Complexation and redox chemistry of neptunium, plutonium and americium with a hydroxylaminato ligand

Abstract

There is significant interest in ligands that can stabilize actinide ions in oxidation states that can be exploited to chemically differentiate 5f and 4f elements. Applications range from developing large-scale actinide separation strategies for nuclear industry processing to carrying out analytical studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-( ^t BuNO)py]^-(interchangeable hereafter with [( ^t BuNO)py]^-), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidation state. An[( ^t BuNO)py]₄ (An = Pu, 1; Np, 2) have been synthesized, characterized by X-ray diffraction, X-ray absorption, ¹H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and analysis. In the case of Pu, oxidation of Pu(iii) to Pu(iv) was observed upon complexation with the [( ^t BuNO)py]^- ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochemical measurements indicate that a Pu(iii) species can be accessed upon one-electron reduction of 1 with a large negative reduction potential (E _1/2 = -2.26 V vs. Fc^+/0). Applying oxidation potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chemistry of U^IV[( ^t BuNO)py]₄ that revealed a stable U(v) product. Treatment of an anhydrous Am(iii) precursor with the [( ^t BuNO)py]^- ligand did not result in oxidation to Am(iv). Instead, the dimeric complex [Am^III(μ₂-( ^t BuNO)py)(( ^t BuNO)py)₂]₂ (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aqueous Am-containing molecular complex prepared using inert atmosphere techniques. Predicted redox potentials from density functional theory calculations show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidation states of actinides (i.e., +5 for Np and Pu and +4 for Am) are not stabilized by [2-( ^t BuNO)py]^-, in good agreement with experimental observations.

Scheme 1. Synthesis of 1, Pu^IV[(^tBuNO)py]₄, via a Pu(iii) precursor.

Fig. 1. Thermal ellipsoid plot (at the 50% probability level) of the solid-state structure of 1, Pu^IV[(^tBuNO)py]₄. Hydrogen atoms have been omitted for clarity, and ^tBu groups are shown as ball-and-stick depictions also for clarity. Atom labelling has been shown only for unique hetero atoms, and the Pu center. Symmetry operations: −X, +Y, 1/2 − Z.

Scheme 2. Synthetic route to 1, Pu^IV[(^tBuNO)py]₄, and 2, Np^IV[(^tBuNO)py]₄ from An(iv) precursors.

Scheme 3. Multi-step synthesis of 3, [Am^III(μ₂-(^tBuNO)py)((^tBuNO)py)₂]₂, an Am(iii) dimeric complex.

Fig. 2. Thermal ellipsoid plot (at the 50% probability level) of the solid-state structure of 3, an Am(iii) dimer. Hydrogen atoms have been omitted for clarity, and ^tBu groups are shown as ball-and-stick depictions also for clarity. Atom labelling has been shown only for unique hetero atoms and the symmetry-equivalent bridging N, O atoms, and the unique Am center. Symmetry operations: −X, 2 − Y, 2 − Z.

Fig. 3. The actinide L₃-edge XANES spectra from An[(^tBuNO)py]₄ [An = Pu (1), bottom; Np (2), top]. Data were compared with oxidation state references, (PPh₄)₂[Np^VIO₂Cl₄], An^IVCl₄(DME)₂ (An = Np, Pu), Pu^IIII₃(NC₅H₅)₄, and an aqueous sample of Pu^VIO₂²⁺ dissolved in HNO_3(aq) (1 M) that contained (NH₄)₂[Ce(NO₃)₆] to hold the Pu oxidation state at +6.

Fig. 4. The EXAFS function k³χ(k) (Å) for solid samples of An[(^tBuNO)py]₄ [An = Pu, 1; bottom; Np, 2, top; Å; ○] and the fits for the data (orange traces).

Fig. 5. Interpretation of the Fourier transform of the k³-EXAFS spectra from An[(^tBuNO)py]₄ [An = Pu, 1; bottom; Np, 2, top; Å; black traces]. Fits are shown as orange dashed traces, while An–O_O–N^tBuNC5 (blue trace), An–N_O–N^tBuNC5 (red trace), An⋯N_O–N^tBuNC5 (olive trace), and An⋯C_O–N^tBuNC5 (brown trace) are shown as inverted functions.

Fig. 6. Fourier transform of k³-EXAFS spectra from An[(^tBuNO)Py]₄ [An = Pu, 1; bottom; Np, 2, top; Å; black traces]. Fits for the data are shown as orange dashed traces. Also included are the real parts of the Fourier transform (olive traces) and the corresponding fit (blue dashed traces).

Fig. 7. Comparison of interatomic distances for An[(^tBuNO)py]₄ (An = Pu, 1; Np, 2) determined by actinide L₃-edge EXAFS, single-crystal X-ray diffraction (XRD), and density functional theory (DFT) with both An(iii) and An(iv).

Fig. 8. Cyclic voltammograms measured for An[(^tBuNO)py]₄ (An = Pu, 1; Np, 2) in THF (2 mM) with 0.1 M [ⁿPr₄N][BAr^F₄] at 200 mV s⁻¹ and compared with previous reports of M[(^tBuNO)py]₄ (M = Ce, Th, U).

Fig. 9. Frontier molecular orbital energy level diagram for U^IV[(^tBuNO)py]₄, Np^IV[(^tBuNO)py]₄, Pu^IV[(^tBuNO)py]₄ and Am^III[(^tBuNO)py]₄⁻ complexes. For all complexes except U^IV[(^tBuNO)py]₄, the envelope of the highest occupied MO (HOMO) and lowest unoccupied MO (LUMO) is given with contour values of 0.03 a.u. and the MO energy levels between the HOMO and singly occupied MOs (SOMOs) are not included. The Z direction is labeled with an arrow. The MOs of An 5f and ligand atomic orbital character are in red and black color, respectively. Only alpha orbital energies are shown. The orbital energies of the Am^III[(^tBuNO)py]₄⁻ complex are shifted by −1.3 eV to facilitate visual presentation.