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Mikeska, Emily R.

Publications and source records attributed to Mikeska, Emily R..

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Cooperative Lanthanide Solvation in an Ionic Liquid for Critical Materials Separations

Effectively separating critical materials, including intra-lanthanide separations, is crucial for meeting growing application demands. Liquid–liquid extraction (LLE) is the industry standard for lanthanide separations, where the selectivity can depend on small changes in metal coordination. In this work, we investigate representative lanthanide Eu coordination with a neutral malonamide extractant in an imidazolium bistriflimide ionic liquid (IL) solvent. Through systematic titrations of the water and extractant and under extraction conditions, we observe surprising cooperative Eu solvation with the IL anion and extractant. Time-resolved fluorescence spectroscopy measurements show strong extractant coordination in water-saturated IL. Lifetime measurements show no water coordination, and extended X-ray fine structure spectroscopy data provide a coordination number of 10. Molecular dynamics simulations confirm this coordination number and reveal IL anion coordination in the final Eu complex, even though it is ordinarily a significantly weaker ligand compared to water. The lack of water in the final extracted complex and IL anion coordination potentially explain the increased extraction in LLE systems using ILs, as evidenced by higher distribution ratios for cation exchange extraction, despite the energetic cost of cation transfer to the aqueous phase. These results highlight the opportunities for tuning metal coordination to drive extraction in unique solvent systems.

Carr, Amanda J.↗
DOE OSTI2025

Reactions of Np(VI) and Pu(VI) with Phenanthroline Result in Bending the [NpVIO2]2+ Unit and a Reduced Pu(V) Species

Despite decades of actinyl chemistry, genuinely bent actinyl structures remain rare beyond uranium. We report the first crystallographic characterization of a bent neptunyl(VI) complex, NpO 2 Cl 2 (phen) 2, along with a linear plutonyl species that, unexpectedly, adopts the +V oxidation state. Coordination of two 1,10-phenanthroline ligands to NpO 2 2+ enforces pronounced bending of the O yl –Np–O yl unit to 162° through steric clashes with the axial phenathroline ligand, representing the sharpest angle reported for any neptunyl(VI) complex. In contrast, synthesis with PuO 2 2+ produces a reduced linear Pu(V) species, PuO 2 Cl(phen) 2, underscoring the redox lability and the reluctance of plutonium to distort. Raman and IR spectra yield the first experimental stretching and interaction force constants for plutonyl(V). Comparison with values for plutonyl(VI) and bent and linear uranyl(VI) and neptunyl(VI) compounds shows that reduction perturbs the actinyl bond more than bending within one oxidation state. Electrochemical and spectroscopic studies clarify phenanthroline binding to Pu(VI) and the system’s reactivity. Quantum chemical calculations indicate that bending is energetically favored in the U–Np–Pu series, but the stabilization energy decreases as the actinide atomic number increases. NBO and QTAIM analyses reveal systematic trends: increasing 5f occupancy and decreasing An–O yl bond covalency from U to Pu. These results demonstrate that ligand-induced bending in neptunyl(VI) species is not only electronically feasible but also synthetically achievable, though plutonyl(VI) is constrained by redox reactivity. This work expands the frontier of nonlinear actinyl chemistry and illuminates how structure, oxidation state, and electronic configuration interrelate across the actinyl series.

actinides↗
DOE OSTI2024

Electroanalytical characterization of Np( VI )/Np( V ) redox in a pentadentate ligand environment and stabilization of [Np V O 2 ] + by hydrogen bonding

The redox chemistry of the actinyl cations (AnO 2 n+ ) heavily influences their reactivity and speciation in solution, but the redox properties of the actinyls in non-aqueous media have received far less attention than they deserve. Here, the non-aqueous electrochemistry of a chemically reversible Np(VI)/Np(V) redox manifold is reported in both protic (CH 3 OH) and aprotic (CH 3 CN) organic media. Using a neutral Np(VI) complex supported by a chelating and strongly donating pentadentate ligand that was fully characterized in prior work, a clean Np(VI)/Np(V) redox couple was found to be accessible under ambient conditions. Coupled electrochemical and spectroscopic studies, as well as simulations of cyclic voltammetry data, confirm the 1e− nature of this couple and establish it to be chemically reversible and nearly electrochemically reversible as well. Bulk electrolysis of a solution of the neutral Np(VI) complex facilitated isolation of the corresponding anionic and monomeric Np(V) species. Data from X-ray diffraction analysis as well as optical and vibrational spectroscopies provide strong evidence in support of metal-centered reduction and the Np(V) oxidation state, findings that are in accord with the measured reduction potentials. Distinctive hydrogen bonding interactions between the terminal (yl) oxo groups and water molecules appear to stabilize the isolated Np(V) species in the solid state, providing insight into the features that afford the uncommon chemically reversible redox encountered in this system.

Mikeska, Emily R.↗
DOE OSTI2024

Preparation of Neptunyl and Plutonyl Acetates To Access Nonaqueous Transuranium Coordination Chemistry

Uranyl diacetate dihydrate is a useful reagent for the preparation of uranyl (UO 2 2+ ) coordination complexes, as it is a well-defined stoichiometric compound featuring moderately basic acetates that can facilitate protonolysis reactivity, unlike other anions commonly used in synthetic actinide chemistry such as halides or nitrate. Despite these attractive features, analogous neptunium (Np) and plutonium (Pu) compounds are unknown to date. Here, in this study, a modular synthetic route is reported for accessing stoichiometric neptunyl(VI) and plutonyl(VI) diacetate compounds that can serve as starting materials for transuranic coordination chemistry. The new NpO 2 2+ and PuO 2 2+ complexes, as well as a corresponding molecular UO 2 2+ complex, are isomorphous in the solid state, and in solution show similar solubility properties that facilitate their use in synthesis. In both solid and solution state, the +VI oxidation state (O.S.) is maintained, as demonstrated by vibrational and optical spectroscopy, confirming that acetate anions stabilize the oxidizing, high-valent +VI states of Np and Pu as they do for the more stable U(VI). All three acetate salts readily react with a model diprotic ligand, affording incorporation of U(VI), Np(VI), and Pu(VI) cores into molecular coordination compounds that occurs concomitantly with elimination of acetic acid; the new complexes are high-valent, yet overall charge neutral, facilitating entry into nonaqueous chemistry by rational synthesis. Computational studies reveal that the dianionic ligand framework assists in stabilizing the +VI O.S. via donation to the 5f shells of the actinides, highlighting the potential usefulness of protonolysis reactivity toward preparation of stabilized high-valent transuranic species.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Synthesis, Isolation, and Study of Heterobimetallic Uranyl Crown Ether Complexes

Although crown ethers can selectively bind many metal cations, little is known regarding the solution properties of crown ether complexes of the uranyl dication, UO 2 2+ . Here, in this work, the synthesis and characterization of isolable complexes in which the uranyl dication is bound in an 18-crown-6-like moiety are reported. A tailored macrocyclic ligand, templated with a Pt(II) center, captures UO 2 2+ in the crown moiety, as demonstrated by results from single-crystal X-ray diffraction analysis. The U(V) oxidation state becomes accessible at a quite positive potential (E 1/2 ) of –0.18 V vs Fc +/0 upon complexation, representing the most positive U VI /U V potential yet reported for the UO 2 n+ core. Isolation and characterization of the U(V) form of the crown complex are also reported here; there are no prior reports of reduced uranyl crown ether complexes, but U(V) is clearly stabilized by crown chelation. Joint computational studies show that the electronic structure of the U(V) form results in significant weakening of U–O oxo bonding despite the quite positive reduction potential at which this species can be accessed, underscoring that crown-ligated uranyl species could demonstrate unique reactivity under only modestly reducing conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗