Host complex

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

Thiourea and selenourea both form canal inclusion compounds. The selenourea compounds, reported only briefly 37), appear to be isostructural with the rhombo-hedral thiourea compounds, although with larger dimensions which are more susceptible to the sizes of guest molecules. For urea, thiourea and selenourea the crystal densities of the compounds are significantly less than those of the hosts values (g cm - 3) for the pairs host/complex are 1.30/1.20 1.40/1.10 2.08/1.60-1.65 for urea, thiourea, selenourea, respectively37). 

In 1994 we published the first chiral dendrimers built from chiral cores and achiral branches [ 1,89], see for instance dendrimer 57 with a core from hydroxy-butanoic acid and diphenyl-acetaldehyde and with twelve nitro-groups at the periphery (Fig. 21). As had already been observed with starburst dendrimers, compound 57 formed stable clathrates with many polar solvent molecules, and it could actually only be isolated and characterized as a complex [2 (57- EtO-Ac (8 H20))]. Because no enantioselective guest-host complex formation could be found, and since compounds of type 57 were poorly soluble, and could thus not be easily handled, we have moved on and developed other systems to investigate how the chirality of the core might be influencing the structure of achiral dendritic elongation units. 

Scheme 12 Formation of guest-host complexes with multiple stoichiometries.

Encapsulation in Self-Assembling Hosts Complex Balance of Enthalpy and Entropy 201... 

The crystalline solid contains a 4 1 stoichiometric guest/host ratio and is composed of 2 1 guest/host complexes with two additional thiourea molecules hydrogen bonded to the guest molecules of the complex. There are three independent crown ether molecules in the solid. Only one of these is shown in Fig. 19, and it lies on a center of symmetry. 

When the concave reagents are compared to other reactions in Supramole-cular Chemistry a distinct difference must be noted Most other approaches try to bind the substrate in a host first. Then this complex reacts with a reagent which either is present in solution or attached to the host. For concave reagents and concave catalysts, however, there is no need for binding of the educt. In contrast, the protonation reactions can be interpreted as a reagent (H ) host complex. 

FIGURE 6 Graphical representation of the guest-host diastereoisomeric complex formation (a) in the presence of acid in the mobile phase and (b) in the absence of acid in the mobile phase, (c) Three-dimensional structures of the guest-host complexes formed between R- and 5-enantiomers of a-phenylethylamine and chiral 18-crown-6 ether CSP. 

Hosomi-Sakurai reaction, allylsilations, 9, 305 Host complexes calixarenes as, 12, 799 crown-ethers as, 12, 813... 

Cyclodextrins and calixarenes are members of a general class of macrocycles commonly referred to as cyclophanes, although the naturally occurring CDs are usually considered separately from the synthetic members of the cyclophane family of compounds. The properties and guest-host complexes of cyclophanes... 

The important part is step 2, the guest release. For the clathrate y form this process occurs over a narrow temperature range bracketed by two plateaus corresponding to the initial (y) and final (a) phases. This behaviour is typical of clathrates. In contrast, for the /1-phase, guest release is continuous over the entire temperature range until the host complex decomposes. This kind of desolvation behaviour in which guests slowly exit a channel that remains intact is characteristic of zeolites. 

Ort/io-bromonitrobenzene as well as ortho- and meta-dinitrobenzenes form isostructural compounds of this type, Zn(NCS)2(4-methylpyridine)4 and Cu(NCS)2(4-methylpyridine)4 (host) complexes with a variety of guest molecules are known as well. 

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