Comparative study of Al 3+ and Fe 3+ ions complexation with a lacunary Dawson type polyoxometalate: stoichiometry and stability analysis

The complexation of Al3+ and Fe3+ ions with the lacunary heteropolyanion (α2P2W12Mo5O61)10− was investigated using UV-Visible spectrophotometry combined with complementary FT-IR analysis. The stoichiometry of the formed complexes was determined by the method of continuous variations (Job’s method) and confirmed by the saturation method. In both cases, the results revealed the formation of predominant 1:1 metal-ligand complexes, as evidenced by the maxima in Job’s plots and the characteristic saturation profiles. The stability constants of the complexes were evaluated from the saturation curves, showing that the Fe³+ complex exhibits higher stability compared to the Al³+ system. This difference is attributed to the stronger coordination ability and higher charge density of Fe³+, as well as the tendency of Al³+ to undergo hydrolysis in aqueous media. FT-IR spectroscopy revealed significant changes in the characteristic P-O, M=O, and M-O-M vibrational bands, confirming the coordination of Fe3+ and Al3+ions within the lacunary Wells-Dawson structure. Overall, this study demonstrates the strong affinity of lacunary polyoxometalates for metal ions and the effectiveness of spectrophotometric and vibrational techniques in characterising metal-ligand interactions, highlighting their potential applications in catalysis, environmental remediation, and metal ion sequestration.