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

Abstract

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 Ac^III cations in the [Ac^III(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 Ac^III 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 → Ac^III 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 Ac^III solution chemistry, and highlight the conversion mechanism between different coordination numbers.

Fig. 1. Optimized geometries of Ac^III aquo complexes under gas-phase (gas) and aqueous solution (aq) conditions at the PBE-D3/TZ2P level of theory. Water molecules marked with green circles belong to the second hydration shell.

Fig. 2. Electronic structure diagram of the [Ac^III(H₂O)₉]³⁺ complex in gas-phase at the PBE-D3/TZ2P level of theory. The MO energies of the H₂O molecule and (H₂O)₉ fragment correspond to the left Y axis, and the MO energies of the Ac³⁺ cation and [Ac^III(H₂O)₉]³⁺ complex are shown on the right Y axis.

Fig. 3. Shapes of deformation densities (isovalue = 0.0005 a.u.) between the interacting fragments of (H₂O)₉ and Ac^III from EDA-NOCV analysis. The cyan and green colors represent density inflow and outflow respectively.

Fig. 4. Electronic structure diagrams of ^gas[Ac^III(H₂O)₉(H₂O)₁]³⁺, ^aq[Ac^III(H₂O)₁₀]³⁺, ^gas[Ac^III(H₂O)₉(H₂O)₂]³⁺, ^aq[Ac^III(H₂O)₁₀(H₂O)₁]³⁺ complexes at the PBE-D3/TZ2P level of theory. The energies of ^aq[Ac^III(H₂O)₁₀]³⁺ and ^aq[Ac^III(H₂O)₁₀(H₂O)₁]³⁺ correspond to the left Y axis, and the energies of ^gas[Ac^III(H₂O)₉(H₂O)₁]³⁺ and ^gas[Ac^III(H₂O)₉(H₂O)₂]³⁺ complex correspond to the right Y axis.

Fig. 5. Relaxed potential energy surface scans for the distance between O atom and Ac^III cation at the PBE-D3/TZ2P level of theory: (a) for ^gas[Ac^III(H₂O)₉(H₂O)₁]³⁺, (b) for ^aq[Ac^III(H₂O)₁₀]³⁺, (c and d) for ^aq[Ac^III(H₂O)₁₀(H₂O)₃₅]³⁺. In the ^aq[Ac^III(H₂O)₁₀(H₂O)₃₅]³⁺ complex, (c and d), two typical O atoms (O8 and O5, see the detailed structural information in Fig. S5 of ESI†) were chosen in the first hydration shell.

Fig. 6. Typical geometries of the relaxed scan for the Ac–O_water bond length in the ^gas[Ac^III(H₂O)₉(H₂O)₁]³⁺ and ^aq[Ac^III(H₂O)₁₀]³⁺ complexes (see Fig. 5).