To rebound or dissociate? This is the mechanistic question in C-H hydroxylation by heme and nonheme metal-oxo complexes

Hydrogen Atom Transfer Reactions of Mononuclear Nonheme Metal-Oxygen Intermediates

Molecular oxygen (O₂), the greenest oxidant, is kinetically stable in the oxidation of organic substrates due to its triplet ground state. In nature, O₂ is reduced by two electrons with two protons to produce hydrogen peroxide (H₂O₂) and by four electrons with four protons to produce water (H₂O) by oxidase and oxygenase metalloenzymes. In the process of the two-electron/two-proton and four-electron/four-proton reduction of O₂ by metalloenzymes and their model compounds, metal-oxygen intermediates, such as metal-superoxido, -peroxido, -hydroperoxido, and -oxido species, are generated depending on the numbers of electrons and protons involved in the O₂ activation reactions. The one-electron reduction of metal-oxygen intermediates is coupled with the binding of one proton. Such a hydrogen atom transfer (HAT) is defined as proton-coupled electron transfer (PCET), and there is a mechanistic dichotomy whether HAT occurs via a concerted PCET pathway or stepwise pathways [i.e., electron transfer followed by proton transfer (ET/PT) or proton transfer followed by electron transfer (PT/ET)]. The metal-oxygen intermediates formed are oxidants that can abstract a hydrogen atom (H-atom) from substrate C-H bonds. The H-atom abstraction from substrate C-H bonds by the metal-oxygen intermediates can also occur via a concerted PCET or stepwise PCET pathways. In the PCET reactions, a proton can be provided not only by the substrate itself but also by an acid that is added to a reaction solution. This Account describes the reactivities of metal-oxygen intermediates, such as metal-superoxido, -peroxido, -hydroperoxido, and -oxido complexes, in HAT reactions, focusing on the mechanisms of PCET reactions of metal-oxygen intermediates and on the mechanistic dichotomy of concerted versus stepwise pathways. Recent developments in the reactivity studies of Cr-, Fe-, and Cu-superoxido complexes in H-atom and hydride transfer reactions are discussed. Reactivities of an iron(III)-hydroperoxido complex and an iron(III)-peroxido complex binding redox-inactive metal ions are also summarized briefly. Mononuclear nonheme iron(IV)- and manganese(IV)-oxido complexes have shown high reactivities in HAT reactions, and their chemistry in PCET reactions is discussed intensively. Acid-catalyzed HAT reactions of metal-oxygen intermediates are also discussed to demonstrate a unified driving force dependence of logarithm of the rate constants of acid-catalyzed oxidation of various substrates by an iron(IV)-oxido complex and that of PCET from one-electron donors to the iron(IV)-oxido complex. PCET reactions of metal-oxygen intermediates are shown to proceed via a concerted pathway (one-step HAT) or a stepwise ET/PT pathway depending on the ET and PCET driving forces (-Δ G). The boundary conditions between concerted versus stepwise PCET pathways are clarified to demonstrate a switchover of the mechanisms only by changing the reaction temperature in the boundary conditions. This Account summarizes recent developments in the HAT reactions by synthetic mononuclear nonheme metal-oxygen intermediates over the past 10 years.

Oxidative Rearrangement Mechanism of Pentalenolactone F Catalyzed by Cytochrome P450 CYP161C2 (PntM)

The CYP161C2 (PntM) from Streptomyces arenae is a member of the cytochrome P450 enzymes, which catalyzes the unusual oxidative rearrangement of pentalenolactone F (1) to the sesquiterpenoid antibiotic pentalenolactone (3). On the basis of the crystal structure of PntM bound with substrate, quantum mechanical/molecular mechanics (QM/MM) calculations have been performed to explore the detailed mechanism of PntM-catalyzed oxidative rearrangement. The conversion from pentalenolactone F (1) to pentalenolactone (3) involves the stereospecific removal of the H-1 si from 1, the syn-1,2-migration of the 2 si methyl group, and the antarafacial loss of H-3 re. The abstraction of H-1 si by Cpd I is calculated to be rate limiting with an energy barrier of 20.3 kcal/mol, which basically agrees with the estimated free energy barrier from experiments (18.6 kcal/mol). It is the unfavorable geometry of Fe-OH-C1 that blocks the oxygen rebound reaction, and the subsequent intramolecular syn-1,2-methyl migration is accompanied by an electron transfer from the substrate to the porphyrin ring via an Fe-OH group, generating the carbocation intermediate. Owing to the positive charge, the intermediate can easily lose a proton to form the final products. Our calculation results indicate that both the carboxyl group of porphyrin and Fe-OH can act as bases to accept the proton of the substrate. The target product pentalenolactone and the three isomeric byproducts correspond to four different modes of deprotonation.

Relationship between Hydrogen-Atom Transfer Driving Force and Reaction Rates for an Oxomanganese(IV) Adduct

Hydrogen atom transfer (HAT) reactions by high-valent metal-oxo intermediates are important in both biological and synthetic systems. While the HAT reactivity of Fe^IV-oxo adducts has been extensively investigated, studies of analogous Mn^IV-oxo systems are less common. There are several recent reports of Mn^IV-oxo complexes, supported by neutral pentadentate ligands, capable of cleaving strong C-H bonds at rates approaching those of analogous Fe^IV-oxo species. In this study, we provide a thorough analysis of the HAT reactivity of one of these Mn^IV-oxo complexes, [Mn^IV(O)(2pyN2Q)]²⁺, which is supported by an N5 ligand with equatorial pyridine and quinoline donors. This complex is able to oxidize the strong C-H bonds of cyclohexane with rates exceeding those of Fe^IV-oxo complexes with similar ligands. In the presence of excess oxidant (iodosobenzene), cyclohexane oxidation by [Mn^IV(O)(2pyN2Q)]²⁺ is catalytic, albeit with modest turnover numbers. Because the rate of cyclohexane oxidation by [Mn^IV(O)(2pyN2Q)]²⁺ was faster than that predicted by a previously published Bells-Evans-Polanyi correlation, we expanded the scope of this relationship by determining HAT reaction rates for substrates with bond dissociation energies spanning 20 kcal/mol. This extensive analysis showed the expected correlation between reaction rate and the strength of the substrate C-H bond, albeit with a shallow slope. The implications of this result with regard to Mn^IV-oxo and Fe^IV-oxo reactivity are discussed.

Stereoselective Metabolism of Omeprazole by Cytochrome P450 2C19 and 3A4: Mechanistic Insights from DFT Study

The efficacy of S-omeprazole as a proton pump inhibitor compared with that of its enantiomer R-omeprazole is studied using density functional theoretical calculations. The pharmacokinetic studies suggest that the efficacy of S-omeprazole presumably depends on metabolic pathway and excretion from the human body. The density functional theory calculations at SMD_water-B3LYP-D3/6-311+G(d,p)/LANL2DZ//B3LYP/6-31G(d)/LANL2DZ with triradicaloid model active species, [Por^•+Fe^IV(SH)O], of CYP2C19 enzyme with high-spin quartet and low-spin doublet states demonstrate C-H bond activation mechanism through a two-state rebound process for the hydroxylation of R-omeprazole and S-omeprazole. The calculated activation free energy barriers for the hydrogen abstraction are 15.7 and 17.5 kcal/mol for R-omeprazole and S-omeprazole, respectively. The hydroxylation of R-omeprazole and S-omeprazole is thermodynamically favored; however, the hydroxylated intermediate of S-omeprazole further disintegrates to metabolite 5- O-desmethylomeprazole with a higher kinetic barrier. We have examined the sulfoxidation of S-omeprazole to omeprazole sulfone metabolite by CYP3A4, and the observed activation free energy barrier is 9.9 kcal/mol. The computational results reveal that CYP2C19 exclusively metabolizes R-omeprazole to hydroxyomeprazole, which is hydrophilic and can easily excrete, whereas CYP3A4 metabolizes S-omeprazole to lipophilic sulfone; hence, the excretion of this metabolite would be relatively slower from the body. The spin density analysis and molecular orbital analysis performed using biorthogonalization calculations indicate that R-omeprazole favors high-spin pathway for metabolism process whereas S-omeprazole prefers the low-spin pathway.

Observation of Radical Rebound in a Mononuclear Nonheme Iron Model Complex

A nonheme iron(III) terminal methoxide complex, [FeIII(N3PyO2Ph)(OCH3)]ClO4, was synthesized. Reaction of this complex with the triphenylmethyl radical (Ph3C•) leads to formation of Ph3COCH3 and the one-electron-reduced iron(II) center, as seen by UV-vis, EPR, 1H NMR, and Mössbauer spectroscopy. These results indicate that homolytic Fe-O bond cleavage occurs together with C-O bond formation, providing a direct observation of the "radical rebound" process proposed for both biological and synthetic nonheme iron centers.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Density Functional Theory Study on the Demethylation Reaction between Methylamine, Dimethylamine, Trimethylamine, and Tamoxifen Catalyzed by a Fe(IV)-Oxo Porphyrin Complex

In this work, we studied computationally the N-demethylation reaction of methylamine, dimethylamine, and trimethylamine as archetypal examples of primary, secondary, and tertiary amines catalyzed by high-field low-spin Fe-containing enzymes such as cytochromes P450. Using DFT calculations, we found that the expected C-H hydroxylation process was achieved for trimethylamine. When dimethylamine and methylamine were studied, two different reaction mechanisms (C-H hydroxylation and a double hydrogen atom transfer) were computed to be energetically accessible and both are equally preferred. Both processes led to the formation of formaldehyde and the N-demethylated substrate. Finally, as an illustrative example, the relative contribution of the three primary oxidation routes of tamoxifen was rationalized through energetic barriers obtained from density functional calculations and docking experiments involving CYP3A4 and CYP2D6 isoforms. We found that the N-demethylation process was the intrinsically favored one, whereas other oxidation reactions required most likely preorganization imposed by the residues close to the active sites.

A competing, dual mechanism for catalytic direct benzene hydroxylation from combined experimental-DFT studies

A dual mechanism for direct benzene catalytic hydroxylation is described. Experimental studies and DFT calculations have provided a mechanistic explanation for the acid-free, Tp ^x Cu-catalyzed hydroxylation of benzene with hydrogen peroxide (Tp ^x = hydrotrispyrazolylborate ligand). In contrast with other catalytic systems that promote this transformation through Fenton-like pathways, this system operates through a copper-oxyl intermediate that may interact with the arene ring following two different, competitive routes: (a) electrophilic aromatic substitution, with the copper-oxyl species acting as the formal electrophile, and (b) the so-called rebound mechanism, in which the hydrogen is abstracted by the Cu-O moiety prior to the C-O bond formation. Both pathways contribute to the global transformation albeit to different extents, the electrophilic substitution route seeming to be largely favoured.

Direct Observation of Oxygen Rebound with an Iron-Hydroxide Complex

The rebound mechanism for alkane hydroxylation was invoked over 40 years ago to help explain reactivity patterns in cytochrome P450, and subsequently has been used to provide insight into a range of biological and synthetic systems. Efforts to model the rebound reaction in a synthetic system have been unsuccessful, in part because of the challenge in preparing a suitable metal-hydroxide complex at the correct oxidation level. Herein we report the synthesis of such a complex. The reaction of this species with a series of substituted radicals allows for the direct interrogation of the rebound process, providing insight into this uniformly invoked, but previously unobserved process.

meso-Tetraphenylporphyrin Iron Chloride Catalyzed Selective Oxidative Cross-Coupling of Phenols

A novel catalytic system for oxidative cross-coupling of readily oxidized phenols with poor nucleophilic phenolic partners based on an iron meso-tetraphenylporphyrin chloride (Fe[TPP]Cl) complex in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) was developed. The unique chemoselectivity of this reaction is attributed to the coupling between a liberated phenoxyl radical with an iron-ligated phenolic coupling partner. The conditions are scalable for preparing a long list of unsymmetrical biphenols assembled from a less reactive phenolic unit substituted with alkyl or halide groups.

Selective photocatalytic hydroxylation and epoxidation reactions by an iron complex using water as the oxygen source

Iron complex catalysed selective and efficient photocatalytic hydroxylation and epoxidation reactions using water as the oxygen atom source has been reported.

The iron complex [(bTAML)Fe^III–OH₂]^– (1) selectively catalyses the photocatalytic hydroxylation and epoxidation reactions of alkanes and alkenes, respectively, using water as the oxygen-atom source. Upon the oxidation of unactivated alkanes, which included several substrates including natural products, hydroxylation was observed mostly at the 3° C–H bonds with 3° : 2° selectivity up to ∼100 : 1. When alkenes were used as the substrates, epoxides were predominantly formed with high yields. In the presence of H₂¹⁸O, more than 90% of the ¹⁸O-labelled oxygen atoms were incorporated into the hydroxylated and epoxide product indicating that water was the primary oxygen source. Mechanistic studies indicate the formation of an active [{(bTAML)Fe^IV}₂-μ-oxo]^2– (2) dimer from the starting complex 1via PCET. The subsequent disproportionation of 2 upon addition of substrate, leading to the formation of Fe^V(O), renders the high selectivity observed in these reactions.

Selective C-H Bond Oxidation Catalyzed by the Fe-bTAML Complex: Mechanistic Implications

Nonheme iron complexes bearing tetradentate N-atom-donor ligands with cis labile sites show great promise for chemoselective aliphatic C-H hydroxylation. However, several challenges still limit their widespread application. We report a mechanism-guided development of a peroxidase mimicking iron complex based on the bTAML macrocyclic ligand framework (Fe-bTAML: biuret-modified tetraamido macrocyclic ligand) as a catalyst to perform selective oxidation of unactivated 3° bonds with unprecedented regioselectivity (3°:2° of 110:1 for adamantane oxidation), high stereoretention (99%), and turnover numbers (TONs) up to 300 using mCPBA as the oxidant. Ligand decomposition pathways involving acid-induced demetalation were identified, and this led to the development of more robust and efficient Fe-bTAML complexes that catalyzed chemoselective C-H oxidation. Mechanistic studies, which include correlation of the product formed with the Fe^V(O) reactive intermediates generated during the reaction, indicate that the major pathway involves the cleavage of C-H bonds by Fe^V(O). When these oxidations were performed in the presence of air, the yield of the oxidized product doubled, but the stereoretention remained unchanged. On the basis of ¹⁸O labeling and other mechanistic studies, we propose a mechanism that involves the dual activation of mCPBA and O₂ by Fe-bTAML, leading to formation of the Fe^V(O) intermediate. This high-valent iron oxo remains the active intermediate for most of the reaction, resulting in high regio- and stereoselectivity during product formation.

Structure and spin state of nonheme FeIVO complexes depending on temperature: predictive insights from DFT calculations and experiments

The spin states (S = 1 and S = 2) of nonheme FeIVO intermediates are believed to play an important role in determining their chemical properties in enzymatic and biomimetic reactions. However, it is almost impossible to investigate the spin state effect of nonheme FeIVO species experimentally, since FeIVO models having the S = 1 and S = 2 spin states at the same time neither exist nor can be synthesized. However, recent synthesis of an FeIVO complex with an S = 1 spin state (triplet), [(Me3NTB)FeIVO]2+ (1), and a structurally similar FeIVO complex but with an S = 2 spin state (quintet), [(TQA)FeIVO]2+ (2), has allowed us to compare their reactivities at 233 K. In the present study, we show that structural variants control the spin-state selectivity and reactivity of nonheme FeIVO complexes. While 1 and 2 were proposed to be in an octahedral geometry based on DFT calculations and spectroscopic characterization done at 4 K, further DFT calculations show that these species may well assume a trigonal bipyramidal structure by losing one coordinated solvent ligand at 233 K. Thus, the present study demonstrates that the structure and spin state of nonheme FeIVO complexes can be different at different temperatures; therefore, the structural and/or spin state information obtained at 4 K should be carefully used at a higher temperature (e.g., 233 K). In addition to 1 and 2, [(TPA)FeIVO]2+ (3) with an S = 1 spin state, whose spin state was determined spectroscopically and theoretically at 233 K, is included in this study to compare the chemical properties of S = 1 and S = 2 FeIVO complexes. The present results add another dimension to the discussion of the reactivites of nonheme FeIVO species, in which the structural preference and spin state of nonheme FeIVO species can vary depending on temperature.

A Mononuclear Nonheme Iron(V)-Imido Complex

Mononuclear nonheme iron(V)-oxo complexes have been reported previously. Herein, we report the first example of a mononuclear nonheme iron(V)-imido complex bearing a tetraamido macrocyclic ligand (TAML), [(TAML)FeV(NTs)]- (1). The spectroscopic characterization of 1 revealed an S = 1/2 Fe(V) oxidation state, an Fe-N bond length of 1.65(4) Å, and an Fe-N vibration at 817 cm-1. The reactivity of 1 was demonstrated in C-H bond functionalization and nitrene transfer reactions.

Nonclassical Single-State Reactivity of an Oxo-Iron(IV) Complex Confined to Triplet Pathways

C-H bond activation mediated by oxo-iron (IV) species represents the key step of many heme and nonheme O₂-activating enzymes. Of crucial interest is the effect of spin state of the Fe^IV(O) unit. Here we report the C-H activation kinetics and corresponding theoretical investigations of an exclusive tetracarbene ligated oxo-iron(IV) complex, [L^NHCFe^IV(O)(MeCN)]²⁺ (1). Kinetic traces using substrates with bond dissociation energies (BDEs) up to 80 kcal mol^-1 show pseudo-first-order behavior and large but temperature-dependent kinetic isotope effects (KIE 32 at -40 °C). When compared with a topologically related oxo-iron(IV) complex bearing an equatorial N-donor ligand, [L^TMCFe^IV(O) (MeCN)]²⁺ (A), the tetracarbene complex 1 is significantly more reactive with second order rate constants k'₂ that are 2-3 orders of magnitude higher. UV-vis experiments in tandem with cryospray mass spectrometry evidence that the reaction occurs via formation of a hydroxo-iron(III) complex (4) after the initial H atom transfer (HAT). An extensive computational study using a wave function based multireference approach, viz. complete active space self-consistent field (CASSCF) followed by N-electron valence perturbation theory up to second order (NEVPT2), provided insight into the HAT trajectories of 1 and A. Calculated free energy barriers for 1 reasonably agree with experimental values. Because the strongly donating equatorial tetracarbene pushes the Fe-d_x²-y² orbital above d_z², 1 features a dramatically large quintet-triplet gap of ∼18 kcal/mol compared to ∼2-3 kcal/mol computed for A. Consequently, the HAT process performed by 1 occurs on the triplet surface only, in contrast to complex A reported to feature two-state-reactivity with contributions from both triplet and quintet states. Despite this, the reactive Fe^IV(O) units in 1 and A undergo the same electronic-structure changes during HAT. Thus, the unique complex 1 represents a pure "triplet-only" ferryl model.

Targeting of High-Valent Iron-TAML Activators at Hydrocarbons and Beyond

TAML activators of peroxides are iron(III) complexes. The ligation by four deprotonated amide nitrogens in macrocyclic motifs is the signature of TAMLs where the macrocyclic structures vary considerably. TAML activators are exceptional functional replicas of the peroxidases and cytochrome P450 oxidizing enzymes. In water, they catalyze peroxide oxidation of a broad spectrum of compounds, many of which are micropollutants, compounds that produce undesired effects at low concentrations-as with the enzymes, peroxide is typically activated with near-quantitative efficiency. In nonaqueous solvents such as organic nitriles, the prototype TAML activator gave the structurally authenticated reactive iron(V)oxo units (Fe^VO), wherein the iron atom is two oxidation equivalents above the Fe^III resting state. The iron(V) state can be achieved through the intermediacy of iron(IV) species, which are usually μ-oxo-bridged dimers (Fe^IVFe^IV), and this allows for the reactivity of this potent reactive intermediate to be studied in stoichiometric processes. The present review is primarily focused at the mechanistic features of the oxidation by Fe^VO of hydrocarbons including cyclohexane. The main topic is preceded by a description of mechanisms of oxidation of thioanisoles by Fe^VO, because the associated studies provide valuable insight into the ability of Fe^VO to oxidize organic molecules. The review is opened by a summary of the interconversions between Fe^III, Fe^IVFe^IV, and Fe^VO species, since this information is crucial for interpreting the kinetic data. The highest reactivity in both reaction classes described belongs to Fe^VO. The resting state Fe^III is unreactive oxidatively. Intermediate reactivity is typically found for Fe^IVFe^IV; therefore, kinetic features for these species in interchange and oxidation processes are also reviewed. Examples of using TAML activators for C-H bond cleavage applied to fine organic synthesis conclude the review.

Spectroscopic and Reactivity Comparisons of a Pair of bTAML Complexes with FeV═O and FeIV═O Units

In this report we compare the geometric and electronic structures and reactivities of [Fe^V(O)]^- and [Fe^IV(O)]^2- species supported by the same ancillary nonheme biuret tetraamido macrocyclic ligand (bTAML). Resonance Raman studies show that the Fe═O vibration of the [Fe^IV(O)]^2- complex 2 is at 798 cm^-1, compared to 862 cm^-1 for the corresponding [Fe^V(O)]^- species 3, a 64 cm^-1 frequency difference reasonably reproduced by density functional theory calculations. These values are, respectively, the lowest and the highest frequencies observed thus far for nonheme high-valent Fe═O complexes. Extended X-ray absorption fine structure analysis of 3 reveals an Fe═O bond length of 1.59 Å, which is 0.05 Å shorter than that found in complex 2. The redox potentials of 2 and 3 are 0.44 V (measured at pH 12) and 1.19 V (measured at pH 7) versus normal hydrogen electrode, respectively, corresponding to the [Fe^IV(O)]^2-/[Fe^III(OH)]^2- and [Fe^V(O)]^-/[Fe^IV(O)]^2- couples. Consistent with its higher potential (even after correcting for the pH difference), 3 oxidizes benzyl alcohol at pH 7 with a second-order rate constant that is 2500-fold bigger than that for 2 at pH 12. Furthermore, 2 exhibits a classical kinteic isotope effect (KIE) of 3 in the oxidation of benzyl alcohol to benzaldehyde versus a nonclassical KIE of 12 for 3, emphasizing the reactivity differences between 2 and 3.

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

Terminal cobalt(IV)-oxo (Co^IV-O) species have been implicated as key intermediates in various cobalt-mediated oxidation reactions. Herein we report the photocatalytic generation of a mononuclear non-haem [(13-TMC)Co^IV(O)]²⁺ (2) by irradiating [Co^II(13-TMC)(CF₃SO₃)]⁺ (1) in the presence of [Ru^II(bpy)₃]²⁺, Na₂S₂O₈, and water as an oxygen source. The intermediate 2 was also obtained by reacting 1 with an artificial oxidant (that is, iodosylbenzene) and characterized by various spectroscopic techniques. In particular, the resonance Raman spectrum of 2 reveals a diatomic Co-O vibration band at 770 cm^-1, which provides the conclusive evidence for the presence of a terminal Co-O bond. In reactivity studies, 2 was shown to be a competent oxidant in an intermetal oxygen atom transfer, C-H bond activation and olefin epoxidation reactions. The present results lend strong credence to the intermediacy of Co^IV-O species in cobalt-catalysed oxidation of organic substrates as well as in the catalytic oxidation of water that evolves molecular oxygen.

Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)-Oxo Complexes

Manganese(IV)-oxo complexes are often invoked as intermediates in Mn-catalyzed C-H bond activation reactions. While many synthetic Mn^IV -oxo species are mild oxidants, other members of this class can attack strong C-H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of Mn^IV -oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the Mn^IV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K-edge X-ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen-atom and oxygen-atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these Mn^IV -oxo complexes, the rate enhancements are correlated with both 1) the energy of a low-lying ⁴ E excited state, which has been postulated to be involved in a two-state reactivity model, and 2) the Mn^III/IV reduction potentials.

Frontier Molecular Orbital Contributions to Chlorination versus Hydroxylation Selectivity in the Non-Heme Iron Halogenase SyrB2

The ability of an Fe^IV═O intermediate in SyrB2 to perform chlorination versus hydroxylation was computationally evaluated for different substrates that had been studied experimentally. The π-trajectory for H atom abstraction (Fe^IV═O oriented perpendicular to the C-H bond of substrate) was found to lead to the S = 2 five-coordinate HO-Fe^III-Cl complex with the C^• of the substrate, π-oriented relative to both the Cl^- and the OH^- ligands. From this ferric intermediate, hydroxylation is thermodynamically favored, but chlorination is intrinsically more reactive due to the energy splitting between two key redox-active dπ* frontier molecular orbitals (FMOs). The splitting is determined by the differential ligand field effect of Cl^- versus OH^- on the Fe center. This makes chlorination effectively competitive with hydroxylation. Chlorination versus hydroxylation selectivity is then determined by the orientation of the substrate with respect to the HO-Fe-Cl plane that controls either the Cl^- or the OH^- to rebound depending on the relative π-overlap with the substrate C radical. The differential contribution of the two FMOs to chlorination versus hydroxylation selectivity in SyrB2 is related to a reaction mechanism that involves two asynchronous transfers: electron transfer from the substrate radical to the iron center followed by late ligand (Cl^- or OH^-) transfer to the substrate.

Ethane C–H bond activation on the Fe(iv)–oxo species in a Zn-based cluster of metal–organic frameworks: a density functional theory study

The generation of a Fe(iv)–oxo complex and its reactivity for C–H bond activation of ethane have been theoretically unraveled.

Beyond ferryl-mediated hydroxylation: 40 years of the rebound mechanism and C-H activation

Since our initial report in 1976, the oxygen rebound mechanism has become the consensus mechanistic feature for an expanding variety of enzymatic C-H functionalization reactions and small molecule biomimetic catalysts. For both the biotransformations and models, an initial hydrogen atom abstraction from the substrate (R-H) by high-valent iron-oxo species (Feⁿ=O) generates a substrate radical and a reduced iron hydroxide, [Fe^n-1-OH ·R]. This caged radical pair then evolves on a complicated energy landscape through a number of reaction pathways, such as oxygen rebound to form R-OH, rebound to a non-oxygen atom affording R-X, electron transfer of the incipient radical to yield a carbocation, R⁺, desaturation to form olefins, and radical cage escape. These various flavors of the rebound process, often in competition with each other, give rise to the wide range of C-H functionalization reactions performed by iron-containing oxygenases. In this review, we first recount the history of radical rebound mechanisms, their general features, and key intermediates involved. We will discuss in detail the factors that affect the behavior of the initial caged radical pair and the lifetimes of the incipient substrate radicals. Several representative examples of enzymatic C-H transformations are selected to illustrate how the behaviors of the radical pair [Fe^n-1-OH ·R] determine the eventual reaction outcome. Finally, we discuss the powerful potential of "radical rebound" processes as a general paradigm for developing novel C-H functionalization reactions with synthetic, biomimetic catalysts. We envision that new chemistry will continue to arise by bridging enzymatic "radical rebound" with synthetic organic chemistry.

Spectroscopic analyses of 2-oxoglutarate-dependent oxygenases: TauD as a case study

A wide range of spectroscopic approaches have been used to interrogate the mononuclear iron metallocenter in 2-oxoglutarate (2OG)-dependent oxygenases. The results from these spectroscopic studies have provided valuable insights into the structural changes at the active site during substrate binding and catalysis, thus providing critical information that complements investigations of these enzymes by X-ray crystallography, biochemical, and computational approaches. This mini-review highlights taurine hydroxylase (taurine:2OG dioxygenase, TauD) as a case study to illustrate the wealth of knowledge that can be generated by applying a diverse array of spectroscopic investigations to a single enzyme. In particular, electronic absorption, circular dichroism, magnetic circular dichroism, conventional and pulse electron paramagnetic, Mössbauer, X-ray absorption, and resonance Raman methods have been exploited to uncover the properties of the metal site in TauD.

Factors Controlling the Chemoselectivity in the Oxidation of Olefins by Nonheme Manganese(IV)-Oxo Complexes

We report the oxidation of cyclic olefins, such as cyclohexene, cyclohexene-d10, and cyclooctene, by mononuclear nonheme manganese(IV)-oxo (Mn(IV)O) and triflic acid (HOTf)-bound Mn(IV)O complexes. In the oxidation of cyclohexene, the Mn(IV)O complexes prefer the C-H bond activation to the C═C double bond epoxidation, whereas the C═C double bond epoxidation becomes a preferred reaction pathway in the cyclohexene oxidation by HOTf-bound Mn(IV)O complexes. In contrast, the oxidation of cyclohexene-d10 and cyclooctene by the Mn(IV)O complexes occurs predominantly via the C═C double bond epoxidation. This conclusion is drawn from the product analysis and kinetic studies of the olefin oxidation reactions, such as the epoxide versus allylic oxidation products, the formation of Mn(II) versus Mn(III) products, and the kinetic analyses. Overall, the experimental results suggest that the energy barrier of the C═C double bond epoxidation is very close to that of the allylic C-H bond activation in the oxidation of cyclic olefins by high-valent metal-oxo complexes. Thus, the preference of the reaction pathways is subject to changes upon small manipulation of the reaction environments, such as the supporting ligands and metal ions in metal-oxo species, the presence of HOTf (i.e., HOTf-bound Mn(IV)O species), and the allylic C-H(D) bond dissociation energies of olefins. This is confirmed by DFT calculations in the oxidation of cyclohexene and cyclooctene, which show multiple pathways with similar rate-limiting energy barriers and depending on the allylic C-H bond dissociation energies. In addition, the possibility of excited state reactivity in the current system is confirmed for epoxidation reactions.

Mononuclear Nonheme High-Spin (S=2) versus Intermediate-Spin (S=1) Iron(IV)-Oxo Complexes in Oxidation Reactions

Mononuclear nonheme high-spin (S=2) iron(IV)-oxo species have been identified as the key intermediates responsible for the C-H bond activation of organic substrates in nonheme iron enzymatic reactions. Herein we report that the C-H bond activation of hydrocarbons by a synthetic mononuclear nonheme high-spin (S=2) iron(IV)-oxo complex occurs through an oxygen non-rebound mechanism, as previously demonstrated in the C-H bond activation by nonheme intermediate (S=1) iron(IV)-oxo complexes. We also report that C-H bond activation is preferred over C=C epoxidation in the oxidation of cyclohexene by the nonheme high-spin (HS) and intermediate-spin (IS) iron(IV)-oxo complexes, whereas the C=C double bond epoxidation becomes a preferred pathway in the oxidation of deuterated cyclohexene by the nonheme HS and IS iron(IV)-oxo complexes. In the epoxidation of styrene derivatives, the HS and IS iron(IV) oxo complexes are found to have similar electrophilic characters.

Modeling Non-Heme Iron Halogenases: High-Spin Oxoiron(IV)-Halide Complexes That Halogenate C-H Bonds

The non-heme iron halogenases CytC3 and SyrB2 catalyze C-H bond halogenation in the biosynthesis of some natural products via S = 2 oxoiron(IV)-halide intermediates. These oxidants abstract a hydrogen atom from a substrate C-H bond to generate an alkyl radical that reacts with the bound halide to form a C-X bond chemoselectively. The origin of this selectivity has been explored in biological systems but has not yet been investigated with synthetic models. Here we report the characterization of S = 2 [Fe(IV)(O)(TQA)(Cl/Br)](+) (TQA = tris(quinolyl-2-methyl)amine) complexes that can preferentially halogenate cyclohexane. These are the first synthetic oxoiron(IV)-halide complexes that serve as spectroscopic and functional models for the halogenase intermediates. Interestingly, the nascent substrate radicals generated by these synthetic complexes are not as short-lived as those obtained from heme-based oxidants and can be intercepted by O2 to prevent halogenation, supporting an emerging notion that rapid rebound may not necessarily occur in non-heme oxoiron(IV) oxidations.