Redox chemistry of neptunium in solutions is sensitive to experimental conditions making it rather challenging to predict, study, and control. Subtle differences in solvent systems, organic ligands, pH, spectator ions, and even concentration of neptunium itself all result in deviations in oxidation state of neptunium ranging from Np(IV) to Np(V)O2+ and Np(VI)O22+. However, it is possible to further promote oxidation of neptunium to Np(VII) under highly alkaline conditions and significant concentration of dissolved lithium metal. Based on known crystal structures, the elusive Np(VII) presents itself as a tetraoxo anion with four oxygen atoms bound to the neptunium in the equatorial plane (Np-O ~1.9Å) leaving two coordination spots open in axial positions. Np(VII)O4- complexes have been crystallized out as hydroxide salts, charge balanced with alkali/alkali-earth metals or guanidinium cation. We hypothesize that extensive actinyl-cation and H-bonding interactions stabilize the Np(VII)O4- anion, promote crystallization, and significant impact the vibrational signatures associated with Np(VII) solid-state complexes and Np(VII) anions in the solution. Herein, we explore the settle differences in Np-O distances in the Np(VII)O4- units across a range of crystal structures resulting in changes between D4h and D2h point groups that affect the number of vibrational bands we observe in vibrational spectra. We utilize single-crystal X-ray diffraction data in combination with Raman and IR spectroscopy along with computational support to deconvolute and assign vibrational bands for Np(VII) complexes in both the solid-state and solution.

Complete vibrational analysis of neptunium(VII) compounds: Point group symmetry challenge

ACS Spring 2022

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