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Renaissance de la chimie de l’ion uranyle {UO2}2+ en solution non aqueuse
Gérald Siffredi
IRAMIS-SCM
Lundi 15/12/2008, 00h00
INSTN, Amphi J. Horowitz, CEA-Saclay

This work deals with new aspects of the chemistry of the uranyle(VI) ion {UO2}2+ observed in anhydrous polar organic solvents such as the activation of the reputedly inert U–Oyl bond and the controlled reduction of this species which represent a particularly active field of research that attracts much attention for both its fundamental aspects and applications.

Treatment of uranyle(VI) compounds UO2X’2 (X’ = I, Otf, Cl) with Me3SiX (X = Cl, Br, I) reagents, in various anhydrous polar organic solvents, has been first considered. In most cases, reduction into tetravalent species with complete deoxygenation of the uranyl {UO2}2+ ion is observed. The reaction is particularly efficient in acetonitrile where the tetravalent [UX4(MeCN)4] complexes, which are useful precursors in uranium chemistry, are fully isolated. In the course of these reactions, the influence of the solvent, the nature of X’ and X in the UO2X’2 precursor and the Me3SiX reagent are pointed out.

Reaction of the uranyl(VI) UO2X2 (X = I, Cl, Otf, NO3) precursors with the anionic MC5R5 (M = K, R = H, Me ; M = Li, R = Me ; M = Tl, R = H) reagents did not lead to the organometallic [(η5-C5R5)nUO2X2-n] species (n = 1, 2) but to pentavalent uranyl(V) complexes. This method is a facile and rapid route towards the formation of stable pentavalent uranyl which offer promising sources for further U(V) chemical developments and for fundamental and applied interests. Their structure is strongly dependent on the the nature of the solvent, the additional ligands X and of the M+ cation. In pyridine, the {UO2(py)5}+ ion appears to be an ubiquitous and a quite stable entity, which however desolvates readily upon exposure to vacuum. The coordinating properties of the basic oxo groups, which coordinate easily to M+ ions (M = Li, K, Tl), favour structural diversity with formation of heteropolymetallic complexes such as [{UO2(py)5}{MX(py)2}] (M = Li, X = I), [{UO2(py)5}{MX2(py)2}](M = K, Tl, X = OTf ; M = K, X = I), [{UO2(py)5}(M2X3)](M = Li, X = OTf) or [{UO2(py)5}2(M3X5)](M = K ; X = OTf). With deposit of insoluble MX salt (TlI in pyridine for example), reactions give mononuclear species like [UO2(py)5][I].

Once again, the results presented here highlight the advantage of handling uranyl(VI) compounds in strictly anhydrous and deoxygenated media and further demonstrate that uranyl chemistry will witness novel developments under such experimental conditions.

 

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