Redox behavior of Ce(IV)/Ce(III) in the presence of nitrilotriacetic acid: A surrogate study for An(IV)/An(III) redox behavior

Complexation of Cyclic Glutarimidedioxime with Cerium: Surrogating for Redox Behavior of Plutonium

The complexation of cerium with glutarimidedioxime (H₂L) was studied by potentiometry, ESI-mass spectrometry, and cyclic voltammetry. Crystallization of [Ce^IV(HL)₃]⁺ from Ce³⁺ starting reactant indicated spontaneous complexation-driven oxidation. In aqueous solution, Ce³⁺ ions form three successive complexes, Ce(HL)²⁺, Ce(HL)₂⁺, and Ce(HL)₃ (where HL^- stands for the singly deprotonated ligand). The interactions of glutarimidedioxime with metal ions are dominantly electrostatic in nature, and the stability constants of the complexes are correlated to the charge density of metal ions. Extrapolations of predicted stability constant (log β) values were made from plotting effective charge and the ionic radius of the metal ion for Pu³⁺ and Pu⁴⁺. The stability constants of Pu^IV(HL)³⁺ and Pu^III(HL)²⁺ are estimated to be 27.74 and 19.75, respectively. The differences of stability constants mean that glutarimidedioxime selectively binds Pu⁴⁺ over Pu³⁺ by a factor of about 8 orders of magnitude, suggesting Pu⁴⁺ would be stabilized by chelation with glutarimidedioxime. The mechanism of reduction of Pu⁴⁺ to Pu³⁺ in acidic solution is explained by decomposition of glutarimidedioxime through acid hydrolysis rather than a chelation-driven mechanism.

Redox Properties of CeIVDOTA in Carbonated Aqueous Solutions. A Radiolytic and an Electrochemical Study

The redox chemistry of Ce^IIIDOTA in cage in carbonate solutions was studied using electrochemistry and radiolysis techniques (continuous radiolysis and pulse radiolysis). Spectroscopic measurements point out that the species present in the solutions at high bicarbonate concentrations are [Ce^IIIDOTA(CO₃)]^3- (or less plausible [Ce^IIIDOTA(HCO₃)]^2-) with the carbonate (bicarbonate) anion as the ninth ligand versus [Ce^IIIDOTA(H₂O)]^- present in the absence of bicarbonate. Electrochemical results show a relatively low increase in the thermodynamic stabilization of the redox couple Ce^IV/III in the presence of carbonate versus its aqueous analogue. [Ce^IVDOTA(CO₃)]^2- and [Ce^IVDOTA(H₂O)], prepared electrochemically, decompose photolytically. However, kept in the dark, both are relatively long lived; [Ce^IVDOTA(H₂O)], though, is orders of magnitude kinetically more stable (a considerably longer half-life). Thus, one concludes that the carbonate species have a different mechanism of decomposition depending also on the presence of dioxygen after its preparation (in deaerated/aerated solutions). The [Ce^IVDOTA(CO₃)]^2- species is produced radiolytically by oxidation of the trivalent species by CO₃^•- with a rate constant, measured using pulse radiolysis, of 3.3 × 10⁵ M^-1 s^-1. This rate constant is at least 1 order of magnitude smaller than most of the rate constants so far reported for the reaction of CO₃^•- with transition metal/lanthanide (cerium)/actinide complexes. This result together with the bulkiness of the reactants might suggest an outer-sphere electron transfer rather than the inner-sphere one so far proposed. The lifetime of the tetravalent cerium species obtained radiolytically in the presence of carbonate is shorter than the electrochemical one, suggesting a different conformer involved.

Influence of Light and Temperature on the Extractability of Cerium(IV) as a Surrogate of Plutonium(IV) and its Effect on the Simulation of an Accidental Fire in the PUREX Process

Modeling of plutonium(IV) behavior during an accidental fire in a reprocessing plant was considered using various non-radioactive metallic surrogates. Among those elements, cerium(IV) was supposed to be a suitable candidate due to possible formation of a complex with TPB, but its extractability and stability have not been studied previously under representative plutonium uranium reduction extraction (PUREX) conditions. In this work, we investigated the chemical analogy between cerium(IV) and plutonium(IV) in this extractive process and combustion thereof. Distribution ratios are reported for acidities of 1-4 mol L^-1 in equal volumes of nitric acid and a 30:70 mixture of tributylphosphate and hydrogenated tetrapropylene. The influences of light, temperature, and extraction time were studied by UV-vis spectroscopy. The results showed that cerium(IV) is extracted quantitatively but is reduced over time to cerium(III) in the organic mixture. Spectrophotometric investigations of this reaction kinetics revealed an apparent rate constant k of 0.021 ± 0.002 mol^0.5 L^0.5 min^-1 at 298 K and an apparent fractional reaction order of 0.5. The activation energy of this reduction was found to be around 82 ± 2 kJ mol^-1 by the Arrhenius plot method. The combustion of mono- and biphasic solutions prepared with a cerium(IV) concentration of 10 g L^-1 revealed that the extracted complexes, Ce₂O·6NO₃·3TBP_(org) or Ce₄O₄·8NO₃·6TBP_(org), are reduced during the combustion. Compositions of the resulting ashes and soot were analyzed and highlighted the presence of pyrophosphates and polycyclic aromatic hydrocarbons, with some traces of cerium. Ce(IV) is not suitable to represent Pu(IV) from a chemical point of view in HNO₃/TBP-HTP solutions.

Inducing selectivity and chirality in group IV metal coordination with high-denticity hydroxypyridinones

The solution- and solid-state interactions between the octadentate siderophore mimic 3,4,3-LI(1,2-HOPO) (343HOPO) and group IV metal ions were investigated using high-resolution mass spectrometry, liquid chromatography, UV-visible spectrophotometry, metal-competition batch titrations, and single crystal X-ray diffraction. 343HOPO forms a neutral 1 : 1 complex, [HfIV343HOPO], that exhibits extreme stability in aqueous solution, with a log β110 value reaching 42.3. These results affirm the remarkable charge-based selectivity of 343HOPO for octacoordinated tetravalent cations with a Hf(iv) complex 1021 more stable than its Lu(iii) analogue. Moreover, [HfIV343HOPO] and its Zr(iv) counterpart show exceptional robustness, with the ligand remaining bound to the cation over a very broad pH range: from pH ∼ 11 to acidic conditions as strong as 10 M HCl. In stark contrast, Ti(iv)-343HOPO species are far less stable and undergo hydrolysis at pH as low as ∼6, likely due to the mismatch between the preferred hexacoordinated Ti(iv) ion and octadentate 343HOPO ligand. The extreme charge-based and denticity-driven selectivity of 343HOPO, now observed across the periodic table, paves the way for new selective sequestration systems for radionuclides including medical 44Ti, 89Zr or 177Lu/Hf isotopes, toxic polonium (Po) contaminants, as well as rutherfordium (Rf) research isotopes. Furthermore, despite the lack of a chiral center in 343HOPO, its complexes with metal ions are chiral and appear to form a single set of enantiomers.

Aqueous chemistry of Ce(iv): estimations using actinide analogues

The prediction of cerium (Ce) aqueous speciation is relevant in many research fields. Indeed, Ce compounds are used for many industrial applications, which may require the control of Ce aqueous chemistry for their synthesis. The aquatic geochemistry of Ce is also of interest. Due to its growing industrial use and its release into the environment, Ce is now considered as an emerging contaminant. Cerium is also used as a proxy of (paleo)redox conditions due to the Ce(iv)/Ce(iii) redox transition. Finally, Ce(iv) is often presented as a relevant analogue of tetravalent actinides (An(iv)). In the present study, quantum chemical calculations were conducted to highlight the similarities between the structures of Ce(iv) and tetravalent actinide (An(iv); An = Th, Pa, U, Np, Pu) aqua-ions, especially Pu(iv). The current knowledge of An(iv) hydrolysis, solubility and colloid formation in water was briefly reviewed but important discrepancies were observed in the available data for Ce(iv). Therefore, new estimations of the hydrolysis constants of Ce(iv) and the solubility of Ce(iv)-(hydr)oxides are proposed, by analogy with Pu(iv). By plotting pH-Eh (Pourbaix) diagrams, we showed that the pH values corresponding to the onset of Ce(iv) species formation (i.e. Ce(iv)-(hydr)oxide or dissolved Ce(iv)) agreed with various experimental results. Although further experimental studies are required to obtain a more accurate thermodynamic database, the present work might yet help to predict more accurately the Ce chemical behavior in aqueous solution.

Toxic heavy metal – Pb, Cd, Sn – complexation by the octadentate hydroxypyridinonate ligand archetype 3,4,3-LI(1,2-HOPO)

The toxicity of heavy metals such as lead (Pb), cadmium (Cd) and tin (Sn) has long been known but accidental exposures of large populations to these elements remain unfortunately a topical issue.

Th(VO3)2(SeO3) and Ln(VO3)2(IO3) (Ln = Ce, Pr, Nd, Sm, and Eu): unusual cases of aliovalent substitution

Six isotypic compounds with rather extreme examples of aliovalent substitution (Th^IVvs. Ln^III; SeO₃²⁻vs. IO₃⁻) have been synthesized.

Solution thermodynamic stability of complexes formed with the octadentate hydroxypyridinonate ligand 3,4,3-LI(1,2-HOPO): a critical feature for efficient chelation of lanthanide(IV) and actinide(IV) ions

The solution thermodynamics of water-soluble complexes formed between Ce(III), Ce(IV), Th(IV) and the octadentate chelating agent 3,4,3-LI(1,2-HOPO) were investigated. Several techniques including spectrofluorimetric and automated spectrophotometric titrations were used to overcome the slow spontaneous oxidation of Ce(III) complexes yielding to stability constants of log β110 = 17.4 ± 0.5, log β11-1 = 8.3 ± 0.4 and log β111 = 21.2 ± 0.4 for [Ce(III)(3,4,3-LI(1,2-HOPO))](-), [Ce(III)(3,4,3-LI(1,2-HOPO)(OH)](2-), and [Ce(III)(3,4,3-LI(1,2-HOPO)H], respectively. Using the spectral properties of the hydroxypyridinonate chelator in ligand competition titrations against nitrilotriacetic acid, the stability constant log β110 = 41.5 ± 0.5 was determined for [Ce(IV)(3,4,3-LI(1,2-HOPO))]. Finally, the extraordinarily stable complex [Ce(IV)(3,4,3-LI(1,2-HOPO))] was used in Th(IV) competition titrations, resulting in a stability constant of log β110 = 40.1 ± 0.5 for [Th(IV)3,4,3-LI(1,2-HOPO))]. These experimental values are in excellent agreement with previous estimates, they are discussed with respect to the ionic radius and oxidation state of each cationic metal, and allow predictions on the stability of other actinide complexes including [U(IV)(3,4,3-LI(1,2-HOPO))], [Np(IV)(3,4,3-LI(1,2-HOPO))], and [Pu(IV)(3,4,3-LI(1,2-HOPO))]. Comparisons with the standard ligand diethylenetriamine pentaacetic acid (DTPA) provide a thermodynamic basis for the observed significantly higher efficacy of 3,4,3-LI(1,2-HOPO) as an in vivo actinide decorporation agent.