W-183 NMR-STUDIES OF TUNGSTATE COMPLEXES OF CARBOHYDRATES .1. CHARACTERIZATION OF 2 STRUCTURAL TYPES IN THE ALDITOL SERIES - EVIDENCE THAT THE TUNGSTATE AND MOLYBDATE THREO COMPLEXES ARE NOT HOMOLOGOUS

Dinuclear tungstate complexes of alditols have been studied in aqueous solution by C-13 and W-183 NMR spectroscopies. Ligands possessing hydroxyl groups in erythro configuration (erythritol, galactitol) formed a series of tetradentate complexes homologous to the known corresponding molybdate erythro compounds. In contrast, ligands with threo hydroxyl groups (threitol, xylitol) formed tridentate complexes different from the tetradentate molybdate threo species. For all alditols, the W-183 NMR spectra showed two sharp tungsten signals separated by 3-7 ppm (erthro series) or congruent-to 60 ppm (threo series). In the erythro complexes, large 3J(W,H) coupling constants (8-10 Hz) were measured and allowed the estimation of the corresponding dihedral angles. A structure that accounts for the asymmetry of the chelating site is proposed for the tridentate threo complexes. The structural difference between the molybdate and tungstate complexes of threo-alditols is discussed in connection with their stabilities and reactivities.