Structure and properties of iron(III) 1,3-propanediaminetetraacetate complex in aqueous solutions

Abstract The structure of iron(III) 1,3-propanediaminetetraacetate (1,3-pdta) in aqueous solutions was studied on the basis of Raman spectroscopy, nuclear magnetic relaxation, and pH titration. It has been found that, in aqueous solution, Fe(1,3-pdta)− exists in a normal sexidentate hexacoordinate geometry. The six-membered chelate ring formed by the diamine moiety of the 1,3-pdta ligand does not adopt a skew-boat conformation having C2 symmetry (as found in sodium salt crystals) hut a twist-boat one having no C2 symmetry (as in the lithium salt crystals). Although edta complexes of iron(III) form a dimeric complex in slightly alkaline solutions, the 1,3-pdta complex of Fe(III) does not show such a tendency. The 1,3-pdta complex is stable only in the pH region of 2–7.5; the corresponding edta complex is stable even in a strongly acidic solution.