Complexation of 2,6-pyridinedicarboxylic and 2,6-pyridinediacetic acids towards aluminium(III) and iron(III)

The complexes formed by 2,6-pyridinedicarboxylic acid (H3X+, pKa1 << 0.4, pKa2 = 2.10 ± 0.01, pKa3 = 4.383 ± 0.004, neutral form H2X) and 2,6-pyridinediacetic acid (H3A+, pKa1 = 2.32 ± 0.01, pKa2 = 3.08 ± 0.01, pKa3 = 5.92 ± 0.01, neutral form H2A) with Al(III) and Fe(III) were studied by means of potentiometric titrations, UV–Vis, and NMR measurements (for Al(III)), at 25 degrees in (Na)Cl 0.6 m aqueous solution. Stoichiometry (and stability constants) of the complexes are the following: AlX+ (4.72 ± 0.01), AlXH-1 (0.37 ± 0.07), AlX2- (7.85 ± 0.02), and AlX2H-12- (2.6 ± 0.3) for Al(III)/H2X; FeX+ (8.840 ± 0.003), FeXH-1 (6.44 ± 0.05), and FeX2- (14.82 ± 0.03) for Fe(III)/H2X; AlAH2+ (8.4 ± 0.1), AlA+ (5.2 ± 0.1), AlA2- (9.1 ± 0.1), and AlA2H-1 2- (3.3 ± 0.3) for Al(III)/H2A; FeA+ (7.53 ± 0.03), FeA2H (16.5 ± 0.3), FeA2- (13.81 ± 0.04), FeA2H-12- (10.0 ± 0.1), and FeA3H2- (22.32 ± 0.08) for Fe(III)/H2A. An Al(III)/H2X complex, probably AlXH-1, was characterised in the solid state. H2X forms stronger complexes towards Al(III) and Fe(III) than does H2A. The main metal/H2X complexes can be hypothesised to display a partial tridentate coordination, and the nitrogen of H2X appears to be involved in the metal chelation. For Al(III)/H2A, the most probable structure of the complexes AlA+ and AlA2- is a bidentate one involving the two acetate arms.