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

Uranium(IV) complexation by 2-furoic acid (2-FA) was examined to better understand the effects of ligand identity and reaction conditions on species formation and stability. Five compounds were isolated: [UCl2 (2-FA)2 (H2 O)2 ]n (1), [U4 Cl10 O2 (THF)6 (2-FA)2 ]⋅2 THF (2), [U6 O4 (OH)4 (H2 O)3 (2-FA)12 ]⋅7 THF⋅H2 O (3), [U6 O4 (OH)4 (H2 O)2 (2-FA)12 ]⋅8.76 H2 O (4), and [U38 Cl42 O54 (OH)2 (H2 O)20 ]⋅m H2 O⋅n THF (5). The structures were determined by single-crystal X-ray diffraction and further characterized by Raman, IR, and optical absorption spectroscopy. The thermal stability and magnetic behavior of the compounds were also examined. Variations in the synthetic conditions led to notable differences in the structural units observed in the solid state. At low H2 O/THF ratios, a tetranuclear oxo-bridged [U4 O2 ] core was isolated. Aging of this solution resulted in the formation a U38 oxo cluster capped by chloro and water ligands. However, at increasing water concentrations only hexanuclear units were observed. In all cases, at temperatures of 100-120 °C, UO2 nanoparticles formed.