Metal–Oxyl Species and Their Possible Roles in Chemical Oxidations

Abstract

Metal−oxyl (M n+ -O • ) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal−oxo (M (n+1)+ O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of M n+ -O • species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of M n+ -O • complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the M n+ -O • species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.

High‐Valent Metal–Oxo Species at the Nodes of Metal–Triazolate Frameworks: The Effects of Ligand Exchange and Two‐State Reactivity for C−H Bond Activation

“…Up to this point, we have simply referred to the [MO] 2+ sites as “metal–oxo” species in line with formal oxidation state counting rules. However, the true electronic structure of these [MO] 2+ sites is expected to exist on a spectrum with a varying degree of oxo and oxyl character (Figure a) . Several complementary methods can be used to assign a “best” resonance structure.…”

“…High-valent transition metal-oxo species have been invoked as reactive intermediates for aw ide range of oxidation reactions with both enzymatic and synthetic catalysts. [1][2][3] Many terminal oxo complexes are powerful oxidants and serve as promising active site motifs for the activation of strong CÀHbonds,such as those of light alkanes, at moderate reaction conditions. [1][2][3][4][5] However,t erminal oxo complexes are often difficult to isolate and probe experimentally as ad irect consequence of their inherent reactivity, especially those containing mid-to-late transition metals.…”

“…[1][2][3] Many terminal oxo complexes are powerful oxidants and serve as promising active site motifs for the activation of strong CÀHbonds,such as those of light alkanes, at moderate reaction conditions. [1][2][3][4][5] However,t erminal oxo complexes are often difficult to isolate and probe experimentally as ad irect consequence of their inherent reactivity, especially those containing mid-to-late transition metals. [6][7][8][9] In the synthesis of molecular metal-oxo complexes,sterically bulky ligands are typically employed to protect highly reactive metal-oxo units,a si tc an be difficult to prevent them from dimerizing or deactivating in solution.…”

Closed Shell Iron(IV) Oxo Complex with an Fe–O Triple Bond: Computational Design, Synthesis, and Reactivity

“…[10] In particular,H AT reactions in non-heme iron(IV)-oxo complexes have been generally predicted to proceed via high spin transition states, [11][12][13] irrespective of the spin state of the initial iron-oxo complexes,aphenomenon known as two-state reactivity. [14,15] TheHAT reactions are frequently of aradical character, [16] but closed-shell oxidants can initiate them as well, [17,18] for example via proton-coupled electron transfer. [19] In this respect, reactivity of so far unknown closed shell ironoxo complexes can elucidate the role of unpaired electrons in Fe-O mediated oxidations.…”

Photoinduced Generation of Superoxidants for the Oxidation of Substrates with High C−H Bond Dissociation Energies

“…However, neither the direct conversion of methane with O 2 to generate methanol nor the direct conversion of benzene with O 2 to produce phenol under mild conditions has been well established. Metal‐oxo species and other strong oxidants are often involved as reactive intermediates responsible for the catalytic oxidation of substrates including methane and benzene …”