The effect of structural hindrance on the complexation of actinides and lanthanides by ligands containing soft nitrogen donors

A serious limiting factor in the continued development of nuclear power is the disposal of high-level radioactive waste from spent nuclear fuel. One strategy for reducing the long term radiotoxicity of spent nuclear fuel is transmutation of the actinides into shorter-lived fission products by bombardment with fast neutrons. An important complicating factor is the presence of large amounts of lanthanides in dissolved spent nuclear fuel. The lanthanides plus yttrium represent approximately 40% of the mass and 1/3 of the atoms created in U/Pu fission. These lanthanides complicate transmutation because of their high neutron absorption cross sections which limits transmutation efficiency.;The separation of lanthanides (Ln) from actinides (An) is therefore critical to the future of nuclear power. Owing to the chemical similarities of trivalent Ln and An, which is the dominant oxidation state for trans Pu actinides, the mutual separation of the two groups is one of the most challenging tasks in separation science that must exploit small differences in their solution chemistry. The prior literature teaches us that donor atoms softer than oxygen, e.g. N, S, Cl, are most effective for accomplishing this separation. It has also been shown in the literature that more sterically hindered ligands which focus their donor groups in a favorable geometry have an increased overall binding affinity due to the reduction in energy required for preorientation when binding.;In this work a series of ligands containing both "soft" and "hard" nitrogen donors with increased steric "focus" of their donor groups have been synthesized and complexed to transition metals, lanthanides and actinides in solution. The emphasis of this work is to consider the interaction of these sterically "focused" ligands with metal centers, and quantify the effect preorientation has on the relative affinity for binding. This information is necessary in developing a basic understanding of a structure/donor group relationship in designing ligands for the selective extraction or binding of metal cations.