Reduction and selective oxo group silylation of the uranyl dication

Uranium on the move Uranium is present almost everywhere in the environment as the highly soluble and mobile uranyl dication UO 2 2+ , which is also a major radioactive pollutant from the nuclear power and mining industries. The compound is chemically inert because of unusually strong bonds between the uranium atom and its two oxo groups. Arnold et al . now report that one uranyl oxo group can be made to undergo radical reactions normally associated only with transition metal oxo groups, if the strong O=U=O bonding is disrupted by placing the dication within an appropriate rigid molecular framework. Once put in the spot, the uranyl dication undergoes both single-electron reduction and oxo-group functionalization to form unique pentavalent uranyl compounds. These transformations might lead to strategies for manipulating and processing uranium in its most common form in solution. Uranium occurs in the environment predominantly as the uranyl dication [UO 2 ] 2+ . Its solubility renders this species a problematic contaminant 1 , 2 , 3 which is, moreover, chemically extraordinarily robust owing to strongly covalent U–O bonds 4 . This feature manifests itself in the uranyl dication showing little propensity to partake in the many oxo group functionalizations and redox reactions typically seen with [CrO 2 ] 2+ , [MoO 2 ] 2+ and other transition metal analogues 5 , 6 , 7 , 8 , 9 . As a result, only a few examples of [UO 2 ] 2+ with functionalized oxo groups are known. Similarly, it is only very recently that the isolation and characterization of the singly reduced, pentavalent uranyl cation [UO 2 ] + has been reported 10 , 11 , 12 . Here we show that placing the uranyl dication within a rigid and well-defined molecular framework while keeping the environment anaerobic allows simultaneous single-electron reduction and selective covalent bond formation at one of the two uranyl oxo groups. The product of this reaction is a pentavalent and monofunctionalized [O = U ... OR] + cation that can be isolated in the presence of transition metal cations. This finding demonstrates that under appropriate reaction conditions, the uranyl oxo group will readily undergo radical reactions commonly associated only with transition metal oxo groups. We expect that this work might also prove useful in probing the chemistry of the related but highly radioactive plutonyl and neptunyl analogues found in nuclear waste.