Formation and High Reactivity of the anti‐Dioxo Form of High‐Spin μ‐Oxodioxodiiron(IV) as the Active Species That Cleaves Strong C−H Bonds

Abstract

Recently, it was shown that μ‐oxo‐μ‐peroxodiiron(III) is converted to high‐spin μ‐oxodioxodiiron(IV) through O−O bond scission. Herein, the formation and high reactivity of the anti‐dioxo form of high‐spin μ‐oxodioxodiiron(IV) as the active oxidant are demonstrated on the basis of resonance Raman and electronic‐absorption spectral changes, detailed kinetic studies, DFT calculations, activation parameters, kinetic isotope effects (KIE), and catalytic oxidation of alkanes. Decay of μ‐oxodioxodiiron(IV) was greatly accelerated on addition of substrate. The reactivity order of substrates is toluene<ethylbenzene≈cumene<trans‐β‐methylstyrene. The rate constants increased proportionally to the substrate concentration at low substrate concentration. At high substrate concentration, however, the rate constants converge to the same value regardless of the kind of substrate. This is explained by a two‐step mechanism in which anti‐μ‐oxodioxodiiron(IV) is formed by syn‐to‐anti transformation of the syn‐dioxo form and reacts with substrates as the oxidant. The anti‐dioxo form is 620 times more reactive in the C−H bond cleavage of ethylbenzene than the most reactive diiron system reported so far. The KIE for the reaction with toluene/[D8]toluene is 95 at −30 °C, which the largest in diiron systems reported so far. The present diiron complex efficiently catalyzes the oxidation of various alkanes with H2O2.

Dioxygen Activation by Nonheme Diiron Enzymes: Diverse Dioxygen Adducts, High-Valent Intermediates, and Related Model Complexes

“…220,232 At lower temperatures, however, it is possible to see the formation of visible chromophores suggestive of the formation of ( μ -1,2-peroxo)diferric species, as discussed in section 4.1. 192,219,220…”

Sc3+-Promoted O–O Bond Cleavage of a (μ-1,2-Peroxo)diiron(III) Species Formed from an Iron(II) Precursor and O2 to Generate a Complex with an FeIV2(μ-O)2 Core

“…A fourth example (along the blue path in Scheme 6) describes the reaction of [Fe III 2 (μ-O)(6-HPA)(OH 2 ) 2 ] 4+ (6-HPA = 1,2-bis{2-[bis(2-pyridylmethyl)aminomethyl]pyridin-6-yl}-ethane) with H 2 O 2 and 2 equiv of Et 3 N to generate a (μ-oxo)(μ-1,2-peroxo)diiron(III) derivative that is in equilibrium with a diiron(IV) intermediate with an OFe IV −O−Fe IV O core. 53,54 In contrast, the formation of high-valent 3 from 2 (along the red path in Scheme 6) occurs by introducing Sc 3+ (and not a proton), representing the first instance of a Lewis acidmediated O−O bond cleavage in a diiron system. In fact, adding a Brønsted acid like HClO 4 or HOTf to 2 results in the protonation of its oxo bridge to form 2 + H + .…”

Iron‐Catalyzed Water Oxidation: O–O Bond Formation via Intramolecular Oxo–Oxo Interaction

“…This process is a reverse process of oxygen activation reported by both Que [15h] and Kodera [15f] . In O 2 activation, Kodera suggested that a syn ‐dioxo form of catalyst 2 was transformed into anti ‐dioxo form through a t ‐oxo‐di(μ‐oxo)diiron(IV) transition state [15f,j] …”