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Highly selective rare-earth separation has become increasingly important due to the indispensable role of these elements in various cutting-edge technologies including clean energy. However, the similar physicochemical properties of rare-earth elements (REEs) render their separation very challenging, and the development of new selective receptors for these elements is potentially of very considerable economic and environmental importance. Herein, we report the development of a series of 4-phosphoryl pyrazolone receptors for the selective separation of trivalent lanthanum, europium, and ytterbium as the representatives of light, middle, and heavy REEs, respectively. X-ray crystallography studies were employed to obtain solid-state structures across 11 of the resulting complexes, allowing comparative structure-function relationships to be probed, including the effect of lanthanide contraction that occurs along the series from lanthanum to europium to ytterbium and which potentially provides a basis for REE ion separation. In addition, the influence of ligand structure and lipophilicity on lanthanide binding and selectivity was systematically investigated via n -octanol/water distribution and liquid-liquid extraction (LLE) studies. Corresponding stoichiometry relationships between solid and solution states were well established using slope analyses. The results provide new insights into some fundamental lanthanide coordination chemistry from a separation perspective and establish 4-phosphoryl pyrazolone derivatives as potential practical extraction reagents for the selective separation of REEs in the future.

Structural investigations of three actinide(iv) 4-phosphoryl 1H-pyrazol-5-olate complexes (An = Th(iv), U(iv), Np(iv)) and their cerium(iv) analogue display the same metal coordination in the solid state. The mononuclear complexes show the metal centre in a square antiprismatic coordination geometry composed by the two O-donor atoms of four deprotonated ligands. Detailed solid state analysis of the U(iv) complex shows that dependent on the solvent used altered arrangements are observable, resulting in a change in the coordination polyhedron of the U(iv) metal centre to bi-capped trigonal prismatic. Further, single crystal analyses of the La(iii) and Ce(iii) complexes show that the ligand can also act as a neutral ligand by protonation of the pyrazolyl moiety. All complexes were comprehensively characterized by NMR, IR and Raman spectroscopy. A single resonance in each of the 31P NMR spectra for the La(iii), Ce(iii), Ce(iv), Th(iv) and Np(iv) complex indicates the formation of highly symmetric complex species in solution. Extended X-ray absorption fine structure (EXAFS) investigations provide evidence for the same local structure of the U(iv) and Np(iv) complex in toluene solution, confirming the observations made in the solid state.

To reduce high-level radiotoxic waste generated by nuclear power plants, highly selective separation agents for minor actinides are mandatory. The mixed N,O-donor ligand N,N,N',N' -tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (H 4 TPAEN; 1 ) has shown good performance as a masking agent in Am 3+ /Eu 3+ separation studies. Adjustments on the pyridyl backbone to raise the hydrophilicity led to a decrease in selectivity and a decrease in M 3+ -N am interactions. An enhanced basicity of the pyridyl N-donors was given as a cause. In this work, we examine whether a decrease in O-donor basicity can promote the M 3+ -N am interactions. Therefore, we replace the deprotonated "charged" carboxylic acid groups of TPAEN 4- by neutral amide groups and introduce N,N,N',N' -tetrakis[(6- N″,N' '-diethylcarbamoylpyridin-2-yl)methyl]ethylenediamine (TPAMEN; 2 ) as a new ligand. TPAMEN was crystallized with Eu(OTf) 3 and Eu(NO 3 ) 3 ·6H 2 O to form positively charged 1:1 [Eu(TPAMEN)] 3+ complexes in the solid state. Alterations in the M-O/N bond distances are compared to [Eu(TPAEN)] - and investigated by DFT calculations to expose the differences in charge/energy density distributions at europium(III) and the donor functionalities of the TPAEN 4- and TPAMEN. On the basis of estimations of the bond orders, atomic charges spin populations, and density of states in the Eu and potential Am and Cm complexes, the specific contributions of the donor-metal interaction are analyzed. The prediction of complex formation energy differences for the [M(TPAEN)] - and [M(TPAMEN)] 3+ (M 3+ = Eu 3+ , Am 3+ ) complexes provide an outlook on the potential performance of TPAMEN in Am 3+ /Eu 3+ separation.