Synthesis and structural characterization of mixed iron-uranium compounds with bidentate N-donor ligands

The reliable assessment of geochemical behavior of uranium (U) is one of the most significant issues for uranium mining sites and geological disposals of radioactive wastes. In addition to the major chemical parameters of groundwater, such as pH, ionic strength, co-existence of anionic species, etc., the oxidation state is one of the fundamental parameters which determine the geochemical behavior of uranium in aquatic environments. Under natural aquatic conditions, the redox chemistry of uranium is dominated either by uranyl(VI) (UO22+) mainly under aerobic conditions or by uranium(IV) (U4+) under anaerobic conditions [1]. The redox behavior of uranium is affected not only by solution conditions, but also by the co-existence of other redox-active ions, such as manganese (Mn) and iron (Fe) [2] which are ubiquitous in the actual aquatic environment. The interaction of U species with other redox species complicates the redox behavior of U, eventually influencing the interaction of U species with other organic ligands and inorganic anions [3,4]. Hence, the understanding of the chemical behavior of U in the presence of other redox active metals is indispensable for the reliable prediction of the geochemical behavior of U.
Based on this background, we performed a systematic study of the reactivity of U(IV) and -(VI) with Fe(II) and -(III), some of the major redox active species abundant in groundwater, in the presence of 2,2’-bipyridine (bipy) and 1,10-phenanthroline (phen) as representatives of naturally occurring N-donor organic ligands. Mixing U(IV)/U(VI) with Fe(II)/Fe(III) and bipy/phen in different solution conditions yielded a wide variety of single crystals of mixed Fe-U compounds, which were further characterized by single crystal / powder X-ray diffraction and other spectroscopic methods.
In the studied iron-uranium systems, the N-donor ligand (L) is always coordinating to Fe, forming [FeII(L)3]2+ or oxo-bridged [(FeIIIL2Cl)2(µ2-O)]2+ cationic units. Uranium is not directly interacting with the ligands, existing as the counter anionic unit of [UIVCl6]2- or [UVIO2Cl4]2- in the structures. This preferable coordination of the N-donor ligands towards iron is independent of the oxidation state of uranium. The interaction of Fe(III) (oxidant) with U(IV) (reductant) naturally resulted in the formation of Fe(II)-U(VI) mixed compounds. This redox reaction is not affected by the presence of the strong N-donor ligands.
These results provide us a molecular insight into the coordination and redox chemistry of the mixed iron-uranium systems in aqueous solutions, which has direct relevance to the geochemistry of uranium. The iron-uranium compounds characterized in this study are also rare examples of such mixed metal-uranium compounds with organic ligands.