Exothermic and Spontaneous Covalent Interaction of Trivalent Lanthanides with Diglycolamide and Carbamoylmethylphosphine Oxide-Functionalized Ionic Liquids: Spectroscopic, Electrochemical, and Computational Investigations

Diglycolamides (DGAs) demonstrated an exceptionally higher affinity for trivalent cations compared to tetravalent ones, attributed to their aggregation behavior, in contrast to the conventional functionalities such as carbamoylmethylphosphine oxide (CMPO), exhibiting a reversal in the affinity. The differing complexation capabilities of the functionalized ionic liquids with DGA and CMPO moieties are reflected in the distinct topology of the electron distribution. Notable additional orbital contributions to the inner s, d, and f subshells of lanthanides suggest a significant covalent character of the complexes. The covalency of the metal-O bonds in the DGA-functionalized ionic liquids was higher than that of the CMPO-functionalized ionic liquids, indicated by Judd-Ofelt parameters. The Eu³⁺complex of CMPO-TSIL was more asymmetric with one inner sphere water molecule compared to the DGA-TSIL complex with two inner sphere water molecules. The DGA-functionalized ionic liquids formed 1:1, 1:2, and 1:3 M:L species, whereas CMPO-TSIL predominantly formed 1:3 and 1:4 species. The complexation was spontaneous and exothermic. Higher complexation constants for Eu³⁺ compared to those of Nd³⁺ were attributed to lanthanide contraction. Shifts in the carbonyl (>C═O) peaks of both DGA-TSIL and CMPO-TSIL confirmed the involvement of carbonyl functionalities in the complexation process. Shifts in the cathodic peak potentials and a reduction in the diffusion coefficient on complexation have been evidenced.