Role of Ligand Straining in Complexation of Eu³⁺–Am³⁺ Ions by TPEN and PPDEN, Scalar Relativistic DFT Exploration in Conjunction with COSMO-RS

To search for new ligands suitable for the separation of minor actinides (MA) from lanthanides (Ln) in nuclear waste reprocessing, theoretical (density functional theory) studies were carried out on the complexation (structures, bonding, and thermodynamics) of La3+, Sm3+, Eu3+, and Am3+ complexes with moderately soft donor ligands TPEN [N,N,N′,N′-tetrakis­(2-pyridylmethyl)­ethylenediamine] and PPDEN [N,N,N′,N″,N″-pentakis­(2-pyridylmethyl) diethylenetriamine] in aqueous and nitrobenzene solutions. B3LYP level of theory was used in conjunction with the conductor-like screening model for real systems (COSMO-RS). The metal ions in [M­(NO3)2(TPEN)]­NO3 and [M­(NO3)­(PPDEN)]­(NO3)2 complexes were deca-coordinated with both TPEN and PPDEN. The enthalpy of the complexation with TPEN in an aqueous solution was found to be negative, indicating the exothermic nature of the reaction as observed in the experiments. The calculated values of free energy of complexation follow the experimental trend: Am3+ > Sm3+ > La3+. Furthermore, the calculated free energy with PPDEN is reduced compared to that with TPEN, which may be attributed to the ligand straining during complex formation, which is also reflected in greater residual charges on both the Eu3+ and Am3+ central ions in the complexes of octadentate PPDEN compared to hexadentate TPEN. The experimental complexation selectivity of Am3+ over Eu3+ with TPEN is established by employing COSMO-RS. Furthermore, TPEN is Am3+-selective, whereas PPDEN is Eu3+-selective, which could be exploited for the efficient separation of MA from Ln