Alkali Counterion-Dependent Crystallization of Uranium(IV)-Chloro Structural Units

The synthesis, structural characterization, and spectroscopic properties of five tetravalent uranium (U) phases including Li₆[U₄(μ₃-O)₂Cl₁₈(H₂O)₂]·10H₂O (1), [U(H₂O)₄Cl₄] (2), [U(H₂O)₄Cl₄]·KCl (3), Rb₂UCl₆ (4), and Cs₂UCl₆ (5) are reported. Notably, a change in the U⁴⁺ solid-state structural unit was observed based on the identity of the alkali counterion used in the synthesis. Li¹⁺ yielded a tetranuclear oxo-bridged cluster, [U₄(μ₃-O)₂Cl₁₈(H₂O)₂]^6-, Na¹⁺ and K¹⁺ yielded two structurally distinct [U(H₂O)₄Cl₄] complexes, and Rb¹⁺ and Cs¹⁺ resulted in [UCl₆]^2- as the dominant phases. The spectroscopic properties of the compounds were analyzed using Raman and UV-vis-NIR absorption spectroscopy. The UV-vis-NIR spectra of compounds 1-5 exhibited transitions consistent with uranium in the +4 oxidation state. Clear differences in the absorption band splitting were observed and are likely attributed to differences in metal ion coordination, crystal field effects, and outer sphere interactions Overall, this work demonstrates the utility of noncovalent interactions in tuning the crystallization of various metal complexes from otherwise identical reaction solutions and provides further evidence that counterions impact the composition and structure of actinide complexes isolated in the solid state. In this way, this work affords important insight into directing and controlling the structure of actinide complexes and clusters.