Host-guest stoichiometry

Formation of Inclusion Compounds, Host-Guest Stoichiometries 56... 

Four out of the five associates in our studies display 1 1 host guest stoichiometry (those of 1, 20, 26, and 37). Only the corresponding inclusion of 41 has a 1 2 stoichiometry. This difference could be linked up with the altered orientation of the vicinal carboxyl groups. 

The H-bonding in the anhydrous 1 Im (Table 24) has topologic properties (Fig. 46) similar to those in the alcohol coordinatoclathrates of 1 with 1 2 host guest stoichiometry (cf. Fig. 17 a). Assuming a perfectly ordered crystal lattice, the resulting central loop of H-bonds should appear to have homodromic directionality with the donor/acceptor functions separated in space. This contrasts to the behavior in the dihydrated l Im where no such characteristic loops are formed. Involvement of the C—H hydrogen atoms of the imidazole molecule, however, is similar in both cases. 

In an attempt to better understand the mechanism for encapsulation of cobaltocenium inside 46 we performed careful titrations of this redox-active cation with variable concentrations of host 4. To our surprise, we quickly realized that only 2-3 equivalents were necessary to fully shut down the electrochemical response of cobaltocenium.51 This result is in strong contrast to the NMR data, which clearly indicate a 6 1 [host/guest] stoichiometry for full encapsulation. There are two important differences between these two types of experiments. In the NMR experiments, solutions were prepared in pure, deuterated CD2CI2 and the only solutes present are cobaltocenium hexafluorophosphate (ca. 1 mM) and host 4 (0-8 mM). In the electrochemical experiments, solutions were prepared in isotopically unenriched CH2CI2 also containing 0.1 M tetradodecylammonium bromide as supporting electrolyte. The concentrations of cobaltocenium hexafluorophosphate and host 4 were similar to those used in the NMR experiments. It clearly became evident that the nature of the supporting electrolyte, especially the nature of its anion, was crucial to... 

MacNicol and Wilson 15) synthesized a series of compounds (20) called hexa-hosts . Such hexa-substituted benzenes can, on crystallization from suitable solvents, from a wide range of inclusion compounds. When Y = SPh, for example, a crystalline complex with CC14 was isolated having a host-guest stoichiometry of 1 2. The CC14 is... 

Apart from the redox-active units, the design strategy was devised to test the efficacy of 18 versus 21 crowns for ion transport and the value of decyl chains or anthrylmethyl units as anchors for the prospective channels. The anthryl units also offered an opportunity to detect cation complexation and establish host-guest stoichiometry by the extremely sensitive technique of fluorescence spectroscopy. 

Ishida et al. [72] prepared a cyclopeptide c(Ala-Aba)3 starting from 2-aminobenzoic acid (Aba). This amino acid restricts conformational freedom of the macrocyclus. p-Ni-trophenylphosphate was found by UV spectroscopy to bind to this macrocyclus with an association constant of K= 1.2 x 106 M"1 in a host-guest stoichiometry of 1 1. 

Another nice example is the formation of well-defined multicomponent crystals under control of the molecular concentration ratio [48]. The design of the oligo(phenylene ethynylene) (OPE) derivative leads to the formation of OPE dimers which upon self-assembly form cavities which can host molecules such as coronene (COR). Depending on the relative molar ratio of both compounds in solution, well-defined supramolecular bicomponent architectures are formed with different host-guest stoichiometries (Fig. 17). 

MacrocycHc receptors 31a and 31b were found to bind both dihydrogen phosphate and chloride in exclusively 1 1 host/guest stoichiometries. Binding constants were calculated for 31a and 31b with dihydrogen phosphate and chloride and revealed that dihydrogenphosphate was bound more strongly (2.5 X 10 M for 31a and 4.0 x 10 M for 31b) than chloride (500 for 31a and <50 M for 31b). 

Kubo. Y. Sugasaki, A. Ikeda, M. Sugiyasu, K. Sonoda. K. Ikeda, A. Takeuehi, M. Shinkai. S. Coperative Cgo binding to a porphyrin tetramer arranged around a p-terphenyl axis in 1 2 host—guest stoichiometry. Org. Lett. 2002, 4. 925 928. 

Unusual control of host-guest stoichiometry in isostructural clathrates of mixed guests was recently re-ported. The diol 14 forms isostructural clathrates H-4A and H 4B (A=dimethylformamide, B=dimethyl sulfoxide). Investigation of the selectivity of H in mixtures of A and B spanning the entire concentration range 0 < Xa < 1 yielded the stepwise competition profile shown in Fig. 3,... 

Multivalent hydrogen bond receptors, (575) have been built from a tri-phenylene (574) core surrounded by six (diaryl)phosphinate groups (Scheme 140). In chloroform, (575) displayed recognition properties toward dihy-droxybenzenes, selectively forming complexes with catechol derivatives in a 1 2 (host guest) stoichiometry. ... 

In fact, although it has a mobile structure in solution and experiences a pseudo rotation process whose energy has been measured, in the solid state it has a "cone structure determined by four intramolecular hydrogen bonds, which create a cavity suitable for the inclusion of neutral molecules. Several inclusion complexes between (1) and aromatic guest molecules have been isolated and their X-ray crystal structure determined. These complexes show either a 1 1 or 2 1 host/guest stoichiometry, although the X-ray structures are rather similar in the two cases. 

Probably the best known of the anticrowns are the mercuracarborands, e.g. 2.82 and 2.83 (Figure 2.17). As with the fluorinated complexes, the carbo-rane substituents act as elaborate electron-withdrawing groups and increase the Lewis acidity of the mercury(ii) ions. Most crown ethers exhibit one-to-one host guest stoichiometry, but most anticrowns form 2 1 complexes on size-fit grounds. One exception to this rule is the chloride complex of 2.83 in which the chloride ion binds within the centre of the host cavity with a long Hg-Cl distance of 2.94 A. 

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