Multiscale investigations of europium(iii) complexation with tetra-n-octyl diglycolamide confined in porous solid supports

Abstract

The microscopic, short-range coordination environments and mesoscopic, long-range structures of Eu³⁺ in contact with tetra-n-octyl diglycolamide (TODGA) confined on ordered mesoporous carbon (OMC) nanoparticles and Amberchrom CG-71 resin were probed using Eu L₃-edge extended X-ray absorption fine structure (EXAFS) as well as small-angle and wide-angle X-ray scattering, SAXS and WAXS, respectively. A homoleptic Eu(TODGA)₃³⁺ coordination complex typical of liquid–liquid extraction (LLE) chemistry is present under low Eu³⁺ loading conditions on both solid supports. Deviations from this traditional structure motif appear at hyperstoichiometric Eu³⁺ loadings, above the 1 : 3 Eu³⁺ to TODGA mole ratio. Microcrystalline-like domains with multinuclear Eu speciation are templated by use of these high Eu³⁺ loading conditions with the functionalized OMC materials, highlighting a major departure of liquid–solid extraction chromatography (EXC) from the coordination chemistry of LLE. No such long-range spatial coherence was observed for analogous polyacrylic resin materials. These results demonstrate both the similarities and differences between multiscale structures in LLE and EXC, underscoring the opportunities for improved separation techniques based on solid supports with long-range spatial coherence (e.g., OMC systems) and without it (e.g., resin materials). Since crystallization was the first approach to adjacent lanthanide separations, the prospect of templating microcrystalline TODGA–lanthanide complexes that are otherwise not prone to crystallization by use of OMC nanoparticles represents a new entry to addressing long-standing issues in purification by liquid–solid phase separation.