Complexation thermodynamics of diglycolamide with f-elements: solvent extraction and density functional theory analysis

Comparative extraction of trivalent lanthanide and actinide ions (La3+, Eu3+, Lu3+, Am3+ and Cm3+) with tetra-n-octyl diglycolamide (TODGA) was studied and showed the trend: Lu3+ > Eu3+ > Cm3+ > Am3+ > La3+. The structure, bonding, energetic and thermodynamic parameters of the trivalent lanthanide and actinide ions (La3+, Eu3+, Lu3+, Am3+ and Cm3+) with a tridentate ligand, tetra-methyl diglycolamide (TMDGA), are reported in the gas and solvent phases in order to understand their complexation and extraction behaviour. The calculations were performed using the generalized gradient approximated BP86 density functional and the hybrid B3LYP functional using SVP and TZVPP basis sets. The calculated structure obtained at the BP86/SVP level of optimization was found to be in close agreement with the X-ray data and also with the structure obtained at the B3LYP/TZVP level of theory. The free energy of extraction was found to be exergonic for the explicit monomer water model. From the solvent extraction experiment the order of extraction was observed as Lu3+ > Eu3+ > Cm3+ > Am3+ > La3+, which was in line with the trends predicted based on the free energy changes in the gas phase calculations (ΔGgp). The Born–Haber thermodynamic cycle and the COSMO (conductor like screening model) solvation model were applied to calculate the free energy of extraction, ΔGext, of lanthanide and actinide ions in the aqueous–dodecane biphasic system and ΔGext, however, predicted different extraction trends. After dispersion correction (B3LYP-D3), the free energy of extraction for the metal ions was found to follow the order: Lu3+ > Eu3+ > La3+, which was also observed in the solvent extraction experiments. Both COSMO and DCOSMO-RS models predict the same metal ion selectivity trend. Different bonding analyses indicate the electrostatic and less covalent nature of interactions between the ligands and the metal ions.