Single-Electron Uranyl Reduction by a Rare-Earth Cation

Abstract

Unlike their transition-metal analogues, the oxo groups of the uranyl dication, [UO2]2+, which has a linear geometry and short, strong U-O bonds are commonly considered inert.[1] Very little Lewis base character has been demonstrated for the uranyl oxo groups,[2,3] which makes them poor models for the heavier, highly radioactive transuranic actinyl cations such as neptunyl [NpO2]n+ (n=1, 2).[4, 5] The heavier actinyls are important components in nuclear waste and demonstrate oxo basicity that can give rise to poorly understood cluster formation and problems in nuclear waste PUREX separation processes.[6] However, it has been shown recently that the more Lewis basic, pentavalent uranyl cation, [UO2]+, can be stabilized indefinitely using suitable equatorial-binding ligands and anaerobic conditions.[7,8] Usually the [UO2]+ cation decomposes by disproportionation, which is also a poorly understood process, but is important in the precipitation of uranium salts out of aqueous environments.[9, 10] The disproportionation is suggested, by analogy with the transuranic metal oxo Lewis base behavior, to involve the formation of cation–cation interactions (CCIs)[11, 12] in which the oxo groups ligate to adjacent actinyl centers forming diamond (A) or T-shaped (B) dimers or clusters which can then allow the transfer of protons and electrons between metals, such as in C.