Involvement of 5f Orbitals in the Covalent Bonding between the Uranyl Ion and Trialkyl Phosphine Oxide: Unraveled by Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory

Progress in phenanthroline-derived extractants for trivalent actinides and lanthanides separation: where to next?

Spent nuclear fuel (SNF) released from reactors possesses significant radioactivity, heat release properties, and high-value radioactive nuclides. Therefore, using chemical methods for reprocessing can enhance economic efficiency and reduce the potential environmental risks of nuclear energy. Due to the presence of relatively diffuse f-electrons, the chemical properties of trivalent lanthanides (Ln(III)) and actinides (An(III)) in SNF solutions are quite similar. Separation methods have several limitations, including poor separation efficiency and the need for multiple stripping agents. The use of novel multi-dental phenanthroline-derived extractants with nitrogen donor atoms to effectively separate An(III) over Ln(III) has been widely accepted. This review first introduces the development history of phenanthroline-derived extractants for extraction and complexation with An(III) over Ln(III). Then, based on structural differences, these extractants are classified into four categories: nitrogen-coordinated, N,O-hybrid coordinated, highly preorganized structure, and unsymmetric structure. Each category's design principles, extraction, and separation performance as well as their advantages and disadvantages are discussed. Finally, we have summarized and compared the unique characteristics of the existing extractants and provided an outlook. This work may offer a reliable reference for the precise identification and selective separation between An(III) and Ln(III), and point the way for future development and exploration.

Selective Separation of U(VI) from Pu(IV) by 2,9-Diamide-1,10-phenanthroline Ligands at High Acidity: Extraction and Coordination Chemistry

During the PUREX process, the separation between U(VI) and Pu(IV) is achieved by reducing Pu(IV) to Pu(III), which is complicated and energy-consuming. To address this issue, we report here the first case of separation of U(VI) from Pu(IV) by o-phenanthroline diamide ligands under high acidity. Two new o-phenanthroline diamide ligands (1,10-phenanthroline-2,9-diyl)bis(indolin-1-ylmethanone) (L1) and (1,10-phenanthroline-2,9-diyl)bis((2-methylindolin-1-yl)methanone) (L2) were synthesized, which can effectively separate U(VI) from Pu(IV) even at 4 mol/L HNO3. The highest separation factor of U(VI) and Pu(IV) can reach over 1000, setting a new record for the separation of U(VI) from Pu(IV) under high acidity. Furthermore, extracted U(VI) can be easily recovered with water or dilute nitric acid, and the extraction performance remains stable even after 150 kGy gamma irradiation, which provides solid experimental support for potential engineering applications. The results of UV-vis titration and single-crystal X-ray diffraction measurements show that the 1:1 complex formed by L1 with U(VI) is more stable than all of the previously reported phenanthroline ligands, which reasonably reveals that the ligand L1 designed in this work has excellent affinity for U(VI). The findings of this work promise to contribute to the facilitation of the PUREX process by avoiding the use of reducing agents. It also provides new clues for designing ligands to achieve efficient separation between U(VI) and Pu(IV) at high acidity.

Are 4f-Orbitals Engaged in Covalent Bonding Between Lanthanides and Triphenylphosphine Oxide? An Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory Study

The bonding covalency between trivalent lanthanides (Ln = La, Pr, Nd, Eu, Gd) and triphenylphosphine oxide (TPPO) is studied by X-ray absorption spectra (XAS) and density functional theory (DFT) calculations on the LnCl3(TPPO)3 complexes. The O, P, and Cl K-edge XAS for the single crystals of LnCl3(TPPO)3 were collected, and the spectra were interpreted based on DFT calculations. The O and P K-edge XAS spectra showed no significant change across the Ln series in the LnCl3(TPPO)3 complexes, unlike the Cl K-edge XAS spectra. The experimental O K-edge XAS spectra suggest no mixing between the Ln 4f- and the O 2p-orbitals in the LnCl3(TPPO)3 complexes. DFT calculations indicate that the amount of the O 2p character per Ln-O bond is less than 0.1% in the Ln 4f-based orbitals in all of the LnCl3(TPPO)3 complexes. The experimental spectra and theoretical calculations demonstrate that Ln 4f-orbitals are not engaged in the covalent bonding of lanthanides with TPPO, which contrasts the involvement of U 5f-orbitals in covalent bonding in the UO2Cl2(TPPO)2 complex. Results in this work reinforce our previous speculation that bonding covalency is potentially responsible for the extractability of monodentate organophosphorus ligands toward metal ions.

Strengths of covalent bonds in LnO2 determined from O K-edge XANES spectra using a Hubbard model

In LnO2 (Ln = Ce, Pr, and Tb), the amount of Ln 4f mixing with O 2p orbitals was determined by O K-edge X-ray absorption near edge (XANES) spectroscopy and was similar to the amount of mixing between the Ln 5d and O 2p orbitals. This similarity was unexpected since the 4f orbitals are generally perceived to be "core-like" and can only weakly stabilize ligand orbitals through covalent interactions. While the degree of orbital mixing seems incompatible with this view, orbital mixing alone does not determine the degree of stabilization provided by a covalent interaction. We used a Hubbard model to determine this stabilization from the energies of the O 2p to 4f, 5d(eg), and 5d(t2g) excited charge-transfer states and the amount of excited state character mixed into the ground state, which was determined using Ln L3-edge and O K-edge XANES spectroscopy. The largest amount of stabilization due to mixing between the Ln 4f and O 2p orbitals was 1.6(1) eV in CeO2. While this energy is substantial, the stabilization provided by mixing between the Ln 5d and O 2p orbitals was an order of magnitude greater consistent with the perception that covalent bonding in the lanthanides is largely driven by the 5d orbitals rather than the 4f orbitals.

Stabilization of Pu(IV) in PuBr4(OPCy3)2 and Comparisons with Structurally Similar ThX4(OPR3)2 (R = Cy, Ph) Molecules

The pursuit of a trivalent plutonium halide phosphine oxide compound, e.g., "PuBr3(OPR)3," instead led to the isolation of the tetravalent trans-PuIVBr4(OPCy3)2, PuBr/Cy, compound by spontaneous oxidation of PuIII. The donating nature of phosphine oxides has allowed the isolation and characterization of PuBr/Cy by crystallographic, multinuclear NMR, solid state, and solution phase UV-vis-NIR spectroscopic techniques. The presence of a putative plutonyl(VI) complex formulated as "trans-PuVIO2Br2(OPCy3)2" was also observed spectroscopically and tentatively by single-crystal X-ray diffraction as a cocrystal of PuBr/Cy. A series of trans-ThX4(OPCy3)2 (X = Cl, ThCl/Cy; Br, ThBr/Cy; I, ThI/Cy) complexes were synthesized for comparison to PuBr/Cy. The triphenylphosphine oxide, OPPh3, complexes, trans-AnI4(OPPh3)2 (An = Th, ThI/Ph; U, UI/Ph), were also synthesized for comparison, completing the series trans-UX4(OPPh3)2 (X = Cl, Br, I), UX/Ph. To enable the synthesis of ThI/Cy and ThI/Ph, a new nonaqueous thorium iodide starting material, ThI4(Et2O)2, was synthesized. The syntheses of organic solvent soluble ThI4L2 (L = Et2O, OPCy3, and OPPh3) are the first examples of crystallographically characterized neutral thorium tetraiodide materials beyond binary ThI4. To show the viability of ThI4(Et2O)2 as a starting material for organothorium chemistry, (C5Me4H)3ThI was synthesized and crystallographically characterized.

Tuning Phosphine Oxide-Substituted ortho-Carboranes for Improved Biphasic Electrochemical UO22+ Capture and Release

We report the synthesis and characterization of a series of new, tunable 1,2-bis(diarylphosphine oxide)-ortho-carboranes, derivatives of our previously reported uranyl (UO22+) capture agent 1,2-(Ph2PO)2-1,2-C2B10H10 (POCb). The series features new cage-substituted variants of POCb, namely, 9-I-POCb (POCbI), 9,12-I2-POCb (POCbI2), 9,12-Me2-POCb (POCbMe2), 9,12-Et2-POCb (POCbEt2), and 4,5,7,8,9,10,11,12-Me8-POCb (POCbMe8). Aryl-substituted variants include 1,2-((4-MeO-Ph)2PO)2-Cb ((OMe)POCb) and 1,2-((4-F-Ph)2PO)2-Cb ((F)POCb). The effects of electron-withdrawing (EWG) and electron-donating (EDG) groups on resulting carborane redox potentials were assessed using electrochemical means, and the resulting Lewis basicities were quantified using empirical and competition-based NMR experiments. In organic solution, carboranes substituted with EWGs exhibited weaker coordination to UO22+, whereas those with EDGs exhibited stronger coordination. Similar to the previously reported unsubstituted POCb, the tunable new series of carboranes were electrochemically reduced and used for the biphasic capture of UO22+ from an aqueous to an organic phase and back again (release) through electrochemical oxidation. Extraction and back-extraction efficiencies were determined by analyses of the aqueous phases by ICP-OES. While all reduced nido-carboranes efficiently extracted UO22+ in high yields (78-88%)─with seemingly no correlation to the aforementioned measured Lewis basicities─we found the back-extraction of UO22+ to be significantly improved from POCb and, surprisingly, more closely related to their hydrophobic rather than their Lewis basic properties.

Evaluating f-Orbital Participation in the UV═E Multiple Bonds of [U(E)(NR2)3] (E = O, NSiMe3, NAd; R = SiMe3)

The reaction of 1 equiv of 1-azidoadamantane with [U^III(NR₂)₃] (R = SiMe₃) in Et₂O results in the formation of [U^V(NR₂)₃(NAd)] (1, Ad = 1-adamantyl) in good yields. The electronic structure of 1, as well as those of the related U(V) complexes, [U^V(NR₂)₃(NSiMe₃)] (2) and [U^V(NR₂)₃(O)] (3), were analyzed with EPR spectroscopy, SQUID magnetometry, NIR-visible spectroscopy, and crystal field modeling. This analysis revealed that, within this series of complexes, the steric bulk of the E^2- (E═O, NR) ligand is the most important factor in determining the electronic structure. In particular, the increasing steric bulk of this ligand, on moving from O^2- to [NAd]^2-, results in increasing U═E distances and E-U-N_amide angles. These changes have two principal effects on the resulting electronic structure: (1) the increasing U═E distances decreases the energy of the f_σ orbital, which is primarily σ* with respect to the U═E bond, and (2) the increasing E-U-N_amide angles increases the energy of f_δ, due to increasing antibonding interactions with the amide ligands. As a result of the latter change, the electronic ground state for complexes 1 and 2 is primarily f_φ in character, whereas the ground state for complex 3 is primarily f_δ.

Selective electrochemical capture and release of uranyl from aqueous alkali, lanthanide, and actinide mixtures using redox-switchable carboranes

We report the selective electrochemical biphasic capture of the uranyl cation (UO₂²⁺) from mixed-metal alkali (Cs⁺), lanthanide (Nd³⁺, Sm³⁺), and actinide (Th⁴⁺, UO₂²⁺) aqueous solutions to an organic, 1,2-dichloroethane (DCE), phase using the ortho-substituted nido-carborane anion, [1,2-(Ph₂PO)₂-1,2-C₂B₁₀H₁₀]²⁻ (^POCb²⁻). The reduced ^POCb²⁻ is generated by electrochemical reduction of the closo-carborane, ^POCb, prior to mixing with the aqueous mixed-metal solution. Subsequent UO₂²⁺ release from the captured product, [UO₂(^POCb)₂]²⁻, was performed by galvanostatic bulk electrolysis of the DCE phase and back-extraction of UO₂²⁺ to a fresh aqueous phase. The selective capture and release of UO₂²⁺ was confirmed by combined ICP-OES and NMR spectral analyses of the aqueous and organic phases, respectively, against the newly synthesized nido-carborane complexes, [[CoCp*₂][Cs(^POCb)]]₂, [CoCp*₂]₃[Nd(^POCb)₃], [CoCp*₂]₃[Sm(^POCb)₃], and [CoCp*₂]₂[Th(^POCb)₃].

Redox-switchable carboranes electrochemically capture and release UO₂²⁺ selectively from mixed metal aqueous solutions, mimicking in part spent nuclear fuel.

Effects of coordinating heteroatoms on molecular structure, thermodynamic stability and redox behavior of uranyl(vi) complexes with pentadentate Schiff-base ligands

Uranyl(vi) complexes with pentadentate N₃O₂-, N₂O₃- and N₂O₂S₁-donating Schiff base ligands, tBu,MeO–saldien–X²⁻ (X = NH, O and S), were synthesized and thoroughly characterized by ¹H NMR, IR, elemental analysis, and single crystal X-ray diffraction. The crystal structures of UO₂(tBu,MeO–saldien–X) showed that the U–X bond strength follows U–O ≈ U–NH > U–S. Conditional stability constants (β_X) of UO₂(tBu,MeO–saldien–X) in ethanol were investigated to understand the effect of X on thermodynamic stability. The log β_X decrease in the order of UO₂(tBu,MeO–saldien–NH) (log β_NH = 10) > UO₂(tBu,MeO–saldien–O) (log β_O = 7.24) > UO₂(tBu,MeO–saldien–S) (log β_S = 5.2). This trend cannot be explained only by Pearson's Hard and Soft Acids and Bases (HSAB) principle, but rather follows the order of basicity of X. Theoretical calculations of UO₂(tBu,MeO–saldien–X) suggested that the ionic character of U–X bonds decreases in the order of U–NH > U–O > U–S, while the covalency increases in the order U–O < U–NH < U–S. Redox potentials of all UO₂(tBu,MeO–saldien–X) in DMSO were similar to each other regardless of the difference in X. Spectroelectrochemical measurements and DFT calculations revealed that the center U⁶⁺ of each UO₂(tBu,MeO–saldien–X) undergoes one-electron reduction to afford the corresponding uranyl(v) complex. Consequently, the difference in X of UO₂(tBu,MeO–saldien–X) affects the coordination of tBu,MeO–saldien–X²⁻ with UO₂²⁺. However, the HSAB principle is not always prominent, but the Lewis basicity and balance between ionic and covalent characters of the U–X interactions are more relevant to determine the bond strengths.

The U–X bond strength and thermodynamic stability of uranyl(vi) complexes with pentadentate N₂O₂X₁-donating ligands (X = NH, O, S) are affected by the difference in X. In contrast, the X atom does not largely affect the redox behavior of the complexes.