Synthesis and Coordination Chemistry of N-Donor Ligands with early Actinides

Understanding the subtle differences between lanthanide and actinide complexation chemistry with different ligand systems is an ongoing field of resarch. Soft donor ligands, especially ligands containing nitrogen, have shown to be promising for the investigation of the small differences in bonding behavior of actinides compared to lanthanides particularly with regards to the covalent contribution within the bond.

BTP type ligands have been used as an extacting ligand for the separation of lanthanides from actinides.1 Therefore, new soft nitrogen donor ligands based on the BTP type ligand (2,6-Bis(1,2,4-Triazin-3-yl)Pyridine) or Schiff base lig-ands have been synthesized to explore the fundamental chemistry of the early 5f-elements. In order to investigate the coordination environment, ligand selectivity, bonding trends and electronic properties a series of actinide complexes ranging from thorium to plutonium has been characterized in solid state as well as in solution.

In the present study the synthesis of the new soft donor ligand L1 was carried out via a copper mediated click reaction of the corresponding alkyne and azide. The bipyridine based ligand L2 was obtainend in a three step synthesis starting from bipyridine via the corresponding N-oxide and cyanide to give the tetrazine.
Both ligands L1 and L2 were succesfully applied in the complexation reaction with trivalent lanthanides e.g. Er and Sm. In this contribution the investigation of the coordination chemistry of these soft N-donor ligands is exended to the early actinides in their tri- and tetravalent oxidation state.

First experimental results show the formation of a 3:1 complex in the case of trivalent Er. In contrast, the same ligand system L1 forms a 2:1 complex with U(IV) including methanolato and iodo ligands for charge compensation. Further results including first structural characterization as well as results from quantum chemical calculations to elucidate the binding situation, will be presented in this contribution.