NMR study of the complexation of D-gulonic acid with tungsten(VI) and molybdenum(VI)

By using multinuclear (H-1, C-13, O-17, Mo-95, W-183) magnetic resonance sp ectroscopy (1D and 2D), D-gulonic acid is found to form ten and seven compl exes, respectively, with tungsten(VI) and molybdenum(VI), in aqueous soluti on, depending on pH and metal-ligand molar ratios. Two isomeric 1:2 (metal- ligand) complexes involving the carboxylate and the adjacent OH group are p resent in the pH range 2-9. At intermediate and high pH, molybdate forms a 2:1 tetradentate complex involving the four secondary hydroxyl groups, wher eas tungstate forms one 2:1 terdentate species. At low and intermediate pH values, three 2:1 complexes are found for both metals, involving the carbox ylate group and three secondary hydroxyl groups, as well as a 5:2 species i nvolving the carboxylate group and all the secondary hydroxyl groups; the c oncentration of this species increases in time mainly at the expense of 2.1 and 1:2 complexes. Tungstate can also form two additional species, probabl y. a 5:2 species involving the carboxylate group and all the hydroxyl group s, and a 2:1 pentadentate species involving the carboxylate group and all t he secondary hydroxyl groups. In alkaline solutions, tungstate is able to f orm an additional 2:1 pentadentate complex involving all the hydroxyl group s. (C) 2000 Elsevier Science Ltd. All rights reserved.