Aqueous Structure of Lanthanide–EDTA Coordination Complexes Determined by a Combined DFT/EXAFS Approach

Abstract

Sustainable production of rare earth elements (REEs) is critical for technologies needed for climate change mitigation, including wind turbines and electric vehicles. However, separation technologies currently used in REE production have large environmental footprints, necessitating more sustainable strategies. Aqueous, affinity-based separations are examples of such strategies. To make these technologies feasible, it is imperative to connect aqueous ligand structure to ligand selectivity for individual REEs. As a step toward this goal, we analyzed the extended X-ray absorption fine structure (EXAFS) of four lanthanides (La, Ce, Pr, and Nd) complexed by a common REE chelator, ethylenediaminetetraacetic acid (EDTA) to determine the aqueous-phase structure. Reference structures from density functional theory (DFT) were used to help fit the EXAFS spectra. We found that all four Ln-EDTA coordination complexes formed 9-coordinate structures with 6 coordinating atoms from EDTA (4 carboxyl oxygen atoms and 2 nitrogen atoms) and 3 oxygen atoms from water molecules. All EXAFS fits were of high quality (R-factor < 0.02) and showed decreasing average first-shell coordination distance across the series (2.62–2.57 Å from La–Nd), in agreement with DFT (2.65–2.56 Å from La–Nd). The insights determined herein will be useful in the development of ligands for sustainable rare earth elements (REE) separation technologies.

Structural Changes to the Gd‐DTPA Complex at Varying Ligand Protonation State

“…[39] Similar coordination bonds (water, amine monodentate carboxylate) have been observed in Ln-EDTA complexes with EXAFS measurements. [40,41] The same coordination structure was observed in our classical MD simulations (Figure 3A), as well as in previous classical MD studies of the [Gd 3 + -DTPA 5À • (H 2 O) n ] 2À complex. [23,24] With a longer time measured in the classical MD simulations, water exchange events were sampled with different water molecules entering and leaving the coordination sphere throughout the trajectory.…”

“…The differing structures arise from shuffling of the positions of the terminal carboxylates between two staggered configurations, and this dynamic exchange process is in agreement with results reported from 1 H and 13 C NMR experiments. [40][41][42] The complex remained in only one conformation throughout the AIMD simulation, though this is expected given its shorter simulation time.…”

Elucidating the Structure of the Eu‐EDTA Complex in Solution at Various Protonation States

“…This supports our classical MD protocol and parameters (see Methods), as this coordination structure is known from X‐ray crystallography, [22] from previous EXAFS measurements, [24] from AIMD simulations, [25,31,32] as well as from previous classical MD simulations [23] . A similar coordination structure is observed for deprotonated Ln‐EDTA complexes with the early Ln 3+ ions [33] …”

“…[23] A similar coordination structure is observed for deprotonated Ln-EDTA complexes with the early Ln 3 + ions. [33] The classical MD simulation for the [Eu 3 + -EDTA 4À • (H 2 O) 3 ] À complex was used to generate a predicted EXAFS spectrum to be compared to the measured experimental EXAFS spectrum shown in Figure 4A. The classical MD spectrum shows that the amplitudes, peak positions, and other minor features mimic those of the measured spectrum.…”

Modeling Lanthanide Ions in Solution: A Versatile Force Field in Aqueous and Organic Solvents

“…The 12-6-4 model did not take into account the strong repulsion between water molecules in the first coordination sphere of cations, resulting in a systematic overestimation of the coordination number (CN), particularly for the most charged cations . Furthermore, in the case of lanthanides, the distances of water molecules in the first coordination sphere do not correctly reproduce the extended X-ray absorption fine structure (EXAFS) values available in the literature, − since the models were adjusted to reproduce Marcus’ distances corresponding to XRD distances even though EXAFS provides more accurate distances. ,, These deviations are more than just numerical differences; they reverberated throughout the lanthanide series, influencing the accuracy of simulations across the series.…”