New look at the uranyl chemistry

The number and diversity of uranyl compounds are witnessing a spectacular growing, revealing novel structural features, in particular with the synthesis of three-dimensional frameworks and nano-materials. All these complexes contain the linear uranium(VI) UO22+ unit which is considered to be thermodynamically and kinetically inert. On the contrary, the pentavalent UO2+ ion, unlike the other actinyl AnO2+ ions (An = Np, Pu), is reputed unstable because of its rapid disproportionation into U(IV) and U(VI) species. These long-lived features of the uranyl complexes were profoundly changed when the syntheses were performed in anhydrous organic media by using new UO2X2 precursors. Thus it was discovered that the uranyl ion could adopt a coordination geometry distinct from the ubiquitous polygonal bipyramidal. The UO22+ ion could be activated and/or reduced under smooth conditions, opening efficient and practical routes to uranium(IV) compounds and to a series of stable pentavalent uranyl complexes, from mononuclear to heteronuclear 3D polymers. (author)