Novel polydentate phosphonic acids: Protonation and stability constants of complexes with Fe(III) and Cu(II) in aqueous medium

The acido‐basic and the complexation properties of di‐, tri‐, and tetra‐phosphonic acids (H 6 L1, H 8 L2, and H 10 L3) toward Fe(III) and Cu(II) were determined by potentiometric titration in aqueous media at 25.0 ± 0.1°C with constant ionic strength (0.1 M, NaClO 4 ). We have determined six, ten, and eight p K a values for the di, tri, and tetra‐phosphonic acids, respectively. In acidic conditions, e.g., 0 ≤ pH ≤ 5; iron and copper presented a high affinity toward these ligands to give complex species. With the ligand H 10 L3, [FeL3H 7 ], and [CuL3H 6 ] 2− were easily obtained at pH 1.8 and 2.7, respectively. We have determined ten stability constants for the H 10 L3/Fe system and nine for the H 10 L3/Cu one; six and four in the cases of H 8 L2/Fe and H 8 L2/Cu systems, respectively. Finally, five stability constants were calculated for the H 6 L1/Fe system and four for the H 6 L1/Cu one. We have not observed any insoluble species in these complexes in acidic medium as well as in alkaline solutions. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:51–62, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20575