Predicted M(H2)12n+ (M = Ac, Th, Pa, U, La and n = 3, 4) complexes with twenty-four hydrogen atoms bound to the metal ion

MOFs with 12-Coordinate 5f-Block Metal Centers

We have constructed an unprecedented MOF platform that accommodates a range of 5f-block metal ions (Th⁴⁺, U⁴⁺, Np⁴⁺, Pu⁴⁺) as the primary building block. The isoreticular actinide metal-organic frameworks (An-MOFs) exhibit periodic trends in the 12-coordinate metal environment, ligand configuration, and resulting ultramicroporosity. It holds potential in distinguishing neighboring tetravalent actinides. The metal ionic radius, carboxylate bite angle, anthracene plane twisting, interligand interactions, and countercation templating collectively determine an interplay between solvation, modulation, and complexation, resulting in a coordination saturation of the central actinide, while lanthanide counterparts are stabilized by the formation of a dimer-based motif. Quantum chemical calculations indicate that this large coordination number is only feasible in the high-symmetry environment provided by the An-MOFs. This category of MOFs not only demonstrates autoluminescence (4.16 × 10⁴ counts per second per gram) but also portends a wide-bandgap (2.84 eV) semiconducting property with implications for a multitude of applications such as hard radiation detection.

High coordination number actinide-noble gas complexes; a computational study

The geometries, electronic structures and bonding of early actinide-noble gas complexes are studied computationally by density functional and wavefunction theory methods, and by ab initio molecular dynamics. AcHe183+ is confirmed as being an 18-coordinate system, with all of the He atoms accommodated in the primary coordination shell, and this record coordination number is reported for the first time for Th4+ and Th3+. For Pa and U in their group valences of 5 and 6 respectively, the largest number of coordinated He atoms is 17. For AnHe17q+ (An = Ac, q = 3; An = Th, q = 4; An = Pa, q = 5; An = U, q = 6), the average An-He binding energy increases significantly across the series, and correlates linearly with the extent of He → Anq+ charge transfer. The interatomic exchange-correlation term Vxc obtained from the interacting quantum atoms approach correlates linearly with the An-He quantum theory of atoms-in-molecules delocalization index, both indicating that covalency increases from AcHe173+ to UHe176+. The correlation energy in AnHe163+ obtained from MP2 calculations decreases in the order Pa > Th > U > Ac, the same trend found in Vxc. The most stable complexes of Ac3+ with the heavier noble gases Ar-Xe are 12 coordinate, best described as Ng12 cages encapsulating an Ac3+ ion. There is enhanced Ng → Ac3+ charge transfer as the Ng gets heavier, and Ac-Ng covalency increases.

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.