Synthesis and Characterization of Water Stable Uranyl(V) Complexes

The importance of uranyl(V) (UO₂⁺) species associated with environmental and geologic applications is becoming increasingly evident, but the tendency of the uranyl(V) cation to disproportionate in water has prevented the isolation of stable complexes. Here we demonstrate that in the presence of the tridentate complexing dipicolinate (dpa²⁻), a ligand highly abundant in soil, the uranyl(V) species can be stabilized and isolated in anoxic basic water. Stable uranyl(V) dipicolinate complexes are readily formed from the reduction of the uranyl(VI) analogue both in organic solution and in basic water, and their solution and solid-state structure were determined. A bis-dpa U^VO₂⁺ complex was obtained from water at pH 10, while at higher pH values, a trinuclear mono-dpa cation-cation complex was isolated. These results present the second ever isolated water stable uranyl(V) complex. Moreover, we demonstrate that dipicolinate complexes of U^VIO₂²⁺, U^VO₂⁺ and U^IV are strongly luminescent with a signature characteristic of each oxidation state. This provides unique examples of luminescent U^V and U^IV compounds.