RuIII(edta) Complex-Mediated Oxidation of Hydrazine to Nitrogen: Spectral, Electrochemical, and Kinetic Studies

The [Ru^III(edta)pz]⁻ complex (edta⁴⁻ = ethylenediaminetetraacetate; pz = pyrazine), despite being a substantially weak electron-accepting oxidant (E_1/2 = 0.24 V vs Normal Hydrogen Electrode for Ru^III/II couple), can effect the oxidation of hydrazinium ion (NH₂NH₃⁺) to nitrogen (N₂) selectively with concomitant formation of the [Ru^II(edta)pz]²⁻ product complex. The time course of the reaction is followed spectrophotometrically at the characteristic band of [Ru^II(edta)(pz)]²⁻ (λ_max = 462 nm) as a function of [NH₂NH₃⁺]_T, pH, and temperature. A mechanism involving hydrogen atom transfer from NH₂NH₃⁺ to the remote N-atom of the coordinated pyrazine in the [Ru^III(edta)pz]⁻ complex is proposed for the aforesaid oxidation reaction. Electrochemical and spectroelectrochemical studies reveal that Ru^III(edta)pz]⁻ can act as a “redox relay” for the electrochemical oxidation of NH₂NH₃⁺ to N₂ at 0.5 V (vs saturated calomel reference electrode) under argon atmosphere. Density functional theory (DFT) calculations are utilized to optimize the structures of the reactant and product complexes. Detailed mechanistic proposals in agreement with the spectral, electrochemical, kinetic, and DFT computation data are presented for the aforesaid oxidation reaction.