Apohost

Figure 7.49 X-ray crystal structures of 7.68 (a) apohost, (b) 1 1 inclusion compound with acetylene (1 bar) and (c) 1 2 acetylene complex (8 bar). The cavity volume is shown as a van der Waals survace -note that each cavity is discrete. Chloride ligands are shown as spheres and the acetylene molecules are shown in space-filling mode (pictures courtesy of Prof. L. J. Barbour, Stellenbosch University).

Common hosts such as urea or p-f-butylcalix[4]arene can exist as various crystal phases, some if which do not contain cavities. The crystal form of the pure host without cavities is called the a-phase. The 30 (apohost) phase contains unfilled cavities while the p -phases have the same host structure but contain different guests. Such structures are sometimes referred to as pseudopoly morphs. Further pure phases (y-phase) or clathrates (y1. phases) may also exist in some cases, as in tri-o-thymotide. Apohosts are usually relatively unstable but allow the inclusion of interesting guests such as gases. 

Dewal, M. B., Lufaso, M. W., Hughes, A. D., Samuel, S. A., Pellechia, P., Shimizu, L. S., Absorption properties of a porous organic crystalline apohost formed by a self-assembled bis-urea macrocycle. Chem. Mat. 2006, 18, 4855-4864. 

The term porous should apply to a specific host phase and not simply to the host molecules as an amorphous or mutating collective. Therefore, in principle, the host framework should remain sub stantially unaffectedby guest uptake and removal. This requirement means that we do not describe, for example, the close-packed, tetragonal a phase of urea as porous, however the description would be appropriate for an empty, hexagonal urea apohost phase (Section 7.3)... 

Here H represents the host molecule in its nonporous a-phase, the apohost, which, when placed in contact with a mixture of guests A and B, which can be liquid or vapour, selects A and forms a solid inclusion compound H An, the p-phasc, while excluding guests B. 

It is noteworthy that, in general, the host framework does not retain its p, porous phase upon guest release, but collapses to the structure of the apohost - the a-phase. There are a few exceptions, in which the host behaves as a zeolite retaining its structural integrity [4-6], and a copper-containing porous open-framework structure has been reported which also withstands partial chemical modification while maintaining its structure [7],... 

The apohost 1-2-cyclohexenone complex was irradiated under UV light at room temperature with a Hanovia 450 W medium-pressure mercury lamp. After 24 h, dimer 2 was the major product and no enone was detectable by 1H NMR or GC/MS. The photoproducts could be separated from apohost 1 by washing with CH2C12, and apohost 1 was recovered. 

A reversible dynamic reorganization between the pure host (apohost) and the host-guest phases requires two conditions a low-cost structural rearrangement between two states represented by the close apohost and the final open host framework, and an easily accessible path of migration for the outcoming and incoming guest molecules. 

Many crystalline solvates of organic and inorganic waad are known in the crystallographic literature [24, 25] for most of them the structure of the corresponding apohost phase has been also reported (Table 4-1). The comparison of the crystal... 

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