Synthesis and structural characterization of a new water soluble actinide(IV) hexanuclear cluster [An6(OH)4O4]12+ (with An = U, Np, Pu).
Résumé
Among the different processes (precipitation, sorption, colloid formation…) affecting the actinide migration in the environment, complexation is particularly important as it increases the amount of actinide in solution, the actinide release and the migration rates.1 It is often assumed that actinide(IV) chemistry in the environment is influenced by its speciation and particularly its hydrolysis. However, soluble oligomer species resulting from hydrolysis-complexation competition are largely absent from thermodynamic descriptions of aqueous speciation.
Actinide(IV) clusters were intermitently studied during the last fifty years before seeing a renewed interest in the last decade.2 Several studies on tetravalent actinides described oxo-hydroxo hexanuclear species, [An6(OH)4O4]12+, stabilized by small organic ligands such as carboxylic acid3,4,5 or small amino acid.6,7,8 These structures were mainly described with Th(IV) and U(IV) whereas only once was reported for Pu(IV) and none for Np(IV).
Herein we report the structure of new actinide(IV) hexanuclear core species [An6(OH)4O4]12+ (An = U, Np, Pu). The cluster is stabilized with a polyamino carboxylic acid, DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), which is an hydrophilic ligand considered in separation processes and in actinide interaction with human body.
The structure determined by single crystal XRD is made of a hexanuclear core built from six plutonium(IV) connected through four oxo (µ3O2-) and four hydroxo (µ3OH-) alternate bridges (Figure 1). This cluster is decorated and stabilized by four DOTA molecules coordinating the four equatorial Pu(IV) cations. Each ligand is linked to one plutonium metallic center by one oxygen atoms of two of its carboxylic acid functions. The third arm acts as a bridging carboxylate group with one oxygen atoms connected to one equatorial adjacent plutonium cation. The last carboxylic branch of the molecule stays uncoordinated to any metallic center. All the water molecules and unbonded oxygen atoms of each carboxylic acid of each DOTA ligand are connected through hydrogen bonds leading to a stable configuration.
X-ray absorption near-edge structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) were employed to characterize the actinide speciation in the solution after dissolution of the crystals in aqueous solution. The best fit spectra based on hexanuclear complexe crystal structure are in excellent agreement with the experimental one. All actinide(IV) shells are well reproduced by the model. Moreover, the UV-visible absorption spectrum of the actinde(IV) solution recorded presents the same signature and overall shape as the crystal UV-vis reflectance ones. These structural characterizations demonstrate the existence of these An(IV) clusters as water soluble complexes. Even though, from U(IV) to Np(IV), the existence of soluble hexanuclear core species has already been demonstrated, it is the first time that Pu(IV) hexanuclear core are identified in aqueous solution. Moreover, these analyses reveal that UV-visible spectroscopy can be used as a probe to identify the soluble An(IV) cluster.
Beyond a complete structural resolution of the solid state cluster (single-crystal XRD) and of the complex in solution (X-ray absorption), some physical-chemical characteristics were investigated. Based on UV-visible spectroscopy, the pH stability range in aqueous solution was determined for each An(IV). The absence of modification in the spectrum shape indicates a large pH range stability: 0.6