Hydrogen‐Atom Abstraction Reactions by Manganese(V)– and Manganese(IV)–Oxo Porphyrin Complexes in Aqueous Solution

Abstract

High-valent manganese(IV or V)-oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated Mn(V)- and Mn(IV)-oxo porphyrins in basic aqueous solution and investigated their reactivities in C-H bond activation of hydrocarbons. We now report that Mn(V)- and Mn(IV)-oxo porphyrins are capable of activating C-H bonds of alkylaromatics, with the reactivity order of Mn(V)-oxo>Mn(IV)-oxo; the reactivity of a Mn(V)-oxo complex is 150 times greater than that of a Mn(IV)-oxo complex in the oxidation of xanthene. The C-H bond activation of alkylaromatics by the Mn(V)- and Mn(IV)-oxo porphyrins is proposed to occur through a hydrogen-atom abstraction, based on the observations of a good linear correlation between the reaction rates and the C-H bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of Mn(IV)-oxo porphyrins to Mn(V)-oxo and Mn(III) porphyrins is not a feasible pathway in basic aqueous solution and that Mn(IV)-oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The C-H bond activation of alkylaromatics by Mn(V)- and Mn(IV)-oxo species proceeds through a one-electron process, in which a Mn(IV)-oxo porphyrin is formed as a product in the C-H bond activation by a Mn(V)-oxo porphyrin, followed by a further reaction of the Mn(IV)-oxo porphyrin with substrates that results in the formation of a Mn(III) porphyrin complex. This result is in contrast to the oxidation of sulfides by the Mn(V)-oxo porphyrin, in which the oxidation of thioanisole by the Mn(V)-oxo complex produces the starting Mn(III) porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the Mn(V)-oxo species occurs by means of a two-electron oxidation process. In contrast, a Mn(IV)-oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.

Modeling the cis‐Oxo‐Labile Binding Site Motif of Non‐Heme Iron Oxygenases: Water Exchange and Oxidation Reactivity of a Non‐Heme Iron(IV)‐Oxo Compound Bearing a Tripodal Tetradentate Ligand

“…[35] On this basis, we propose that the reaction takes place via a rate determining hydrogen atom abstraction (HAT) that can be regarded as a proton coupled electron transfer (PCET), followed by a fast electron-transfer. [68, 76, 83, 84]…”

Mn K-Edge X-ray Absorption Studies of Oxo- and Hydroxo-manganese(IV) Complexes: Experimental and Theoretical Insights into Pre-Edge Properties

“…9 With regard to synthetic systems, Mn K-edge XAS has played a critical role in the characterization of high-valent oxo- and hydroxo-manganese species 10−16 that model intermediates commonly proposed in biological and synthetic oxidation reactions. 13,15,17−22 Although several Mn V –oxo adducts have been structurally characterized by X-ray diffraction, 23−26 Mn K-edge XAS has been commonly employed to determine metal–ligand bond lengths for high-valent oxo- and hydroxo-manganese adducts that have thus far eluded structural characterization by X-ray diffraction. 10−22,27 A major advantage of the XAS technique is that crystalline samples are not required.…”

Accelerated Oxygen Atom Transfer and C−H Bond Oxygenation by Remote Redox Changes in Fe3Mn‐Iodosobenzene Adducts

“…[3] Nonetheless, studying these effects on C–H bond activation or oxygen atom transfer is challenging due to limitations in accessing metal complexes with properties suitable for meaningful comparison. [2d,4] Such studies are even more challenging in multinuclear systems, which represent more accurate models of enzymes’ active sites. [1f,5] Only recently, a study of bimetallic complexes demonstrated that changing the oxidation state from [HO-Fe IV -O-Fe IV =O] to [HO-Fe III -O-Fe IV =O] resulted in a change in spin-state, concomitant with a remarkable million-fold increase in the rate of C–H bond cleavage.…”