Insight into the Complexation of Actinides and Lanthanides with Diglycolamide Derivatives: Experimental and Density Functional Theoretical Studies.

Extraction of actinide (Pu4+, UO22+, Am3+) and lanthanide (Eu3+) ions was carried out using different diglycolamide (DGA) ligands with systematic increase in the alkyl chain length from n-pentyl to n-dodecyl. The results show a monotonous reduction in the metal ion extraction efficiency with increasing alkyl chain length and this reduction becomes even more prominent in case of the branched alkyl (2-ethylhexyl) substituted DGA (T2EHDGA) for all the metal ions studied. Steric hindrance provided by the alkyl groups has a strong influence in controlling the extraction behavior of the DGAs. The distribution ratio reduction factor, defined as the ratio of the distribution ratio values of different DGAs to that of T2EHDGA, in n-dodecane follows the order UO22+ > Pu4+ > Eu3+ > Am3+. Complexation of Nd3+ was carried out with the DGAs in methanol by carrying out UV-vis spectrophotometric titrations. The results indicate a significant enhancement in the complexation constants upon going from methyl to n-pentyl substituted DGAs. They decreased significantly for DGAs containing alkyl substituents beyond the n-pentyl group, which corresponds to the observed trend from the solvent extraction studies. DFT-based calculations were performed on the free and the Nd3+ complexes of the DGAs both in the gas and the solvent (methanol) phase and the results were compared the experimental observations. Luminescence spectroscopic investigations were carried out to understand the complexation of Eu3+ with the DGA ligands and to correlate the nature of the alkyl substituents on the photophysical properties of the Eu(III)-DGA complexes. The monoexponential nature of the decay profiles of the complex revealed the predominant presence of single species, while no water molecules were present in the inner coordination sphere of the Eu3+ ion.