Synthesis and characterization of the solid uranium ( VI ) dioxo - diacetohydroxamate complex

A novel dry synthesis for the uranium(VI) dioxodiacetohydroxamate (UAHA) complex has been developed. The complex was generated in > 80% yield by mechanically grinding solid uranyl acetate dihydrate (UAc) with solid acetohydroxamic acid in stoichiometric amounts. The resulting UO2(AHA)2 ·4H2O solid is purified by washing with acetone. The stoichiometry was confirmed via colorimetric assays for U(VI) and AHA. The analogous ferric trisacetohydroxamate complex (FeAHA) was synthesized for comparison. The UAHA solid was extensively characterized by ultraviolet-visible (UV-vis), Fourier-transform infrared (FT-IR), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The compound did not fluoresce after laser excitation. Proton nuclear magnetic resonance (NMR) spectra were obtained of the complex in D2O, acidified acetonitrile-d3, and DMSO-d6. The solubility was determined over a range of solvents. It was determined that in the purified solid, two bidentate AHA molecules bind to uranyl via the carbonyl and hydroxamate oxygen atoms, a structure analogous to known ferric, nickel, and lanthanum AHA complexes. In an acidic environment, binding is monodentate through the hydroxamate oxygen. And in aqueous solution, the UAHA complex assumes both binding moieties, depending on the pH. This pH-dependent speciation change is demonstrated for the first time. The easy synthesis and purification of UAHA enables researchers to strictly control reaction conditions; to eliminate interfering salts and water; and to study the complex in the solid-phase.