Aromatic guests

Indeed, the conformational softness of the GS brick framework, and its importance to structure prediction based on the native 2D packing of aromatic guests, is evident in the structures of G2V-3(naph-thalene), G2VI-3(biphenyl), and G2VII-3(anthracene). These compounds are isostructural, their host... 

Diederich and coworkers [10] synthesized so-called dendrophanes (Figure 13.6) containing a paracyclophane core embedded in dendritic poly(ether-amide) shells. X-ray crystal-structure analysis indicated that these dendrimers had an open cavity binding site in the center, suitable for the binding of aromatic guests. NMR and fluorescence titration experiments revealed a site specific binding between these dendrimers and 6-(p-toluidino)naphthalene-2-sulfonate (TNS) with a 1 1 association. Also, the fluorescence spectral shift of TNS, which is... 

The fact that the dendritic shell can produce localized microenvironments has been used by Diederich et al. who developed water-soluble dendritic cyclophanes (dendrophanes) as models for globular proteins (Figure 16.13) [5, 170, 171], These dendrimers contain well-defined cyclophane recognition sites as initiator cores for the complexation of small aromatic guests [172-174] and steroids [174-176], Enlargement of the cyclophane core could be used as a tool to complex larger steroid molecules. 

Since this work, a number of other molecular clips have been synthesised. Extensive studies in the synthesis of these compounds have been carried out by Pan and Wu who have described a new class of clips which bind aromatic guests and are soluble in nonpolar organic solvents <2006SL49>. The synthesis of this new class of molecular clip is also based on a novel synthetic pathway utilising aromatic amines outlined in Scheme 17. 

In order to increase the n-n interaction with the aromatic guests, two molecular clips with naphthalene walls (compounds 8 and 9) were synthesized. These molecules had quite unexpected properties. Surprisingly, the clip with 1,4-dimethoxynaphthalene cavity walls (8) did not bind guest molecules [15]. The reason for this behavior became clear when the X-ray structure was solved [16] (Fig. 6). All four methoxy groups of 8 were found to point into the cavity, completely blocking the access of a potential guest to the carbonyl groups. A similar structure may also be present in solution. 

Of the three isomers, only the aa conformer is able to stabilize complexes with aromatic guests through n-n stacking interactions with both naphthyl... 

Addition of an alkali metal ion to 23 should also make this host molecule a better binder of aromatic guest molecules. In the aa conformation unlike the as conformation-the 7r-electron rich naphthalene walls are capable of sandwiching rc-electron poor molecules (see Fig. 12). Indeed, upon addition of a potassium salt to a solution of 23, the for binding of 1,3-dinitrobenzene increases by a factor of 2 to 6, depending on the solvent systems [26]. Consistent with the proposed structure of the complex of 23 with sodium ions, addition of these ions has no effect on the of 23 with 1,3-dinitrobenzene. 

The compounds M(NH3)2Ni(CN)4 (M = Zn or Cd), which consist of two-dimensional polymeric sheets of tetracyanonickelate ions bridged by coordinating diamminemetal(II) cations, function as host lattices for clathration of small aromatic molecules such as thiophene, furan, pyrrole or pyridine IR studies indicate the presence of hydrogen bonding between the host lattice ammonia and the aromatic guest molecules.132,133 A crystal structure determination of the related clathrate Cd(en)Ni(CN)4(pyrrole)2 has been reported.134 Similarly, the complex Cd(py)2Ni(CN)4 consists of polymeric [Cd—Ni(CN)4] layers held together by Cd-bound pyridine.135... 

Scheme 6.5 Conformational change in molecular tweezers 6.44 upon aromatic guest binding.29...

Figure 6.37 (a) small cyclophane core dendrophane for complexation of small aromatic guests such... 

The work of Thomas et al. was preceded by the formation of a range of related molecular squares and triangles such as Fujita s molecular square (10.10), which is able to function as a solution host for aromatic guests such as naphthalene with a binding constant, Ku of 1800 M 1. Rigid spacers (edges)... 

Figure 10.32 Molecular panels and the structures formed with Pd(en)2+ (aromatic guests shown as spacefilling representations).30...

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