Computational study of core modified dipyriamethyrin for the competitive complexation of Am3+/Cm3+ from their trichlorides

In the process of handling and storage of radioactive actinides it is essential to selectively sequester the minor actinides, such as Am and Cm, through a competitive complexation process. Herein we computationally designed two core modified ligands (L2(1-) and L3) through systematic oxygen substitution at the NH sites of dipyriamethyrin (L1_2H), a hexadentate expanded porphyrin, and studied their competitive complexation towards trivalent actinides (An = Am/Cm) from their trichlorides using density functional theory (DFT). We observed shorter An-N bonds and longer An-O bonds in complexes based on core modified ligands (L2(1-) and L3). The An-Cl bond length increases with increasing axial coordination number (i.e., from L1(2-) to L3) to accommodate the ligands. All the bonds were identified to be electrostatic in nature. L1(2-) exhibits shorter bonds and larger bond orders on complexing with Am than with Cm. On moving from complexes of L2(1-) to L3, the An-N bond lengths are shortened, while An-O bond lengths become larger. Between the complexes of Am and Cm, there is marginal difference in their bond distances with L2(1-) and L3. Charge analysis shows ligand to metal charge transfer during coordination, with back-donation from An to N/O and Cl. The calculated spin-density analysis indicates that An remains in its trivalent oxidation state on complexation, while orbital occupation analysis shows that the 5f and 6d orbitals are involved in bonding; this was confirmed by molecular orbital (MO) analysis that shows the complexes of L2(1-) and L3 to exhibit higher degeneracy in their overlapping MOs. Further, the energy decomposition analysis (EDA) confirms that all ionic bonds are primarily due to electrostatic contributions, where the orbital contributions increase from L1(2-) to L3 complexes and maximum covalency was observed in Cm complexes due to the energy matching between the 5f orbitals of Cm and the 2p orbitals of N and Cl, compared to Am. To confirm the competitiveness in the complexation of the ligand towards Am vs. Cm, the thermodynamic parameters were analysed for the ligand and metal substitution reactions. L1(2-) shows more affinity towards Am than Cm, while L2(1-) and L3 prefer Cm.