Mechanism of oxidation of iron(II) complexes by the azide radical

Abstract

The reactions of azide radicals (N₃˙) with both iron(II) aquo- and iron(II) azide complexes have been investigated using pulse radiolysis, the second-order rate constants for the reactions being measured as (3.2 ± 1.0)× 10⁶ and (5.2 ± 0.5)× 10⁸ dm³ mol^–1 s^–1, respectively. From the transient spectra, it was proposed that the immediate product of the reaction of N₃˙ with iron(II) azide complexes was a novel iron(II)-complexed azide radical (Fe²⁺N₃˙)_aq having a characteristic optical absorption at 300 nm and 410 nm (ε= 1700 and 1100 dm³ mol^–1 cm^–1, respectively).

From the effect of both pH and azide ion concentration on the kinetic fate of (Fe²⁺N₃˙)_aq, it is proposed that intramolecular electron transfer occurs for both (Fe²⁺N₃˙)_aq and its deprotonated form, [(Fe²⁺N₃˙)(OH^–)]_aq, with first-order rate constants of (1.2 ± 0.3)× 10⁴ and (5.4 ± 0.4)× 10⁴ s^–1, respectively. At relatively high azide concentrations ( > 10^–3 mol dm^–3), the observable product of these reaction is the iron(III) species, [Fe³⁺(OH^–)(N₃^–)]_aq, whereas at lower azide concentrations, the hydroxo product, Fe³⁺(OH^–)_aq, is observed.