Host-guest complexes, molecular structures

In the modelling of structures of host-guest complexes, molecular mechanics, particularly when allied with molecular graphics, has proved to be particularly useful. Molecular mechanics has the virtue of being simple to understand and easy to program. The technique can be applied to a large number of atoms without excessive computer power. It is now available in several relatively cheap and easy to use packages on many hardware platforms. Paradoxically its main drawback is caused by its virtues. Results can be obtained easily from the software but are very often unsatisfactory because the parameters provided are either inaccurate or not applicable to the systems under study. This problem is common with commercial software where parameters are often added... 

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

Three molecules of the host-guest complex can associate in three-molecular associate (Figure 9.10). This structure was confirmed by light scattering measurements. When free matrix was added to the associated system, dissociation occurs (Figure 9.11). 

Clathrasils are host/guest complexes comprised of covalent guest molecules entrapped within cages formed by a silica host framework (1, 2). Like all zeolitic materials, clathrasils have enormous potential as advanced optical and electronic materials whose composite character permits synthetic manipulation of both the molecular structure of the guest species and the extended structure of the host framework (3, 4). Like other zeolites, however, clathrasils also suffer severe handicaps as advanced materials due to a reluctance to form large single crystals and a tendency to form stoichiometrically and structurally defective crystals (5 -10). 

Fujiwara, T., Tanaka, N., Ooshita, R., Hino, R., Mori, K., and Toda, F. (1990) Crystal and Molecular Structure of the Crystalline Host-Guest Complex between (R)-(+)-2,2 -Dihydroxy-l,l -binaphthyl and (5)-(-)-(Ethyl m-Tolyl Selenoxide), Bull. Chem. Soc. Jpn., 63, 249-251. 

Molecular structure of the tetrahedral Fe(II) host-guest complex cation. The capping BF groups and the phenyl rings of the tripod ligands have been omitted for clarity. From S. Mann, G. Huttner,... 

Pearlman RS (1987) Rapid generation of high quality approximate 3D molecular structures. Chem Des Auto News 2 1-7 Sadowski J, Rudolph C, Gasteiger J (1992) The generation of 3D models of host-guest complexes. Anal Chim Acta 265 233-241... 

Small structural changes are especially important in molecular recognition. It has been noted, e.g., that ... subtle changes of molecular structure may result in severe changes of inclusion behavior of a potential host molecule due to the complicated interplay of weak inter-molecular forces that govern host-guest complex formation [41],... 

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