Biomimetic hydrocarbon oxidation catalyzed by nonheme iron(III) complexes with peracids: evidence for an Fe(V)=O species

Mononuclear nonheme iron(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Fe(Me(2)bpb)Cl(H(2)O)] (3 a) and [Fe(bpc)Cl(H(2)O)] (4 a), were prepared by substitution reactions involving the previously synthesized iron(III) complexes [Et(3)NH][Fe(Me(2)bpb)Cl(2)] (3) and [Et(3)NH][Fe(bpc)Cl(2)] (4). Complexes 3 a and 4 a were characterized by IR and elemental analysis, and complex 3 a also by X-ray crystallography. Nonheme iron(III) complexes 3, 3 a, 4, and 4 a catalyze olefin epoxidation and alcohol oxidation on treatment with m-chloroperbenzoic acid. Pairwise comparisons of the reactivity of these complexes revealed that the nature of the axial ligand (Cl(-) versus H(2)O) influences the yield of oxidation products, whereas an electronic change in the supporting chelate ligand has little effect. Hydrocarbon oxidation by these catalysts was proposed to involve an iron(V) oxo species which is formed on heterolytic O-O bond cleavage of an iron acylperoxo intermediate (FeOOC(O)R). Evidence for this iron(V) oxo species was derived from KIE (k(H)/k(D)) values, H(2) (18)O exchange experiments, and the use of peroxyphenylacetic acid (PPAA) as the peracid. Our results suggest that an Fe(V)=O moiety can form in a system wherein the supporting chelate ligand comprises a mixture of neutral and anionic nitrogen donors. This work is relevant to the chemistry of mononuclear nonheme iron enzymes that are proposed to oxidize organic substrates via reaction pathways involving high-valent iron oxo species.     

Enhanced selectivity in non-heme iron catalysed oxidation of alkanes with peracids: evidence for involvement of Fe(IV)=O species

Catalytic alkane oxidation with high selectivity using peracids and an (N4Py)Fe complex is presented and the role of [(N4Py)Fe(IV)=O]2+ species, molecular oxygen and hydroxyl radicals in the catalysis is discussed.     

Biomimetic aryl hydroxylation derived from alkyl hydroperoxide at a nonheme iron center. Evidence for an Fe(IV)=O oxidant

Many nonheme iron-dependent enzymes activate dioxygen to catalyze hydroxylations of arene substrates. Key features of this chemistry have been developed from complexes of a family of tetradentate tripodal ligands obtained by modification of tris(2-pyridylmethyl)amine (TPA) with single alpha-arene substituents. These included the following: -C(6)H(5) (i.e., 6-PhTPA), L(1); -o-C(6)H(4)D, o-d(1)-L(1); -C(6)D(5), d(5)-L(1); -m-C(6)H(4)NO(2), L(2); -m-C(6)H(4)CF(3), L(3); -m-C(6)H(4)Cl, L(4); -m-C(6)H(4)CH(3), L(5); -m-C(6)H(4)OCH(3), L(6); -p-C(6)H(4)OCH(3), L(7). Additionally, the corresponding ligand with one alpha-phenyl and two alpha-methyl substituents (6,6-Me(2)-6-PhTPA, L(8)) was also synthesized. Complexes of the formulas [(L(1))Fe(II)(NCCH(3))(2)](ClO(4))(2), [(L(n)())Fe(II)(OTf)(2)] (n = 1-7, OTf = (-)O(3)SCF(3)), and [(L(8))Fe(II)(OTf)(2)](2) were obtained and characterized by (1)H NMR and UV-visible spectroscopies and by X-ray diffraction in the cases of [(L(1))Fe(II)(NCCH(3))(2)](ClO(4))(2), [(L(6))Fe(II)(OTf)(2)], and [(L(8))Fe(II)(OTf)(2)](2). The complexes react with tert-butyl hydroperoxide ((t)()BuOOH) in CH(3)CN solutions to give iron(III) complexes of ortho-hydroxylated ligands. The product complex derived from L(1) was identified as the solvated monomeric complex [(L(1)O(-))Fe(III)](2+) in equilibrium with its oxo-bridged dimer [(L(1)O(-))(2)Fe(III)(2)(mu(2)-O)](2+), which was characterized by X-ray crystallography as the BPh(4)(-) salt. The L(8) product was also an oxo-bridged dimer, [(L(8)O(-))(2)Fe(III)(2)(mu(2)-O)](2+). Transient intermediates were observed at low temperature by UV-visible spectroscopy, and these were characterized as iron(III) alkylperoxo complexes by resonance Raman and EPR spectroscopies for L(1) and L(8). [(L(1))Fe(II)(OTf)(2)] gave rise to a mixture of high-spin (S = 5/2) and low-spin (S = 1/2) Fe(III)-OOR isomers in acetonitrile, whereas both [(L(1))Fe(OTf)(2)] in CH(2)Cl(2) and [(L(8))Fe(OTf)(2)](2) in acetonitrile afforded only high-spin intermediates. The L(1) and L(8) intermediates both decomposed to form respective phenolate complexes, but their reaction times differed by 3 orders of magnitude. In the case of L(1), (18)O isotope labeling indicated that the phenolate oxygen is derived from the terminal peroxide oxygen via a species that can undergo partial exchange with exogenous water. The iron(III) alkylperoxo intermediate is proposed to undergo homolytic O-O bond cleavage to yield an oxoiron(IV) species as an unobserved reactive intermediate in the hydroxylation of the pendant alpha-aryl substituents. The putative homolytic chemistry was confirmed by using 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH) as a probe, and the products obtained in the presence and in the absence of air were consistent with formation of alkoxy radical (RO(*)). Moreover, when one ortho position was labeled with deuterium, no selectivity was observed between hydroxylation of the deuterated and normal isotopomeric ortho sites, but a significant 1,2-deuterium shift ("NIH shift") occurred. These results provide strong mechanistic evidence for a metal-centered electrophilic oxidant, presumably an oxoiron(IV) complex, in these arene hydroxylations and support participation of such a species in the mechanisms of the nonheme iron- and pterin-dependent aryl amino acid hydroxylases.     

A density functional study on a biomimetic non-heme iron catalyst: insights into alkane hydroxylation by a formally HO-FeV=O oxidant

The reactivity of [HO-(tpa)Fe(V)=O] (TPA=tris(2-pyridylmethyl)amine), derived from O-O bond heterolysis of its [H(2)O-(tpa)Fe(III)-OOH] precursor, was explored by means of hybrid density functional theory. The mechanism for alkane hydroxylation by the high-valent iron-oxo species invoked as an intermediate in Fe(tpa)/H(2)O(2) catalysis was investigated. Hydroxylation of methane and propane by HO-Fe(V)=O was studied by following the rebound mechanism associated with the heme center of cytochrome P450, and it is demonstrated that this species is capable of stereospecific alkane hydroxylation. The mechanism proposed for alkane hydroxylation by HO-Fe(V)=O accounts for the experimentally observed incorporation of solvent water into the products. An investigation of the possible hydroxylation of acetonitrile (i.e., the solvent used in the experiments) shows that the activation energy for hydrogen-atom abstraction by HO-Fe(V)=O is rather high and, in fact, rather similar to that of methane, despite the similarity of the H-CH(2)CN bond strength to that of the secondary C-H bond in propane. This result indicates that the kinetics of hydrogen-atom abstraction are strongly affected by the cyano group and rationalizes the lack of experimental evidence for solvent hydroxylation in competition with that of substrates such as cyclohexane.     

Olefin cis-dihydroxylation versus epoxidation by non-heme iron catalysts: two faces of an Fe(III)-OOH coin

The oxygenation of carbon-carbon double bonds by iron enzymes generally results in the formation of epoxides, except in the case of the Rieske dioxygenases, where cis-diols are produced. Herein we report a systematic study of olefin oxidations with H(2)O(2) catalyzed by a group of non-heme iron complexes, i.e., [Fe(II)(BPMEN)(CH(3)CN)(2)](2+) (1, BPMEN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) and [Fe(II)(TPA)(CH(3)CN)(2)](2+) (4, TPA = tris(2-pyridylmethyl)amine) and their 6- and 5-methyl-substituted derivatives. We demonstrate that olefin epoxidation and cis-dihydroxylation are different facets of the reactivity of a common Fe(III)-OOH intermediate, whose spin state can be modulated by the electronic and steric properties of the ligand environment. Highly stereoselective epoxidation is favored by catalysts with no more than one 6-methyl substituent, which give rise to low-spin Fe(III)-OOH species (category A). On the other hand, cis-dihydroxylation is favored by catalysts with more than one 6-methyl substituent, which afford high-spin Fe(III)-OOH species (category B). For catalysts in category A, both the epoxide and the cis-diol product incorporate (18)O from H(2)(18)O, results that implicate a cis-H(18)O-Fe(V)=O species derived from O-O bond heterolysis of a cis-H(2)(18)O-Fe(III)-OOH intermediate. In contrast, catalysts in category B incorporate both oxygen atoms from H(2)(18)O(2) into the dominant cis-diol product, via a putative Fe(III)-eta(2)-OOH species. Thus, a key feature of the catalysts in this family is the availability of two cis labile sites, required for peroxide activation. The olefin epoxidation and cis-dihydroxylation studies described here not only corroborate the mechanistic scheme derived from our earlier studies on alkane hydroxylation by this same family of catalysts (Chen, K.; Que, L, Jr. J. Am. Chem. Soc. 2001, 123, 6327) but also further enhance its credibility. Taken together, these reactions demonstrate the catalytic versatility of these complexes and provide a rationale for Nature's choice of ligand environments in biocatalysts that carry out olefin oxidations.     

Olefin cis-dihydroxylation with bio-inspired iron catalysts. evidence for an Fe(II)/Fe(IV) catalytic cycle

Iron(II) complexes of a series of N-acylated dipyridin-2-ylmethylamine ligands (R-DPAH) have been investigated as catalysts for the cis-dihydroxylation of olefins to model the action of Rieske dioxygenases that catalyze arene cis-dihydroxylation. The Rieske dioxygenases have a mononuclear iron active site coordinated to a 2-histidine-1-carboxylate facial triad motif. The R-DPAH ligands are designed to provide a facial N,N,O-ligand set that mimics the enzyme active site. The iron(II) complexes of the R-DPAH ligands activate H(2)O(2) to effect the oxidation of olefin substrates into cis-diol products. As much as 90% of the H(2)O(2) oxidant is converted into cis-diol, but a large excess of olefin is required to achieve the high conversion efficiency. Reactivity and mechanistic comparisons with the previously characterized Fe(TPA)/H(2)O(2) catalyst/oxidant combination (TPA = tris(pyridin-2-ylmethyl)amine) lead us to postulate an Fe(II)/Fe(IV) redox cycle for the Fe(R-DPAH) catalysts in which an Fe(IV)(OH)(2) oxidant carries out the cis-hydroxylation of olefins. This hypothesis is supported by three sets of observations: (a) the absence of a lag phase in the conversion of the H(2)O(2) oxidant into a cis-diol product, thereby excluding the prior oxidation of the Fe(II) catalyst to an Fe(III) derivative as established for the Fe(TPA) catalyst; (b) the incorporation of H(2)(18)O into the cis-diol product, thereby requiring O-O bond cleavage to occur prior to cis-diol formation; and (c) the formation of cis-diol as the major product of cyclohexene oxidation, rather than the epoxide or allylic alcohol products more commonly observed in metal-catalyzed oxidations of cyclohexene, implicating an oxidant less prone to oxo transfer or H-atom abstraction.     

Mechanism of benzene hydroxylation by high-valent bare Fe(IV)=O2+: explicit electronic structure analysis

The conversion of benzene to phenol by high-valent bare FeO(2+) was comprehensively explored using a density functional theory method. The conductor-like screen model (COSMO) was used to mimic the role of solvent effect with acetonitrile chosen as the solvent. Two radical mechanisms and one oxygen insertion mechanism were tested for this conversion. The first radical mechanism can also be named as the concerted mechanism in which the hydrogen-atom abstraction process is accomplished via a four-centered transition state. The second radical mechanism is initiated by a direct hydrogen-atom abstraction with a collinear C-H-O transition structure. It is actually the same as the well-accepted rebound mechanism for the C-H bond activation by heme and nonheme iron-oxo catalysts. The third is an oxygen insertion mechanism which is essentially an aromatic electrophilic attack leading to an arenium σ-complex intermediate. The formation of a precomplex with an η(4) coordinate environment in the first radical mechanism is energetically more favorable. However, the relatively lower activation energy barrier of the oxygen insertion mechanism compared to the radical ones makes it highly competitive if the Fe=O(2+) collides with benzene in the proper orientation. The detailed potential energy surfaces also indicate that the second radical mechanism, i.e., the benzene C-H bond activation through the rebound mechanism, is less favorable. This thorough theoretical study, especially the electronic structure analysis, may offer very important clues for understanding and studying C-H bond activation by iron-based catalysts and enzymatic reactions in protein active pockets.     

First direct evidence for stereospecific olefin epoxidation and alkane hydroxylation by an oxoiron(IV) porphyrin complex

We report in this study that an oxoiron(IV) porphyrin complex bearing electron-deficient porphyrin ligand, (TPFPP)FeIV=O (TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion), shows reactivities similar to those found in oxoiron(IV) porphyrin pi-cation radicals. In the epoxidation of olefins by the (TPFPP)FeIV=O complex, epoxides were yielded as major products; cyclohexene oxide was the sole product formed in the epoxidation of cyclohexene, and stilbenes were stereospecifically oxidized to the corresponding epoxide products. More striking results were obtained in alkane hydroxylation reactions; the hydroxylation of adamantane afforded a high degree of selectivity for tertiary C-H bonds over secondary C-H bonds, and the hydroxylation of cis-1,2-dimethylcyclohexane yielded a tertiary alcohol product with >99% retention of stereochemistry. The latter result demonstrates that an oxoiron(IV) porphyrin complex hydroxylates alkanes with a high stereospecificity. Isotope labeling studies performed with H218O and 18O2 in the olefin epoxidation and alkane hydroxylation reactions demonstrated that oxygen atoms in oxygenated products derived from the oxoiron(IV) porphyrin complex.     

Alkane hydroxylation catalyzed by metalloporhyrins : evidence for different active oxygen species with alkylhydroperoxides and iodosobenzene as oxidants.

A comparative study of cyclohexane and n-heptane hydroxylations by cumylhydroperoxide and iodosobenzene, catalyzed by various metalloporphyrins, indicates that different active oxygen species, presumably the cumyloxy radical and a metal-oxo intermediate, are involved in these reactions.     

Unusual efficiency of a non-heme iron complex as catalyst for the hydroxylation of aromatic compounds by hydrogen peroxide: comparison with iron porphyrins

The non-heme iron complex, Fe(TPAA = tris-〚N-(2-pyridylmethyl)-2-aminoethyl〛amine)(ClO₄)₂, is a bad catalyst for the epoxidation of alkenes such as cyclooctene, cyclohexene and cis-stilbene and for the hydroxylation of alkanes such as adamantane by H₂O₂, when compared to the iron porphyrin Fe〚TDCPN₅P = meso-tetra-(2,6-dichlorophenyl)-β-pentanitroporphyrin〛Cl. At the opposite, Fe(TPAA)(ClO₄)₂ is a much better catalyst for the hydroxylation of arenes by H₂O₂; in its presence, anisole, toluene, ethylbenzene, benzene and chlorobenzene are transformed into the corresponding phenols, with respective yields of 53, 17, 24, 22 and 13% based on H₂O₂. Interestingly, in Fe(TPAA)-catalysed oxidations of anisole, toluene and ethylbenzene by H₂O₂, hydroxylation of the aromatic ring is by far the major reaction, even when compared to usually favoured reactions such as benzylic oxidation and oxidative demethylation.     

Amide-based nonheme cobalt(III) olefin epoxidation catalyst: partition of multiple active oxidants Co(V)=O, Co(IV)=O, and Co(III)-OO(O)CR

A mononuclear nonheme cobalt(III) complex of a tetradentate ligand containing two deprotonated amide moieties, [Co(bpc)Cl(2)][Et(4)N] (1; H(2)bpc = 4,5-dichloro-1,2-bis(2-pyridine-2-carboxamido)benzene), was prepared and then characterized by elemental analysis, IR, UV/Vis, and EPR spectroscopy, and X-ray crystallography. This nonheme Co(III) complex catalyzes olefin epoxidation upon treatment with meta-chloroperbenzoic acid. It is proposed that complex 1 shows partitioning between the heterolytic and homolytic cleavage of an O-O bond to afford Co(V)=O (3) and Co(IV)=O (4) intermediates, proposed to be responsible for the stereospecific olefin epoxidation and radical-type oxidations, respectively. Moreover, under extreme conditions, in which the concentration of an active substrate is very high, the Co-OOC(O)R (2) species is a possible reactive species for epoxidation. Furthermore, partitioning between heterolysis and homolysis of the O-O bond of the intermediate 2 might be very sensitive to the nature of the solvent, and the O-O bond of the Co-OOC(O)R species might proceed predominantly by heterolytic cleavage, even in the presence of small amounts of protic solvent, to produce a discrete Co(V) ?O intermediate as the dominant reactive species. Evidence for these multiple active oxidants was derived from product analysis, the use of peroxyphenylacetic acid as the peracid, and EPR measurements. The results suggest that a less accessible Co(V)=O moiety can form in a system in which the supporting chelate ligand comprises a mixture of neutral and anionic nitrogen donors.     

Synthetic models for the cysteinate-ligated non-heme iron enzyme superoxide reductase: observation and structural characterization by XAS of an Fe(III)-OOH intermediate

Superoxide reductases (SORs) belong to a new class of metalloenzymes that degrade superoxide by reducing it to hydrogen peroxide. These enzymes contain a catalytic iron site that cycles between the Fe(II) and Fe(III) states during catalysis. A key step in the reduction of superoxide has been suggested to involve HO(2) binding to Fe(II), followed by innersphere electron transfer to afford an Fe(III)-OO(H) intermediate. In this paper, the mechanism of the superoxide-induced oxidation of a synthetic ferrous SOR model ([Fe(II)(S(Me2)N(4)(tren))](+) (1)) to afford [Fe(III)(S(Me2)N(4)(tren)(solv))](2+) (2-solv) is reported. The XANES spectrum shows that 1 remains five-coordinate in methanolic solution. Upon reaction of 1 with KO(2) in MeOH at -90 degrees C, an intermediate (3) is formed, which is characterized by a LMCT band centered at 452(2780) nm, and a low-spin state (S = 1/2), based on its axial EPR spectrum (g(perpendicular) = 2.14; g(parallel) = 1.97). Hydrogen peroxide is detected in this reaction, using both (1)H NMR spectroscopy and a catalase assay. Intermediate 3 is photolabile, so, in lieu of a Raman spectrum, IR was used to obtain vibrational data for 3. At low temperatures, a nu(O-O) Fermi doublet is observed in the IR at 788(2) and 781(2) cm(-)(1), which collapses into a single peak at 784 cm(-1) upon the addition of D(2)O. This vibrational peak diminishes in intensity over time and essentially disappears after 140 s. When 3 is generated using an (18)O-labeled isotopic mixture of K(18)O(2)/K(16)O(2) (23.28%), the vibration centered at 784 cm(-1) shifts to 753 cm(-1). This new vibrational peak is close to that predicted (740 cm(-1)) for a diatomic (18)O-(18)O stretch. In addition, a nu(O-O) vibrational peak assigned to free hydrogen peroxide is also observed (nu(O-O) = 854 cm(-1)) throughout the course of the reaction between Fe(II)-1 and superoxide and is strongest after 100 s. XAS studies indicate that 3 possesses one sulfur scatterer at 2.33(2) A and four nitrogen scatterers at 2.01(1) A. Addition of two Fe-O shells, each containing one oxygen, one at 1.86(3) A and one at 2.78(3) A, improved the EXAFS fits, suggesting that 3 is an end-on peroxo or hydroperoxo complex, [Fe(III)(S(Me2)N(4)(tren))(OO(H))](+). Upon warming above -50 degrees C, 3 is converted to 2-MeOH. In methanol and methanol:THF (THF = tetrahydrofuran) solvent mixtures, 2-MeOH is characterized by a LMCT band at lambda(max) = 511(1765) nm, an intermediate spin-state (S = 3/2), and, on the basis of EXAFS, a relatively short Fe-O bond (assigned to a coordinated methanol or methoxide) at 1.94(10) A. Kinetic measurements in 9:1 THF:MeOH at 25 degrees C indicate that 3 is formed near the diffusion limit upon addition of HO(2) to 1 and converts to 2-MeOH at a rate of 65(1) s(-1), which is consistent with kinetic studies involving superoxide oxidation of the SOR iron site.     

Efficient hydroxylation of aromatic compounds catalyzed by an iron(II) complex with H2O2

A mononuclear iron(II) complex, Et₄N[Fe(C₁₀H₆NO₂)₃], coordinated by three 1‐nitroso‐2‐naphtholate ligands in a fac‐N₃O₃ geometry, was initiated to catalyze the direct hydroxylation of aromatic compounds to phenols in the presence of H₂O₂ under mild conditions. Various reaction parameters, including the catalyst dosage, temperature, mole ratio of H₂O₂ to benzene, reaction time and solvents which could affect the hydroxylation activity of the catalyst, were investigated systematically for benzene hydroxylation to obtain ideal benzene conversion and high phenol distribution. Under the optimum conditions, the benzene conversion was 10.2% and only phenol was detected. The catalyst was also found to be active towards hydroxylation of other aromatic compounds with high substrate conversions. The hydroxyl radical formed due to the reaction of the catalyst and H₂O₂ was determined to be the crucial active intermediate in the hydroxylation. A rational pathway for the formation of the hydroxyl radical was proposed and justified by the density functional theory calculations. Copyright © 2014 John Wiley & Sons, Ltd.     

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 (Fe^III–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 Fe^III–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.

A photoinduced iron(III)-hydroperoxo intermediate (Fe^III-OOH) was trapped by bypassing the charge accumulation process, that triggers the oxygen atom transfer reaction to an alkene with O₂ as sole oxygen source in water.     

A density functional study of O-O bond cleavage for a biomimetic non-heme iron complex demonstrating an Fe(v)-intermediate

Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H(2)O(2) is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(III)(TPA)-OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an Fe(V) oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por.)Fe(IV)=O](+1) in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O-O bond cleavage, yielding the two oxidants Fe(IV)=O and the OH. radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been performed for the O-O cleavage in the Fe(III)(TPA)-alkylperoxide species. In this case, the barrier for O-O homolysis is found to be slightly lower, leading to loss of stereospecificity and supporting the experimental conclusion that this is the preferred pathway for alkylperoxide oxidants. The difference between hydroperoxide and alkylperoxide as oxidant derives from the higher O-O bond strength for hydrogen peroxide (by 8.0 kcal/mol).     

EXAFS spectroscopic evidence for an Fe=O unit in the Fe(IV) intermediate observed during oxygen activation by taurine:alpha-ketoglutarate dioxygenase

The Fe(II)- and alpha-ketoglutarate-dependent dioxygenases catalyze hydroxylation reactions of considerable biomedical and environmental significance. Recently, the first oxidized iron intermediate in the reaction of a member of this family, taurine:alpha-ketoglutarate dioxygenase (TauD), was detected and shown to be a high-spin Fe(IV) complex. In this study we have used X-ray absorption spectroscopy to demonstrate the presence of a short (1.62 A) interaction between the iron and one of its ligands in the Fe(IV) intermediate but not in the Fe(II) starting complex. The detection of this interaction strongly corroborates the hypothesis that the intermediate contains an Fe=O structural motif.