Uranium Chemistry: From Unusual Oxidation States to Polymetallic Clusters

Uranium chemistry plays a crucial role in many aspects of material science and nuclear technology including the development of new uranium materials as potential nuclear fuel, nuclear fuel reprocessing and safe disposal and in determining the mobility of actinides in the environment. The disproportionation reaction, the oxydation and the hydrolysis are major aspects of uranium chemistry leading to the simultaneous presence of different oxidation states in aqueous solutions. The complexity of these aqueous systems renders very difficult the rational design of preprogrammed species and the structural analysis of the final species which become even more complicated in the presence of different metal ions, organic or inorganic ligands. In our group we use rigorously anaerobic and anhydrous conditions to stabilize uranium in unusual oxidation states, to promote original reactivity and to develop the supramolecular assembly of polymetallic uranium complexes which can mediate inter-metallic communication or act as model of environmentally relevant nanoparticles.

 

Fig. 1 : Self-assembly of oxide and nitride clusters

Molecular actinide clusters play a very important role in the environmental migration of actinides and related remediation strategy. The formation of polymetallic complexes is also a key problem in nuclear fuel reprocessing. Moreover actinide-based clusters are of interest as precursor of nuclear fuel and for the design of new functional actinide material such as molecular nanomagnets. In spite of its relevance the cluster chemistry of actinides remains limited to few examples because of the lack of versatile synthetic methods. We have identified different routes to the supramolecular assembly of actinide clusters with different properties. One approach takes advantage of the organizational or template role of the nitride ion for the self-assembly of a unique molecular nitride clusters that could provide important molecular precursor for the convenient synthesis of uranium nitrides (see fig. 1). Furthermore we have developed an original approach for the synthesis of large uranium polyoxometalate.

 

The controlled hydrolysis of low-valent uranium species in anaerobic non-aqueous conditions has provided an original route to the self-assembly of large discrete uranium mixed-valent (U(IV)/U(V)) oxoclusters with the U6O8 core or with the unprecedented U12O20 core depending on the supporting ligand. This approach presents a very large scope for the synthesis of new uranium-based materials and could provide important model systems for actinide aggregates involved in the transport of actinides in the environment. Moreover the controlled hydrolysis reaction provides an efficient route to the stabilisation of pentavalent uranyl. Aside from its fundamental interest, the chemistry of pentavalent uranyl has important environmental implications. UO2+ is an elusive species which undergoes facile disproportionation to UO2(VI) and U(IV) which has been proposed to involve the formation of a dimeric complex through the mutual binding of two uranyl(V) groups (commonly known as cation-cation interaction, CCI).

 

Fig. 2 : Magnetic coupling in UO2+-UO2+ complexes

In our group we have been able to isolate the first examples of monometallic and polymetallic complexes of pentavalent uranyl through the careful combined tuning of the electronics and sterics of the ligand, the cation and the solvent. DFT quasi-relativistic calculations, carried out on selected complexes, clearly point out the presence of a stabilizing covalent interaction with adapted ligands. Moreover, the computed electronic structures correlate quite well with experimental redox and magnetic data. Cation-cation interaction provide an original route to the self-assembly of polymetallic complexes of actinides. The chemistry of these species is of high interest because of the important role they could play in the formation of problematic aggregates in nuclear waste. Moreover we have shown that cation-cation interaction can mediate magnetic communication between uranium ions and the isolated tetrameric and dimeric complexes are well poised to provide the first example of a true magneto-structural correlation in actinide chemistry (see fig. 2).

 

Selected publication(s) )  L. Natrajan et al. J. Am. Chem. Soc. 128 (2006) 7152.;F. Burdet et al. J. Am. Chem. Soc. 128, (2006) 16512; G. Nocton et al. Angew. Chem., Int. Ed. 46 (2007) 7574; G. Nocton et al. J. Am. Chem. Soc. 130  (2008) 16633-16645

 

Maj : 20/07/2012 (563)

 

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