Complex Formation of Alkyl-N-iminodiacetic Acids and Hard Metal Ions in Aqueous Solution and Solid State

Abstract

The calcium(II), iron(III) and chromium(III) alkyl-N-iminodiacetate systems have been studied in aqueous solution with respect to stability, acid–base properties and structure. The calcium(II) ion forms only one weak complex with methyl-N-iminodiacetic acid in water, K1 = 12.9 (2) mol–1⋅dm3, while iron(III) and chromium(III) form very stable complexes with alkyl-N-iminodiacetic acids. The calcium(II)–methyl-N-iminodiacetate complex is octahedral in the solid state with most probably water in the remaining positions giving a mean Ca–O bond distance of ca. 2.36 Å. The iron(III) alkyl-N-iminodiacetate complexes have low solubility due to a strong tendency to form polymeric structures. Depending on pH in the solution at their preparation, the degree of hydrolysis in the resulting compound(s) may differ. In the solid state, the polymeric iron(III) alkyl-N-iminodiacetate compounds seem to have the mean composition Fe2O(Cx-IDA)5; the mean Fe–O bond distances to the oxo group and the alkyl-N-iminodiacetate ligands are 1.92 and 2.02 Å, respectively. In these complexes the nitrogen atoms are bound at much longer bond distances, 0.1–0.2 Å, than the carboxylate oxygens. This distribution with short strong Fe–O bonds and much longer and weaker Fe–N bonds is also found in most other structurally characterized iron(III) carboxylated amine/polyamine complexes. The chromium(III) alkyl-N-iminodiacetate complexes are octahedral in both solution and solid state, and the low solubility of the solid compounds indicates a polymeric structure with the ligands bridging chromium(III) ions. Also, chromium(III) binds oxygen atoms in carboxylated amines at significantly shorter distance than the nitrogen stoms. The chromium(III) alkyl-N-iminodiacetate complexes display such slow kinetics at titration with strong base that the back-titration with strong acid shows completely different acid–base properties, thus the acid–base reactions are irreversible.