Guest complexes preparation

Host-guest complexes such as (67) have been prepared from molecular squares involving Lewis base receptor sites, such as cyclobis[(cw-(dppp)Pt(4-ethynylpyridyl)2)(cM-LM)]Ag2 6+(OTf)6, where M = Pdn or Ptn and L = dppp or 2PEt3, by reaction with pyridine, pyrazine, phenazine, or 4,4 -dipyridyl ketone.519... 

A number of capped cyclodextrins which are able to bind a metal ion at one end of their cavity, together with an organic guest in the cavity, have been synthesized. Such species parallel in several respects the family of completely synthetic vaulted transition-metal complexes prepared by Busch and coworkers and already discussed in Chapter 3 (section 3.5). 

Vogtle has developed this approach further and employed a series of anionic templates to prepare rotaxanes (instead of the neutral template in the above reaction) [65-67]. In this approach a phenolate, thiophenolate or sulfonamide anion is non-covalently bound to the tetralactam macrocycle (46) forming a host-guest complex via hydrogen bonding (see Scheme 21). 

Moreover, these dendrimers proved to be good hosts for anionic dye molecules. Films prepared from these host-guest complexes showed efficient ( > 90%) energy transfer from the OPV units to the encapsulated dye molecules... 

Some modifications to the cyclodextrin structure have also been found to improve their complexing ability. Casu and coworkers prepared 2,3,6-tri-O-methyl and 2,6-di-O-methyl derivatives of alpha and beta cyclodextrin. They observed that tri-O-methyl-alpha cyclodextrin shows an almost ten-fold increased stability of the complex with the guest, Methyl Orange, compared with the unmodified alpha cyclodextrin. A possible reason for this increase in stability is that the methyl groups are responsible for an extension of the hydrophobic cavity of the cyclodextrin. Other workers,however, observed a much smaller enhancement of stability of complexes on methylation of the cyclodextrin, and a decrease in stability has even been reportedfor the one host-two guests complex of tropaeolin with beta cyclodextrin. Thus, the effect of methylation on the stability of a complex varies with the guest species involved, and cannot be readily predicted. 

Actually, coordination complexes of different metal salts of DBTA with hydroxycarboxylic acid esters, hydroxycarboxylic acids and alcohols as well as host-guest complexes of DBTA with chiral phosphine oxides and racemic alcohols can be prepared and used for separation of optical isomers. In the next subchapters theoretical and practical aspects of these recent resolution processes are summarised. 

Figure 4.21 Formation of rotaxanes 53 and 54. Each preparation is directed by the formation of the corresponding intermediate host-guest complex shown ...

In 1967, Pedersen described the preparation and properties of crown ethers,3 which are macrocyclic polyethers capable of sequestering metal cations. These catalysts can enhance the solubility and reactivity of salts in nonpolar solvents. For example, 18-crown-6, i.e., yxo-anhydro-hexaethylene glycol, forms a host-guest complex with potassium cation (K+) (Equation (1)). This association enables ionic potassium fluoride (KF) to dissociate in nonpolar benzene. And since the nucleophilic F counterions are not complexed,4 the yield of the Finkelstein reaction,5 i.e., halide-halide exchange, is increased 6... 

The supramolecular approach mentioned above employs a dimeric zinc porphyrin reagent, now available under the name Zn-tweezer . The latter is capable of forming 1 1 host-guest complexes upon adding a solution of A7,A-bidentate conjugate, prepared by reacting the chiral substrate with an achiral trifiinctional bidentate carrier as shown in Figure 20.55 56... 

Toda procedure for obtaining enantiomeri-cally pure compounds will find broad application very soon. This development could make preparative HPLC with chiral columns obsolete and be applied to distillable amino acid derivatives as well. After all, analytical resolution of amino acids was quite successful by host/guest complexation chromatography with reversed-phase packings loaded with Cram s chiral 1,1 -binaphthyl crown ethers (similar to 1). [20]... 

First examples of second-sphere coordination adduct between a CD and such organometallic complexes were observed with the ferrocene and its derivatives. Thus, Breslow was the first to report in 1975 that ferrocene forms a 1 1 adduct with the /3-CD in both A/T-dimethylformamide and dimethyl sulfoxide. Nevertheless, the first crystalline inclusion complexes of ferrocene and its derivatives were prepared by Harada and Takahashi in 1984. The inclusion complexes were obtained by direct addition of crystals of ferrocene or its derivatives into aqueous solutions of CD. The stoichiometry of the inclusion complexes was found to be dependent on the sizes of the CD. The /3-CD and y-CD formed 1 1 stoichiometric inclusion complexes whereas a-CD formed a 2 1 (CD guest) complex with ferrocene. From induced circular dichroism experiments, three structures were proposed for these adducts as shown in Figure... 

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