X-ray absorption spectroscopic studies of high-spin nonheme (alkylperoxo)iron(III) intermediates

Mechanistic Insights into Amphoteric Reactivity of an Iron-Bispidine Complex

The reactivity of FeIII -alkylperoxido complexes has remained a riddle to inorganic chemists owing to their thermal instability and impotency towards organic substrates. These iron-oxygen adducts have been known as sluggish oxidants towards oxidative electrophilic and nucleophilic reactions. Herein, we report the synthesis and spectroscopic characterization of a relatively stable mononuclear high-spin FeIII -alkylperoxido complex supported by an engineered bispidine framework. Against the notion, this FeIII -alkylperoxido complex serves as a rare example of versatile reactivity in both electrophilic and nucleophilic reactions. Detailed mechanistic studies and computational calculations reveal a novel reaction mechanism, where a putative superoxido intermediate orchestrates the amphoteric property of the oxidant. The design of the backbone is pivotal to convey stability and reactivity to alkylperoxido and superoxido intermediates. Contrary to the well-known O-O bond cleavage that generates an FeIV -oxido species, the FeIII -alkylperoxido complex reported here undergoes O-C bond scission to generate a superoxido moiety that is responsible for the amphiphilic reactivity.

Room Temperature Aerobic Peroxidation of Organic Substrates Catalyzed by Cobalt(III) Alkylperoxo Complexes

Room temperature aerobic oxidation of hydrocarbons is highly desirable and remains a great challenge. Here we report a series of highly electrophilic cobalt(III) alkylperoxo complexes, Co^III(qpy)OOR supported by a planar tetradentate quaterpyridine ligand that can directly abstract H atoms from hydrocarbons (R'H) at ambient conditions (Co^III(qpy)OOR + R'H → Co^II(qpy) + R'^• + ROOH). The resulting alkyl radical (R'^•) reacts rapidly with O₂ to form alkylperoxy radical (R'OO^•), which is efficiently scavenged by Co^II(qpy) to give Co^III(qpy)OOR' (Co^II(qpy) + R'OO^• → Co^III(qpy)OOR'). This unique reactivity enables Co^III(qpy)OOR to function as efficient catalysts for aerobic peroxidation of hydrocarbons (R'H + O₂ → R'OOH) under 1 atm air and at room temperature.

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 )(H₂ O)(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.

Acid-Triggered O-O Bond Heterolysis of a Nonheme FeIII (OOH) Species for the Stereospecific Hydroxylation of Strong C-H Bonds

A novel hydroperoxoiron(III) species [Fe^III (OOH)(MeCN)(PyNMe₃ )]²⁺ (3) has been generated by reaction of its ferrous precursor [Fe^II (CF₃ SO₃ )₂ (PyNMe₃ )] (1) with hydrogen peroxide at low temperatures. This species has been characterized by several spectroscopic techniques and cryospray mass spectrometry. Similar to most of the previously described low-spin hydroperoxoiron(III) compounds, 3 behaves as a sluggish oxidant and it is not kinetically competent for breaking weak C-H bonds. However, triflic acid addition to 3 causes its transformation into a much more reactive compound towards organic substrates that is capable of oxidizing unactivated C-H bonds with high stereospecificity. Stopped-flow kinetic analyses and theoretical studies provide a rationale for the observed chemistry, a triflic-acid-assisted heterolytic cleavage of the O-O bond to form a putative strongly oxidizing oxoiron(V) species. This mechanism is reminiscent to that observed in heme systems, where protonation of the hydroperoxo intermediate leads to the formation of the high-valent [(Porph^. )Fe^IV (O)] (Compound I).

Facile and Reversible Formation of Iron(III)-Oxo-Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce^IV (NH₄ )₂ (NO₃ )₆ is often used in artificial water oxidation and generally considered to be an outer-sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe^III -O-Ce^IV (OH₂ )(NO₃ )₄ ]⁺ (3), a complex obtained from the reaction of [(N4Py)Fe^II (NCMe)]²⁺ with 2 equiv CAN or [(N4Py)Fe^IV =O]²⁺ (2) with Ce^III (NO₃ )₃ in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S=1 Fe^IV in 2 to S=5/2 in 3, which is found to be facile despite the formal spin-forbidden nature of this process. This observation suggests that Fe^IV =O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe-O-M Complexes Derived from [FeIV(Oanti/syn)(TMC)]2+ Isomers

Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two faces in supporting an oxoiron(IV) moiety, as exemplified by the prototypical [(TMC)FeIV(Oanti)(NCCH3)](OTf)2, where anti indicates that the O atom is located on the face opposite all four methyl groups, and the recently reported syn isomer [(TMC)FeIV(Osyn)(OTf)](OTf). The ability to access two isomers of [(TMC)FeIV(Oanti/syn)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CH3CN)(TMC)FeIII-Oanti-CrIII(OTf)4(NCCH3)] (1) by inner-sphere electron transfer between Cr(OTf)2 and [(TMC)FeIV(Oanti)(NCCH3)](OTf)2. Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf)2 and [(TMC)FeIV(Osyn)(NCCH3)](OTf)2, which is formulated as [(TMC)FeIII-Osyn-CrIII(OTf)4(NCCH3)] based on its characterization by UV-vis, resonance Raman, Mössbauer, and X-ray absorption spectroscopic methods, as well as electrospray mass spectrometry. Its pre-edge area (30 units) and Fe-O distance (1.77 Å) determined by X-ray absorption spectroscopy are distinctly different from those of 1 (11-unit pre-edge area and 1.81 Å Fe-O distance) but more closely resemble the values reported for [(TMC)FeIII-Osyn-ScIII(OTf)4(NCCH3)] (3, 32-unit pre-edge area and 1.75 Å Fe-O distance). This comparison suggests that 2 has a square pyramidal iron center like 3, rather than the six-coordinate center deduced for 1. Density functional theory calculations further validate the structures for 1 and 2. The influence of the distinct TMC topologies on the coordination geometries is further confirmed by the crystal structures of [(Cl)(TMC)FeIII-Oanti-FeIIICl3] (4Cl) and [(TMC)FeIII-Osyn-FeIIICl3](OTf) (5). Complexes 1-5 thus constitute a set of complexes that shed light on ligand topology effects on the coordination chemistry of the oxoiron moiety.

Characterization of a heterobimetallic nonheme Fe(III)-O-Cr(III) species formed by O2 activation

We report the generation and spectroscopic characterization of a heterobimetallic [(TMC)Fe(III)-O-Cr(III)(OTf)4] species (1) by bubbling O2 into a mixture of Fe(TMC)(OTf)2 and Cr(OTf)2 in NCCH3. Complex 1 also formed quantitatively by adding Cr(OTf)2 to [Fe(IV)(O)(TMC)(NCCH3)](2+). The proposed O2 activation mechanism involves the trapping of a Cr-O2 adduct by Fe(TMC)(OTf)2.

Spectroscopic identification of an Fe(III) center, not Fe(IV), in the crystalline Sc-O-Fe adduct derived from [Fe(IV)(O)(TMC)]²⁺

The apparent Sc(3+) adduct of [Fe(IV)(O)(TMC)](2+) (1, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) has been synthesized in amounts sufficient to allow its characterization by various spectroscopic techniques. Contrary to the earlier assignment of a +4 oxidation state for the iron center of 1, we establish that 1 has a high-spin iron(III) center based on its Mössbauer and EPR spectra and its quantitative reduction by 1 equiv of ferrocene to [Fe(II)(TMC)](2+). Thus, 1 is best described as a Sc(III)-O-Fe(III) complex, in agreement with previous DFT calculations (Swart, M. Chem. Commun. 2013, 49, 6650.). These results shed light on the interaction of Lewis acids with high-valent metal-oxo species.

Correlation between structural, spectroscopic, and reactivity properties within a series of structurally analogous metastable manganese(III)-alkylperoxo complexes

Manganese-peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn(III)-OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn(III)-OOR compounds extend the number of known end-on Mn(III)-(η(1)-peroxos) to six. The ligand backbone is shown to alter the metal-ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O-O and Mn···N(py,quin) distances), spectroscopic (E(πv*(O-O) → Mn CT band), ν(O-O)), and kinetic (ΔH(‡) and ΔS(‡)) parameters for these complexes provide compelling evidence for rate-limiting O-O bond cleavage. Products identified in the final reaction mixtures of Mn(III)-OOR decay are consistent with homolytic O-O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O-O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O-O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo πv*(O-O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function.

X-ray absorption near-edge spectroscopy in bioinorganic chemistry: Application to M-O2 systems

Metal K-edge X-ray absorption spectroscopy (XAS) has been extensively applied to bioinorganic chemistry to obtain geometric structure information on metalloprotein and biomimetic model complex active sites by analyzing the higher energy extended X-ray absorption fine structure (EXAFS) region of the spectrum. In recent years, focus has been on developing methodologies to interpret the lower energy K-pre-edge and rising-edge regions (XANES) and using it for electronic structure determination in complex bioinorganic systems. In this review, the evolution and progress of 3d-transition metal K-pre-edge and rising-edge methodology development is presented with particular focus on applications to bioinorganic systems. Applications to biomimetic transition metal-O₂ intermediates (M = Fe, Co, Ni and Cu) are reviewed, which demonstrate the power of the method as an electronic structure determination technique and its impact in understanding the role of supporting ligands in tuning the electronic configuration of transition metal-O₂ systems.

Oxygen atom transfer mediated by an iron(IV)/iron(II) macrocyclic complex containing pyridine and tertiary amine donors

A new non-heme iron model complex containing a high-spin iron(II) complexed with N-methylated pyridine-containing macrocycle was synthesized and crystallographically characterized. The complex generates peroxo- and high-valent iron-oxo intermediates in reactions with tert-butylhydroperoxide and isopropyl 2-iodoxybenzoate, respectively, allowing to gain insight into the formation and reactivity of enzyme-like intermediates related to biological oxygen activation. The formation and reactivity of these intermediate species were investigated by the stopped-flow methodology. The mechanism of oxygen transfer to organic substrates involving reaction of oxoiron(IV) intermediate was elucidated on the basis of spectroscopic and kinetic data. Incorporation of a pyridine ring into the macrocycle increased the reactivity of the Fe(IV)=O intermediates in comparison with polyamine tetraaza macrocyclic complexes: ferryl (Fe(IV)=O) species derived from 3 demonstrated electrophilic reactivity in transferring an oxygen atom to substituted triarylphosphines and to olefins (such as cyclooctene). However, iron(III) alkylperoxo intermediate was unreactive with cyclooctene.

Role of Fe(IV)-oxo intermediates in stoichiometric and catalytic oxidations mediated by iron pyridine-azamacrocycles

An iron(II) complex with a pyridine-containing 14-membered macrocyclic (PyMAC) ligand L1 (L1 = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene), 1, was prepared and characterized. Complex 1 contains low-spin iron(II) in a pseudo-octahedral geometry as determined by X-ray crystallography. Magnetic susceptibility measurements (298 K, Evans method) and Mössbauer spectroscopy (90 K, δ = 0.50(2) mm/s, ΔE(Q) = 0.78(2) mm/s) confirmed that the low-spin configuration of Fe(II) is retained in liquid and frozen acetonitrile solutions. Cyclic voltammetry revealed a reversible one-electron oxidation/reduction of the iron center in 1, with E(1/2)(Fe(III)/Fe(II)) = 0.49 V vs Fc(+)/Fc, a value very similar to the half-wave potentials of related macrocyclic complexes. Complex 1 catalyzed the epoxidation of cyclooctene and other olefins with H(2)O(2). Low-temperature stopped-flow kinetic studies demonstrated the formation of an iron(IV)-oxo intermediate in the reaction of 1 with H(2)O(2) and concomitant partial ligand oxidation. A soluble iodine(V) oxidant, isopropyl 2-iodoxybenzoate, was found to be an excellent oxygen atom donor for generating Fe(IV)-oxo intermediates for additional spectroscopic (UV-vis in CH(3)CN: λ(max) = 705 nm, ε ≈ 240 M(-1) cm(-1); Mössbauer: δ = 0.03(2) mm/s, ΔE(Q) = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (L1)Fe(IV)═O intermediate was established in rapid (k(2) = 26.5 M(-1) s(-1) for oxidation of PPh(3) at 0 °C), associative (ΔH(‡) = 53 kJ/mol, ΔS(‡) = -25 J/K mol) oxidation of substituted triarylphosphines (electron-donating substituents increased the reaction rate, with a negative value of Hammet's parameter ρ = -1.05). Similar double-mixing kinetic experiments demonstrated somewhat slower (k(2) = 0.17 M(-1) s(-1) at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (L1)Fe(IV)═O that could be generated from (L1)Fe(II) and H(2)O(2) or isopropyl 2-iodoxybenzoate. Independently determined rates of ferryl(IV) formation and its subsequent reaction with cyclooctene confirmed that the Fe(IV)-oxo species, (L1)Fe(IV)═O, is a kinetically competent intermediate for cyclooctene epoxidation with H(2)O(2) at room temperature. Partial ligand oxidation of (L1)Fe(IV)═O occurs over time in oxidative media, reducing the oxidizing ability of the ferryl species; the macrocyclic nature of the ligand is retained, resulting in ferryl(IV) complexes with Schiff base PyMACs. NH-groups of the PyMAC ligand assist the oxygen atom transfer from ferryl(IV) intermediates to olefin substrates.

Structure and reactivity of a mononuclear non-haem iron(III)-peroxo complex

Oxygen-containing mononuclear iron species--iron(III)-peroxo, iron(III)-hydroperoxo and iron(IV)-oxo--are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes. It has been difficult to generate synthetic analogues of these three active iron-oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution crystal structure of a mononuclear non-haem side-on iron(III)-peroxo complex, [Fe(III)(TMC)(OO)](+). We also report a series of chemical reactions in which this iron(III)-peroxo complex is cleanly converted to the iron(III)-hydroperoxo complex, [Fe(III)(TMC)(OOH)](2+), via a short-lived intermediate on protonation. This iron(III)-hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)](2+), via homolytic O-O bond cleavage of the iron(III)-hydroperoxo species. All three of these iron species--the three most biologically relevant iron-oxygen intermediates--have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)-hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)-oxo complex in C-H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)-hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes.

Four-coordinate trispyrazolylboratomanganese and -iron complexes with a pyrazolato co-ligand: syntheses and properties as oxidation catalysts

A series of complexes of the type [(Tp(R1,R2))M(X)] (Tp = trispyrazolylborato) with R(1)/R(2) combinations Me/tBu, Ph/Me, iPr/iPr, Me/Me and for M = Mn or Fe coordinating [Pz(Me,tBu)](-) (Pz = pyrazolato) or Cl(-) as co-ligand X has been synthesised. Although the chloride complexes were very unreactive and stable in air, the pyrazolato series was far more reactive in contact with oxidants like O(2) and tBuOOH. The [(Tp(R1,R2))M(Pz(Me,tBu))] complexes proved to be active pre-catalysts for the oxidation of cyclohexene with tBuOOH, reaching turnover frequencies (TOFs) ranging between moderate and good in comparison to other manganese catalysts. Cyclohexene-3-one and cyclohexene-3-ol were always found to represent the main products, with cyclohexene oxide occasionally formed as a side product. The ratios of the different oxidation products varied with the reaction conditions: in the case of a peroxide/alkene ratio of 4:1, considerably more ketone than alcohol was obtained and cyclohexene oxide formation was almost negligible, whereas a ratio of 1:10 led to a significant increase of the alcohol proportion and to the formation of at least small amounts of the epoxide. Pre-treatment of the dissolved [(Tp(R1,R2))M(Pz(Me,tBu))] pre-catalysts with O(2) led to product distributions and TOFs that were very similar to those found in the absence of O(2), so that it may be argued that tBuOOH and O(2) both lead to the same active species. The results of EPR spectroscopy and ESI-MS suggest that the initial product of the reaction of [(Tp(Me,Me))Mn(Pz(Me,tBu))] with O(2) contains a Mn(III)(O)(2)Mn(IV) core. Prolonged exposure to O(2) leads to a different dinuclear complex containing three O-bridges and resulting in different TOFs/product distributions. Analogous findings were made for other complexes and formation of these overoxidised products may explain the deviation of the catalytic performances if the reactions are carried out in an O(2) atmosphere.

Reactivity of copper(II)-alkylperoxo complexes

Copper(II) complexes 1a and 1b, supported by tridentate ligand bpa [bis(2-pyridylmethyl)amine] and tetradentate ligand tpa [tris(2-pyridylmethyl)amine], respectively, react with cumene hydroperoxide (CmOOH) in the presence of triethylamine in CH(3)CN to provide the corresponding copper(II) cumylperoxo complexes 2a and 2b, the formation of which has been confirmed by resonance Raman and ESI-MS analyses using (18)O-labeled CmOOH. UV-vis and ESR spectra as well as DFT calculations indicate that 2a has a 5-coordinate square-pyramidal structure involving CmOO(-) at an equatorial position and one solvent molecule at an axial position at low temperature (-90 °C), whereas a 4-coordinate square-planar structure that has lost the axial solvent ligand is predominant at higher temperatures (above 0 °C). Complex 2b, on the other hand, has a typical trigonal bipyramidal structure with the tripodal tetradentate tpa ligand, where the cumylperoxo ligand occupies an axial position. Both cumylperoxo copper(II) complexes 2a and 2b are fairly stable at ambient temperature, but decompose at a higher temperature (60 °C) in CH(3)CN. Detailed product analyses and DFT studies indicate that the self-decomposition involves O-O bond homolytic cleavage of the peroxo moiety; concomitant hydrogen-atom abstraction from the solvent is partially involved. In the presence of 1,4-cyclohexadiene (CHD), the cumylperoxo complexes react smoothly at 30 °C to give benzene as one product. Detailed product analyses and DFT studies indicate that reaction with CHD involves concerted O-O bond homolytic cleavage and hydrogen-atom abstraction from the substrate, with the oxygen atom directly bonded to the copper(II) ion (proximal oxygen) involved in the C-H bond activation step.

X-ray absorption spectroscopy structural investigation of early intermediates in the mechanism of DNA repair by human ABH2

Human ABH2 repairs DNA lesions by using an Fe(II)- and αKG-dependent oxidative demethylation mechanism. The structure of the active site features the facial triad of protein ligands consisting of the side chains of two histidine residues and one aspartate residue that is common to many non-heme Fe(II) oxygenases. X-ray absorption spectroscopy (XAS) of metallated (Fe and Ni) samples of ABH2 was used to investigate the mechanism of ABH2 and its inhibition by Ni(II) ions. The data are consistent with a sequential mechanism that features a five-coordinate metal center in the presence and absence of the α-ketoglutarate cofactor. This aspect is not altered in the Ni(II)-substituted enzyme, and both metals are shown to bind the cofactor. When the substrate is bound to the native Fe(II) complex with α-ketoglutarate bound, a five-coordinate Fe(II) center is retained that features an open coordination position for O(2) binding. However, in the case of the Ni(II)-substituted enzyme, the complex that forms in the presence of the cofactor and substrate is six-coordinate and, therefore, features no open coordination site for oxygen activation at the metal.

X-ray absorption spectroscopy and reactivity of thiolate-ligated Fe(III)-OOR complexes

The reaction of a series of thiolate-ligated iron(II) complexes [Fe(II)([15]aneN(4))(SC(6)H(5))]BF(4) (1), [Fe(II)([15]aneN(4))(SC(6)H(4)-p-Cl)]BF(4) (2), and [Fe(II)([15]aneN(4))(SC(6)H(4)-p-NO(2))]BF(4) (3) with alkylhydroperoxides at low temperature (-78 °C or -40 °C) leads to the metastable alkylperoxo-iron(III) species [Fe(III)([15]aneN(4))(SC(6)H(5))(OOtBu)]BF(4) (1a), [Fe(III)([15]aneN(4))(SC(6)H(4)-p-Cl)(OOtBu)]BF(4) (2a), and [Fe(III)([15]aneN(4))(SC(6)H(4)-p-NO(2))(OOtBu)]BF(4) (3a), respectively. X-ray absorption spectroscopy (XAS) studies were conducted on the Fe(III)-OOR complexes and their iron(II) precursors. The edge energy for the iron(II) complexes (∼7118 eV) shifts to higher energy upon oxidation by ROOH, and the resulting edge energies for the Fe(III)-OOR species range from 7121-7125 eV and correlate with the nature of the thiolate donor. Extended X-ray absorption fine structure (EXAFS) analysis of the iron(II) complexes 1-3 in CH(2)Cl(2) show that their solid state structures remain intact in solution. The EXAFS data on 1a-3a confirm their proposed structures as mononuclear, 6-coordinate Fe(III)-OOR complexes with 4N and 1S donors completing the coordination sphere. The Fe-O bond distances obtained from EXAFS for 1a-3a are 1.82-1.85 Å, significantly longer than other low-spin Fe(III)-OOR complexes. The Fe-O distances correlate with the nature of the thiolate donor, in agreement with the previous trends observed for ν(Fe-O) from resonance Raman (RR) spectroscopy, and supported by optimized geometries obtained from density functional theory (DFT) calculations. Reactivity and kinetic studies on 1a- 3a show an important influence of the thiolate donor.