In engineered or environmental systems, uranium (U) often exists as a hexavalent state (6+). This highly positive charge of 6+ results in the formation of a unique trans-dioxo cationic unit with a linear O-U-O arrangement (namely uranyl(VI), often written as “UO22+”). This linearly arranged O-U-O unit (i.e. uranyl unit) is remarkably robust and it maintains its linear arrangement even under harsh conditions (e.g. acidic, basic, high temperature, etc.). Due to this chemical stability, uranyl(VI) is the most common and primary form of uranium in the engineered and environmental systems.
The remarkable stability of the uranyl(VI) unit also determines the chemical behavior of U in the actual engineered and environmental systems. In particular, the rigid linearity of the uranyl unit restricts the interaction of U with other substances, such as chemical ligands, exclusively on the equatorial plane perpendicular to the linear O-U-O unit, eventually limiting the chemical reactivity of U in the actual systems. This also indicates that the robust uranyl unit governs the fundamental chemistry of uranium that has been established to date.
In order to achieve a breakthrough in the chemistry of uranium, researchers have performed many attempts to break the linearity of the uranyl unit with a wide variety of approaches, including the use of specially functionalized chemical ligands or the contrivance of new synthetic routes. Thanks to these scientific efforts by many researchers, we can now chemically bend the linear uranyl unit to slightly below 170°, mostly in the range between 166 and 168°. The major challenge that the precedent researchers were facing to bend the uranyl unit is the complication of its chemistry.
More concretely, the precedent successful attempts required either the synthesis of new and complex/bulky chemical ligands or the development of intricate multi-step synthetic routes (or both). These technical difficulties have certainly retarded the development of our knowledge and capability to “bend” the uranyl unit, which is actually prerequisite to explore and/or control the chemical reactivity of U. Our recent research at the Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany, has aimed to tackle this issue and has succeeded in bending the robust uranyl unit with a simple one-step synthesis using only simple and commercially available chemicals.
The new “bent” uranyl compound, [UO2Cl2(phen)2] (phen = 1,10-phenanthroline), was obtained by a simple mixing of uranyl(VI) chloride (UO2Cl2·n(H2O)) and phen in acetone, immediately producing a yellow precipitate. The X-ray structure characterization (X-ray diffraction) on the resultant yellow compound revealed that the arrangement of the uranyl unit in the compound deviates significantly from its ideal linearity with the O-U-O angle close to 160° (161.8°). This is the most bent uranyl unit reported thus far. Our study further proved that the observed significant bending of the uranyl unit arises from the sum of different factors including several intra- and inter-molecular interactions and the bent uranyl unit is active in chemical reactions.
Our finding indicates that the new “bent” uranyl compound is anticipated to be chemically reactive and, hence, can be potentially employed as a starting compound to further explore the synthetic and coordination chemistry of uranium that have a large potential for practical and industrial applications to, e.g., nuclear industry.
This study, [UO2Cl2(phen)2], a Simple Uranium(VI) Compound with a Significantly Bent Uranyl Unit (phen=1,10-phenanthroline) was recently published in the journal Chemistry – A European Journal.