Abstract

Abstract 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 mononuclear 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 (Km) 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 Km values decrease in the following order: [Mn(salen)Cl] (728 mol−1 dm3) >> [Mn(saltnOH)Cl] (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−2 dm6 s−1) > [Mn(salen)Cl] (1.67 × 105 mol−2 dm6 s−1) > [Mn(saltnOCOPh)Cl] (3.34 × 104mol−2 dm6 s−1) > [Mn(salen)Cl] (6.15 × 103 mol−2 dm6 s−1). In spite of the small Km values for saltnOH, saltnOCOPh, and saltn complexes with the 1,3-diamine ligand, compared to that for the salen complex, the large k1 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.

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