Vincent J. Smith

Tunable anisotropic thermal expansion of a porous zinc(II) metal-organic framework

A novel three-dimensional metal-organic framework (MOF) that displays anisotropic thermal expansion has been prepared and characterized by single-crystal X-ray diffraction (SCD) and thermal analysis. The as-prepared MOF has one-dimensional channels containing guest molecules that can be removed and/or exchanged for other guest molecules in a single-crystal to single-crystal fashion. When the original guest molecules are replaced there is a noticeable effect on the host mechanics, altering the thermal expansion properties of the material. This study of the thermal expansion coefficients of different inclusion complexes of the host MOF involved systematic alteration of guest size, i.e., methanol, ethanol, n-propanol, and isopropanol, showing that fine control over the thermal expansion coefficients can be achieved and that the coefficients can be correlated with the size of the guest. As a proof of concept, this study demonstrates the realizable principle that a single-crystal material with an exchangeable guest component (as opposed to a composite) may be used to achieve a tunable thermal expansion coefficient. In addition, this study demonstrates that greater variance in the absolute dimensions of a crystal can be achieved when one has two variables that affect it, i.e., the host-guest interactions and temperature.

Giant Negative Area Compressibility Tunable in a Soft Porous Framework Material

A soft porous material [Zn(L)2(OH)2]n·Guest (where L is 4-(1H-naphtho[2,3-d]imidazol-1-yl)benzoate, and Guest is water or methanol) exhibits the strongest ever observed negative area compressibility (NAC), an extremely rare property, as at hydrostatic pressure most materials shrink in all directions and few expand in one direction. This is the first NAC reported in metal-organic frameworks (MOFs), and its magnitude, clearly visible and by far the highest of all known materials, can be reversibly tuned by exchanging guests adsorbed from hydrostatic fluids. This counterintuitive strong NAC of [Zn(L)2(OH)2]n·Guest arises from the interplay of flexible [-Zn-O(H)-]n helices with layers of [-Zn-L-]4 quadrangular puckered rings comprising large channel voids. The compression of helices and flattening of puckered rings combine to give a giant piezo-mechanical response, applicable in ultrasensitive sensors and actuators. The extrinsic NAC response to different hydrostatic fluids is due to varied host-guest interactions affecting the mechanical strain within the range permitted by exceptionally high flexibility of the framework.

Preparation, Thermal Behaviour and Solid-state Structures of Inclusion Complexes of Permethylated-β-cyclodextrin with the Garlic-derived Antithrombotics (E)- and (Z)-Ajoene

The E and Z isomers of the antithrombotic ajoene (4,5,9-trithiadodeca-1,6,11-triene 9-oxide), components of garlic (Allium Sativa, L.), were complexed with heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB) to yield solid inclusion compounds with the compositions TRIMEB·(E)-ajoene·0.5H2O (1) and TRIMEB·(Z)-ajoene (2). These species were investigated by thermal analysis techniques (HSM, TGA, DSC), powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction at 173 K. Thermal analysis of 1 revealed dehydration followed by melting at 143.8°C, and only melting at 140.5°C for 2. The complexes yielded different PXRD traces, indicating crystallization in different arrangements. Single-crystal X-ray methods revealed the space groups P21 (1) and P212121 (2) and the simultaneous presence of the guest stereoisomers in both complexes. Refinement of guest site-occupancies showed that each complex crystal consists of a mixture of diastereomers in 1:1 molar ratio. The different crystal packing arrangements for 1 and 2 are induced by the distinctly different modes of inclusion of the E and Z isomers.

Single-crystal to single-crystal guest exchange and phase transformations in a porous metallocycle

Single crystals of a previously reported porous metallocycle [Ag2L2](BF4)2·2CH3CN (1), where L is the ligand 1,4-bis(2-methylimidazol-1-ylmethyl)benzene, were grown from acetonitrile and immersed in different organic solvents. The crystals thus treated were subjected to single-crystal X-ray diffraction analysis, which revealed that the acetonitrile guest molecules had been replaced by the solvent that the compound was exposed to, yielding five different solvates: [Ag2L2](BF4)2·2(CH3)2CO (2), [Ag2L2](BF4)2·2CHCl3 (3), [Ag2L2](BF4)2·C6H6 (4), [Ag2L2](BF4)2·C6H4F2 (5), [Ag2L2](BF4)2·C7H8 (6). Thermogravimetric analysis supports these findings.

Inclusion complexes of 2-methoxyestradiol with dimethylated and permethylated β-cyclodextrins: models for cyclodextrin–steroid interaction

Summary The interaction between the potent anticancer agent 2-methoxyestradiol (2ME) and a series of cyclodextrins (CDs) was investigated in the solid state using thermal analysis and X-ray diffraction, while the possibility of enhancing its poor aqueous solubility with CDs was probed by means of equilibrium solubility and dissolution rate measurements. Single crystal X-ray diffraction studies of the inclusion complexes between 2ME and the derivatised cyclodextrins heptakis(2,6-di-O-methyl)-β-CD (DIMEB) and heptakis(2,3,6-tri-O-methyl)-β-CD (TRIMEB) revealed for the first time the nature of the encapsulation of a bioactive steroid by representative CD host molecules. Inclusion complexation invariably involves insertion of the D-ring of 2ME from the secondary side of each CD molecule, with the 17-OH group generally hydrogen bonding to a host glycosidic oxygen atom within the CD cavity, while the A-ring and part of the B-ring of 2ME protrude from the secondary side. In the case of the TRIMEB·2ME complex, there is evidence that complexation proceeds with mutual conformational adaptation of host and guest molecules. The aqueous solubility of 2ME was significantly enhanced by CDs, with DIMEB, TRIMEB, randomly methylated β-CD and hydroxypropyl-β-CD being the most effective hosts. The 2:1 host–guest β-CD inclusion complex, prepared by two methods, yielded very rapid dissolution in water at 37 °C relative to untreated 2ME, attaining complete dissolution within 15 minutes (co-precipitated complex) and 45 minutes (complex from kneading).

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

Sublimation of p-iso-propylcalix[4]arene under reduced pressure results in the concomitant formation of two new polymorphs (forms IIP and IIIP). Both forms consist of interdigitated dimers of calixarene molecules, as does the known form IP. Despite similar inclusion behaviour to that of p-tert-butylcalix[4]arene, p-iso-propylcalix[4]arene does not appear to favour the formation of a transiently porous material.

Solvent- and Pressure-Induced Phase Changes in Two 3D Copper Glutarate-Based Metal–Organic Frameworks via Glutarate (+gauche ⇄ −gauche) Conformational Isomerism

Two isoreticular three-dimensional copper(II) glutarate-based pillared-layered metal-organic frameworks (MOFs) with flexible pillars, [Cu2(glu)2(bpa)] and [Cu2(glu)2(bpp)] (bpa = 1,2-bis(4-pyridyl)ethane; bpp = 1,3-bis(4-pyridyl)propane), undergo spontaneous phase changes upon solvent loss at room temperature. Using single-crystal X-ray diffraction analysis (SCXRD), we show that the phase changes result in new narrow-channel forms that experience a large reduction in solvent-accessible volume. Moreover, the [Cu2(glu)2(bpa)] MOF displays a stepped sorption isotherm for the uptake of CO2 at room temperature. This is indicative of reversion of the framework to the wide-channel form under CO2 pressure. Supercritical CO2 was used to isolate the gas-included structures, and by means of SCXRD we were able to determine the positions of the CO2 molecules in the channels of the frameworks. Finally, we report the use of molecular modeling simulations to elucidate the phase-change mechanism, including the energetic changes involved. Structural limitations in both MOFs allow for only direct gauche-gauche enantiomeric interconversion of the glutarate moieties.

Ring size effect on the solid state assembly of propargyl substituted hexa- and octacyclic peptoids

The investigation of the solid state assembly of propargyl substituted hexa- and octacyclic peptoids highlights the effect of ring size in determining the packing arrangement of the macrocycles. A layered arrangement is obtained in the case of the hexacyclic peptoid 1 and a tubular arrangement in the case of the octacyclic peptoid 2. Guest molecules either intercalate between the layers as in 1 or are located within the peptoid nanotube as in 2.

Creation of new guest accessible space under gas pressure in a flexible MOF: multidimensional insight through combination of in situ techniques

Metal-organic frameworks (MOFs) are an important class of porous materials with numerous potential applications. Molecular level understanding of various processes involving MOFs is very important in order to design porous materials with improved properties. Here we describe the elucidation by means of single-crystal X-ray diffraction (SCD) of three different phases of Zn2(bdc)2(bpy) at different pressures of CO2. Moreover, this is a rare example where new space is created in the structure under gas pressure - space that was not previously occupied by guest molecules in any of the related structures. In addition to in situ SCD and sorption analysis, pressure-gradient differential scanning calorimetry (PG-DSC) provides very useful information about the system. This study represents the first instance where PG-DSC has been used to study the sorption behavior of a flexible porous framework.

18-Crown-6 templates offset-linked pyrogallol[4]arene dimers

Offset-linked dimers of C-ethylpyrogallol[4]arene (PgC2) ⊂ 18-crown-6 have been synthesised and structurally elucidated. The 18-crown-6 acts as a template/guest for the assembly of the first observed tri-solvent-bridged, extended pyrogallol[4]arene-based dimers. Oppositely, adjacently and diagonally positioned macrocycles are connected through a complex network of solvent-mediated H bonds.