Apohost structures

Figure 4-2. Occurrence of d values for the interactions between pairs of waad molecules in the apohost structures, d values span from 3.6A to 7.2 A, corresponding to a range of contacts from wheel... axle to wheel... wheel. Analogue apohost molecules and polymorphs are identified by bar meshing, and it can be seen that they are grouped around similar d values. The insets show the collocation of some of the structures discussed in the paper (from left XISPOE, FIFHAE, SOGGIE, KUVWON, IFAJIO)...

In general, solvates of the same parent apohost tend to cluster about host-dependent d values these are larger than those found in the corresponding apohost structures... 

Figure 4-3. Correlation between d and A for pairs of waad molecules in the apohost structures. Shortest axle. .. axle distances d correspond to displacements of about one half of the molecule, which optimize the packing between the partners. Some cases are exemplified in the insets, from left IFAJIO, XISPOE, SOGGIE (in the direction perpendicular to the one shown in Figme 4-2)...

Figure 4-4. Effect of guest inclusion on the geometry of the pair of wood molecules. Solvates of the same parent host (indicated by bar meshing as in Figure 4-2) tend to cluster arotmd host dependent d values, which are larger than those found in the corresponding apohost structure. A values for solvates are grouped between 0.2 and 0.7. The correlation between d and A disappears...

As described earlier under "Analysis of apohost structures, the assembly of waad molecules is based on OH... X (X=N, anion, tt) or hydrogen bonds between host molecules, or can be mediated by the guest (G) via OH... G interactions. In the former case we define the strucmre as a self-mediated network, in the latter as a guest-mediated network (Scheme 4.4). 

Figure 4-6. Layer organization and metrics for Pd(LOH)2X2 and Pd(LOH)2X2-2G (in the picture G=acetone). According to the Venetian blinds machinery, the transition between solvate and apohost structures does not alter significantly the location of palladium centers...

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. 

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],... 

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... 

Figure 4-1. Typical interactions between waad pairs in the crystal structures of apohost phases (from top left, clockwise XISPOE, SOGGIE, KUVWON, FIFHAE)...

The conversion from apohost to clathrate structure requires that the diol molecules can reversibly switch from a situation in which the host molecules interact with themselves (self-mediated network) to another in which they interact with the guests (guest-mediated network), with a concomitant rearrangement of geometric parameters d and A. 

Figure 4-5. The comparison between apohost (left) and solvate structures (right) for families (b-d) of similar organic vraad shows that in many cases the molecules are arranged in columns with similar spacings and different inclination. The columns respond to guest inclusion or removal by changing machinery olecular axles relatively to the column axis, according to the Venetian blinds ...

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