Engineering Cryptand-like Coordination Sites in Multisite Metal Acyl-Organic Cages for Specific Recognition and Deep Purification of Radioactive Strontium(II)

Tailored design of organic linkers or metal nodes can implant desirable binding sites in metal-organic cages (MOCs), expanding the types of guests they can encapsulate. In this work, we propose a feasible method of engineering acyl-type metal nodes to endow MOCs with selective recognition ability towards metal ions without compromising structural robustness, and a novel uranyl-sealed multisite metal supramolecular cage (UOC) is synthesized as a prototype compound. In UOC, peroxide-bridged dimeric uranyl units create a well-defined cryptand-like cavity as a recognition site for efficient encapsulation of Sr2+ through precise size-matching effect. Bonding analysis of Sr@UOC reveals significant electrostatic interaction and orbital overlap between Sr 4d and O 2p, explaining the high binding strength. Hydrophobic cavities at both ends of UOC allow further co-inclusion of organic guests, facilitating the co-encapsulation of two different types of guest species. Inspired by UOC's strong binding affinity for Sr2+, it is employed as absorbents to capture low concentrations of Sr2+ in aqueous solutions. A removal rate of 99.9% for Sr2+ ions at an initial concentration as low as 0.01 mM is achieved, with a record high distribution coefficient (Kd) of 1.36×107 mL/g, demonstrating UOC’s potential for deep purification of trace amounts of radioactive 90Sr2+.