Unlocking Neptunium(IV) Aqueous Chemistry via Polyoxometalate-Mediated Reduction and Stabilization

Distinct from the rest of the periodic table, including its actinide neighbors, neptunium exhibits truly unique chemistry as its speciation and chemical behavior are dominated by the neptunyl(V) ion, i.e., NpO2+. Here, we describe the spontaneous destabilization and reduction of neptunyl(V) in aqueous solution via complexation to the Keggin-type polyoxometalate (POM) ligand PW11O397-. The POM-mediated reduction of NpO2+ does not require any reducing agent and occurs within minutes, at room temperature, and in aqueous solution. The POM complexation allows for the thermodynamic stabilization of Np4+ by forming the complex [Np(PW11O39)2]10- (Np(PW11)2), which remains soluble, water-stable, and air-stable for weeks. The Np(PW11)2 complex also persists over an extended acidity range. Crystallization of Np(PW11)2 and characterization via single-crystal X-ray diffraction revealed a rare case where both the alpha and beta isomers of the Keggin ligand are present in the structure, forming an unprecedented 50:50 mixture of Np(α-PW11)2 and Np(α-PW11)(β-PW11). Comparative crystal structures obtained with other tetravalent cation-PW11 complexes (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. The solution-state and solid-state characterization of the Np-PW11 system via UV-vis-NIR absorbance, Raman spectroscopy, 31P NMR, VT NMR, and relaxometry further elucidated the speciation (complex stoichiometry and oxidation state) and the mixed rotational isomerism. Moreover, comparative experiments with uranium revealed that the two actinyl ion types (UO22+ vs. NpO2+) undergo drastically different reactions in the presence of PW11. While NpO2+ reduces to Np4+ and forms a stable complex, UO22+ is not reduced but instead uses PW11 as a phosphate reservoir and precipitates as uranyl phosphate. As the neptunyl(V) ion is traditionally highly stable and soluble under environmental conditions, this study paves the way for POM-mediated immobilization of Np and other actinides via in situ reduction and precipitation.