Interactions of Fe(III) with adrenaline, l-dopa and other catechol derivatives : Equilibria and kinetics of complex formation in acidic perchlorate media

Abstract

Equilibrium and kinetic data are reported at 10, 25 and 35°C and μ = 1·0 M for complex formation between Fe(III) and a series of catechols. In acidic media (0·100–0·010 M, HClO 4), a single 1:1 complex is formed according to: Fe 3+ + H 2 cat ⇄ K eq Fecat + + 2 H + , and the equilibrium constants increase in the order 4-methylcatechol < 4-t-butyl- < catechol < Dopa < Adrenaline < 4-COOH- < 4-CN- < 1,2-dihydroxy-3,5-benzenedisulphonic acid. Kinetic data, obtained by the stopped-flow technique, showed that the reactions progress through an S N 1 mechanism according to: FeOH 2+ + H 2 cat ⇄ k r k ƒ Fecat + + H + + H 2 O , and k f values are independent of substituent groups present on the ligand, which affect only the reverse reaction rates.