Carcerand molecules

Again, this effect can be understood in terms of the templating of hyperbolic structures, due to the presence of hyperbolic TFS, set up by the caesium ions in solution. For example, the carcerand molecule resides on the P-surface (Fig. 2.21), which is a common TFS in aqueous zeolite syntheses. The improved yield of large ring compoimds in the presence of cesium is also understandable on this basis, since the precursors to these compounds prefer to wrap around tunnels of the TFS [13]. 

An uneventful coupling of two hemispherical cavitand molecules — a tetrameth-anethiol and a tetrakis(chloromethyl)precursor (see p. 169) — yielded D.J. Cram s (1988) carcerand . ft entraps small molecules such as THF or DMF, cesium or chloride ions, or argon atoms as permanently imprisoned guests . Only water molecules are small enough to pass through the two small pores of this molecular (prison) cell. 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

Bowl-shaped molecules like 245 and 246 inspired Cram to propose cage molecules like 247 [2] that could host incarcerated smaller guests, hence the name carcerands. The lattermolecule remained hypothetical butthe Cram group... 

It should be stressed that calixarenes, hemispherands and spherands, carcerands and hemicarcerands, and some other molecules discussed elsewhere in this book, belong to the cyclophane group of compounds. 

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

A large carcerand has a cavity volume of 120 A3. If the molecular volume of methane is 28.5 A3, calculate the occupancy factor, p, for a 1 1 methane carceplex of this host. What host volume would result in an occupancy factor of 0.67, equivalent to solid methane Calculate the notional pressure of one molecule of methane in a cavity of this size. Do you think that such a carceplex is likely to form ... 

The anion-binding carcerand 11 was described by the Amouri group [31]. This complex contains a tetrafluoroborate anion coordinated to two cobalt(II) ions. Each cobalt ion adopts a square-pyramidal geometry. Four benzimidazole arms of the bridging ligands fill the equatorial positions, and solvent molecules (acetonitrile) coordinate to the outside axial positions. Inside the complex the included tetrafluoroborate anions interacts with the cobalt ions whose inside axial positions are otherwise coordinatively unsaturated. No exchange of the anion was observed even at 60 °C. A detailed study of the anion-binding properties in the crystal state of similar metalla-macrotricyclic cryptands has been performed by Adarsh et al. [32],... 

The synthesis of hollow architectures based on cavitands started in 1985, when Cram et al. reported on the inclusion of solvent molecules in a carcerand obtained by covalent linkage of two cavitands [30]. Since then, molecular and supramolecular capsules have been prepared from cavitands and calix[n]arenes due to two important properties the bowl-shaped form and the various functionalizations which can be introduced at the upper rim of the cavitands or the wider rim of the calix[n]arenes (Fig. 1). 

Cram [49a] elaborated further on this concept by enclosing space in his carcerands and hemicarcerands (See Scheme 1) to form a new inner phase which he has referred to as a new phase of matter . In contrast to the hollow space found inside clathrates and zeolites for instance, the cages of these molecules are independent of the form and physical state. For example, hemicarcerands and related supramolecular systems (i.e. hemicarceplexes) prevail in the solid, liquid, or gas phase. This characteristic-hollow space, the inside surface — is maintained across all phase transitions. The inner surfaces and spaces of these systems are not manifested as bulk properties. An extensive review on the synthesis of these materials has been published recently [205]. 

More sophisticated methods have been developed for the assembly of very complex two dimensional molecules (i.e., palytoxin, Mwt. 2680 or vitamin B12) or three dimensional architecture (i.e., cryptates/carcerands) involving a convergent covalent approach. The resulting assemblies of covalently linked atoms are precise... 

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