The Role of the Organic Solvent Polarity in Isolating Uranyl Peroxide Capsule Fragments

Uranyl peroxide capsules are a fascinating class of polyoxometalates (POMs), discovered only in the 21st century. Understanding the reactivity between peroxide, alkali cations, and uranyl in alkaline solutions is important in nuclear science disciplines including mineralogy, nuclear energy, and legacy nuclear wastes. Here, we have developed a general procedure to isolate different fragments of the uranyl-peroxide POM capsules, using organic solvents to partially remove K⁺ salts from crude solids of the monomer building block UO₂(O₂)₃^4– (K–U₁), leading to stabilization of these reactive fragments. Higher polarity organic solvents remove more K⁺ salts from the crude solid, owed to higher solubility, resulting in more extensive linking of uranyl peroxide building units. By this strategy we have isolated and structurally characterized a dimer K₆[(UO₂)₂(O₂)₄(OH)₂]·7H₂O (K–U₂) and a hexamer face frequently observed in the capsules, K₁₂[(UO₂)₆(O₂)₉(OH)₆]·xH₂O (K–U₆). Comparing experimental and computed Raman spectra shows that these intermediates crystallize by a solid-to-solid transformation, via polymerization of the monomer building block. By small-angle X-ray scattering (SAXS), we track the conversion of the fragments to POM capsules; the reaction rate increases from K–U₁ (days) < K–U₂ (hours) < K–U₆ (instantaneous). This study provides a general synthetic procedure to isolate metastable uranyl peroxide oligomers and control the oligomerization, which will be later applied to systems with the heavier alkalis that are even less stable.