Synthesis and spectroscopic characterization of mixed-fluoro complexes of U0₂²⁺ containing amino acids, acetylacetonate or acetate as the co-ligands and the first report on the heptafluoroxioxouranate(VI) complex [UO₂F₇]^5-

Abstract

The synthesis of mixed-ligand fluoro complexes of UO₂₂₊ of the types A₃[UO₂(GlyH)₂F₅] . 3H₂O [A = K (1) or NH₄⁺ (2)], K₃[UO₂(AlanH)₂F₅ . 2H₂O (3), (NH₄)₅[UO₂(CysH)₂F₅].2H₂O (4), [UO₂(acac)F(H₂O)₂] .3H₂O (6), K₂[UO₂(acac)F3] (8) and [UO2(CH₃COO)F(H₂O)₂] (9) (GlyH = glycine, AlanH = alanine, CysH^- = cysteinate and acac^- = acetylacetonate) is described. The complexes have been characterized by a combination of chemical analyses, solution conductance measurements and spectroscopic studies. Vibrational spectroscopy has been used for their structural assessment. Laser Raman spectrum could be recorded only for K₂[UO₂(acac)F₃] (8), while an extensive fluorescence foiled such attempts on the other complexes. Each of the three amino acid co-ligands acts in a unidentate manner, being coordinated to UO₂²⁺ through the caroxylate oxygen atom. While glycine and alanine occur in the zwitter-ionic form, cysteine seems to be present as a uninegative ligand. The reaction of [UO₂(acac)F(H₂O)₂].3H₂O (6) with aqueous HF produced [UO₂F₂] . 3H₂O (7). Treatment of K₃[UO₂(GlyH)₂F₅] . 3H₂O (1) with water afforded the hitherto unreported potassium heptafluorodioxouranate(VI) dihydrate, K₅[UO₂F₇ . 2H₂O (5), in a high yield with satisfactory ana measurement (590 ω^-1 cm² mil^-1). IR and laser Raman spectra provide clear evidence for the presence of trans-linked O---U---O and coordinated fluoride. The solution Raman spectrum of 5 is similar to that of its solid indicating that the structure in aqueous solution is the same as the solid. Scanning electron microscopy has been used to ascertain its homogeneity and crystalline nature.