From Actinide Bonding to Extraction Performance: Trends in An(IV) N-Heterocyclic Diamide Complexes

Abstract

Understanding periodic trends in actinide chemistry is central to the rational design of advanced separation processes in the nuclear fuel cycle. Here we present a systematic investigation of actinide complexes supported by the N-heterocyclic diamide ligand, integrating crystallographic, spectroscopic, theoretical, and solvent-extraction studies within a unified coordination framework. Single-crystal X-ray diffraction reveals a series of ( 4Et DABipy)An( NO3)4 (An = Th, U, Np, Pu) complexes, with Np and Pu structures representing the first structurally characterized Np(IV) and Pu(IV) species in a mixed N,O-donor environment with coordination number 12. Structural parameters display a monotonic contraction along the actinide series, with the largest variation between Th and U and only minor changes across U-Pu. Quantum-chemical analysis using QTAIM metrics indicates predominantly electrostatic metal-ligand interactions, with increased ionicity from Th to U and nearly indistinguishable bonding characteristics among U-Pu complexes. Spectroscopic studies confirm that the neutral complexes persist in solution and correspond to the species extracted into the organic phase. Despite the structural and bonding similarities, solvent extraction experiments reveal a decrease in extraction efficiency in the order U(IV) > Np(IV) > Pu(IV), together with pronounced time dependence that enables non-equilibrium separation. Inclusion of U(VI) affords a separation factor of up to 22.6 for the U(VI)/Pu(IV) pair, demonstrating the potential of N-heterocyclic diamide systems for U/Pu separation without a redox step. The remarkable molecular-level discrimination exhibited by the employed ligand framework, allows separation of f-block elements with nearly identical structural and electronic characteristics, guiding the design of next-generation separation strategies.