Leonard J. Barbour

X-Seed - A Software Tool for Supramolecular Crystallography

Abstract X-Seed is a software tool for X-ray crystallographers and runs under any of the 32-bit Microsoft Windows operating systems including 95, 98, Millennium Edition, 2000 and XP. Many of its features are tailored to meet the needs of scientists interested in solid-state supramolecular chemistry. The two primary functions of the software are (i) to serve as a graphical interface to the SHELX-97 program suite, and (ii) to produce high-quality molecular graphics images. In addition to these features, many other useful functions are also implemented in order to facilitate the process of crystal structure solution, refinement, analysis and presentation.

An intermolecular (H2O)10 cluster in a solid-state supramolecular complex

Chemical self-assembly is the process by which ‘programmed’ molecular subunits spontaneously form complex supramolecular frameworks,. This approach has been applied to many model systems, in which hydrogen bonds,, metal–ligand coordination or other non-covalent interactions typically control the self-assembly process. In biology, self-assembly is generally dynamic and depends on the cooperation of many such non-covalent interactions. Water can play an important role in these biological self-assembly processes, for example by stabilizing the native conformation of biopolymers. Hydrogen-bonded (H2O)n clusters, can play an important role in stabilizing some supramolecular species, both natural and synthetic, in aqueous solution. Here we report the preparation and crystal structure of a self-assembled, three-dimensional supramolecular complex that is stabilized by an intricate array of non-covalent interactions involving contributions from solvent water clusters, most notably a water decamer ((H2O)10) with an ice-like molecular arrangement. These findings show that the degree of structuring that can be imposed on water by its surroundings, and vice versa, can be profound.

Metal sulfonatocalix[4,5]arene complexes: bi-layers, capsules, spheres, tubular arrays and beyond

Abstract The main focus of this review is the self-assembly in aqueous solutions of bowl-shaped sodium p-sulfonatocalix[4,5]arenes with main group, transition metal and lanthanoid species, and with various organic molecules as additional supramolecular building components, for example 18-crown-6, and other macrocycles, pyridine N-oxide, amino-acids, and more. The versatility of building up new materials based on these components is demonstrated by the formation of a diverse range of complex inclusion structures assembled through π-stacking, hydrogen bonding and coordination interactions. There are up–down arrangements of calix[4]arenes in hydrophobic–hydrophobic bi-layer structures with the positively charged species and included molecules between the layers. A variant of this is prevalent in structures incorporating 18-crown-6 which, in essence, are built up of globular superanions or ionic capsules, for example {Na+⊂(18-crown-6)(H2O)n}⊂{(p-sulfonatocalix[4]arene4−)2}7−, n=0 or 2. These can crystallize, often selectively, polynuclear hydrolytic M(III) cations [M2(OH)2(H2O)8]4+, [M3(OH)4(H2O)10]5+, [M4(OH)6(H2O)12]6+, M=Cr or Rh, or [A113O4(OH)24(H2O)12]7+, depending on the pH and other synthetic parameters. Lanthanide(III) ions form a range of complexes at specific pH in the presence of the calixarene and crown ether, including complexes containing the capsule [{18-crown-6}⊂{(M(H2O)73+)1.33(p-sulfonatocalix[4]arene4−)}2], for the smaller lanthanides, or the Ferris-wheel type structure [{La3+⊂(18-crown-6)(OH2)3}⋂{(p-sulfonatocalix[4]arene4−+2H+)}]+, for the larger lanthanide. In the presence of pyridine N-oxide, at pH 4 where the calixarenes take on 5− charge, an up–up arrangement of sulfonated calixarenes results, either assembled in icosahedral spheres, or infinite chiral, helical nano-tubes.

Exceptionally large positive and negative anisotropic thermal expansion of an organic crystalline material.

In general, the relatively modest expansion experienced by most materials on heating is caused by increasing anharmonic vibrational amplitudes of the constituent atoms, ions or molecules. This phenomenon is called positive thermal expansion (PTE) and usually occurs along all three crystallographic axes. In very rare cases, structural peculiarities may give rise either to anomalously large PTE, or to negative thermal expansion (NTE, when lattice dimensions shrink with heating). As NTE and unusually large PTE are extremely uncommon for molecular solids, mechanisms that might give rise to such phenomena are poorly understood. Here we show that the packing arrangement of a simple dumbbell-shaped organic molecule, coupled with its intermolecular interactions, facilitates a cooperative mechanical response of the three-dimensional framework to changes in temperature. A series of detailed structural determinations at 15-K intervals has allowed us to visualize the process at the molecular level. The underlying mechanism is reminiscent of a three-dimensional (3D) folding trellis and results in exceptionally large and reversible uniaxial PTE and biaxial NTE of the crystal. Understanding such mechanisms is highly desirable for the future design of sensitive thermomechanical actuators.

Organization of the interior of molecular capsules by hydrogen bonding

The enclosure of functional entities within a protective boundary is an essential feature of biological systems. On a molecular scale, free-standing capsules with an internal volume sufficiently large to house molecular species have been synthesized and studied for more than a decade. These capsules have been prepared by either covalent synthesis or self-assembly, and the internal volumes have ranged from 200 to 1,500 Å3. Although biological systems possess a remarkable degree of order within the protective boundaries, to date only steric constraints have been used to order the guests within molecular capsules. In this article we describe the synthesis and characterization of hexameric molecular capsules held together by hydrogen bonding. These capsules possess internal order of the guests brought about by hydrogen bond donors within, but not used by, the framework of the capsule. The basic building blocks of the hexameric capsules are tetrameric macrocycles related to resorcin[4]arenes and pyrogallol[4]arenes. The former contain four 1,3-dihydroxybenzene rings bridged together by -CHR- units, whereas the latter contain four 1,2,3-trihydroxybenzene rings bridged together. We now report the synthesis of related mixed macrocycles, and the main focus is on the macrocycle composed of three 1,2,3-trihydroxybenzene rings and one 1,3-dihydroxybenzene ring bridged together. The mixed macrocycles self-assemble from a mixture of closely related compounds to form the hexameric capsule with internally ordered guests.

Macrocyclic polyethers as probes to assess and understand alkali metal cation-π interactions

Abstract Cation-π interactions of alkali metals with arenes have been known in the gas phase for two decades but solid-state structural data have become available only recently. The quest for solid-state evidence is described here. Complexation of Na+ and K+ by arene-terminated lariat ethers has provided important insights into the cation-π interaction.

C60 and C70 Compounds in the Pincerlike Jaws of Calix[6]arene

Isostructural species are found in the solid state for the supramolecular 1:2 complexes of a calix[6]arene molecule and either C60 or C70 (see the structure of the [(calix[6]arene)(C60 )2 ] complex on the right). The calixarene assumes a double-cone conformation, and the overall structure is a result of the complementarity of the building blocks with respect to size and form-in other words, the shallow calixarene cavity and the fullerene surface have similar curvatures.

The liquid phase epitaxy approach for the successful construction of ultra-thin and defect-free ZIF-8 membranes: pure and mixed gas transport study

The liquid-phase epitaxy (LPE) method was effectively implemented to deliberately grow/construct ultrathin (0.5-1 μm) continuous and defect-free ZIF-8 membranes. Permeation properties of different gas pair systems (O2-N2, H2-CO2, CO2-CH4, C3H6-C3H8, CH4-n-C4H10) were studied using the time lag technique.

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.

Direct evidence for single-crystal to single-crystal switching of degree of interpenetration in a metal-organic framework.

A known doubly interpenetrated metal-organic framework with the formula [Zn2(ndc)2(bpy)] possesses minimal porosity when activated. We show not only that the material converts to its triply interpenetrated analogue upon desolvation, but also that the transformation occurs in a single-crystal to single-crystal manner under ambient conditions. The mechanism proposed for the conversion is supported by computational methods and by analogy with the solid-state behavior of an analogous system.