Structure, isomerism, and ligand dynamics in dioxouranium(VI) complexes

The ligand exchange reactions in complexes of the type UO2LFn(H2O)(3-n), where n = 1-3 and L is one of the bidentate ligands picolinate (pic), oxalate (ox), carbonate, and acetate (ac), UO2L2F (L = picolinate or oxalate), have been investigated by using an array of F-19-, C-13-, O-17-, and H-1-NMR techniques. The rates of exchange were sufficiently low to allow the identification of isomers,which are those expected for a pentagonal bipyramid coordination geometry, with the ''yl'' oxygens in the apical positions. All ligand exchange reactions were independent of the concentration of free ligand. Two different fluoride exchange pathways were identified, intermolecular exchange between coordinated and free fluoride and intramolecular exchange between different fluoride sites within a given complex. The intramolecular exchange reaction in UO2(pic)F-3(3-) was shown to be a result of opening/closing of the chelate ring, equivalent to a site exchange between the two ''edge'' fluorides. Dissociation of acetate is the major factor determining the intramolecular fluoride exchange reactions in UO2(ac)F-3(2-). In this same way the broad F-19-NMR peaks or UO2LF2(H2O)(2-) and UO2LF(H2O)(2)(-) are a result of a comparatively slow exchange between coordinated and bulk water. At pH > 6 UO2(ox)F(H2O)(2)(-) forms a hydroxide-bridged dimer (UO2)(2)(ox)(2)F-2(OH)(2)(4-), which exists in two isomeric forms.