Koichiro Takao

Actinide Chemistry in Ionic Liquids

This Forum Article provides an overview of the reported studies on the actinide chemistry in ionic liquids (ILs) with a particular focus on several fundamental chemical aspects: (i) complex formation, (ii) electrochemistry, and (iii) extraction behavior. The majority of investigations have been dedicated to uranium, especially for the 6+ oxidation state (UO2(2+)), because the chemistry of uranium in ordinary solvents has been well investigated and uranium is the most abundant element in the actual nuclear fuel cycles. Other actinides such as thorium, neptunium, plutonium, americium, and curiumm, although less studied, are also of importance in fully understanding the nuclear fuel engineering process and the safe geological disposal of radioactive wastes.

X-ray absorption fine structures of uranyl(V) complexes in a nonaqueous solution.

The structures of three different U(V) complexes, [U(V)O(2)(salophen)DMSO](-), [U(V)O(2)(dbm)(2)DMSO](-), and [U(V)O(2)(saldien)](-), in a dimethyl sulfoxide (DMSO) solution were determined by X-ray absorption fine structure for the first time.

Neptunium Carbonato Complexes in Aqueous Solution: An Electrochemical, Spectroscopic, and Quantum Chemical Study

The electrochemical behavior and complex structure of Np carbonato complexes, which are of major concern for the geological disposal of radioactive wastes, have been investigated in aqueous Na(2)CO(3) and Na(2)CO(3)/NaOH solutions at different oxidation states by using cyclic voltammetry, X-ray absorption spectroscopy, and density functional theory calculations. The end-member complexes of penta- and hexavalent Np in 1.5 M Na(2)CO(3) with pH = 11.7 have been determined as a transdioxo neptunyl tricarbonato complex, [NpO(2)(CO(3))(3)](n-) (n = 5 for Np(V), and 4 for Np(VI)). Hence, the electrochemical reaction of the Np(V/VI) redox couple merely results in the shortening/lengthening of bond distances mainly because of the change of the cationic charge of Np, without any structural rearrangement. This explains the observed reversible-like feature on their cyclic voltammograms. In contrast, the electrochemical oxidation of Np(V) in a highly basic carbonate solution of 2.0 M Na(2)CO(3)/1.0 M NaOH (pH > 13) yielded a stable heptavalent Np complex of [Np(VII)O(4)(OH)(2)](3-), indicating that the oxidation reaction from Np(V) to Np(VII) in the carbonate solution involves a drastic structural rearrangement from the transdioxo configuration to a square-planar-tetraoxo configuration, as well as exchanging the coordinating anions from carbonate ions (CO(3)(2-)) to hydroxide ions (OH(-)).

Complex formation and molecular structure of neptunyl(VI) and -(V) acetates.

Stability and coordination of neptunyl(VI) and -(V) acetate complexes in aqueous solution were studied by using UV-vis-near-IR (NIR) and X-ray absorption fine structure (XAFS) spectroscopy. In the neptunyl(VI) acetate system, the formation of Np(VI)O(2)(AcO)(+), Np(VI)O(2)(AcO)(2)(aq), and Np(VI)O(2)(AcO)(3)(-) was detected. Both spectroscopic methods provided similar stability constants: log K(1) = 2.98 +/- 0.01, log beta(2) = 4.60 +/- 0.01, and log beta(3) = 6.34 +/- 0.01 from UV-vis-NIR and log K(1) = 2.87 +/- 0.03, log beta(2) = 4.20 +/- 0.06, and log beta(3) = 6.00 +/- 0.01 from XAFS at I = 0.30 M (H,NH(4))ClO(4). Extended XAFS (EXAFS)-derived structural data for Np(VI)O(2)(2+)(aq), Np(VI)O(2)(AcO)(+), and Np(VI)O(2)(AcO)(3)(-) were consistent with their stoichiometry, showing a bidentate coordination of acetate (Np-O(ax) = 1.76-1.77 A; Np-O(eq) = 2.43-2.47 A; Np-C(c) = 2.87 A; Np-C(t) = 4.38 A). Similar to Np(VI), Np(V) forms also three different complexes with acetate. The stability constants of Np(V)O(2)(AcO)(aq), Np(V)O(2)(AcO)(2)(-), and Np(V)O(2)(AcO)(3)(2-) were determined by UV-vis-NIR titration to log K(1) = 1.93 +/- 0.01, log beta(2) = 3.11 +/- 0.01, and log beta(3) = 3.56 +/- 0.01 at I = 0.30 M (H,NH(4))ClO(4). The present result is corroborated by the structural information from EXAFS (Np-O(ax) = 1.83-1.85 A; Np-O(eq) = 2.51 A; Np-C(c) = 2.90-2.93 A) and by the electrochemical behavior of the Np(V/VI) redox couple in the presence of AcOH as a function of the pH.

Experimental and Theoretical Approaches to Redox Innocence of Ligands in Uranyl Complexes: What Is Formal Oxidation State of Uranium in Reductant of Uranyl(VI)?

Redox behavior of [UO2(gha)DMSO](-)/UO2(gha)DMSO couple (gha = glyoxal bis(2-hydroxanil)ate, DMSO = dimethyl sulfoxide) in DMSO solution was investigated by cyclic voltammetry and UV-vis-NIR spectroelectrochemical technique, as well as density functional theory (DFT) calculations. [UO2(gha)DMSO](-) was found to be formed via one-electron reduction of UO2(gha)DMSO without any successive reactions. The observed absorption spectrum of [UO2(gha)DMSO](-), however, has clearly different characteristics from those of uranyl(V) complexes reported so far. Detailed analysis of molecular orbitals and spin density of the redox couple showed that the gha(2-) ligand in UO2(gha)DMSO is reduced to gha(•3-) to give [UO2(gha)DMSO](-) and the formal oxidation state of U remains unchanged from +6. In contrast, the additional DFT calculations confirmed that the redox reaction certainly occurs at the U center in other uranyl(V/VI) redox couples we found previously. The noninnocence of the Schiff base ligand in the [UO2(gha)DMSO](-)/UO2(gha)DMSO redox couple is due to the lower energy level of LUMO in this ligand relative to those of U 5f orbitals. This is the first example of the noninnocent ligand system in the coordination chemistry of uranyl(VI).

Electrochemical behavior of [UO 2 Cl 4 ] 2− in 1-ethyl-3-methylimidazolium based ionic liquids

In order to examine the chemical form of uranyl species in 1-ethyl-3-methylimidazolium (EMI) based ionic liquids, UV-visible absorption spectra of solutions prepared by dissolving [EMI]2[UO2Cl4] into a mixture of EMICl and EMIBF4 (50:50 mol%) were measured. As a result, it was confirmed that uranyl species in the mixture of EMICl and EMIBF4 existed as [UO2Cl4]2−. Cyclic voltammograms (CVs) of [UO2Cl4]2− in the mixture were measured at 25 °C using a Pt working electrode, a Pt wire counter electrode, and an Ag/Ag+ reference electrode (0.01 M AgNO3, 0.1 M tetrabutylammonium perchlorate in acetonitrile) in a glove box under an Ar atmosphere. Peaks corresponding to one redox couple were observed around −1.05 V (Epc) and −0.92 V (Epa) vs. ferrocene/ferrocenium ion (Fc/Fc+). The potential differences between two peaks (ΔEp) increased from 101 to 152 mV with an increase in the scan rate from 50 to 300 mV s−1, while the (Epc + Epa)/2 value was constant, −0.989 V vs. Fc/Fc+ regardless of the scan rate. Furthermore, the diffusion coefficient of [UO2Cl4]2− and the standard rate constant were estimated to be 3.7 × 10−8 cm2 s−1 and (2.7–2.8) × 10−4 cm s−1 at 25 °C. By using the diffusion coefficient and the standard rate constant, the simulation of CVs was performed based on the reaction, [UO2Cl4]2− + e− = [UO2Cl4]3−. The simulated CVs were found to be consistent with the experimental ones. From these results, it is concluded that [UO2Cl4]2− in the mixture of EMICl and EMIBF4 is reduced to [UO2Cl4]3− quasi-reversibly at −0.989 V vs. Fc/Fc+.

μ-η2:η2-Peroxido-bis­[nitratodioxido­bis(pyrrolidin-2-one)uranium(VI)]

In the crystal structure of the title compound, [U2(NO3)2O4(O2)(C4H7NO)4], two UO2 2+ ions are connected by a μ-η2:η2-O2 unit. The O2 unit shows ‘side-on’ coordination to both U atoms. An inversion center is located at the midpoint of the O—O bond in the O2 unit, affording a centrosymmetrically expanded dimeric structure. The U—O(axial) bond lengths are 1.777 (4) Å and 1.784 (4) Å, indicating that the oxidation state of U is exclusively 6+, i.e., UO2 2+. Furthermore, the O—O distance is 1.492 (8) Å, which is typical of peroxide, O2 2–. The U atom is eight-coordinated in a hexagonal-bipyramidal geometry. The coordinating atoms of the nitrate and pyrrolidine-2-one ligands and the μ-η2:η2-O2 2– unit are located in the equatorial plane and form an irregular hexagon. An intermolecular hydrogen bond is found between N—H of the pyrrolidine-2-one ligand and the coordinating O of the same ligand in a neighboring complex. A second intermolecular hydrogen bond is found between the N—H of the other pyrrolidine-2-one ligand and one of the uranyl oxido atoms.

Development of Advanced Reprocessing System Based on Precipitation Method Using Pyrrolidone Derivatives as Precipitants: —Precipitation Behavior of U(VI), Pu(IV), and Pu(VI) by Pyrrolidone Derivatives with Low Hydrophobicity—

An advanced reprocessing system for spent FBR fuels based on two precipitation processes has been proposed. In the first process, only U(VI) species is precipitated using a pyrrolidone derivative (NRP) with lower hydrophobicity and donicity, which should yield a lower precipitation ability. In the second process, residual U(VI) and Pu(IV, VI) are precipitated simultaneously using an NRP with higher hydrophobicity and donicity, which should yield a higher precipitation ability. In order to select the precipitants for the first precipitation process, we have examined the precipitation behavior of U(VI), Pu(IV), and Pu(VI) species in HNO3 using N-n-propyl-2-pyrrolidone (NProP), N-n-butyl-2-pyrrolidone (NBP), and N-isobutyl-2-pyrrolidone (NiBP) with lower hydrophobicity and donicity than N-cyclohexyl-2-pyrrolidone (NCP) previously proposed as the precipitant. It was found that NRPs could precipitate U(VI) nearly stoichiometrically and that the decontamination factors for simulated fission products were higher than those in NCP systems. Furthermore, as seen in NCP, it was found that in the U(VI)-Pu(IV) mixtures, a small amount of Pu(IV) was temporarily coprecipitated with U(VI) by NRPs in spite of their lower precipitation ability and then the coprecipitated Pu(IV) component was redissolved with continuous stirring. From these results, NRPs can be proposed as candidate precipitants for the first precipitation process. In particular, NBP is considered to be the most promising precipitant, because of the relatively high solubility of the NProP precipitant, the increases in viscosity of NiBP slurry with stirring, and the relatively fast sedimentation rate of NBP precipitates.

An aqueous electrolyte of the widest potential window and its superior capability for capacitors

A saturated aqueous solution of sodium perchlorate (SSPAS) was found to be electrochemically superior, because the potential window is remarkably wide to be approximately 3.2 V in terms of a cyclic voltammetry. Such a wide potential window has never been reported in any aqueous solutions, and this finding would be of historical significance for aqueous electrolyte to overcome its weak point that the potential window is narrow. In proof of this fact, the capability of SSPAS was examined for the electrolyte of capacitors. Galvanostatic charge-discharge measurements showed that a graphite-based capacitor containing SSPAS as an electrolyte was stable within 5% deviation for the 10,000 times repetition at the operating voltage of 3.2 V without generating any gas. The SSPAS worked also as a functional electrolyte in the presence of an activated carbon and metal oxides in order to increase an energy density. Indeed, in an asymmetric capacitor containing MnO2 and Fe3O4 mixtures in the positive and negative electrodes, respectively, the energy density enlarged to be 36.3 Whkg−1, which belongs to the largest value in capacitors. Similar electrochemical behaviour was also confirmed in saturated aqueous solutions of other alkali and alkaline earth metal perchlorate salts.

Solubility of Uranyl Nitrate Precipitates with N-Alkyl-2-pyrrolidone Derivatives (Alkyl = n-Propyl, n-Butyl, iso-Butyl, and Cyclohexyl)

The solubility of UO2(NO3)2(NRP)2 (NRP = N-alkyl-2-pyrrolidone) in aqueous solutions with HNO3 (0–5.0 M) and the corresponding NRP (0–0.50M) has been studied. As a result, the solubility of each speciesof UO2(NO3)2(NRP)2 generally decreases with increasing concentrations of HNO3 and the corresponding NRP (C HNO3 and C NRP, respectively) in the supernatant. The solubility of UO2(NO3)2(NRP)2 also depends on the type of NRP; a higher hydrophobicity of NRP generally leads to a lower solubility of UO2(NO3)2(NRP)2. The logarithms of effective solubility products (K eff) of UO2(NO3)2(NProP)2, UO2(NO3)2(NBP)2, UO2(NO3)2(NiBP)2, and UO2(NO3)2(NCP)2 at different CHNO3 values and 293K were evaluated. For instance, at CHNO3 = 3:0 M, logK NProP eff = −1:07 ± 0:03, log K NBP eff = −2:23 ± 0:02, log K NiBP eff = −2:59 ± 0:03, and log K NCP eff = −3:80 ± 0:05. The solubility of UO2(NO3)2(NRP)2 is determined by the balance among the common-ligand effect, ionic strength, and variation of log K eff with C HNO3.