Synthesis
and Structural Characterization of Hydrolysis
Products within the Uranyl Iminodiacetate and Malate Systems
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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 (<i>ida</i>) (Na<sub>3</sub>[(UO<sub>2</sub>)<sub>3</sub>(OH)<sub>3</sub>(ida)<sub>3</sub>]·8H<sub>2</sub>O (<b>1</b>)) or malate (<i>mal</i>) (K(<i>pip</i>)<sub>2</sub>[(UO<sub>2</sub>)<sub>3</sub>O(<i>mal</i>)<sub>3</sub>]·6H<sub>2</sub>O (<b>2a</b>) (<i>pip</i> = C<sub>4</sub>N<sub>2</sub>H<sub>12</sub>), (<b>2b</b>) (<i>pip</i>)<sub>3</sub>[(UO<sub>2</sub>)<sub>3</sub>O(<i>mal</i>)<sub>3</sub>]·H<sub>2</sub>O, and (<i>pip</i>)<sub>6</sub>[(UO<sub>2</sub>)<sub>11</sub>(O)<sub>4</sub>(OH)<sub>4</sub>(<i>mal</i>)<sub>6</sub>(CO<sub>3</sub>)<sub>2</sub>]·23H<sub>2</sub>O (<b>3</b>)) 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 <b>1</b> and <b>2</b> is a trimeric 3:3 uranyl <i>ida</i> or <i>mal</i> species, but different bridging groups between
the metal centers create variations in the structural topologies of
the molecular units. Compound <b>3</b> contains a large polynuclear
cluster with 11 U atoms, which is composed of trimeric and pentameric
building units chelated by <i>mal</i> ligands and linked
through hydroxyl groups and carbonate anions. The characterized compounds
represent novel structural topologies for U<sup>6+</sup> hydrolysis
products that may be important molecular species in near-neutral aqueous
systems