Rigidified cyclic diglycolamides: coordination and extraction of trivalent f-elements

Nuclear energy contributes to reducing greenhouse gas emissions, but the reprocessing of spent nuclear fuel generates high-level waste that requires safe and efficient management. Diglycolamides (DGAs) are promising CHON-compliant, non-selective extractants widely investigated for actinide and lanthanide separation. Their extraction performance is strongly dependent on ligand structure, particularly on conformational flexibility and preorganization. Structural preorganization through cyclization of DGA-based ligands represents an effective strategy to modulate coordination behavior and extraction properties. In this work, a series of new cyclic diglycolamide ligands (L1-L4) were designed using computational modeling and subsequently synthesized. Their coordination behavior was investigated both in solution by NMR titration and in the solid state by single-crystal X-ray diffraction. Their extraction performance toward trivalent f-elements was systematically evaluated. For the first time, the formation of lanthanum complexes with the composition [LaL(NO₃)₄]₂[LaL₂(NO₃)(H₂O)₂], corresponding to 1 : 1 and 2 : 1 ligand-to-metal stoichiometries, was identified for DGA-based ligands. These findings demonstrate that ligand cyclization significantly affects coordination modes and extraction behavior, highlighting the potential of cyclic DGAs as extractants for nuclear waste management.