Reactivity, Formation, and Solubility of Polyoxometalates Probed by Calorimetry

Room temperature calorimetry methods were developed to describe the energy landscapes of six polyoxometalates (POMs), Li-U₂₄, Li-U₂₈, K-U₂₈, Li/K-U₆₀, Mo₁₃₂, and Mo₁₅₄, in terms of three components: enthalpy of dissolution (ΔH_diss), enthalpy of formation of aqueous POMs (ΔH_f,(aq)), and enthalpy of formation of POM crystals (ΔH_f,(c)). ΔH_diss is controlled by a combination of cation solvation enthalpy and the favorability of cation interactions with binding sites on the POM. In the case of the four uranyl peroxide POMs studied, clusters with hydroxide bridges have lower ΔH_f,(aq) and are more stable than those containing only peroxide bridges. In general for POMs, the combination of calorimetric results and synthetic observations suggest that spherical topologies may be more stable than wheel-like clusters, and ΔH_f,(aq) can be accurately estimated using only ΔH_f,(c) values owing to the dominance of the clusters in determining the energetics of POM crystals.