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

Abstract

Reduction of uranyl(VI) to UV and to UIV is important in uranium environmental migration and remediation processes. The anaerobic reduction of a uranyl UVI complex supported by a picolinate ligand in both organic and aqueous media is presented. The [UVIO2(dpaea)] complex is readily converted into the cis‐boroxide UIV 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 UIV oxo‐hydroxo cluster. This report provides the first example of direct conversion of a uranyl(VI) compound into a well‐defined molecular UIV species in aqueous conditions.

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

“…Each uranium ion is octacoordinated by the dpaea ligand, one O sulp and two bridging O oxo /O hydr atoms. The value of the UÀO oxo bond distances ranges from 2.0477(17) to 2.1699 (17) and UÀO hydroxo averages at 2.327(3) which are both in the range of typical for uranium(IV) oxo and U IV hydroxo bond distances. [12] The value of the U-O-U angles in the trimer range from 138.20(9)8 for U-m-OH-U to 144.39(9)8 and 148.83(8)8 (U-m-O-U) and is similar to those found for the only three other trinuclear U IV clusters reported so far.…”

“…Slow evaporation of a water solutions generated by addition of 1 equiv of 2. [17] Scheme 2. One and two electrons reduction of 1 in water at pH 8.5 to afford 6 and 4.…”

“…One and two electrons reduction of 1 in water at pH 8.5 to afford 6 and 4. [17] with the presence of rigid C s symmetric trinuclear solution species. A diffusion coefficient of 2.2.10 À6 cm 2 s À1 and a hydrodynamic radius of 9 were calculated, close to the spherical radius estimated from the crystal structure of the complex 5 (8 ; Supporting Information, Table S1).…”

“…A similar proton-coupled electron transfer mechanism was computed for the reduction of uranyl(V) carbonate to U IV by Fe 2+ in aqueous solution, but it was found to have a high thermodynamic barrier preventing U V /U IV conversion in the presence of carbonate. [15] Here, the putative bis-hydroxo U IV intermediate formed from the reduction of 1 and 2 undergoes condensation due to the basicity of the hydroxo ligand to yield the trinuclear oxo-hydroxo bridged cluster 4 via exclusion of [17] Angewandte Chemie Communications one oxo group as a water molecule. The formation of actinide oxo clusters from condensation reactions is an important process in the water chemistry of actinides that has attracted numerous studies both in organic and water solution owing to its relevance in the migration of actinides in the environment.…”

Synthesis and Characterization of Water Stable Uranyl(V) Complexes

“…[13] We demonstrated that the stepwise reduction of the uranyl(VI) complex [UO 2 -(dpaea)] can be effected with stoichiometric amounts of sodium dithionite in water to yield as table uranyl(V) analogue and awell-defined molecular U IV species. [14] Theh igh denticity of the dpaea 2À ligand, which tightly occupies five coordination sites in the equatorial plane of the UO 2 + species,i sb elieved to be crucial in stabilizing [UO 2 -(dpaea)] À against disproportionation. Notably,t he ligand prevents the interaction between the basic uranyl(V) oxo group and the Ucenter of another UO 2 + species,also known as cation-cation interactions (CCI), [15] that were identified as intermediates in uranyl(V) disproportionation.…”