Actinide Chemistry Research Articles

A Mathematical Model of Reduction Potentials of Lanthanide Alloys Correlation with Atomic Radius and Electronegativity Based on the Hume-Rothery Rules

To achieve closed nuclear fuel cycle and sustainable development of nuclear energy, efficient pyroprocessing of spent fuel becomes one of the major concerns in Generation IV nuclear energy [1]. Molten salt based electrolysis is a promising method in future pyroprocessing technology. As neutron poison, lanthanides are the typical detrimental elements that encumber the realization of the so-called partitioning & transmutation (P&T) strategy, and thus must be separated from actinides [2]. Therefore, investigating the electrochemical properties of actinides and lanthanides is of pivotal importance. In fact, in light of the chemical similarity of trivalent actinides and lanthanides, the chemistry of actinides can be revealed somewhat through exploring the lanthanide elements which are more readily obtained [3, 4]. In this work, it was found that the reduction potentials of Cu(Ni, Al, Zn)-rich Cu(Ni, Al, Zn)-Ln alloys manifest an evident “Bimodal Effect”. Hume-Rothery rules pointed out that the main factors affecting the formation of alloy solid solution of two metal elements are: 1) the difference of the atomic radius of the solvent and the solute metals, and 2) the electronegativity difference between the solvent and the solute metals. By fitting the data of reduction potentials, differences of the atomic radii and electronegativity differences, a formula was built up, which provides a useful and reliable approach for predicting related important reduction potentials of actinide alloys. The result of multiple linear regression analysis (Fig. 1) indicated that the good reliability of forecast and goodness of fit. Acknowledgments This paper is funded by the International Exchange Program of Harbin Engineering University for Innovation-oriented Talents Cultivation.

Structural and Stability Trends of the Complexation of Hexavalent Actinides with Two Dipicolinic Acid Derivatives: An Experimental and Theoretical Study.

Revealing the complexation behavior of high-valent actinides in solutions is of great importance to better understand the fundamental chemistry of actinides as well as to control the separation property of actinides in nuclear fuel cycles. In this work, the complexation of hexavalent actinide cations U(VI), Np(VI), and Pu(VI) with two dipicolinic acid derivatives, 6-(dimethylcarbamoyl)picolinic acid (DMAPA, denoted as HL) and N2,N2,N6,N6-tetramethylpyridine-2,6-dicarboxamide (TMPDA), in aqueous solutions were investigated by absorption spectrophotometry, X-ray crystallography, and DFT calculations. Formation of 1/1 and 1/2 (metal:ligand) complexes of U(VI), Np(VI), and Pu(VI) with DMAPA were identified and the corresponding stability constants were determined. The binding strengths of the three hexavalent actinide cations with DMAPA follow the order of U(VI) > Np(VI) > Pu(VI) in both 1/1 and 1/2 complexes, indicating that the driving force for the complexation is mainly electrostatic interactions. In addition, the relationships between the features of the absorption spectra and the symmetry of Pu(VI) and Np(VI) complexes with DMAPA have been established. The crystal structure of the 1/2 U(VI)/DMAPA complex as well as the DFT optimized structures of An(VI)/DMAPA complexes shed additional light on the structural characters of the hexavalent actinide cation complexes. In contrast to DMAPA, TMPDA exhibits no observable complexation with the three hexavalent actinide cations in aqueous solution, which could be attributed to low electron density on the donor atoms of TMPDA as well as steric hindrance effect by dimethyl carbamoyl groups as elucidated by DFT calculations. The results from this work provide new insights into the complexation trends of hexavalent actinide cations in aqueous solutions.

From Thorium to Plutonium: Trends in Actinide(IV) Chloride Structural Chemistry.

A series of eighteen tetravalent actinide (An = Th, U, Pu) compounds were synthesized from acidic aqueous solutions containing thorium, uranium, or plutonium and a series of protonated nitrogen heterocycles. The compounds were characterized using Raman, IR, and optical absorption spectroscopies. The structures were determined using single-crystal X-ray diffraction and found to consist of [An(H2O)xCly]4-y (x = 4-7 and y = 2-4) or AnCl62- molecular units. Breaks in the structural chemistry of the early actinides were observed, with Th adopting exclusively Th-aquo-chloro species and Pu forming only PuCl62-; U crystallized as both U-aquo-chloro and UCl62-. The relationship between the solid-state structural units and the solution species was interrogated using UV-vis-near-IR absorption spectroscopy. A comparison of the solution and solid-state spectra suggested that, although prevalent in the solid state, particularly for U and Pu, AnCl62- does not exist to an appreciable extent in the reaction solution. Despite the identification of U-aquo-chloro species in solution, there are limited reports of these complexes in the solid state. Isolation of these unique actinide(IV) chlorides as reported in this work may point to the importance of nonbonding interactions in the stabilization and precipitation of AnIV structural units.

Inverse Trans Influence in Low-Valence Actinide-Group 10 Metal Complexes of Phosphinoaryl Oxides: A Theoretical Study via Tuning Metals and Donor Ligands.

The recognition and in-depth understanding of inverse trans influence (ITI) have successfully guided the synthesis of novel actinide complexes and enriched actinide chemistry. Those complexes, however, are mainly limited to the involvement of high-valence actinide and/or metal-ligand multiple bonds. Examples containing both low oxidation state actinide and metal-metal single bond remain rare. Herein, more than 20 actinide-transition metal (An-TM) complexes of phosphinoaryl oxide ligands have been designed in accordance with several experimentally known analogs, by changing the metal atoms (An = Th, Pa, U, Np, and Pu; and TM = Ni, Pd, and Pt), actinide oxidation states (IV and III) and metal-metal axial donor ligands (X = Me3SiO, F, Cl, Br, and I). The relativistic density functional theory study of structural (trans-An-X and cis-An-O toward An-TM), bonding (topological electron/energy density), and electronic properties reveals the order of the ITI stabilizing actinide-metal bond. Computed electron affinity (EA) values, related to the electrochemical reduction, linearly correlate with experimentally measured reduction potentials. Although the same ITI order for the ligand donors was shown as in a previous study, the correlation between electrochemical reduction and the ITI was found to be weak when the actinide atoms were changed. For most complexes, the reduction is primarily of an actinide-based mechanism with minor participation of transition metal and phosphinoaryl oxide, whereas that of thorium-nickel complexes is different.

Perspectives for the Development of Chemistry of Actinides in Kazakhstan

Perspectives for the Development of Chemistry of Actinides in Kazakhstan

Emergence of the structure-directing role of f-orbital overlap-driven covalency

FEUDAL (f’s essentially unaffected, d’s accommodate ligands) is a longstanding bonding model in actinide chemistry, in which metal-ligand binding uses 6d-orbitals, with the 5f remaining non-bonding. The inverse-trans-influence (ITI) is a case where the model may break down, and it has been suggested that ionic and covalent effects work synergistically in the ITI. Here, we report an experimentally grounded computational study that quantitatively explores the ITI, and in particular the structure-directing role of f-orbital covalency. Strong donor ligands generate a cis-ligand-directing electrostatic potential (ESP) at the metal centre. When f-orbital participation, via overlap-driven covalency, becomes dominant via short actinide-element distances, this ionic ESP effect is overcome, favouring a trans-ligand-directed geometry. This study contradicts the accepted ITI paradigm in that here ionic and covalent effects work against each other, and suggests a clearly non-FEUDAL, structure-directing role for the f-orbitals.

Actinide Chemistry at the Extreme.

Actinide Chemistry at the Extreme.

Mixed-valent neptunium oligomer complexes based on cation-cation interactions.

Mixed-valent tri- and tetranuclear complexes of neptunium, [{NpIVCl4}{NpVO2Cl(THF)3}2]·THF and [{NpIVCl3}{NpVO2(μ2-Cl)(THF)2}3{μ3-Cl}] (THF = tetrahydrofuran), were synthesised and characterised. Both the complexes are formed via the cation-cation interactions between the Np(iv) centre and the axial oxygens of the neptunyl(v) unit (i.e. trans-dioxo NpO2+ cation), demonstrating the potential of cation-cation interactions for further exploring the oligomer/cluster chemistry of actinides.

SEPARATION OF AMERICIUM FROM AGED PLUTONIUM DIOXIDE

The Recycle Fuel Fabrication Laboratory (RFFL) is a licensed nuclear facility designed to produce experimental quantities of mixed oxide fuel for reactor physics tests and demonstration irradiations. Since its refurbishment in 1996, several fuel fabrication campaigns and research projects have been completed. The RFFL is the only facility in Canada capable of handling significant quantities of plutonium and other actinides. In this context, effort has been put forth in recent years to demonstrate capabilities in new fields, such as actinide chemistry. One of the first experiments in actinide chemistry conducted in the Facility was devoted to the separation and purification of 241Am from aged PuO2. The experimental work presented here, was conducted to establish actinide separation capability at the RFFL and demonstrate the staff expertise to perform these types of separations. The separation experiments were carried out using ion exchange columns packed with basic anion exchange AG1-X4 resin. More than 92% of the 241Am contained in the starting PuO2 solution was recovered in the experiments; however, some plutonium was also found in the washing effluent fraction. The final americium concentration of the washing effluent fraction was estimated to be about 50% of the total heavy elements present. More than 93% of the purified plutonium retained in the column was eluted. The experimental results are discussed in terms of the speciation behaviour of the Am–Pu–N–H2O systems using E-pH thermodynamic equilibrium diagrams and the maximum ionic exchange capacity of the resin.

Relativistic quantum chemical calculations show that the uranium molecule U2 has a quadruple bond

Understanding the bonding, reactivity and electronic structure of actinides is lagging behind that of the rest of the periodic table. This can be partly explained by the challenges that one faces in experimental studies of such radioactive compounds and also by the need to properly account for relativistic effects in theoretical studies. A further challenge is the very complicated electronic structures encountered in actinide chemistry, as vividly illustrated by the naked diuranium molecule U2. Here we report a computational study of this emblematic molecule using state-of-the-art relativistic quantum chemical methods. Notably, the variational inclusion of spin-orbit interactions leads not only to a different electronic ground state, but also to a lower bond multiplicity compared with those in previous studies.

Pentavalent Curium, Berkelium, and Californium in Nitrate Complexes: Extending Actinide Chemistry and Oxidation States.

Pentavalent actinyl nitrate complexes AnVO2(NO3)2- were produced by elimination of two NO2 from AnIII(NO3)4- for An = Pu, Am, Cm, Bk, and Cf. Density functional theory (B3LYP) and relativistic multireference (CASPT2) calculations confirmed the AnO2(NO3)2- as AnVO2+ actinyl moieties coordinated by nitrates. Computations of alternative AnIIIO2(NO3)2- and AnIVO2(NO3)2- revealed significantly higher energies. Previous computations for bare AnO2+ indicated AnVO2+ for An = Pu, Am, Cf, and Bk, but CmIIIO2+: electron donation from nitrate ligands has here stabilized the first CmV complex, CmVO2(NO3)2-. Structural parameters and bonding analyses indicate increasing An-NO3 bond covalency from Pu to Cf, in accordance with principles for actinide separations. Atomic ionization energies effectively predict relative stabilities of oxidation states; more reliable energies are needed for the actinides.

A diuranium carbide cluster stabilized inside a C80 fullerene cage

Unsupported non-bridged uranium–carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer–Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@Ih(7)-C80·[NiII-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium–carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.

Complexation of Trivalent Lanthanides (Eu) and Actinides (Cm) with Aqueous Phosphates at Elevated Temperatures.

In this study, the complexation of Eu(III) and Cm(III) with aqueous phosphates was investigated using laser-induced luminescence spectroscopy. Experiments at 25 °C and different ionic strengths (0.6-3.1 mol·L-1 NaClO4) established the formation of EuH2PO42+ and CmH2PO42+. From the conditional stability constants, the respective values at infinite dilution as well as the ε(Me(H2PO4)2+;ClO4-) (Me = Eu or Cm) ion interaction coefficients (using the specific ion interaction theory - SIT) were derived. Further experiments (at constant ionic strength of 1.1 mol·L-1) showed that upon increasing the temperature (25-80 °C), the formation of both EuH2PO42+ and CmH2PO42+ was favored. Using the van't Hoff equation, the molal enthalpy Δ R H m° and molal entropy Δ R S m° of these reactions were derived, corroborating an endothermic and entropy driven complexation process. This work contributes to a better understanding of the coordination chemistry of both trivalent lanthanides and actinides with phosphate ions.

(Invited) Molecular Structures and Unique Bindings of Actinide Endohedral Fullerenes

The successful isolation and characterization of Th@C82, U@C82, U@C74 have shown that actinide fullerenes have unique properties which are previously unknown to the endohedral fullerene world. Now, with the further exploration of actinide endohedral fullerenes based on U and Th, we found these endohedral fullerenes are unique for not only fullerene chemistry but also to actinide chemistry. U@C80 and U@C76 have been found to process Non-IPR cages, which are rare cases for the mono metallic endohedral fullerenes. Th@C86 and U@C86, on the other hand were found to share the same IPR cages, where have never been reported before. The crystallographic and computational studies of U2C@I h-C80 reveal reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unpreced­entedly short uranium-carbon distances (2.03 Å) of genuine U=C double-bond character. The isolation of U2C@C80 gives an unprecedented look into a long sought after but heretofore unknown U=C bond which lacks heteroatom support, allowing for detailed structural and electronic insights.

Exploring Synthetic Routes to Heteroleptic UIII, UIV, and ThIV Bulky Bis(silyl)amide Complexes

The bis(silyl)amide {N(SiMe3)2} (N′′) has supported spectacular actinide (An) chemistry for over 40 years, yet surprisingly there are only a handful of An complexes containing larger bis(silyl)amides, for example [U(N**)3] [N** = {N(SiMe2tBu)2}, 1]. Herein we report the structural characterization of the UIII complexes [U(N**)2(µ‐I)]2 (2), [U(N†′)2(µ‐I)]2 [N†′ = {N(SiiPr3)(SiMe3)}, 3], [U(N††)(I)2(THF)2] [N†† = {N(SiiPr3)2}, 4], [U(N††)2(I)] (5), and [U(N†′)2(I)(THF)] (6), and the AnIV complexes [An(N**){N(SiMe2tBu)(SiMetBuCH2‐κ2‐N,C)}(µ‐Cl)]2 (7‐An, An = U, Th) and [Th(N**)2{N(SiMe2tBu)(SiMetBuCH2‐κ2‐N,C)}] (8). Low crystalline yields were obtained in all cases, presumably due to facile cyclometallation. Although this precluded full characterization of UIII 4 and 6, and AnIV 7 and 8, for UIII complexes 2, 3, and 5 yields were high enough to perform NMR, EPR, NIR/UV/Vis, and FTIR spectroscopy, elemental analysis, and magnetic measurements.

Future Directions for Transuranic Single Molecule Magnets

Single Molecule Magnets (SMMs) based on transition metals and rare earths have been the object of considerable attention for the past 25 years. These systems exhibit slow relaxation of the magnetization, arising from a sizeable anisotropy barrier, and magnetic hysteresis of purely molecular origin below a given blocking temperature. Despite initial predictions that SMMs based on 5f-block elements could outperform most others, the results obtained so far have not met expectations. Exploiting the versatile chemistry of actinides and their favorable intrinsic magnetic properties proved, indeed, to be more difficult than assumed. However, the large majority of studies reported so far have been dedicated to uranium molecules, thus leaving the largest part of the 5f-block practically unexplored. Here, we present a short review of the progress achieved up to now and discuss some options for a possible way forward.

Protactinium and the intersection of actinide and transition metal chemistry

The role of the 5f and 6d orbitals in the chemistry of the actinide elements has been of considerable interest since their discovery and synthesis. Relativistic effects cause the energetics of the 5f and 6d orbitals to change as the actinide series is traversed left to right imparting a rich and complex chemistry. The 5f and 6d atomic states cross in energy at protactinium (Pa), making it a potential intersection between transition metal and actinide chemistries. Herein, we report the synthesis of a Pa-peroxo cluster, A6(Pa4O(O2)6F12) [A = Rb, Cs, (CH3)4N], formed in pursuit of an actinide polyoxometalate. Quantum chemical calculations at the density functional theory level demonstrate equal 5f and 6d orbital participation in the chemistry of Pa and increasing 5f orbital participation for the heavier actinides. Periodic changes in orbital character to the bonding in the early actinides highlights the influence of the 5f orbitals in their reactivity and chemical structure.

Atomic properties of actinide ions with particle-hole configurations

We study the effects of higher-order electronic correlations in the systems with particle-hole excited states using a relativistic hybrid method that combines configuration interaction and linearized coupled-cluster approaches. We find the configuration interaction part of the calculation sufficiently complete for eight electrons while maintaining good quality of the effective coupled-cluster potential for the core. Excellent agreement with experiment was demonstrated for a test case of La$^{3+}$. We apply our method for homologue actinide ions Th$^{4+}$ and U$^{6+}$ which are of experimental interest due to a puzzle associated with the resonant excitation Stark ionization spectroscopy (RESIS) method. These ions are also of interest to actinide chemistry and this is the first precision calculation of their atomic properties.

Protactinium and the intersection of actinide and transition metal chemistry

Protactinium and the intersection of actinide and transition metal chemistry

Actinide-based MOFs: a middle ground in solution and solid-state structural motifs.

In this review, we highlight how recent advances in the field of actinide structural chemistry of metal-organic frameworks (MOFs) could be utilized towards investigations relative to efficient nuclear waste administration, driven by the interest towards development of novel actinide-containing architectures as well as concerns regarding environmental pollution and nuclear waste storage. We attempt to perform a comprehensive analysis of more than 100 crystal structures of the existing An (U,Th)-based MOFs to establish a correlation between structural density and wt% of actinide and bridge structural motifs common for natural minerals with ones typically observed in the solution chemistry of actinides. In addition to structural considerations, we showcase the benefits of MOF modularity and porosity towards the stepwise building of hierarchical material complexity and the capture of nuclear fission products, such as technetium and iodine. We expect that these facets not only contribute to the fundamental science of actinide chemistry, but also could foreshadow pathways for more efficient nuclear waste management.