H₂O coordination in macropa complexes of f elements (Ac, La, Lu): feasibility of the 11th coordination site

The feasibility of an additional ligand coordination at the 11th coordination site of actinium, lanthanum, and lutetium ions in 10-fold coordinated macropa complexes has been studied by means of density functional theory calculations. The study covered the two main macropa conformers, Δ(δλδ)(δλδ) and Δ(λδλ)(λδλ), favoured by larger (Ac³⁺, La³⁺) and smaller (Lu³⁺) ions, respectively. At the molecular level, the coordination of H₂O is the most favourable to the largest Ac³⁺ while only slightly less to La³⁺. Protonation of the picoline arms enhances the coordination by shifting the metal ion closer to the open site of the ligand. The choice of macropa conformer has only a slight influence on the strength and bonding properties of the H₂O coordination. Aqueous solution environment decreases considerably the energy gain of H₂O coordination at the 11th coordination site.