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.

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Metal-organic frameworks featuring ligands with open chelating groups are versatile platforms for the preparation of a diverse set of heterogeneous catalysts through postsynthetic metalation. The crystalline nature of these materials allows them to be characterized via X-ray diffraction, which provides valuable insight into the structure of the metal sites that facilitate catalysis. A highly porous and thermally robust zirconium-based metal-organic framework, Zr6O4(OH)4(bpydc)6 (bpydc(2-) = 2,2'-bipyridne-5,5'-dicarboxylate), bears open bipyridine sites that readily react with a variety of solution- and gas-phase metal sources to form the corresponding metalated frameworks. Remarkably, Zr6O4(OH)4(bpydc)6 undergoes a single-crystal-to-single-crystal transformation upon metalation that involves a change in space group from Fm3̅m to Pa3̅. This structural transformation leads to an ordering of the metalated linkers within the framework, allowing structural characterization of the resulting metal complexes. Furthermore, Zr6O4(OH)4(bpydc)6 yields an active heterogeneous catalyst for arene C-H borylation when metalated with [Ir(COD)2]BF4 (COD = 1,5-cyclooctadiene). These results highlight the unique potential of metal-organic frameworks as a class of heterogeneous catalysts that allow unparalleled structural characterization and control over their active sites.

Developing novel drug carriers for pulmonary delivery is necessary to achieve higher efficacy and consistency for treating pulmonary diseases while limiting off-target side effects that occur from alternative routes of administration. Metal–organic frameworks (MOFs) have recently emerged as a class of materials with characteristics well-suited for pulmonary drug delivery, with chemical tunability, high surface area, and pore size, which will allow for efficient loading of therapeutic cargo and deep lung penetration. UiO-66, a zirconium and terephthalic acid-based MOF, has displayed notable chemical and physical stability and potential biocompatibility; however, its feasibility for use as a pulmonary drug delivery vehicle has yet to be examined. Here, we evaluate the use of UiO-66 nanoparticles (NPs) as novel pulmonary drug delivery vehicles and assess the role of missing linker defects in their utility for this application. We determined that missing linker defects result in differences in NP aerodynamics but have minimal effects on the loading of model and therapeutic cargo, cargo release, biocompatibility, or biodistribution. This is a critical result, as it indicates the robust consistency of UiO-66, a critical feature for pulmonary drug delivery, which is plagued by inconsistent dosage because of variable properties. Not only that, but UiO-66 NPs also demonstrate pH-dependent stability, with resistance to degradation in extracellular conditions and breakdown in intracellular environments. Furthermore, the carriers exhibit high biocompatibility and low cytotoxicity in vitro and are well-tolerated in in vivo murine evaluations of orotracheally administered NPs. Following pulmonary delivery, UiO-66 NPs remain localized to the lungs before clearance over the course of seven days. Our results demonstrate the feasibility of using UiO-66 NPs as a novel platform for pulmonary drug delivery through their tunable NP properties, which allow for controlled aerodynamics and internalization-dependent cargo release while displaying remarkable pulmonary biocompatibility.

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.