Characterization of the nature of Cd(Ⅱ) ion binding sites on natural and synthetic polymers using 113Cd NMR spectroscopy

Abstract

The $^{113}Cd$ NMR spectroscopy has been used to probe the cadmium ion binding sites in humic substances with the aid of variation of the $^{113}Cd$ chemical shift that are highly sensitive to the donor atoms, coordination number and geometry. The elucidation of metal ions binding sites on humic substances is complicated by two factors, namely macromolecular nature (conformational change) and heterogeneity (polyfunctionality). Therefore, the contribution of each of these factors that may cause $^{113}Cd$ chemical shift changes was investigated separately using synthetic polymers and model ligands including carboxylates, hydroxycarboxylates, and carboxylate possessing the nitrogen moiety. A variety of complexes of cadmium with mono- and di-carboxylic acids used as model ligands give a single averaged chemical shift even at reduced temperature, demonstrating that the cadmium-monocarboxylate complexes are undergoing rapid exchange in solution. Using the known stability constants, the individual chemical shifts were calculated and were found to be in the range -22 to -24ppm for $CdL^+$ and -39 to -40ppm for $CdL_2$ (L = carboxylate) species: Carboxylates with higher basicity tend to increase the shielding of the cadmium in the complexes. With dicarboxylic acid ligands $HO_2C(CH_2)_nCO_2H$ (n = 0-3), the $^{113}Cd$ nucleus showed a more shielding trend as n increased: Cd(oxalate) +11.0ppm < Cd(malonate) -19.1ppm < Cd(succinate) -35.3ppm~Cd(glutarate) -35.9ppm. Only oxalate and malonate seem to behave as chelate ligands to form ring with cadmium. However the steric effect seem to hinder chelate ring formation in dialkylsubstituted malonate complexes of cadmium. The $^{113}Cd$ NMR studies were extended to single crystals of cadmium(II)-malonate complexes formed in aqueous solution. Polymeric tetraaquobis(malonato)dicadmium(II) 1 was formed initially. But two other complexes, namely triaquo(malonato)-cadmium(Ⅱ) monohydrate 9 and diaquo(malonato)cadmium(II) 13 were obta...