Coordination of Tetravalent Actinides (An=Th^IV, U^IV, Np^IV, Pu^IV) with DOTA: From Dimers to Hexamers

Abstract

Three tetravalent actinide (An ) hexanuclear clusters with the octahedral core [An (OH) O ] (An =U , Np , Pu ) were structurally characterized in the solid state and in aqueous solution by using single-crystal X-ray diffraction, X-ray absorption, IR, Raman, and UV/Vis spectroscopy. The observed structure, [An (OH) O (H O) (HDOTA) ]⋅HCl/HNO ⋅n H O (An=U(I), Np(II), Pu(III)), consists of a An hexanuclear pseudo-octahedral cluster stabilized by DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) ligands. The six actinide atoms are connected through alternate μ -O and μ -OH groups. Extended X-ray absorption fine structure (EXAFS) investigations combined with UV/Vis spectroscopy provide evidence for the same local structure in moderate acidic and neutral aqueous solutions. The synthesis mechanism was partially elucidated and the main physicochemical properties (pH range stability, solubility, and protonation constant) of the cluster were determined. The results underline the importance of: 1) considering such polynuclear species in thermodynamic models, and 2) competing reactions between hydrolysis and complexation. It is interesting to note that the same synthesis route with thorium(IV) led to the formation of a dimer, Th (H O) (H DOTA) ⋅4 NO ⋅x H O (IV), which contrasts to the structure of the other An hexamers.

On the Aqueous Chemistry of the UIV–DOTA Complex

“…The absorption spectrum of the aqueous side product (Figure b) was found to be similar to that reported for the [U 6 (OH) 4 (O) 4 (DOTAH) 4 ] cluster by Tamain et al …”

“…The nonaqueous synthesis of the [U IV (κ 8 ‐DOTA)(dmso)] complex enabled the identification of a different monomeric U IV –DOTA species, the [U IV (κ 4 ‐DOTA)(dmso) 4 ] complex, in which the U IV cation was located above the ligand O 4 plane and bonded to four other solvent ligands. A similar species was also suggested as the precursor for the eventual formation of the U IV –DOTA oxohydroxo cluster, as shown for the equivalent Pu IV cluster . The three different species described above and the possible chemical equilibrium that may form between each of them in solution indicate that the U IV ‐DOTA system is an interesting probe for the study of the aqueous chemistry of the U IV cation.…”

“…Similar coordination spheres were reported in aqueous media for DOTA complexes of Pu III ; Am III ; and Ln III , such as Ce III , Eu III , and Yb III . The separation and dissolution in aqueous solutions of the U IV out‐of‐plane complex may have led to its rapid oligomerization, as shown by Tamain et al . for the equivalent Pu IV hexameric cluster formation from an initially formed monomeric out‐of‐plane complex.…”

“…Therefore, only a few attempts to study its Ln/An (III,IV) ‐equivalent cPAC aqueous complexes formation have been reported. Two examples are the U IV –HEHA (HEHA=1,4,7,10,13,16‐hexaazacyclooctadecane‐ N , N ′, N ′′, N ′′′, N ′′′′, N ′′′′′‐hexaacetic acid) complex, which was found to hydrolyze rapidly upon its formation, and the oligomeric oxohydroxo [U 6 (OH) 4 (O) 4 (DOTAH) 4 ] cluster, formed by the aqueous reaction of the cation with the DOTA chelate . The tendency of aqueous U IV cations to form soluble oligomeric oxohydroxo clusters in aqueous solutions is well documented, with recent reports discussing their pH‐ and temperature‐dependent oligomerization reactions .…”

Extension of the Plutonium Oxide Nanocluster Family to Include {Pu16} and {Pu22}

“…Actinides (An) in the +4 oxidation state are of particular interest due to the thermodynamic favorability and insolubility of hydrolysis products. Yet, recent studies show that the hexanuclear actinide nanoclusters described below are favored over the hydrolysis products predicted by most aqueous speciation models . Therefore, while metal‐oxide nanoclusters can provide opportunities for the tailored design of materials, they can also adversely impact process and environmental chemistry that is targeted at simple dissolved species.…”