Substitution and oxidation kinetics in substituted catechol - iron ( 111 ) systems

The equilibrium constants and kinetics for the complexing of aqueous iron(m) by 2,3dihydroxybenzoic acid (DHB) and 1,2-dihydroxy-3~-benzenedisulfo~te (Tiron) have been studied with iron(IIl) in excess over ligand. Formation constant comparisons indicate that DHB complexes in a mode analogous to salicylate. The reaction rate has the hydrogen ion dependence typical for iron(m) complexation, but there is an unusual contribution from a term second order in irm(III). The DHB-iron(III) complex shows a subsequent redox reaction with the rate first order in iron(IU) and inverse second order in iron(II). The mechanistic implications for the redox mechanism are described. INTRODUCTION This study is concerned with the equilibrium constants and kinetics for the complexing of iron(III) by 2,3dihydroxybenzoic acid(DHB) and 1,2-dihydroxy-3,5-benzenedisulfonate(Tiron). The kinetics of the oxidation of DHB by iron@) also has been studied. There have been several equilibrium studies of the Tiron systeml~2~3 but several features remain uncertain. With DHB, the previous observations495 seem not to fully account for the added complexity of the ligand oxidation reaction. RESULTS The complexation of Tiron has been studied mainly as a point of comparison with DHB. For example the formation constants are used to assess whether DHB is complexed through the carboxylate and phenoxide groups as in salicylate or through two phenoxides as in catechol. When a solution of DBH is mixed with iron(II1) in dilute aqueous acid the solution rapidly tu rns blue and then fades to yellow in 5 to 10 minutes, and finally changes to brown after several hours. Qualitative tests show that iron@) is not present at the blue stage but is formed at the yellow stage. These obsevations indicate that the blue species is an iron(III)-DHB complex, and that the blue to yellow change involves oxidation of DHB by iron(III). When the same experiment is done with Tiron, there is immediate formation of a green color. This color appears to be stable, but spectrophotometric observations show that the absorbance actually decreases slowly on the tens of minutes time scale. Spectrophotometric methods were used to determine the formation constants of the initial blue and green complexes with DHB and Tiron, respectively. These studies were done with [iron(III)]>>[ligand] at various acidities in 1 M NaClOq-HC104 at 25". The results are plotted in Figure 1. The linearity of the plots in Figure 1 is consistent with the following reactions. Fe(OH2)?+ + (TironHd24 (H@)$e(Tiron)+ 2H+ The equilibrium constants are 3.2 M.2 and 7.0 M.6 M for Tiron and DHB respectively.