Lanthanide Speciation in Potential SANEX and GANEX Actinide/Lanthanide Separations Using Tetra-N-Donor Extractants

Lanthanide­(III) complexes with N-donor extractants, which exhibit the potential for the separation of minor actinides from lanthanides in the management of spent nuclear fuel, have been directly synthesized and characterized in both solution and solid states. Crystal structures of the Pr³⁺, Eu³⁺, Tb³⁺, and Yb³⁺ complexes of 2,9-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe₄-BTPhen) and the Pr³⁺, Eu³⁺, and Tb³⁺ complexes of 6,6′-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2′-bypyridine (CyMe₄-BTBP) were obtained. The majority of these structures displayed coordination of two of the tetra-N-donor ligands to each Ln³⁺ ion, even when in some cases the complexations were performed with equimolar amounts of lanthanide and N-donor ligand. The structures showed that generally the lighter lanthanides had their coordination spheres completed by a bidentate nitrate ion, giving a 2+ charged complex cation, whereas the structures of the heavier lanthanides displayed tricationic complex species with a single water molecule completing their coordination environments. Electronic absorption spectroscopic titrations showed formation of the 1:2 Ln³⁺/L_N₄‑donor species (Ln = Pr³⁺, Eu³⁺, Tb³⁺) in methanol when the N-donor ligand was in excess. When the Ln³⁺ ion was in excess, evidence for formation of a 1:1 Ln³⁺/L_N₄‑donor complex species was observed. Luminescent lifetime studies of mixtures of Eu³⁺ with excess CyMe₄-BTBP and CyMe₄-BTPhen in methanol indicated that the nitrate-coordinated species is dominant in solution. X-ray absorption spectra of Eu³⁺ and Tb³⁺ species, formed by extraction from an acidic aqueous phase into an organic solution consisting of excess N-donor extractant in pure cyclohexanone or 30% tri-<i>n</i>-butyl phosphate (TBP) in cyclohexanone, were obtained. The presence of TBP in the organic phase did not alter lanthanide speciation. Extended X-ray absorption fine structure data from these spectra were fitted using chemical models established by crystallography and solution spectroscopy and showed the dominant lanthanide species in the bulk organic phase was a 1:2 Ln³⁺/L_N‑donor species