Synthesis, reactivity, and environmental relevance of uranyl(V) complexes.

Knowledge of fundamental chemical properties of all environmentally relevant uranium species is essential to understand environmental uranium mobility and develop novel remediation strate-gies. A myriad of uranium(VI) and uranium(IV) compounds has been studied for decades but the uranyl(V) analog was traditionally thought of as a highly unstable species of limited environmen-tal importance. This understanding has changed in the last decade with studies demonstrating uranyl(V) to be a persistent species in Fe rich environments and an intermediate in the biological /abiotic transformations of soluble uranyl(VI) compounds into insoluble uranium(IV) polynuclear species. These processes are imperative for drinking water remediation but their mechanism and the role of U(V) remains unclear. A stable uranyl(V) complex in organic media was first reported more than 10 years ago and this led to rapid development in the field. However, a stable compound in aqueous media at environ-mentally relevant pH could not be obtained and this was selected as the primary goal of this the-sis. Our research produced the first uranyl(V) complex that is stable in both organic and aqueous me-dia. This was achieved using an aminopicolinate ligand, combining the pentadentate binding mode with the ability to form stable complexes with metal ions in water. This molecule has al-lowed us to investigate the persistence of U(V) in certain environments, its conversion to insolu-ble complexes, and the mechanism of bacterial reduction of uranium with collaborators in micro-biology. The effect of Fe2+ on the stability of U(V) towards proton-induced disproportionation and redox reactions was investigated. Cation-cation interaction between uranyl(V) oxygen and Fe2+ was shown to stabilize U(V), explaining its unusual persistence in Fe-rich environments. Additionally, the effect of iron and the impact of a supporting ligand on the electronic structure of U(V) was investigated in collaboration by a high-resolution X-ray absorption spectroscopy and computational study.
Several rational routes for the preparation of uranium polyoxometalates (POM) were developed. The resulting clusters provide a good model of uranate species formed upon environmental reduc-tion of uranyl(VI). An insoluble well-defined molecular trinuclear U(IV) oxo/hydroxo cluster was isolated in a good yield from the direct reduction of a uranyl(VI) complex in aqueous media. This unprecedented 2-electron reduction was accomplished by utilizing a readily available and non-toxic reductant (Na2S2O4). Finally, controlled hydrolysis of UCl4 in organic solution was explored to investigate the factors that govern the assembly of uranium POMs. The formation of discrete uranium(IV) clusters of various nuclearities (U6 – U38) was shown to be dependent on multiple factors, however, the time variable was found to be paramount.