Coordination Chemistry of U(Ⅵ) With Acetohydroxamic Acid

The deprotonation of acetohydroxamic acid(AHA) and its coordination chemistry with uranyl ion(\begin{document}${\mathrm{UO}}_2^{2+ }$\end{document}) in 1.0 mol/L NaClO4 has been investigated with potentiometric titration and Raman spectroscopy at 25 ℃. For the first time, the deprotonations of AHA in aqueous solutions are quantitatively investigated with potentiometric titration method. It is found that in aqueous solution AHA acts as a monobasic acid and the protonation constants calculated to be pKa1=9.24±0.11(errors are given by the HyperQuad software fitting calculations). The determined pKa1 is in good agreement with the previously reported one-step protonation. In a wide pH range from acidic to basic aqueous solutions, the two step-wise protonation of AHA has been found, at the same time, the complexation of U(Ⅵ) with AHA has been re-visited. For AHA at different degrees of deprotonation, various complexes of U(Ⅵ) with AHA at ratios of U(Ⅵ) and AHA from 1∶1 to 1∶3 are recognized by potentiometric titration method. Amongst, the 1∶1 set of complexes consists of UO2A+ and UO2(OH)A, the 1∶2 includes UO2A2 and UO2(OH)\begin{document}${\mathrm{A}}_2^{-} $\end{document}, and the 1∶3 is composed of three complexes UO2\begin{document}${\mathrm{A}}_3^{-} $\end{document}, UO2(OH)\begin{document}${\mathrm{A}}_3^{2-} $\end{document} and UO2(OH)2\begin{document}${\mathrm{A}}_3^{3-} $\end{document}. There are maybe more complex species formed during the titrations, especially in very basic solutions, but those cannot be well quantitatively characterized due to their very low solubilities. The stability constants of the mentioned seven species have been determined by HyperQuad software, with the values of UO2A+ and UO2A2 in good consistence with those reported previously. All other five new-detected complexes are detected in solutions of relatively higher pH values. In the potentiometric titrations at relatively low AHA concentration, both protons of AHA can be deprotonated to form higher proportioned UO2(OH)A in comparison with UO2A2, which is an equal species to UO2A+ in term of releasing/consuming H+ during the titration process. At greatly excessive concentrations of AHA, three complexes, UO2A+, UO2A2, and UO2\begin{document}${\mathrm{A}}_3^{-} $\end{document} are successively formed with only one proton deprotonated A− as the pH being increased; further, two species, UO2(OH)\begin{document}${\mathrm{A}}_3^{2-} $\end{document} and UO2(OH)2\begin{document}${\mathrm{A}}_3^{3-} $\end{document}, are formed with partial of the ligands in the form of completely deprotonated (OH)A2−. The Raman spectroscopic titrations show that the Raman shift of O=U=O moves to lower wavenumbers with the increase in the number of ligands, with 849 cm−1, 828 cm−1 and 807 cm−1 corresponding to the three sets of U(Ⅵ) to AHA ratios of 1∶1, 1∶2, and 1∶3, respectively. Moreover, the results demonstrate that the effect of deprotonation on the Raman shifts of uranyl ion is negligible, but the effect on the Raman intensity is significant. The almost same step-wise Raman shifts at about 21 cm−1 for the three sets of complexes, 1∶1, 1∶2 and 1∶3, indicates a similar coordination mode for all the AHA ligands with different degrees of deprotonation, which provides informative hints for the structures of the complexes.