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Ihringer, Johannes (2006): Cyclodextrin-Komplexe hochgeladener Metall-Kationen. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

The use of carbohydrates as ligands in coordination chemistry is an important area of research with many potential applications. Few studies have been done on the complexation behaviour of main group metals under aqueous conditions. In this thesis the complexation of carbohydrates in the presence of tin(IV) and lead(IV) central metals in aqueous solution was studied. Tin(IV) was investigated using 119Sn NMR techniques. It could be shown that a mixture of hexahydroxidostannate and the threefold amount of ethanediol forms a mixture of heteroleptic mono- and bis- (ethanediolato)hydroxidostannates and homoleptic tris(ethandiolato)stannate. In contrast to these observations, the threefold amount of beta-cyclodextrin with hexahydroxidostannate(IV) showed a single signal, whose chemical shift indicates a new, hexacoordinate, tin-containing species. When using lithium as a counterion, crystals suitable for X-ray diffraction grew over the course of one week. The results from the structural analysis were consistent with the spectroscopic results and show a supramolecular assembly with threefold rotational symmetry that consists of three components: three beta-cyclodextrin dianions, three tetrahedral tetraaqualithium cations and the tetravalent metal center. Attempts to prepare chelates using lead(IV) as the central metal by adding potential chelators such as oxalate, catecholate, or glycosides to hexahydroxidoplumbat(IV) solutions result in the formation of decomposition products. Surprisingly, beta-cyclodextrin could avoid decomposition and formed single crystals. X-ray analysis of the product showed lead(IV) complexed by three cyclodextrin dianions isotypic to the tin(IV)-cyclodextrin complex. It is the first described carbohydrate-lead(IV) complex. It could be shown, that the functional groups of the oligosaccharide cyclodextrin are able to assemble into a nanoscaled trimer that provides a mononuclear binding site for high valent metal centers. These findings may be helpful to develop other cyclodextrin-metal complexes with metals which show catalytic activity. One possible use for the developed system could be the treatment of heavy-metal-contaminated water.