Quantitative Precipitation of Uranyl or Plutonyl Nitrate with N-(1-Adamantyl)acetamide in Nitric Acid Aqueous Solution

The reactivity of the N-(1-adamantyl)acetamide ligand (L = adam) has been evaluated as precipitating agent for the hexavalent uranyl cation ([U] 20-60 g L^-1) in concentrated nitric acid aqueous solution (0.5-5 M). It results in the formation of a crystalline complex (UO₂)(adam)₂(NO₃)₂.2(adam), in which the uranyl center is 8-fold coordinated to two chelating nitrate groups and two N-(1-adamantyl)acetamide (= adam) ligands through the oxygen atom of the amide function. Two other non coordinating adam moieties are also observed in the crystal structure packing and interact through a hydrogen-bond scheme with the uranyl-centered species. A similar molecular assembly has been obtained with the plutonyl(VI) cation, in the complex (PuO₂)(adam)₂(NO₃)₂.2(adam). Precipitation studies indicate high (UO₂)(adam)₂(NO₃)₂.2(adam) formation yields (up to 99%(U) for an L/U molecular ratio of 5/1) for HNO₃ concentration in the 0.5-5 M range. However, the precipitation kinetics is rather slow and the reaction is completed after several hours (3-4 h). The calcination of the resulting solid under an air atmosphere led to the formation of the U₃O₈ oxide from 400 degrees C through a transient phase UO₂ fluorite-type (from 200 degrees C). (author)