Host-guest compounds

Fig. 1. The principle of formation of an inclusion compound, (a) concave host (b) convex guest component (c) host—guest compound.

Much attention has recently been focused on organoboronic acids and their esters because of their practical usefulness for synthetic organic reactions including asymmetric synthesis, combinatorial synthesis, and polymer synthesis [1, 3, 7-9], molecular recognition such as host-guest compounds [10], and neutron capture therapy in treatment of malignant melanoma and brain tumor ]11]. New synthetic procedures reviewed in this article wiU serve to find further appHcations of organoboron compounds. 

The wheel-and-axle design as source for host-guest compounds was originally proposed by Toda and Hart in 1981 for hosts containing hydroxyl functions 481 (see Ch. 3, Sect. 2.1 of Vol. 140). The l,l,6,6-tetraphenylhexa-2,4-diyne-l,6-diol (24) provides a representative compound. It forms 1 2 crystalline inclusion complexes with a large number of small guest molecules, including a variety of ketones, amines, amides and a sulfoxide 48). 

Cavity Shapes Steric Fit of Host-Guest Compounds.103... 

The d-spacings of the 211 (d = 33.1 A) and 220 (d = 28.6 A) reflections of the pristine MCM-48 silica are observable in the host/guest compounds whereas the higher order reflections (20 4-6°) disappear. In addition, the 113/021 (d = 2.53 A 1=100%) and 208/220 (d = 1.48 A 1=36%) reflections of the inverse cobalt iron structure appear in sample A and B. In contrast to the bulk material of CoFe24 (figure 2) synthesized under exact the same conditions as the phases A and B, the reflections of the host/guest compounds are much broader and weaker in intensity, indicating the occurrence of very small particles. 

Table T. Characterization of the pristine MCM-48 silica and the host/guest compound A and B with respect to specific surface area, pore volume and mean pore diameter.

Figure 5 Comparison of the Fe K-edge (a) and the Co K-edge (b) XANES spectra of the nanostructured host/guest compounds A and B and reference materials.

Summarizing one can say that in case of these complex nanostructured host/guest compounds only the application of several complementary characterization techniques give reasonable results on the structure of the system. 

Table 1.2 Classification of common host-guest compounds of neutral hosts.

Solid state host-guest compounds or clathrates are examples of co-crystals where the host packs in such a way as to leave a gap in the crystal lattice suitable for inclusion of the guest. 

Clathrates are the original host-guest compounds with chlorine clathrate hydrate - a compounds in which a water hydrogen bonded network hosts a molecule of Cl2 - being known since 1810. 

The copolymerization of a rue thy latcd-/ -cy c 1 odextri n 1 1 host-guest compound of styrene with various molar ratios of sodium 4-(acrylamido)-phenyldiazosulfonate carried out in water with free radical initiator is described [40]. Depending on the amount of sodium 4-(aciylamido)-phenyldiazosulfonate incorporated in the copolymer, water- or DMF-soluble copolymers of high molar mass were obtained. Irradiation of the copolymers with UV light in solution resulted in rapid decomposition of the azo chromophore. Irradiation of the polymers as films led to crosslinking and thus to insolubility. 

Thermal gravimetry (TG) and differential scanning calorimetry (DSC) are useful techniques in the analysis of inclusion compounds. If we consider the desolvation of a host-guest compound with a volatile guest ... 

The method relies on the p and n salts having different solubilities, and they must not form solid solutions or double salts. The more insoluble salt is filtered and the purified acid recovered by adding mineral acid. This method of chiral resolution is well established, and lists of resolving agents for many classes of racemic compounds are available [22], Inclusion chemistry may be employed for the same purpose by preparing host-guest compounds with a chiral host ... 

Toda, F., D. L. Ward., and Hart, H. (1981) Wheel-and-Axile Design as a Source of Host-Guest Compounds. The Crystal Structure of the 2 1 Acetone Tetraphenyl-2,4-hexadiyne-l,6-diol Complex, Tetrahedron Lett., 22, 3865-3868. 

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