Stabilization of hydrated AcIII cation: the role of superatom states in actinium-water bonding.

225Ac-based radiopharmaceuticals have the potential to become invaluable in designated cancer therapy. However, the limited understanding of the solution chemistry and bonding properties of actinium has hindered the development of existing and emerging targeted radiotherapeutics, which also poses a significant challenge in the discovery of new agents. Herein, we report the geometric and electronic structural properties of hydrated AcIII cations in the [AcIII(H2O) n ]3+ (n = 4-11) complexes in aqueous solution and gas-phase using density functional theory. We found that nine water molecules coordinated to the AcIII cation is the most stable complex due to an enhanced hydration Gibbs free energy. This complex adopts a closed-shell 18-electron configuration (1S 21P 61D 10) of a superatom state, which indicates a non-negligible covalent character and involves H2O → AcIII σ donation interaction between s-/p-/d-type atomic orbitals of the Ac atom and 2p atomic orbitals of the O atoms. Furthermore, potentially existing 10-coordinated complexes need to overcome an energy barrier (>0.10 eV) caused by hydrogen bonding to convert to 9-coordination. These results imply the importance of superatom states in actinide chemistry generally, and specifically in AcIII solution chemistry, and highlight the conversion mechanism between different coordination numbers.