Unlocking Solid-State Organometallic Photochemistry with Optically Transparent, Porous Salt Thin Films.

Synthetic porous materials continue to garner attention as platforms for solid-state chemistry and as designer heterogeneous catalysts. Applications in photochemistry and photocatalysis, however, are plagued by poor light harvesting efficiency due to light scattering resulting from sample microcrystallinity and poor optical penetration that arises from inner filter effects. Here we demonstrate the layer-by-layer growth of optically transparent, photochemically active thin films of porous salts. Films are grown by sequential deposition of cationic Zr-based porous coordination cages and anionic Mn porphyrins. Photolysis facilitates the efficient reduction of Mn(III) sites to Mn(II) sites, which can be observed in real-time by transmission UV-vis spectroscopy. Film porosity enables substrate access to the Mn(II) sites and facilitates reversible O2 activation in the solid state. These results establish optically transparent, porous salt thin films as versatile platforms for solid-state photochemistry and in operando spectroscopy.

Adaptive Pore Opening to Form Tailored Adsorption Sites in a Cooperatively Flexible Framework Enables Record Inverse Propane/Propylene Separation.

A proposed low-energy alternative to the separation of alkanes from alkenes by energy-intensive cryogenic distillation is separation by porous adsorbents. Unfortunately, most adsorbents preferentially take up the desired, high-value major component alkene, requiring frequent regeneration. Adsorbents with inverse selectivity for the minor component alkane would enable the direct production of purified, reagent-grade alkene, greatly reducing global energy consumption. However, such materials are exceedingly rare, especially for propane/propylene separation. Here, we report that through adaptive and spontaneous pore size and shape adaptation to optimize an ensemble of weak noncovalent interactions, the structurally responsive metal-organic framework CdIF-13 (sod-Cd(benzimidazolate)2) exhibits inverse selectivity for propane over propylene with record-setting separation performance under industrially relevant temperature, pressure, and mixture conditions. Powder synchrotron X-ray diffraction measurements combined with first-principles calculations yield atomic-scale insight and reveal the induced fit mechanism of adsorbate-specific pore adaptation and ensemble interactions between ligands and adsorbates. Dynamic column breakthrough measurements confirm that CdIF-13 displays selectivity under mixed-component conditions of varying ratios, with a record measured selectivity factor of α ≈ 3 at 95:5 propylene:propane at 298 K and 1 bar. When sequenced with a low-cost rigid adsorbent, we demonstrated the direct purification of propylene under ambient conditions. This combined atomic-level structural characterization and performance testing firmly establishes how cooperatively flexible materials can be capable of unprecedented separation factors.

Porous Salts as Platforms for Heterogeneous Catalysis.

The preparation of a new class of reactive porous solids, prepared via straightforward salt metathesis reactions, is described here. Reaction of the dimethylammonium salt of a magnesium-based porous coordination cage with the chloride salt of [CrII Cl(Me4 cyclam)]+ affords a porous solid with concomitant removal of dimethylammonium chloride. The salt consists of the ions combined in the expected ratio based on their charge as confirmed by UV-vis and X-ray photoelectron spectroscopies, ion chromatography (IC), and inductively coupled plasma mass spectrometry (ICP-MS). The porous salt boasts a Brunauer-Emmett-Teller (BET) surface area of 213 m2  g-1 . Single crystal X-ray diffraction reveals the chromium(II) cations in the structure reside in the interstitial space between porous cages. Importantly, the chromium(II) centers, previously shown to react with O2 to afford reactive chromium(III)-superoxide adducts, are still accessible in the solid state as confirmed by UV-vis spectroscopy. The site-isolated reactive centers have competence toward hydrogen atom abstraction chemistry and display significantly increased stability and reactivity as compared to dissolved ions.

Aluminum-based metal-organic framework nanoparticles as pulmonary vaccine adjuvants.

The adoption of pulmonary vaccines to advantageously provide superior local mucosal protection against aerosolized pathogens has been faced with numerous logistical and practical challenges. One of these persistent challenges is the lack of effective vaccine adjuvants that could be well tolerated through the inhaled route of administration. Despite its widespread use as a vaccine adjuvant, aluminum salts (alum) are not well tolerated in the lung. To address this issue, we evaluated the use of porous aluminum (Al)-based metal-organic framework (MOF) nanoparticles (NPs) as inhalable adjuvants. We evaluate a suite of Al-based MOF NPs alongside alum including DUT-4, DUT-5, MIL-53 (Al), and MIL-101-NH2 (Al). As synthesized, MOF NPs ranged between ~ 200 nm and 1 µm in diameter, with the larger diameter MOFs matching those of commercial alum. In vitro examination of co-stimulatory markers revealed that the Al-based MOF NPs activated antigen presenting cells more effectively than alum. Similar results were found during in vivo immunizations utilizing ovalbumin (OVA) as a model antigen, resulting in robust mucosal humoral responses for all Al MOFs tested. In particular, DUT-5 was able to elicit mucosal OVA-specific IgA antibodies that were significantly higher than the other MOFs or alum dosed at the same NP mass. DUT-5 also was uniquely able to generate detectable IgG2a titers, indicative of a cellular immune response and also had superior performance relative to alum at equivalent Al dosed in a reduced dosage vaccination study. All MOF NPs tested were generally well-tolerated in the lung, with only acute levels of cellular infiltrates detected and no Al accumulation; Al content was largely cleared from the lung and other organs at 28 days despite the two-dose regime. Furthermore, all MOF NPs exhibited mass median aerodynamic diameters (MMADs) of ~ 1.5-2.5 µm when dispersed from a generic dry powder inhaler, ideal for efficient lung deposition. While further work is needed, these results demonstrate the great potential for use of Al-based MOFs for pulmonary vaccination as novel inhalable adjuvants.

Solvent-Induced Spin-State Change in Copper Corroles.

The electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. The ground state of these compounds is best described as an antiferromagnetically coupled Cu(II) corrole radical cation. In coordinating solvents, these molecules become paramagnetic, and this is often accompanied by a color change. The underlying chemistry of these solvent-induced properties is currently unknown. Here, we show that a coordinating solvent, such as pyridine, induces a change in the ground spin state from an antiferromagnetically coupled Cu(II) corrole radical cation to a ferromagnetically coupled triplet. Over time, the triplet reacts to produce a species with spectral signatures that are characteristic of the one-electron-reduced Cu(II) corrole. These observations account for the solvent-induced paramagnetism and the associated color changes that have been observed for copper corroles in coordinating solvents.

Utilization of a Mixed-Ligand Strategy to Tune the Properties of Cuboctahedral Porous Coordination Cages.

Ligand functionalization has been thoroughly leveraged to alter the properties of paddlewheel-based coordination cages where, in the case of ligand-terminated cages, functional groups are positioned on the periphery of synthesized cages. While these groups can be used to optimize solubility, porosity, crystal packing, thermal stability toward desolvation, reactivity, or optical activity, optimization of multiple properties can be challenging given their interconnected nature. For example, installation of functional groups to increase the solubility of porous cages typically has the effect of decreasing their porosity and stability toward thermal activation. Here we show that mixed-ligand cages can potentially address these issues as the benefits of various functional groups can be combined into one mixed-ligand cage. We further show that although ligand exchange reactions can be employed to obtain mixed ligand copper(II)-based cages, direct synthesis of mixed-ligand products is necessary for molybdenum(II) paddlewheel-based cages as these substitutionally inert clusters are resistant to ligand exchange. We ultimately show that highly soluble, highly porous, and thermally stable cuboctahedral cages are isolable by this strategy.

Permanently Microporous Metal-Organic Polyhedra.

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.

Elaboration of Porous Salts.

A large library of novel porous salts based on charged coordination cages was synthesized via straightforward salt metathesis reactions. For these, solutions of salts of oppositely charged coordination cages are mixed to precipitate MOF-like permanently porous products where metal identity, pore size, ligand functional groups, and surface area are highly tunable. For most of these materials, the constituent cages combine in the ratios expected based on their charge. Additional studies focused on the rate of salt metathesis or reaction stoichiometry as variables to tune particle size or product composition, respectively. It is expected that the design principles outlined here will be widely applicable for the synthesis of new porous salts based on a variety of charged porous molecular precursors.

Gas Storage in Porous Molecular Materials.

Molecules with permanent porosity in the solid state have been studied for decades. Porosity in these systems is governed by intrinsic pore space, as in cages or macrocycles, and extrinsic void space, created through loose, intermolecular solid-state packing. The development of permanently porous molecular materials, especially cages with organic or metal-organic composition, has seen increased interest over the past decade, and as such, incredibly high surface areas have been reported for these solids. Despite this, examples of these materials being explored for gas storage applications are relatively limited. This minireview outlines existing molecular systems that have been investigated for gas storage and highlights strategies that have been used to understand adsorption mechanisms in porous molecular materials.

Stabilizing Porosity in Organic Cages through Coordination Chemistry.

The number of studies concerning the permanent porosity of molecular materials, especially porous organic cages (POCs) and porous coordination cages (PCCs), have increased substantially over the past decade. The work presented here outlines novel approaches to the preparation of porous molecular structures upon metalation of nonporous, amine-based organic cages. Reduction of the well-known CC3 and CC1 imine-based POCs affords nonporous, highly flexible amine cages. These materials can be endowed with significant levels of structural rigidity via post-synthetic metalation of their ethylenediamine-type binding pockets. The hybrid metal-organic cages accessed through this approach combine aspects of POC and PCC chemistry, with structures of this type providing a potentially promising new direction for the design and development of porous molecular materials with tunability in overall charge, metal cation, porosity, and solubility.

Synthesis and Characterization of an Isoreticular Family of Calixarene-Capped Porous Coordination Cages.

Functionalization of permanently porous coordination cages has been used to tune phase, surface area, stability, and solubility in this promising class of adsorbents. For many cages, however, these properties are intricately tied together, and installation of functional groups, for example, to increase solubility often leads to a decrease in surface area. Calixarene-capped cages offer the advantage in that they are cluster-terminated cages whose solid-state packing, and thus surface area, is typically governed by the nature of the capping ligand rather than the bridging ligand. In this work we investigate the influence of ligand functionalization on two series of isoreticular Ni(II)- and Co(II)-based calixarene-capped cages. The two types of materials described are represented as octahedral and rectangular prismatic coordination cages and can be synthesized in a modular manner, allowing for the substitution of dicarboxylate bridging ligands and the introduction of functional groups in specific locations on the cage. We ultimately show that highly soluble cages can be obtained while still having access to high surface areas for many of the isolated phases.

Cooperative insertion of CO2 in diamine-appended metal-organic frameworks.

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as 'phase-change' adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg(2+) within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.

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.

Electrochemically Mediated Syntheses of Titanium(III)-Based Metal-Organic Frameworks.

Although metal-organic frameworks featuring coordinatively unsaturated transition metal sites are relatively common, examples with redox-active cations are rare. In this report, we describe the electrochemically mediated synthesis of TiIII-MIL-101 from the inexpensive Ti4+ precursor TiCl4. The framework obtained via electrosynthesis is identical to that prepared from the significantly more expensive and air-sensitive starting material TiCl3. The above electrosynthetic strategy was also extended to prepare TiIII-MIL-100 and two high-quality extended TiIII-MIL structures, for the first time. These materials represent examples of titanium-based MOFs with extended pore structures. Several physical methods demonstrate that these materials are superior in quality to samples of the analogous MOFs prepared via conventional routes from starting exogenous TiCl3. Given the ease with which the electrosyntheses may be carried out and their compatibility with a broad range of bridging ligands, we expect that this new methodology will find utility for the synthesis of a number of novel materials containing coordinatively unsaturated, redox-active metal cations.

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.

Separation of Xylene Isomers through Multiple Metal Site Interactions in Metal-Organic Frameworks.

Purification of the C8 alkylaromatics o-xylene, m-xylene, p-xylene, and ethylbenzene remains among the most challenging industrial separations, due to the similar shapes, boiling points, and polarities of these molecules. Herein, we report the evaluation of the metal-organic frameworks Co2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) and Co2( m-dobdc) ( m-dobdc4- = 4,6-dioxido-1,3-benzenedicarboxylate) for the separation of xylene isomers using single-component adsorption isotherms and multicomponent breakthrough measurements. Remarkably, Co2(dobdc) distinguishes among all four molecules, with binding affinities that follow the trend o-xylene > ethylbenzene > m-xylene > p-xylene. Multicomponent liquid-phase adsorption measurements further demonstrate that Co2(dobdc) maintains this selectivity over a wide range of concentrations. Structural characterization by single-crystal X-ray diffraction reveals that both frameworks facilitate the separation through the extent of interaction between each C8 guest molecule with two adjacent cobalt(II) centers, as well as the ability of each isomer to pack within the framework pores. Moreover, counter to the presumed rigidity of the M2(dobdc) structure, Co2(dobdc) exhibits an unexpected structural distortion in the presence of either o-xylene or ethylbenzene that enables the accommodation of additional guest molecules.

Design and Synthesis of Porous Nickel(II) and Cobalt(II) Cages.

Coordination assemblies containing transition-metal cations with coordinatively unsaturated sites remain a challenging target in the synthesis of porous molecules. Herein, we report the design, synthesis, and characterization of three porous hybrid inorganic/organic porous molecular assemblies based on cobalt(II) and nickel(II). Precise tuning of ligand functionalization allows for the isolation of molecular species in addition to two- and three-dimensional metal-organic frameworks. The cobaltous and nickelous cage compounds display excellent thermal stabilities in excess of 473 K and Brunauer-Emmett-Teller surface areas on the order of 200 m2/g. The precise ligand functionalization utilized here to control phases between discrete molecules and higher-dimensional solids can potentially further be tuned to optimize the porosity and solubility in future molecular systems.

Metal Insertion in a Methylamine-Functionalized Zirconium Metal-Organic Framework for Enhanced Carbon Dioxide Capture.

The reaction of ZrCl4 with 2',3',5',6'-tetramethylamino-p-terphenyl-4,4″-dicarboxylic acid (H2tpdc-4CH2NH2·3HCl) in the presence of NaF affords Zr6O4(OH)2.1F1.9(tpdc-4CH2NH2·3HCl)6 (1), which is a new member of the Zr6O4(OH)4(dicarboxylate linker)12 or UiO-68 family, and exhibits high porosity with BET and Langmuir surface areas of 1910 m2/g and 2220 m2/g, respectively. Remarkably, fluoride ion incorporation in the zirconium clusters results in increased thermal stability, marking the first example of enhancement in the stability of a UiO framework by this defect-restoration approach. Although material 1 features four alkylamine groups on each organic linker, the framework does not exhibit the high CO2 uptake that would be expected for reaction between CO2 and the amine groups to form carbamic acid or ammonium carbamate species. The absence of strong CO2 adsorption can likely be attributed to protonation at some of the amine sites and the presence of counterions. Indeed, exposure of material 1 to acetonitrile solutions of the organic bases 1,8-bis(dimethylamino)naphthalene (DMAN) or trimethylamine, affords a partially deprotonated material, which exhibits enhanced CO2 uptake. Exposure of basic amine sites also facilitates the postsynthetic chelation of copper(I) ([Cu(MeCN)4]·CF3SO3) to yield material 2 with an enhanced CO2 uptake of 4 wt % at 0.15 bar, which is double that of the parent framework 1.