O−O Bond Formation and Liberation of Dioxygen Mediated by N₅‐Coordinate Non‐Heme Iron(IV) Complexes

Abstract

Formation of the O−O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O−O bond in solution, from non‐heme, N5‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O−O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [FeIV(O)] and [FeV(O)] being the resting and active principles of the O−O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O−O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

“…Horner et al deduced the detailed O−O bond formation pathway of a non-heme pentadentate iron complex 3 (Figure 61) via UV−vis−NIR spectroelectrochemistry and isotope labeling experiments. 770 Complex 3 catalyzes water oxidation in the presence of a peroxyacid, namely, mCPBA, which on the other hand acts as an oxygen atom donor, as evidenced from 18 O isotope labeling experiments. In situ UV−vis−NIR studies displayed the evolution of a spectral signature at 752 nm followed by its disappearance to evolve a peak at 320 nm.…”

Non-heme oxoiron complexes as active intermediates in the water oxidation process with hydrogen/oxygen atom transfer reactions

“…This study explores the water oxidation process by the iron-N 5 ligated non-heme complex, 29 which was reported as the catalyst for oxygen evolution in an aqueous solution of m -chloroperoxybenzoic acid (mCPBA). Under the experimental conditions, the oxo iron( iv ) was formed from the [Fe II (L)(MeCN)] 2+ complex in the presence of excess mCPBA.…”

“…To overcome this, researchers have focused on developing WOCs based on the first-row transition metals 14,15 such as manganese, 16–18 cobalt, 19–21 copper, 22–25 and nickel. 26–28 In particular, iron 29–33 is readily available and less toxic.…”

Mechanistic Insight into the O2 Evolution Catalyzed by Copper Complexes with Tetra- and Pentadentate Ligands

“…As reported earlier, few pentadentate complexes did not act as water oxidation catalysts , due to the inability to stabilize the higher oxidation state. But very few pentadentate complexes with first-row metals − were reported as WOCs. The studied copper pentadentate complex also acts as a suitable catalyst for the oxygen elevation process.…”