An europium(iii) diglycolamide complex: insights into the coordination chemistry of lanthanides in solvent extraction

Abstract

The synthesis, stoichiometry, and structural characterization of a homoleptic, cationic europium(III) complex with three neutral tetraalkyldiglycolamide ligands are reported. The tri(bismuth tetrachloride)tris(N,N,N′,N′-tetra-n-octyldiglycolamide)Eu salt, [Eu(TODGA)₃][(BiCl₄)₃] obtained from methanol was examined by Eu L₃-edge X-ray absorption spectroscopy (XAS) to reveal an inner-sphere coordination of Eu³⁺ that arises from 9 O atoms and two next-nearest coordination spheres that arise from 6 carbon atoms each. A structural model is proposed in which each TODGA ligand with its OC_a–C_b–O–C_b–C_aO backbone acts as a tridentate O donor, where the two carbonyl O atoms and the one ether O atom bond to Eu³⁺. Given the structural rigidity of the tridentate coordination motif in [Eu(TODGA)₃]³⁺ with six 5-membered chelate rings, the six Eu–C_a and six Eu–C_b interactions are readily resolved in the EXAFS (extended X-ray absorption fine structure) spectrum. The three charge balancing [BiCl₄]⁻ anions are beyond the cationic [Eu(TODGA)₃]³⁺ cluster in an outer sphere environment that is too distant to be detected by XAS. Despite their sizeable length and propensity for entanglement, the four n-octyl groups of each TODGA (for a total of twelve) do not perturb the Eu³⁺ coordination environment over that seen from previously reported single-crystal structures of tripositive lanthanide (Ln³⁺) complexes with tetraalkyldiglycolamide ligands (of the same 1 : 3 metal-to-ligand ratio stoichiometry) but having shorter i-propyl and i-butyl groups. The present results set the foundation for understanding advanced solvent extraction processes for the separation of the minor, tripositive actinides (Am, Cm) from the Ln³⁺ ions in terms of the local structure of Eu³⁺ in a solid state coordination complex with TODGA.