Kinetics and mechanism of hydroxylamine oxidation by [Fe2III(µ-O)(phen)4(H2O)2]4+ in aqueous media

Abstract

Equilibrium studies show that in aqueous solutions containing excess 1,10-phenanthroline (phen) in the range pH 3-9, the complex ion [Fe2III ( μ-O)(phen)4(H2O)2]4+ (1) undergoes rapid but partial hydrolysis and coexists with [Fe2III ( μ-)(phen)3(H2O)4]4+ (1d),[Fe2 III ( μ-O)(phen)4(H2O)(OH)]3+ (2), and [Fe2III ( μ-O)(phen)4(OH)2]2+ (3). The solution oxidizes hydroxylamine quantitatively to N2O and is itself reduced to [Fe(phen)3]2+. The reactions in the range pH 3-6 are first-order in concentrations of complex and hydroxylamine but exhibits complex [H+] dependence, suggesting kinetic contributions from 1, 1d, and 2 but not from 3. Rapid formation of inner-sphere adducts between NH2OH and different {Fe2O}4+ species followed by rate-determining one-electron transfer to produce NHOH and {Fe2O}3+ is proposed. All subsequent steps are rapid. Ambient light does not affect kinetics and reaction products.Key words: kinetics, equilibrium, oxo bridge, iron (III), hydroxylamine.