Coordination motifs of binary neodymium(III) D-gluconate, D-galactonate and L-gulonate complexes and the transition from inner- to outer-sphere coordination in neutral to strongly alkaline medium

Abstract

The structural aspects of the complexation of Nd(III) in aqueous solutions with D-gluconate, D-galactonate and L-gulonate in the neutral to hyperalkaline pH range have been studied by using 1H and 13C NMR, CD and Raman spectroscopies. Beside the identification of the binding sites, an attempt was made to reveal whether the structure of the complexes depends on the type of the ligand. It was concluded that the carboxylate group of each ligand has an essential role in the coordination to Nd(III). In solutions of pH > 5, other hydroxyl or alkoxide groups are also coordinated. Based on the NMR spectra, the coordination of several C-OH groups can be inferred, and CD measurements strongly suggest that beside the carboxylate, the C4-OH group takes part in the metal ion coordination in the Gluc– and Gal– complexes. On the basis of our observations from all the three spectroscopic methods, Nd(OH)3 units covered by the polyhydroxy carboxylate is the most appropriate model for describing the systems at pH > 12.