Coordination Chemistry of Water-Soluble N, N-Dialkyl-Diglycolamic Acid With U(Ⅵ)

Abstract

In this work, the coordination chemistry of the uranyl ion(\begin{document}${\mathrm{UO}}_2^{2+} $\end{document}) with N, N-dimethyl-diglycolamic acid(HDMDGA, HLI) and N, N-diethyl-diglycolamic acid(HDEDGA, HLII) has been systematically studied through thermodynamic and structural analysis, by means of both potentiometric and spectroscopic titration, as well as Raman spectroscopy and X-ray diffractometry. The protonation constants of L−(L=LI, LII) and the stability constants of the uranyl complexes were determined by potentiometric and spectroscopic titrations in 1.0 mol/L NaClO4, with the molar absorption spectra of all species deconvoluted in the process of the latter titration. Besides, the standard Raman spectra of various complexes were confirmed by Raman spectroscopic titration. Furthermore, the single crystal of compound \begin{document}${\mathrm{UO}}_2{\mathrm{L}}^{{\mathrm{II}}}_2 $\end{document} was obtained by slow evaporation of aqueous solution containing uranyl ion and LII ligands at room temperature. X-ray diffraction analysis shows a monoclinic crystal system in P21/c space group, with lattice parameters: a=15.5096(6) Å（1 Å=0.1 nm）, b=7.7934(3) Å, c=11.3732(4) Å, α=90°, β=97.388(4)°, γ=90° and Z=2. Although both ligands show a similar structure and protonation constant, the different values of stability constant in aqueous solution and the different bond length of U-O in two complexes indicate the distinct coordination ability for two ligands with \begin{document}${\mathrm{UO}}_2^{2+} $\end{document}, where HDEDGA performs better. Like the crystal structure of \begin{document}$ {\mathrm{UO}}_2{\mathrm{L}}^{{\mathrm{I}}}_2$\end{document}, the ligand LII in the \begin{document}${\mathrm{UO}}_2{\mathrm{L}}^{{\mathrm{II}}}_2 $\end{document} complex coordinates with \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} in a tridentate manner, with three oxygen atoms(amide oxygen, ether oxygen and carboxylic acid oxygen) coordinating with the linear \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} at the equatorial plane. In addition, the diffuse reflectance spectra of the \begin{document}${\mathrm{UO}}_2{\mathrm{L}}^{{\mathrm{II}}}_2 $\end{document} crystal show characteristics significantly different from the absorption spectra of the corresponding complex in the aqueous solution. Meanwhile, the Raman shift of the characteristic peak of uranyl ion between hydrated uranyl ion and the 1∶1 complex(U∶ LII) is significantly larger than that between 1∶1 and 1∶2. Therefore, it is speculated that the structure of the 1∶2 complex in aqueous solution is supposed to be different from that in the solid crystal.

Keywords

• \begin{document}$ {\mathrm{uo}}_2^{2+}$\end{document}
• n, n-dialkyl-diglycoamic acid
• stability constant
• crystal structure
• raman spectrum