Porous Molecular Solids and Liquids

Engineering Permanent Porosity into Liquids

The possibility of engineering well-defined pores into liquid materials is fascinating from both a conceptual and an applications point of view. Although the concept of porous liquids was proposed in 2007, these materials had remained hypothetical due to the technical challenges associated with their synthesis. Over the past five years, however, reports of the successful construction of porous liquids based on existing porous scaffolds, such as coordination cages, organic cages, metal-organic frameworks, porous carbons, zeolites, and porous polymers, have started to emerge. Here, the focus is on these early reports of porous liquids as prototypes in the field, classified according to the previously defined types of porous liquids. Particular attention will be paid to design strategies and structure-property relationships. Porous liquids have already exhibited promising applications in gas storage, transportation, and chemical separations. Thus, they show great potential for use in the chemical industry. The challenges of preparation, scale-up, volatility, thermal and chemical stability, and competition with porous solids will also be discussed.

Diversity-oriented synthesis of polymer membranes with ion solvation cages

Microporous polymers feature shape-persistent free volume elements (FVEs), which are permeated by small molecules and ions when used as membranes for chemical separations, water purification, fuel cells and batteries^1-3. Identifying FVEs that have analyte specificity remains a challenge, owing to difficulties in generating polymers with sufficient diversity to enable screening of their properties. Here we describe a diversity-oriented synthetic strategy for microporous polymer membranes to identify candidates featuring FVEs that serve as solvation cages for lithium ions (Li⁺). This strategy includes diversification of bis(catechol) monomers by Mannich reactions to introduce Li⁺-coordinating functionality within FVEs, topology-enforcing polymerizations for networking FVEs into different pore architectures, and several on-polymer reactions for diversifying pore geometries and dielectric properties. The most promising candidate membranes featuring ion solvation cages exhibited both higher ionic conductivity and higher cation transference number than control membranes, in which FVEs were aspecific, indicating that conventional bounds for membrane permeability and selectivity for ion transport can be overcome⁴. These advantages are associated with enhanced Li⁺ partitioning from the electrolyte when cages are present, higher diffusion barriers for anions within pores, and network-enforced restrictions on Li⁺ coordination number compared to the bulk electrolyte, which reduces the effective mass of the working ion. Such membranes show promise as anode-stabilizing interlayers in high-voltage lithium metal batteries.

The Ionic Organic Cage: An Effective and Recyclable Testbed for Catalytic CO2 Transformation

Porous organic cages (POC) are a class of relatively new molecular porous materials, whose concept was raised in 2009 by Cooper’s group and has rarely been directly used in the area of organic catalysis. In this contribution, a novel ionic quasi-porous organic cage (denoted as Iq-POC), a quaternary phosphonium salt, was easily synthesized through dynamic covalent chemistry and a subsequent nucleophilic addition reaction. Iq-POC was applied as an effective nucleophilic catalyst for the cycloaddition reaction of CO2 and epoxides. Owing to the combined effect of the relatively large molecular weight (compared with PPh3+I−) and the strong polarity of Iq-POC, the molecular catalyst Iq-POC displayed favorable heterogeneous nature (i.e., insolubility) in this catalytic system. Therefore, the Iq-POC catalyst could be easily separated and recycled by simple centrifugation method, and the catalyst could be reused five times without obvious loss of activity. The molecular weight augmentation route in this study (from PPh3+I− to Iq-POC) provided us a “cage strategy” of designing separable and recyclable molecular catalysts.

An In Situ Coupling Strategy toward Porous Carbon Liquid with Permanent Porosity

An in situ coupling approach is used to fabricate the porous carbon liquid with permanent porosity by directly dispersing hollow carbon nanospheres in polymerized ionic liquids. It is a kind of homogenous and stable type III porous liquid at room temperature. Because of the well-preserved permanent porosity, this unique porous carbon liquid is capable of absorbing the largest quantity of carbon dioxide than the reference PILs and solid carbon liquid, thus, can function as a promising candidate for application in gas storage. More importantly, this approach not only provides an easy method to tune the properties of those specific porous liquids, but also is suitable for fabricating other porous liquid based on varied porous structures (e.g., porous carbon nitride, porous boron nitride, and polymer with intrinsic microporosity), thus paving a viable path for the rational design and synthesis of novel porous liquids with functional properties for specific applications.

Structural and computational investigation of an imine-based propeller-shaped macrocyclic cage

In this study, we present the synthesis, spectroscopic and structural characterization of self-assembling gem-dimethyl imine based molecular cage (IMC). Self-assembling macrocycles and cages have well-defined cavities and have extensive functionalities ranging from energy storage, liquid crystals, and catalysts to water splitting photo absorber. IMC has large voids i.e., 25% of the total crystal volume thus could accommodate wide substrates. The synthesized imine-based molecular cages are stabilized by coaxial π bonded networks and long-range periodic van der Waal and non-bonded contacts as observed from the crystal structure. IMC also has typical properties of soft condensed matter materials, hence theoretical prediction of stress and strain tensor along with thermophysical properties were computed on crystal system and were found to be stable. Molecular dynamics revealed IMC is stabilized by, strong interactions between the interstitial phenyl rings. Density functional theory (DFT) based physicochemical properties were evaluated and has band gap of around 2.38ev (520 nm) similar to various photocatalytic band gap materials.

A hydrogen-bonded organic framework based on redox-active tri(dithiolylidene)cyclohexanetrione

Redox-active hexakis(4-carboxyphenyl) tri(dithiolylidene)cyclohexanetrione (CPDC) was synthesized. The CPDC-based porous framework, constructed via anomalistic helical hydrogen-bonding, exhibites permanent porosity and photoconductivity.

Guest-dependent single-crystal-to-single-crystal transformations in porous adamantane-bearing macrocycles

An adamantane-bearing macrocycle exhibited permanent intrinsic porosity and adsorption of small guests in single-crystal-to-single-crystal fashions. The guest capture resulted in the structural transformations of supramolecular organic frameworks.

Solution processable metal-organic frameworks for mixed matrix membranes using porous liquids

The combination of well-defined molecular cavities and chemical functionality makes crystalline porous solids attractive for a great number of technological applications, from catalysis to gas separation. However, in contrast to other widely applied synthetic solids such as polymers, the lack of processability of crystalline extended solids hampers their application. In this work, we demonstrate that metal-organic frameworks, a type of highly crystalline porous solid, can be made solution processable via outer surface functionalization using N-heterocyclic carbene ligands. Selective outer surface functionalization of relatively large nanoparticles (250 nm) of the well-known zeolitic imidazolate framework ZIF-67 allows for the stabilization of processable dispersions exhibiting permanent porosity. The resulting type III porous liquids can either be directly deployed as liquid adsorbents or be co-processed with state-of-the-art polymers to yield highly loaded mixed matrix membranes with excellent mechanical properties and an outstanding performance in the challenging separation of propylene from propane. We anticipate that this approach can be extended to other metal-organic frameworks and other applications.

Porous Aromatic Framework Nanosheets Anchored with Lewis Pairs for Efficient and Recyclable Heterogeneous Catalysis

Lewis pairs (LPs) with outstanding performance for nonmetal-mediated catalysis reactions have high fundamental interest and remarkable application prospects. However, their solubility characteristics lead to instability and deactivation upon recycling. Here, the layered porous aromatic framework (PAF-6), featuring two kinds of Lewis base sites (NPiperazine and NTriazine), is exfoliated into few-layer nanosheets to form the LP entity with the Lewis acid. After comparison with various porous networks and verification by density functional theory (DFT) calculations, the NTriazine atom in the specific spatial environment is determined to preferably coordinate with the electron-deficient boron compound in a sterically hindered pattern. LP-bare porous product displays high catalytic activity for the hydrogenation of both olefin and imine compounds, and demonstrates ≈100% activity after 10 successful cycles in hydrogenation reactions. Considering the natural advantage of porous organic frameworks to construct LP groups opens up novel prospects for preparing other nonmetallic heterogeneous catalysts for efficient and recyclable catalysis.

Reticular Chemistry in the Construction of Porous Organic Cages

Reticular chemistry offers the possibility of systematic design of porous materials with different pores by varying the building blocks, while the emerging porous organic cage (POC) system remains generally unexplored. A series of new POCs with dimeric cages with odd-even behaviors, unprecedented trimeric triangular prisms, and the largest recorded hexameric octahedra have been prepared. These POCs are all constructed from the same tetratopic tetraformylresorcin[4]arene cavitand by simply varying the diamine ligands through Schiff-base reactions and are fully characterized by X-ray crystallography, gas sorption measurements, NMR spectroscopy, and mass spectrometry. The odd-even effects in the POC conformation changes of the [2 + 4] dimeric cages have been confirmed by density functional theory calculations, which are the first examples of odd-even effects reported in the cavitand-based cage system. Moreover, the "V" shape phenylenediamine linkers are responsible for the novel [3 + 6] triangular prisms. The window size and environment can be easily functionalized by different groups, providing a promising platform for the construction of multivariate POCs. Use of linear phenylenediamines led to record-breakingly large [6 + 12] truncated octahedral cages, the maximum inner cavity diameters and volumes of which could be readily modulated by increasing the spacer length of the phenylenediamine linkers. This work can lead to an understanding of the self-assembly behaviors of POCs and also sheds light on the rational design of POC materials for practical applications.

Modulation of Crystal Packing via the Tuning of Peripheral Functionality for a Family of Dinuclear Mesocates

A family of four novel pyrazinyl-hydrazone based ligands have been synthesized with differing functionality at the 5-position of the central aromatic ring. Previous work has shown such ligands to form dinuclear triple mesocates which pack to form hexagonal channels capable of gas sorption. The effect of the peripheral functionality of the ligand on the crystal packing was investigated by synthesizing complexes 1 to 4 which feature amino, bromo, iodo and methoxy substituents respectively. Complexes 1 to 3 crystallized in the same hexagonal space group P6₃ /m and featured 1D channels. However, on closer inspection while the packing of 1 is mediated by hydrogen bonding interactions, the packing of complexes 2 and 3 are not, due to a subtlety different π-π stacking interaction enforced by the halogen substituent. The more bulky nature of the methoxy substituent of 4 results in the complex crystallizing in the triclinic space group P-1, featuring an entirely different crystal packing.

Molecularly Imprinted Porous Aromatic Frameworks for Molecular Recognition

Porous aromatic frameworks (PAFs) are an important class of porous materials that are well-known for their ultralarge surface areas and superb stabilities. Basically, PAF solids are constructed from periodically arranged phenyl fragments connected via C-C bonds (generally), which provide vast accessible surfaces that can be modified with functional groups and intrinsic pathways for rapid mass transfer. Molecular imprinting technology (MIT) is an effective method for producing binding sites with a specific geometry and size that complement a template object. This review focuses on the integration of MIT into PAF structures via state-of-the-art coupling chemistry to expand the application of porous materials in the fields of metal ion extraction (including the nuclear element uranium) and selective catalysis. Additionally, a concise outlook on the rational construction of molecularly imprinted porous aromatic frameworks is discussed in terms of developing next-generation porous materials for broader applications.

Geometric landscapes for material discovery within energy-structure-function maps

Porous molecular crystals are an emerging class of porous materials formed by crystallisation of molecules with weak intermolecular interactions, which distinguishes them from extended nanoporous materials like metal-organic frameworks (MOFs). To aid discovery of porous molecular crystals for desired applications, energy-structure-function (ESF) maps were developed that combine a priori prediction of both the crystal structure and its functional properties. However, it is a challenge to represent the high-dimensional structural and functional landscapes of an ESF map and to identify energetically favourable and functionally interesting polymorphs among the 1000s to 10 000s of structures typically on a single ESF map. Here, we introduce geometric landscapes, a representation for ESF maps based on geometric similarity, quantified by persistent homology. We show that this representation allows the exploration of complex ESF maps, automatically pinpointing interesting crystalline phases available to the molecule. Furthermore, we show that geometric landscapes can serve as an accountable descriptor for porous materials to predict their performance for gas adsorption applications. A machine learning model trained using this geometric similarity could reach a remarkable accuracy in predicting the materials' performance for methane storage applications.

Hybrid Catalysts for Artificial Photosynthesis: Merging Approaches from Molecular, Materials, and Biological Catalysis

Increasing demand for sustainable energy sources continues to motivate the development of new catalytic processes that store intermittent energy in the form of chemical bonds. In this context, photosynthetic organisms harvest light to drive dark reactions reducing carbon dioxide, an abundant and accessible carbon source, to store solar energy in the form of glucose and other biomass feedstocks. Inspired by this biological process, the field of artificial photosynthesis aims to store renewable energy in chemical bonds spanning fuels, foods, medicines, and materials using light, water, and CO₂ as the primary chemical feedstocks, with the added benefit of mitigating the accumulation of CO₂ as a greenhouse gas in the atmosphere. As such, devising new catalyst platforms for transforming CO₂ into value-added chemical products is of importance. Historically, catalyst design for artificial photosynthesis has been approached from the three traditional fields of catalysis: molecular, materials, and biological. In this Account, we show progress from our laboratory in constructing new hybrid catalysts for artificial photosynthesis that draw upon design concepts from all three of these traditional fields of catalysis and blur the boundaries between them. Starting with molecular catalysis, we incorporated biological design elements that are prevalent in enzymes into synthetic systems. Specifically, we demonstrated that proper positioning of intramolecular hydrogen bond donors or addition of intermolecular multipoint hydrogen bond donors with classic iron porphyrin and nickel cyclam platforms can substantially increase rates of CO₂ reduction and break electronic scaling relationships. In parallel, we incorporated a key materials design element, namely, high surface area and porosity for maximizing active site exposure, into molecular systems. A supramolecular porous organic cage molecule was synthesized with iron porphyrin building blocks, and the porosity was observed to facilitate substrate and charge transport through the catalyst film. In turn, molecular design elements can be incorporated into materials catalysts for CO₂ reduction. First, we utilized molecular synthons in a bottom-up reticular approach to drive polymerization/assembly into a bulk framework material. Second, we established an organometallic approach in which molecular ligands, including chelating ones, are adsorbed onto a bulk inorganic solid to create and tune new active sites on surfaces. Finally, we describe two examples in which molecular, materials, and biological design elements are all integrated to catalyze the reduction of CO₂ into CH₄ using a hybrid biological-materials interface with sustainably generated H₂ as the reductant or to reduce CO into value-added C₂ products acetate and ethanol using a hybrid molecular-materials interface to construct a biomimetic, bimetallic active site. Taken together, our program in catalysis for energy and sustainability has revealed that combining more conventional design strategies in synergistic ways can lead to advances in artificial photosynthesis.

Solvent-controlled self-assembly of tetrapodal [4 + 4] phosphate organic molecular cage

Two flexible subcomponents, namely tris(4-formylphenyl)phosphate and tris(2-aminoethyl)amine, are assembled into a tetrapodal [4 + 4] cage depending on the solvent effect. Single-crystal structure analysis reveals that the caivity is surrounded by four phosphate uints. Good selectivity of CO₂ adsorption over CH₄ is demonstrated by the gas adsorption experiment.

Embedding alkenes within an icosahedral inorganic fullerene {(NH4)42[Mo132O372(L)30(H2O)72]} for trapping volatile organics

Eight alkene-functionalized molybdenum-based spherical Keplerate-type (inorganic fullerene) structures have been obtained via both direct and multistep synthetic approaches. Driven by the opportunity to design unique host-guest interactions within hydrophobic, π-electron rich confined environments, we have synthesised {(NH₄)₄₂[Mo₁₃₂O₃₇₂(L)₃₀(H₂O)₇₂]}, where L = (1) acrylic acid, (2) crotonic acid, (3) methacrylic acid, (4) tiglic acid, (5) 3-butenoic acid, (6) 4-pentenoic acid, (7) 5-hexenoic acid, and (8) sorbic acid. The compounds, which are obtained in good yield (10-40%), contain 30 carboxylate-coordinated alkene ligands which create a central cavity with hydrophobic character. Extensive Nuclear Magnetic Resonance (NMR) spectroscopy studies contribute significantly to the complete characterisation of the structures obtained, including both 1D and 2D measurements. In addition, single-crystal X-ray crystallography and subsequently-generated electron density maps are employed to highlight the distribution in ligand tail positions. These alkene-containing structures are shown to effectively encapsulate small alkyl thiols (1-propanethiol (A), 2-propanethiol (B), 1-butanethiol (C), 2-butanethiol (D) and 2-methyl-1-propanethiol (E)) as guests within the central cavity in aqueous solution. The hydrophobically driven clustering of up to 6 equivalents of volatile thiol guests within the central cavity of the Keplerate-type structure results in effective thermal protection, preventing evaporation at elevated temperatures (ΔT ≈ 25 K).

Hydrogen-bonded porous frameworks constructed by rigid π-conjugated molecules with carboxy groups

This review covers construction and properties of porous molecular crystals (PMCs) constructed through hydrogen-bonding of C3-symmetric, rigid, π-conjugated molecular building blocks possessing carboxyaryl groups, which was reported in the last 5 years by the author’s group. PMCs with well-defined, self-standing pores have been attracted attention due to various functionalities provided by selective and reversible inclusion of certain chemical species into the pores. However, it has been recognized for long time that construction of PMCs with permanent porosity is not easy due to weakness of noncovalent intermolecular interactions. Systematic construction of PMCs have been limited so far. To overcome this problem, the author has proposed a unique molecular design concept based on C3-symmetric π-conjugated molecules (C3PIs) possessing o-bis(4-carboxyphenyl)benzene moieties in their periphery and demonstrated that C3PIs systematically yielded hydrogen-bonded organic frameworks (HOFs) composed of H-bonded 2D hexagonal networks (H-HexNets) or interpenetrated 3D pcu-networks, which exhibit permanent porosity, significant thermal stability, polar solvent durability, robustness/flexibility, and/or multifunctionality.

Transforming surface-modified metal organic framework powder into room temperature porous liquids via an electrical balance strategy

The design of MOF-based room temperature porous liquids via an electrical balance strategy.

Switching porosity of stable triptycene-based cage via solution-state assembly processes

It is a great challenge to tune the porosity of porous materials. As most porous organic cages are soluble, solution processability can be a possible way to regulate the porosity of such materials. Herein, a triptycene-based cage (TC) is demonstrated to be stable in acid, base or boiling water. Meanwhile, its porosity can be tuned by adjusting the solution-state assembly processes. TC molecules crystallized slowly from solution exhibit nearly no porosity to nitrogen (off-state). While, after rapid precipitating from methanol/dichloromethane solution, the obtained TC (TC-rp) is in a porous state and exhibit a high BET surface area of 653 m² g⁻¹ (on-state).

Here, a kind of triptycene-based cage is demonstrated to have good chemical stability in acid, base and boiling water. Moreover, its porosity can be tuned by varying the solution-state assembly processes.

Toward crystalline porosity estimators for porous molecules

Our data-mining of crystalline molecular materials reveals the correlations between the molecular and crystalline porosity.

Immobilisation of Homogeneous Pd Catalysts within a Type I Porous Liquid

An N-heterocyclic carbene-based palladium complex was successfully immobilised on the inner surfaces of hollow silica nanospheres. The external surfaces of these spheres were functionalised with a corona-canopy to produce a Type I porous liquid. To confirm the successful immobilisation of the catalytic precursor, the porous liquid system was explored using the Heck reaction as a model reaction. This work demonstrated that homogeneous catalysts can be successfully immobilised within porous liquids in principle and that the approach used could be readily adapted for the immobilisation of other systems.

Hysteresis in the gas sorption isotherms of metal–organic cages accompanied by subtle changes in molecular packing

Cooperative gas uptake in metal–organic cages is tuned using supramolecular chemistry.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

A Molecular Coordination Template Strategy for Designing Selective Porous Aromatic Framework Materials for Uranyl Capture

Uranium capture from seawater could solve increasing energy demand and enable a much needed relaxing from fossil fuels. Low concentration (∼3 ppb), competing cations (especially vanadium) and pH-dependent speciation prohibit highly efficient uranium uptake. Despite intensive research, selective extraction of uranyl ions over vanadyl units remains a tremendous challenge. Here, we adopted a molecular coordination template strategy to design a uranyl-specific bis-salicylaldoxime entity and decorated it into a highly porous aromatic framework (PAF-1) by programmable assembly. The superstructure (MISS-PAF-1) gives a strong affinity that removes 99.97% of uranium in 120 min. Notably, it binds to the uranyl ion at least 100 times more selectively than 14 different cations tested, including the vanadyl ion, in simulated seawater at ambient pH. Real seawater samples collected from the Bohai Sea achieve 5.79 mg g^-1 of uranium capacity over 56 days without PAF degradation, exceeding a 4-fold higher amount than commercial adsorbents.

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-bdc^2- = 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-bdc^2- = 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.

Soft Porous Crystal Based upon Organic Cages That Exhibit Guest-Induced Breathing and Selective Gas Separation

Soft porous crystals (SPCs) that exhibit stimuli-responsive dynamic sorption behavior are attracting interest for gas storage/separation applications. However, the design and synthesis of SPCs is challenging. Herein, we report a new type of SPC based on a [2 + 3] imide-based organic cage (NKPOC-1) and find that it exhibits guest-induced breathing behavior. Various gases were found to induce activated NKPOC-1 crystals to reversibly switch from a "closed" nonporous phase (α) to two porous "open" phases (β and γ). The net effect is gate-opening behavior induced by CO₂ and C3 hydrocarbons. Interestingly, NKPOC-1-α selectively adsorbs propyne over propylene and propane under ambient conditions. Thus, NKPOC-1-α has the potential to separate binary and ternary C3 hydrocarbon mixtures, and the performance was subsequently verified by fixed bed column breakthrough experiments. In addition, molecular dynamics calculations and in situ X-ray diffraction experiments indicate that the gate-opening effect is accompanied by reversible structural transformations. The adsorption energies from molecular dynamics simulations aid are consistent with the experimentally observed selective adsorption phenomena. The understanding gained from this study of NKPOC-1 supports the further development of SPCs for applications in gas separation/storage because SPCs do not inherently suffer from the recyclability problems often encountered with rigid materials.