The Energy Landscape of Uranyl–Peroxide Species

Nanoscale uranyl peroxide clusters containing UO₂²⁺ groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. There is an extensive family of clusters containing 28 uranium atoms (U₂₈ clusters), with an encapsulated anion in the center, for example, [UO₂(O₂)_3−x(OH)_x⁴⁻], [Nb(O₂)₄³⁻], or [Ta(O₂)₄³⁻]. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U₂₈ compounds, one of which is uranyl centered and the other is peroxotantalate centered. The [(Ta(O₂)₄]-centered U₂₈ capsule is energetically more stable than the [(UO₂)(O₂)₃]-centered capsule. These data, along with our prior studies on other uranyl–peroxide solids, are used to explore the energy landscape and define thermochemical trends in alkali–uranyl–peroxide systems. It was suggested that the energetic role of charge-balancing alkali ions and their electrostatic interactions with the negatively charged uranyl–peroxide species is the dominant factor in defining energetic stability. These experimental data were supported by DFT calculations, which agree that the [(Ta(O₂)₄]-centered U₂₈ capsule is more stable than the uranyl-centered capsule. Moreover, the relative stability is controlled by the interactions of the encapsulated alkalis with the encapsulated anion. Thus, the role of alkali-anion interactions was shown to be important at all length scales of uranyl–peroxide species: in both comparing clusters to clusters; and clusters to monomers or extended solids.