Porous Organic Cages for Sulfur Hexafluoride Separation

Ligand-Based Phase Control in Porous Molecular Assemblies

Functionalization of isophthalic acid ligands with linear alkoxide groups from ethoxy through pentoxy is shown to have a pronounced effect on both the synthesis of porous paddlewheel-based molecular assemblies and their resulting surface areas and gas adsorption properties. Shorter chain length is compatible with either tetragonal or hexagonal two-dimensional materials, with the hexagonal phase favored with longer chain length. Precise tuning of reaction conditions affords discrete molecular species that are soluble in a variety of organic solvents. The isolated porous molecules display BET surface areas ranging from 125 m²/g to 545 m²/g. The pentoxide-based molecular assembly shows considerable promise for the separation of hydrocarbons with average isosteric heats of adsorption of -48 and -31 kJ/mol for ethylene and ethane, respectively.

A Priori Theoretical Model for Discovery of Environmentally Sustainable Perfluorinated Compounds

Since SF₆ is the most potent greenhouse gas, the search for a viable alternative is taking on great urgency for several decades but without success. The demanding combination of performance, safety, and environmental properties for the new chemistry superior to SF₆ was thought to be nearly impossible to achieve. In contrast to the commonly used mixtures with two or three individual gases, a hybrid model has been proposed to create the new perfluorinated compounds with multiple unsaturated chemical bonds by means of full or partial integration of the parent molecules. A unique combination of a series of paradoxical properties that is high in dielectric strength and stability, low in boiling point, and significantly lower in global warming potential is achieved for the first time. The present a priori theoretical predictions shed new lights on the rational molecular design of the perfluorinated compounds and will greatly inspire experimental synthesis and field tests on the new chemistry for dielectric use.

Encapsulation and solid state sequestration of gases by calix[6]arene-based molecular containers

Two calix[6]arene-based molecular containers were synthesized in high yields. These containers can encapsulate small guests through a unique "rotating door" complexation process. The sequestration of greenhouse gases is clearly demonstrated. They can be stored in the solid state for long periods and released via dissolution of the inclusion complex.

Molecular recognition and activation by polyaza macrocyclic compounds based on host-guest interactions

The design and syntheses of supramolecular hosts for the recognition and activation of molecules and anions are one of the most active research fields in supramolecular chemistry, in which polyaza macrocyclic ligands and their complexes have drawn particular attention due to their strong host-guest interactions. This review mainly focuses on the recent progress in the recognition of molecules and anions by polyaza macrocyclic compounds including polyaza macrocycles, polyaza macrobicycles and polyaza macrotricycles, as well as the activation of molecules by polyaza macrocyclic ligands and their metal complexes.

Shape-Persistent [4+4] Imine Cages with a Truncated Tetrahedral Geometry

The synthesis of shape-persistent organic cage compounds is often based on the usage of multiple dynamic covalent bond formation (such as imines) of readily available precursors. By careful choice of the precursors geometry, the geometry and size of the resulting cage can be accurately designed and indeed a number of different geometries and sizes have been realized to date. Despite of this fact, little is known about the precursors conformational rigidity and steric preorganization of reacting functional groups on the outcome of the reaction. Herein, the influence of conformational rigidity in the precursors on the formation of a [4+4] imine cage with truncated tetrahedral geometry is discussed.

Porous organic cage stabilised palladium nanoparticles: efficient heterogeneous catalysts for carbonylation reaction of aryl halides

Porous organic cage stabilised palladium nanoparticles were prepared using methanol as a mild reductant and displayed high catalytic activity for the carbonylation reaction of aryl halides under mild conditions.

Oriented Two-Dimensional Porous Organic Cage Crystals

The formation of two-dimensional (2D) oriented porous organic cage crystals (consisting of imine-based tetrahedral molecules) on various substrates (such as silicon wafers and glass) by solution-processing is reported. Insight into the crystallinity, preferred orientation, and cage crystal growth was obtained by experimental and computational techniques. For the first time, structural defects in porous molecular materials were observed directly and the defect concentration could be correlated with crystal growth rate. These oriented crystals suggest potential for future applications, such as solution-processable molecular crystalline 2D membranes for molecular separations.

Inside information on xenon adsorption in porous organic cages by NMR

A solid porous molecular crystal formed from an organic cage, CC3, has unprecedented performance for the separation of rare gases. Here, xenon was used as an internal reporter providing extraordinarily versatile information about the gas adsorption phenomena in the cage and window cavities of the material. 129Xe NMR measurements combined with state-of-the-art quantum chemical calculations allowed the determination of the occupancies of the cavities, binding constants, thermodynamic parameters as well as the exchange rates of Xe between the cavities. Chemical exchange saturation transfer (CEST) experiments revealed a minor window cavity site with a significantly lower exchange rate than other sites. Diffusion measurements showed significantly reduced mobility of xenon with loading. 129Xe spectra also revealed that the cage cavity sites are preferred at lower loading levels, due to more favourable binding, whereas window sites come to dominate closer to saturation because of their greater prevalence.

Computationally-Guided Synthetic Control over Pore Size in Isostructural Porous Organic Cages

The physical properties of 3-D porous solids are defined by their molecular geometry. Hence, precise control of pore size, pore shape, and pore connectivity are needed to tailor them for specific applications. However, for porous molecular crystals, the modification of pore size by adding pore-blocking groups can also affect crystal packing in an unpredictable way. This precludes strategies adopted for isoreticular metal-organic frameworks, where addition of a small group, such as a methyl group, does not affect the basic framework topology. Here, we narrow the pore size of a cage molecule, CC3, in a systematic way by introducing methyl groups into the cage windows. Computational crystal structure prediction was used to anticipate the packing preferences of two homochiral methylated cages, CC14-R and CC15-R, and to assess the structure-energy landscape of a CC15-R/CC3-S cocrystal, designed such that both component cages could be directed to pack with a 3-D, interconnected pore structure. The experimental gas sorption properties of these three cage systems agree well with physical properties predicted by computational energy-structure-function maps.

Porous Molecular Solids and Liquids

Until recently, porous molecular solids were isolated curiosities with properties that were eclipsed by porous frameworks, such as metal-organic frameworks. Now molecules have emerged as a functional materials platform that can have high levels of porosity, good chemical stability, and, uniquely, solution processability. The lack of intermolecular bonding in these materials has also led to new, counterintuitive states of matter, such as porous liquids. Our ability to design these materials has improved significantly due to advances in computational prediction methods.

Topological landscapes of porous organic cages

We define a nomenclature for the classification of porous organic cage molecules, enumerating the 20 most probable topologies, 12 of which have been synthetically realised to date. We then discuss the computational challenges encountered when trying to predict the most likely topological outcomes from dynamic covalent chemistry (DCC) reactions of organic building blocks. This allows us to explore the extent to which comparing the internal energies of possible reaction outcomes is successful in predicting the topology for a series of 10 different building block combinations.

Highly selective CO2vs. N2 adsorption in the cavity of a molecular coordination cage

Two M₈L₁₂ cubic coordination cages, as desolvated crystalline powders, preferentially adsorb CO₂ over N₂ with ideal selectivity CO₂/N₂ constants of 49 and 30 at 298 K. A binding site for CO₂ is suggested by crystallographic location of CS₂ within the cage cavity at an electropositive hydrogen-bond donor site, potentially explaining the high CO₂/N₂ selectivity compared to other materials with this level of porosity.

Understanding gas capacity, guest selectivity, and diffusion in porous liquids

An in-depth study of porous liquids using measurement techniques, molecular simulations, and control experiments to advance their quantitative understanding.

Porous liquids are a new class of material that could have applications in areas such as gas separation and homogeneous catalysis. Here we use a combination of measurement techniques, molecular simulations, and control experiments to advance the quantitative understanding of these liquids. In particular, we show that the cage cavities remain unoccupied in the absence of a suitable guest, and that the liquids can adsorb large quantities of gas, with gas occupancy in the cages as high as 72% and 74% for Xe and SF₆, respectively. Gases can be reversibly loaded and released by using non-chemical triggers such as sonication, suggesting potential for gas separation schemes. Diffusion NMR experiments show that gases are in dynamic equilibrium between a bound and unbound state in the cage cavities, in agreement with recent simulations for related porous liquids. Comparison with gas adsorption in porous organic cage solids suggests that porous liquids have similar gas binding affinities, and that the physical properties of the cage molecule are translated into the liquid state. By contrast, some physical properties are different: for example, solid homochiral porous cages show enantioselectivity for chiral aromatic alcohols, whereas the equivalent homochiral porous liquids do not. This can be attributed to a loss of supramolecular organisation in the isotropic porous liquid.

Modular assembly of porous organic cage crystals: isoreticular quasiracemates and ternary co-crystal

Co-crystallisation of helically chiral porous organic cage molecules has enabled the formation of isoreticular quasiracemates and a rare porous organic ternary co-crystal.

Transforming a chemically labile [2+3] imine cage into a robust carbamate cage

Turning a pH labile porous cage into a highly pH stable porous organic cage by fixation with carbamate units.

Exploring the gem-dimethyl effect in the formation of imine-based macrocycles and cages

A series of imine-based macrocycles and cages has been synthesized using the gem-dimethyl effect as a platform. Following this strategy, imine-based macrocyclization could be performed even at higher concentration.

Three-dimensional protonic conductivity in porous organic cage solids

Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells. To maximize proton conduction, three-dimensional conduction pathways are preferred over one-dimensional pathways, which prevent conduction in two dimensions. Many crystalline porous solids to date show one-dimensional proton conduction. Here we report porous molecular cages with proton conductivities (up to 10(-3) S cm(-1) at high relative humidity) that compete with extended metal-organic frameworks. The structure of the organic cage imposes a conduction pathway that is necessarily three-dimensional. The cage molecules also promote proton transfer by confining the water molecules while being sufficiently flexible to allow hydrogen bond reorganization. The proton conduction is explained at the molecular level through a combination of proton conductivity measurements, crystallography, molecular simulations and quasi-elastic neutron scattering. These results provide a starting point for high-temperature, anhydrous proton conductors through inclusion of guests other than water in the cage pores.

Understanding static, dynamic and cooperative porosity in molecular materials

The practical adsorption properties of molecular porous solids can be dominated by dynamic flexibility but these effects are still poorly understood. Here, we combine molecular simulations and experiments to rationalize the adsorption behavior of a flexible porous organic cage.

The practical adsorption properties of molecular porous solids can be dominated by dynamic flexibility but these effects are still poorly understood. Here, we combine molecular simulations and experiments to rationalize the adsorption behavior of a flexible porous organic cage.

Endohedrally functionalised porous organic cages

The synthesis and characterisation of two novel, endohedrally functionalised porous organic cages are presented.

Cavity partition and functionalization of a [2+3] organic molecular cage by inserting polar PO bonds

The cavity of a [2+3] organic molecular cage was partitioned and functionalized by inserting inner-directed PO bonds, which shows CO₂ capture and CH₄ exclusion due to the size-matching and polarity effects.