The kinetics and mechanisms of the reaction of iron(III) with gallic acid, gallic acid methyl ester and catechin

The kinetics and mechanisms of the reactions of a number of pyrogallol-based ligands with iron(III) have been investigated in aqueous solution at 25 degrees C and ionic strength 0.5 M NaClO(4). Mechanisms have been proposed which account satisfactorily for the kinetic data. These are generally consistent with a mechanism in which the 1:1 complex that is formed initially when the metal reacts with the ligand subsequently decays through an electron transfer reaction. There was also some evidence for the formation of a 1:2 ligand-to-metal complex at higher pH values. The kinetics of complex formation were investigated with either the ligand or metal in pseudo-first-order excess. Rate constants for k(1) of 2.83(+/-0.09)x10(3), 1.75(+/-0.045)x10(3) and 3300(+/-200) M(-1) s(-1) and k(-1) of 20(+/-6.0), 35(+/-13) and 25+/-7.6 M(-1) s(-1) have been evaluated for the reaction of Fe(OH)(2+) with gallic acid, gallic acid methyl ester and catechin, respectively. The stability constant of each [Fe(L)](+) complex has been calculated from the kinetic data. The iron(III) assisted decomposition of the initial iron(III) complex formed was investigated. Analysis of the kinetic data yielded both the equilibrium constants for protonation of the iron(III) complexes initially formed together with the rate constants for the intramolecular electron transfers for gallic acid and gallic acid methyl ester. All of the suggested mechanisms and calculated rate constants are supported by calculations carried out using global analysis of time-dependent spectra.