Kinetics and mechanism of complex formation reaction of iron(III) with 4-methyl-8-(carboxymetahyl_aminomethyl umbelliferrone and related ligands: role of substituent groups on nitrogen donor atoms on the reaction

Abstract

The formation of the iron(III) complex of 4-methyl-8-(carboxymethyl)aminomethyl umbelliferrone ( 1, H 2mug) and its N-methyl ( 2, H 2mugm) and N-carboxymethyl ( 3, H 3muia) derivatives has been investigated spectrophotometrically in an aqueous sodium nitrate solution (0.10 mol dm −3) at 25°C. A 1 : 1 (metal : ligand) complex forms uniquely for all three ligands, indicating a steric interference between the coordinated and a coordinating ligand. The stability constant [log β 11/(mol −1 dm 3(] is 15.74 ± 0.20 for 4 [Fe(mug) +], 15.69 ± 0.20 for 5 [Fe(mugm) +] and 18,67 ± 0.20 for 6 [Fe(muia)]. The complex formation reaction proceeds through two parallel pathways of Fe 3+ and Fe(OH) 2+ with diprotonated ligand species of 1, 2 and 3; a slow coordination of the phenolato oxygen, followed by rapid donations of the amino nitrogen and carboxylato oxygen(s) to form a mono_terdentato- and tetradentato)iron(III) complex was deduced as the reaction mechanism. The possibility of changing the thermodynamic stability with no variation in the reaction mechanism by the introduction of substituent groups on the amino nitrogen of 1 is suggested.