High-valent actinyl (AnO₂; An = U, Np and Pu) complexation with TEtraQuinoline (TEQ) ligand - a DFT study

A recently synthesised novel macrocycle composed of four quinoline units called TEtraquinoline (TEQ) (J. Am. Chem. Soc., 2023, 145(4), 2609-2618) was reported to exhibit transition metal complexation ability. Meanwhile, there is a growing interest in different binding motifs for radioactive and toxic actinides. In this study, we modelled high-valent actinyl (AnO₂), An = U, Np, Pu; n = 1, 2, 3 complexes of TEQ and studied their geometric and electronic properties using scalar relativistic density functional theory (SR-DFT). The calculated results showed that the equatorial An-N and axial AnO bonds were polar bonds with a high degree of covalence, the former being longer than the latter. Natural bond orbital (NBO) analysis of the An-N bond orders decreased from complexes of uranyl to plutonyl and from complexes of heptavalent to pentavalent actinyls. This was due to the localization of the 5f orbital in the heavier actinyl and the high charge on An. The charge analysis showed a ligand-to-metal charge transfer (LMCT) on complexation. It was interesting to observe metal-to-ligand spin delocalization only in the [An^VI/VIIO₂L]^2+/3+ complexes, where the spin density on An was observed to increase on complexation. Based on the assigned oxidation states, it was observed that the heptavalent neptunyl cation retained its formal high oxidation state on complexation with TEQ. The energetics associated with the formation reaction of all the actinyl-TEQ complexes suggest spontaneity at lower temperatures (i.e., lower than 298.15 K). The energy decomposition analysis (EDA) indicates that the electrostatic energy contributions were predominant in the [An^VO₂L]⁺ complexes, while covalent (orbital) energy contributions were higher in the [An^VIO₂L]²⁺ and [An^VIIO₂L]³⁺ complexes. The extended transition state-natural orbitals for chemical valence (ETS-NOCV) analysis confirmed the prominent covalent character in [An^VIIO₂L]³⁺ over [An^VIO₂L]²⁺ and [An^VO₂L]⁺and the back donation of charges from An to N that stabilizes TEQ.