Cyclodextrin-Komplexe hochgeladener Metall-Kationen

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.