Storage of methane and freon by interstitial van der Waals confinement

Single-crystal to Single-crystal Transformations - Guest Removal and Substitution in a Robust Solvent-templated Metallocyclic Compound

The formation, structure, and single-crystal to single-crystal transformations of the dinuclear metallocycle [Ag2L2](PF6)2·2CH3CN (L = 2,3,5,6-tetrafluoro-1,4-bis(2-methylimidazole-1-yl-methyl)benzene) are described. The acetonitrile guest molecules can be removed without loss of single crystallinity to afford a porous material. CO2 sorption isotherms were recorded at various temperatures and the structures and sorption behaviour of the crystals are compared with those of a previously reported analogous system. Upon exposure to acetone vapour, the desolvated crystals undergo a single-crystal to single-crystal transformation to form the acetone solvate.

Porous organic cages

Porous materials are important in a wide range of applications including molecular separations and catalysis. We demonstrate that covalently bonded organic cages can assemble into crystalline microporous materials. The porosity is prefabricated and intrinsic to the molecular cage structure, as opposed to being formed by non-covalent self-assembly of non-porous sub-units. The three-dimensional connectivity between the cage windows is controlled by varying the chemical functionality such that either non-porous or permanently porous assemblies can be produced. Surface areas and gas uptakes for the latter exceed comparable molecular solids. One of the cages can be converted by recrystallization to produce either porous or non-porous polymorphs with apparent Brunauer-Emmett-Teller surface areas of 550 and 23 m2 g(-1), respectively. These results suggest design principles for responsive porous organic solids and for the modular construction of extended materials from prefabricated molecular pores.

Soft porous crystals

The field of host-guest complexation is intensely attractive from diverse perspectives, including materials science, chemistry and biology. The uptake and encapsulation of guest species by host frameworks are being investigated for a wide variety of purposes, including separation and storage using zeolites, and recognition and sensing by enzymes in solution. Here we focus on the concept of the cooperative integration of 'softness' and 'regularity'. Recent developments on porous coordination polymers (or metal-organic frameworks) have provided the inherent properties that combine these features. Such soft porous crystals exhibit dynamic frameworks that are able to respond to external stimuli such as light, electric fields or the presence of particular species, but they are also crystalline and can change their channels reversibly while retaining high regularity. We discuss the relationship between the structures and properties of these materials in view of their practical applications.

Nanoporous crystals of calixarene/porphyrin supramolecular complex functionalized by diffusion and coordination of metal ions

A highly nanoporous material has been obtained by self-assembly of calixarene and porphyrin building blocks. This supramolecular zeolite-like structure was successively functionalized by diffusion and coordination of metal ions to form a new bifunctionalized nanoporous material containing a porphyrinic pigment together with a metal center.

Computational studies of load-dependent guest dynamics and free energies of inclusion for CO2 in low-density p-tert-butylcalix[4]arene at loadings up to 2:1

The structure, dynamics, and free energies of absorption of CO(2) by a low-density structure (P4/n) of calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO(2):TBC4 have been studied by using molecular dynamics simulations with two sources of initial TBC4 structures (TBC4-T and TBC4-U). The CO(2)/TBC4 complex structure is very sensitive to the initial lattice spacing of TBC4. From the computed radial distribution functions of CO(2) molecules, a CO(2) dimer is observed for TBC4-T and a cage-interstitial CO(2) structure is suggested for TBC4-U. The dynamics of the CO(2) molecules show little initial TBC4 structural dependency. The free energy of inclusion for a single CO(2) in this TBC4 structure for various loadings is -4.0 kcal/mol at 300 K and -1.8 kcal/mol at 450 K, showing that CO(2) inclusion is favored. The fully loaded 1:1 CO(2):TBC4 system is slightly less favorable at -3.9 and -1.2 kcal/mol at 300 and 450 K, respectively. The first CO(2) added beyond 1:1 loading shows a significant drop in absorption energy to -1.9 and +1.9 kcal/mol at 300 and 450 K. These data are consistent with experimental results showing that low-density structures of TBC4 are able to absorb CO(2) at loadings greater than 1:1 but retention is lower than for 1:1 loaded systems indicating the free energy of inclusion for addition of the CO(2) above 1:1 is less favorable.

Substituent effects on xenon binding affinity and solution behavior of water-soluble cryptophanes

A water-soluble triacetic acid cryptophane-A derivative (TAAC) was synthesized and determined by isothermal titration calorimetry and fluorescence quenching assay to have a xenon association constant of 33,000 M(-1) at 293 K, which is the largest value measured for any host molecule to date. Fluorescence lifetime measurements of TAAC in the presence of varying amounts of xenon indicated static quenching by the encapsulated xenon and the presence of a second non-xenon-binding conformer in solution. Acid-base titrations and aqueous NMR spectroscopy of TAAC and a previously synthesized tris(triazole propionic acid) cryptophane-A derivative (TTPC) showed how solvation of the carboxylate anions can affect the aqueous behavior of the large, nonpolar cryptophane. Specifically, whereas only the crown-crown conformer of TTPC was observed, a crown-saddle conformer of TAAC was also assigned in aqueous solution.

Discrete MnAlPO-5 nanocrystals synthesized by an ionothermal approach

Discrete MnAlPO-5 nanocrystals with monomodal particle size distribution (diameter of 80 nm) were synthesized using an ionothermal technique; the pore filling effect of the ionic liquid molecules during the synthesis of microporous nanocrystals is verified.

Closed-pore crystal capable of adsorbing CO2 onto isolated cavities generated by disorderly mixing of substituents on host skeleton

A single crystal adsorbent, [Rh(II)(2)(bza)(4)(2,3-empyz)](n) (2,3-empyz = 2-ethyl-3-methylpyrazine) (1), was synthesized by self-assembly reaction of a Rh(2) benzoate complex and substituted pyrazine linker. The compound consists of one-dimensional zigzag chains, which generated a closed-pore structure without channels. The cavities were statistically generated by the static disorder of substituents on pyrazine and are separated by long intervals within the crystal. The property of CO(2) absorption was characterized in this closed-pore system. The CO(2) inclusion structure was determined by single-crystal X-ray diffraction measurements. These studies suggest that CO(2) molecules were adsorbed and diffused in the nonporous crystal with the isolated cavities.

A molecular turnstile in para-octanoyl calix[4]arene nanocapsules

The thermal treatment of different inclusion complexes of para-octanoyl calix[4]arene leads to the formation of a guest-free van der Waals nanocapsular structure possessing a remarkable stability caused by the high mobility of alkanoyl arms.

Engineering void space in organic van der Waals crystals: calixarenes lead the way

Seemingly non-porous organic solids have the ability for guest transport and have also been shown to absorb gases, including hydrogen, methane and acetylene, to varied extents. These materials also show potential for gas separation technology, display remarkable water transport through hydrophobic crystals, and clearly show that molecules within crystals are capable of cooperating with guests as they move through non-porous environments. This work is presented within a broader topic which also encompasses crystal engineering and (microporous) metal-organic frameworks (MOF's).

Evaluation of the adsorption free energy of light guest molecules in nanoporous host structures

We present a simple method for estimation of the adsorption free energy of lightweight featureless molecules in nanoporous structures. The partition function of the system is computed quantum mechanically within the ideal gas approximation, and leads directly to the free adsorption energy. Storage capacities can be estimated by combining the ideal gas results with the real gas equation of state. Two possible implementations of the approach are discussed. The assumptions and potential sources of errors in the calculations are analyzed. The performance of the method is illustrated for graphitic slit pores and solid C60.

Sorption of nitrogen oxides in a nonporous crystal

The uptake of various nitrogen oxides was studied with the well known nonporous p-tert-butylcalix[4]arene under ambient conditions.

Frustrated Organic Solids Display Unexpected Gas Sorption

Between nonsorptive solvate and desolvate forms of p-tert-butylcalix[5]arene lies a frustrated crystalline region in which there is rapid sorption of gases. From sorption studies, we conclude that the frustrated form is porous and that the intermediate form represents a new type of material that was previously unrecognized by traditional gas sorption trends or ideals for other active systems.

Carbon Nanorings: A Challenge to Theoretical Chemistry

High-level quantum-chemical methods show that the binding in the inclusion complex of hexamethylbenzene (HMB) in 6-cycloparaphenilacetylene (6-CPPA) cannot be explained only in terms of electrostatic interactions-caused by the polarization associated to curved pi-conjugated systems-and the inclusion of dispersion forces is definitely needed. The theoretical description of van der Waals interactions is notoriously complicated and in fact some DFT methods cannot even predict the existence of the relatively small supramolecular nanoring studied here. However, ab initio MP2 calculations agree with experimental data and show that, in the considered complex, the HMB fragment is placed at the center of the 6-CPPA ring. The binding energy, which is not available experimentally, is calculated to be around -14 kcal mol(-1) with a lower limit of -19 kcal mol(-1).