UV-Vis Absorption Spectroscopy of Polonium(IV) Chloride Complexes: An Electronic Structure Theory Study

More than a hundred years after its discovery, the chemistry of the polonium radioelement is still largely unknown. However, it is quite clear that the properties of this heavy element ( Z = 84) may be affected by relativistic effects, in particular scalar relativistic effects and the so-called spin-orbit coupling (SOC). In this Article, we revisit the interpretation of UV-vis absorption spectra of polonium(IV) complexes in HCl medium, reported decades ago. From the data, two complexes were hypothesized, complex I with a maximum of absorption at 344 nm (at low HCl concentration) and complex II with a maximum at 418 nm (the only visible peak for HCl concentrations above 0.5 M). By monitoring the absorbance at 344 and 418 nm as a function of both the HCl concentration and the pH, complex I was formulated as [Po(OH)Cl _x]^{3- x} while complex II was formulated as [PoCl_{2+ x}]^{2- x}. In this work, we study the ground-state geometries of the [Po(OH)Cl _x]^{3- x} and [PoCl_{2+ x}]^{2- x} complexes for x = 4-2, i.e. for the most probable complexes, with density functional theory (DFT), demonstrating that solvation can remarkably change the geometries of such systems. The electronic excitation energies are then computed with time-dependent DFT (TD-DFT), second-order N-electron valence state perturbation theory (NEVPT2), and contracted spin-orbit configuration interaction (c-SOCI), showing (i) that the SOC must be at play to obtain excitation energies in the right energy domain and (ii) that the quantum chemical calculations point toward x = 4, i.e., toward the [Po(OH)Cl₄]^- and [PoCl₆]^2- complexes.

Inducing selectivity and chirality in group IV metal coordination with high-denticity hydroxypyridinones

The solution- and solid-state interactions between the octadentate siderophore mimic 3,4,3-LI(1,2-HOPO) (343HOPO) and group IV metal ions were investigated using high-resolution mass spectrometry, liquid chromatography, UV-visible spectrophotometry, metal-competition batch titrations, and single crystal X-ray diffraction. 343HOPO forms a neutral 1 : 1 complex, [HfIV343HOPO], that exhibits extreme stability in aqueous solution, with a log β110 value reaching 42.3. These results affirm the remarkable charge-based selectivity of 343HOPO for octacoordinated tetravalent cations with a Hf(iv) complex 1021 more stable than its Lu(iii) analogue. Moreover, [HfIV343HOPO] and its Zr(iv) counterpart show exceptional robustness, with the ligand remaining bound to the cation over a very broad pH range: from pH ∼ 11 to acidic conditions as strong as 10 M HCl. In stark contrast, Ti(iv)-343HOPO species are far less stable and undergo hydrolysis at pH as low as ∼6, likely due to the mismatch between the preferred hexacoordinated Ti(iv) ion and octadentate 343HOPO ligand. The extreme charge-based and denticity-driven selectivity of 343HOPO, now observed across the periodic table, paves the way for new selective sequestration systems for radionuclides including medical 44Ti, 89Zr or 177Lu/Hf isotopes, toxic polonium (Po) contaminants, as well as rutherfordium (Rf) research isotopes. Furthermore, despite the lack of a chiral center in 343HOPO, its complexes with metal ions are chiral and appear to form a single set of enantiomers.