Detection and characterization of an oxomanganese(V) porphyrin complex by rapid-mixing stopped-flow spectrophotometry

The first detection and characterization of oxomanganese(V) porphyrin complexes under ambient catalytic conditions is described. The reaction of (tetra-(N-methylpyridyl)porphyrinato)manganese(III) [Mn(III)TMPyP] with a variety of oxidants such as m-chloroperoxybenzoic acid (m-CPBA), HSO ₅ ^-, and ClO- has been shown to produce the same, short-lived intermediate (1) by stopped-flow spectrophotometry. The Soret maximum of 1 was found at 443 nm, intermediate between that of oxomanganese(IV) (428 nm) and Mn(III)TMPyP (462 nm), thus facilitating its detection. The rate of formation of 1 from Mn(III)TMPyP followed second-order kinetics, first order in Mn(III) porphyrin and first order in oxidant. The rate constants have the following order: m- CPBA (2.7 x 10 ⁷ M ¹ S ¹)> HSO ₅ ^- 6.9 x 10 ⁵ M ¹ s ¹ ≃ CIO (6.3 x 10 ⁵ M ¹ s ^-1). Once formed, the intermediate species 1 was rapidly converted to oxoMn(IV) (2) by one-electron reduction with a first-order rate constant of 5.7 s ^-1. The oxoMn(IV) species 2 was relatively stable under the reaction conditions, decaying slowly to Mn(III)TMPyP with a first-order rate constant of 0.027 s ^-1. The identity of 1 as an oxomanganese(V) complex was indicated by its reactivity. The one-electron reduction of I to oxoMn(IV) was greatly accelerated by nitrite ion (k = 1.5 x 10 ⁷ M ^-1 s ^-1). However, the reaction between nitrite and oxoMn(IV) is much slower (k = 1.4 x 10 ² M ^-1 s ^-1). The oxoMn(V) intermediate 1 was shown to be highly reactive toward olefins, affording epoxide products. By contrast, oxoMn(IV) (2) was not capable of effecting the same reaction under these conditions. In the presence of carbamazepine (3) efficient oxygen transfer from the highly reactive oxoMn(V) (1) to the olefin (second-order rate constant of 6.5 x 10 ⁵ M ^-1 s ^-1) resulted in the conversion of I directly back to Mn(III)TMPyP without the appearance of the stable oxoMn(IV) intermediate 2. With m-CPBA as the oxidant in the presence of H ₂ ¹⁸O, the product epoxide was shown to contain 35% ¹⁸O, consistent with an O-exchange-labile oxoMn(V) intermediate. Nitrite ion inhibited the epoxidation reaction competitively by one electron reduction of the oxoMn(V) intermediate to the unreactive oxoMn(IV).