A “Push−Pull” Mechanism for Heterolytic O−O Bond Cleavage in Hydroperoxo Manganese Porphyrins

A water-soluble manganese porphyrin, 5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrinatomanganese(III) (MnIIITDMImP) is shown to react with H2O2 to generate a relatively stable dioxomanganese(V) porphyrin complex (a compound I analog). Stopped-flow kinetic studies revealed Michaelis Menton-type saturation kinetics for H2O2. The visible spectrum of a compound 0 type intermediate, assigned as MnIII(OH)(OOH)TDMImP, can be directly observed under saturating H2O2 conditions (Soret band at 428 nm and Q bands at 545 and 578 nm). The rate-determining O−O heterolysis step was found to have a very small activation enthalpy (ΔH≠ = 4.2 ± 0.2 kcal mol−1) and a large, negative activation entropy (ΔS≠ = −36 ± 1 cal mol−1 K−1). The O−O bond cleavage reaction was pH independent at 8.8 < pH < 10.4 with a first-order rate constant of 66 ± 12 s−1. These observations indicate that the O−O bond in MnIII(OH)(OOH)TDMImP is cleaved via a concerted “push−pull” mechanism. In the transition state, the axial (proximal) −OH...