Energetic Trends in Monomer Building Blocks for Uranyl Peroxide Clusters

The uranyl triperoxide anionic monomer is a fundamental building block for uranyl peroxide polyoxometalate capsules. The reaction pathway from the monomer to the capsule can be greatly altered by the counterion: both the reaction rate and the resulting capsule structure. We synthesized and characterized uranyl triperoxides Mg₂UO₂(O₂)₃·13H₂O (MgUT), Ca₂UO₂(O₂)₃·9H₂O (CaUT), Sr₂UO₂(O₂)₃·9H₂O (SrUT), and K₄UO₂(O₂)₃·3H₂O (KUT) and compared their thermodynamic stabilities. The enthalpies of formation from oxides and elements of these compounds were calculated by thermochemical cycles from measurements by high temperature oxide melt drop solution calorimetry. Their formation enthalpies from oxides become more negative linearly as a function of the increasing basicity of the respective oxides on the Smith scale. This relationship holds for previously Li and Na analogues. Further affirming the trend, Δ H_f,ox of MgUT departs from linearity, due to the distinct bonding environment of Mg²⁺, as compared to the other alkalis and alkaline earths in the series.