Storage of methane and freon by interstitial van der Waals confinement

Calix[4]arene-Based Porous Organic Nanosheets

Calixarenes are a common motif in supramolecular chemistry but have rarely been incorporated in structurally well-defined covalent 2D materials. Such a task is challenging, especially without a template, because of the nonplanar configuration and conformational flexibility of the calixarene ring. Here, we report the first-of-a-kind solvothermal synthesis of a calix[4]arene-based 2D polymer (CX4-NS) that is porous, covalent, and isolated as few-layer thick (3.52 nm) nanosheets. Experimental and theoretical characterization of the nanosheets is presented. Atomic force microscopy and transmission electron microscopy results are consistent with the calculated lowest energy state of the polymer. In the lowest energy state, parallel layers are tightly packed, and the calixarenes adopt the 1,2-alternate conformation, which gives rise to a two-dimensional pattern and a rhombic unit cell. We tested the material's ability to adsorb I₂ vapor and observed a maximum capacity of 114 wt %. Molecular simulations extended to model I₂ capture showed excellent agreement with experiments. Furthermore, the material was easily regenerated by mild ethanol washings and could be reused with minimal loss of efficiency.

Importance of Molecular Meshing for the Stabilization of Solvophobic Assemblies

The effect of the methyl groups in neutral gear-shaped amphiphiles (GSAs) on the stability of nanocubes was investigated using a novel C_{2 v}-symmetric GSA, which was synthesized using selective alternate trilithiation of a pentabrominated hexaphenylbenzene derivative. The lack of only one methyl group in the GSA decreased the association constant for the assembly of the nanocube by 3 orders of magnitude. A surface analysis recently developed by the authors (SAVPR: surface analysis with varying probe radii) was carried out for characteristic isomers of the nanocube consisting of C_{2 v}-symmetric GSAs. It was found that the methyl groups near the equator of the nanocube play a significant role in the stabilization of the nanocubes.

Porous Polycalix[4]arenes for Fast and Efficient Removal of Organic Micropollutants from Water

Organic micropollutants are hazards to the environment and human health. Conventional technologies are often inefficient at removing them from wastewater. For example, commercial activated carbon (AC) exhibits slow uptake rates, limited capacities, and is costly to regenerate. Here, we report the utility of porous calix[4]arene-based materials, CalPn (n = 2-4), for water purification. Calixarenes are a common motif in supramolecular chemistry but have rarely been incorporated into extended, porous networks such as organic polymers. CalPn exhibit pollutant uptake rates (k_obs) and adsorption capacities (q_max) that are among the highest reported. For example, the k_obs of CalP4 for bisphenol A (BPA) is 2.12 mg/g·min, which is significantly higher (16 to 240 times) than k_obs for ACs and 1.4 times higher than that of the most efficient material previously reported; the q_max of CalP4 for BPA is 403 mg/g. The CalPn polymers can be regenerated several times, with performance levels left undiminished, by a simple wash procedure that is less energy intensive than that required for ACs. These findings demonstrate the potential of calixarene-based materials for organic micropollutant removal.

The Structure of Diphenyl Ether-Methanol in the Electronically Excited and Ionic Ground States: A Combined IR/UV Spectroscopic and Theoretical Study

Diphenyl ether offers competing docking sites for methanol: the ether oxygen acts as a common hydrogen-bond acceptor and the π system of each phenyl ring allows for OH-π interactions driven by electrostatic, induction, and dispersion forces. Based on investigations in the electronic ground state (S₀ ), we present a detailed study of the electronically excited state (S₁ ) and the ionic ground state (D₀ ), in which an impact on the structural preference is expected compared with the S₀ state. Dispersion forces in the electronically excited state were analyzed by comparing the computed binding energies at the coupled-cluster-singles (CCS) and approximate coupled-cluster-singles-doubles levels of theory (CC2 approximation). By applying UV/IR/UV spectroscopy, we found a more strongly bound OH-π structure in the S₁ state compared with the S₀ state, in agreement with spin-component-scaled CC2 calculations. A structural rearrangement into a non-hydrogen-bonded structure takes places upon ionization in the D₀ state, which was revealed by using IR photodissociation spectroscopy and confirmed by theory.

Luminescent Lanthanoid Calixarene Complexes and Materials

This review aims to provide an overview of recent examples of lanthanoid-calixarene complexes incorporated into light-emitting materials. Background information on the antenna effect and early work on lanthanoid complexes on calixarenes is provided to set the context. Classes of materials discussed include polymers, nanoparticles, and metal clusters.

Chemical reaction within a compact non-porous crystal containing molecular clusters without the loss of crystallinity

A yellow crystal with {FeII4O₄} cubes is modified to a black crystal with {FeIII4O₄} cubes via a SC–SC transformation.

The very rare occurrence of a gas–solid chemical reaction has been found to take place on a molecule within a compact non-porous crystal without destroying its long-range structural order and retaining similar crystal structures when yellow crystals of FeII4(mbm)₄Cl₄(MeOH)₄ were exposed to air to give black [FeIII4(mbm)₄Cl₄(OH)₄]·2H₂O. The latter cannot be synthesised directly. The original cluster underwent an exchange of methanol to hydroxide, an oxidation of Fe(ii) to Fe(iii), a change in stereochemistry and hydration while the packing and space-group remained unaltered.

Structural characterization of framework-gas interactions in the metal-organic framework Co2(dobdc) by in situ single-crystal X-ray diffraction

The crystallographic characterization of framework-guest interactions in metal-organic frameworks allows the location of guest binding sites and provides meaningful information on the nature of these interactions, enabling the correlation of structure with adsorption behavior. Here, techniques developed for in situ single-crystal X-ray diffraction experiments on porous crystals have enabled the direct observation of CO, CH4, N2, O2, Ar, and P4 adsorption in Co2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate), a metal-organic framework bearing coordinatively unsaturated cobalt(ii) sites. All these molecules exhibit such weak interactions with the high-spin cobalt(ii) sites in the framework that no analogous molecular structures exist, demonstrating the utility of metal-organic frameworks as crystalline matrices for the isolation and structural determination of unstable species. Notably, the Co-CH4 and Co-Ar interactions observed in Co2(dobdc) represent, to the best of our knowledge, the first single-crystal structure determination of a metal-CH4 interaction and the first crystallographically characterized metal-Ar interaction. Analysis of low-pressure gas adsorption isotherms confirms that these gases exhibit mainly physisorptive interactions with the cobalt(ii) sites in Co2(dobdc), with differential enthalpies of adsorption as weak as -17(1) kJ mol-1 (for Ar). Moreover, the structures of Co2(dobdc)·3.8N2, Co2(dobdc)·5.9O2, and Co2(dobdc)·2.0Ar reveal the location of secondary (N2, O2, and Ar) and tertiary (O2) binding sites in Co2(dobdc), while high-pressure CO2, CO, CH4, N2, and Ar adsorption isotherms show that these binding sites become more relevant at elevated pressures.

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.

Hydrophobic metallo-supramolecular Pd2L4 cages for zwitterionic guest encapsulation in organic solvents

Hydrophobic metallo-supramolecular cages are selectively encapsulating hydrophilic zwitterionic guests in organic solvents via synergetic multivalent recognition.

The role of non-covalent interaction for the adsorption of CO2 and hydrocarbons with per-hydroxylated pillar[6]arene: a computational study

A systematic study has been performed with DFT calculations for the physisorption of CO₂, CH₄, and n-butane gases by pillar[6]arene (PA[6]) in gas phase.

Reversible structural switching of a metal–organic framework by photoirradiation

A photoresponsive metal organic framework material undergoes switching of its pore volume and sorption capacity.

Pillar[n]arene-based supramolecular organic frameworks with high hydrocarbon storage and selectivity

The high hydrocarbon storage capacity and adsorption selectivity of two low-density, solution-processable pillar[n]arene-based SOFs have been investigated for the first time.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure I Clathrate Hydrate

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates.

Redox-Active Metal-Organic Composites for Highly Selective Oxygen Separation Applications

A redox-active metal-organic composite material shows improved and selective O2 adsorption over N2 with respect to individual components (MIL-101 and ferrocene). The O2 sensitivity of the composite material arises due to the formation of maghemite nanoparticles with the pore of the metal-organic framework material.

Hydration-dependent anomalous thermal expansion behaviour in a coordination polymer

A polar coordination polymer has been synthesised that exhibits anomalous anisotropic thermal expansion. The guest water molecules of the as-synthesised complex can be removed on activation without loss of crystal singularity to yield the fully dehydrated form that shows considerably different thermal expansion behaviour compared to its hydrated analogue.

A crystalline sponge based on dispersive forces suitable for X-ray structure determination of included molecular guests

A crystalline porous material showing one-dimensional (1-D) rectangular micropores (12 × 9 Å2) has been assembled from a semirigid macrocyclic tetraimine and EtOAc as the templating agent. The 1-D nature of the material is intrinsic to the conformationally rigid structure of a macrocyclic sub-unit bearing four cyclohexylidene residues. The multiple dispersive forces established among the aliphatic residues glutted the 1-D channels and provided thermal stability to the material at temperatures below 160 °C. Upon removal of the template, the structure of the empty solid exhibited permanent microporosity (SBET = 342 m2 g-1). Being a true molecular sponge, the channel framework of this material allowed the inclusion of a variety of molecular sample guests without compromising its crystalline nature. Remarkably, this crystalline material enabled the structure determination by X-ray diffraction of the included molecules. Theoretical studies demonstrated the vital role played by the dispersive forces in the overall stabilization of the crystal packing.

Structurally Flexible C₃-Symmetric Receptors for Molecular Recognition and Their Self-Assembly Properties

The bioinspired design and synthesis of building blocks and their assemblies by the supramolecular approach has ever fascinated scientists to utilize such artificial systems for numerous purposes. Flexibility is a basic feature of natural systems. However, in artificial systems this is difficult to control, especially if there is no preorganization of the component(s) of a system. We have designed and synthesized a series of C3 -symmetric N-bridged flexible receptors and successfully utilized them to selectively entrap the notorious and toxic nitrate anion in aqueous medium. This was the first report of highest binding affinity for the nitrate anion in aqueous medium. An impressive self-sorting phenomenon of reversibly formed hydrogen-bonded capsules, which self-assembled from flexible tripodal receptors having branches of similar size and bearing the same amide functionality, has been disclosed. Encapsulated nitrate anion has been further utilized for the photochemical [2+2] cycloaddition reaction for the synthesis of strained four-membered ring structures through dynamic self-assembly. In this Personal Account, we summarize these results showing the utility of naturally inspired flexibility in artificial systems.

Tuning fluorocarbon adsorption in new isoreticular porous coordination frameworks for heat transformation applications

We report adsorption behaviors of a typical fluorocarbon R22 (CHClF₂) in a new series of isoreticular porous coordination frameworks [Zn₄O(bpz)₂(ldc)].

Adsorption heat transformation is one of the most energy-efficient technologies, which relies much on the type and performance of the adsorbent–adsorbate pair. Here, we report adsorption behaviors of a typical fluorocarbon R22 (CHClF₂) in a new series of isoreticular porous coordination polymers [Zn₄O(bpz)₂(ldc)], in which the typical Zn₄O clusters are connected by hydrophobic 3,3′,5,5′-tetramethyl-4,4′-bipyrazolate (bpz^2–) and different linear dicarboxylates (ldc^2–) to form non-interpenetrated pcu networks with variable pore sizes, shapes, and volumes. Fluorocarbon sorption measurements of these materials revealed high R22 uptakes of 0.73–0.97 g g^–1 (0.62–0.65 g cm^–3) at 298 K and 1 bar and working capacities of 0.41–0.72 g g^–1 (0.35–0.47 g cm^–3) between 273 and 313 K at about 0.13, 0.11 and 0.52 bar, respectively, as well as very large diffusion coefficients of 5.1–7.3 × 10^–7 cm² s^–1. Noteworthily, the R22 sorption performance can be dramatically improved by subtle modification of the pore size and shape, demonstrating porous coordination polymer–fluorocarbon as a promising adsorbent–adsorbate pair for heat transformation applications.

Selective CO2 gas adsorption in the narrow crystalline cavities of flexible peptide metallo-macrocycles

The BF₄⁻ salt of peptide Ni(ii)-macrocycle demonstrated selective CO₂ gas adsorption (ca. 6–7 molecules per macrocycle) into its narrow cavities.

In silico studies on the origin of selective uptake of carbon dioxide with cucurbit[7]uril amorphous material

The efficient capture and storage of flue gases is of current interest due to environmental problems. The DFT calculation demonstrates the origin of the physisorption of flue gases (CO₂, N₂and CH₄) on amorphous solid cucurbit[7]uril.

Controlling the assembly of cyclotriveratrylene-derived coordination cages

Ligand-functionalised cyclotriveratrylene derivatives self-assemble to afford coordination cages and topologically non-trivial constructs, including controlled assembly of M₃L₂ metallo-cryptophane and M₆L₈ cages.

Concomitant polymorphs of p-iso-propylcalix[4]arene

Sublimation of p-iso-propylcalix[4]arene (form I_P) under reduced pressure results in the concomitant formation of two new polymorphs (forms II_P and III_P).