Kinetics and mechanism of the formation of an iron(III) complex with Nα-pyridoxyl-L-tryptophan

The formation of an iron(III) complex with a new multidentate ligand, Nα-pyridoxyl-L-tryptophan(1, H2pltp), has been investigated thermodynamically and kinetically in an aqueous 0.10 mol dm−3 sodium perchlorate solution at 25°C. A 1:2(metal : ligand) complex of 1 forms with much higher stability constant (logβ12/mol−2 dm6 = 38.29±0.28) than the related complex, bis(pyridoxylidene-L-tryptophan)iron(III), having the azomethine group (log β12/mol−2 dm6 = 18.38±0.10 [3]). This effect was explained by the contribution of amino nitrogen of higher basicity to the thermodynamic stability. The complex formation reaction proceeds through two parallel pathways of Fe3+ +H3pltp+ and FeOH2+ +H4pltp2+ and also another pathway of FeOH2+ +H3pltp+ with slower coordination of the first ligand to iron(III). The mechanistic rate-determining step for the formation of the complex was the coordination of the phenolato oxygen to a central metal atom, followed by the rapid donations of the amino nitrogen and carboxylato oxygen atoms.