Uranyl | EPFL Graph Search

The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula UO22+. It has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands may be bound to the uranyl ion in an equatorial plane around the uranium atom. The uranyl ion forms many complexes, particularly with ligands that have oxygen donor atoms. Complexes of the uranyl ion are important in the extraction of uranium from its ores and in nuclear fuel reprocessing. Structure and bonding The uranyl ion is linear and symmetrical, with both U–O bond lengths of about 180 pm. The bond lengths are indicative of the presence of multiple bonding between the uranium and oxygen atoms. Since uranium(VI) has the electronic configuration of the preceding noble gas, radon, the electrons used in forming the U–O bonds are supplied by the oxygen atoms. The electrons are donated into empty atomic orbitals on the uranium atom.

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

Radmila Faizova

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 â U38) was shown to be dependent on multiple factors, however, the time variable was found to be paramount.

EPFL2020

The effect of iron binding on uranyl(V) stability

Radmila Faizova, Marinella Mazzanti, Rosario Scopelliti

Here we report the effect of UO2+⋯Fe2+ cation–cation interactions on the redox properties of uranyl(V) complexes and on their stability with respect to proton induced disproportionation. The tripodal heptadentate Schiff base trensal3− ligand allowed the synthesis and characterization of the uranyl(VI) complexes [UO2(trensal)K], 1 and [UO2(Htrensal)], 2 and of uranyl(V) complexes presenting UO2+⋯K+ or UO2+⋯Fe2+ cation–cation interactions ([UO2(trensal)K]K, 3, [UO2(trensal)] [K(2.2.2crypt)][K(2.2.2crypt)], 4, [UO2(trensal)Fe(py)3], 6). The uranyl(V) complexes show similar stability in pyridine solution, but the presence of Fe2+ bound to the uranyl(V) oxygen leads to increased stability with respect to proton induced disproportionation through the formation of a stable Fe2+–UO2+–U4+ intermediate ([UO2(trensal)Fe(py)3U(trensal)]I, 7) upon addition of 2 eq. of PyHCl to 6. The addition of 2 eq. of PyHCl to 3 results in the immediate formation of U(IV) and UO22+ compounds. The presence of an additional UO2+ bound Fe2+ in [(UO2(trensal)Fe(py)3)2Fe(py)3]I2, 8, does not lead to increased stability. Redox reactivity and cyclic voltammetry studies also show an increased range of stability of the uranyl(V) species in the presence of Fe2+ with respect both to oxidation and reduction reactions, while the presence of a proton in complex 2 results in a smaller stability range for the uranyl(V) species. Cyclic voltammetry studies also show that the presence of a Fe2+ cation bound through one trensal3− arm in the trinuclear complex [{UO2(trensal)}2Fe], 5 does not lead to increased redox stability of the uranyl(V) showing the important role of UO2+⋯Fe2+ cation–cation interactions in increasing the stability of uranyl(V). These results provide an important insight into the role that iron binding may play in stabilizing uranyl(V) compounds in the environmental mineral-mediated reduction of uranium(VI).

2018

Ligand-Supported Facile Conversion of Uranyl(VI) into Uranium(IV) in Organic and Aqueous Media

Rizlan Bernier-Latmani, Farzaneh Fadaei Tirani, Radmila Faizova, Marinella Mazzanti

Reduction of uranyl(VI) to U-V and to U-IV is important in uranium environmental migration and remediation processes. The anaerobic reduction of a uranyl U-VI complex supported by a picolinate ligand in both organic and aqueous media is presented. The [(UO2)-O-VI(dpaea)] complex is readily converted into the cis-boroxide U-IV species via diborane-mediated reductive functionalization in organic media. Remarkably, in aqueous media the uranyl(VI) complex is rapidly converted, by Na2S2O4, a reductant relevant for chemical remediation processes, into the stable uranyl(V) analogue, which is then slowly reduced to yield a water-insoluble trinuclear U-IV oxo-hydroxo cluster. This report provides the first example of direct conversion of a uranyl(VI) compound into a well-defined molecular U-IV species in aqueous conditions.

2020