As compared to porous network solids, including metal–organic frameworks, covalent–organic frameworks, porous aromatic frameworks, and zeolites, porous molecular materials are relatively unexplored. Additionally, within porous molecular space, porous organic cages (POCs) have been the most widely reported over the past decade. Relatively recently, however, porous hybrid metal–organic molecular complexes have received considerable attention with a large fraction of surface areas for these coordination cages reported over the past three years. This review focuses on advances in this area. We highlight the recent work with permanently microporous metal–organic polyhedra (MOPs). Analogous to early work in the area of MOFs, the vast majority of MOPs for which surface areas have been reported have been based on paddlewheel building units and carboxylate ligands. We describe the synthesis of porous cages and highlight those based on monometallic, bimetallic, trimetallic, tetrametallic, and higher nuclearity clusters. Finally, we showcase work wherein the porosity of MOPs has been leveraged for applications related to the storage and separation of small molecules and the incorporation of these porous and potentially porous cages into membranes.

Although gas adsorption properties of extended three-dimensional metal−organic materials have been widely studied, they remain relatively unexplored in porous molecular systems. This is particularly the case for porous coordination cages for which surface areas are typically not reported. Herein, we report the synthesis, characterization, activation, and gas adsorption properties of a family of carbazole-based cages. The chromium analog displays a coordination cage record BET (Brunauer−Emmett−Teller) surface area of 1235 m2/g. With precise synthesis and activation procedures, two previously reported cages similarly display high surface areas. The materials exhibit high methane adsorption capacities at 65 bar with the chromium (II) cage displaying CH4 capacities of 194 cm3/g and 148 cm3/cm3. This high uptake is a result of optimal pore design, which was confirmed via powder neutron diffraction experiments.

Porous molecular solids are promising materials for gas storage and gas separation applications. However, given the relative dearth of structural information concerning these materials, additional studies are vital for further understanding their properties and developing design parameters for their optimization. Here, we examine a series of isostructural cuboctahedral, paddlewheel-based coordination cages, M24(tBu-bdc)24 (M = Cr, Mo, Ru; tBu-bdc2− = 5-tert-butylisophthalate), for high-pressure methane storage. As the decrease in crystallinity upon activation of these porous molecular materials precludes diffraction studies, we turn to a related class of pillared coordination cage-based metal-organic frameworks, M24(Me-bdc)24(dabco)6 (M = Fe, Co; Me-bdc2− = 5-methylisophthalate; dabco = 1,4-diazabicyclo[2.2.2]octane) for neutron diffraction studies. The five porous materials display BET surface areas from 1,057 – 1,937 m2/g and total methane uptake capacities of up to 143 cm3(STP)/cm3. Both the porous cages and cage-based frameworks display methane adsorption enthalpies of −15 to −22 kJ/mol. Also supported by molecular modeling, neutron diffraction studies indicate that the triangular windows of the cage are favorable methane adsorption sites with CD4-arene interactions between 3.7 and 4.1 Å. At both low and high loadings, two additional methane adsorption sites on the exterior surface of the cage are apparent for a total of 56 adsorption sites per cage. These results show that M24L24 cages are competent gas storage materials and further adsorption sites may be optimized by judicious ligand functionalization to control extra-cage pore space.

Metal–organic frameworks and porous coordination cages have shown incredible promise as a result of their high tunability. However, syntheses pursuing precisely targeted mixed functionalities, such as multiple ligand types or mixed-metal compositions are often serendipitous, require postsynthetic modification strategies, or are based on complex ligand design. Herein, we present a new method for the controlled synthesis of mixed functionality metal–organic materials via the preparation of porous salts. More specifically, the combination of porous ionic molecules of opposite charge affords framework-like materials where the ratio between cationic cage and anionic cage is potentially tunable. The resulting doubly porous salt displays the spectroscopic signatures of the parent cages with increased gas uptake capacities as compared to starting materials. This approach will be widely applicable to all families of porous ions and represents a new and powerful method for the synthesis of porous solids with tailored functionalities.

Zirconium-based coordination cages have received considerable recent attention as a result of their structural tunability and amenability to postsynthetic modification. Although these structures have adopted tetrahedral (4-vertex) or cigar (2-vertex) geometries depending on the shape and nuclearity of the ligand used in their assembly, their structures have not previously been shown to be either tunable or dynamic. Here, we examine the speciation of a series of zirconium-based cages where the geometry and nuclearity of the product cage can be tuned via judicious functionalization of dicarboxylate ligands. We further show that many of these materials exist as both cage structures in solution or the solid state. With appropriate solvent exchange and activation conditions, the cigar structures can display Brunauer–Emmett–Teller (BET) surface areas as high as 123 m2/g. As a result of their expanded pore structures, the tetrahedra display significantly increased BET surface areas approaching 700 m2/g. These results show that by appropriate ligand functionalization the structures of zirconium-based cages can be modified to tune their phase and surface area. Finally, the isolation of phase-pure cages allows for the utilization of anion metathesis reactions to tune their solubility.