calixarene concentrations. We conclude that the species with a


Ramı´rez et al.

TABLE 4: Distribution Coefficients for UO22+, Th4+, and Eu3+ (CL ) 8 × 10-4 M*; CM e 1.19 × 10-4 M) and Separation Factors between Actinides (An) and the Rare Earths (RE) B6bL6 /org. phase 4 An and RE ions

aq. ph. (1)

aq. ph. (2)

aq. ph. (3)

B6bL6 /org. phase 5 aq. ph. (1)

aq. ph. (2)

aq. ph. (3)

Distribution Coefficients* Th4+ UO22+ Eu3+ Th/U Th/La U/La Th/Eu U/Eu Th/Y U/Y a

Separation Factors R 1.2 11.7 9.8 3.6 3.1 8.3 7.0

1.4 1.9 1.3 80.2 55.9 13.4 9.3

1.4 7.1 5.3 16.7 12.4 a a

1.2 12.4 10.3 5.2 4.3 11.0 9.2

2.6 2.3 0.2 1.1 2.2 2.0 32.5 29.5 4.7 4.3

0.9 8.5 8.6 4.4 4.5 a a

Metal ion (III) is not extracted.

this means that 2.47 × 10-4 M B6bL6 can still separate thorium from aqueous solutions richer in thorium nitrate. No formation of a microemulsion nor a third phase27b,c were noticed. In summary, the extraction study of La3+, Eu3+, Y3+, Th4+, and UO22+ ions with the hexaphosphinoylated p-tert-butylcalix[6]arene, B6bL6, shows that the extraction ability does not parallel the hard character of these cations (UO22+< La3+ < Eu3+ La g Y, (aqueous phase 1 and 3 and organic phase 5) is similar to that obtained with (phosphine oxide-CH2)4-p-tert-butylcalix[4]arene (log D: Eu > La > Y) in a more polar solvent, 1,2-dichloroethane, and with 1 M HNO3/0.9 M Al(NO3)3 aqueous phase.7c The extraction behavior of these rare earths with B6bL6 is probably due to several factors, including the degree of hardness of the cations along with the steric interactions between the phosphinoyl groups and these cations, as well as the extent of the distorted alternate in-out cone conformation of the semirigid B6bL6 calixarene. IV. Conclusions

Figure 6. Loading capacity of B6bL6, 2.47 X10-4 M in chloroform, by thorium nitrate from 1 M HNO3/3.5 M NaNO3 aqueous solution at 291 ( 2 K.

1 metal ion/1 calixarene stoichiometry is the most stable from the thermodynamic point of view. At 291 K, at a concentration > 8.3 × 10-4 M of B6bL6, an incipient emulsified phase is noticed when uranyl and thorium ions are extracted. The separation factors RAn/Ln and RAn/Y given in Table 4 demonstrate the good selectivity of the studied calixarene toward actinides over the rare earths. In particular, actinides could be separated efficiently from Eu(III) using aqueous phase 2 and completely from yttrium. The aqueous samples can be treated again with nitric acid to separate remaining actinides. We propose then that the extraction selectivity of B6bL6 toward actinide ions present in different aqueous media over that of the studied rare earths could be useful in the declassification of radioactive solutions containing those elements. III.5.4. Influence of the Actinide Salt Concentration. The loading capacity of B6bL6 toward actinides was investigated for the thorium ion in the aqueous phase 2, varying the concentration of thorium nitrate from 3.3 × 10-4 to 0.3 M at constant concentration of the calixarene in the chloroformic organic phase (2.47 × 10-4 M). Figure 6 displays the graph of the log of the concentration of thorium in the organic phase after its extraction from the aqueous phase versus the log of thorium concentration in the aqueous phase before extraction. It is observed that, up to 0.1 M thorium nitrate, the organic phase is not yet saturated;

The hexaphosphinoylated p-tert-butylcalix[6]arene ligand, B6bL6, forms stable actinide complexes with 1An/1L and 1An/ 2L stoichiometries in organic media. It is concluded that in organic medium, B6bL6 recognizes the uranyl ion better than the thorium ion since the uranyl 1:2 B6bL6 compound yield is 15% larger than that corresponding to the thorium complex, confirming the greater affinity of this calixarene toward the uranyl ion. The experimental and molecular modeling studies support that in compound 1, the six phosphinoyl pendant arms of the calixarene are coordinated to the uranyl ion in an asymmetrical arrangement, while in 3, each calixarene affords three phosphinoyl pendant arms to the uranyl, acquiring a more symmetrical structure. In thorium calixarene complex 2, its coordination sphere is completed with one nitrate and water molecule, while in 4, it includes four phosphinoyl pendant arms per calixarene and a water molecule. The εs values indicate no dissociation of the actinide ion. The luminescence emission of the uranyl B6bL6 complex 1 reveals a good energy transfer from the ligand to the uranyl ion, whereas the emission of compound 3 is almost quenched. The noticeable absolute quantum yield found for 1 is evidence of an antenna effect. The biexponential decay of the uranyl complexes does not reflect dissociation but, rather, equilibrium between two species complexed to B6bL6 in different conformations or possibly with some of the secondsphere water molecules replaced by acetonitrile molecules in one of the species. The capability of B6bL6 toward the extraction of actinides has been demonstrated. The formation of 1:1 species is predominant during the extraction of uranyl, thorium, and europium with B6bL6 from an aqueous medium rich in nitrate ions. The extraction percentages, the distribution coefficients, and the separation factors found for the actinides using aqueous phase 2 allow us to propose that the actinides contained in 1 M HNO3/3.5 M NaNO3 radioactive solutions will be separated efficiently with B6bL6 from europium, while from lanthanum

Actinide Complexes with para-tert-Butylcalix[6]arene and yttrium, they will be separated if they are in 3 M HNO3/ 3.5 M NaNO3 solutions. It is concluded that aqueous phase 2 and B6bL6 as the extractant in chloroform form a good liquid-liquid system which could be an acceptable option for extracting actinides from highly salty nuclear-waste-containing rare earths and sodium. Acknowledgment. This work is supported through grants from CONACYT (Me´xico), Project No. 36689-E and the Swiss National Science Foundation project SCOPES 2000-2003 No. 7BUPJ062293. We thank Q. E. Ricardo Soria from the Analytical Department for his help in the uranium determination by ICP and the technicians from the Chemistry Department of ININ. We thank Mr. Claudio Fernańdez from the Library of ININ for his help anytime we needed it. Supporting Information Available: Absorption spectra of compounds 1-4 and free calixarene (Figure S1), fluorescence emission spectra of compound 1 and uranyl nitrate solutions (Figure S2), fluorescence emission spectra of frozen solutions of compound 1 and uranyl nitrate salt (Figure S3), luminescence emission spectra of frozen solutions of compound 3 and uranyl nitrate salt (Figure S4), and the modeled molecule of compound 4 (Figure S5). Table S1 collects calculated structural parameters of the thorium complexes using CAChe software. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Chamberlain, D.; Leonard, R.; Hoh, J.; Gray, E.; Vandergrift, G. TRUEX Hot Demonstration ANL-80/37; Argonne National Laboratory: Argonne, IL, 1990. (b) Chopin, G. R. Research in Actinide Chemistry; Report DE97005235, DOE, Office for Energy Research, 1997. (c) Horwitz, E. P.; Diamond, H.; Martin, K. A. SolVent Extr. Ion Exch. 1987, 5, 447. (d) Peters, M. W.; Werner, E. J.; Scott, M. J. Inorg. Chem. 2002, 41, 1707. (e) Clark, D. L.; Hobart, D. E.; Neu, M. P. Chem. ReV. 1995, 95, 25. (2) (a) Alexandratos, S. D.; Natesan, S. Ind. Eng. Chem. Res. 2000, 39, 3998. (b) Talanova, G. G. Ind. Eng. Chem. Res. 2000, 39, 3550. (c) Gokel, G. W. Crown Ethers and Cryptands; The Royal Society of Chemistry: Cambridge, U.K., 1994. (d) Calixarenes for Separations; Lumetta, G. J., Rogers, R. D., Gopalan, A. S., Eds.; ACS Symposium Series 757, American Chemical Society: Washington, DC, 2000. (e) S´liwa, W. Croat. Chim. Acta ReV. 2002, 75, 131. (3) (a) Barboso, S.; Garcıá Carrera, A.; Matthews, S. E.; Arnaud-Neu, F.; Bo¨hmer, V.; Dozol, J. F.; Rouquette, H.; Schwing-Weill, M.-J. J. Chem. Soc., Perkin Trans. 2 1999, 719. (b) Arnaud-Neu, F.; Browne, J. K.; Byrne, D.; Marrs, D. J.; McKervey, M. A.; O’Hagan, P.; Schwing-Weill, M.-J.; Walker, A. Chem.sEur. J. 1999, 5, 175. (c) Schmidt, C.; Saadioui, M.; Bo¨hmer, V.; Host, V.; Spirlet, M.-R.; Desreux, J. F.; Brisach, F.; ArnaudNeu, F.; Dozol, J.-F. Org. Biomol. Chem. 2003, 1, 4089. (4) (a) Meera, R.; Luxmi Varma, R.; Reddy, M. L. P. SolVent Extr. Res. DeV. Jpn. 2003, 10, 13. (b) Van Leeuwen, F. W. V.; Verboom, W.; Reinhoudt, D. N. Chem. Soc. ReV. 2005, 34, 753. (5) (a) Gutsche, C. D. Calixarenes ReVisited; The Royal Society of Chemistry: Cambridge, U.K., 1998. (b) Shinkai, S. Tetrahedron 1993, 49, 8933. (c) Gutsche, C. D. Calixarenes; The Royal Society of Chemistry: Cambridge, U.K., 1989. (d) Mandolini, L.; Ungaro, R. Calixarenes in Action; Imperial College Press: London, 2000. (e) Asfari, Z.; Bo¨hmer, V.; Harrowfield, J. M.; Vicens, J. Calixarenes 2001; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2001. (6) Roundhill, D. M. Prog. Inorg. Chem. 1995, 43, 533. (7) (a) Nagasaki, T.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1991, 1063. (b) Arnaud-Neu, F.; Cremin, S.; Harris, S.; McKervey, M. A.; Schwing-Weill, M.-J.; Schwinte´, P.; Walker, A. J. Chem. Soc., Dalton Trans. 1997, 329. (c) Yaftian, M. R.; Burgard, M.; Matt, D.; Dieleman, C. B.; Rastegar, F. SolVent Extr. Ion Exch. 1997, 15, 975. (d) Yaftian, M. R.; Burgard, M.; Wieser, C.; Dieleman, C. B.; Matt, D. SolVent Extr. Ion Exch. 1998, 16, 1131. (e) Dalgarno, S. J.; Raston, C. L. Chem. Commun. 2002, 2216. (8) (a) Lambert, T. M.; Jarvinen, G. D.; Golapan, A. S. Tetrahedron Lett. 1999, 40, 1613. (b) Oshima, T.; Yamamoto, T.; Ohto, K.; Goto, M.; Nakashio, F.; Furusaki, S. SolVent Extr. Res. DeV. Jpn. 2001, 8, 194. (c) Rudzevich, Y. I.; Drapailo, A. B.; Rudzevich, V. L.; Miroshnichenko, V. I.; Kal’chenko, V. I.; Smirnov, I. V.; V Babain, A.; Varnek, A. A.; Wipff, G. Russ. J. Gen. Chem. 2002, 72, 1736.

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10987 (9) (a) Arnaud-Neu, F.; Bo¨hmer, V.; Dozol, J.-F.; Gru¨ttner, C.; Jakobi, R. A.; Kraft, D.; Mauprivez, O.; Rouquette, H.; Schwing-Weill, M.-J.; Simon, N.; Vogt, W. J. Chem. Soc., Perkin Trans. 2 1996, 1175. (b) Arduini, A.; Bo¨hmer, V.; Delmau, L.; Desreux, J.-F.; Dozol, J. F.; Garcia Carrera, M. A.; Lambert, B.; Musigmann, C.; Pochini, A.; Shivanyuk, A.; Ugozzoli, F. Chem.sEur. J. 2000, 6, 2135. (10) (a) Harrowfield, J. M.; Mocerino, M.; Peachey, B. J.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1996, 1687. (b) Lambert, B.; Jacques, V.; Shivanyuk, A.; Matthews, S. E.; Tunayar, A.; Baaden, M.; Wipff, G.; Bo¨hmer, V.; Desreux, J. F. Inorg. Chem. 2000, 39, 2033. (11) (a) Baaden, M.; Berny, F.; Muzet, N.; Troxler, L.; Wipff, G. In Calixarenes for Separations; Lumetta, G. J., Rogers, R. D., Gopalan, A. S., Eds.; ACS Symposium Series 757, American Chemical Society: Washington, DC, 2000. (b) Wipff, G.; Engler, E.; Guilbard, P.; Lauterbach, M.; Troxler, L.; Varnek, A. New J. Chem. 1996, 20, 403. (c) Varnek, A.; Wipff, G. J. Comput. Chem. 1996, 17, 1520. (12) (a) Le Saulnier, L.; Varbanov, S.; Scopelliti, R.; Elhabiri, M.; Bu¨nzli, J.-C. G. J. Chem. Soc., Dalton Trans. 1999, 3919. (b) Ramı´rez, F. de M.; Charbonnie`re, L.; Muller, G.; Scopelliti, R.; Bu¨nzli, J.-C. G. J. Chem. Soc., Dalton Trans. 2001, 3205. (c) Ramı´rez, F. de M.; Varbanov, S.; Cećile, C.; Muller, G.; Fatin-Rouge, N.; Scopelliti, R.; Bu¨nzli, J.-C. G. J. Chem. Soc., Dalton Trans. 2002, 4505. (d) Ramı´rez, F. de M.; Charbonnie`re, L.; Muller, G.; Bu¨nzli, J.-C. G. Eur. J. Inorg. Chem. 2004, 2348. (e) Puntus, L. N.; Chauvin, A. S.; Varbanov, S.; Bu¨nzli, J.-C. G. Eur. J. Inorg. Chem. 2007, 2315. (13) (a) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part A. Theory and Applications in Inorganic Chemistry; John Wiley Interscience Publ.: New York, 1997. (b) Ross, S. D. Inorganic Infrared and Raman Spectra; McGraw-Hill Book Company: London, 1972. (c) Dik, T. A. J. Appl. Spectrosc. 2001, 68, 55. (d) Cotton, S. Lanthanides and Actinides,1st ed.; MacMillan Education LT: England, 1991. (e) Denning, R. G.; Snellgrove, T. R.; Woodwark, D. R. Mol. Phys. 1976, 32, 419. (14) (a) Agarwal, R. K.; Agarwal, H.; Arora, K. ReV. Inorg. Chem. 2000, 20, 1. (b) Bagnall, K. W.; Wakerley, M. W. J. Chem. Soc., Dalton Trans. 1974, 889. (c) Drew, M. G. B. Coord. Chem. ReV. 1977, 24, 179. (d) Sarsfield, Mark, J.; Helliwell, M.; Raftery, J. Inorg. Chem. 2004, 43, 3170. (e) Haigh, C. W. Polyhedron 1995, 14, 2871. (f) Thue´ry, P.; Nierlich, M.; Masci, B.; Asfari, Z.; Vicens, J. J. Chem. Soc., Dalton Trans. 1999, 3151. (15) (a) Deshayes, L.; Keller, N.; Lance, M.; Navaza, A.; Nierlich, M.; Vigner, J. Polyhedron 1994, 1725. (b) Sessler, J. L.; Seidel, D.; Vivian, A. E.; Lynch, V.; Scott, B. L.; Keogh, D. W. Angew. Chem., Int. Ed. 2001, 40, 591. (c) Thue´ry, P.; Keller, N.; Lance, M.; Vigner, J.-D.; Nierlich, M. New J. Chem. 1995, 19, 619. (d) Bilsel, O.; Milam, S. N.; Girolami, G. S.; Suslick, K. S.; Holten, D. J. Phys. Chem. 1993, 97, 7216. (e) Sessler, L.; Mody, T. D.; Dulay, M. T.; Espinosa, R.; Lynch., V. Inorg. Chim. Acta 1996, 246, 23. (f) Abd El-Wahab, Z. H.; Mashaly, M. M.; Faheim, A. A. Chem. Pap. 2005, 59, 25. (16) Malone, J. F.; Marrs, D. J.; McKervey, M. A.; O’Hagan, P.; Thompson, N.; Walker, A.; Arnaud-Neu, F.; Mauprivez, O.; Schwing-Weill, M.-J.; Dozol, J.-F.; Rouquette, H.; Simon, N. J. Chem. Soc., Chem. Commun. 1995, 2151. (17) (a) Dieke, G. H.; Duncan, A. B. F. Spectroscopic Properties of Uranium Compounds, 1st ed., Division III; McGraw-Hill Book Company, Inc.: New York, 1949. (b) Tanner, P. A. Appl. Spectrosc. 1993, 47, 1949. (c) Billard, I. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr; Bu¨nzli, J.-C. G.; Pecharsky, V. K., Eds.; Elsevier Science B.V: Amsterdam, The Netherlands, 2003; Vol. 33, Ch. 216. (18) (a) Alcock, N. W.; Espera´s, S. J. Chem. Soc., Dalton Trans. 1977, 893. (b) Oda, Y.; Koyama, T.; Funasaka, H. Genshiryoku Bakkuendo Kenkyu 2001, 7, 41. (c) Choppin, G. R.; Rizkalla, E. N. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr.; Eyring, L.; Choppin, G. R.; Lander, G. H., Eds.; Elsevier Science B.V.: Amsterdam, The Netherlands, 1994; Vol. 18, Ch. 128. (19) (a) Ueki, T.; Zalkin, A.; Templeton, D. H. Acta Crystallogr. 1966, 20, 836. (b) Bowen, S. M.; Duesler, E. N.; Paine, R. T. Inorg. Chem. 1982, 21, 261. (c) Malik, K. M. A.; Jeffery, J. W. Acta Crystallogr. 1973, B29, 2687. (20) (a) De Aquino, A. R.; Bombieri, G.; Isolani, P. C.; Vicentini, G.; Zukerman-Schpector, J. Inorg. Chim. Acta 2000, 306, 101. (b) Alcock, N. W.; Roberts, M. M. J. Chem. Soc., Dalton Trans. 1982, 25. (c) Rao, L. F.; Choppin, G. R. Inorg. Chem. 1990, 29, 3589. (d) Morss, L. R.; Nash, K. L.; Ensor, D. D. J. Chem. Soc., Dalton Trans. 2000, 285. (e) De Almeida, K. C. S.; Martins, T. S.; Isolani, P. C.; Vicentini, G.; Zukerman-Schpector, J. J. Solid State Chem. 2003, 171, 230. (f) Umeda, K.; Zukerman-Schpector, J.; Isolani, P. C. Polyhedron 2006, 25, 2447. (21) (a) Denning, R. G. In Gmelin Handbook of Inorganic Chemistry; Springer-Verlag: Weinheim, Germany, 1983; Vol. A6, p 31. (b) Jorgensen, C. K.; Reisfield, R. Struct. Bonding (Berlin) 1982, 50, 121. (c) Denning, R. G. Struct. Bonding (Berlin) 1992, 79, 215. (d) Flint, C. D.; Tanner, P. A. J. Chem. Soc., Faraday Trans. 2 1984, 80, 219. (e) Denning, R. G. J. Phys. Chem. A 2007, 111, 4125.

10988 J. Phys. Chem. B, Vol. 112, No. 35, 2008 (22) (a) Lopez, M.; Birch, D. J. S. Chem. Phys. Lett. 1997, 268, 125. (b) Azenha, M. E. D. G.; Burrows, H. D.; Formosinho, S. J.; Miguel, M. G. M.; Daramanyan, A. P.; Khudyakov, I. V. J. Lumin. 1991, 48, 522. (c) Moulin, C.; Laszak, I.; Moulin, V.; Tondre, C. Appl. Spectrosc. 1998, 52, 528. (d) Daramanyan, A. P.; Khudyakov, I. V. Photochem. Photobiol. 1990, 52, 293. (e) Massad, W. A.; Repossi, P.; Arguëllo, G. A. J. Colloid Interface Sci. 2002, 255, 189. (f) Formosinho, S. J.; Miguel, M. G. M. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1745. (23) (a) Burrows, H. D.; Miguel, M. G. M. AdV. Colloid Interface Sci. 2001, 89, 485. (b) Massad, W. A.; Arguëllo, G. A. J. Radioanal. Nucl. Chem. 2000, 245, 407. (c) Burrows, H. D. Inorg. Chem. 1990, 29, 139. (d) Burrows, H. D.; Cardoso, A. C.; Formosinho, S. J.; Miguel, M. G. M. J. Chem. Soc., Faraday Trans. 1 1985, 81, 49. (e) De Houver, S.; Servaes, K.; Go¨rller-Warlrand, C. Phys. Chem. Chem. Phys. 2003, 5, 1164.

Ramı´rez et al. (24) (a) Hall, I.; Nicholson, G. P.; Piper, T. J.; Taylor, D. M.; Williams, G.; Williams, D. R. Radiochim. Acta 1995, 69, 225. (b) Nagasaki, T.; Shinkai, S.; Matsuda, T. J. Chem. Soc., Perkin Trans. 1 1990, 2617. (25) Bartlett, J. R.; Cooney, R. P. J. Mol. Struct. 1989, 193, 295. (26) Bu¨nzli, J.-C.G. In Lanthanide Probes in Life, Chemical and Earth Sciences. Theory and Practice; Bu¨nzli, J.-C.G., Choppin, G. R., Eds.; Elsevier Science B.V.: Amsterdam, The Netherlands, 1989; Ch. 7. (27) (a) Rice, N. M.; Irving, N. H.; Leonard, M. A. Pure Appl. Chem. 1993, 65, 2373. (b) Vasudeva Rao, P. R.; Kolarik, Z. SolVent Extr. Ion Exch. 1996, 14, 955. (c) Shrinivasan, T. G.; Vijayasaradhi, S.; Dhamodaran, R.; Suresh, A.; Vasudeva Rao, P. R. SolVent Extr. Ion Exch. 1998, 16, 1001.

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Physicochemical Properties and Theoretical Modeling of Actinide

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