Quest for environmentally benign ligands for actinide separations: thermodynamic, spectroscopic, and structural characterization of U(VI) complexes with Oxa-diamide and related ligands

Complexation of U(VI) with N,N,N',N'-tetramethyl-3-oxa-glutaramide (TMOGA) and N,N-dimethyl-3-oxa-glutaramic acid (DMOGA; see picture) was studied in comparison with their dicarboxylate analogue, oxydiacetic acid (ODA). The stability constants, enthalpy, and entropy of complexation all decrease in the order ODA> DMOGA>TMOGA, showing that the complexation is entropy driven and the substitution of a carboxylate group with an amide group reduces the strength of complexation with U(VI) due to the decrease in the entropy of complexation (see figure).Complexation of U(VI) with N,N,N',N'-tetramethyl-3-oxa-glutaramide (TMOGA) and N,N-dimethyl-3-oxa-glutaramic acid (DMOGA) was studied in comparison with their dicarboxylate analogue, oxydiacetic acid (ODA). Thermodynamic parameters, including stability constants, enthalpy, and entropy of complexation, were determined by spectrophotometry, potentiometry, and calorimetry. Single-crystal X-ray diffraction, EXAFS spectroscopy, FT-IR absorption spectroscopy, and laser-induced luminescence spectroscopy were used to obtain structural information on the U(VI) complexes. Like ODA, TMOGA and DMOGA form tridentate U(VI) complexes, with three oxygen atoms (the amide, ether, and/or carboxylate oxygen) coordinating to the linear UO(2) (2+) ion through the equatorial plane. The stability constants, enthalpy, and entropy of complexation all decrease in the order ODA>DMOGA>TMOGA, showing that the complexation is entropy driven and the substitution of a carboxylate group with an amide group reduces the strength of complexation with U(VI) due to the decrease in the entropy of complexation. The trend in the thermodynamic stability of the complexes correlates very well with the structural and spectroscopic data obtained by single-crystal X-ray diffraction, FT-IR spectroscopy, and laser-induced luminescence spectroscopy.