Ph.D. projects

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Synthesis and characterization of actinide complexes with (N,S)-donor ligands

Johannes Balas

Prof. Dr. Thorsten Stumpf, Dr. Robert Gericke (HZDR)

Chemistry of the f-elements

11/2023-11/2026

Actinides (An) is the name given to the elements in the 5f-block of the periodic table of elements. All of the corresponding isotopes are radioactive, whereby those of the transuranium elements are only accessible for experimental investigations through artificial production. Compared to the lanthanides, whose 4f-valence orbitals are more shielded against external influences, the fundamental properties of the actinides differ significantly. Thus, for the early actinides from thorium to plutonium, a variety of oxidation states in the range from +I to +VII are accessible. Furthermore, the 5f-orbitals, whose occupation is the most important property of the actinides, are involved in the formation of chemical bonds. This results in characteristic electronic and magnetic properties, which are particularly relevant for spectroscopic investigation methods.

The behavior of these radionuclides in the environment and the safe treatment and disposal of nuclear waste are of particular interest in actinide chemistry. An important basis for this is a comprehensive understanding of the binding properties of actinides, as well as their reactivity and interactions with environmental systems. These fundamental properties can be derived from studies of An-complexes with organic ligands. However, previous studies on the coordination chemistry of actinides lag far behind those of transition metals and lanthanides and focus mainly on the coordination of thorium and uranium with hard donor atoms according to the HSAB principle. For a better understanding of the electronic and magnetic properties of the 5f-elements, systematic studies of An-complexes with different donor atoms at a fundamental level are therefore necessary.

This work is part of the project FENABIUM-II, funded by the German Federal Ministry of Education and Research (BMBF, 02NUK077B), which focuses on the interactions of f-elements with biologically relevant structural motifs. The aim of the work is to conduct complex syntheses for the early actinides thorium, uranium, neptunium and plutonium in their tetravalent oxidation state with (N,S)-donor ligands and to comprehensively characterize the obtained compounds in solution and solid phase. For example, the bidentate ligands 2-mercaptopyridine and 2-mercapto-1-methylimidazole are used, which contain soft sulfur donors and intermediate nitrogen donors and enable a comparison of the resulting coordinative bonds. In addition to the structural identification of the corresponding An-complexes by single-crystal X-ray diffraction, a variety of methods are used for characterization, such as IR, EA, MS, NMR, CV, HERFD-XANES and SQUID. In addition to quantum chemical calculations, the bonding properties of the 5f-elements can be investigated comparatively and trends along the series of early actinides can be identified. Furthermore, bimetallic complex syntheses, for example with Pd²⁺, using the different HSAB hardnesses of the donor atoms, provide deeper insights into the binding behavior of the early actinides.

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