Metal–organic cages for molecular separations

Abstract

Separation technology is central to industries as diverse as petroleum, pharmaceuticals, mining, and life sciences. Metal-organic cages, a class of molecular containers formed via coordination-driven self-assembly, show great promise as separation agents. Precise control of the shape, size, and functionalization of cage cavities enables them to selectively bind and distinguish a wide scope of physicochemically similar substances in solution. Extensive research has thus been performed involving separations of high value targets with coordination cages, ranging from gases and liquids to compounds dissolved in solution. Enantiopure capsules also show great potential for the separation of chiral molecules. The use of crystalline cages as absorbents, or the incorporation of cages into polymer membranes, could increase the selectivity and efficiency of separation processes. This review covers recent progress in using metal-organic cages to achieve separations, with discussion of the many methods of using them in this context. Challenges and potential future developments are also discussed.Compared to metal-organic frameworks (MOFs), a class of solid, porous coordination-based materials that have been widely used for separation, [29][30][31][32] an advantage of coordination cages is their tailorable solubilities. 33,34 Cages in solution, in contrast to solid MOFs, can deform or de-ligate so as to encapsulate a guest much larger than the cage pores. Cages can also be incorporated into polymers, [35][36][37][38] as fillers, to prepare mixed-matrix membranes for performing separations. [39][40][41][42] The solubility of these capsules in different solvents enables them, in contrast to MOFs, to form homogeneous mixtures with polymer matrices, avoiding agglomeration or precipitation of fillers during membrane formation, which may lead to poor separation performance.This review summarizes recent advances in using metal-organic cages for separations, with emphasis on their multiple modes of use, ranging from cages in solution and in the solid, to polymer membranes and separation columns with cages incorporated. Cages in solution for extractionCoordination cages have been explored as liquid-phase extractants to extract target molecules from an immiscible liquid phase (liquid-liquid extraction) or a solid phase (solid-liquid extraction).These strategies require binding affinities that are strong enough to pull target molecules from the initial phase into another. Cages bring guests into solvents in which the guests are not normally soluble; the cages are thus useful liquid-phase extractants. Successful application of this extraction strategy requires cages that are stable in different solvents. The high stability of a host-guest complex after extraction can impede extractant recycling and cargo recovery, however stimuli-responsive guest release can be used to overcome these challenges. [43][44][45][46][47] Liquid-liquid extractionThe Nitschke group recently reported Fe II 4L4 tetrahedron 1 (Figure 1A), prepared from an azaphosp...

A Cavity‐Tailored Metal‐Organic Cage Entraps Gases Selectively in Solution and the Amorphous Solid State

“…The system described herein may serve as the foundation for hydrocarbon-binding cage-based porous liquids [7a] or new fluid processes for the separation, purification, or storage of hydrocarbon gases. [20] Figure 4. Gas adsorption isotherms of 1 for CH 4 (circles), C 2 H 6 (squares), and C 2 H 4 (triangles) at 295 K.…”

From Mechanically Interlocked Structures to Host–Guest Chemistry Based on Twisted Dimeric Architectures by Adjusting Space Constraints

“…[7][8][9] ) have been extensively applied to construct numerous discrete architectures. [10][11][12] On the basis of discrete structures, more complex systems formed by intermolecular interactions, such as hierarchical selfassembly, [13][14][15] the mechanically interlocked molecules (MIMs), [16][17][18][19] and host-guest systems, [20][21][22][23][24] have received considerable attention over the past few decades. As widespread structures playing a unique role in living organisms, complex systems spark the fabrication of aesthetic molecular structures and provide impetus to the development of molecular machines and smart materials.…”

Structural Flexibility in Metal-Organic Cages

“…Metal-organic cages (MOCs) are self-assembled structures derived from carefully selected combinations of metal ions and ligands ( Cook and Stang, 2015 ; Debata et al, 2019 ; Pilgrim and Champness, 2020 ). Porous MOCs of myriad shape and size are capable of binding guest molecules in their cavities ( Rizzuto et al, 2019 ), leading to applications in catalysis ( Yoshizawa et al, 2009 ; Sinha and Mukherjee, 2018 ), storage of reactive species ( Galan and Ballester, 2016 ), molecular separations ( Zhang et al, 2021 ), and drug delivery ( Casini et al, 2017 ) amongst others. Guest-binding is a complex process involving contributions from enthalpic and entropic factors for both the encapsulation of the guest(s) and the liberation of solvent molecules (or displacement of extant guest molecules) ( Metherell et al, 2018 ).…”