Insights into the Extraction of Actinides from Lanthanides Using 3,3'-Dimethoxy-phenyl-bis-1,2,4-triazinyl-2,6-pyridine Ligand - A DFT Study

Recently reported 3,3'-dimethoxy-phenyl-bis-1,2,4-triazinyl-2,6-pyridine (DMOP- BTP) ligand is recognized as an efficient candidate for the stripping of actinides (An) from lanthanides (Ln) in nuclear wastes in nitric acid medium. However, the complexation behaviour and selectivity of this DMOP-BTP ligand towards An(III)/Ln(III) is still unclear. In order to understand the origin of complexation of this ligand, a series of metal ions (M3+=Ce, Nd, Eu, U, Am, Cm) have been chosen and its binding ability to form ML(NO3)(3) (L=DMOP- BTP) complex is studied using scalar relativistic ZORA/DFT calculations. Metal -Nitrogen bond lengths obtained from optimized geometries reveal that An(III) prefer binding to DMOP-BTP ligand compared to Ln(III). To elucidate the metal-ligand bonding nature, Mayer and Nalewajsk- Mrozek (NM) bond order analyses have been carried out. Quantum Theory of Atoms In Molecules (QTAIM) and Energy Decomposition Analysis (EDA) have been done using the optimized geometries to comprehend the bonding situation in these ML(NO3)(3) complexes. Results reveal that the bonds of An(III) ions tend to have a higher covalency compared to that of their lanthanide ions. Furthermore, it is understood that the NO3 ions around the metal ion also take part in interaction with the ligand and help in the stabilization of the metal complex. Thus, this work throws light on the preferential selectivity of the DMOP- BTP ligand for An(III) ions over Ln(III) ions.