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

Solvent-free mechanochemistry for the preparation of mixed-ligand cuboctahedral porous coordination cages

This study investigates post-synthetic ligand exchange in a series of copper(II) and chromium(II) cuboctahedral cages of the formula M24(R-bdc)24 through solvent-free mechanochemistry for the preparation of mixed-ligand cages. While solvent-based ligand exchange does not proceed when the cages are insoluble or when they are dissolved in non-coordinating solvents, solvent-free mechanochemistry can be used to prepare a number of mixed-ligand cages featuring a variety of functional groups regardless of cage solubility. We further extend this strategy to intercage ligand exchange reactions where the solid-state reaction of cages proceeds in just ten minutes while corresponding solvent-based reactions require more than one week of reaction time. The results highlight mechanochemically-facilitated ligand exchange as an exceptionally facile and efficient method for the production of mixed-ligand cuboctahedral cages.

Coalescence of Porous Coordination Cages into Crystalline and Amorphous Bulk Solids

Discrete porous coordination cages are attractive as a solution processable material whose porosity is not predicated on a network structure. Here, we leverage the peripheral functionalization of these cage structures to obtain 12 novel, solution processable, porous coordination cages that afford crystalline and amorphous single-phase millimeter-scale monolithic bulk structures (six of each) upon solidification. These structures are based upon prototypal metal-organic polyhedra [Cu24(5-x-isophthalate)24] (where x = NH2, OH), wherein meta-substitution of linker ligands with acyl chloride or isocyanate moieties afforded amide and urethane functional groups, respectively. These porous cage structures were obtainable via direct synthesis between a metal salt and a ligand as well as postsynthetic modification of the cage and formed monoliths following centrifugation and drying of the product. We rationalize their self-assembly as colloidal packing of nanoscale cuboctahedral cages through weak interactions between their hydrophobic alkyl/aromatic surfaces. In general, amorphous solids were obtained via rapid precipitation from the mother liquor upon methanol addition, while crystalline solids could be obtained only following further chloroform and pyridine additions. The structure of the materials is confirmed via gas sorption and spectroscopic methods, while powder X-ray diffraction and transmission electron microscopy are used to determine the nature of these bulk solids.

Stabilization of Lantern-Type Metal-Organic Cages (MOCs) by Protective Control of Ligand Exchange Rates

Self-assembling systems in nature display remarkable complexity with assemblies of different sub-units to generate functional species. Synthetic analogues of such systems are a challenge, often requiring the ability to bias distributions that are under thermodynamic assembly control. Using lantern-type MOCs (metal-organic cages) as a prototypical self-assembling system, herein we explore the role that steric bulk plays in controlling the exchange rate of ligands in paddlewheel-based assemblies, and thus the stability of cages, in competitive self-assembling scenarios. The effective lifetime of the lantern-type MOCs varies over an order of magnitude depending on the steric bulk proximal to the metal nodes with lifetimes of the cages ranging from tens of minutes to several hours. The bulk of the coordinating solvents likewise reduces the rate of ligand exchange, and thus yields longer-lived species. Understanding this subtle effect has implications for controlling the stability of complex assemblies in competitive environments with implications for guest release and application.

Statistical Distribution of Binary Ligands within Rhodium-Organic Octahedra Tunes Microporosity in Their Assemblies

Structure-porosity relationships for metal-organic polyhedra (MOPs) are hardly investigated because they tend to be amorphized after activation, which inhibits crystallographic characterization. Here, we show a mixed-ligand strategy to statistically distribute two distinct carbazole-type ligands within rhodium-based octahedral MOPs, leading to systematic tuning of the microporosity in the resulting amorphous solids.

Pore-Networked Soft Materials Based on Metal-Organic Polyhedra

ConspectusThe last two decades have witnessed a tremendous development of crystalline microporous adsorbents in a wide range of applications including molecular adsorption, storage and separation, purification, as well as catalysis. The main players as porous materials that have contributed to the developments are extended molecular frameworks (e.g., metal-organic frameworks, MOFs; covalent-organic frameworks, COFs) or discrete porous molecules (e.g., metal-organic cages, MOCs; porous organic cages, POCs) thanks to the high degrees of freedom in their structural designability and tunability. To overcome the processability issue originating from their powder forms after synthesis, one main strategy is to hybridize the microporous adsorbents as pore-containing fillers with solvents or polymers as processable matrices to produce porous soft materials, such as porous liquids, gels/aerogels, and mixed-matrix membranes, depending on the form of matrix used. Nevertheless, the fabrication of "ideal" hybrid materials relies on the homogeneous distribution of the pore-containing fillers within the matrices. It is still challenging to find a versatile way to solve the aggregation issues of fillers and their insufficient interaction with the matrices, which are concerned with inhibiting the translation of the distinctive properties of microporous adsorbents into the obtained hybrid soft materials.Herein, we describe a new bottom-up approach for the fabrication of "pore-networked soft materials" based on the concept of directly interconnecting the pore-containing fillers into a continuous pore network within the matrices. The advantages of the pore-networking strategy lie in two main aspects: (i) the elimination of the need to struggle with the aggregation issue of fillers due to their overall interconnection throughout the matrices; (ii) the generation of continuous pore networks that guarantee the efficient molecular mass transfer in the materials. In this Account, we summarize our state-of-the-art progress of pore-networked soft materials based on the use of MOCs, alternatively called metal-organic polyhedra (MOPs) herein, as pore units for the pore network construction. The good solubility of MOPs in organic solvents allows them to be feasibly processed in solution, wherein the coordination of MOPs with organic linkers leads to the formation of linked MOP gels featuring not only intrinsic MOP cavities but also tunable extrinsic porosities generated between linked MOPs through the control of MOP/linker structures and network connectivity. Furthermore, the matrix of the linked MOP network, here referred to as the continuous phase with respect to the entire porous MOP network, is not limited to the solvents. We anticipate that the implementation of air, liquids, and polymers as the matrices could result in different forms of pore-networked soft materials like aerogels, foams, gels, monoliths, and membranes. For instance, we demonstrate the fabrication of linked MOP aerogel and permanently porous gel with their potential applications on selective CO2 photoreduction and gas sorption, respectively. We believe that the pore-network strategies will advance the development of porous soft materials featuring unique advantages and properties beyond the current hybrid systems.

(Bio)Functionalisation of Metal-Organic Polyhedra by Using Click Chemistry

The surface chemistry of Metal-Organic Polyhedra (MOPs) is crucial to their physicochemical properties because it governs how they interact with external substances such as solvents, synthetic organic molecules, metal ions, and even biomolecules. Consequently, the advancement of synthetic methods that facilitate the incorporation of diverse functional groups onto MOP surfaces will significantly broaden the range of properties and potential applications for MOPs. This study describes the use of copper(I)-catalysed, azide-alkyne cycloaddition (CuAAC) click reactions to post-synthetically modify the surface of alkyne-functionalised cuboctahedral MOPs. To this end, a novel Rh(II)-based MOP with 24 available surface alkyne groups was synthesised. Each of the 24 alkyne groups on the surface of the "clickable" Rh-MOP can react with azide-containing molecules at room temperature, without compromising the integrity of the MOP. The wide substrate catalogue and orthogonal nature of CuAAC click chemistry was exploited to densely functionalise MOPs with diverse functional groups, including polymers, carboxylic and phosphonic acids, and even biotin moieties, which retained their recognition capabilities once anchored onto the surface of the MOP.

Rapid post-synthetic modification of porous coordination cages with copper-catalyzed click chemistry

Novel cobalt calixarene-capped and zirconium-based porous coordination cages were prepared with alkyne and azide functionality to leverage post-synthetic modification by click chemistry. While the calixarene-capped cages showed impressive stability when exposed to the most straightforward copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reaction conditions with copper(II) sulfate and sodium ascorbate as the reducing agent, milder reaction conditions were necessary to perform analogous CuAAC reactions on zirconium-based cages. Reaction kinetics were monitored by IR spectroscopy, confirming rapid reaction times (<3 hours).

Porous Metal-Organic Cages Based on Rigid Bicyclo[2.2.2]oct-7-ene Type Ligands: Synthesis, Structure, and Gas Uptake Properties

Three new ligands containing a bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxydiimide unit have been used to assemble lantern-type metal-organic cages with the general formula [Cu4 L4 ]. Functionalisation of the backbone of the ligands leads to distinct crystal packing motifs between the three cages, as observed with single-crystal X-ray diffraction. The three cages vary in their gas sorption behaviour, and the capacity of the materials for CO2 is found to depend on the activation conditions: softer activation conditions lead to superior uptake, and one of the cages displays the highest BET surface area found for lantern-type cages so far.

Selectivity Rule of Cryptands for Anions: Molecular Rigidity and Bonding Site

Cryptands utilize inside CH or NH groups as hydrogen bond (H-bond) donors to capture anions such as halides. In this work, the nature and selectivity of confined hydrogen bonds inside cryptands were computationally analyzed with the energy decomposition scheme based on the block-localized wavefunction method (BLW-ED), aiming at an elucidation of governing factors in the binding between cryptands and anions. It was revealed that the intrinsic strengths of inward hydrogen bonds are dominated by the electrostatic attraction, while the anion preferences (selectivity) of inner CH and NH hydrogen bonds are governed by the Pauli exchange repulsion and electrostatic interaction, respectively. Typical conformers of cages are classified into two groups, including the C_3(h) -symmetrical conformers, in which all halide anions are located near the centroids of cages, and the "semi-open" conformers, which exhibit shifted bonding sites for different halide anions. Accordingly, the difference in governing factors of selectivity is attributed to either the rigidity of cages or the binding site of anions for these two groups. In details, the C₃ conformers of NH cryptands can be enlarged more remarkably than the C_3(h) -symmetrical conformers of CH cryptands as the size of anion (ionic radius) increases, resulting in the relaxation of the Pauli repulsion and a dramatic reduction in electrostatic attraction, which eventually rules the selectivity of NH cryptands for halide anions. By contrary, the CH cryptands are more rigid and cannot effectively reduce the Pauli repulsion, which subsequently governs the anion preference. Unlike C₃ conformers whose rigidity determines the selectivity, semi-open conformers exhibit different binding sites for different anions. From F^- to I^- , the bonding site shifts toward the outside end of the pocket inside the semi-open NH cryptand, leading to the significant reduction of the electrostatic interaction that dominates the anion preference. Differently, binding sites are much less affected by the size of anion inside the semi-open CH cryptand, in which the Pauli exchange repulsion remains the key factor for the selectivity of inner hydrogen bonds.

Anion Binding as a Strategy for the Synthesis of Porous Salts

Porous salts have recently emerged as a promising new class of ultratunable permanently microporous solids. These adsorbents, which were first reported as ionic solids based on porous cations and anions, can be isolated from a wide variety of charged, permanently porous coordination cages. A challenge in realizing the full tunability of such systems, however, lies in the fact that the majority of coordination cages for which surface areas have been reported are comprised of charge-balanced inorganic and organic building blocks that result in neutral cages. As such, most reported permanently porous coordination cages cannot be used as reagents in the synthesis of porous salts. Here, we show that the facile reaction of TBAX (TBA⁺ = tetra-n-butylammonium; X = F^- and Cl^-) with molybdenum paddlewheel-based coordination cages of the M₄L₄ and M₂₄L₂₄ lantern and cuboctahedra structure types, respectively, affords charged cages by virtue of coordination of halide anions to the internal and/or external metal sites on these structures, as confirmed by single-crystal X-ray diffraction, X-ray photoelectron spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. At a practical level, the TBAX/cage reactions, which are fully reversible upon isolation of the cage with the appropriate solvent, solubilize otherwise rigorously insoluble cages. This method significantly increases the solution processability of these highly porous solids. Toward the formation of new porous salts, halide binding also serves to incorporate charge on neutral cages and make them amenable to simple salt metathesis reactions to afford new porous salts based on anions and cations with intrinsic porosity. A combination of diffraction methods and a suite of spectroscopic tools confirms speciation of the isolated solids, which represent a new class of highly tunable porous salts. Ultimately, this work represents a roadmap for the preparation of new porous solids and showcases the utility and broad applicability of anion binding as a strategy for the synthesis of porous salts.

Post‐Synthetic Modification of a Porous Hydrocarbon Cage to Give a Discrete Co 24 Organometallic Complex**

A new alkyne‐based hydrocarbon cage was synthesized in high overall yield using alkyne‐alkyne coupling in the cage forming step. The cage is porous and displays a moderately high BET surface area (546 m² g⁻¹). The cage loses crystallinity on activation and thus is porous in its amorphous form, while very similar cages have been either non‐porous, or retained crystallinity on activation. Reaction of the cage with Co₂(CO)₈ results in exhaustive metalation of its 12 alkyne groups to give the Co₂₄(CO)₇₂ adduct of the cage in good yield.

Post-synthetic modifications of metal–organic cages

Metal-organic cages (MOCs) are discrete, supramolecular entities that consist of metal nodes and organic linkers, which can offer solution processability and high porosity. Thereby, their predesigned structures can undergo post-synthetic modifications (PSMs) to introduce new functional groups and properties by modifying the linker, metal node, pore or surface environment. This Review explores current PSM strategies used for MOCs, including covalent, coordination and noncovalent methods. The effects of newly introduced functional groups or generated complexes upon the PSMs of MOCs are also detailed, such as improving structural stability or endowing desired functionalities. The development of the aforementioned design principles has enabled systematic approaches for the development and characterization of families of MOCs and, thereby, provides insight into structure-function relationships that will guide future developments.

Metal-Organic Polyhedra as Building Blocks for Porous Extended Networks

Metal-organic polyhedra (MOPs) are a subclass of coordination cages that can adsorb and host species in solution and are permanently porous in solid-state. These characteristics, together with the recent development of their orthogonal surface chemistry and the assembly of more stable cages, have awakened the latent potential of MOPs to be used as building blocks for the synthesis of extended porous networks. This review article focuses on exploring the key developments that make the extension of MOPs possible, highlighting the most remarkable examples of MOP-based soft materials and crystalline extended frameworks. Finally, the article ventures to offer future perspectives on the exploitation of MOPs in fields that still remain ripe toward the use of such unorthodox molecular porous platforms.

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.

Surface chemistry of metal-organic polyhedra

Metal-organic polyhedra (MOPs) are discrete, intrinsically-porous architectures that operate at the molecular regime and, owing to peripheral reactive sites, exhibit rich surface chemistry. Researchers have recently exploited this reactivity through post-synthetic modification (PSM) to generate specialised molecular platforms that may overcome certain limitations of extended porous materials. Indeed, the combination of modular solubility, orthogonal reactive sites, and accessible cavities yields a highly versatile molecular platform for solution to solid-state applications. In this feature article, we discuss representative examples of the PSM chemistry of MOPs, from proof-of-concept studies to practical applications, and highlight future directions for the MOP field.

Polymer networks of imine-crosslinked metal–organic cages: tuneable viscoelasticity and iodine adsorption

The solution-state structure of MOP-15 is elucidated, enabling its direct use as a porous monomer for covalent polymer networks.

Controlling phase in low-nuclearity calixarene-capped porous coordination cages with ligand functionalization

Porosity in low-nuclearity coordination cages is relatively rare as cages with larger pore sizes are usually targeted as a way to increase gas adsorption capabilities in this promising class of...

Linking metal-organic cages pairwise as a design approach for assembling multivariate crystalline materials

Using metal-organic cages (MOCs) as preformed supermolecular building-blocks (SBBs) is a powerful strategy to design functional metal-organic frameworks (MOFs) with control over the pore architecture and connectivity. However, introducing chemical complexity into the network via this route is limited as most methodologies focus on only one type of MOC as the building-block. Herein we present the pairwise linking of MOCs as a design approach to introduce defined chemical complexity into porous materials. Our methodology exploits preferential Rh-aniline coordination and stoichiometric control to rationally link Cu4L4 and Rh4L4 MOCs into chemically complex, yet extremely well-defined crystalline solids. This strategy is expected to open up significant new possibilities to design bespoke multi-functional materials with atomistic control over the location and ordering of chemical functionalities.

A Family of Externally-Functionalised Coordination Cages

New synthetic routes are presented to derivatives of a (known) M8L12 cubic coordination cage in which a range of different substituents are attached at the C4 position of the pyridyl rings at either end of the bis(pyrazolyl-pyridine) bridging ligands. The substituents are (i) –CN groups (new ligand LCN), (ii) –CH2OCH2–CCH (containing a terminal alkyne) groups (new ligand LCC); and (iii) –(CH2OCH2)3CH2OMe (tri-ethyleneglycol monomethyl ether) groups (new ligand LPEG). The resulting functionalised ligands combine with M2+ ions (particularly Co2+, Ni2+, Cd2+) to give isostructural [M8L12]16+ cage cores bearing 24 external functional groups; the cages based on LCN (with M2+ = Cd2+) and LCC (with M2+ = Ni2+) have been crystallographically characterised. The value of these is twofold: (i) exterior nitrile or alkene substituents can provide a basis for further synthetic opportunities via ‘Click’ reactions allowing in principle a diverse range of functionalisation of the cage exterior surface; (ii) the exterior –(CH2OCH2)3CH2OMe groups substantially increase cage solubility in both water and in organic solvents, allowing binding constants of cavity-binding guests to be measured under an increased range of conditions.

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.

Integrated Photoresponsive Alkaline Earth Metal Coordination Networks: Synthesis, Topology, Photochromism and Photoluminescence Investigation

Photoresponsive materials are a key part of the age of smart technology that have potential in a broad range of applications. Coordination networks (CNs) are widely used due to their designability and stability. In this work, three novel alkaline earth metal coordination networks (AEM-CNs): [Mg(CMNDI)(H₂ O)₂ ], [Ca(CMNDI)(H₂ O)₂ ]⋅H₂ O, and [Sr(CMNDI)(H₂ O)(DMF)] with fsl, cds, and scn topology nets were synthetized via N,N'-bis(carboxymethyl)-1,4,5,8-naphthalenediimide (H₂ CMNDI); the scn net is not found in the Reticular Chemistry Structure Resource or ToposPro. The reusable and sensitive photochromic properties of the three CNs enable them to be used as secret inks or ultraviolet detectors. In addition, the CNs also exhibited reusable photoluminescent turn-off toward the drug molecules, balsalazide disodium (Bal.) and colchicine (Col.), with good limits of detection of 0.16 and 0.70 μM. To the best of our knowledge, this is the first study of a fluorescence sensor for Bal. Thus, the AEM-CNs provide a design idea for integrated photoresponsive materials that could be further improved in the near future by further study.

Assembly and Covalent Cross-Linking of an Amine-Functionalised Metal-Organic Cage

The incorporation of reactive functional groups onto the exterior of metal-organic cages (MOCs) opens up new opportunities to link their well-defined scaffolds into functional porous solids. Amine moieties offer access to a rich catalogue of covalent chemistry; however, they also tend to coordinate undesirably and interfere with MOC formation, particular in the case of Cu₂ paddlewheel-based MOCs. We demonstrate that tuning the basicity of an aniline-functionalized ligand enables the self-assembly of a soluble, amine-functionalized Cu₄L₄ lantern cage (1). Importantly, we show control over the coordinative propensity of the exterior amine of the ligand, which enables us to isolate a crystalline, two-dimensional metal-organic framework composed entirely of MOC units (2). Furthermore, we show that the nucleophilicity of the exterior amine of 1 can be accessed in solution to generate a cross-linked cage polymer (3) via imine condensation.

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.

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.

Porous metal-organic alloys based on soluble coordination cages

Diverse strategies for the preparation of mixed-metal three-dimensional porous solids abound, although many of them lend themselves only moderate levels of tunability. Herein, we report the design and synthesis of surface functionalized permanently microporous coordination cages and their use in the isolation of mixed metal solids. Judicious alkoxide-based ligand functionalization was utilized to tune the solubility of starting copper(ii)-based cages and their resulting compatibility with the mixed-cage approach described here. We further prepared a family of isostructural molybdenum(ii) cages for a subset of the ligands. The preparation of mixed-metal cage solids proceeds under facile conditions where solutions of parent cages are mixed and product phases isolated. A suite of spectroscopic and characterization tools confirm the starting cages are intact in the amorphous product. Finally, we show that utilization of precise ligand functional groups can be used to prepare mixed cage solids that can be easily and cleanly separated into their constituent components through simple solvent washing or solvent extraction techniques.

Using Low-Pressure Methane Adsorption Isotherms for Higher-Throughput Screening of Methane Storage Materials

A useful correlation between the low-pressure (up to 1.2 bar), low-temperature (195 K) and high-pressure (up to 65 bar), room temperature (298 K) methane storage properties of a range of porous materials is reported. Methane isotherms under these two sets of conditions show a remarkable agreement in both equilibrium adsorption and deliverable capacities for materials with pore volumes that are less than approximately 0.80 cm³/g. This trend holds well for the suite of metal-organic frameworks and porous coordination cages we studied, in addition to a zeolite and porous organic cage. Although it is well known that gravimetric gas storage capacity trends with gravimetric surface area, the 1.2 bar, 195 K excess adsorption capacity of a given framework is a better indicator of its room temperature, 65 bar capacity. Given the significantly smaller sample quantities needed for low-pressure measurements, greater accessibility to researchers around the world, accuracy of the measurement, and higher throughput, we envision this method as a rapid screening tool for the identification of methane storage materials. As excess/total adsorption and gravimetric/volumetric adsorption can be interconverted by simple utilization of the scalar quantities of pore volume or density, respectively, this method can be easily adapted to obtain both gravimetric and volumetric total adsorption capacities for a given adsorbent. In terms of volumetric methane adsorption, we further investigate the relationship between crystallographic and bulk density for the adsorbents studied here. With this analysis, it becomes apparent that in the absence of novel synthetic approaches, reported volumetric storage capacities should be viewed as an optimistic upper limit for a given material and not necessarily a true reflection of its actual adsorption properties as most MOFs have bulk densities that are less than half of their crystallographic values.