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 (^4EtDABipy)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. Beyond tetravalent actinide separations, the investigated ligand system demonstrates a separation factor of up to 22.6 for the U(VI)/Pu(IV) pair, highlighting 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.