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

Area of Science:

• Inorganic Chemistry
• Materials Science
• Radiochemistry

Background:

• Organic ligands with carboxylate groups significantly influence the behavior of tetravalent metal ions.
• Previous research often used simple monocarboxylates for actinide complexation.
• The impact of the organic backbone on solution and solid-state chemistry of Th(IV) and U(IV) complexes requires further investigation.

Purpose of the Study:

• To investigate the complexation of thorium (Th(IV)) and uranium (U(IV)) by 4-hydroxybenzoic acid in aqueous solutions.
• To understand how the organic structure of 4-hydroxybenzoic acid affects the solution and solid-state chemistry of Th(IV) and U(IV) complexes.
• To compare the formation of hexanuclear clusters and nanoparticles with 4-hydroxybenzoic acid versus previously studied monocarboxylate ligands.

Main Methods:

• Synthesis of Th(IV) and U(IV) compounds using AnCl4 and 4-hydroxybenzoic acid at room temperature.
• Characterization using single-crystal X-ray diffraction, Raman, infrared, optical spectroscopies, and thermogravimetry.
• Solution studies involving small-angle X-ray scattering, UV-Vis spectroscopy, and dynamic light scattering to analyze speciation and cluster evolution.

Main Results:

• Two isomorphous compounds, [An6O4(OH)4(H2O)6(4-HB)12]·nH2O (An = Th, U), were synthesized and structurally characterized.
• The structures feature ligand-decorated oxo/hydroxo-bridged hexanuclear units.
• 4-hydroxybenzoic acid promotes hexamer formation without the need for pH adjustment, unlike previous studies.

Conclusions:

• 4-hydroxybenzoic acid plays a crucial role in the formation of hexanuclear Th(IV) and U(IV) clusters.
• The ligand's structure influences the solution speciation and the evolution of clusters to nanoparticles.
• This work provides insights into the impact of organic ligands on actinide solution and solid-state chemistry.