Guest encapsulation

Douglas, T. and Young, M. (1998) Host-guest encapsulation of materials by assembled virus protein cages. Nature, 393, 152-155. 

The carbonyl complex [Ag(L9)(C0)] (12), also of quite remarkable stability, is obtained by reaction of [Ag(L9)(C2H4)] (11) with CO in hexane. Nevertheless, the CO can be easily removed by increasing the temperature of the solution or by purging with an inert gas. Hence, such a reversible guest encapsulation within a molecular container might find applications for gas separation and storage. Again, one likely reason for the stability of the complexes is the protection offered by the bulky mesityl substituents that surround the ethylene or CO unit. 

Thus, the PEO segment actually becomes hydrophobic at higher temperatures. This temperature-dependent change converts the amphiphilic block copolymer to a water-insoluble hydrophobic polymer (Topp et al. 1997 Chung et al. 2000). The temperature at which the polymer exhibits this transition is called its lower critical solution temperature (LCST). In addition to PEO, substituted poly(A -isopropyl acrylamide) (PNIPAM Chart 2.1) exhibits temperature sensitivity, where the LCST can be tuned by varying the alkyl fimctionahty. The guest encapsulation combined with the temperature-sensitive precipitation of the polymers has been exploited to sequester and separate guest molecules from aqueous solutions (Fig. 2.4). 

Marinakos SM, Novak JP, Brousseau LC, House AB, Edeki EM, Feldhaus JC, Feldheim DL. Gold particles as templates for the synthesis of hollow polymer capsules. Control of capsule dimensions and guest encapsulation. J Am Chem Soc 1999 121 8518-8522. 

One of the simplest forms of guest encapsulation is the formation of an inclusion complex with a host containing a well-defined, preformed cavity. In this regard, the... 

Schmuck, C. (2007) Guest encapsulation within self assembled molecular containers. Angew. Chem., Int. Ed., 46 (31), 5830-5833. 

Scheme 10.8 Guest encapsulation by softball hosts is driven by entropic (solvophobic) effects.

Fig. 27 A A chiral object like a hand in an achiral environment leaves a chiral space available. B Spherical supramolecular capsules are much less suitable compared to C cylindrical ones for enantiorecognition because of the greater number of possible reciprocal orientations of two co-guests encapsulated...

In order to answer some of these outstanding questions, this chapter presents and analyzes a variety of chiral architectures based on metal-ligand coordinative interactions. Although there are a number of ways in which chirality can be introduced into coordination assemblies [2], we will concentrate on ensembles formed from achiral components, where chirality arises from metal coordination geometries or asymmetric ligand arrangements. Particular focus lies with assemblies that show host-guest encapsulation potential and maintain their chirality in solution. 

Fig. 15 Formation of trigonal prismatic coordination complexes from tripyridine ligand 20 and angular connector 21. Upon guest encapsulation, a dynamic equilibrium between chiral cage 22 and achiral 23 is observed. Reprinted with permission from [75]. Copyright 1999 American Chemical Society...

The only example of a twisted cuboctahedron was reported by Kimura and coworkers [159]. Cuboctahedron 63 was obtained by the 4 4 self-assembly of fris(Znn-cyclen) complex 61 with trianionic trithiocyanurate (TCA3-, 62) upon guest encapsulation and was characterized both in the solid state and in solution (Fig. 33). Suitable guests are lipophilic organic molecules of matching size, such as, ([D4]-2,2,3,3)-3-(trimethylsilyl)propionic acid (TSP), 1-adamantanecarboxylic acid, 2,4-dinitrophenol (2,4-DNP), adamantine (ADM), or the tetra-n-propylammonium (TPA) cation. 

Fig. 33 Schematic representation of cuboctahedron 63 formation. Equimolar amounts of fris(Znn-cyclen) complex 61 react with trianionic trithiocyanurate (TCA3-, 62) upon suitable guest encapsulation to form 63. Adapted with permission from [159]. Copyright 2002 Wiley, Inc.

A subsequent detailed thermodynamic study on guest encapsulation within the above-mentioned capsule revealed that not only CH/n interactions between the methyl groups on the guest ends and the aromatic cavity, but also desolvation of the inner cavity, play a key role in guest encapsulation [33],... 

Kobayashi K, Ishii K, Yamanaka M (2005) Orientational isomerism and binding ability of nonsymmetrical guests encapsulated in a self-assembling heterodimeric capsule. Chem Eur J... 

Haino T, Kobayashi M, Fukazawa Y (2006) Guest encapsulation and self-assembly of a cavitand-based coordination capsule. Chem Eur J 12 3310... 

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