Preparation of Neptunyl and Plutonyl Acetates to Access Non-Aqueous Transuranium Coordination Chemistry

Uranyl diacetate dihydrate is a useful reagent for the preparation of uranyl coordination complexes, as it is a well-defined stoichiometric compound featuring moderately basic acetates that can facilitate protonolysis reactivity, unlike other anions commonly used in synthetic actinide chemistry such as halides or nitrate. Despite these attractive features, analogous neptunium (Np) and plutonium (Pu) compounds are unknown to date. Here, a modular synthetic route is reported for accessing stoichiometric neptunyl(VI) and plutonyl(VI) diacetate compounds that can serve as starting materials for transuranic coordination chemistry. The new NpO22+ and PuO22+ complexes, as well as a corresponding molecular UO22+ complex, are isomorphous in the solid state, and in solution show similar solubility properties that facilitate their use in synthesis. In both solid and solution state, the +VI oxidation state (O.S.) is maintained, as demonstrated by vibrational and optical spectroscopy, confirming that acetate anions can stabilize the oxidizing, high-valent +VI states of Np and Pu as they do for the relatively more stable U(VI). All three acetate salts react smoothly with a model diprotic ligand, affording incorporation of U(VI), Np(VI), and Pu(VI) cores into molecular coordination compounds that occurs concomitantly with elimination of acetic acid; the new complexes are high-valent, yet overall charge neutral, facilitating entry into non-aqueous chemistry by rational synthesis. Joint computational studies reveal that the dianionic ligand framework assists in stabilizing the +VI O.S. through both strong sigma donation to the actinide centers as well as through chelation enabled by appropriate geometric flexibility in the ligand backbone.