Kinetics of Redox Reactions between Tetrakis (2, 2ˈ- bipyridine)-µ- oxodiiron (III) Complex and Two Derivatives of Thiourea in Aqueous Acidic Medium

Abstract

The kinetics and mechanism of the redox reactions of tetrakis(2,2′-bipyridine)-μ-oxodiiron(III) chloride, [Fe2(bpy)4O]Cl4 (Fe2O4+), with N-methylthiourea (MTU) and N-allylthiourea (ATU) have been investigated spectrophotometrically in aqueous acidic medium. Stoichiometric studies revealed a 1:1 oxidant-to-reductant ratio for both systems, corresponding to the oxidation of the thiourea derivatives to their respective urea analogues with concomitant reduction of the iron(III) complex to Fe2+. The reactions were first order with respect to both Fe2O4+ and the reductants, giving an overall second-order rate law. The second-order rate constants at [H+] = 1.0 × 10−3 mol dm−3, I = 0.30 mol dm−3 (NaCl), T = 27.0 ± 1.0 °C and λmax = 520 nm were (27.99 ± 0.03) × 10−2 dm3 mol−1 s−1 and (6.25 ± 0.02) × 10−2 dm3 mol−1 s−1 for the MTU and ATU systems, respectively, indicating that MTU is approximately 4.5 times more reactive than ATU. The reaction rates were independent of hydrogen ion concentration within the range (4.0 × 10−4–1.4 × 10−3 mol dm−3) and ionic strength within the range 0.10–0.60 mol dm−3. Addition of sulfate and acetate ions significantly retarded the reaction rates, whereas Mg2+ and K+ ions exhibited negligible effects. Acrylamide polymerization tests provided no evidence for free-radical intermediates, while spectroscopic monitoring and Michaelis–Menten analyses showed no detectable intermediate complex formation. The combined kinetic, spectroscopic, and product studies support an outer-sphere electron-transfer mechanism for the oxidation of MTU and ATU by Fe2O4+ in aqueous acidic medi