Mechanistic studies on oxidation of hydrazine by a μ-oxo diiron(iii,iii) complex in aqueous acidic media—proton coupled electron transfer

Abstract

[Fe₂(μ-O)(phen)₄(H₂O)₂]⁴⁺ (1), one of the simplest μ-oxo diiron(III) complexes, quantitatively oxidises hydrazine to dinitrogen and itself is reduced to two moles of ferroin, [Fe(phen)₃]²⁺ in presence of excess phenanthroline. The weak dibasic acid, 1 (pK_a1 = 3.71 ± 0.05 and pK_a2 = 5.28 ± 0.10 at 25.0 °C, I = 1.0 mol dm⁻³ (NaNO₃)) and its conjugate bases, [Fe₂(μ-O)(phen)₄(H₂O)(OH)]³⁺ (2) and [Fe₂(μ-O)(phen)₄(OH)₂]²⁺ (3) are involved in the redox process with the reactivity order 1 > 2 > 3 whereas N₂H₄ and not N₂H₅⁺ was found to be reactive in the pH interval studied 3.45–5.60. Cyclic voltammetric studies indicate poor oxidizing capacity of the title substitution-labile diiron complex, yet it oxidizes N₂H₄ with a moderate rate—a proton coupled electron transfer (1e, 1H⁺) drags the energetically unfavourable reaction to completion. The rate retardation in D₂O media is substantially higher at higher pH due to the increasing basicity of the oxo-ligand in the order 3 > 2 > 1. Marcus calculations result an unacceptably high one-electron self-exchange rate for the iron center indicating an inner-sphere nature of the electron-transfer.