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

Abstract

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 [AnO(OH)(HO)(4-HB)]· nHO [An = Th (Th-1) and U (U-1); 4-HB = 4-hydroxybenzoate] were synthesized via room-temperature reactions of AnCl 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 U-1 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 U-1 and those that exist in solution prior to crystallization as well as upon dissolution of U-1 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 UO 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.

Mononuclear to Polynuclear UIV Structural Units: Effects of Reaction Conditions on U‐Furoate Phase Formation

“…By comparison, compound 1 was only observed above ambient temperatures. Heating the solutions from which 1 – 4 crystallized above 100 °C ( 1 – 3 ) or 120 °C ( 4 ) resulted in a color change consistent with the formation of UO 2 nanoparticles as has been previously reported . Indeed, the powder X‐ray diffraction was consistent with the formation of UO 2 (Figures S11–S14 in Supporting Information).…”

“…Reaction temperature is similarly well known to influence product formation . Compounds 2 – 4 were observed over a wide temperature range, 25–80 °C, though as mentioned previously the optimal temperature for crystallization and product yield was found to be 50 °C.…”

“…With respect to the third possibility, the disordering of μ 3 ‐OH/O has been observed for hexanuclear clusters . In many cases, the disordered sites have U−O bond distances intermediate (2.34–2.37 Å) those of U‐μ 3 ‐O (2.19–2.28 Å) and U‐μ 3 ‐OH (2.42–2.48 Å) . For example, in 4 the U‐μ 3 ‐O/OH bond distances were found to be 2.35(2) Å, consistent with partial (50 %) occupancy of μ 3 ‐O and μ 3 ‐OH.…”

“…In all cases, the occupancy of Cl refined to 1; disordering of the terminal Cl sites with H 2 O resulted in an unsatisfactory refinement. With respect to the third possibility, the disordering of μ 3 ‐OH/O has been observed for hexanuclear clusters . In many cases, the disordered sites have U−O bond distances intermediate (2.34–2.37 Å) those of U‐μ 3 ‐O (2.19–2.28 Å) and U‐μ 3 ‐OH (2.42–2.48 Å) .…”

“…The increase in nuclearity that is observed from the tetrameric species in 2 at lower water content compared to hexameric units in 3 and 4 at higher water content is consistent with expected trends in hydrolysis and condensation. Indeed, a number of related hexanuclear An IV ‐carboxylate clusters have been isolated from aqueous solution under relatively similar reaction conditions …”