Mechanism for the Homolytic Cleavage of Alkyl Hydroperoxides by the Manganese(III) Dimer Mn^III₂(2-OHsalpn)₂

Abstract

The oxidation of MnIII 2(2-OHsalpn)2, 1 (2-OHsalpn = 1,3-bis(salicylideneamino)-2-propanol), with tert-butyl hydroperoxide was studied in organic media. A one-electron reaction occurs resulting in initial formation of the oxidized MnIIIMnIV(2-OHsalpn)2OH species 2 and a tert-butoxy radical. The MnIIIMnIV(2-OHsalpn)2OH complex can be readily protonated to yield MnIIIMnIV(2-OHsalpn)2 +. Upon addition of excess tert-butyl hydroperoxide, singlet dioxygen is evolved in an exothermic reaction indicative of radical-induced disproportionation of the hydroperoxide. In the presence of a radical scavenger, the oxygen evolution is strongly attenuated. This system was observed to oxygenate cyclohexene to yield 2-cyclohexen-1-one, 2-cyclohexen-1-ol, and traces of cyclohexene oxide. Cumene is oxygenated to 2-phenyl-2-propanol and acetophenone, and cyclohexane is oxygenated to cyclohexanone and cyclohexanol. However, 18O-labeling experiments show that the oxygen in the products results exclusively from reactions with aerobic dioxygen and not from the tert-butyl hydroperoxide oxidant. These results indicate that oxygenation occurs by radical-initiated aerobic autoxidation and not via oxo transfer from a high-valent manganese oxo species. 2 was studied by UV−vis, IR, NMR, EXAFS, XANES, and EPR spectroscopies, which support the assignment of a dinuclear manganese(III/IV) terminal hydroxo structure. 2 was also formed by the direct reaction of MnIIIMnIV(2-OHsalpn)2 + with OH-, further supporting the description of 2 as having a terminal hydroxide ligand. The kinetics of its formation from 1 and tert-butyl hydroperoxide were examined and found to be first-order in peroxide. Titration experiments confirm that 1 equiv of peroxide is required to convert 1 to 2, supporting the conclusion that the peroxide undergoes a one-electron reduction. This system demonstrates that oxy radicals formed from homolytic one-electron cleavage of a peroxide by a manganese(III) complex can give small amounts of olefin oxidation species (i.e. epoxides) normally considered evidence for high-valent manganese oxo-transfer chemistry. It therefore underscores the need to test any system for which high-valent metal oxo transfer is proposed to occur using rigorous oxygen-labeling experiments to determine the source of oxygen.

Reflections on small molecule manganese models that seek to mimic photosynthetic water oxidation chemistry

“…As an example, protonation or methylation of the oxo-bridge destroyed the catalase activity of these compounds. Possibly most important, these were the first manganese oxo bridged molecules for which homolytic bond dissociation energies were reported [104]. These data demonstrated that the energetics of water oxidation were precisely tuned to both the proton and the electron and that proton coupled electron transfer (PCET) or H-atom abstraction were likely to be involved in water oxidation chemistry [9,89,143].…”

“…They showed the first 2.7Å Mn-Mn separations [99], the distance that forms the structural hallmark of the catalytic center and served as the basis for testing the famous "Dimer of dimers" structural model for the enzyme [99][100][101]. In addition, studies of the reactivity to form and destroy the dimers, along with perturbations of the properties upon protonation of these oxo bridges has proved critical to the viability of more recent water oxidation proposals [102][103][104].…”

Synthesis, Structure and Characterisation of a New Trinuclear Di‐μ‐phenolato‐μ‐carboxylato MnIIIMnIIMnIII Complex with a Bulky Pentadentate Ligand: Chemical Access to Mononuclear MnIV–OH Entities

“…For clarity, only the terminal Mn III ions of complex 1 are considered. We propose that after the coordination of the tBuOOH oxidant to the Mn III centre, the homolytic breakage of the O-O bond [79] would lead to the release of the radical tBuO…”

X-ray and vibrational spectroscopy of manganese complexes relevant to the oxygen-evolving complex of photosynthesis

“…No compounds are available containing Mn and Ca, which are bridged by mono-µ-oxo or carboxylate bridges. Even though a great many Mn compounds, relevant to the OEC, have been published, only a few of these of compounds have been studied by XANES 35,[186][187][188][189][190][191][192][193][194][195][196] or Mn Kβ XES. 46,[197][198][199] In fact, some of the compounds involved are manganese salts, such as Mn II Cl 2 , or oxides, such as Mn 2 O 3 , which are less relevant to the OEC.…”

“…46,[197][198][199] In fact, some of the compounds involved are manganese salts, such as Mn II Cl 2 , or oxides, such as Mn 2 O 3 , which are less relevant to the OEC. 38,190,193,194,198 The focus of these studies is on either the influence of an oxidation-state change 191,192,196,197 or a structural change [186][187][188]195 …”