NMR Study of uronic acids and their complexation with molybdenum(VI) and tungsten(VI) oxoions

A multinuclear 1D and 2D NMR study of d-galacturonic and d-glucuronic acids in aqueous solution and their complexation with tungstate and molybdate ions for variable concentration and pH conditions has been undertaken. The acids exist mainly in the pyranose forms, but complexes were detected involving the less stable α- and β-furanose anomers as well as the α-pyranose form. Thus, NMR evidence was gathered for the formation of two 1:2 (metal-ligand) complexes of W(VI) with the furanose forms. These are stronger with d-galacturonic acid and when the β forms are involved. The same was found with Mo(VI), but, in addition, 2:1 complexes also form. In the case of d-galacturonic acid, three such complexes were detected, two involving the α-pyranose form, in an approximately 4C1, and a 1C4 conformation, respectively, and the other presumably involving the β-furanose isomer. With d-glucuronic acid, only one such complex could be characterized, involving the α-pyranose isomer in a distorted 1C4 conformation. More detailed information on the structure of the various complexes was obtained from 1H, 13C, 17O, 95Mo, and 183W NMR data. The 2:1 complexes with the α-pyranose forms, insofar as they involve metal binding to the ring oxygen atom, are considered to play an important role in the oxidation of the acids especially by Mo(VI).