Activation of Water by Pentavalent Actinide Dioxide Cations: Characteristic Curium Revealed by a Reactivity Turn after Americium.

Swapping of an oxygen atom of water with that of a pentavalent actinide dioxide cation, AnO ₂ ⁺ also called an "actinyl", requires activation of an An-O bond. It was previously found that such oxo exchange in the gas phase occurs for the first two actinyls, PaO ₂ ⁺ and UO ₂ ⁺ , but not the next two, NpO ₂ ⁺ and PuO ₂ ⁺ . The An-O bond dissociation energies (BDEs) decrease from PaO ₂ ⁺ to PuO ₂ ⁺ , such that the observation of a parallel decrease in the An-O bond reactivity is intriguing. To elucidate oxo exchange, we here extend experimental studies to AmO ₂ ⁺ , americyl(V), and CmO ₂ ⁺ , curyl(V), which were produced in remarkable abundance by electrospray ionization of Am ³⁺ and Cm ³⁺ solutions. Like other AnO ₂ ⁺ , americyl(V) and curyl(V) adsorb up to four H ₂ O molecules to form tetrahydrates AnO ₂ (H ₂ O) ₄ ⁺ with the actinide hexacoordinated by oxygen atoms. It was found that AmO ₂ ⁺ does not oxo-exchange, whereas CmO ₂ ⁺ does, establishing a "turn" to increasing the reactivity from americyl to curyl, which validates computational predictions. Because oxo exchange occurs via conversion of an actinyl(V) hydrate, AnO ₂ (H ₂ O) ⁺ , to an actinide(V) hydroxide, AnO(OH) ₂ ⁺ , it reflects the propensity for actinyl(V) hydrolysis: PaO ₂ ⁺ hydrolyzes and oxo-exchanges most easily, despite the fact that it has the highest BDE of all AnO ₂ ⁺ . A reexamination of the computational results for actinyl(V) oxo exchange reveals distinctive properties and chemistry of curyl(V) species, particularly CmO(OH) ₂ ⁺ .