Host-guest inclusion complexes calixarene hosts

Only few investigations have been published on the gas-phase ion chemistry of host-guest complexes of calixarenes. With the advent of ESI mass spectrometry, especially when combined with ion-trap and FT-ICR mass spectrometry, this field has started to be developed. Binding selectivities of alkali metal ions to cahxarene-based crown ethers and open-chain ethers have been studied , the inclusion of neutral guests into the protonated resorcarene-based cavitand hosts by gas-phase ion-molecule reactions with amines have been studied and the formation of capsules from various calixarene tetraether derivatives and alkylammonium ions as ionic guests (notably enabling their detection by mass spectrometry) have been described recently . ... 

It should be stressed that there is not alwaysjustice in reseach evaluation. The selective formation of inclusion complexes by cyclodextrins (such as 11) was established by Cramer [6] at least 15 years earlier than that by crown ethers. However, cyclodextrin studies forming an independent branch of host-guest chemistry seem underestimated in spite of their considerably greater practical importance at present than that of other host macrocycles (crown ethers 17, calixarenes 18, etc.). Sometimes they are even totally neglected by discussing inclusion phenomena [7]. 

Common adsorbents are oxides such as silica, alumina, alumino-silicates, and clays, among others. Rarer studies were also presented for cellulose and cellulose derivatives or starch [2a]. Cyclodextrins and calixarenes are good examples of organic host molecules, which can form inclusion complexes with many guest molecules. 

Harrowfield, J.M. Richmond. W.R. Sobolev. A. Inclusion of quaternary ammonium compounds by calixarenes. J. Inch Phenoin. Mol. Recognit. Chem. 1994. 19. 257. Lippmann, T. Wilde. H. Pink. M. Schaefer. A. Hesse. M. Mann. G. Host-guest complexes between resorcinol-derived cal ix [4] arenas and alkyl ammonium ions. Angew. Chem. 1993, 105, 1258-1260. ... 

The first examples reported of totally synthetic uncharged host-uncharged guest intramolecular inclusion complexes elucidated by x-ray crystal structures were the calixarenes (125,126). This class of macrocyclic cyclophanes contains phenolic residues in a cyclic array linked by methylene groups all ortho to the hydroxy groups (Figure 2.53). 

In these solid inclusion componnds, the association of host and guest components is strictly a solid state phenomenon, and the examples covered in this chapter focns almost entirely on solid inclnsion componnds of this type. However, it is relevant to note that there is also another class of inclnsion compound in which the host is a molecnle containing an appropriate cavity or binding site that can inclnde gnest molecules. Such molecular host-guest complexes generally exist as associated entities both in the solntion state and in the crystalline state, and thns the incln-sion phenomenon is not exclusively a property of the solid state. Examples of host molecules of this type are crown ethers, cyclodextrins, cryptands, and calixarenes. 

Inclusion of cationic molecules by hosts in organic solvents is also observed with other host molecules, such as crown ethers and calixarenes. Cat-ion/n and coordination interactions are the main driving forces in these complexes. However, the formation of host-guest complexes with neutral... 

One of the first examples of calixarene-capped AuNPs employed for host-guest studies was published by Arduini and co-workers in 2005 [30]. The calixarene derivatives chosen for these studies were l,3-diaIkoxycalix[4]arenes, which were known to form endo-cavity inclusion complexes with tetraalkylammonium and N-alkyl pyridinium ion pairs in weakly polar solvents [31, 32]. 

It took 8 years to publish another structure. In 1976 K. J. Palmer et al. reported two conformers of another resorcin[4] arene derivative, i.e. cis- and trans-C-para-bromophenyl-4,6,10,12,16,18,22,24-0-octaacetylcalix[4]resorcinarene [3]. Than in next 3 years G. D. Andreetti et al. published the first crystal structure of inclusion compound formed by para-ferf-butylcalix[4] arene with toluene [4]. The calixarene molecule was found in cone conformation and the guest molecule could not be located within the host cavity due to its disorder (the molecular complex was located on a crystallographic fourfold axis). The Authors also noticed that the structure undergoes the order-disorder phase transition at 248 K. In 1996 the disorder high temperature phase was resolved with the aid of solid-state NMR data by E. B. Brouwer et al. [5] (unfortunately, the atoms coordinates are not deposited in CSD) and in 1998 the low temperature ordered phase was reported by A. Arduini et al. [6] (Fig. 38.2). The inclusion complex is stabilized by week interaction. The crystal and molecular structure of this... 

Chlorhexidine forms an inclusion complex with para-sulphonatocalix[4]arene. I this complex chlorhexidine adopts V-shaped conformation (Fig. 38.8a). Two other derivatives of cahx[4]arene, i.e. the dimethoxycarboxylic acid and dihydroxyphosphonic acid which are derivatized on lower rim the calixarenes form inclusion dimers and their internal cavities are protected against inclusion of guest molecule (Fig. 38.8b, c). The co-crystals are formed instead and the chlorhexidine molecule adopts S and Z-shaped conformations, respectively. This may suggest that the conformation of guest molecule may be controlled by appropriate selection of host molecule. 

In the last example of the para-sulphonatocalix[5]arene inclusion complex with ethane-l,2-bis(4,4 -bipyridinium) cation the host molecule adopts 1-alternate conformation (Fig. 38.16d). It is interesting that the part of the guest is not included into another calixarene cavity but it interacts with hydrophilic surrounding formed by water molecules and sulphonate group of the inverted ring of host molecule [36]. 

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