Hemicarcerand, thermodynamically

Fig. 19 (A) Thermodynamically controlled synthesis of hemicarcerand 37.129,130 (B) The delivery of ferrocene from 34 to bulk solution occurred at a faster rate in the presence of TFA and/or 1,3-diaminobenzene. (For color version of this figure, the reader is referred to the web version of this book.)...

The relatively low thermodynamic stability of complexes of hemicarcerands or other container-type hosts is a direct consequence of structural aspects of the walls that make up the inner surface of such compounds. These walls are lined by aromatic subunits while free electron pairs of heteroatoms such as those of the ether oxygen atoms are preferentially oriented to the outside. Complexes are therefore enthalpically stabilized only by weak dispersive interactions. In the case of positively charged guests cation-re interactions can contribute to binding enthalpy as in a self-assembled calixarene-derived capsule [9], but directed interactions such as hydrogen-bonding interactions are usually absent. 

All these hemicarcerands demonstrate hosting properties, the thermodynamic and kinetic aspects of which are intrinsically tied to the sizes and shapes of the cavity, portal, and guest. There is, however, a limitation to the... 

Carcerands and Hemicarcerands, p. 189 Cryptophanes, p. 340 Glycoluril-Based Hosts, p. 597 Hydrogen Bonding, p. 658 Micelles and Vesicles, p. 861 Molecular Squares, Boxes, and Cubes, p. 909 Platonic and Archimedean Solids, p. 1100 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 Self-Assembly Terminology, p. 1263 Soft and Smart Materials, p. 1302... 

Figure 13 Thermodynamically controlled assembly of octaimine hemicarcerand 36 from 2 equivalents of cavitand 37 and four phenylene-1,3-diamines (38).

The efficiency of these hemicarcerand and polycavitand nanocapsule syntheses approaches those of self-assembly processes using hydrogen bonding or metal-ligand coordination and results from selection of the least strained capsule through the thermodynamic control and reversibility of the system. 

Piatnitski et al. carried out a detailed thermodynamic analysis of the binding properties of the water-solnble hemicarcerand 65. ° This host lacks one of the linkers of 64, which facilitates gnest exchange. Thus, 65 displays thermodynamic selectivity in its binding properties, which differs from many other hemicarcerands, for which constrictive binding controls selectivity. 

A second example in which through-shell electron transfer was examined quantitatively is the electrochemical oxidation of Fc incarcerated inside hemicarcerand 36. Electron transfer was strongly hindered kineticaUy and thermodynamically compared to free Fc (Figure 28). " The half-way potential for the oxidation was more positive, due to the hydrophobicity of the inner phase, and the electron transfer rate was reduced 10-fold. The latter may result partially from the higher mass of 36 Fc compared to Fc and also from a reduction of the electronic coupling between the Fc center and the electrode surface which is affected by the increase in distance from 3.5 to about 9 A. Whether the hemicarcerand s aromatic structure mediates the electron coupling is not clear. 

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