Hexa-host

In 1974, Vogtle and Weber disclosed the preparation of a class of molecules which they reported showed remarkable phenomenological parallelisms to the mode of food capture by an octopus using its suction pads . Such molecules have also been referred to as hexa-hosts (see below). 

Molecular complexation or clathration was observed somewhat earlier using similar compounds referred to by MacNicol and Wilson as hexa-hosts ". These compounds formed clathrates with carbon tetrachloride, toluene, dioxane, bromoform and a num-... 

Figure 7.24 A comparison of a typical hexa-host (A) with its Piedfort analogue (B), composed of two trisubstituted aromatic rings juxtaposed. The light dotted circle is symbolic of the projected guest binding region in each case. (Reproduced by permission of The Royal Society of Chemistry).

MacNicol and Wilson 15) synthesized a series of compounds (20) called hexa-hosts . Such hexa-substituted benzenes can, on crystallization from suitable solvents, from a wide range of inclusion compounds. When Y = SPh, for example, a crystalline complex with CC14 was isolated having a host-guest stoichiometry of 1 2. The CC14 is... 

Fig. 6. Hexa-host strategy Close connection between (a) a hydrogen-bonded hexamer unit typical of phenolic hosts (hydroquinone, Dianin s compound cf. Fig. 4) and (b) of a hexasubstituted benzene analogue (follow up the shaded hexagons) (c) characteristic constitution of hexa-host molecules and (d) host-guest packing of a representative clathrate inclusion compound (dioxane clathrate of I3a, 1 1 dioxane molecules hatched)...

Recent review MacNicol, D. D. Structure and design of inclusion compounds the hexa-hosts and symmetry considerations, in Inclusion Compounds, Vol. 2, (Atwood J. L., Davies, J. E. D., MacNicol, D. D., ed.) Academic Press, London 1984, p. 123... 

The prototype hexa-host 8 forms an isostructural series of H G ratio 1 2 with halogenated species (e.g., CCI4, CCl3Br) crystallizing in space group The latter... 

Hofmann-type clathrates Werner clathrates clathrate hydrates inclusion compounds of urea, thiourea, and selenourea cyclodextrins calixarenes gossypol hexa-hosts hydroquinone phenol and Dianin s compounds graphite intercalates natural and pillared clays and others. Such studies contributed to the birth of supramolecular chemistry, with relevance to a new understanding of the world of materials that was emphasized with a Nobel Prize. 

The classification of literature was carried out with the controlled key-word phrases denoting broader groupings of the host-species concerned such as hexa-host, Hofmann-type, quinol, urea adduct, Werner-type, etc. Crystal structure analysis, spectroscopic technique, and other physico-chemical methods applied were also included in the keyword phrases. In the original literature various terms have been used to express the group of inclusion complexes, and the selection appears to depend on the author s preference. Such confusion or controversy causes severe difficulties to retrieve important information from vast amount of chemical literatures. Therefore, we applied rather loosely controlled groupings as cited above. The citation number in the Chemical Abstracts (Chem. Abstr.) was added to ease the reference of the original items written in various unfamiliar languages. 

Even though solution crystallization is always the method of choice, it has limited applicability in cases where the molecules have a strong tendency to form solvates. Solvent-free techniques such as melt crystallization and sublimation are suitable for certain compounds. For example. Das and Barbour obtained four polymorphs of a hexa-host, hexakis(4-cyanophenyloxy)benzene, upon melt crystallization, which otherwise forms a series of solvates with the solvent of crystallization. Sublimation is yet... 

The hexa-host molecules are the first synthetic clathrands with no direct structural relationship to a previously known host. They were designed from scratch in the first supramolecular clathrand synthesis. MacNicol realized that appropriate hexa-substituted benzenes would be more robust analogs of the (0-H)6 hydrogen-bonded motif found in inclusion compounds such as the j6-structure of hydro-quinone 49 " and Dianin s compound 8 (Section S)." If the aromatic substituents were attached to the central core by a flexible linker atom or group (L e.g., S or CH2-O), then the new molecule would act as a covalent mimic of the earlier hydrogen-bonded motif (Figure 13). 

Structures 50-53 are representative of the 50 or so of these highly successful hexa-hosts that have been shown to form clathrate compounds. The crystal structure of (52) (dioxane), illustrated in Figure 14, is a representative example. Hexa-host synthesis and inclusion has been reviewed in detail. There have been occasional reports of hexa-host molecules adopting alternative conformations. 

Figure 13 The hexa-host concept. Diagrammatic comparison of the (0-H)e hydrogen-bonded motif present in the R3 Dianin s componnd and j8-hydroqninone inclusion structures (a), and a hexaaryl substituted benzene using linkers (L) (b). In both cases, the substituents are subtended alternatively up then down (ababab) around the central ring, and the 0-0 and L-L distances are comparable.

Figure 14 Part of the crystal structure of the hexa-host clathrate (52) (dioxane) projected on the ab plane. Color code O, red S, yellow host C, green and guest C, purple. The H atoms are omitted, and the central aromatic ring of 52 is colored dark green, for clarity. ...

Hexaarylbenzenes combine design elements of the inclined aromatic plane (Section 4) and the hexa-host (Section 5.1) strategies. These clathrands are able to include terminal alkynes by ntilizing multiple C-H- -Jt interactions (including the sp C-H- -Jt motif). This outcome is illustrated in Figure 26 for the case of the clathrate formed between hex-amethylbenzene 98 and phenylethyne. ... 

There is also a close relationship between the inclusion crystal structures of hydroquinone 49, Dianin s compound 8, and the alicyclic diol 103. This is revealed if the two motifs used by MacNicol in the hexa-host design, namely the aromatic ring and (0-H)6 cycle structures, are regarded as equivalent supramolecular nodal points. The network connectivity (see Network and Graph Set Analysis, Supramolecular Materials Chemistry) and familial relationship of these three lattices is then revealed. ... 

The extension of the naturally occurring hexameric units to synthetic analogs is significant. The hexa hosts, shown in Fig. 23, have the same overall geometry as the phenolic hosts, but the hydrogen bonding in the latter has been replaced with full covalent bonds in the former. The result is particularly important since it shows that molecules can be constructed or engineered to have a specific shape. 

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