Characterization of the Fe^V=O Complex in the Pathway of Water Oxidation

Abstract

Hypervalent FeV=O species are implicated in a multitude of oxidative reactions of organic substrates, as well as in catalytic water oxidation, a reaction crucial for artificial photosynthesis. Spectroscopically characterized FeV species are exceedingly rare and, so far, were produced by the oxidation of Fe complexes with peroxy acids or H2O2: reactions that entail breaking of the O−O bond to form a FeV=O fragment. The key FeV=O species proposed to initiate the O−O bond formation in water oxidation reactions remained undetected, presumably due to their high reactivity. Here, we achieved freeze quench trapping of six coordinated [FeV=O,(OH)(Pytacn)]2+ (Pytacn=1‐(2′‐pyridylmethyl)‐4,7‐dimethyl‐1,4,7‐triazacyclononane) (2) generated during catalytic water oxidation. X‐ray absorption spectroscopy (XAS) confirmed the FeV oxidation state and the presence of a FeV=O bond at ≈1.60 Å. Combined EPR and DFT methods indicate that 2 contains a S=3/2 FeV center. 2 is the first spectroscopically characterized high spin oxo‐FeV complex and constitutes a paradigmatic example of the FeV=O(OH) species proposed to be responsible for catalytic water oxidation reactions.

Low‐Spin and High‐Spin Perferryl Intermediates in Non‐Heme Iron Catalyzed Oxidations of Aliphatic C−H Groups

“…In 2020, Pushkar with co‐workers reported freeze quench trapping of the oxoiron(V) intermediate [(PyTACN)Fe V =O(OH)](OTf) 2 (Figure 3). [8b] X‐ray absorption spectroscopy (XAS) confirmed the Fe V oxidation state and the presence of a Fe V =O bond at ∼1.60 Å. EPR spectrum of this intermediate displays broad features at g ∼ 3.00, 1.97 and 1.44, corresponding to the high‐spin S =3/2 Fe V center. This spectroscopic evidence for the formation of the high‐spin oxoiron(V) intermediate is in line with our assignment of 4a to the high‐spin perferryl species.…”

Formation of CoIV=O intermediate at the Boundary of the “Oxo-wall” Induces Water Oxidation

“…This pathway is similar to WOR catalyzed by group 8 transition metals, forming intermediates in high oxidation states. 28,50 Formation of Co V has not been supported experimentally or with DFT. Thus, O-O bond formation likely proceeds through the…”

Systematic Influence of Electronic Modification of Ligands on the Catalytic Rate of Water Oxidation by a Single‐Site Ru‐Based Catalyst

“…Water oxidation typically involves multiple proton‐coupled electron transfer (PCET) steps producing intermediates with various valence states. The O−O bond formation is triggered by high valent metal‐oxo species, often postulated as the crucial intermediates [21,37,38] . Over decades, both dinuclear and mononuclear Ru II polypyridine catalysts have attracted great attention from researchers.…”

“…[5][6][7][8][9] This functional aspect of the OEC may provide a possible path for the design of efficient catalysts for artificial photosynthesis. In the last decade, various water oxidation catalysts (WOCs) have been developed based on ruthenium, [10][11][12][13][14] manganese, [15][16][17][18][19] iron, [20][21][22][23] and iridium. [24][25][26][27] Integrated systems with chromophore-catalyst assemblies [28][29][30][31] and catalysts incorporated into metal-organic frameworks (MOF)s were also reported.…”

“…The OÀ O bond formation is triggered by high valent metal-oxo species, often postulated as the crucial intermediates. [21,37,38] Over decades, both dinuclear and mononuclear Ru II polypyridine catalysts have attracted great attention from researchers. During this time, many different series of Ru-based WOCs were synthesized and analyzed.…”