Kinetics and Mechanisms of Hydrogen Peroxide Disproportionation Catalyzed by Mononuclear Schiff Base Manganese(III) Complexes: Effects of Chelate and Hydroxide Ion on Hydrogen Peroxide Disproportionation in N,N-Dimethylformamide

Article abstract of DOI:10.1246/bcsj.71.609

The preparation of [Mn(saltnOCOPh)Cl]·DMF (H₂saltnOCOPh: N,N'-(2-benzoyloxypropane-1,3-diyl)bis(salicylideneamine)) and kinetics and mechanisms of H₂O₂ disproportionation catalyzed by mononucleur Schiff base manganese(III) complexes, such as [Mn(salen)Cl) (H₂salen: N,N'-ethylenebis(salicylideneamine)), [Mn(saltn)Cl] (H₂saltn: N,N'-propane- 1,3-diylbis(salicylideneamine)), [Mn(saltnOH)Cl] (H₂saltnOH: N,N'-(2-hydroxypropane-1,3-diyl)bis(salicylideneamine)), and [Mn(saltnOCOPh)Cl] in N,N'-dimethylformamide (DMF), have been investigated. The disproportionation of H₂O₂ to O₂ and H₂O proceeds coupled with the redox cycle between the Mn(III) complex and the Mn(IV) intermediate: The first step is the fast equilibrium (K_m) of the Mn(III) complex and the Mn(IV) intermediate formed by the reaction of the Mn(III) complex with H₂O₂, followed by a slow reaction (k₁) of the Mn(IV) intermediate with H₂O₂ to produce O₂ and H₂O recovering the original Mn(III) complex. The K_m values decrease in the following order: [Mn(salen)Cl] (728 mol^-1 dm³) ? [Mn(saltnOH)CI] (28.0 mol^-1 dm³) > [Mn(saltn)Cl] (6.28 mol^-1 dm³) > [Mn(saltnOCOPh)Cl] (1.83 mol^-1 dm³), reflecting an increased distortion along the axis containing the coordination of H₂O₂ to the Mn(III) complex. On the other hand, the rate constants (k₁) fall into the following sequences: [Mn(saltnOH)Cl] (4.29 × 10⁵ mol^-2dm⁶s^-1) > [Mn(salen)Cl] (1.67 × 10⁵ mol^-2 dm⁶ s^-1) > [Mn(saltnOCOPh)Cl] (3.34 × 10⁴ mol^-2 dm⁶ s^-1) > [Mn(salen)Cl] (6.15 × 10³ mol^-2 dm⁶ s^-1). In spite of the small K_m values for saltnOH, saltnOCOPh, and saltn complexes with the 1,3-diamine ligand, compared to that for the salen complex, the large k_l value for the saltnOH complex strongly suggests stabilization of the transition state for the formation of hydrogen-bondings among the Mn(IV) intermediates and H₂O₂. Futhermore, the effect of the OH^- ion on the H₂O₂ disproportionation catalyzed by [Mn(salen)Cl] has been reported. On account of the formation of [Mn(salen)OH] coordinated by the OH^- ion, the appearance of the reaction path involving not only the Mn(III)-Mn(IV) cycle, but also the Mn(II)-Mn(III) cycle, is shown based on the ESR and visible spectral studies. The activity for the Mn(II)-Mn(III) cycle is 20 times larger than that for the Mn(III)-Mn(IV) cycle.