Oxidation Of Organic Substrates Research Articles

Selective Catalytic Oxidation of Organic Sulfides to Sulfoxides without Forming Sulfones over Solid Molybdenum Blue: Kinetic and Thermodynamic Studies

The present investigation reports studies on the selective catalytic oxidation of organic sulfide substrates over molybdenum blue catalyst supported on boron phosphate. The catalyst was synthesized through partial precipitation method and characterized by XRD, FTIR and SEM techniques. The sulfoxidation was carried out in a batch reactor using benzyl phenyl sulfide as the substrate over the present catalyst and the reaction parameters were varied and optimized. The results were compared with the MoO3 impregnated boron phosphate. The catalyst was also studied for its performance over other sulfide substrates and the results were compared with available studies in literature. The reaction followed pseudo first-order kinetics and rate of the reaction under optimized condition was 10.1 × 10-3 min-1, with energy of activation of 29.3 kJ/mol. The Mo-O-Mo bridging and -Mo=O bonds present in molybdenum blue were participating in the reaction and possible mechanism has been proposed. The 100% selectivity of the product towards sulfoxide has been attributed to the big-wheel structure of molybdenum blue as it sterically hinders further reaction of sulfoxides formed to sulfones.

Metal-Organic-Framework-Supported and -Isolated Ceria Clusters with Mixed Oxidation States.

The formation of oxygen vacancies via reversible transitions between Ce(IV) and Ce(III) plays a crucial role in the propensity of cerium oxide to act as a supporting promoter in oxidative heterogeneous catalysis. An open challenge is, however, preparation of high-porosity, supported arrays of isolated ceria(IV, III) clusters with high porosity. Herein, we report two examples of oxy-Ce(IV, III) clusters supported and spatially isolated on an oxy-zirconium MOF, NU-1000. The clusters are introduced using either of two Ce complexes (precursors): CeIV(tmhd)4 (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate) or CeIII(iPrCp)3 (iPrCp = tris(isopropyl-cyclopenta-dienyl), via SIM (solvothermal installation in MOFs). The prepared materials are named Ce-l-SIM-NU-1000 and Ce-n-SIM-NU-1000, respectively. X-ray photoelectron spectroscopy characterization shows that the ratio of Ce(III) to Ce(IV) oxidation states can be modulated. Difference envelope density analyses of X-ray scattering show that CexOyHz clusters in Ce-n-SIM-NU-1000 are located between pairs of Zr6 nodes, whereas in Ce-l-SIM-NU-1000, they are sited on MOF linkers throughout the micropores of NU-1000. Cluster size differences were further evaluated by pair function distribution (PDF) analyses of total X-ray scattering reveal that the node sited clusters contain of only a few cerium ions, whereas the linker-sited clusters each contain ∼90 cerium ions. The observed size appears to be defined by the size of NU-1000s triangular pores, that is, cluster formation appears to be pore templated. The Ce-SIM functionalized materials are catalytically active for hydrolysis of DMNP (dimethyl 4-nitrophenyl phosphate), a nerve-agent simulant. Conversion of a small fraction of the Ce(IV) ions in which the presence of small fractions of the cerium(IV) ions in Ce-l-SIM-NU-1000 to cerium(III) significantly enhances catalytic activity-perhaps by labilizing aqua ligands and facilitating simulant binding to the clusters Lewis-basic metal ions. While not explored here, the larger clusters, when partially reduced, are, we believe, candidate catalysts for O2 activation and subsequent selective oxidation of organic substrates.

Anodic vs cathodic potentiostatic control of a methane producing microbial electrolysis cell aimed at biogas upgrading

A fully biological Microbial Electrolysis Cell (MEC) aimed at biogas upgrading has been operated under different operating conditions in order to enhance CO2 removal from a synthetic biogas. Specifically, CO2 reduction into CH4 occurred at the MEC biocathode with the oxidation of organic substrates in the anodic chamber partially sustaining the energy demand of the process. In the cathode chamber, methane formation was the main driver of current generation which, in turn, sustained alkalinity generation and related CO2 sorption. This study mainly focused on the minimization of the anodic and cathodic overpotentials to maximize the process efficiency. To accomplish this objective, an innovative strategy of MEC polarization was adopted, consisting in the shift of the potentiostatic control of the process from the anode (at +0.2 V vs SHE) to the cathode (at −0.65, −0.90 and −1.00 V vs SHE), along with the control of the fluid dynamic conditions of the anode chamber. An almost complete (99%) energy recovery was obtained by methane production with the cathode potential controlled at −0.65 V vs SHE. Finally, at the MEC cathode, current was utilized to reduce CO2 into CH4 (with a cathodic capture efficiency of about 70%) as well as to promote CO2 sorption into HCO3−. The latter represents the main CO2 removal mechanism that accounted for 85% of the CO2 removal.

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water

AbstractUsing light energy and O2 for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O2 by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)3]2+, generating the highly oxidized chromophore and the powerful reductant methyl‐viologen radical MV+.. MV+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O2 to O2.− in an aqueous medium to generate an active iron(III)‐(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water.

Using light energy and O2 for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O2 by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)3 ]2+ , generating the highly oxidized chromophore and the powerful reductant methyl-viologen radical MV+. . MV+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O2 to O2.- in an aqueous medium to generate an active iron(III)-(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.

Dinitrosyl Iron Complexes in the Sensitized Oxidation of Organic Substrates

It is shown that the efficacy of the photosensitized oxidation of organic substrates in aqueous media (with the oxidation of tryptophan being used as an example) does not depend on adding biologically active dinitrosyl iron complexes (DNICs) with thiol-containing ligands to the reaction medium if it contains Pluronic F127, a poly(ethylene oxide)–poly(propylene oxide)– poly(ethylene oxide) triblock copolymer, at concentrations higher than the critical micelle concentration. Photosensitizer molecules are localized in Pluronic micelles and are shielded from the damaging impact of NO• radicals that form upon the photodegradation of DNIC molecules. Photodynamic therapy (PDT) sessions (the photoinduced necrosis and apoptosis of cells in pathologically altered tissues and the simultaneous initiation of regeneration and repair of the treated tissues due to the photodecomposition of DNIC molecules) thus become fundamentally possible.

Review of the principal mechanisms, prospects, and challenges of bioelectrochemical systems

AbstractBioelectrochemical systems (BES) are commonly utilized to generate green electricity, chemicals, and materials through bioelectrocatalytic processes. Over the years, the growing interests in low carbon energy, wastes valorization and the sustainable bioremediation of environmental pollutants have generated interests in BES such as microbial fuel cells (MFC) and bioelectrochemical fuel cells (BFC). The MFCs are the most advanced BES that can ensure the microbial conversion of chemical energy into electrical energy. Therefore, this article seeks to review and present valuable literature on the fundamental operational principles, mechanisms and understanding of BES such as MFCs. It seeks to highlight the schematics of these systems along with the processes and mechanisms such as the oxidation of organic substrates ranging from acetate compounds to complex mixtures. Furthermore, the prospects, challenges, and future applications of BES technologies are presented. The findings indicate that BFCs and MFCs are hampered by low efficiencies, energy output, mass transfer, porosity, and proton conductivity of the electrode and membrane materials along with mechanical strength, scalability, biocompatibility, and chemical stability. However, BES could potentially impact on clean energy production, greenhouse gases mitigation, wastewater treatment, bioanalysis, biosensors, and environmental remediation in the future.

(Invited) Electrochemical Analogues of Dioxygenase and Monooxygenase Activity in Synthetic Iron Porphyrins

Dioxygen adducts of heme proteins are ubiquitous in all O2 activating/reducing heme proteins in nature. While artificial dioxygen adducts of heme are known for six decades, till date these have not been demonstrated to be able to oxidize organic substrates in sharp contrast to their non-heme analogues and naturally occurring enzymes like heme dioxygenases. To address this apparent anomaly an iron porphyrin complex is synthesized which includes a pendant quinol group. The corresponding dioxygen bound iron porphyrin species is demonstrated to perform Hydrogen atom transfer (HAT) from a quinol group appended to the porphyrin ligand. The resultant ferric peroxide, formed by the first HAT, performs a 2nd HAT generating a ferryl species (FeIV=O) and resulting in the 2e-/2H+ oxidation of the pendant hydroquinol to quinone. All the intermediate species are trapped at cryogenic temperatures and spectroscopically characterized using EPR and resonance Raman spectroscopy. DFT calculations reproduce the experimental kinetic parameters and provide insight into the nature of TS involved. These results demonstrate that artificial analogues of both heme ferric superoxide and ferric hydroperoxide species can perform HAT from an organic substrate when appropriately pre-organized to hydrogen bond to the bound superoxide and peroxide species. Siilarly, catalytic oxidation of organic substrates, using a green oxidant like O2, has been a long-term goal of the scientific community. In nature, these oxidations are performed by metalloenzymes which generate highly oxidizing species from O2 which, in turn, can oxidize inert organic substrates e.g. mono/di oxygenases. The same oxidants are produced during O2 reduction during respiration in the mitochondria but are reduced by electron transfer i.e. reductases. Iron porphyrin mimics of the active site of Cytochrome P450 (Cyt P450) are created atop self-assembled monolayer covered electrode. The rate of electron transfer from the electrode to the iron porphyrin site is attenuated to derive monooxygenase reactivity from these constructs which generally show reductase activity and catalytic hydroxylation of inert C-H bonds to alcohol and epoxidation alkenes, using molecular O2, is demonstrated with turnover numbers > 104. Mechanistic investigations suggest that a compound I analogue, formed during O2 reduction, is the primary oxidant and the selectivity is determined by the shape of the distal environment of the catalyst.

Electron Transfer Control of Reductase versus Monooxygenase: Catalytic C-H Bond Hydroxylation and Alkene Epoxidation by Molecular Oxygen.

Catalytic oxidation of organic substrates, using a green oxidant like O2, has been a long-term goal of the scientific community. In nature, these oxidations are performed by metalloenzymes that generate highly oxidizing species from O2, which, in turn, can oxidize very stable organic substrates, e.g., mono-/dioxygenases. The same oxidants are produced during O2 reduction/respiration in the mitochondria but are reduced by electron transfer, i.e., reductases. Iron porphyrin mimics of the active site of cytochrome P450 (Cyt P450) are created atop a self-assembled monolayer covered electrode. The rate of electron transfer from the electrode to the iron porphyrin site is attenuated to derive monooxygenase reactivity from these constructs that otherwise show O2 reductase activity. Catalytic hydroxylation of strong C–H bonds to alcohol and epoxidation of alkenes, using molecular O2 (with 18O2 incorporation), is demonstrated with turnover numbers >104. Uniquely, one of the two iron porphyrin catalysts used shows preferential oxidation of 2° C–H bonds of cycloalkanes to alcohols over 3° C-H bonds without overoxidation to ketones. Mechanistic investigations with labeled substrates indicate that a compound I (FeIV=O bound to a porphyrin cation radical) analogue, formed during O2 reduction, is the primary oxidant. The selectivity is determined by the shape of the distal pocket of the catalyst, which, in turn, is determined by the substituents on the periphery of the porphyrin macrocycle.

Proposal of the reverse flow model for the origin of the eukaryotic cell based on comparative analyses of Asgard archaeal metabolism

The origin of eukaryotes represents an unresolved puzzle in evolutionary biology. Current research suggests that eukaryotes evolved from a merger between a host of archaeal descent and an alphaproteobacterial endosymbiont. The discovery of the Asgard archaea, a proposed archaeal superphylum that includes Lokiarchaeota, Thorarchaeota, Odinarchaeota and Heimdallarchaeota suggested to comprise the closest archaeal relatives of eukaryotes, has helped to elucidate the identity of the putative archaeal host. Whereas Lokiarchaeota are assumed to employ a hydrogen-dependent metabolism, little is known about the metabolic potential of other members of the Asgard superphylum. We infer the central metabolic pathways of Asgard archaea using comparative genomics and phylogenetics to be able to refine current models for the origin of eukaryotes. Our analyses indicate that Thorarchaeota and Lokiarchaeota encode proteins necessary for carbon fixation via the Wood-Ljungdahl pathway and for obtaining reducing equivalents from organic substrates. By contrast, Heimdallarchaeum LC2 and LC3 genomes encode enzymes potentially enabling the oxidation of organic substrates using nitrate or oxygen as electron acceptors. The gene repertoire of Heimdallarchaeum AB125 and Odinarchaeum indicates that these organisms can ferment organic substrates and conserve energy by coupling ferredoxin reoxidation to respiratory proton reduction. Altogether, our genome analyses suggest that Asgard representatives are primarily organoheterotrophs with variable capacity for hydrogen consumption and production. On this basis, we propose the 'reverse flow model', an updated symbiogenetic model for the origin of eukaryotes that involves electron or hydrogen flow from an organoheterotrophic archaeal host to a bacterial symbiont.

In situ plankton community respiration measurements show low respiratory quotients in a eutrophic lake.

Planktonic community respiration is an important carbon cycling process, typically quantified by converting measured values of dissolved O2 consumption rates into CO2 production rates assuming a respiratory quotient of 1 (RQ = CO2 per O2 by moles). However, the true variability in planktonic RQs between different aquatic ecosystems is poorly understood. We conducted in situ RQ measurements in a eutrophic lake dominated by algal-derived substances and found that RQs were significantly below 1. In fact, many RQ values were extremely low (0.2-0.6), below theoretical RQs for oxidation of algal organic matter substrates (0.7-0.8), suggesting that other factors than substrate control need to be considered to understand the RQ. This view was further supported by lack of correlations between RQ and microbial variables known to be strongly substrate dependent, including bacterial growth efficiency and the functional capacity of the bacterioplankton community to degrade different compounds. Based on the measured dynamics in methane and nutrient pools, we discuss that methane oxidation and nitrification likely occurred in the lake, contributing to the unusually low RQs. Our findings demonstrate that planktonic RQs in productive lakes can systematically be below 1, suggesting that CO2 emissions from these lakes may currently be overestimated.

Iron-Decorated, Guanidine Functionalized Metal-Organic Framework as a Non-heme Iron-Based Enzyme Mimic System for Catalytic Oxidation of Organic Substrates

A novel porous functionalized metal-organic framework (MOF) as a non-heme iron-based enzyme mimic system was achieved via two-step post-synthetic modification of the MIL-101(Cr)-NH2, and characterized by FT-IR, PXRD, TGA, SEM, EDS, CHN, BET surface area, and ICP-OES analyses. This new modified MOF (MIL-101(Cr)-guanidine-Fe) has been demonstrated to be a highly efficient, active, and reusable catalyst for oxidation of various organic substrates, including alcohols, alkenes and alkyl arenes at room temperature using H2O2 as an oxidant.

Catalyzed Oxidation of Carotenoids by Lactoperoxidase in the Presence of Ethanol.

The discovery of the lactoperoxidase system as a biocatalyst in milk was a landmark finding. The activation of this system using hydrogen peroxide (H2O2) raised hopes for oxidation of various organic substrates. The involvement of lactoperoxidase system in the catalyzed-oxidation of carotenoids in the whey proteins, and the effect of various solvents on carotenoids' oxidation reaction rate has been studied. However, there is no evidence for this reaction without the addition of oxidizing agents, such as peroxides. Here, we reveal that carotenoids are oxidized through the addition of just ethanol in the presence of lactoperoxidase. The oxidation of carotenoids through this exquisite strategy is ∼360 times faster than harnessing the lactoperoxidase system in whey proteins via the addition of hydrogen peroxide. Bearing in mind that ethanol is not an oxidizing agent, this observation suggests a potential paradigm shift in our understanding of lactoperoxidase and catalyzed oxidation in biochemical systems.

A Bpp-based dinuclear ruthenium photocatalyst for visible light-driven oxidation reactions

The photocatalytic oxidation of organic substrates in water using a diruthenium chromophore-catalyst dyad molecule can be tuned by the nature of the bridging ligand.

Reactive Cobalt⁻Oxo Complexes of Tetrapyrrolic Macrocycles and N-based Ligand in Oxidative Transformation Reactions.

High-valent cobalt–oxo complexes are reactive transient intermediates in a number of oxidative transformation processes e.g., water oxidation and oxygen atom transfer reactions. Studies of cobalt–oxo complexes are very important for understanding the mechanism of the oxygen evolution center in natural photosynthesis, and helpful to replicate enzyme catalysis in artificial systems. This review summarizes the development of identification of high-valent cobalt–oxo species of tetrapyrrolic macrocycles and N-based ligands in oxidation of organic substrates, water oxidation reaction and in the preparation of cobalt–oxo complexes.

Preparation, characterization and catalytic performance of polyoxometalate immobilized on the surface of halloysite

Polyoxometalates (POMs) are well known to have excellent catalytic performances for oxidation of organic substrates. In this paper, we immobilized POM, Cs3(NH4)[{Ru4O6(H2O)9}2Sb2W20O68(OH)2]·9H2O (SbWRu), on the surface of halloysite nanotubes (HNTs) functionalized by 3-aminopropyltriethoxysilane (Apts) to prepare a novel heterogeneous catalyst, HNTs/Apts/SbWRu, which is never reported in the literature to our knowledge. The catalyst HNTs/Apts/SbWRu was characterized by elemental analysis, IR spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N2 adsorption measurements to determine its composition, structure and morphology. The catalytic performance of this catalyst was tested in green oxidation system of n-tetradecane using air as oxidant under the mild reaction condition with normal atmospheric pressure and low temperature and without adding any solvents and additives. Furthermore, in order to find the optimum catalytic reaction condition, we prepared five catalysts containing different amounts of SbWRu, 0.97%, 1.94%, 2.80%, 4.23% and 5.87%. The results of the controlled experiments confirmed that the catalyst containing SbWRu of 1.94% exhibited high activity with a conversion (53.30%) and turnover frequency (TOF: 52396 h−1) at the optimal reaction condition. Moreover, this catalyst can be recovered and reused by filtration without significant loss of its catalytic performance for at least five times. The final conversion of n-tetradecane runs up to 87.97% after five consecutive cycles without the separation of the catalyst HNTs/Apts/SbWRu (1.94%).

Aminoiron(III)–porphyrin–alumina catalyst obtained by non-hydrolytic sol-gel process for heterogeneous oxidation of hydrocarbons

An aminoiron(III) porphyrin immobilized on an alumina matrix was prepared and used as catalyst for the oxidation of organic substrates. Powder alumina had been prepared by a non-hydrolytic sol-gel method through condensation of aluminum chloride with anhydrous ethanol. Then, iron(III) [5,10,15,20-tetrakis(2,6-dichloro-3-aminophenyl)-porphyrin] was immobilized on the alumina powder under magnetic stirring, reflux, and inert atmosphere. Ultraviolet–visible and infrared spectroscopies, powder X-ray diffraction, scanning electron microscopy and thermal analysis were applied for characterizing the resulting material, confirming that the ironporphyrin was immobilized on the alumina support. The catalytic activity of ironporphyrin/alumina was evaluated in the oxidation of (Z)-cyclooctene and cyclohexane and in the Baeyer-Villiger oxidation of cyclohexanone using iodosylbenzene or hydrogen peroxide as oxygen donors. The novel immobilized catalyst proved to be a promising system for the efficient and selective oxidation of the organic substrates with 85–92% selectivity to the epoxide in the oxidation of alkenes and 25–41% to the ketone in the oxidation of cyclohexane. As for the Baeyer-Villiger oxidation of cyclohexanone, good conversion to ԑ-caprolactone was observed as well. The material is a reusable heterogeneous catalyst, which makes it more economically feasible than its homogeneous counterpart.

Electron Fluxes in Biocathode Bioelectrochemical Systems Performing Dechlorination of Chlorinated Aliphatic Hydrocarbons.

Bioelectrochemical systems (BESs) are regarded as a promising approach for the enhanced dechlorination of chlorinated aliphatic hydrocarbons (CAHs). However, the electron distribution and transfer considering dechlorination, methanogenesis, and other bioprocesses in these systems are little understood. This study investigated the electron fluxes in biocathode BES performing dechlorination of three typical CAHs, 1,1,2,2-tetrachloroethene (PCE), 1,1,2-trichloroethene (TCE) and 1,2-dichloroethane (1,2-DCA). Anaerobic sludge was inoculated to cathode and biocathode was acclimated by the direct acclimation and selection. The constructed biocathode at −0.26 V had significantly higher dechlorination efficiency (E24h > 99.0%) than the opened circuit (E24h of 17.2–27.5%) and abiotic cathode (E24h of 5.5–10.8%), respectively. Cyclic voltammetry analysis demonstrated the enhanced cathodic current and the positive shift of onset potential in the cathodic biofilm. Under autotrophic conditions with electrons from the cathode as sole energy source (columbic efficiencies of 80.4–90.0%) and bicarbonate as sole carbon source, CAHs dechlorination efficiencies were still maintained at 85.0 ± 2.0%, 91.4 ± 1.8%, and 84.9 ± 3.1% for PCE, TCE, and 1,2-DCA, respectively. Cis-1,2-dichloroethene was the final product for PCE and TCE, while 1,2-DCA went through a different dechlorination pathway with the non-toxic ethene as the final metabolite. Methane was the main by-product of the heterotrophic biocathode, and methane production could be enhanced to some extent by electrochemical stimulation. The various electron fluxes originating from the cathode and oxidation of organic substrates might be responsible for the enhanced CAHs dechlorination, while methane generation and bacterial growth would probably reduce the fraction of electrons provided for CAH dechlorination. The study deals with the dechlorination and competitive bioprocesses in CAH-dechlorinating biocathodes with a focus on electron fluxes.

Metal–organic frameworks as a new platform for molecular oxygen and aerobic oxidation of organic substrates: Recent advances

Since metal–organic frameworks (MOFs) as a novel kind of crystalline porous materials containing metal ions and organic linkers have emerged in the early 1990s, the number of publications associated with MOFs have remarkably grown in various fields including synthesis methods, structure analysis, gas storage, separation, drug delivery and catalysis. In this review, we outline the recent progress in the application of MOFs as heterogeneous catalysts for oxidations, focusing on the processes that have used the molecular oxygen or air as an oxidant. These two types of oxidants comply with the philosophy of green chemistry and thus provide the greener method concerning environmental benignity and sustainability. Also, the tandem oxidation processes (TOP) which are catalyzed in a manner mentioned above and their proposed mechanisms are presented in detail.

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.