A Charged Coordination Cage-Based Porous Salt.

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.

Atomically Precise Crystalline Materials Based on Kinetically Inert Metal Ions via Reticular Mechanopolymerization.

Atomistic control of the coordination environment of lattice ions and the distribution of metal sites within crystalline mixed-metal coordination polymers remain significant synthetic challenges. Herein is reported the mechanochemical synthesis of a reticular family of crystalline heterobimetallic metal-organic frameworks (MOFs) is now achieved by polymerization of molecular Ru2 [II,III] complexes, featuring unprotected carboxylic acid substituents, with Cu(OAc)2 . The resulting crystalline heterobimetallic MOFs are solid solutions of Ru2 and Cu2 sites housed within [M3 L2 ] phases. The developed mechanochemical strategy is modular and allows for systematic control of the primary coordination sphere of the Ru2 sites within an isoreticular family of materials. This strategy is anticipated to provide a rational approach to atomically precise mixed-metal materials.

Understanding Gas Storage in Cuboctahedral Porous Coordination Cages.

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 1057-1937 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 extracage pore space.

Methane Storage in Paddlewheel-Based Porous Coordination Cages.

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.

Tuning water adsorption, stability, and phase in Fe-MIL-101 and Fe-MIL-88 analogs with amide functionalization.

Metal-organic frameworks (MOFs) of the MIL series of materials have been widely studied as a result of their high tunability and the diversity of structure types that exist for these typically M3+ containing frameworks. We explored the use of amide-functionalized ligands in the synthesis of Fe-MIL-101 as a means to tune the water stability and water vapor adsorption in this important class of frameworks. We further show that slow leaching of Fe from NdFeB magnets can afford MIL-101 or MIL-88 under various conditions where the phase of the framework is controlled by length of the carbon chains on amide substituents. NdFeB can also be used to prepare these materials at room temperature in the absence of additional metal salts.

Design and synthesis of aryl-functionalized carbazole-based porous coordination cages.

A subset of coordination cages have garnered considerable recent attention for their potential permanent porosity in the solid state. Herein, we report a series of functionalized carbazole-based cages of the structure type M12(R-cdc)12 (M = Cr, Cu, Mo) where the functional groups include a range of aromatic substituents. Single-crystal X-ray structure determinations reveal a variety of intercage interactions in these materials, largely governed by pi-pi stacking. Density functional theory for a subset of these cages was used to confirm that the nature of the increased stability of aryl-functionalized cages is a result of inter-cage ligand interactions.

Elucidating the Structure of the Metal-Organic Framework Ru-HKUST-1.

Ru-HKUST-1 (Ru3(btc)2X1.5;btc3-=1,3,5-benzenetricarboxylate; X-=chloride,acetate,trimesate,hydroxide) has received considerable attention as a result of its structure type, tunability, and the redox-active nature of its constituent metal paddlewheel building units. As compared to some of the other members of the HKUST-1 family, its surface area is typically reported as ~25% lower than expected. In contrast to this, a related ruthenium-based porous coordination cage, Ru24(tBu-bdc)24Cl12, displays the expected surface area when compared to Cr2+ and Mo2+ analogs. Here, we examine the factors that result in this decreased surface area for the MOF. We show that with appropriate solvent exchange and activation conditions, Ru-HKUST-1 can display a B.E.T. surface areas as high as 1439 m2/g. We utilize a combination of spectroscopic and diffraction techniques to accurately determine the structure of the MOF, which is most accurately described here as Ru3(btc)2(OAc)1.07Cl0.43, as prepared under our conditions. Further, by simply treating the sample as air-sensitive upon isolation, adsorption selectivities toward unsaturated molecu les greatly improve.

Tuning the Porosity, Solubility, and Gas-Storage Properties of Cuboctahedral Coordination Cages via Amide or Ester Functionalization.

The molecular nature of porous coordination cages can endow these materials with significant advantages as compared to extended network solids. Chiefly among these is their solubility in volatile solvents, which can be leveraged in the synthesis, characterization, modification, and utilization of these adsorbents. Although cuboctahedral, paddlewheel-based coordination cages have shown some of the highest surface areas for coordination cages, they often have limited solubility. Here, we show that amide and ester functionalization, which has been widely utilized in porous solids to tune material properties, can be used to tune the solubility, porosity, and bulk adsorbent properties of copper-, chromium-, and molybdenum-based cuboctahedral coordination cages. In addition, we demonstrate that the solubility of a set of diphenylamide-functionalized cages can be utilized to increase their bulk densities for gas storage applications. For a subset of these cages, we further show that amide and ester functional groups can be added postsynthetically, a strategy that is particularly important for the latter where direct cage syntheses with these groups are challenging.

Synthesis and characterization of low-nuclearity lantern-type porous coordination cages.

Permanent porosity in lantern-type M4L4 paddlewheel-based cages is rare and has only been reported for naphthalene, naphthyridine, and diethynylbenzene-based linkers. This work presents the design, synthesis, and characterization of small lanterns that exhibit CO2 accessible BET surface areas in excess of 200 m2 g-1. The crystal packing and porosity of these cages can be tuned by either ligand functionalization or the choice of M2+ source used in their synthesis. Given their low nuclearity, these cages with internal M-M distances of less than 5 Å represent the lower size limit for permanently microporous coordination cages.

Design and synthesis of capped-paddlewheel-based porous coordination cages.

To leverage the structural diversity of metal-organic frameworks, the ability to controllably terminate them for the isolation of porous coordination cages is advantageous. However, the strategy has largely been limited to ligand termination methods, particularly for paddlewheel-based materials. Here, we show a paddlewheel-capping strategy can be employed to afford previously unattainable coordination cage structures that are mimetic of metal-organic framework pores.

Gas adsorption in an isostructural series of pillared coordination cages.

The synthesis and characterization of two novel pillared coordination cages is reported. By utilizing 1,4-diazabicyclo[2.2.2]octane (dabco) as a pillar with increased basicity as compared to pyrazine or 4,4'-bipyridine, a stable copper-based material was prepared. Extending this strategy to iron(ii) afforded an isostructural material that similarly retains high porosity and crystallinity upon solvent evacuation. Importantly, the iron solid represents a rare example of porous iron paddlewheel-based metal-organic material that is stable to solvent evacuation. Neutron powder diffraction studies on these materials indicate the triangular and square windows of the cage are prime ethane and ethylene adsorption sites.