Synthesis and Structural Characterization of Hydrolysis Products within the Uranyl Iminodiacetate and Malate Systems

Abstract

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.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

“…Spectral features from chelating ligands were specifically noted for the citrate [16], malate [133], pyromellitate [134], benzenedicarboxylate [135], and hydrobenzoate [136] systems. Similarly, uranyl peroxide spectra contain overlapping modes from uranyl and peroxide stretches.…”

Uranyl Ion Complexes with Chiral Malic and Citramalic, and Prochiral Citric and Tricarballylic Acids: Influence of Coligands and Additional Metal Cations

“…One of our objectives in the present work is to explore further the coordination modes adopted by these versatile ligands. Several uranyl malate complexes have previously been characterized, in which either a 2:2 dimeric motif analogous to that found with citrate is present, with additional metal cations providing formation of 2D or 3D assemblies in some cases,, or a more intricate uranyl oligomer resulting from hydrolysis as a central core . Citramalic acid, used as its pure R enantiomer, has also given several complexes based on the dimeric motif,, , or with a different bonding mode, enabling the formation of a 2D network .…”

Directing dimensionality in uranyl malate and copper uranyl malate compounds

“…All bands are blue shifted from the previously reported uranyl malate compounds, which observed a ν 1 stretch at 816 cm -1 [41]. In these molecular species, hydrolysis of the uranyl moiety leads to the formation of a trimer which has an additional u 3 -O group, leading to additional red shift of the symmetric stretching vibration [41].…”

“…Addition of strong sigma donors lead to a red shift from the uranyl pentaaqua complex ([(UO 2 )(H 2 O) 5 ] 2+ ) band located at 870 cm -1 and the carboxylate groups of the malate ligand functions in this capacity [17]. All bands are blue shifted from the previously reported uranyl malate compounds, which observed a ν 1 stretch at 816 cm -1 [41]. In these molecular species, hydrolysis of the uranyl moiety leads to the formation of a trimer which has an additional u 3 -O group, leading to additional red shift of the symmetric stretching vibration [41].…”