Synthetic High‐Valent M–O–X Oxidants

Abstract

High‐valent metal–oxygen species (M–O–X species, as opposed to terminal metal‐oxo, M=O, species) have been found to constitute a large family of capable oxidants. Herein, synthetic high‐valent complexes with hydroxide, alkoxide and other anionic O‐atom ligands, as well as O–Lewis acid adducts, are reviewed with a focus on their reactivity. Mn, Fe and Ru complexes are described, as well as recent additions to the field containing late transition metals (Co, Ni, Cu). These species are competent in a range of oxidations, and in particular they are capable of hydrogen atom transfer from strong C–H bonds. We explore how their reaction mechanisms and oxidising potency are modulated by the O‐atom ligand.

Preparation and Characterisation of a Bis‐μ‐Hydroxo‐NiIII2 Complex

“…In these complexes, and often for terminal metal–oxo ones (M=O), tunnelling has been attributed to the hydrogen‐atom‐transfer mechanism resulting in exceptionally high KIE values. In contrast, for M‐O‐X oxidants, KIE values are almost always less than the classical limit of 7, ruling out a tunnelling mechanism . This observation points towards complexes 2 and 3 having hydroxide and not oxo‐ligands.…”

“…In contrast, for M-O-X oxidants, KIE values are almost always less than the classical limit of 7, ruling out at unnelling mechanism. [36] This observation points towards complexes 2 and 3 havingh ydroxide and not oxo-ligands.…”

Controlling the Reactivity of a Metal-Hydroxo Adduct with a Hydrogen Bond

“…Metal–oxygen adducts (i.e., metal-oxo, -peroxo, and -hydroxo species) feature prominently in the proposed mechanisms of a variety of metalloenzymes and small-molecule, synthetic catalysts. − In many cases, these metal–oxygen species are involved in critical substrate oxidation steps in the catalytic cycle. While it is now well established that high-valent metal-oxo species can be involved in such reactions, there are increasing examples of mid- and high-valent metal-hydroxo species that can effect substrate oxidation reactions. − Two metalloenzymes that rely on midvalent metal­(III)-hydroxo adducts to perform their function are manganese superoxide dismutase (MnSOD) and manganese lipoxygenase (MnLOX). MnSOD regulates the levels of reactive oxygen species in the cell by catalyzing the disproportionation of superoxide to hydrogen peroxide and dioxygen. − The MnLOX enzyme catalyzes the oxidation of polyunsaturated fatty acids into their hydroperoxides, which are further metabolized into biologically active oxylipins such as a leukotrienes and jasmonates.…”

A Mononuclear Non-heme Manganese(III)–Aqua Complex as a New Active Oxidant in Hydrogen Atom Transfer Reactions

“…Lipoxygenases, which are widely distributed in plants, animals, and fungi, employ iron­(III)–hydroxo or manganese­(III)–hydroxo complexes as reactive intermediates to activate allylic C–H bonds via hydrogen atom transfer (HAT), which is believed to proceed via proton-coupled electron transfer (PCET). − These enzymes catalyze the regio- and stereo­specific dioxygenation of poly­unsaturated fatty acids by abstracting a H-atom with the Fe III –OH or Mn III –OH intermediates and generating hydro­peroxides for further reactions. − For example, the use of Fe III –OH as a HAT agent to cleave a substrate C–H bond produces [Fe II –OH 2 R • ] by concerted PCET, in which proton is transferred to the OH ligand to form a H 2 O ligand along with the electron transfer (ET) to the Fe III metal center to produce Fe II –OH 2 . − This strategy can avoid the formation of high-valent metal–oxo intermediates in activating substrate C–H bonds. There have been a number of studies on the HAT reactions using synthetic Fe III –OH and Mn III –OH complexes for HAT as well as PCET. − Synthetic Fe III –OCH 3 , Mn III –OCH 3 , and Mn IV –OH complexes have also been used as lipoxygenase models in activating substrate C–H bonds. − …”

“…In contrast to the extensive studies performed with the Fe III –OH and Mn III –OH complexes, − HAT and/or PCET reactions by metal­(III)–aqua complexes, M III –OH 2 , have never been explored previously. Herein, we report for the first time a high reactivity of a mono­nuclear non-heme Mn­(III)–aqua complex, [(dpaq)­Mn III ­(OH 2 )] 2+ ( 1 ; see Figure for the structure, formation, and reactivity of 1 ), in HAT and PCET reactions; the reactivity of 1 is much greater than that of the corresponding Mn­(III)–hydroxo complex, [(dpaq)­Mn III ­(OH)] + ( 2 ).…”