Four-electron Reduction Of O2 Research Articles

Membrane-Bound Redox Enzyme Cytochrome bd-I Promotes Carbon Monoxide-Resistant Escherichia coli Growth and Respiration.

The terminal oxidases of bacterial aerobic respiratory chains are redox-active electrogenic enzymes that catalyze the four-electron reduction of O2 to 2H2O taking out electrons from quinol or cytochrome c. Living bacteria often deal with carbon monoxide (CO) which can act as both a signaling molecule and a poison. Bacterial terminal oxidases contain hemes; therefore, they are potential targets for CO. However, our knowledge of this issue is limited and contradictory. Here, we investigated the effect of CO on the cell growth and aerobic respiration of three different Escherichia coli mutants, each expressing only one terminal quinol oxidase: cytochrome bd-I, cytochrome bd-II, or cytochrome bo3. We found that following the addition of CO to bd-I-only cells, a minimal effect on growth was observed, whereas the growth of both bd-II-only and bo3-only strains was severely impaired. Consistently, the degree of resistance of aerobic respiration of bd-I-only cells to CO is high, as opposed to high CO sensitivity displayed by bd-II-only and bo3-only cells consuming O2. Such a difference between the oxidases in sensitivity to CO was also observed with isolated membranes of the mutants. Accordingly, O2 consumption of wild-type cells showed relatively low CO sensitivity under conditions favoring the expression of a bd-type oxidase.

Four-Electron Reduction of O2 Using Distibines in the Presence of ortho-Quinones.

This study, which aims to identify atypical platforms for the reduction of dioxygen, describes the reaction of O2 with two distibines, namely, 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene and 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethyldihydroacridine, in the presence of an ortho-quinone such as phenanthraquinone. The reaction proceeds by oxidation of the two antimony atoms to the + V state in concert with reductive cleavage of the O2 molecule. As confirmed by 18O labeling experiments, the two resulting oxo units combine with the ortho-quinone to form an α,α,β,β-tetraolate ligand that bridges the two antimony(V) centers. This process, which has been studied both experimentally and computationally, involves the formation of asymmetric, mixed-valent derivatives featuring a stibine as well as a catecholatostiborane formed by oxidative addition of the quinone to only one of the antimony centers. Under aerobic conditions, the catecholatostiborane moiety reacts with O2 to form a semiquinone/peroxoantimony intermediate, as supported by NMR spectroscopy in the case of the dimethyldihydroacridine derivative. These intermediates swiftly evolve into the symmetrical bis(antimony(V)) α,α,β,β-tetraolate complexes via low barrier processes. Finally, the controlled protonolysis and reduction of the bis(antimony(V)) α,α,β,β-tetraolate complex based on the 9,9-dimethylxanthene platform have been investigated and shown to regenerate the starting distibine and the ortho-quinone. More importantly, these last reactions also produce two equivalents of water as the product of O2 reduction.

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand.

The reaction of TaMe3 Cl2 with the rigid acridane-derived trisamine H3 NNN yields the tantalum(V) complex [TaCl2 (NNNcat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O2 yielding the oxido-bridged bimetallic complex [{TaCl2 (NNNsq )}2 O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O2 activation was investigated by DFT calculations revealing initial binding of O2 to the tantalum(V) center followed by complete O2 scission to produce a terminal oxido-complex.

First-Principles Study on Pt-Based Sub-Nanocluster Carbon-Metal Hybrid Catalysts for the Oxygen Reduction Reaction

Recently, the polymer electrolyte membrane fuel cell (PEMFC) has been drawing huge attention as an eco-friendly energy conversion device which can replace an internal combustion engine without environmental contaminant such as CO2 and NOx. However, the use of expensive noble metal catalysts such as Pt is essential for the cathode because the oxygen reduction reaction (ORR) at cathode are kinetically much slower than the hydrogen oxidation at anode. For this reason, many of researchers have vigorously worked on the development of Pt-based alloy catalysts such as Pt-Co and Pt-Ni alloys to reduce noble metal usage, but there are still limitations in catalytic activity and durability for the commercialization. Herein, to overcome the limitations, we developed Pt-based carbon-metal hybrid catalyst showing high performance for ORR using density functional theory (DFT) calculation. The slab model of Pt-based carbon-metal hybrid catalyst was constructed by Pt sub-nanocluster supported on graphite with graphene-shaped carbon shell on top of the Pt sub-nanocluster. The most stable geometry of the hybrid catalyst was determined by the formation energy, and the ORR mechanism on surface of hybrid catalyst as well as electrochemical activity was investigated. As a result of investigation, we confirmed that the oxygen reduction occurs on active graphene-shaped carbon shell sites. On pure Pt sub-nanocluster hybrid catalyst, once the hydrogen superoxide (OOH) is adsorbed on carbon shell surface after the oxygen adsorption and protonation, the hydrogen peroxide (H2O2) is preferentially formed at higher onset potential than that of four-electron O2 reduction, which the final product is water (H2O). However, the formation of hydrogen peroxide in PEMFC should be avoided owing to creation of radical, resultant degradation of membrane and low energy conversion efficiency. Surprisingly, we identified that the hydrogen peroxide formed on pure Pt sub-nanocluster hybrid catalyst can be decomposed into OH and H2O with low energy barrier, whose value is much lower than desorption energy of produced hydrogen peroxide, so that subsequent reduction reaction can occur. When this four-electron ORR via hydrogen peroxide decomposition takes place, the ORR activity is improved compared to Pt(111) catalyst, where the onset potential of pure Pt sub-nanocluster hybrid catalyst presents is higher than that of Pt(111). Moreover, we introduced Pt-based alloy sub-nanocluster to reduce Pt usage further. The Pt-based alloy sub-nanocluster hybrid catalyst also exhibited increased ORR activity. For Pt-based alloy sub-nanocluster hybrid catalyst, we carried out analysis of density of state and figured out that the more orbital overlap between carbon shell and sub-nanocluster is generated, the higher catalytic activity is shown due to enrichment of electrons on carbon shell surface. We expect this work can help understanding the mechanism of activity improvement of carbon metal hybrid catalyst, suggesting promising way to design high-performance ORR catalyst with low Pt loadings.

Bioinspired Trinuclear Copper Catalyst for Water Oxidation with a Turnover Frequency up to 20000 s-1.

Solar-powered water splitting is a dream reaction for constructing an artificial photosynthetic system for producing solar fuels. Natural photosystem II is a prototype template for research on artificial solar energy conversion by oxidizing water into molecular oxygen and supplying four electrons for fuel production. Although a range of synthetic molecular water oxidation catalysts have been developed, the understanding of O-O bond formation in this multielectron and multiproton catalytic process is limited, and thus water oxidation is still a big challenge. Herein, we report a trinuclear copper cluster that displays outstanding reactivity toward catalytic water oxidation inspired by multicopper oxidases (MCOs), which provides efficient catalytic four-electron reduction of O2 to water. This synthetic mimic exhibits a turnover frequency of 20000 s-1 in sodium bicarbonate solution, which is about 150 and 15 times higher than that of the mononuclear Cu catalyst (F-N2O2Cu, 131.6 s-1) and binuclear Cu2 complex (HappCu2, 1375 s-1), respectively. This work shows that the cooperation between multiple metals is an effective strategy to regulate the formation of O-O bond in water oxidation catalysis.

Catalytic Four-Electron Reduction of Dioxygen by Ferrocene Derivatives with a Nonheme Iron(III) TAML Complex.

A mononuclear nonheme iron(III) complex with a tetraamido macrocyclic ligand (TAML), [(TAML)FeIII]- (1), is a selective precatalyst for four-electron reduction of dioxygen by ferrocene derivatives in the presence of acetic acid (CH3COOH) in acetone. This is the first work to show that a nonheme iron(III) complex catalyzes the four-electron reduction of O2 by one-electron reductants. An iron(V)-oxo complex, [(TAML)FeV(O)]- (2), was produced by oxygenation of 1 with O2 via the formation of triacetone triperoxide (TATP), acting as an autocatalyst that shortened the induction time for the generation of 2. Decamethylferrocene (Me10Fc) and octamethylferrocene (Me8Fc) reduced 2 to 1 by two electrons in the presence of CH3COOH to produce decamethylferrocenium cation (Me10Fc+) and octamethylferrocenium cation (Me8Fc+), respectively. Then, 1 was oxygenated by O2 to regenerate 2 via the formation of TATP. In the cases of ferrocene (Fc), bromoferrocene (BrFc) and 1,1'-dibromoferrocene (Br2Fc), initial electron transfer from ferrocene derivatives to 2 occurred; however, neither a second proton-coupled electron transfer from ferrocene derivatives to 2 nor a catalytic four-electron reduction of O2 occurred.

Role of a Tyrosine Radical in Human Ceruloplasmin Catalysis.

Multicopper oxidases (MCOs) are a large family of diverse enzymes found in both eukaryotes and prokaryotes that couple one-electron oxidations of various substrates to the four-electron reduction of O2 to H2O, functioning through a set of metallocofactors consisting of one type 1 copper (T1 Cu) and one trinuclear copper cluster (TNC). Human serum ceruloplasmin (Cp) is a unique member of MCOs composed of six cupredoxin domains and harbors six Cu ions arranged as three T1 Cu and one TNC. The native substrate of Cp is Fe2+. It is an essential ferroxidase critical for iron homeostasis and is closely associated with metal-mediated diseases and metal neurotoxicity. In human serum, Cp operates under substrate-limiting low [Fe2+] but high [O2] conditions, implying the possible involvement of partially reduced intermediates in Cp catalysis. In this work, we studied for the first time Cp reactivities at defined partially reduced states and discovered a tyrosine radical weakly magnetically coupled to the native intermediate (NI) of the TNC via a hydrogen bond. Our results lead to a new hypothesis that human iron transport is regulated as the paired transfer of iron from ferroportin to Cp to transferrin, and the tyrosine residue in Cp acts as a gate to avoid reactive oxygen species (ROS) formation when Fe2+ delivery is dysregulated.

Photocatalytic H2O2 production from O2 under visible light irradiation over phosphate ion-coated Pd nanoparticles-supported BiVO4

H2O2 production from O2 over Pd nanoparticle (NP) co-catalysts-supported BiVO4 was studied. Several deposition methods and conditions (active metal, co-catalyst loading, solution pH) were investigated for catalyst preparation, and the photo-assisted deposition (PAD) of Pd from an aqueous phosphate buffer solution (pH = 7) was found to be optimal, resulting in small and uniform phosphate ion (PO43−)-coated Pd NPs being selectively deposited on the reductive surfaces of the BiVO4. This catalyst exhibited better H2O2 production from O2 in the presence of methanol (CH3OH) as a hole scavenger under visible light irradiation than that by the catalysts prepared by the other methods and under other PAD conditions. It is surmised that PO43− partially coats smaller Pd NPs with a narrow size distribution on the reductive surfaces of the BiVO4, and that these NPs promote H2O2 generation synergistically by catalyzing the two-electron reduction of O2 while simultaneously inhibiting the four-electron reduction of O2 and the two-electron reduction of H2O2 to H2O.

Enhanced anodic oxidation and energy saving for dye removal by integrating O2-reducing biocathode into electrocatalytic reactor

Simultaneous enhancement of dye removal and reduction of energy consumption is critical for electrochemical oxidation in treating dyeing wastewater. To address this issue, this work presented a novel process termed biocathode-electrocatalytic reactor (BECR). The dual-chamber BECR employed O2-reducing biocathode instead of normal stainless steel (SS) cathode and MnOx/Ti anode to reduce O2 in the cathode chamber and treat methylene blue (MB) in the anode chamber, respectively. BECR successfully started up at 0.7 and 1 V and substantially improved MB and total organic carbon (TOC) removal compared with the electrocatalytic reactor with SS cathode (ECR-SS), e.g., removal of MB (150 mg L−1) increased from 27.0 ± 0.2% to 78.1 ± 0.4% at 1 V. To achieve the same TOC removal, BECR reduced the energy consumption by approximately 45.7% compared with ECR-SS (19.5 and 35.9 kWh (kg TOC) −1 for BECR and ECR, respectively). To explain the above merits of BECR, M(·OH) (·OH adsorbed on the anode surface) generation, potential of MnOx/Ti anode (Ea), and their correlation were investigated. When coupled with O2-reducing biocathode, MnOx/Ti anode considerably accelerated M(·OH) generation because Ea increased. The increased Ea in BECR was due to the fact that its cathodic reaction was converted to the four-electron O2 reduction, which exhibited a higher cathodic potential than hydrogen evolution reaction on SS cathode in ECR-SS. Thereby, BECR simultaneously promoted dye removal and reduced energy consumption, showing promise in treating dyeing wastewater.

Fate of oxygen species from O2 activation at dimetal cofactors in an oxidase enzyme revealed by 57Fe nuclear resonance X-ray scattering and quantum chemistry

Oxygen (O2) activation is a central challenge in chemistry and catalyzed at prototypic dimetal cofactors in biological enzymes with diverse functions. Analysis of intermediates is required to elucidate the reaction paths of reductive O2 cleavage. An oxidase protein from the bacterium Geobacillus kaustophilus, R2lox, was used for aerobic in-vitro reconstitution with only 57Fe(II) or Mn(II) plus 57Fe(II) ions to yield [FeFe] or [MnFe] cofactors under various oxygen and solvent isotopic conditions including 16/18O and H/D exchange. 57Fe-specific X-ray scattering techniques were employed to collect nuclear forward scattering (NFS) and nuclear resonance vibrational spectroscopy (NRVS) data of the R2lox proteins. NFS revealed Fe/Mn(III)Fe(III) cofactor states and Mössbauer quadrupole splitting energies. Quantum chemical calculations of NRVS spectra assigned molecular structures, vibrational modes, and protonation patterns of the cofactors, featuring a terminal water (H2O) bound at iron or manganese in site 1 and a metal-bridging hydroxide (μOH−) ligand. A procedure for quantitation and correlation of experimental and computational NRVS difference signals due to isotope labeling was developed. This approach revealed that the protons of the ligands as well as the terminal water at the R2lox cofactors exchange with the bulk solvent whereas 18O from 18O2 cleavage is incorporated in the hydroxide bridge. In R2lox, the two water molecules from four-electron O2 reduction are released in a two-step reaction to the solvent. These results establish combined NRVS and QM/MM for tracking of iron-based oxygen activation in biological and chemical catalysts and clarify the reductive O2 cleavage route in an enzyme.

De Novo Design of Four-Helix Bundle Metalloproteins: One Scaffold, Diverse Reactivities.

De novo protein design represents an attractive approach for testing and extending our understanding of metalloprotein structure and function. Here, we describe our work on the design of DF (Due Ferri or two-iron in Italian), a minimalist model for the active sites of much larger and more complex natural diiron and dimanganese proteins. In nature, diiron and dimanganese proteins protypically bind their ions in 4-Glu, 2-His environments, and they catalyze diverse reactions, ranging from hydrolysis, to O2-dependent chemistry, to decarbonylation of aldehydes. In the design of DF, the position of each atom-including the backbone, the first-shell ligands, the second-shell hydrogen-bonded groups, and the well-packed hydrophobic core-was bespoke using precise mathematical equations and chemical principles. The first member of the DF family was designed to be of minimal size and complexity and yet to display the quintessential elements required for binding the dimetal cofactor. After thoroughly characterizing its structural, dynamic, spectroscopic, and functional properties, we added additional complexity in a rational stepwise manner to achieve increasingly sophisticated catalytic functions, ultimately demonstrating substrate-gated four-electron reduction of O2 to water. We also briefly describe the extension of these studies to the design of proteins that bind nonbiological metal cofactors (a synthetic porphyrin and a tetranuclear cluster), and a Zn2+/proton antiporting membrane protein. Together these studies demonstrate a successful and generally applicable strategy for de novo metalloprotein design, which might indeed mimic the process by which primordial metalloproteins evolved. We began the design process with a highly symmetrical backbone and binding site, by using point-group symmetry to assemble the secondary structures that position the amino acid side chains required for binding. The resulting models provided a rough starting point and initial parameters for the subsequent precise design of the final protein using modern methods of computational protein design. Unless the desired site is itself symmetrical, this process requires reduction of the symmetry or lifting it altogether. Nevertheless, the initial symmetrical structure can be helpful to restrain the search space during assembly of the backbone. Finally, the methods described here should be generally applicable to the design of highly stable and robust catalysts and sensors. There is considerable potential in combining the efficiency and knowledge base associated with homogeneous metal catalysis with the programmability, biocompatibility, and versatility of proteins. While the work reported here focuses on testing and learning the principles of natural metalloproteins by designing and studying proteins one at a time, there is also considerable potential for using designed proteins that incorporate both biological and nonbiological metal ion cofactors for the evolution of novel catalysts.

Oxygen Reduction Reactions of Fe-N-C Catalysts: Current Status and the Way Forward

Currently, Fe-N-C materials are considered to be among the most important oxygen reduction reaction (ORR) catalysts, because they are potential substitutes for Pt-based catalysts and are therefore promising in the development of non-noble metal-based catalysts. However, challenges such as electron transfer kinetics still exist and need to be improved upon. From a chemical stand point, improvements can be made through the better understanding of mechanisms in Fe-N-C-based ORR catalysis along with a deeper understanding of the chemical origin of active sites on Fe-N-C catalyst surfaces. Based on these, this comprehensive review will focus on the energy conversion, transformation kinetics and electron transfer of the ORR process as catalyzed by Fe-N-C catalysts. And by taking these and other relevant analytical results for Fe-N-C materials into consideration, primary strategies in the improvement in Fe-N-C catalyst activity will be presented. As the promising Pt substrate for oxygen reduction catalysis, the Fe-N-C materials are active toward the four-electron reduction of O2 to H2O. This review focuses on the profound understanding of heterogeneous oxygen reduction reaction on Fe-N-C materials from the following aspects: (1) thermodynamics of energy conversion in ORR processes, (2) kinetics of ORR processes based on Fe-N-C catalysts, (3) the textural features of Fe-N-C and analytic results known as far, (4) fundamental principle for Fe-N-C materials synthesis and (5) practical application for fuel cell and metal–air batteries.

Electrophoretically Sheathed Carbon Fiber Microelectrodes with Metal/Nitrogen/Carbon Electrocatalyst for Electrochemical Monitoring of Oxygen in Vivo.

Metal/nitrogen/carbon (M/N/C, M = Co or Fe) nanomaterials have been demonstrated to catalyze oxygen reduction reaction (ORR) with implication in fuel cell studies; however, their potential application as sensing materials for in vivo monitoring of oxygen (O2) in the central nervous system has never been reported. This study reports a first demonstration that M/N/C nanomaterials can be used as sensing materials to form an electrochemical assay for in vivo O2 monitoring. To demonstrate this application, the M/N/C nanocomposites prepared by pyrolysis of zeolitic imidazolate framework (ZIF-67) are used as an example and is electrophoretically deposited onto carbon fiber microelectrodes (CFEs) to catalyze a four-electron reduction of O2 without producing cell-toxic hydrogen peroxide intermediate. The M/N/C-sheathed CFEs have high catalytic performance toward ORR in a neutral solution, selectivity toward O2 sensing in the presence of ascorbic acid (AA), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), uric acid (UA), and 5-hydroxytryptamine (5-HT) at their physiological levels in rat brain, and capability to real-time monitor O2 fluctuation during respiring gases. This study offers a new electrochemical approach to in vivo O2 monitoring with nonplatinum catalyst for ORR.

Changing the Selectivity of O2 Reduction Catalysis with One Ligand Heteroatom

The development of catalytic systems that selectively reduce O2 to water is needed to continue the advancement of fuel cell technologies. As an alternative to platinum catalysts, derivatives of iron (Fe) and cobalt (Co) porphyrin molecular catalysts provide one benchmark for catalyst design, but incorporation of these catalysts into heterogeneous platforms remains a challenge. Co-porphyrins can be heterogeneous O2 reduction catalysts when immobilized on to edge plane graphite (EPG) electrodes, but their selectivity for the desired four-electron reduction of O2 to H2O is often poor. Herein, we demonstrate substantial improvements in the O2 reduction selectivity using a Co-porphyrin that incorporates a 2-pyridyl group at one of the meso-positions of a Co-tetraarylporphyrin (cobalt(II) 5-(2-pyridyl)-10,15,20-triphenylporphyrin, CoTPPy). The properties of CoTPPy immobilized on EPG were investigated using cyclic voltammetry, rotating disk, and rotating ring-disk electrochemistry. The presence of a single 2-pyr...

Bimetallic M/N/C catalysts prepared from π-expanded metal salen precursors toward an efficient oxygen reduction reaction.

Nonprecious metal electrocatalysts are being explored as alternatives to platinum-group metal electrocatalysts for the oxygen reduction reaction (ORR) which is required for cathode materials in fuel cells. Herein, we describe a new method for preparing bimetallic nitrogen-containing carbon catalysts with high ORR activity using π-expanded M(salen) precursors. The M/N/C and bimetallic FeM/N/C ORR catalysts were obtained by pyrolysis of a mixture of a carbon support (Vulcan XC-72R) and the metal complex as a precursor. The bimetallic FeCu catalyst prepared from Fe and Cu complexes with the N,N′-bis(2-hydroxy-1-naphthylidene)-1,2-phenylenediamine ligand (2NAPD) is found to have an onset potential of 0.87 V, which is positively shifted by 50 mV from that of the catalyst prepared from the monometallic Fe(2NAPD) complex. The FeCu/N/C catalyst promotes efficient four-electron reduction in the ORR. High-resolution transmission electron microscopy studies reveal that both Fe and Cu metals together with pyridinic nitrogen species are highly dispersed within the carbonaceous structure in FeCu/2NAPD@VC, suggesting that the N-coordinated Fe and Cu sites promote efficient four-electron reduction of O2. This new methodology facilitates design of nonprecious bimetallic carbon catalysts with excellent ORR activity.

Preparation of nitrogen-doped carbon materials based on polyaniline fiber and their oxygen reduction properties

The nitrogen-doped carbon materials express excellent oxygen reduction reaction (ORR) electrocatalytic activity, which can be used as cathode catalyst for fuel cells to replace platinum electrocatalysts. In this paper, polyaniline and glucose as precursors were used to prepare nitrogen-doped carbon materials by hydrothermal reaction combined with high temperature carbonization activation. The results showed that the onset potential of nitrogen-doped porous materials calcined at 900 ℃ (PCPG-900) was −0.171 V, which was close to commercial Pt/C catalyst (−0.03 V) and the limiting current density was 5.4 mA cm−2, which was 0.9 mA cm−2 higher than that of Pt/C catalyst (4.5 mA cm−2). And the (average) number of electrons transfer number per molecule of O2 was 4.2, indicating that oxygen reduction reaction mainly via four-electron reduction of O2 to H2O. The resulting product may be a promising candidate as alternative catalysts for the cathode side in fuel cells.

Enhanced H2O2 production by selective electrochemical reduction of O2 on fluorine-doped hierarchically porous carbon

Electrochemical synthesis of hydrogen peroxide (H2O2) via two-electron pathway of oxygen reduction reaction is a promising alternative to the current anthraquinone process. The H2O2 production from O2 is a competing reaction with four-electron O2 reduction to H2O, and the selectivity is related to the adsorption energy of the OOH intermediate on electrocatalysts surface. Generally, the properties for binding of OOH intermediate on catalysts can be controlled by changing its electronic structure. Herein, the electronic structure of porous carbon materials was tuned by doping different types and contents of fluorine species. The yield of H2O2 generation depended on the F content and the best catalytic activity toward H2O2 electrosynthesis was obtained with F content of 3.41 at.%. The resultant F-doped porous carbon (FPC) catalysts exhibited good H2O2 selectivity of 97.5–83.0% and the H2O2 production rate could reach 112.6–792.6 mmol h−1 g−1 over the potential range of 0.2 V to −0.3 vs. RHE (pH 1). The density functional theory (DFT) calculations and experiments revealed that the incorporation of CF2, 3 into carbon plane promotes the activation of O2 molecule and facilitates desorption of OOH intermediate, which was crucial to H2O2 synthesis.

Molecular Cobalt Catalysts for O2 Reduction: Low-Overpotential Production of H2O2 and Comparison with Iron-Based Catalysts.

A series of mononuclear pseudomacrocyclic cobalt complexes have been investigated as catalysts for O2 reduction. Each of these complexes, with CoIII/II reduction potentials that span nearly 400 mV, mediate highly selective two-electron reduction of O2 to H2O2 (93-99%) using decamethylferrocene (Fc*) as the reductant and acetic acid as the proton source. Kinetic studies reveal that the rate exhibits a first-order dependence on [Co] and [AcOH], but no dependence on [O2] or [Fc*]. A linear correlation is observed between log(TOF) vs E1/2(CoIII/II) for the different cobalt complexes (TOF = turnover frequency). The thermodynamic potential for O2 reduction to H2O2 was estimated by measuring the H+/H2 open-circuit potential under the reaction conditions. This value provides the basis for direct assessment of the thermodynamic efficiency of the different catalysts and shows that H2O2 is formed with overpotentials as low as 90 mV. These results are compared with a recently reported series of Fe-porphyrin complexes, which catalyze four-electron reduction of O2 to H2O. The data show that the TOFs of the Co complexes exhibit a shallower dependence on E1/2(MIII/II) than the Fe complexes. This behavior, which underlies the low overpotential, is rationalized on the basis of the catalytic rate law.

Oxidase catalysis via aerobically generated hypervalent iodine intermediates.

The development of sustainable oxidation chemistry demands strategies to harness O2 as a terminal oxidant. Oxidase catalysis, in which O2 serves as a chemical oxidant without necessitating incorporation of oxygen into reaction products, would allow diverse substrate functionalization chemistry to be coupled to O2 reduction. Direct O2 utilization suffers from intrinsic challenges imposed by the triplet ground state of O2 and the disparate electron inventories of four-electron O2 reduction and two-electron substrate oxidation. Here, we generate hypervalent iodine reagents-a broadly useful class of selective two-electron oxidants-from O2. This is achieved by intercepting reactive intermediates of aldehyde autoxidation to aerobically generate hypervalent iodine reagents for a broad array of substrate oxidation reactions. The use of aryl iodides as mediators of aerobic oxidation underpins an oxidase catalysis platform that couples substrate oxidation directly to O2 reduction. We anticipate that aerobically generated hypervalent iodine reagents will expand the scope of aerobic oxidation chemistry in chemical synthesis.

Temperature Dependence of the Catalytic Two- versus Four-Electron Reduction of Dioxygen by a Hexanuclear Cobalt Complex.

The synthesis and characterization of a hexanuclear cobalt complex 1 involving a nonheme ligand system, L1, supported on a Sn6O6 stannoxane core are reported. Complex 1 acts as a unique catalyst for dioxygen reduction, whose selectivity can be changed from a preferential 4e-/4H+ dioxygen-reduction (to water) to a 2e-/2H+ process (to hydrogen peroxide) only by increasing the temperature from -50 to 25 °C. A variety of spectroscopic methods (119Sn-NMR, magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV-vis absorption, and X-ray absorption spectroscopy (XAS)) coupled with advanced theoretical calculations has been applied for the unambiguous assignment of the geometric and electronic structure of 1. The mechanism of the O2-reduction reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The reason why the same catalyst can act in either the two- or four-electron reduction of O2 can be explained by the constraint provided by the stannoxane core that makes the O2-binding to 1 an entropically unfavorable process. This makes the end-on μ-1,2-peroxodicobalt(III) intermediate 2 unstable against a preferential proton-transfer step at 25 °C leading to the generation of H2O2. In contrast, at -50 °C, the higher thermodynamic stability of 2 leads to the cleavage of the O-O bond in 2 in the presence of electron and proton donors by a proton-coupled electron-transfer (PCET) mechanism to complete the O2-to-2H2O catalytic conversion in an overall 4e-/4H+ step. The present study provides deep mechanistic insights into the dioxygen reduction process that should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.