Solution and Solid State Structural Chemistry of Th(IV) and U(IV) 4‑Hydroxybenzoates

Organic ligands with carboxylate functionalities have been shown to affect the solubility, speciation, and overall chemical behavior of tetravalent metal ions. While many reports have focused on actinide complexation by relatively simple monocarboxylates such as amino acids, in this work we examined Th­(IV) and U­(IV) complexation by 4-hydroxybenzoic acid in water with the aim of understanding the impact that the organic backbone has on the solution and solid state structural chemistry of thorium­(IV) and uranium­(IV) complexes. Two compounds of the general formula [An₆O₄(OH)₄(H₂O)₆(4-HB)₁₂]·<i>n</i>H₂O [An = Th (<b>Th-1</b>) and U (<b>U-1</b>); 4-HB = 4-hydroxybenzoate] were synthesized via room-temperature reactions of AnCl₄ and 4-hydroxybenzoic acid in water. Solid state structures were determined by single-crystal X-ray diffraction, and the compounds were further characterized by Raman, infrared, and optical spectroscopies and thermogravimetry. The magnetism of <b>U-1</b> was also examined. The structures of the Th and U compounds are isomorphous and are built from ligand-decorated oxo/hydroxo-bridged hexanuclear units. The relationship between the building units observed in the solid state structure of <b>U-1</b> and those that exist in solution prior to crystallization as well as upon dissolution of <b>U-1</b> in nonaqueous solvents was investigated using small-angle X-ray scattering, ultraviolet–visible optical spectroscopy, and dynamic light scattering. The evolution of U solution speciation as a function of reaction time and temperature was examined. Such effects as well as the impact of the ligand on the formation and evolution of hexanuclear U­(IV) clusters to UO₂ nanoparticles compared to prior reported monocarboxylate ligand systems are discussed. Unlike prior reported syntheses of Th and U­(IV) hexamers where the pH was adjusted to ∼2 and 3, respectively, to drive hydrolysis, hexamer formation with the HB ligand appears to be promoted only by the ligand