Speaker
Description
Diglycolamides (DGAs) are an important class of extractants due to their modular chemical structure which enables systematic tuning of steric and electronic properties. They are used in americium selective extraction processes such as the AmSel process to facilitate the separation of trivalent ions from spent nuclear fuel. The TODGA/SO$_3$-Ph-BTBP system, for example, allows selective separation of Am(III) from Cm(III) and lanthanides with separation factors up to SF$_{\text{Cm/Am}}$ = 2.5. Substituting SO$_3$-Ph-BTBP with PrOH-BPTD reduces Am selectivity, whereas replacing TODGA with unsymmetrically substituted diglycolamides (UDGAs) enhances Cm/Am separation. These findings underline the pronounced sensitivity of extraction performance to subtle structural variations and emphasize the need for systematic studies of the underlying coordination chemistry.
We present a comprehensive spectroscopic study of Cm(III) complexation with a series of unsymmetrically substituted UDGA, called R-DdDGA (R = nPr, iPr, nBu and iBu; DdDGA = didodecyldiglycolamide) using time-resolved laser fluorescence spectroscopy. All investigated ligands form isostructural [Cm(UDGA)$_\text{n}$]$^{3+}$ complexes (n = 1-3). Data analysis demonstrates that both alkyl chain length and branching significantly affect complex stability. Propyl-substituted derivatives form more stable complexes than their butyl analogues, indicating that increased steric demand diminishes efficient ligand packing around the metal center. Shorter or suitably branched substituents mitigate repulsive interactions between coordinated ligands and thereby enhance stability. Additionally, the position of branching also plays a decisive role in complex stability. The α-branched iPr derivative forms complexes approx. 2.5 orders of magnitude more stable than those obtained with the β-branched iBu analogue. This observation aligns with extraction data showing reduced distribution ratios for β-branched ligands. In addition to steric effects, electronic contributions are likely involved. α-Branched substituents can transfer electron density more effectively to the amide group. This increases the donor strength of the coordinating oxygen atoms. Despite their strong binding affinity, UDGAs remain sufficiently flexible to enable ternary complex formation in the presence of coordinating anions. In nitrate media, for example, [Cm(UDGA)$_\text{n}$(NO$_3$)]$^{2+}$ (n = 1,2) species are observed. Overall, unsymmetric substitution not only modifies complex stability but also enables additional coordination modes that may be relevant for extraction processes.