Characterization of tetravalent actinide complexes with imine type ligands (salen and its derivative)


Characterization of tetravalent actinide complexes with imine type ligands (salen and its derivative)

Radoske, T.; März, J.; Kaden, P.; Walter, O.; Stumpf, T.; Ikeda-Ohno, A.

Although the geochemistry of uranium is typically dominated by uranium(VI) (as UO22+), tetravalent uranium (U(IV)) could also occur under the geochemical conditions relevant to uranium mines or geological repositories of radioactive wastes, particularly under anaerobic conditions. In this context, the geochemical behaviour of U(IV) should be fully understood for the reliable safety/environmental assessment of such nuclear-related activities. The complexation and speciation of metal ions in groundwater is affected not only by inorganic ligands, but also many other natural organic ligands with a wide variety of different functional groups. Among the functionalities of naturally occurring organic ligands, oxygen(O)- and nitrogen(N)-donor groups play the most significant role in interacting with metal ions. The interaction of U(IV) with O- and N-donor ligands is, therefore, one of the fundamental information to understand the geochemical behaviour of uranium. Given this fact, this study focuses on the complexation of U(IV), as well as another tetravalent actinide of Th(IV), with the imine ligand salen and its derivative (Figure 1), which possess both O- and N-donor groups in the structure, as simplified model of naturally relevant organic O-/N-donor ligands. The complex structure in the solid state was determined by single crystal X-ray diffraction (SC-XRD), while the complexation in solution was studies by NMR and UV-visible absorption spectroscopy.
A series of single crystals of the U(IV)-salen complexes were obtained as a function of M:L ratio and pH by a liquid-liquid diffusion method. SC-XRD measurements on the obtained crystals revealed the new crystal structures, all showing the eight-fold coordination of the U centre with a trigonal dodecahedral geometry with the ligand on the primary coordination sphere of U.
UV-visible absorption measurements of U(IV)-salen solution as a function of M:L ratio indicate the existence of two independent solution species in the system, assigning as the U(IV)-salen complexes with the M:L ratios of 1:1 and 1:2. 1H-NMR spectra of solution samples in which the complex [UIV(Le)2] was dissolved indicate that the protons on the bridging ethyl group between the two imine groups show a significant high-field shift due to the paramagnetic effect of uranium(IV). The spectra also indicate that the metal is positioned at the centre of the coordination polyhedron formed by the two ligand molecules.

Keywords: uranium; imine; ligands; complex; thorium

  • Lecture (Conference)
    Migration 2017, 10.-15.09.2017, Barcelona, Spain

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