Complexes host-guest

Host-guest complexes involving pyridines and their benzene derivatives can be divided into categories depending upon the interactions involved. In the largest group, pyridinium derivatives act as Jt-acceptors and their interaction with electron rich 7i-donors stabilizes the complexes. Within this group, there is still much diversity. 

The simplest complexes of this group are those that involve a pyridinium guest included in a host containing electron-rich aromatics. An example of this mode of complexation is the complex 66, formed by complexation of a tweezer-like aromatic host 67 and a dendritically appended bipyridinium dication 68 2005AGE477, 2006JA637 . 

A further subgroup consists of those complexes that can be considered as a pyridinium guest included in a rt-donor cavity. It is noteworthy that in all such cases, the relative size of the cavity and the guest is important in determining 

Notably, while the most common complexes contain the two components in a 1 1 ratio, other ratios are possible depending on relative concentrations of the components along with steric requirements. This is demonstrated by the 1 2 complex 78 2003PCA4669 and the extended hyperbranched polymer 79 2004JA14738 . 

While there are very many simple complexes based on this motif, there are also some in which the inclusion is facilitated by multiple crown ether linkages between the electron-rich aromatics this is exemplified by the complex 80 2003JA9367, 2005JOC3231 . Similar 2 1 complexes with larger bipyridinium-based guests have also been reported 2003JA9272, 2005T10242 . 

Bound molar fractions X are determined from the measured diffusion coefficients as weighted averages of the values for the bound and free states and association constants X, are derived from changes in diffusion coefficients as a function of the host-guest ratio. 

Adapted with permission from [41], Copyright 2002, American Chemical Society. 

In the field of organometaUic chemishy in particular, diffusion measurements have been used extensively by a number of pioneering groups in various studies of complex 

In a separate study on a series of zirconium-based metallocenes complexed with [MeB(C6F5)3] there has been shown a good correlation between their hydrodynamic volumes estimated from diffusion measurements (using Si -tolyl)4 as internal reference in benzene) and the van der Waals volumes determined for the 1 1 ion pairs from their crystal structures [45] (Fig. 9.29), suggesting that appropriate application of the diffusion NMR methodology can be an effective means of determining the sizes of molecular complexes in solution. 

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Fig. 1. Schematic representation of a receptor—substrate (host—guest) complex involving cavity inclusion of the substrate and the formation of different types of weak supramolecular interactions between receptor (hatched) and substrate (dotted).

From the kinetic point of view the facts are different and the order is reverse, ie, the rigid highly preorganized spherands are slow, as contrasted with the flexible barely preorganized podands that are fast both in formation and decomposition of the receptor—substrate (host—guest) complex (20,21). 

F. Viigde and E. Weber, eds., Host-Guest Complex Chemisty—Macroycles, Springer, Bedin-Heidelberg, 1985. 

These molecules are significant in the field of research devoted to host—guest complexation. Synthetic routes to a number of calixarenes have been developed (11). 

Optically active thiiranes have been obtained by resolution of racemic mixtures by chiral tri-o-thymotide. The dextrorotatory thymotide prefers the (5,5)-enantiomer of 2,3-dimethylthiirane which forms a 2 1 host guest complex. A 30% enantiomeric excess of (5,5)-(—)-2,3-dimethylthiirane is obtained (80JA1157). 

Carceplexes and hemicarceplexes, host-guest complexes in which carcerands and hemicarcerands, in particular those including heterocyclic fragments, are hosts 99CRV931. 

Fig. 3-4. (A) Changes in chemical shift of protons of cyclophane -CH - groups between bipyridinium and phenyl in H NMR spectra of 3 as a function of (R)-DOPA concentration (a) 0, (b) 0.111, and (c) 0.272 mol (B) Change in chemical shift plotted against the analytical concentration of (R)- and (5)-DOPA. The solid line is calculated for 1 1 host - guest complexation. (Reprinted with permission from ref. [79]. Copyright 1998, American Chemical Society.)...

Vogtle, F, (ed.) Host Guest Complex Chemistry I, Top. Curr. Chem. 98 (1981), Berlin Heidelberg New York, Springer-Verlag 1982... 

In this chapter we discuss host-guest complexes of arenediazonium salts with crown ethers and related compounds. Transition metal complexes of arenediazonium ions are treated together with those of dinitrogen and of diazoalkanes in our second book (Zollinger, 1995, Sec. 10.1). 

P-Cyclodextrin is assumed to form host-guest complexes with diazonium ions (Fukunishi et al., 1982, 1985), but if so, complexation increases the extent of thermal dediazoniation, i. e., it has the contrary effect to that usually seen. 

Checking the literature of the second half of the 1980s for applications of crown complexes of arenediazonium salts for synthetic purposes, one gets the impression that these complexes are not widely used. The two comprehensive reviews by Izatt et al. (1985, 1991) show a definite decline of interest in host —guest complexation of arenediazonium ions in the second half of the 1980s. The subject of arenediazonium... 

HMPT, see Hexamethylphosphoric triamide Host-guest complexation, general 289ff., 293ff., 321, see also Crown ether complexes... 

Another related host-guest complex has been constructed from citric acid, boric acid, and a strontium salt in a 2 1 1 stoichiometry. In this complex the strontium cation is surrounded by four water molecules, two monodentate carboxyl groups and one oxygen atom of the BO4 unit [161]. 

Fig. 30. Compound 118 is a cage-like host-guest complex with the simultaneous inclusion of two metal cations...

A polyrotaxane with a dendrimer-like structure is known [60]. Based on the observation that [3-CD and sodium deoxycholate (NaDC) 54 forms a 2 1 host guest complex in water, Tato et al. constructed hyperbranched polyrotaxanes 55 by slowly reacting triply branched receptor 53 containing P-CD and NaDC... 

Host-guest complex Irradiation time (h) Product ... 

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