Jerry L. Atwood

A chiral spherical molecular assembly held together by 60 hydrogen bonds

Spontaneous self-assembly processes that lead to discretespherical molecular structures are common in nature. Sphericalviruses (such ashepatitis B) and fullerenes are well-known examples inwhich non-covalent and covalent forces,respectively, direct the assembly of smaller subunits intolarger superstructures. A common feature of theseshell-like architectures is their ability to encapsulateneutral and/or charged guests whose size, shape and chemicalexteriors complement those of the hosts innersurface,. Their interiors can often beregarded as a new phase of matter, capable of controlling the flowof reactants, transients and products, and of catalysingreactions of both chemical and biological relevance. Suchproperties have inspired the recent emergence ofmonomolecular and supramolecular dimeric molecularcapsules,, many of which have been basedon the head-to-head alignment of bowl-shapedpolyaromatic macrocycles such as calix[4]arenes,,. But true structural mimicry offrameworks akin to viruses and fullerenes, which are based onthe self-assembly of n > 3 subunits,and where surface curvature is supplied by edge sharing of regularpolygons, has remained elusive. Here we present anexample of such a system: a chiral spherical molecular assemblyheld together by 60 hydrogen bonds (1) (Fig. 1). We demonstrate the ability of 1, which consists of six calix[4]resorcinarenes 2 and eight water molecules, to self-assemble and maintain its structure in apolar media and to encapsulate guest species within a well-defined cavity that possesses an internal volume of about 1,375 Å3. Single crystal X-ray analysis shows that its topology resembles that of a spherical virus and conforms to the structure of a snub cube, one of the 13 Archimedean solids.

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.

Flexible (Breathing) Interpenetrated Metal-Organic Frameworks for CO2 Separation Applications

A breathing 2-fold interpenetrated microporous metal-organic framework was synthesized with a flexible tetrahedral organic linker and Zn(2) clusters that sorb CO(2) preferably over N(2) and H(2).

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.

Flexible metal–organic supramolecular isomers for gas separation

Three interpenetrated metal-organic supramolecular isomers were synthesised using a flexible tetrahedral organic linker and Zn(2) clusters that sorb CO(2) preferably over N(2), H(2) and methane at room temperature.

Supramolecular blueprint approach to metal-coordinated capsules

An important problem in designing any large network is the assembly of systems that are resilient to change. From a chemical point of view, an analogy can be used where one requires supramolecular assemblies to maintain their dimensionality combined with limited structural perturbation in response to variation in its intermolecular framework. The identification of hydrogen-bonded framework patterns within experimentally known supramolecular assemblies that are structurally robust to disruption and selective hydrogen substitution are envisioned to act as a supramolecular blueprint or template for metal-ion retroinsertion. Here, we report the formation of a large neutral discrete pseudo-spherical coordination capsule assembled from 6 pyrogallol[4]arene ligands and 24 Cu(II) metal ions. Amazingly, this coordination capsule is structurally analogous to its hydrogen-bonded counterpart. This result shows a robust ability of pyrogallol[4]arene molecules to self-assemble into large hexameric cage structures from either the hydrogen-bonding or metal-ligand coordination process. The identification of robust supramolecular assemblies that conserve their structure in response to interchangeability between hydrogen-bonded networks for metal coordination, or inversely, represents an important advancement in supramolecular design.

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.

Gas-induced transformation and expansion of a non-porous organic solid

Organic solids composed by weak van der Waals forces are attracting considerable attention owing to their potential applications in gas storage, separation and sensor applications. Herein we report a gas-induced transformation that remarkably converts the high-density guest-free form of a well-known organic host (p-tert-butylcalix[4]arene) to a low-density form and vice versa, a process that would be expected to involve surmounting a considerable energy barrier. This transformation occurs despite the fact that the high-density form is devoid of channels or pores. Gas molecules seem to diffuse through the non-porous solid into small lattice voids, and initiate the transition to the low-density kinetic form with approximately 10% expansion of the crystalline organic lattice, which corresponds to absorption of CO2 and N2O (refs 4,5). This suggests the possibility of a more general phenomenon that can be exploited to find more porous materials from non-porous organic and metal-organic frameworks that possess void space large enough to accommodate the gas molecules.

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.

Preparation and crystal structures of the complexes (η5-C5H4CPh2-η5-C13H8) MCl2 (M Zr, Hf) and the catalytic formation of high molecular weight high tacticity syndiotactic polypropylene

Abstract The reaction of MCl 4 (M  Zr, Hf) with the dilithium salt of 1-cycolopentadienyl-1-fluorenyl-1,1-diphenylmethane in pentane at ambient temperature leads to the formation of the complexes (η 5 -C 5 H 4 CPh 2 -η 5 -C 13 H 8 )MCl 2 . NMR and X-ray diffraction data confirm that the fluorenyl groups are η 5 -bonded in these complexes. When activated with aluminoxane or other appropriate ionizing agents both complexes catalyze the stereospecific polymerization of propylene to syndiotactic polypropylene of high molecular weight and tacticity.