W-183 NMR-STUDIES OF TUNGSTATE COMPLEXES OF CARBOHYDRATES .3. SPECIESFORMED WITH ALL-THREO ALDITOLS ACTING AS TRIDENTATE, TETRADENTATE, ORPENTADENTATE LIGANDS

Tungstate complexes of all-three alditols HOCH2(CHOH)(n-2)CH2OH (DL-th reitol, xylitol, L-iditol, meso-glyceroido-heptitol) have been studied by multinuclear C-13 and W-183 NMR. Contrary to the corresponding mol ybdate species in which the ligands are always tetradentate, the tungs tate species exhibit structural variety, depending on the pH of format ion. Only the tungstate complexes (type M) formed at pH 7 are homologo us to the molybdate species. At pH 7-9, the ligands are tridentate and chelate a ditungstate group in an asymmetrical way (type T). The corr esponding tungstate complex of glycerol is reported for the first time . At pH 9-12, dinuclear species of a previously unknown type P are slo wly formed by the pentadentate xylitol and iditol. The heptitol afford s two complexes of type P, in which the sites of chelation are HO-2,3, 3,5,6 (major complex) and HO-1,2,3,4,5. A possible structure, in agree ment with C-13 and W-183 NMR data, is proposed for complexes of type P in which the presence of two WO6 octahedra linked by an edge is remin iscent of structures of ditungstate subunits of polytungstate ions.