Redox Reaction between an Oxo-Bridged Diiron Complex and Hydrazine: A Kinetic Study

Objective: Mechanistic investigation of the reaction between the complex ion [Fe^III2(μ-O)(phen)4(H2O)2]⁴⁺ (1) (phen = 1,10-phenanthroline) and its hydrolytic derivatives [Fe^III2(μ-O)(phen)4(H2O)(OH)]³⁺ (1a) and [Fe^III2(μ-O)(phen)4(OH)2]²⁺ (1b) which coexist in rapid equilibria in the range pH = 3.00 to pH = 5.00 (pKa1 = 3.71 ± 0.03, pKa2 = 5.28 ± 0.07) with N2H5⁺ ion to produce [Fe(phen)3]²⁺ has been covered in this study. Methods: Rise in absorbance of the product with time was measured and rates have been determined using initial rate methods. Dependence of rate on pH and concentration of N2H5⁺ are studied in detail. Results: In the presence of excess phenanthroline, the reaction follows simple first-order exponential profile. Interestingly, in the absence of any added phenanthroline, the reaction becomes faster and the reaction profile changes to nearly linear with curvature near the end of the reaction. Slight autocatalytic nature has been observed only for the slower reactions. The observed rate constants obtained using initial rate method show first-order dependence on the concentration of N2H5⁺. An increase in rate with increasing pH has been noticed. The plot of rate versus 1/[H⁺] shows saturation beginning near pH 4.6, suggesting that the mono-deprotonated form of the complex (i.e. 1a) is the active oxidant in this pH range. Conclusion: Presumably, the reaction becomes faster in the absence of added phenanthroline because ligand dissociation from the parent complex becomes facilitated when there is no external phenanthroline ligand present. This produces a coordinatively unsaturated Fe(III) species which is a faster oxidant.