Polarizable interaction potential for molecular dynamics simulations of actinoids(III) in liquid water

In this work, we have developed a polarizable classical interaction potential to study actinoids(III) in liquid water. This potential has the same analytical form as was recently used for lanthanoid(III) hydration [M. Duvail, P. Vitorge, and R. Spezia, J. Chem. Phys. 130, 104501 (2009)]. The hydration structure obtained with this potential is in good agreement with the experimentally measured ion-water distances and coordination numbers for the first half of the actinoid series. In particular, the almost linearly decreasing water-ion distance found experimentally is replicated within the calculations, in agreement with the actinoid contraction behavior. We also studied the hydration of the last part of the series, for which no structural experimental data are available, which allows us to provide some predictive insights on these ions. In particular we found that the ion-water distance decreases almost linearly across the series with a smooth decrease of coordination number from nine to eight at the end.     

Synergistic extraction of lanthanoids(III) with 2-thenoyltrifluoroacetone and benzoic acid: thermodynamic parameters in the complexation in organic phases and the hydration

In order to examine the reason why the magnitude of the synergistic effect observed in the extraction of lanthanoids(III) with a β-diketone and a monodentate Lewis base generally decreases along with increasing atomic number, the hydration number of the extracted species when lanthanoids(III) are extracted with TTA (2-thenoyltrifluoroacetone, HA) and benzoic acid (HB) into chloroform by Karl Fischer titration and the enthalpy change in complexation between LnA₃ and HB by calorimetric titration were determined across the lanthanoid series at 25 °C.It has been concluded that since the decrement of entropy change caused by the change in the number of released water molecules and in the coordination number of lanthanoids(III) upon complexation is larger than the increment of the enthalpy change, the values of the second formation constants of the complexes decrease with increasing the atomic number across lanthanoid series so that the magnitude of the synergistic extraction decreases with increasing the atomic number.     

Recent advances in computational actinoid chemistry

We briefly review advances in computational actinoid (An) chemistry during the past ten years in regard to two issues: the geometrical and electronic structures, and reactions. The former addresses the An-O, An-C, and M-An (M is a metal atom including An) bonds in the actinoid molecular systems, including actinoid oxo and oxide species, actinoid-carbenoid, dinuclear and diatomic systems, and the latter the hydration and ligand exchange, the disproportionation, the oxidation, the reduction of uranyl, hydroamination, and the photolysis of uranium azide. Concerning their relevance to the electronic structures and reactions of actinoids and their importance in the development of an advanced nuclear fuel cycle, we also mentioned the work on actinoid carbides and nitrides, which have been proposed to be candidates of the next generation of nuclear fuel, and the oxidation of PuO(x), which is important to understand the speciation of actinoids in the environment, followed by a brief discussion on the urgent need for a heavier involvement of computational actinoid chemistry in developing advanced reprocessing protocols of spent nuclear fuel. The paper is concluded with an outlook.     

An overview of eight- and nine-coordinate N-donor solvated lanthanoid(III) and actinoid(III) ions

Journal of Radioanalytical and Nuclear Chemistry - The use of replacement lanthanoid ions in actinoid chemistry is commonplace, which requires a full understanding of the similarities and...     

High-energy X-ray absorption spectroscopy: a new tool for structural investigations of lanthanoids and third-row transition elements

This is the first systematic study exploring the potential of high-energy EXAFS as a structural tool for lanthanoids and third-row transition elements. The K-edge X-ray absorption spectra of the hydrated lanthanoid(III) ions both in aqueous solution and in solid trifluoromethanesulfonate salts have been studied. The K-edges of lanthanoids cover the energy range from 38 (La) to 65 keV (Lu), while the corresponding energy range for the L(3)-edges is 5.5 (La) to 9.2 keV (Lu). We show that the large widths of the core-hole states do not appreciably reduce the potential structural information in the high-energy K-edge EXAFS data. Moreover, for lanthanoid compounds, more accurate structural parameters are obtained from analysis of K-edge than from L(3)-edge EXAFS data. The main reasons are the much wider k range available and the absence of double-electron transitions, especially for the lighter lanthanoids. A comparative K- and L(3)-edge EXAFS data analysis of nonahydrated crystalline neodymium(III) trifluoromethanesulfonate demonstrates the clear advantages of K-edge analysis over conventionally performed studies at the L(3)-absorption edge for structural investigations of lanthanoid and third-row transition metal compounds. The coordination chemistry of the hydrated lanthanoid(III) ions in aqueous solution and solid trifluoromethanesulfonate salts, based on the results of both the K- and L(3)-edge EXAFS data, is thoroughly discussed in the next paper in this series (I. Persson, P. D'Angelo, S. De Panfilis, M. Sandstr?m, L. Eriksson, Chem. Eur. J. 2008, 14, DOI: 10.1002/chem.200701281).     

Complexing Properties of Uranium and Lanthanoids With 4-(2-Thiazolylazo)-6-Chlororesorcinol

Abstract

4-(2-Thiazolylazo)-6-chlororesorcinol (TAR-Cl) reacts sensitively with uranyl(II) and lanthanoids(III), and forms reddish-brown 1:1 and 1:2 complexes. The complexing behaviors were examined spectrophotometrically. The absorption maxima of the complexes are focused near 553 nm and the optimum pH for complexation lies between 6.5–8.8. Beer's law holds up to about 2 × 10^?5 mol 1^?1, with a molar absorptivity of 3.00 × 10⁴ 1 mol^?1 cm^?1 for uranium and 6 × 10⁴ 1 mol^?1 cm^?1 level for each lanthanoid. The absorptivities are increased with the atomic number, especially in light lanthanoids, that are correlative both to the lanthanoid contraction and the basicity of ortho hydroxyl group in the resorcinol ring, but such effects are not clearly recognized in heavy lanthanoids. Effect of masking agents was also examined, and uranium could be determined selectively in the presence of lanthanoid mixtures by the addition of CyDTA.     

Highly hydrated cations: deficiency, mobility, and coordination of water in crystalline nonahydrated scandium(III), yttrium(III), and lanthanoid(III) trifluoromethanesulfonates

Trivalent lanthanide-like metal ions coordinate nine water oxygen atoms, which form a tricapped trigonal prism in a large number of crystalline hydrates. Water deficiency, randomly distributed over the capping positions, was found for the smallest metal ions in the isomorphous nonahydrated trifluoromethanesulfonates, [M(H2O)n](CF3SO3)3, in which M = Sc(III), Lu(III), Yb(III), Tm(III) or Er(III). The hydration number n increases (n = 8.0(1), 8.4(1), 8.7(1), 8.8(1) and 8.96(5), respectively) with increasing ionic size. Deuterium (2H) solid-state NMR spectroscopy revealed fast positional exchange between the coordinated capping and prism water molecules; this exchange started at temperatures higher than about 280 K for lutetium(III) and below 268 K for scandium(III). Similar positional exchange for the fully nonahydrated yttrium(III) and lanthanum(III) compounds started at higher temperatures, over about 330 and 360 K, respectively. An exchange mechanism is proposed that can exchange equatorial and capping water molecules within the restrictions of the crystal lattice, even for fully hydrated lanthanoid(III) ions. Phase transitions occurred for all the water-deficient compounds at approximately 185 K. The hydrated scandium(III) trifluoromethanesulfonate transforms reversibly (DeltaH degrees = -0.80(1) kJ mol(-1) on cooling) to a trigonal unit cell that is almost nine times larger, with the scandium ion surrounded by seven fully occupied and two partly occupied oxygen atom positions in a distorted capped trigonal prism. The hydrogen bonding to the trifluoromethanesulfonate anions stabilises the trigonal prism of water ligands, even for the crowded hydration sphere of the smallest metal ions in the series. Implications for the Lewis acid catalytic activity of the hydrated scandium(III) and lanthanoid(III) trifluoromethanesulfonates for organic syntheses performed in aqueous media are discussed.     

Hydrated structures and dehydration processes of lanthanoid(III) chloranilates studied by EXAFS

The local structures of lanthanoid(III) chloranilate complexes of Pr(III), Nd(III), Tb(III) and Er(III) have been studied by EXAFS (extended X-ray absorption fine structure). Hydrated structures of the lanthanoid(III) ions in these complexes have been investigated with respect to their coordination numbers and interatomic distances. Six or four water molecules coordinate to the lanthanoid(III) ion of Pr(III) or Nd(III), respectively, just after preparation of the complexes. The temperature dependence of the first coordinated structures has been studied in order to reveal the behavior of the coordinated water molecules in dehydration process. The coordination number around the central lanthanoid(III) ion decreases stepwise as temperature increases, depending on the type of central lanthanoid(III) ion present. The interatomic distance between the central lanthanoid(III) ion and oxygen atoms in the first shell decreases, accompanying the decrease of the coordination numbers. A parameter representing proportion shows the reduction of interatomic distance as one coordinated water molecule removes from the central ion, depending on the type of lanthanoid(III) ions.     

Thermodynamic and Spectroscopic Study of the Adduct Formation of Tris(β-diketonato)lanthanoids with Heteroamines

A remarkable enhancement of the extraction of lanthanoids(Ⅲ)(Ln) with β-diketones in the presence of a Lewis base, so-called synergistic effects, would be caused by the adduct formation of the β-diketonates with the Lewis base. The trend of the variation of the adduct formation constants across the lanthanoid series may be different among β-diketones used. It has also been observed that the trend across lanthanoid series and also the values of the enthalpy change in the adduct formation of the 2-thenoyltrifluoroacetonates(TTA) with 1,10-phenanthroline(phen) are very similar to those with 2,25-bipyridyl(bpy), although the values of the adduct formation constants with the former are larger than those with the latter.     

Lanthanoid element recognition on surface-imprinted polymers containing dioleylphosphoric acid as a functional host.

Surface-imprinted polymers have been newly developed for the separation of lanthanoid elements: i.e. La(III), Ce(III), and Dy(III). The imprinted polymers were prepared by surface template polymerization with dioleylphosphoric acid, which exhibits a high affinity to lanthanoids, as a functional host molecule. Separation behavior of La(III), Ce(III) and Dy(III) was investigated with the imprinted polymers, and the imprinting effect of the polymers was evaluated in comparison with that of the unimprinted polymers and also with a conventional solvent extraction method for the same lanthanoid ions. The results indicate that the increase of selectivity for Dy(III) compared to the rest of the ions by the surface-imprinted polymers originated from a synergistic effect of both the affinity with the functional host molecule in nature and the size exclusion by the cavity formed on the polymer surface.     

X-ray absorption fine structure spectroscopic studies of Octakis(DMSO)lanthanoid(III) complexes in solution and in the solid iodides

Octakis(DMSO)lanthanoid(III) iodides (DMSO = dimethylsulfoxide), [Ln(OS(CH3)2)8]I3, of most lanthanoid(III) ions in the series from La to Lu have been studied in the solid state and in DMSO solution by extended X-ray absorption fine structure (EXAFS) spectroscopy. L3-edge and also some K-edge spectra were recorded, which provided mean Ln-O bond distances for the octakis(DMSO)lanthanoid(III) complexes. The agreement with the average of the Ln-O bond distances obtained in a separate study by X-ray crystallography was quite satisfactory. The crystalline octakis(DMSO)lanthanoid(III) iodide salts have a fairly broad distribution of Ln-O bond distances, ca. 0.1 A, with a few disordered DMSO ligands. Their EXAFS spectra are in excellent agreement with those obtained for the solvated lanthanoid(III) ions in DMSO solution, both of which show slightly asymmetric distributions of the Ln-O bond distances. Hence, all lanthanoid(III) ions are present as octakis(DMSO)lanthanoid(III) complexes in DMSO solution, with the mean Ln-O distances centered at 2.50 (La), 2.45 (Pr), 2.43 (Nd), 2.41 (Sm), 2.40 (Eu), 2.39 (Gd), 2.37 (Tb), 2.36 (Dy), 2.34 (Ho), 2.33 (Er), 2.31 (Tm), and 2.29 A (Lu). This decrease in the Ln-O bond distances is larger than expected from the previously established ionic radii for octa-coordination. This indicates increasing polarization of the LnIII-O(DMSO) bonds with increasing atomic number. However, the S(1s) electron transition energies in the sulfur K-edge X-ray absorption near-edge structure (XANES) spectra, probing the unoccupied molecular orbitals of lowest energy of the DMSO ligands for the [Ln(OS(CH3)2)8](3+) complexes, change only insignificantly from Ln = La to Lu. This indicates that there is no appreciable change in the sigma-contribution to the S-O bond, probably due to a corresponding increase in the contribution from the sulfur lone pair to the bonding.     

Enhanced separation of trivalent lanthanoids by solvent extraction with 18-crown-6 and EDTA complexonate

The selectivities during solvent extraction of lanthanoids with macrocycles can be modified with complexonates in the aqueous phase. In the case of solvent extraction of lanthanoids with 18-crown-6 and trichloroacetic acid (TCA), addition of EDTA to the aqueous phase enhances the selectivities of lanthanoids by 3-7 times compared to those without the complexonate. This is due to the fact that the stability of lanthanoid-EDTA complexes increases in the opposite direction to the crown-TCA complexes across the lanthanoid series. The selectivities observed in this system are among the largest reported for the light lanthanoids. The effect of the complexonate on lanthanoid extraction can be explained by a simple model presented in this paper.     

Intramolecular Sandwich Complexation of Light Lanthanoid Nitrates with Bis(benzo-12-crown-4)s: Enhanced Selectivity for Eu

Spectrophotometric titrations performed in anhydrous acetonitrile at 25°C give the complex stability constants (log Ks) and the Gibbs free energy changes (-G° for the stoichiometric 1 : 1 intramolecular sandwich complexation of light lanthanoid (III) nitrates (La Gd) with the polymethylene-bridged bis(benzo-12-crown-4)s 1, the corresponding dioxo derivative 2 and its dihydroxy analogue 3. The complex stability sequence as a function of reciprocal ionic radius of lanthanoids showed similar profiles in stability constants (log Ks) and maximum stabilities were obtained at Eu3+ for the complexation of light lanthanoids with the three bis(benzo-12-crown-4)s 1–3. The cation binding abilities and relative selectivities for the trivalent lanthanoid ions of these structurally related bis(benzo-12-crown-4)s 1–3 are discussed according to the derivatization of the bridging.     

Yttrium(III) bonding to organic ligands: A comparison with the lanthanoid(III) cations

Summary The yttrium(III) bonding to organic substrates (oximes, -diketonates and (poly)amino-(poly)carboxylates) has been compared with that of the lanthanoid(III) cations. The complexation constants of Y³⁺ with the examined organic ligands are similar to those of some cations of the first half of the lanthanoid series, in contrast with the fact that the Y³⁺ ionic dimensions are similar to those of Ho³⁺. This has been explained by correlating the formation constants of the Y³⁺ and the lanthanoids(III) complexes by the equation logK ₁=C _AC_B+E _AE_B, where the parametersC andE indicate the tendency of each Lewis acidA and Lewis baseB to undergo covalent or ionic bonding, and where the ratioH=E/C indicates the charge control on the bond formation tendency of each speciesA orB. The results are commented in terms of the utility of Y³⁺ in assisting organic reactions.

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Bindung von Yttrium(III) an organische Liganden: Vergleich mit Lanthanoid(III)-Kationen

Zusammenfassung Es wurde die Bindung von Yttrium(III) an organische Substanzen [Oxime, -Diketonate und (Poly)Amino(poly)carboxylate] im Vergleich mit Lanthanoid(III)-Kationen behandelt. Die Komplexierungskonstanten von Y³⁺ sind ähnlich denen einiger Kationen der ersten Hälfte der Lanthanoidenserie; dies steht im Gegensatz zur Tatsache, daß die Dimensionen des Y³⁺-Ions denen des Ho³⁺ entsprechen. Die Erklärung wurde mittels der für die Bildungskonstanten der Y³⁺- und Lanthanoid(III)-Komplexe gültigen Gleichung logK ₁=C _AC_B+E _AE_B gefunden, wobeiC undE Parameter sind, die die Tendenz der Lewis-SäurenA und der Lewis-BasenB zum Eingehen von kovalenten oder ionischen Bindungen charakterisieren und wo das VerhältnisH=E/C den Steuerungseffekt der Ladung auf die Bindungstendenz der SpeziesA oderB beschreibt. Die Ergebnisse werden im Hinblick auf den Nutzen von Y³⁺ zur Unterstützung organischer Reaktionen diskutiert.

     

Revised ionic radii of lanthanoid(III) ions in aqueous solution

A new set of ionic radii in aqueous solution has been derived for lanthanoid(III) cations starting from a very accurate experimental determination of the ion-water distances obtained from extended X-ray absorption fine structure (EXAFS) data. At variance with previous results, a very regular trend has been obtained, as expected for this series of elements. A general procedure to compute ionic radii in solution by combining the EXAFS technique and molecular dynamics (MD) structural data has been developed. This method can be applied to other ions allowing one to determine ionic radii in solution with an accuracy comparable to that of the Shannon crystal ionic radii.     

Hydration of lanthanoid(III) ions in aqueous solution and crystalline hydrates studied by EXAFS spectroscopy and crystallography: the myth of the "gadolinium break"

The structures of the hydrated lanthanoid(III) ions including lanthanum(III) have been characterized in aqueous solution and in the solid trifluoromethanesulfonate salts by extended X-ray absorption fine structure (EXAFS) spectroscopy. At ambient temperature the water oxygen atoms appear as a tricapped trigonal prism around the lanthanoid(III) ions in the solid nonaaqualanthanoid(III) trifluoromethanesulfonates. Water deficiency in the capping positions for the smallest ions starts at Ho and increases with increasing atomic number in the [Ln(H(2)O)(9-x)](CF(3)SO(3))(3) compounds with x=0.8 at Lu. The crystal structures of [Ho(H(2)O)(8.91)](CF(3)SO(3))(3) and [Lu(H(2)O)(8.2)](CF(3)SO(3))(3) were re-determined by X-ray crystallography at room temperature, and the latter also at 100 K after a phase-transition at about 190 K. The very similar Ln K- and L(3)-edge EXAFS spectra of each solid compound and its aqueous solution indicate indistinguishable structures of the hydrated lanthanoid(III) ions in aqueous solution and in the hydrated trifluoromethanesulfonate salt. The mean Ln--O bond lengths obtained from the EXAFS spectra for the largest ions, La-Nd, agree with estimates from the tabulated ionic radii for ninefold coordination but become shorter than expected starting at samarium. The deviation increases gradually with increasing atomic number, reaches the mean Ln-O bond length expected for eightfold coordination at Ho, and increases further for the smallest lanthanoid(III) ions, Er-Lu, which have an increasing water deficit. The low-temperature crystal structure of [Lu(H(2)O)(8.2)](CF(3)SO(3))(3) shows one strongly bound capping water molecule (Lu-O 2.395(4) A) and two more distant capping sites corresponding to Lu-O at 2.56(1) A, with occupancy factors of 0.58(1) and 0.59(1). There is no indication of a sudden change in hydration number, as proposed in the "gadolinium break" hypothesis.     

Hydration structures of U(III) and U(IV) ions from ab initio molecular dynamics simulations

We apply DFT+U-based ab initio molecular dynamics simulations to study the hydration structures of U(III) and U(IV) ions, pertinent to redox reactions associated with uranium salts in aqueous media. U(III) is predicted to be coordinated to 8 water molecules, while U(IV) has a hydration number between 7 and 8. At least one of the innershell water molecules of the hydrated U(IV) complex becomes spontaneously deprotonated. As a result, the U(IV)-O pair correlation function exhibits a satellite peak at 2.15 A? associated with the shorter U(IV)-(OH(-)) bond. This feature is not accounted for in analysis of extended x-ray absorption fine structure and x-ray adsorption near edge structure measurements, which yield higher estimates of U(IV) hydration numbers. This suggests that it may be useful to include the effect of possible hydrolysis in future interpretation of experiments, especially when the experimental pH is close to the reported hydrolysis equilibrium constant value.     

Synergic extraction of lanthanoids(III) with thenoyl trifluoroacetone and terpyridine

The synergic extraction of trivalent lanthanoids (Ln: La, Nd, Sm, Eu, Yb and Lu) with 2-thenoyltrifluoroacetone (Htta) and 2,26, 2-terpyridine (tpy) into benzene has been studied. The partition coefficient (P_s) of tpy was obtained experimentally in order to calculate the equilibrium concentration of tpy in the organic phase. From the slope analysis, it was shown that these lanthanoids were extracted as Ln(tta)₃(tpy). The adduct formation constant (_s,1) and the synergic extraction constant were obtained for each lanthanoid. The (_s,1) decreases with increasing atomic number of lanthanoids and the trend of (_s,1) is compared with that for bidentate and unidentate heterocyclic amines.