Supramolecular Assembly of U(IV) Clusters and Superatoms with Unconventional Countercations

Abstract

Superatoms are nanometer-sized molecules or particles that form ordered lattices, mimicking their atomic counterparts. Hierarchical assembly of superatoms gives rise to emergent properties in lattices of quantum dots, p-block clusters, and fullerenes. Here, we introduce a family of uranium-oxysulfate cluster anions whose hierarchical assembly in water is controlled by two parameters: acidity and the lanthanide or transition-metal countercation. In acid, larger Ln^III (Ln = La-Ho) link hexamer (U₆) oxoclusters into body-centered cubic frameworks, while smaller Ln^III (Ln = Er-Lu and Y) promote linking of 14 U₆ clusters into hollow superclusters (U₈₄ superatoms). U₈₄ assembles into superlattices including cubic-closest packed, body-centered cubic, and interpenetrating networks, bridged by interstitial countercations and U₆ clusters. Divalent transition metals (TM = Mn^II and Zn^II) charge-balance and promote the fusion of 10 U₆ and 10 U monomers into a wheel-shaped cluster (U₇₀). Dissolution of U₇₀ in organic media reveals (by small-angle X-ray scattering) that differing supramolecular assemblies are accessed, controlled by TM^II-linking of U₇₀ clusters. Magnetic measurements of these assemblies reveal Curie-Weiss behavior at high temperatures, without pairing of the 5f²-electrons down to 2 K.