Heptavalent Actinide Tetroxides NpO₄^– and PuO₄^–: Oxidation of Pu(V) to Pu(VII) by Adding an Electron to PuO₄

The highest known actinide oxidation states are Np­(VII) and Pu­(VII), both of which have been identified in solution and solid compounds. Recently a molecular Np­(VII) complex, NpO₃(NO₃)₂^–, was prepared and characterized in the gas phase. In accord with the lower stability of heptavalent Pu, no Pu­(VII) molecular species has been identified. Reported here are the gas-phase syntheses and characterizations of NpO₄^– and PuO₄^–. Reactivity studies and density functional theory computations indicate the heptavalent metal oxidation state in both. This is the first instance of Pu­(VII) in the absence of stabilizing effects due to condensed phase solvation or crystal fields. The results indicate that addition of an electron to neutral PuO₄, which has a computed electron affinity of 2.56 eV, counterintuitively results in oxidation of Pu­(V) to Pu­(VII), concomitant with superoxide reduction