Synthesis of Nonadentate Ligand Diethylene Glycol-Bis(3-Aminopropyl Ether)-<i>N</i>,<i>N</i>,<i>N′</i>,<i>N′</i>-Tetraacetic Acid DEGTA and Its Complexation Behavior toward Trivalent Lanthanides and Actinides

A new nonadentate ligand, DEGTA (diethylene glycol-bis(3-aminopropyl ether)-<i>N</i>,<i>N</i>,<i>N'</i>,<i>N'</i>-tetraacetic acid), from the polyaminopolycarboxylate family, was synthesized in a two-step reaction. The ligand's pH-dependent behavior (structure and p<i>K</i>_a values) was determined by nuclear magnetic resonance (NMR) spectroscopy. The complexation ability of the ligand toward trivalent lanthanides and actinides was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) using Eu(III) and Cm(III) as representatives. For Eu(III), two species occurring at different pH values were observed and corroborated by concentration- and pD-dependent NMR-titration series, viz. [EuH₂(DEGTA)]⁺ and [Eu(DEGTA)]^-. The latter is shown to be nine-coordinate, forming isostructural complexes with Cm(III) and Sm(III) as inferred from TRLFS and 2D NMR experiments, respectively. Since DEGTA can be seen as a consecutive derivative of EDTA and EGTA with an elongated backbone, the structures of their Eu(III) complexes were calculated using density functional theory (DFT) and the same aminoacetate binding motif proven by Fourier-transform infrared (FT-IR) spectroscopy. Upon comparison of structure-property relationships (denticity and chain length vs coordination geometry and complex stability) one can draw conclusions on DEGTA's complexation behavior in particular, and some generalizable trends in complexation properties within the complexone series are discussed. Looking further ahead, this knowledge will help in further developing decontamination, decommissioning, and decorporation strategies.