Add time:07/31/2019         Source:sciencedirect.com

Colourless tungstate complexes of the sugar acids, l-manonnic, d-glucaric and galactaric acids have been studied in aqueous solutions. Multinuclear 13C and 183W NMR have been used to identify the structures of the complexes and the sites of chelation of each ligand. Indirect photometry has been used to determine the stoichiometry and the stability constants of the complexes.For mannonic acid, two complexes of the lactic type with a mono-nuclear and a di-nuclear tungsten core have been identified at acidic pH. In these complexes, a (OH) group at the carbon atom in α position is necessary to the formation of the complex and only the hydroxyl groups (OH) of the carboxyl function and of the carbon atom in α position belongs to the site of chelation. Upon increasing the pH, the lactic complexes disappear: first the mono-nuclear, then also the di-nuclear lactic complexes, however a new complex of mixed (lactic + erythro) type with a (5, 2) composition has been observed. Upon increasing the pH a single erythro complex appears and at pH 10 a small amount of threo (tetradentate) type complex has been observed.For glucaric and galactaric acids , a new tri-nuclear complex (3, 3, 6) is formed in acid medium, this complex is composed of three mono-nuclear bidentate sites of chelation related one to the other. Upon increasing the pH, these new tri-nuclear complexes disappear and only for the W-galactarate system there is formation of a di-nuclear tetradentate complex of type erythro analogous to the complex of the mannonic acid.

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