Electron-transfer kinetics and mechanism of di-imine bond formation in tetracyano(ethylenediamine)ferrate(II)

Abstract

The oxidative dehydrogenation of the ethylenediamine (en) ligand in the complex [Fe(CN)₄(en)]^2– has been investigated by cyclic voltammetry, spectroelectrochemistry, and by stopped-flow kinetics with the [Fe(CN)₆]^3– ion, at 25 °C, and I= 0.100 mol dm^–3(KCl). The first step in the oxidation process is assigned to a reversible electron-transfer reaction in the [Fe(CN)₄(en)]^2– complex, with E_½= 0.303 V versus a normal hydrogen electrode. The oxidized ion undergoes successive electron transfer in the presence of OH^– ions (pH > 11), forming the red Fe^II–di-imine complex. The observed rate constant is given by k_obs.=(1.9 × 10⁴)[Fe(CN)₆^3–][OH^–]. The di-imine complex is further oxidized in the presence of [Fe(CN)₆]^3– ions, with k_obs.=(4.7 × 10³)[Fe(CN)₆^3–][OH^–]. A mechanism is proposed involving Fe^III intermediate species which undergo induced electron transfer assisted by OH^– ions.