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

Abstract

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.