Chemistry of tetravalent uranium carboxylates

In the natural state, uranium is mostly found in the form of uranyl cation (UO₂)²⁺, or the metal has the oxidation degree +VI. Under anaerobic conditions, the presence of reducing agents (bacteria, iron, complexing agents, UV) can lead to the formation of tetravalent uranium. Although uranium is not very soluble in water, thus limiting its transport in the ecosystem, this cation precipitates in the form of uranium oxide nanoparticles and colloids (UO₂), which are very mobile in the environment. The formation of these species and their transport in nature are notably conditioned by the influence of natural complexing molecules. The 'Solid State Chemistry and Nuclear Materials' team of the 'Catalysis and Solid State Chemistry' unit (UCCS) is interested in the synthesis of coordination complexes of light actinides (thorium, uranium). The objective of this study is to crystallize molecular organizations inserting tetravalent uranium with the help of carboxylate ligand. The elaboration of crystals facilitates structural determination by accessing the molecular organization of the species formed. The choice of carboxylate ligands is motivated by their similarities with natural complexing molecules, such as humic acids, or pollutants (e.g. phthalate). This strategy makes it possible to identify the structural organization of uranium complexes that may be present in the natural state, to better understand the behavior of this metal during accidental contamination, and to propose effective cleanup methods. This approach has led to the production of many uranium(IV) molecular assemblies, including a giant poly-oxo cluster inserting 38 uranium atoms. This assembly is particularly interesting because it has dimensions (about 10 nm) and a chemistry close to those of uranium nanoparticles existing in soils contaminated by nuclear waste