Axial Ligand and Spin‐State Influence on the Formation and Reactivity of Hydroperoxo–Iron(III) Porphyrin Complexes

The present study focuses on the formation and reactivity of hydroperoxo–iron(III) porphyrin complexes formed in the [Fe^III(tpfpp)X]/H₂O₂/HOO^? system (TPFPP=5,10,15,20‐tetrakis(pentafluorophenyl)‐21H,23H‐porphyrin; X=Cl^? or CF₃SO₃^?) in acetonitrile under basic conditions at ?15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high‐spin [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] could be observed with the application of a low‐temperature rapid‐scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O? O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo‐ to heterolytic O? O bond cleavage is observed for high‐ [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron‐rich porphyrin ligands, electron‐deficient [Fe^III(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe^III(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)–oxo porphyrin π‐cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.     

Mechanistic Insight into Peroxo‐Shunt Formation of Biomimetic Models for Compound II,Their Reactivity toward Organic Substrates,and the Influence of N‐Methylimidazole Axial Ligation

High‐valent iron‐oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)Fe^III(OH) porphyrin ([meso‐tetrakis(2,4,6‐trimethyl‐3‐sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H₂O₂ with (TMPS)Fe^III(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)Fe^IV?O(OH) point to the overall associative nature of the process. A pH‐dependence study on the formation of (TMPS)Fe^IV?O(OH) revealed a very high reactivity of OOH^? toward (TMPS)Fe^III(OH) in comparison to H₂O₂. The influence of N‐methylimidazole (N‐MeIm) ligation on both the formation of iron(IV)‐oxo species and their oxidising properties in the reactions with 4‐methoxybenzyl alcohol or 4‐methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)Fe^III(H₂O)(N‐MeIm) is highly reactive toward H₂O₂ to form the iron(IV)‐oxo species, (TMPS)Fe^IV?O(N‐MeIm). The latter species can also be formed in the reaction of (TMPS)Fe^III(N‐MeIm)₂ with H₂O₂ or in the direct reaction of (TMPS)Fe^IV?O(OH) with N‐MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)Fe^IV?O(OH) and (TMPS)Fe^IV?O(N‐MeIm) do not display a pronounced effect of the N‐MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH^? substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH^? or N‐MeIm) in the trans position to the oxo group in the iron(IV)‐oxo species does not significantly affect the activation barriers calculated for C?H dehydrogenation of the selected organic substrates.     

Selective oxidation of benzyl alcohol to benzaldehyde, 1‐phenylethanol to acetophenone and fluorene to fluorenol catalysed by iron (II) complexes supported by pincer‐type ligands: Studies on rapid degradation of organic dyes

Hexacoordinated non‐heme iron complexes [Fe^II(L¹)₂](ClO₄)₂ ( 1 ) and [Fe^II(L²)₂](PF₆)₂ ( 2 ) have been synthesized using ligands L¹ = (E)‐2‐chloro‐6‐(2‐(pyridin‐2ylmethylene) hydrazinyl)pyridine and L² = (E)‐2‐chloro‐6‐(2‐(1‐(pyridin‐2‐yl)ethylidene)hydrazinyl) pyridine]. These complexes are highly active non‐heme iron catalysts to catalyze the C (sp³)?H bonds of alkanes. These iron complexes have been characterized using ESI?MS analysis and molecular structures were determined by X‐ray crystallography. ESI ? MS analysis also helped to understand the generation of intermediate species like Fe^III?OOH and Fe^IV=O. DFT and TD?DFT calculations revealed that the oxidation reactions were performed through high‐valent iron center and a probable reaction mechanism was proposed. These complexes were also utilized for the degradation of orange II and methylene blue dyes.     

Lability‐Controlled Syntheses of Heterometallic Clusters

A bulky bidentate ligand was used to stabilize a macrocyclic [Fe^III₈Co^II₆] cluster. Tuning the basicity of the ligand by derivatization with one or two methoxy groups led to the isolation of a homologous [Fe^III₈Co^II₆] species and a [Fe^III₆Fe^II₂Co^III₂Co^II₂] complex, respectively. Lowering the reaction temperatures allowed isolation of [Fe^III₆Fe^II₂Co^III₂Co^II₂] clusters with all three ligands. Temperature‐dependent absorption data and corresponding experiments with iron/nickel systems indicated that the iron/cobalt self‐assembly process was directed by the occurrence of solution‐state electron‐transfer‐coupled spin transition (ETCST) and its influence on reaction intermediate lability.     

Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

The binding of a series of substituted phenols as axial ligands onto a diiron(III)? bisporphyrin framework have been investigated. Spectroscopic characterization revealed high‐spin states of the iron centers in all of the phenolate complexes, with one exception in the 2,4,6‐trinitrophenolate complex of diiron(III)? bisporphyrin, which only stabilized the pure intermediate‐spin (S=3/2) state of the iron centers. The average Fe? N (porphyrin) and Fe? O (phenol) distances that were observed with the 2,4,6‐trinitrophenolate complex were 1.972(3) Å and 2.000(2) Å, respectively, which are the shortest and longest distances reported so far for any Fe^III? porphyrin with phenoxide coordination. The alternating shift pattern, which shows opposite signs of the chemical shifts for the meta versus ortho/para protons, is attributed to negative and positive spin densities on the phenolate carbon atoms, respectively, and is indicative of π‐spin delocalization onto the bound phenolate. Electrochemical data reveals that the E_1/2 value for the Fe^III/Fe^II couple is positively shifted with increasing acidity of the phenol. However, a plot of the E_1/2 values for the Fe^III/Fe^II couple versus the pK_a values of the phenols shows a linear relationship for all of the complexes, except for the 2,4,6‐trinitrophenolate complex. The large deviation from linearity is probably due to the change of spin for the complex. Although 2,4,6‐trinitrophenol is the weakest axial ligand in the series, its similar binding with the corresponding Fe^III? monoporphyrin only results in stabilization of the high‐spin state. The porphyrin macrocycle in the 2,4,6‐trinitrophenolate complex of diiron(III)? bisporphyrin is the most distorted, whilst the “ruffling” deformation affects the energy levels of the iron d orbitals. The larger size and weaker binding of 2,4,6‐trinitrophenol, along with heme? heme interactions in the diiron(III)? bisporphyrin, are responsible for the larger ring deformations and eventual stabilization of the pure intermediate‐spin states of the iron centers in the complex.     

Aromatic Hydroxylation at a Non‐Heme Iron Center: Observed Intermediates and Insights into the Nature of the Active Species

Mechanism of substrate oxidations with hydrogen peroxide in the presence of a highly reactive, biomimetic, iron aminopyridine complex, [Fe^II(bpmen)(CH₃CN)₂][ClO₄]₂ ( 1 ; bpmen=N,N'‐dimethyl‐N,N'‐bis(2‐pyridylmethyl)ethane‐1,2‐diamine), is elucidated. Complex 1 has been shown to be an excellent catalyst for epoxidation and functional‐group‐directed aromatic hydroxylation using H₂O₂, although its mechanism of action remains largely unknown. 1 , 2 Efficient intermolecular hydroxylation of unfunctionalized benzene and substituted benzenes with H₂O₂ in the presence of 1 is found in the present work. Detailed mechanistic studies of the formation of iron(III)–phenolate products are reported. We have identified, generated in high yield, and experimentally characterized the key Fe^III(OOH) intermediate (λ_max=560 nm, rhombic EPR signal with g=2.21, 2.14, 1.96) formed by 1 and H₂O₂. Stopped‐flow kinetic studies showed that Fe^III(OOH) does not directly hydroxylate the aromatic rings, but undergoes rate‐limiting self‐decomposition producing transient reactive oxidant. The formation of the reactive species is facilitated by acid‐assisted cleavage of the O? O bond in the iron–hydroperoxide intermediate. Acid‐assisted benzene hydroxylation with 1 and a mechanistic probe, 2‐Methyl‐1‐phenyl‐2‐propyl hydroperoxide (MPPH), correlates with O? O bond heterolysis. Independently generated Fe^IV?O species, which may originate from O? O bond homolysis in Fe^III(OOH), proved to be inactive toward aromatic substrates. The reactive oxidant derived from 1 exchanges its oxygen atom with water and electrophilically attacks the aromatic ring (giving rise to an inverse H/D kinetic isotope effect of 0.8). These results have revealed a detailed experimental mechanistic picture of the oxidation reactions catalyzed by 1 , based on direct characterization of the intermediates and products, and kinetic analysis of the individual reaction steps. Our detailed understanding of the mechanism of this reaction revealed both similarities and differences between synthetic and enzymatic aromatic hydroxylation reactions.     

Iron‐Promoted ortho‐ and/or ipso‐Hydroxylation of Benzoic Acids with H2O2

Regioselective hydroxylation of aromatic acids with hydrogen peroxide proceeds readily in the presence of iron(II) complexes with tetradentate aminopyridine ligands [Fe^II(BPMEN)(CH₃CN)₂](ClO₄)₂ ( 1 ) and [Fe^II(TPA)(CH₃CN)₂](OTf)₂ ( 2 ), where BPMEN=N,N′‐dimethyl‐N,N′‐bis(2‐pyridylmethyl)‐1,2‐ethylenediamine, TPA=tris‐(2‐pyridylmethyl)amine. Two cis‐sites, which are occupied by labile acetonitrile molecules in 1 and 2 , are available for coordination of H₂O₂ and substituted benzoic acids. The hydroxylation of the aromatic ring occurs exclusively in the vicinity of the anchoring carboxylate functional group: ortho‐hydroxylation affords salicylates, whereas ipso‐hydroxylation with concomitant decarboxylation yields phenolates. The outcome of the substituent‐directed hydroxylation depends on the electronic properties and the position of substituents in the molecules of substrates: 3‐substituted benzoic acids are preferentially ortho‐hydroxylated, whereas 2‐ and, to a lesser extent, 4‐substituted substrates tend to undergo ipso‐hydroxylation/decarboxylation. These two pathways are not mutually exclusive and likely proceed via a common intermediate. Electron‐withdrawing substituents on the aromatic ring of the carboxylic acids disfavor hydroxylation, indicating an electrophilic nature for the active oxidant. Complexes 1 and 2 exhibit similar reactivity patterns, but 1 generates a more powerful oxidant than 2 . Spectroscopic and labeling studies exclude acylperoxoiron(III) and Fe^IV?O species as potential reaction intermediates, but strongly indicate the involvement of an Fe^III? OOH intermediate that undergoes intramolecular acid‐promoted heterolytic O? O bond cleavage, producing a transient iron(V) oxidant.     

A Non‐Heme Iron Photocatalyst for Light‐Driven Aerobic Oxidation of Methanol

Non‐heme (L)Fe^III and (L)Fe^III‐O‐Fe^III(L) complexes (L=1,1‐di(pyridin‐2‐yl)‐N,N‐bis(pyridin‐2‐ylmethyl)ethan‐1‐amine) underwent reduction under irradiation to the Fe^II state with concomitant oxidation of methanol to methanal, without the need for a secondary photosensitizer. Spectroscopic and DFT studies support a mechanism in which irradiation results in charge‐transfer excitation of a Fe^III?μ‐O?Fe^III complex to generate [(L)Fe^IV=O]²⁺ (observed transiently during irradiation in acetonitrile), and an equivalent of (L)Fe^II. Under aerobic conditions, irradiation accelerates reoxidation from the Fe^II to the Fe^III state with O₂, thus closing the cycle of methanol oxidation to methanal.     

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

The reactivity of a mononuclear high‐spin iron(III)‐alkylperoxo intermediate [Fe^III(t‐BuL^Urea)(OOCm)(OH₂)]²⁺( 2 ), generated from [Fe^II(t‐BuL^Urea)(H₂O)(OTf)](OTf) ( 1 ) [t‐BuL^Urea=1,1′‐(((pyridin‐2‐ylmethyl)azanediyl)bis(ethane‐2,1‐diyl))bis(3‐(tert‐butyl)urea), OTf=trifluoromethanesulfonate] with cumyl hydroperoxide (CmOOH), toward the C?H and C=C bonds of hydrocarbons is reported. 2 oxygenates the strong C?H bonds of aliphatic substrates with high chemo‐ and stereoselectivity in the presence of 2,6‐lutidine. While 2 itself is a sluggish oxidant, 2,6‐lutidine assists the heterolytic O?O bond cleavage of the metal‐bound alkylperoxo, giving rise to a reactive metal‐based oxidant. The roles of the urea groups on the supporting ligand, and of the base, in directing the selective and catalytic oxygenation of hydrocarbon substrates by 2 are discussed.     

Two‐Color Valence‐to‐Core X‐ray Emission Spectroscopy Tracks Cofactor Protonation State in a Class I Ribonucleotide Reductase

Proton transfer reactions are of central importance to a wide variety of biochemical processes, though determining proton location and monitoring proton transfers in biological systems is often extremely challenging. Herein, we use two‐color valence‐to‐core X‐ray emission spectroscopy (VtC XES) to identify protonation events across three oxidation states of the O₂‐activating, radical‐initiating manganese–iron heterodinuclear cofactor in a class I‐c ribonucleotide reductase. This is the first application of VtC XES to an enzyme intermediate and the first simultaneous measurement of two‐color VtC spectra. In contrast to more conventional methods of assessing protonation state, VtC XES is a more direct probe applicable to a wide range of metalloenzyme systems. These data, coupled to insight provided by DFT calculations, allow the inorganic cores of the Mn^IVFe^IV and Mn^IVFe^III states of the enzyme to be assigned as Mn^IV(μ‐O)₂Fe^IV and Mn^IV(μ‐O)(μ‐OH)Fe^III, respectively.     

Dendritic Iron Porphyrins with a Tethered Axial Ligand as New Model Compounds for Heme Monooxygenases

The novel iron(III) porphyrin dendrimers of generation zero ([ 1 ⋅Fe^III]Cl), one ([ 2 ⋅Fe^III]Cl), and two ([ 3 ⋅Fe^III]Cl) (Fig. 1) were prepared (Schemes 1 and 3) as models of heme monooxygenases. They feature controlled axial ligation at the Fe center by one imidazole tethered to the porphyrin core and possess a vacant coordination site available for ligand binding and catalysis. The high purity of the dendrimers and the absence of structural defects was demonstrated by matrix‐assisted laser‐desorption‐ionization time‐of‐flight (MALDI‐TOF) mass spectrometry (Fig. 3). The electronic properties of the Fe^III porphyrin dendrimers and comparison compounds [ 4 ⋅Fe^III]Cl and [ 12 ⋅Fe^III(1,2‐Me₂Im)]Cl (1,2‐Me₂Im=1,2‐dimethylimidazole) were investigated by UV/VIS and EPR (electronic paramagnetic resonance) spectroscopy, as well as by measurements of the magnetic moments by the Evans‐Scheffold method. Epoxidation of olefins and oxidation of sulfides to sulfoxides, catalyzed by the new dendritic metalloporphyrins, were investigated in CH₂Cl₂ with iodosylbenzene as the oxidant (Tables 1 and 2). The total turnover numbers were found to increase with the size of the dendrimer, due to improved catalyst stability at higher dendritic generations (Figs. 4 and 5). The second‐generation complex [ 3 ⋅Fe^III]Cl was, therefore, the most efficient catalyst in the series, despite the fact that its active site is considerably hindered by the encapsulation inside the sterically demanding, fluctuating dendritic wedges. Very high product selectivities were observed in all oxidation reactions, regardless of dendrimer generation.     

Disproportionation Equilibrium of a μ‐Oxodiiron(III) Complex Giving Rise to C−H Activation Reactivity: Structural Snapshot of a Unique Oxoiron(IV) Adduct

μ‐Oxodiiron(III) species are air‐stable and unreactive products of autoxidation processes of monomeric heme and non‐heme iron(II) complexes. Now, the organometallic [(L^NHC)Fe^III‐(μ‐O)‐Fe^III(L^NHC)]⁴⁺ complex 1 (L^NHC is a macrocyclic tetracarbene) is shown to be reactive in C?H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) ( 2 ) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2 . Kinetic analysis gave an equilibrium constant for the disproportionation of 1 , which is very small (K_eq=7.5±2.5×10^?8 m ). Increasing K_eq might by a useful strategy for circumventing the formation of dead‐end μ‐oxodiiron(III) products during Fe‐based homogeneous oxidation catalysis.     

Stabilization of Low‐Valent Iron(I) in a High‐Valent Vanadium(V) Oxide Cluster

Low‐valent iron centers are critical intermediates in chemical and bio‐chemical processes. Herein, we show the first example of a low‐valent Fe^I center stabilized in a high‐valent polyoxometalate framework. Electrochemical studies show that the Fe^III‐functionalized molecular vanadium(V) oxide (DMA)[Fe^IIIClV^V₁₂O₃₂Cl]³⁻ (DMA=dimethylammonium) features two well‐defined, reversible, iron‐based electrochemical reductions which cleanly yield the Fe^I species (DMA)[Fe^IClV^V₁₂O₃₂Cl]⁵⁻. Experimental and theoretical studies including electron paramagnetic resonance spectroscopy and density functional theory computations verify the formation of the Fe^I species. The study presents the first example for the seemingly paradoxical embedding of low‐valent metal species in high‐valent metal oxide anions and opens new avenues for reductive electron transfer catalysis by polyoxometalates.     

Development and validation of a LC method for the separation and determination of the anticancer‐active FeIII(4‐methoxy‐salophene) using the new second‐generation monolith

LC method with the newly introduced second‐generation monolithic silica RP‐18e column has been developed for the separation of Fe^III(salophene) and four methoxy‐substituted Fe^III(salophene) complexes. The method has been validated for the quantitation of Fe^III(4‐OMe‐salophene), a highly active anticancer substance in vitro, bound to serum albumin. Our routinely used high‐resolution continuum‐source atomic absorption spectroscopy method based on the determination of the central iron atom was unsuitable in this case because serum originally contains significant amounts of iron as revealed by a blank sample of serum albumin. The developed LC method depends on detecting the whole complex rather than the bound iron. Two morphologically different first‐ and second‐generation HPLC monolithic columns have been compared for this purpose. The newly introduced second‐generation monolithic silica column Chromolith® HighResolution RP‐18e column (100 × 4.6 mm, Merck) separated the mixture successful within 13 min. A mobile phase consisting of 25 mM phosphate buffer pH 3/methanol (60:40, v/v) was used at a flow rate of 1 mL/min. The dynamic linear working range of the calibration curve for Fe^III(4‐OMe‐salophene) was found to be between 1 and 200 μg/mL. Detection and quantitation limits were 0.3 and 1 μg/mL, respectively.     

Spectroscopic and Kinetic Evidence for the Crucial Role of Compound 0 in the P450cam‐Catalyzed Hydroxylation of Camphor by Hydrogen Peroxide

The hydroperoxo iron(III) intermediate P450_camFe^III–OOH, being the true Compound 0 (Cpd 0) involved in the natural catalytic cycle of P450_cam, could be transiently observed in the peroxo‐shunt oxidation of the substrate‐free enzyme by hydrogen peroxide under mild basic conditions and low temperature. The prolonged lifetime of Cpd 0 enabled us to kinetically examine the formation and reactivity of P450_camFe^III–OOH species as a function of varying reaction conditions, such as pH, and concentration of H₂O₂, camphor, and potassium ions. The mechanism of hydrogen peroxide binding to the substrate‐free form of P450_cam differs completely from that observed for other heme proteins possessing the distal histidine as a general acid–base catalyst and is mainly governed by the ability of H₂O₂ to undergo deprotonation at the hydroxo ligand coordinated to the iron(III) center under conditions of pH≥p${K{{{\rm P450}\hfill \atop {\rm a}\hfill}}}$ . Notably, no spectroscopic evidence for the formation of either Cpd I or Cpd II as products of heterolytic or homolytic O?O bond cleavage, respectively, in Cpd 0 could be observed under the selected reaction conditions. The kinetic data obtained from the reactivity studies involving (1R)‐camphor, provide, for the first time, experimental evidence for the catalytic activity of the P450Fe^III–OOH intermediate in the oxidation of the natural substrate of P450_cam.     

Nitrous‐Acid‐Mediated Synthesis of Iron–Nitrosyl–Porphyrin: pH‐Dependent Release of Nitric Oxide

Two iron–nitrosyl–porphyrins, nitrosyl[meso‐tetrakis(3,4,5‐trimethoxyphenylporphyrin]iron(II) acetic acid solvate ( 3 ) and nitrosyl[meso‐tetrakis(4‐methoxyphenylporphyrin]iron(II) CH₂Cl₂ solvate ( 4 ), were synthesized in quantitative yield by using a modified procedure with nitrous acid, followed by oxygen‐atom abstraction by triphenylphosphine under an argon atmosphere. These nitrosyl porphyrins are in the {FeNO}⁷ class. Under an argon atmosphere, these compounds are relatively stable over a broad range of pH values (4–8) but, under aerobic conditions, they release nitric oxide faster at high pH values than that at low pH values. The generated nitric‐oxide‐free iron(III)–porphyrin can be re‐nitrosylated by using nitrous acid and triphenylphosphine. The rapid release of NO from these Fe^II complexes at high pH values seems to be similar to that in nitrophorin, a nitric‐oxide‐transport protein, which formally possesses Fe^III. However, because the release of NO occurs from ferrous–nitrosyl–porphyrin under aerobic conditions, these compounds are more closely related to nitrobindin, a recently discovered heme protein.     

Olefin cis‐Dihydroxylation and Aliphatic CH Bond Oxygenation by a Dioxygen‐Derived Electrophilic Iron–Oxygen Oxidant

Many iron‐containing enzymes involve metal–oxygen oxidants to carry out O₂‐dependent transformation reactions. However, the selective oxidation of C? H and C?C bonds by biomimetic complexes using O₂ remains a major challenge in bioinspired catalysis. The reactivity of iron–oxygen oxidants generated from an Fe^II–benzilate complex of a facial N₃ ligand were thus investigated. The complex reacted with O₂ to form a nucleophilic oxidant, whereas an electrophilic oxidant, intercepted by external substrates, was generated in the presence of a Lewis acid. Based on the mechanistic studies, a nucleophilic Fe^II–hydroperoxo species is proposed to form from the benzilate complex, which undergoes heterolytic O? O bond cleavage in the presence of a Lewis acid to generate an Fe^IV–oxo–hydroxo oxidant. The electrophilic iron–oxygen oxidant selectively oxidizes sulfides to sulfoxides, alkenes to cis‐diols, and it hydroxylates the C? H bonds of alkanes, including that of cyclohexane.     

Electrochemical study of a nonheme Fe(ii) complex in the presence of dioxygen. Insights into the reductive activation of O2 at Fe(ii) centers

Recent efforts to model the reactivity of iron oxygenases have led to the generation of nonheme Fe^III(OOH) and Fe^IV(O) intermediates from Fe^II complexes and O₂ but using different cofactors. This diversity emphasizes the rich chemistry of nonheme Fe(ii) complexes with dioxygen. We report an original mechanistic study of the reaction of [(TPEN)Fe^II]²⁺ with O₂ carried out by cyclic voltammetry. From this Fe^II precursor, reaction intermediates such as [(TPEN)Fe^IV(O)]²⁺, [(TPEN)Fe^III(OOH)]²⁺ and [(TPEN)Fe^III(OO)]⁺ have been chemically generated in high yield, and characterized electrochemically. These electrochemical data have been used to analyse and perform simulation of the cyclic voltammograms of [(TPEN)Fe^II]²⁺ in the presence of O₂. Thus, several important mechanistic informations on this reaction have been obtained. An unfavourable chemical equilibrium between O₂ and the Fe^II complex occurs that leads to the Fe^III-peroxo complex upon reduction, similarly to heme enzymes such as P450. However, unlike in heme systems, further reduction of this latter intermediate does not result in O–O bond cleavage.     

A Mixed‐Valence {Fe13} Cluster Exhibiting Metal‐to‐Metal Charge‐Transfer‐Switched Spin Crossover

It is promising and challenging to manipulate the electronic structures and functions of materials utilizing both metal‐to‐metal charge transfer (MMCT) and spin‐crossover (SCO) to tune the valence and spin states of metal ions. Herein, a metallocyanate building block is used to link with a Fe^II‐triazole moiety and generates a mixed‐valence complex {[(Tp^4‐Me)Fe^III(CN)₃]₉[Fe^II₄(trz‐ph)₆]}?[Ph₃PMe]₂?[(Tp^4‐Me)Fe^III(CN)₃] ( 1 ; trz‐ph=4‐phenyl‐4H‐1,2,4‐triazole). Moreover, MMCT occurs between Fe^III and one of the Fe^II sites after heat treatment, resulting in the generation of a new phase, {[(Tp^4‐Me)Fe^II(CN)₃][(Tp^4‐Me)Fe^III(CN)₃]₈ [Fe^IIIFe^II₃(trz‐ph)₆]}? [Ph₃PMe]₂?[(Tp^4‐Me)Fe^III(CN)₃] ( 1 a ). Structural and magnetic studies reveal that MMCT can tune the two‐step SCO behavior of 1 into one‐step SCO behavior of 1 a . Our work demonstrates that the integration of MMCT and SCO can provide a new alternative for manipulating functional spin‐transition materials with accessible multi‐electronic states.     

A Strongly Spin‐Frustrated FeIII7 Complex with a Canted Intermediate Spin Ground State of S=7/2 or 9/2

A disk‐shaped [Fe^III₇(Cl)(MeOH)₆(μ₃‐O)₃(μ‐OMe)₆ (PhCO₂)₆]Cl₂ complex with C₃ symmetry has been synthesised and characterised. The central tetrahedral Fe^III is 0.733 Å above the almost co‐planar Fe^III₆ wheel, to which it is connected through three μ₃‐oxide bridges. For this iron‐oxo core, the magnetic susceptibility analysis proposed a Heisenberg–Dirac–van Vleck (HDvV) mechanism that leads to an intermediate spin ground state of S=7/2 or 9/2. Within either of these ground state manifolds it is reasonable to expect spin frustration effects. The ⁵⁷Fe Mössbauer (MS) analysis verifies that the central Fe^III ion easily aligns its magnetic moment antiparallel to the externally applied field direction, whereas the other six peripheral Fe^III ions keep their moments almost perpendicular to the field at stronger fields. This unusual canted spin structure reflects spin frustration. The small linewidths in the magnetic Mössbauer spectra of polycrystalline samples clearly suggest an isotropic exchange mechanism for realisation of this peculiar spin topology.