Interaction of Tetravalent Actinides (An(IV)) with Mixed N/O-Donor Imine Type Ligands

Because of their unique electronic properties originating from 5f-orbitals, the coordination chemistry of actinides (An) is still an attractive research field in terms not only of nuclear engineering but also of basic chemistry. In particular, the early An show profound complex chemistry due to a wide variety of possible oxidation states ranging from +II to +VII, which is in contrast to the dominant trivalent state for their chemical analog of lanthanides. The aim of our research activities is to gain knowledge about the interaction of An with a variety of hard- and soft-donor ligands, eventually providing a comprehensive understanding of the electronic nature of actinide compounds. To this end, the focus of this study lies on the characterization of Th(IV) and U(IV) complexes with the imine ligand salen and its derivative (Figure 1). The ligands possess both O- (i.e. hard) and N-donor (soft) groups in the structure, which could be also considered as a simplified model of naturally relevant organic O-/N-donor ligands.

Results and Discussion
A series of single crystals of the U(IV)-salen complexes were obtained as a function of M:L ratio and pH by liquid-liquid diffusion methods. 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 ligands 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 the dissolved complex [UIV(Le)2] and the pure ligand in solution were recorded. All expected multiplets of the complex can be clearly identified and confirm complexation. The spectra also showed a significant high-field shift due to proximity of the paramagnetic uranium(IV) centre, indicating that the metal is positioned at the centre of the coordination polyhedron formed by the two ligand molecules.