Uranyl–Peroxide Capsule Self‐Assembly in Slow Motion

: Uranyl-peroxide capsules are the newest family of polyoxometalates. Although discovered 13 years previously with over 70 topologies reported, there is a lack in the fundamental understanding of assembly mechanisms, particularly the role of the alkali counterions. Herein, the reaction pathway and assembly of uranyl peroxide capsules is reported by tracking the conversion from K+ uranyl triperoxide monomer to the K+ uranyl-peroxide U28 capsule by means of small-angle X-ray scattering and Raman spectroscopy. For the first time, the K+ uranyl-peroxide pentamer face is isolated and structurally characterized, giving credence to the long-held belief that these geometric faces serve as building blocks to the fully formed capsules. Once isolated and re-dissolved, the pentamer face undergoes rapid conversion to capsule forms, underlining its high reactivity that challenges its isolation. Calorimetric measurements of the studied species confirms the pentamer lies on the energy landscape between the monomer and capsule.