Indirect photometry and multinuclear 13C and 183W NMR study of tungstate complexes of sugar acids: Stabilities and structures

The tungstate complexes of the sugar acids, meso-tartaric and l-gulonic acids, have been studied in aqueous solutions. Indirect photometry has been used to determine the stability constants and the stoichiometry of these colourless complexes. Multinuclear 13C and 183W NMR spectroscopies have been used to specify the structures of the complexes and the sites of chelation of the ligand. For meso-tartaric acid, complexes of the lactic type with a mononuclear and with a dinuclear tungsten core have been identified. In these type lactic complexes, only the hydroxyl groups of the carboxyl function and the atom carbon in α position belong to the site of chelation of the tungstate complexes. Both α-hydroxyacid moieties of the meso-tartaric acid are involved in the chelation.For l-gulonic acid, lactic type complexes with a mononuclear and a dinuclear tungsten core have been observed at acidic pH. Upon increasing the pH, the lactic complexes disappear: first the mononuclear, then the dinuclear complexes. In pH range 4–8, for gulonic acid, dual lactic, threo (tetradentate) type complexes occur, the dual lactic, erythro and lactic, threo complexes exist. At pH 10, for gulonic acid, a single complex has been identified in which the ligand is pentadentate. In this complex, all carbon atoms of the ligand are involved in the site of chelation except the carboxyl atom.