Engineering Multisite Metal-Organic Cages with Metal Acyl Nodes 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) and greatly expand the type of guest 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 of MOCs. A novel MOC with a uranyl-sealed calix[4]resorcinarene (C[4]R)-based multisite cavity, namely UOC, is synthesized as a prototype compound. In UOC, peroxide-bridged dimeric uranyl units on both sides of the coordination cage offer abundant coordination-available oxygen sites, creating a cryptand-like cavity that enables high-efficient recognition and encapsulation of Sr2+ ion due to precise size-matching effect. Bonding analysis of the resultant Sr@UOC suggest that, although electrostatic interaction predominates in host-guest interactions between UOC and Sr2+, there is significant degree of overlapping between Sr 4d and O 2p orbitals, thus conforming the origin of high binding strength of UOC for Sr2+. Meanwhile, hydrophobic binding cavities at both ends of UOC allow it to further encapsulate organic guests, thus facilitating co-inclusion of two different kinds of guest species in UOC for the first time. Inspired by the strong binding affinity of UOC to Sr2+, it is employed as absorbents to capture Sr2+ ion of low concentrations in aqueous solution. A removal efficiency of 99.9% could be achieved for Sr2+ ions at the initial concentration as low as 0.01 mM, with a record distribution coefficient (Kd) as high as 1.36×107 mL/g, demonstrating its huge potential for deep purification of trace amount of radioactive 90Sr2+.