Iron-oxo Species Research Articles

Axial g-C3N4 coordinated iron(III) phthalocyanine mediated ultra-efficient peroxymonosulfate activation for high-valent iron species generation

Herein, a new axial g-C3N4 coordinated iron(III) phthalocyanine (FeP/CN) was fabricated for peroxymonosulfate (PMS) activation. Around 100% degradation of acetaminophen (AP), 2,4-dichlorophenol (2,4-DP), sulfadiazine (SDZ), and methyl orange (MO) (30 mg L-1) were achieved within 20 min by adding 0.02 g L-1 FeP/CN5 (3.62 wt% Fe) and 0.2 mM PMS. High-valent iron-oxo species (FeIV=O) was demonstrated as the main reactive species, which mediated a two-electron transfer process with pollutants. Characterizations and computational analysis revealed that the axial g-C3N4 ligand provided Fe(III) coordination environments to generate FeIV=O species directly through PMS activation, and increased the reactivity of the FeIV=O species in pollutants oxidation due to the narrowed HOMO-LUMO gap. Besides, small displacement of Fe atom (0.23 Å) from the macrocycles plane due to the axial g-C3N4 ligand decreased the iron demetalization rate from 3.54% to 0.28% in the catalyst/PMS system. This work offered an excellent strategy to design high-efficiency catalysts for FeIV=O generation.

Magnetically separable Fe-base deposited on different carbon sources for ultrasound/persulfate-like heterogeneous activation: Optimized synthesis and field driving process

The eco-friendly composite materials, micro-nano Fe-base/glucose-derived hydro-chars (Feb/HCs), were prepared, whose magnetic separation can be achieved in both preparation and water treatment stages. The performances of Feb/HCs after N2 heat-treatment to activate persulfate, chlorite, hydrogen peroxide, etc., in ultrasound field, obtained great improvement by extracting “O”. For comparison, other different sized and magnetic iron-carbon based composites based on different carbon sources of activated carbons, cornstalk-derived bio-chars, and glucose-derived pyrolytic carbons were prepared and applied to systematically compare the performance of activation. The Feb/HCs with optimizing preparation were utilized as activators to well treat different structures (triphenylmethane-, azo-, and xanthene-) contaminants. The detected p-Benzoquinone and 2-chloro-p-Benzoquinone could be considered as transitional and characteristic intermediates from carbocyclic compounds to chain compounds. The degradation mechanisms were evoked by pH and absorption to trigger via free/non-free radicals processes: high valence iron-oxo species, sulfate radicals, hydroxyl radicals, Cl-based substances, etc. The findings contrastively provided the potential applications of magnetically separable iron-carbon based composites for heterogeneous activation in environmental remediation.

Cu/Fe oxide integrated on graphite felt for degradation of sulfamethoxazole in the heterogeneous electro-Fenton process under near-neutral conditions

In the heterogeneous electro-Fenton (EF) system, high-efficiency and durable materials have attracted widespread attention as cathodes for degradation of refractory organic pollutants. In this study, a stable Cu/Fe oxide modified graphite felt electrode (Cu0.33Fe0.67NBDC-300/GF) was fabricated via a one-step hydrothermal method and subsequent thermal treatment, which used a bimetallic metal–organic framework (MOF) with 2-aminoterephthalic acid (NH2BDC) ligand as the precursor. The Cu0.33Fe0.67NBDC-300/GF electrode was used as the cathode for sulfamethoxazole (SMX) degradation in the heterogeneous EF process. The coexistence of the FeII/FeIII and CuI/CuII redox couples significantly accelerates the regeneration of FeII and promotes the generation of active free radicals (•OH and •O2−). FeIV was detected during the process, which indicates that the high-valent iron-oxo species was produced in near-neutral pH conditions. The removal efficiency of SMX (10 mg L−1) can reach 100.0% within 75 min over a wide pH range (4.0–9.0). After five cycles, the electrode retained a high stability and an outstanding catalytic capacity. Furthermore, the mechanisms and pathways for SMX degradation were proposed, the products and intermediates of SMX were analyzed, and the toxicity was evaluated. It was found that the toxicity decreased after degradation. This study displays a novel strategy for building an efficient and stable self-supporting electrode for treating antibiotic wastewater.

Peroxymonosulfate Activated by Natural Porphyrin Derivatives for Rapid Degradation of Organic Pollutants Via Singlet Oxygen and High-Valent Iron-Oxo Species

Peroxymonosulfate Activated by Natural Porphyrin Derivatives for Rapid Degradation of Organic Pollutants Via Singlet Oxygen and High-Valent Iron-Oxo Species

Photocatalytic generation of a non-heme Fe(iii)-hydroperoxo species with O2 in water for the oxygen atom transfer reaction.

Coupling a photoredox module and a bio-inspired non-heme model to activate O2 for the oxygen atom transfer (OAT) reaction requires a vigorous investigation to shed light on the multiple competing electron transfer steps, charge accumulation and annihilation processes, and the activation of O2 at the catalytic unit. We found that the efficient oxidative quenching mechanism between a [Ru(bpy)3]2+ chromophore and a reversible electron mediator, methyl viologen (MV2+), to form the reducing species methyl viologen radical (MV˙+) can convey an electron to O2 to form the superoxide radical and reset an Fe(iii) species in a catalytic cycle to the Fe(ii) state in an aqueous solution. The formation of the Fe(iii)-hydroperoxo (FeIII-OOH) intermediate can evolve to a highly oxidized iron-oxo species to perform the OAT reaction to an alkene substrate. Such a strategy allows us to bypass the challenging task of charge accumulation at the molecular catalytic unit for the two-electron activation of O2. The FeIII-OOH catalytic precursor was trapped and characterized by EPR spectroscopy pertaining to a metal assisted catalysis. Importantly, we found that the substrate itself can act as an electron donor to reset the photooxidized chromophore in the initial state closing the photocatalytic loop and hence excluding the use of a sacrificial electron donor. Laser Flash Photolysis (LFP) studies and spectroscopic monitoring during photocatalysis lend credence to the proposed catalytic cycle.

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

In this study, we explore the water oxidation process with the help of density functional theory. The formation of an oxygen-oxygen bond is crucial in the water oxidation process. Here, we report the formation of the oxygen-oxygen bond by the N5-coordinate oxoiron species with a higher oxidation state of FeIV and FeV. This bond formation is studied through the nucleophilic addition of water molecules and the transfer of the oxygen atom from meta-chloroperbenzoic acid (mCPBA). Our study reveals that the oxygen-oxygen bond formation by reacting mCPBA with FeVO requires less activation barrier (13.7 kcal mol-1) than the other three pathways. This bond formation by the oxygen atom transfer (OAT) pathway is more favorable than that achieved by the hydrogen atom transfer (HAT) pathway. In both cases, the oxygen-oxygen bond formation occurs by interacting the σ*dz2-2pz molecular orbital of the iron-oxo intermediate with the 2px orbital of the oxygen atom. From this study, we understand that the oxygen-oxygen bond formation by FeIVO with the OAT process is also feasible (16 kcal mol-1), suggesting that FeVO may not always be required for the water oxidation process by non-heme N5-oxoiron. After the oxygen-oxygen bond formation, the release of the dioxygen molecule occurs with the addition of the water molecule. The release of dioxygen requires a barrier of 7.0 kcal mol-1. The oxygen-oxygen bond formation is found to be the rate-determining step.

High-valent iron-oxo species mediated cyclic oxidation through single-atom [tbnd]Fe-N6 sites with high peroxymonosulfate utilization rate

Fe singly anchored onto graphite carbon nitride (Fe-SA/PHCNS) as Fe-N6 was synthesized to realize efficient peroxymonosulfate (PMS) activation and superior organic pollutant oxidation. The Fe-N6 was the core catalytic site of activation via forming high-valent iron–oxo species (FeN6 =O) as the transient reactive intermediate as evidenced by X-ray adsorption fine spectroscopy (XAFS), in-situ Raman spectrum and theoretical calculation. The cycle of Fe-N6 and FeN6 =O interconversion was sustainably observed with organic pollutant as the electron-donor and PMS as the electron-acceptor. As results, the equivalent steady state concentration of FeN6 =O was as high as 2.39 × 10−8 M and this system exhibited 97.2% average PMS utilization rate. Fe-SA/PHCNS can be immobilized onto carbon felt for the simultaneous filtration and oxidation with stable and efficient performance. This study elucidated the mechanism and superiority of FeN6 =O mediated oxidation pathway and can advance the research and application of this new approach in advanced oxidation processes.

High efficiency degradation of tetracycline by peroxymonosulfate activated with Fe/NC catalysts: Performance, intermediates, stability and mechanism

Carbon-based catalysts have the advantages of biological cleaning, eco-friendly and cost-effective in water treatment. While, nitrogen doped biochar promotes the development of non-radical peroxymonosulfate (PMS) activation in environmental remediation. Thus, three-dimensional sponge-like porous Fe and N co-doped biochar (Fe/CN–30) with high catalytic activity for PMS activation was synthesized. In a wide pH range (1–11), the Fe/CN–30 catalyst can efficiently degrade tetracycline (TC) with a small amount of PMS. The non-radical pathways are prominent in the TC decomposition process according to the quenching experiments, electron paramagnetic resonance (EPR) and gas chromatograph-mass spectrometer (GC–MS) analysis, in which the contribution of high-valent iron-oxo species (Fe(IV) = O) was dominant. X-ray photoelectron spectroscopy and reaction kinetic experiments confirmed that the coordination sites of Fe and N in the Fe/CN–30 are the reactive centers for TC degradation. Moreover, the successive addition of low concentration PMS into the system was confirmed to favor the PMS utilization, and the high selectivity of the Fe/CN–30 was confirmed by the analysis of pollutant structure. Furthermore, by-products of TC degradation in the Fe/CN-30/PMS system and the possible TC degradation pathways were proposed via liquid chromatography-mass spectrometry (LC-MS). Therefore, this study dedicates to providing new insights into the non-radical pathway-catalyzed AOPs.

Nonheme Iron Imido Complexes Bearing a Non-Innocent Ligand: A Synthetic Chameleon Species in Oxidation Reactions.

High-valent iron-imido complexes can perform C-H activation and sulfimidation reactions, but are far less studied than the more ubiquitous iron-oxo species. As case studies, we have looked at a recently published iron(V)-imido ligand π-cation radical complex, which is formally an iron(VI)-imido complex [FeV (NTs)(TAML+. )] (1; NTs=tosylimido), and an iron(V)-imido complex [FeV (NTs)(TAML)]- (2). Using a theoretical approach, we found that they have multiple energetically close-lying electromers, sometimes even without changing spin states, reminiscent of the so-called Compound I in Cytochrome P450. When studying their reactivity theoretically, it is indeed found that their electronic structures may change to perform efficient oxidations, emulating the multi-spin state reactivity in FeIV O systems. This is actually in contrast to the known [FeV (O)(TAML)]- species (3), where the reactions occur only on the ground spin state. We also looked into the whole reaction pathway for the C-H bond activation of 1,4-cyclohexadiene by these intermediates to reproduce the experimentally observed products, including steps that usually attract no interest (neither theoretically nor experimentally) due to their non-rate-limiting status and fast reactivity. A new "clustering non-rebound mechanism" is presented for this C-H activation reaction.

Bleach catalysis in aqueous medium by iron(III)-isoindoline complexes and hydrogen peroxide

Hydrogen peroxide and peroxymonocarbonate anion-based bleach reactions are important for many applications such as paper bleach, waste water treatment and laundry. Nonheme iron(III) complexes, [Fe III (L 1-4 )Cl 2 ] with the 1,3-bis(2 ′ -Ar-imino)isoindolines ligands (HL n , n=1–4, Ar = pyridyl, thiazolyl, benzimidazolyl and N-methylbenzimidazolyl, respectively) have been shown to catalyze the oxidative degradation of morin as a soluble model of a bleachable stain by H 2 O 2 in buffered aqueous solution. In these experiments the bleaching activity of the catalysts was significantly influenced by the Lewis acidity and redox properties of the metal centers, and showed a linear correlation with the Fe III /Fe II redox potentials (in the range of 197–415 mV) controlled by the modification of the electron donor properties of the ligand introducing various aryl groups on the bis-iminoisoindoline moiety. A similar trend but with low yields was observed for the disproportionation of H 2 O 2 (catalase-like reaction) which is a major side reaction of catalytic bleach with transition metal complexes. The effect of bicarbonate ions might be explained by the reduction of Fe(III) ions and/or the formation of peroxymonocarbonate monoanion, which is a much stronger oxidant and could increase the formation of the catalytically active high-valent oxoiron species.

Surface-mediated periodate activation by nano zero-valent iron for the enhanced abatement of organic contaminants

Periodate (PI)-based advanced oxidation processes have recently received increasing attentions. Herein, PI was readily activated by nano zero-valent iron (nZVI) and subsequently led to the enhanced oxidation of organic contaminants, with the removal performance of sulfadiazine (SDZ) in the nZVI/PI process even higher than that in the nZVI/peroxydisulfate process under identical conditions. Kinetic experiments indicated that the decay of SDZ was susceptible to the dosage of nZVI and PI, but was barely affected by pH values (4.0–7.0) under buffered conditions, suggesting the promising performance of the nZVI/PI process in a relatively wide pH range. Selective degradation of contaminants and 18O-isotope labeling assays collectively demonstrated that iodate radical (•IO3), high-valent iron-oxo species (Fe(IV)) and hydroxyl radical (•OH) were responsible for the abatement of organic contaminants. More importantly, due to the relatively weak steric hindrance effect of PI, PI easily adsorbed on the surface of nZVI and no iron leaching was detected throughout the reaction, implying that PI activation induced by nZVI was a surface-mediated process. Besides, PI was not transformed into harmful reactive iodine species. This study proposed an environmental-friendly approach for PI activation and shed new lights on the PI-based processes.

NRVS investigation of ascorbate peroxidase compound II: Observation of Iron(IV)oxo stretching

The protonation state of ascorbate peroxidase compound II (APX-II) has been a subject of debate. A combined X-ray/neutron crystallographic study reported that APX-II is best described as an iron(IV)hydroxide species with an FeO distance of 1.88 Å (Kwon, et al. Nat Commun2016, 7, 13,445), while X-ray absorption spectroscopy (XAS) experiments (utilizing extended X-ray absorption fine structure (EXAFS) and pre-edge analyses) indicate APX-II is an authentic iron(IV)oxo species with an FeO distance 1.68 Å (Ledray, et al. Journal of the American Chemical Society2020,142, 20,419). Previous debates concerning ferryl protonation states have been resolved through the application of Badger's rule, which correlates FeO bond distances with FeO vibrational frequencies. To obtain the required vibrational data, we have collected Nuclear Resonance Vibrational Spectroscopy (NRVS) data for APX-II. We observe a broad vibrational feature in the range associated with iron(IV)oxo stretching (700–800 cm−1). This feature appears to have two peaks at 732 cm−1 and 770 cm−1, corresponding to FeO distances of 1.69 and 1.67 Å, respectively. The broad vibrational envelope and the presence of multiple resonances could reflect a distribution of hydrogen bonding interactions within the active-site pocket.

PH Changes That Induce an Axial Ligand Effect on Nonheme Iron(IV) Oxo Complexes with an Appended Aminopropyl Functionality

Nonheme iron enzymes often utilize a high-valent iron(IV) oxo species for the biosynthesis of natural products, but their high reactivity often precludes structural and functional studies of these complexes. In this work, a combined experimental and computational study is presented on a biomimetic nonheme iron(IV) oxo complex bearing an aminopyridine macrocyclic ligand and its reactivity toward olefin epoxidation upon changes in the identity and coordination ability of the axial ligand. Herein, we show a dramatic effect of the pH on the oxygen-atom-transfer (OAT) reaction with substrates. In particular, these changes have occurred because of protonation of the axial-bound pendant amine group, where its coordination to iron is replaced by a solvent molecule or anionic ligand. This axial ligand effect influences the catalysis, and we observe enhanced cyclooctene epoxidation yields and turnover numbers in the presence of the unbound protonated pendant amine group. Density functional theory studies were performed to support the experiments and highlight that replacement of the pendant amine with a neutral or anionic ligand dramatically lowers the rate-determining barriers of cyclooctene epoxidation. The computational work further establishes that the change in OAT is due to electrostatic interactions of the pendant amine cation that favorably affect the barrier heights.

Role of Ferryl Ion Intermediates in Fast Fenton Chemistry on Aqueous Microdroplets.

In the aqueous environment, FeII ions enhance the oxidative potential of ozone and hydrogen peroxide by generating the reactive oxoiron species (ferryl ion, FeIVO2+) and hydroxyl radical (·OH) via Fenton chemistry. Herein, we investigate factors that control the pathways of these reactive intermediates in the oxidation of dimethyl sulfoxide (Me2SO) in FeII solutions reacting with O3 in both bulk-phase water and on the surfaces of aqueous microdroplets. Electrospray ionization mass spectrometry is used to quantify the formation of dimethyl sulfone (Me2SO2, from FeIVO2+ + Me2SO) and methanesulfonate (MeSO3-, from ·OH + Me2SO) over a wide range of FeII and O3 concentrations and pH. In addition, the role of environmentally relevant organic ligands on the reaction kinetics was also explored. The experimental results show that Fenton chemistry proceeds at a rate ∼104 times faster on microdroplets than that in bulk-phase water. Since the production of MeSO3- is initiated by ·OH radicals at diffusion-controlled rates, experimental ratios of Me2SO2/MeSO3- > 102 suggest that FeIVO2+ is the dominant intermediate under all conditions. Me2SO2 yields in the presence of ligands, L, vary as volcano-plot functions of E0(LFeIVO2++ O2/LFe2+ + O3) reduction potentials calculated by DFT with a maximum achieved in the case of L≡oxalate. Our findings underscore the key role of ferryl FeIVO2+ intermediates in Fenton chemistry taking place on aqueous microdroplets.

Activation of peroxymonosulfate by molybdenum disulfide-mediated traces of Fe(III) for sulfadiazine degradation

The activation of persulfate by ferrous iron (Fe(II)) is of great interest to the environmental remediation community, but the reduction of ferric iron (Fe(III)) to Fe(II) is slow and the accumulation of iron sludge resulted from the precipitation of Fe(III) is a great concern. Here, molybdenum disulfide (MoS2) was studied as a co-catalyst to improve the activation of peroxymonosulfate (PMS) by Fe(III) for sulfadiazine (SDZ) degradation and different characterization technologies were used to reveal the reactive species. The results showed that a strong synergy existed between MoS2 and Fe(III); approximately 94.3% of the SDZ was removed by MoS2-Fe(III)-PMS after reaction for 30 min, while only 8.5% and 56.4% of the SDZ was removed by Fe(III)-PMS and MoS2-PMS, respectively. Both hydroxyl radicals and sulfate radicals were generated and the latter was the primary species. In addition to the radicals, singlet oxygen was found to be generated and contributed to the degradation of SDZ. The chemical probe reaction with methyl phenyl sulfoxide showed that the generation of high-valent iron-oxo species was not obvious by MoS2-Fe(III)-PMS under both acidic and neutral conditions. MoS2 had good stability. No noticeable deactivation was observed during the 1st to 5th run and no obvious oxidation of surface Mo(IV) occurred. Based on the characterization of catalyst and oxidizing species, a mechanism for the activation of PMS by MoS2-Fe(III) was proposed. The results from this study are expected to clarify the reactive species and deepen the understanding of MoS2-promoted persulfate activation by Fe(II)/Fe(III).

Activation of peroxymonosulfate by single-atom Fe-g-C3N4 catalysts for high efficiency degradation of tetracycline via nonradical pathways: Role of high-valent iron-oxo species and Fe–Nx sites

A Fenton-like catalyst was prepared based on isolated Fe single-atom and Fe clusters anchored onto a g-C3N4 framework. It exhibited high activity and stability in the heterogeneous activation of peroxymonosulfate (PMS) for tetracycline (TC) degradation. Both experimental and density functional theory calculation results demonstrated that the unique N-coordinated single Fe atom (Fe–N4) served as active sites with optimal binding energy for PMS activation. The experimental analysis indicated that the single Fe atoms displayed superior catalytic activity to Fe clusters. Furthermore, both high-valent iron-oxo species and single oxygen-dominated nonradical processes caused TC degradation. Among them, single Fe atoms anchored onto Fe-g-C3N4 served as Fe–N4 reactive sites can directly activate PMS to produce high-valent iron-oxo species, which was the key active nonradical species causing TC degradation.

Mechanistic insight into the generation of high-valent iron-oxo species via peroxymonosulfate activation: An experimental and density functional theory study

The generation of high-valent iron-oxo species via the activation of peroxymonosulfate (PMS) by Fe–N moieties has shown promise for selective degradation of contaminants, but the underlying electron-transfer mechanism remains unclear. To fill this gap, we here investigated the generation of high-valent iron-oxo species using hemin as a model Fe–N moiety–containing activator of PMS and used density functional theory (DFT) calculation to gain mechanistic insights from a molecular level. The results showed that hemin had great reactivity for the target contaminant–bisphenol A degradation; around 100% degradation was achieved after reaction for 30 min. Radicals including hydroxyl radicals, sulfate radicals, and superoxide anion radicals and singlet oxygen did not contribute to the degradation. Instead, FeV = O was the dominant reactive species that was revealed by the generation of dimethyl sulfone from dimethyl sulfoxide oxidation. The selectivity of FeV = O was demonstrated by the negligible inhibitory effects of common anions. The chemical bond and Mulliken charge analyses consistently suggested that the electron transfer from hemin led to the cleavage of O–O bond and the activation of PMS. Although we do not point out the exact pathway for the generation of FeV = O, the results from this study are expected to deepen the understanding of the interaction between PMS and Fe-containing porphyrin materials and the generation of high-valent iron-oxo species.

Enhanced Oxidation of Organic Contaminants by Iron(II)-Activated Periodate: The Significance of High-Valent Iron-Oxo Species.

Potassium periodate (PI, KIO4) was readily activated by Fe(II) under acidic conditions, resulting in the enhanced abatement of organic contaminants in 2 min, with the decay ratios of the selected pollutants even outnumbered those in the Fe(II)/peroxymonosulfate and Fe(II)/peroxydisulfate processes under identical conditions. Both 18O isotope labeling techniques using methyl phenyl sulfoxide (PMSO) as the substrate and X-ray absorption near-edge structure spectroscopy provided conclusive evidences for the generation of high-valent iron-oxo species (Fe(IV)) in the Fe(II)/PI process. Density functional theory calculations determined that the reaction of Fe(II) with PI followed the formation of a hydrogen bonding complex between Fe(H2O)62+ and IO4(H2O)-, ligand exchange, and oxygen atom transfer, consequently generating Fe(IV) species. More interestingly, the unexpected detection of 18O-labeled hydroxylated PMSO not only favored the simultaneous generation of ·OH but also demonstrated that ·OH was indirectly produced through the self-decay of Fe(IV) to form H2O2 and the subsequent Fenton reaction. In addition, IO4- was not transformed into the undesired iodine species (i.e., HOI, I2, and I3-) but was converted to nontoxic iodate (IO3-). This study proposed an efficient and environmental friendly process for the rapid removal of emerging contaminants and enriched the understandings on the evolution mechanism of ·OH in Fe(IV)-mediated processes.

Efficient removal of pyrene by biochar supported iron oxide in heterogeneous Fenton-like reaction via radicals and high-valent iron-oxo species

Biochar supported different doses of ferro ferric oxide (Fe3O4/BC) was synthesized with different pyrolytic temperatures and compared in catalyze oxidation of pyrene as a typical polycyclic aromatic hydrocarbon through heterogeneous Fenton-like reaction. The results illustrated that the 99.5% of pyrene in aqueous solutions was removed at the initial pH of 7.5 by the synthesized Fe3O4/BC5 with composite mass ratio of 1:5 without pyrolysis after 720 min reaction. The high catalytic efficiency was probably ascribed to the catalyzed oxidation of hydroxyl radicals and high-valent iron-oxo species which was confirmed by the changes of functional groups on Fe3O4/BC5 surface, electron paramagnetic resonance, dimethyl sulfoxide as a probe compound experiment and radical quenching experiment. Furthermore, the accelerated electron transfer of pyrene catalyzed degradation in Fe3O4/BC5/H2O2 system was measured by linear sweep voltammetry and electrochemical impedance spectroscopy. Six degradation products were identified by GC–MS, then the radical and electron transfer oxidation pathways of pyrene were proposed.

Spin State Tunes Oxygen Atom Transfer towards FeIV O Formation in FeII Complexes.

Oxoiron(IV) complexes bearing tetradentate ligands have been extensively studied as models for the active oxidants in non-heme iron-dependent enzymes. These species are commonly generated by oxidation of their ferrous precursors. The mechanisms of these reactions have seldom been investigated. In this work, the reaction kinetics of complexes [FeII (CH3 CN)2 L](SbF6 )2 ([1](SbF6 )2 and [2](SbF6 )2 ) and [FeII (CF3 SO3 )2 L] ([1](OTf)2 and [2](OTf)2 (1, L=Me,H Pytacn; 2, L=nP,H Pytacn; R,R' Pytacn=1-[(6-R'-2-pyridyl)methyl]-4,7- di-R-1,4,7-triazacyclononane) with Bu4 NIO4 to form the corresponding [FeIV (O)(CH3 CN)L]2+ (3, L=Me,H Pytacn; 4, L=nP,H Pytacn) species was studied in acetonitrile/acetone at low temperatures. The reactions occur in a single kinetic step with activation parameters independent of the nature of the anion and similar to those obtained for the substitution reaction with Cl- as entering ligand, which indicates that formation of [FeIV (O)(CH3 CN)L]2+ is kinetically controlled by substitution in the starting complex to form [FeII (IO4 )(CH3 CN)L]+ intermediates that are converted rapidly to oxo complexes 3 and 4. The kinetics of the reaction is strongly dependent on the spin state of the starting complex. A detailed analysis of the magnetic susceptibility and kinetic data for the triflate complexes reveals that the experimental values of the activation parameters for both complexes are the result of partial compensation of the contributions from the thermodynamic parameters for the spin-crossover equilibrium and the activation parameters for substitution. The observation of these opposite and compensating effects by modifying the steric hindrance at the ligand illustrates so far unconsidered factors governing the mechanism of oxygen atom transfer leading to high-valent iron oxo species.