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

The preparation of [Mn(saltnOCOPh)Cl]. DMF (H(2)saltnOCOPh: N,N'-(2-benzoyloxypropane-1,3-diyl)bis(salicylideneamine)) and kinetics and mechanisms of H2O2 disproportionation catalyzed by mononuclear Schiff base manganese(III) complexes, such as [Mn(saltn)Cl] (H(2)salen: N,N'-ethylenebis(salicylideneamine)), [Mn(saltn)Cl] (H(2)saltn: N,N'-propane-1,3 -diylbis(salicylideneamine)), [Mn(saltnOH)Cl] (H(2)saltnOH: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 O-2 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 (k(1)) of the Mn(IV) intermediate with H2O2 to produce O-2 and H2O recovering the original Mn(III) complex. The K-m values decrease in the following order. [Mn(salen)-Cl] (728 mol(-1) dm(3))much greater than[Mn(saltnOH)Cl] (28.0 mol(-1) dm(3)) > [Mn(saltn)Cl] (6.28 mol(-1) dm(3))> [Mn(saltnOCOPh)Cl] (1.83 mol(-1) dm(3)), reflecting an increased distortion along the axis containing the coordination of H2O2 to the Mn(III) complex. On the other hand, the rate constants (k(1)) fall into the following sequences: [Mn(saltnOH)Cl] (4.29x10(5) mol(-2)dm(6) s(-1)) > [Mn(salen)Cl] (1.67 x 10(5) mol(-2) dm(6) s(-1)) > [Mn(saltnOCOPh)Cl] (3.34x10(4) mol(-2) dm(6) s(-1))> [Mn-(salen)Cl] (6.15x10(3) mol(-2) dm(6) 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(1) 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(m)-Mn(IV) cycle, but also the Mn(II)-Mn(m) cycle, is shown based on the ESR and visible spectral studies. The activity for the Mn(II)-Mn(m) cycle is 20 times larger than that for the Mn(m)-Mn(IV) cycle.