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

Neptunium exhibits truly unique chemistry as its speciation is dominated by the neptunyl(V) ion (NpO2+). Here, we describe the spontaneous destabilization and reduction of neptunyl(V) 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 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.