Ionic-Liquid-Based Synthesis of U₂₄ Uranyl Peroxide Cage Clusters with Encapsulated Hexanuclear Lanthanide Oxide/Hydroxide Clusters

Six lanthanide-containing (Pr-Tb, excluding Pm) uranyl peroxide cage clusters (UPCs) formed following the dissolution of studtite, [(UO₂)(O₂)(H₂O)₂](H₂O)₂, and lanthanide nitrates in the ionic liquid 1-ethyl-3-methylimidazolium diethyl phosphate (EMIm-DEP). Crystals of the compounds were isolated and characterized using single crystal X-ray diffraction, Raman and infrared spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. The six structures are composed of U₂₄, [(UO₂)₂₄(O₂)₂₄(OH)₂₄]^24-, cage clusters with the sodalite topology encapsulating [Ln₆O(OH)₈]⁸⁺ hexanuclear units. The crystal packing of the cage clusters produces a highly porous crystal structure and nonlocalized EMIm-DEP ions reside in pores within the crystal structure, providing charge-balance to the anionic cage clusters. Adjacent U₂₄ cages are directly linked through hydrogen bonds donated by hydroxy groups of one cluster and accepted by uranyl ion oxygen atoms of another. The typical UPC synthesis involves decomposition of studtite in alkaline aqueous conditions, making the formation of Ln-containing UPCs impractical owing to the precipitation of insoluble lanthanide compounds under such conditions. This is the first report of studtite dissolving in an ionic liquid and subsequently forming UPCs, as well as the first report of the formation of U₂₄ containing lanthanide cations.