Breaking the Neptunyl Barrier: Direct Access to Neptunium(IV) in Aqueous Solution via Polyoxometalate-Mediated Reduction and Stabilization

Hastings, Ashley M.; Colliard, Ian; Kaseman, Derrick C.; Colla, Christopher A.; Deblonde, Gauthier J.-P.

doi:10.1021/acs.inorgchem.5c02949

Breaking the Neptunyl Barrier: Direct Access to Neptunium(IV) in Aqueous Solution via Polyoxometalate-Mediated Reduction and Stabilization

Journal Article · Fri Sep 05 00:00:00 EDT 2025 · Inorganic Chemistry

DOI:https://doi.org/10.1021/acs.inorgchem.5c02949· OSTI ID:2999043

^[1]; Colliard, Ian ^[1]; ^[1]; ^[1]; ^[1]

Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

Neptunium exhibits truly unique chemistry as its speciation is dominated by the neptunyl(V) ion (NpO₂⁺). Here, in this study, we describe the spontaneous destabilization and reduction of neptunyl(V) via complexation to the Keggin-type polyoxometalate (POM) ligand PW₁₁O₃₉^7–. The POM-mediated reduction of NpO₂⁺ does not require any reducing agent and occurs within minutes, at room temperature, and in aqueous solution. The resulting [Np(PW₁₁O₃₉)₂]^10- complex (Np(PW₁₁)₂) remains soluble, water-stable, and air-stable for weeks and persists over an extended acidity range. Single-crystal structure, solid-state Raman and UV–visible absorbance characterization of Np(PW₁₁)₂ revealed a mixed alpha/beta isomery of the Keggin ion, forming an unprecedented 50:50 mixture of Np(α-PW₁₁)₂ and Np(α-PW₁₁)(β-PW₁₁). Experiments with other tetravalent ions (i.e., Zr⁴⁺, Hf⁴⁺, Ce⁴⁺, and Th⁴⁺) indicate that the occurrence of the beta isomer is specific to Np⁴⁺ and independent of the cation’s size. Solution-state characterization of the Np-PW₁₁ system via UV-visible-NIR absorbance, ³¹P NMR, VT NMR, and relaxometry further elucidated the speciation. Moreover, comparative experiments with uranium revealed that the two types of actinyl ions (UO₂²⁺ vs NpO₂⁺) undergo drastically different reactions in the presence of PW₁₁. UO₂²⁺ is not reduced but instead uses PW₁₁ as a phosphate reservoir and precipitates as uranyl phosphate.

View Accepted Manuscript (DOE)

Research Organization:: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

Sponsoring Organization:: USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC), Basic Energy Sciences (BES)

Grant/Contract Number:: AC52-07NA27344

OSTI ID:: 2999043

Report Number(s):: LLNL--JRNL-2001514

Journal Information:: Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 37 Vol. 64; ISSN 0020-1669; ISSN 1520-510X

Publisher:: American Chemical Society (ACS)Copyright Statement

Country of Publication:: United States

Language:: English

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