A multinuclear NMR spectroscopy study of the tungstate and molybdate complexes of d-fructose and l-sorbose

The formation of dinuclear tungstate and molybdate complexes of d-fructose and l-sorbose was studied in aqueous solution by 1H, 13C, and 183W NMR spectroscopy. These ketoses react only in acyclic forms, because HO-3,4 are trans in the cyclic forms. With tungstate, both sugars form a major complex of the same type in which the hydrated ketose is bound through the O-1,2,4 site of chelation. The ligands are tetradentate, since both O-2 atoms bridge the metal atoms. Complexes of the same type prevail in the molybdate systems, but are accompanied by one (sorbose) or two (fructose) minor complexes formed by the acyclic keto forms in which the sites of chelation are the O-3,4,5,6 systems. Multinuclear NMR data are provided for the identification of the various types of complexes.