Actinide(II) Dioxo Stabilization in the Dipyriamethyrin Ligand Environment: A DFT Study

This study aims to understand the properties of mid- to late-actinide dioxo cations, [An^II(η¹-³O₂)]²⁺ (An = Am-Lr) in their +II oxidation state and their interaction with the dipyriamethyrin ligand. The DFT calculations of the total binding energies of the complex isomers with 7 and 8 coordination to the actinide point to the latter as relatively more stable. In the complexes, Am, Cm, and Lr shift to a more stable +III oxidation state, Bk to Es were assigned oxidation states between +II and +III, while Fm to No retained their formal +II oxidation state. The triplet dioxygen transitioned to a doublet superoxide upon complexation, as observed from Mayer bond orders (∼1.5 for O*-O bonds) and spin density (∼0.5 on O). Based on bond lengths and bond orders, the An-N bonds show weak covalency, while the An-O* and An···O bonds exhibited stronger covalent interactions. The calculated thermodynamic parameters indicate the formation of [An(O₂)L] to be spontaneous and exothermic, with [Cm(O₂)L] and [Lr(O₂)L] being the most thermodynamically and energetically feasible. Energy decomposition analysis quantifies more covalent character in the former than the latter. Conversely, [No(O₂)L] is the least stable due to the reduced availability of 6d and 7s orbitals for bonding.