Uranyl citrate forms trimeric species at pH > 5.5, but exact structural characteristics of these important oligomers have not previously been reported. Crystallization and structural characterization of the trimers suggests the self-assembly of the 3 : 3 and 3 : 2 U : Cit complexes into larger sandwich and macrocyclic molecules. Raman spectroscopy and ESI-MS have been utilized to investigate the relative abundance of these species in solution under varying pH and citrate concentrations. Additional dynamic light scattering experiments indicate that self-assembly of the larger molecules does occur in aqueous solution.

The interplay of hydrolysis and chelation by organic ligands results in the formation of novel uranium species in aqueous solutions. Many of these molecular complexes have been identified by spectroscopic and potentiometric techniques, but a detailed structural understanding of these species is lacking. Identification of possible uranyl hydrolysis products in the presence of organic functional groups has been achieved by the crystallization of molecular species into a solid-state compound, followed by structural and chemical characterization of the material. The structures of three novel molecular complexes containing either iminodiacetate (ida) (Na3[(UO2)3(OH)3(ida)3]·8H2O (1)) or malate (mal) (K(pip)2[(UO2)3O(mal)3]·6H2O (2a) (pip = C4N2H12), (2b) (pip)3[(UO2)3O(mal)3]·H2O, and (pip)6[(UO2)11(O)4(OH)4(mal)6(CO3)2]·23H2O (3)) ligands have been determined by single-crystal X-ray diffraction and have been chemically characterized by IR, Raman, and NMR spectroscopies. A major structural component in compounds 1 and 2 is a trimeric 3:3 uranyl ida or mal species, but different bridging groups between the metal centers create variations in the structural topologies of the molecular units. Compound 3 contains a large polynuclear cluster with 11 U atoms, which is composed of trimeric and pentameric building units chelated by mal ligands and linked through hydroxyl groups and carbonate anions. The characterized compounds represent novel structural topologies for U(6+) hydrolysis products that may be important molecular species in near-neutral aqueous systems.

Organic acids are important metal chelators in environmental systems and tend to form soluble complexes in aqueous solutions, ultimately influencing the transport and bioavailability of contaminants in surface and subsurface waters. This is particularly true for the formation of uranyl citrate complexes, which have been utilized in advanced photo- and bioremediation strategies for soils contaminated with nuclear materials. Given the complexity of environmental systems, the formation of ternary or heterometallic uranyl species in aqueous solutions are also expected, particularly with Al(iii) and Fe(iii) cations. These ternary forms are reported to be more stable in aqueous solutions, potentially enhancing contaminant mobility and uptake by organisms, but the exact coordination geometries of these soluble molecular complexes have not been elucidated. To provide insight into the nature of these species, we have developed a series of geochemical model compounds ([(UO(2))(2)Al(2)(C(6)H(4)O(7))(4)](6-) (U(2)Al(2)), [(UO(2))(2)Fe(2)(C(6)H(4)O(7))(4)](6-) (U(2)Fe(2)-1) and [(UO(2))(2)Fe(2)(C(6)H(4)O(7))(4)(H(2)O)(2)](6-) (U(2)Fe(2)-2) and [(UO(2))(2)Fe(4)(OH)(4)(C(6)H(4)O(7))(4)](8-) (U(2)Fe(4))) that were characterized by single-crystal X-ray diffraction and vibrational spectroscopy. Mass spectroscopy was then employed to compare the model compounds to species present in aqueous solutions to provide an enhanced understanding of the ternary uranyl citrate complexes that could be relevant in natural systems.

Uranyl hybrid compounds are complex materials due to variability in coordination geometry, flexibility in ligand chelation, and metal hydrolysis, which leads to difficulty in controlling the secondary building units. The presence of transition metals in uranyl hybrid materials adds to the complexity, but also leads to an increase in the dimensionality of the topology from infinite chains and 2-D sheets, to 3-D framework lattices. In this study, five uranyl malate compounds were synthesized at room temperature: ((C 4 H 12 N 2)[(UO 2) 2 (C 4 H 3 O 5) 2 ] • 4 H 2 O (UMal1), (C 4 H 12 N 2)[(UO 2) 2 (C 4 H 3 O 5) 2 ] (UMal1-b), [(UO 2)(C 4 H 3 O 5)Cu(C 10 H 8 N 2)Cl(H 2 O)] • 2 H 2 O (UCuMal1), [(UO 2) 2 (C 4 H 3 O 5) 2 Cu(C 5 H 5 N) 2 (H 2 O) 2 ] • 2 H 2 O (UCuMal2), [(UO 2) 2 (C 4 H 3 O 5) 2 Cu(C 5 H 5 N) 2 (H 2 O) 2 ] • 2 H 2 O (UCuMal3)). These compounds were characterized using single-crystal X-ray diffraction, thermogravimetric analysis and Raman spectroscopy. All five compounds contain an identical uranyl malate secondary building unit that could be further linked through the Cu(II) cation. In this system, the identity of the ligands bonded to the Cu(II) cation impacted dimensionality and could be the key to designing materials with a known uranyl building unit.