Selective Formation of an Fe^IVO or an Fe^IIIOOH Intermediate From Iron(II) and H₂O₂: Controlled Heterolytic versus Homolytic Oxygen–Oxygen Bond Cleavage by the Second Coordination Sphere

Abstract

We demonstrate that the devised incorporation of an alkylamine group into the second coordination sphere of an FeII complex allows to switch its reactivity with H2O2 from the usual formation of FeIII species towards the selective generation of an FeIV‐oxo intermediate. The FeIV‐oxo species was characterized by UV/Vis absorption and Mössbauer spectroscopy. Variable‐temperature kinetic analyses point towards a mechanism in which the heterolytic cleavage of the O−O bond is triggered by a proton transfer from the proximal to the distal oxygen atom in the FeII‐H2O2 complex with the assistance of the pendant amine. DFT studies reveal that this heterolytic cleavage is actually initiated by an homolytic O−O cleavage immediately followed by a proton‐coupled electron transfer (PCET) that leads to the formation of the FeIV‐oxo and release of water through a concerted mechanism.

Catalytically Active Iron(IV)oxo Species Based on a Bis(pyridinyl)phenanthrolinylmethane

“…mCPBA to the very same complex solution at −20 °C, a set of two new quadrupole doublets appears with a distribution of 37 % and 63 %, respectively. The minor quadrupole doublet with an isomer shift of 0.04 mm s −1 close to zero and a corresponding quadrupole splitting of 0.54 mm s −1 is well in line with the formation of a high‐spin Fe(IV)oxo compound and comparable values were previously reported for tetradentate tpa Ph Fe(IV)O (δ=0.09 mm s −1 ; ΔE Q =0.51 mm s −1 ; S=2; tpa Ph : tris(5‐phenylpyrrole‐2‐ylmethyl)amine) [24] and other ferryl species [25,26] . In addition, a ferric species (δ=0.48 mm s −1 ; ΔE Q =1.91 mm s −1 ) is observed and a similar Mößbauer spectrum upon Fe(IV)oxo formation was previously reported by Nordlander and colleagues and assigned to an oxo‐bridged iron dimer impurity [27] .…”

High‐spin square planar iron(II) alkali metal siloxide complexes – influence of the alkali metal and reactivity towards O2 and NO

“…One of them, which accounts for almost 50 % of the sample, exhibits an isomeric shift of δ = 0.42 mm/s and a quadrupole splitting of ΔE Q = 0.53 mm/s and we assign it to an iron(III) product. It has not escaped our attention that these parameters strongly resemble those that were assigned to the iron(IV) oxo species formed upon treatment of the α-Fe sites in Fe-BEA (iron modified beta-zeolite) after treatment with N 2 O (δ = 0.41 mm/s, ΔE Q = 0.55 mm/s), [18] however, it is unlikely that a corresponding complex would be stable at room temperature, as in our case, and in addition a band between λ = 550-750 nm in the UV-vis spectrum which is characteristic for iron(IV) oxo compounds [19] cannot be found in the spectra shown in Figure 4. The two other compounds have an isomeric shift of around δ = 0.4 mm/s, too.…”

Highly Selective and Catalytic Oxygenations of C−H and C=C Bonds by a Mononuclear Nonheme High‐Spin Iron(III)‐Alkylperoxo Species

“…In the Fe−O bond homolysis, a high‐spin Fe III −OOR would produce peroxyl radical, which often leads to uncontrolled oxidation of substrates in alkylperoxide‐dependent catalytic oxidations by iron complexes . Recent reports indicated that different strategies, such as use of protic/Lewis acid or use of secondary coordination interactions, could direct the heterolytic O−O cleavage of Fe−OOR (R=H, acyl) species . Direct formation of iron(IV)‐oxo species from iron(II) precursor complexes and hydrogen peroxide have been reported without generation of iron(III)‐hydroperoxo intermediates .…”

“…Recent reports indicated that different strategies, such as use of protic/Lewis acid or use of secondary coordination interactions, could direct the heterolytic O−O cleavage of Fe−OOR (R=H, acyl) species . Direct formation of iron(IV)‐oxo species from iron(II) precursor complexes and hydrogen peroxide have been reported without generation of iron(III)‐hydroperoxo intermediates . A putative Fe II −OOR(H), generated in situ upon one‐electron reduction of the iron(III) intermediate, has been proposed to undergo heterolytic O−O bond cleavage, resulting in the formation of the corresponding iron(IV)‐oxo species .…”