Density functional theory studies of actinide(III) motexafins (An-Motex²⁺, An = Ac, Cm, Lr). Structure, stability, and comparison with lanthanide(III) motexafins

Newly developed relativistic energy-consistent 5f-in-core actinide pseudopotentials and corresponding (7s6p5d1f)/ [5s4p3d1f] basis sets in the segmented contraction scheme, combined with density functional theory methods, have been used to study the molecular structure and chemical properties of selected actinide(III) motexafins (An-Motex²⁺, An = Ac, Cm, Lr). Structure and stability are discussed, and a comparison to the lanthanide(III) motexafins (Ln-Motex²⁺, Ln = La, Gd, Lu) is made. The actinide element is found to reside above the mean N₅ motexafin plane, and the larger the cation, the greater the observed out-of-plane displacement. It is concluded that the actinium(III), curium(III), and lawrencium(III) cations are tightly bound to the macrocyclic skeleton, yielding stable structures. However, the calculated metal-ligand gas-phase binding energy for An-Motex²⁺ is about 1-2 eV lower than that of Ln-Motex²⁺, implying a lower stability of An-Motex²⁺ compared to Ln-Motex²⁺. Results including solvent effects imply that Ac-Motex²⁺ is the most stable complex in aqueous solution and should be the best candidate for experimentalists to get stable actinide(III) motexafin complexes.