Engineering Multisite Metal-Organic Cages with Metal Acyl Nodes for Specific Recognition and Deep Purification of Radioactive Strontium(II)

Abstract

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, with a multisite anionic metal supramolecular cage (UOC) with uranyl nodes assembled with calix[4]resorcinarene linkers (C[4]R) as a prototype compound, to endow MOCs with selective recognition ability towards metal ions without compromising structural robustness of MOCs. 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 Sr^(2+) 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 Sr^(2+), there is significant degree of overlapping between Sr 4d and O 2p orbitals, thus conforming the origin of high binding strength of UOC for Sr^(2+). 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 Sr^(2+), it is employed as absorbents to capture Sr^(2+) 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 1.36×10^7 mL/g, demonstrating its huge potential for deep purification of trace amount of radioactive 90Sr^(2+).