Unexpected Roles of Alkali-Metal Cations in the Assembly of Low-Valent Uranium Sulfate Molecular Complexes

The directing effect of coordinating ligands in the formation of uranium molecular complexes has been well established, but the role of counterions in metal-ligand interactions remains ambiguous and requires further investigation. In this work, we describe the targeted isolation, through the choice of alkali-metal ions, of a family of tetravalent uranium sulfates, showing the influence of the overall topology and, unexpectedly, the U^IV nuclearity upon the inclusion of such countercations. Analyses of the structures of uranium(IV) oxo/hydroxosulfate oligomeric species isolated from consistent synthetic conditions reveal that the incorporation of Na⁺ and Rb⁺ promotes the crystallization of 0D discrete clusters with a hexanuclear [U₆O₄(OH)₄(H₂O)₄]¹²⁺ core, whereas the larger Cs⁺ ion allows for the isolation of a 2D-layered oligomer with a less condensed trinuclear [U₃(O)]¹⁰⁺ center. This finding expands the prevalent view that counterions play an innocent role in molecular complex synthesis, affecting only the overall packing but not the local oligomerization. Interestingly, trends in nuclearity appear to correlate with the hydration enthalpies of alkali-metal cations, such that the alkali-metal cations with larger hydration enthalpies correspond to more hydrated complexes and cluster cores. These findings afford new insights into the mechanism of nucleation of U^IV, and they also open a new path for the rational design and synthesis of targeted molecular complexes.