Storage of methane and freon by interstitial van der Waals confinement

Isostructurality, polymorphism and mechanical properties of some hexahalogenated benzenes: the nature of halogen...halogen interactions

The nature of intermolecular interactions between halogen atoms, X...X (X = Cl, Br, I), continues to be of topical interest because these interactions may be used as design elements in crystal engineering. Hexahalogenated benzenes (C6Cl(6-n)Br(n), C6Cl(6-n)I(n), C6Br(6-n)I(n)) crystallise in two main packing modes, which take the monoclinic space group P2(1)/n and the triclinic space group P1. The former, which is isostructural to C6Cl6, is more common. For molecules that lack inversion symmetry, adoption of this monoclinic structure would necessarily lead to crystallographic disorder. In C6Cl6, the planar molecules form Cl...Cl contacts and also pi...pi stacking interactions. When crystals of C6Cl6 are compressed mechanically along their needle length, that is, [010], a bending deformation takes place, because of the stronger interactions in the stacking direction. Further compression propagates consecutively in a snakelike motion through the crystal, similar to what has been suggested for the motion of dislocations. The bending of C6Cl6 crystals is related to the weakness of the Cl...Cl interactions compared with the stronger pi...pi stacking interactions. The triclinic packing is less common and is restricted to molecules that have a symmetrical (1,3,5- and 2,4,6-) halogen substitution pattern. This packing type is characterised by specific, polarisation-induced X...X interactions that result in threefold-symmetrical X3 synthons, especially when X = I; this leads to a layered pseudohexagonal structure in which successive planar layers are inversion related and stacked so that bumps in one layer fit into the hollows of the next in a space-filling manner. The triclinic crystals shear on application of a mechanical stress only along the plane of deformation. This shearing arises from the sliding of layers against one another. Nonspecificity of the weak interlayer interactions here is demonstrated by the structure of twinned crystals of these compounds. One of the compounds studied (1,3,5-tribromo-2,4,6-triiodobenzene) is dimorphic, adopting both the monoclinic and triclinic structures, and the reasons for polymorphism are suggested. To summarise, both chemical and geometrical models need to be considered for X...X interactions in hexahalogenated benzenes. The X...X interactions in the monoclinic group are nonspecific, whereas in the triclinic group some X...X interactions are anisotropic, chemically specific and crystal-structure directing.

Energetics of small molecule and water complexation in hydrophobic calixarene cavities

Calixarenes grafted on silica are energetically uniform hosts that bind aromatic guests with 1:1 stoichiometry, as shown by binding energies that depend upon the calixarene upper rim composition but not on their grafted surface density (0.02-0.23 nm(-2)). These materials are unique in maintaining a hydrophilic silica surface, as probed by H2O physisorption measurements, while possessing a high density of hydrophobic binding sites that are orthogonal to the silica surface below them. The covalently enforced cone-shaped cavities and complete accessibility of these rigidly grafted calixarenes allow the first unambiguous measurements of the thermodynamics of guest interaction with the same calixarene cavities in aqueous solution and vapor phase. Similar to adsorption into nonpolar protein cavities, adsorption into these hydrophobic cavities from aqueous solution is enthalpy-driven, which is in contrast to entropy-driven adsorption into water-soluble hydrophobic hosts such as beta cyclodextrin. The adsorption thermodynamics of several substituted aromatics from vapor and liquid are compared by (i) describing guest chemical potentials relative to pure guest, which removes differences among guests because of aqueous solvation and van der Waals contacts in the pure condensed phase, and (ii) passivating residual guest binding sites on exposed silica, titrated by water during adsorption from aqueous solution, using inorganic salts before vapor adsorption. Adsorption isotherms depend only upon the saturation vapor pressure of each guest, indicating that guest binding from aqueous or vapor media is controlled by van der Waals contacts with hydrophobic calixarene cavities acting as covalently assembled condensation nuclei, without apparent contributions from CH-pi or other directional interactions. These data also provide the first direct quantification of free energies for interactions of water with the calixarene cavity interior. The calixarene-water interface is stabilized by approximately 20 kJ/mol relative to the water-vapor interface, indicating that water significantly competes with the aromatic guests for adsorption at these ostensibly hydrophobic cavities. This result is useful for understanding models of water interactions with other concave hydrophobic surfaces, including those commonly observed within proteins.

Boron Macrocycles Having a Calix-Like Shape. Synthesis, Characterization, X-ray Analysis, and Inclusion Properties

The syntheses, structure, and inclusion properties of trinuclear boron compounds having a calix-like shape are described. The compounds have been obtained via self-assembly reactions between salicylaldehyde derivatives and 3-aminophenylboronic acid, whereby the formation of three N --> B coordination bonds favored the oligomerization. The products have high melting points (>370 degrees C), are stable to moisture, and have good solubility in organic solvents; the latter property is useful for host-guest recognition experiments. The structural analysis by X-ray diffraction revealed that diverse conformations are possible because of the presence of two different units of aromatic rims. A cone-cone (double-cone) conformation is observed for three of these compounds, while the remaining one has a cone-partial cone conformation. An analysis of the molecular packing showed that the molecules are stacked in columns in two different orientations in relation to the organization of the macrocycles when referred to the N-B bonds. The inclusion properties toward primary amines and ammonium chlorides were analyzed by titration experiments and monitored by UV spectroscopy, whereby association constants on the order of 10(2)-10(3) M(-1) were determined.

Crystal porosity and the burden of proof

The study of porosity in the context of crystal engineering is rapidly growing in intensity. However, claims of porosity are often highly subjective and use of the term "porous" is susceptible to abuse. This contribution discusses some of the criteria to be considered when stating that a particular crystal structure is porous.

Direct Synthesis of Deep Cavity p-Phenylcalix[4]arene in Poly(Ethylene Glycol), and its Self-Association in the Solid State

p-Phenylcalix[4]arene is formed directly from p-phenylphenol in 66% yield (50% isolated yield) using poly(ethylene glycol) as the reaction medium, with crystallization of the pure cavitand from toluene mediated by p-carborane. The solid-state structure comprises interlocking columnar arrays.

Formation and Characterization of Crystalline Molecular Arrays of Gas Molecules in a 1-Dimensional Ultramicropore of a Porous Copper Coordination Polymer

Molecules and atoms confined in a nanospace may have properties distinctly different from those of the bulk fluid, owing to the formation of a specific molecular array characteristic of nanospace. In situ synchrotron powder X-ray diffraction measurements have been used to observe confined guest molecules such as N2, O2, Ar, and CH4 in the well-regulated ultramicropore of a copper coordination polymer, 1 ([Cu2(pzdc)2pyz]: pzdc = 2,3-pyrazinedicarboxylate and pyz = pyrazine). The obtained crystal structures indicate that guest molecules are confined in a linear fashion to form crystalline-like regular ordered arrays, in contrast to the situation in the gas and liquid state, even at temperatures above the boiling point, and the ordered arrays are characteristic of the kind of gas molecule and the geometrical and potential properties of the ultramicropore of 1.

Guest-Induced Supramolecular Isomerism in Inclusion Complexes of T-Shaped Host 4,4-Bis(4′-hydroxyphenyl)cyclohexanone

The T-shaped host molecule 4,4-bis(4'-hydroxyphenyl)cyclohexanone (1) has an equatorial phenol group and a cyclohexanone group along the arms and an axial phenol ring as the stem. The equatorial phenyl ring adopts a "shut" or "open" conformation, like a windowpane, depending on the size of the guest (phenol or o/m-cresol), for the rectangular voids of the hydrogen-bonded ladder host framework. The adaptable cavity of host 1 expands to 11x15-18 A through the inclusion of water with the larger cresol and halophenol guests (o-cresol, m-cresol, o-chlorophenol, and m-bromophenol) compared with a size of 10x13 A for phenol and aniline inclusion. The ladder host framework of 1 is chiral (P2(1)) with phenol, whereas the inclusion of isosteric o- and m-fluorophenol results in a novel polar brick-wall assembly (7x11 A voids) as a result of auxiliary C-H...F interactions. The conformational flexibility of strong O-H...O hydrogen-bonding groups (host 1, phenol guest), the role of guest size (phenol versus cresol), and weak but specific intermolecular interactions (herringbone T-motif, C-H...F interactions) drive the crystallization of T-host 1 towards 1D ladder and 2D brick-wall structures, that is, supramolecular isomerism. Host 1 exhibits selectivity for the inclusion of aniline in preference to phenol as confirmed by X-ray diffraction, 1H NMR spectroscopy, and thermogravimetry-infrared (TG-IR) analysis. The T(onset) value (140 degrees C) of aniline in the TGA is higher than those of phenol and the higher-boiling cresol guests (T(onset)=90-110 degrees C) because the former structure has more O-H...N/N-H...O hydrogen bonds than the clathrate of 1 with phenol which has O-H...O hydrogen bonds. Guest-binding selectivity for same-sized phenol/aniline molecules as a result of differences in hydrogen-bonding motifs is a notable property of host 1. Host-guest clathrates of 1 provide an example of spontaneous chirality evolution during crystallization and a two-in-one host-guest crystal (phenol and aniline), and show how weak C-H...F interactions (o- and m-fluorophenol) can change the molecular arrangement in strongly hydrogen-bonded crystal structures.

Gas Adsorption Sites in a Large-Pore Metal-Organic Framework

The primary adsorption sites for Ar and N2 within metal-organic framework-5, a cubic structure composed of Zn4O(CO2)6 units and phenylene links defining large pores 12 and 15 angstroms in diameter, have been identified by single-crystal x-ray diffraction. Refinement of data collected between 293 and 30 kelvin revealed a total of eight symmetry-independent adsorption sites. Five of these are sites on the zinc oxide unit and the organic link; the remaining three sites form a second layer in the pores. The structural integrity and high symmetry of the framework are retained throughout, with negligible changes resulting from gas adsorption.

Grand canonical monte carlo simulation study of methane adsorption at an open graphite surface and in slit-like carbon pores at 273 K

Grand canonical Monte Carlo (GCMC) simulation was used for the systematic investigation of the supercritical methane adsorption at 273 K on an open graphite surface and in slit-like micropores of different sizes. For both considered adsorption systems the calculated excess adsorption isotherms exhibit a maximum. The effect of the pore size on the maximum surface excess and isosteric enthalpy of adsorption for methane storage at 273 K is discussed. The microscopic detailed picture of methane densification near the homogeneous graphite wall and in slit-like pores at 273 K is presented with selected local density profiles and snapshots. Finally, the reliable pore size distributions, obtained in the range of the microporosity, for two pitch-based microporous activated carbon fibers are calculated from the local excess adsorption isotherms obtained via the GCMC simulation. The current systematic study of supercritical methane adsorption both on an open graphite surface and in slit-like micropores performed by the GCMC summarizes recent investigations performed at slightly different temperatures and usually a lower pressure range by advanced methods based on the statistical thermodynamics.

Head-to-tail self-assembly of a calix[4]arene inclusion polymer controlled by a pendant arm

A calix[4]arene functionalized at one phenolic group with a pendant ethoxy acetate group, forms an inclusion complex that is stable even in the presence of other potential guest molecules.

Organic crystals absorb hydrogen gas under mild conditions

We have studied the hydrogen sorption on three well-known organic hosts that possess vacant lattice voids large enough to accommodate H2 molecules.

Crystal engineering of nonporous organic solids for methane sorption

The low density polymorph of the well-known host p-tert-butylcalix[4]arene absorbs more methane than p-tert-pentylcalix[4]arene at room temperature and 1 atm pressure, but the order of absorption is reversed at 38 atm with p-tert-pentylcalix[4]arene absorbing more.

Enclathration of morpholinium cations by Dianin's compound: salt formation by partial host-to-guest proton transfer

In spite of partial deprotonation upon inclusion of morpholine, Dianin's compound maintains its well-known clathrate structure in the solid state.

Bilayers, Corrugated Bilayers, and Coordination Polymers of p-Sulfonatocalix[6]arene

Exploration into the host-guest supramolecular chemistry of p-sulfonatocalix[6]arene with pyridine N-oxide and 4,4'-dipyridine N,N'-dioxide has resulted in the characterization of three new structural motifs with the calixarene in the "up-down" double partial cone conformation. Two are hydrogen-bonded network structures formed with pyridine N-oxide and either nickel or lanthanide metal counterions (1 and 2, respectively). Complex 1 displays host-guest interactions between pyridine N-oxide and the calixarene in the presence of hexaaquanickel(II) counterions. Complex 2 demonstrates selective coordination modes for different lanthanides involving the calixarene and pyridine N-oxide. The third structure, 3, is a coordination polymer which is formed with 4,4'-dipyridine N,N'-dioxide molecules which span a hydrophilic layer and join lanthanide/p-sulfonatocalix[6]arene fragments. Although complexes 1-3 all have the calixarene in the "up-down" double partial cone conformation, 1 and 3 form bilayer arrangements within the extended structures while 2 forms a previously unseen corrugated bilayer arrangement.

Molecular Encapsulation of Gases

The principles and techniques of molecular encapsulation, as applied to environmentally, biologically, and commercially important gases, are reviewed. The gases may be trapped within natural (clathrates, cyclodextrins) or synthetic (cryptophanes, carcerands, calixarenes) cavities. The physical and chemical features of the cavities are key to understanding which gases may be trapped and to what extent. These trapping materials possess a host of applications, from gas sensing and separation to acting as storage devices and microreaction vessels.

Boron–nitrogen macrocycles: a new generation of calix[3]arenes

The simple condensation reaction of 3,5-di-tert-butyl salicylaldehyde and 3-aminophenylboronic acid leads to a trimeric macrocyclic compound. The ability of this molecule to include small organic molecules was in a first approximation analyzed by (1)H NMR spectroscopy and X-ray crystallography.

Thermally programmable gas storage and release in single crystals of an organic van der Waals host

A single crystal of a low density form of guest-free p-tert-butylcalix[4]arene can take up and release small guest molecules by controlling the temperature and pressure without changing the structure. Using NMR spectroscopy with flowing hyperpolarized xenon, we have shown that at room temperature access of xenon to the pore system is difficult, whereas it is relatively easy at 100 degrees C. There are good prospects for simple van der Waals materials such as the title material to be used as programmable zeolite mimics.

Physicochemical aspects of host-guest compounds

The macro properties of crystalline inclusion compounds depend on their structures. Their thermal stabilities are a function of the strength and the directionality of the various nonbonded interactions occurring in the host-guest assembly. Their lattice energies, as measured by the method of atom-atom potentials, correlate with the thermodynamics of the guest-release reactions and the selectivity that a given host displays for a particular guest. The kinetics of solid-host:vapor-guest reactions and of guest exchange are important in our understanding of catalytic processes. Crystal engineering, in which materials of predetermined properties may be synthesized, is still at the empirical stage.

Formation of a novel 1D supramolecule [HgCl2(ptz)]2.HgCl2 (ptz = phenothiazine): a new precursor to submicrometer Hg2Cl2 rods

A novel supramolecule [HgCl(2)(ptz)](2).HgCl(2) (ptz = phenothiazine) with uncoordinated inorganic salt HgCl(2) presented in a 1D chain was first prepared and then successfully applied as a new precursor in the preparation of submicrometer Hg(2)Cl(2) rods. Single crystal X-ray analysis showed that the 1D chain structure is stabilized by hydrogen bonds between adjacent chains and the coordination mode of the ligand phenothiazine is unusual with large steric inhibition other than the chain directions. The results revealed that the particular chain structure plays a significant role in the formation of the Hg(2)Cl(2) rods.

Hydrogen-bonded superstructures of a small host molecule and lanthanide aquo ions

Despite containing only relatively small molecular components, the structure of [Eu(H(2)O)(9)](1.5)(CTV)(6)(CH(3)CN)(5.5)(H(2)O)(7.5)[Co(C(2)B(9)H(11))(2)](4.5) (C(191)H(337.5)B(81)Co(4.5)Eu(1.5)N(5.5)O(57), a = 54.590(2) A, b = 37.5788(17) A, c = 31.8067(14) A, beta = 116.573(2) degrees, monoclinic, C2/c, Z = 8) is of an unusually large size and level of complexity, with an intricate hydrogen-bonding network formed between lanthanide aquo ions, water, and the small host molecule cyclotriveratrylene (=CTV). The 3D hydrogen-bonded network structure that results is unique and too complicated to apply the usual topological analysis. Instead a simplified model using structural subunits is described.

Guest transport in a nonporous organic solid via dynamic van der Waals cooperativity

A well-known organic host compound undergoes single-crystal-to-single-crystal phase transitions upon guest uptake and release. Despite a lack of porosity of the material, guest transport through the solid occurs readily until a thermodynamically stable structure is achieved. In order to actively facilitate this dynamic process, the host molecules undergo significant positional and/or orientational rearrangement. This transformation of the host lattice is triggered by weak van der Waals interactions between the molecular components. In order for the material to maintain its macroscopic integrity, extensive cooperativity must exist between the molecules throughout the crystal, such that rearrangement can occur in a well-orchestrated fashion. We demonstrate here that even weak dispersive forces can exert a profound influence over solid-state dynamics.

Hydrogen bonds seal single-molecule capsules

In the presence of methanol the tetrakis(benzoxazines) complex tetramethylammonium cation within the cavity, and the cavity is completely sealed by two intramolecular hydrogen bonds between amide groups. The Cl(-) anion is found external to the cavity. In CHCl(3), Me(4)N(+) is complexed within the cavity, but the Cl(-) anion acts as a stopper in the upper rim of the cavity, hydrogen-bonded to the amide groups. The solution results are supported by single-crystal X-ray structural studies of both the single-molecule molecular capsules, and those stoppered by Cl(-).