Coordination of Cd(II) by N-alkylamino sugars in aqueous solution as studied by potentiometry and NMR spectroscopy

The Cd(II) coordination of 1-(2-aminoethylamino)-1-deoxy-d-alditols (1a–b) and of 1-(2-aminopropylamino)-1-deoxy-d-alditols (2a–b) was investigated by means of 113Cd NMR spectroscopy and potentiometry. The influence of the sugar chains on the basicity of the amino groups was determined with 13C chemical shift pH titration curves. It appears that the inductive effects of the sugar moieties give rise to a decrease in pKa of the secondary amino function. The Cd(II) stability constants for ligands 1a–b and 2a–b and the Ni(II) stability constants for 1a were determined by potentiometry. 113Cd NMR spectroscopy was employed to study the structures of the Cd(II) complexes of 1a–b and 2a–b. From the 113Cd chemical shifts for CdL and CdL2, it could be concluded that at neutral pH both primary and secondary nitrogen atoms are involved in coordination. 13C NMR shows that additional coordination of a hydroxyl group of the carbohydrate chain occurs at high pH (pH 12).