Reductive Activation Of Dioxygen Research Articles

Bio-inspired copper complexes with Cu2S cores: (solvent) effects on oxygen reduction reactions.

The need for effective alternative energy sources and "green" industrial processes is a more crucial societal topic than ever. In this context, mastering oxygen reduction reactions (ORRs) is a key step to develop fuel cells or to propose alternatives to energy-intensive setups such as the anthraquinone process for hydrogen peroxide production. Achieving this goal using bio-inspired metal complexes based on abundant and non-toxic elements could provide an environmentally friendly option. Given the prevalence of Cu-containing active sites capable of reductive activation of dioxygen in nature, the development of Cu-based catalysts for the ORR thus appears to be a relevant approach. We herein report the preparation, full characterization and (TD)DFT investigation of a new dinuclear mixed-valent copper complex 6 exhibiting a Cu2S core and a bridging triflate anion. Its ORR activity was compared with that of its parent catalyst 1. Two types of solvents were used, acetonitrile and acetone, and various catalyst/Me8Fc (electron source) ratios were tested. Our results highlight a counterintuitive solvent effect for 1 and a drastic drop in the activity for 6 in coordinating acetonitrile together with the modification of its chemical structure.

The Role of the Redox Non-Innocent Hydroxyl Ligand in the Activation of O2 Performed by [Ni(H)(OH)

The cationic complex [Ni(H)(OH)]+ was previously found to activate dioxygen and methane in gas phase under single collision conditions. These remarkable reactivities were thought to originate from a non-classical electronic structure, where the Ni-center adopts a Ni(II), instead of the classically expected Ni(III) oxidation state by formally accepting an electron from the hydroxo ligand, which formally becomes a hydroxyl radical in the process. Such radicaloid oxygen moieties are envisioned to easily react with otherwise inert substrates, mimicking familiar reactivities of free radicals. In this study, the reductive activation of dioxygen by [Ni(H)(OH)]+ to afford the hydroperoxo species was investigated using coupled cluster, multireference ab initio and density functional theory calculations. Orbital and wave function analyses indicate that O2 binding tranforms the aforementioned non-classical electronic structure to a classical Ni(III)-hydroxyl system, before O2 reduction takes place. Remarkably, we found no evidence for a direct involvement of the radicaloid hydroxyl in the reaction with O2 , as is often assumed. The function of the redox non-innocent character of the activator complex is to protect the reactive electronic structure until the complex engages O2 , upon which a dramatic electronic reorganization releases internal energy and drives the chemical reaction to completion.

Recent progress in the development of new artificial metalloenzymes as biocatalysts for selective oxidations and Diels‐Alder reaction ‐ Mini‐Review

AbstractOur recent research is turning towards the elaboration of artificial metalloenzymes that catalyze reactions of interest for organic chemistry under eco‐compatible conditions. First, totally artificial metalloenzymes that catalyze selective oxidations in water are described following three main lines: (i) Insertion of microperoxidase 8 into Metal Organic Frameworks leading to artificial metalloenzymes as new biocatalysts for the selective sulfoxydation of sulfides and oxidation of dyes and by H2O2; (ii) Design of a new polyimine polymer‐based artificial reductase that allows the reductive activation of dioxygen and its use as an oxygen atom source for selective oxidations catalyzed by metal complexes including metalloporphyrins, copper complexes or Polyoxometalates and, (iii) Design of new artificial metalloenzymes that catalyze the photoreduction of H2O in the presence of photoactivable ruthenium complexes and the concommitant oxidation of sulfides. Second, the synthesis of new stereoselective Diels‐Alderases is described following three strategies: (i) Covalent insertion of metal complexes into thermostable artificial proteins issued from a new family of alpha‐helical repeated motifs (αReps), (ii) Substitution of the native Fe ion of a cupin‐like protein, ACCO oxidase, by a copper(II) ion and (iii) Insertion of a copper(II) complex‐antagonist conjugate into an adenosine receptor located at the surface of living HEK cells.

Effect of Ligand Fields on the Reactivity of O2‐Activating Iron(II)‐Benzilate Complexes of Neutral N5 Donor Ligands

Three new iron(II)-benzilate complexes [(N4Py)Fe II (benzilate)]ClO 4 ( 1 ), [(N4Py Me2 )Fe II (benzilate)]ClO 4 ( 2 ) and [(N4Py Me4 )Fe II (benzilate)]ClO 4 ( 3 ) of neutral pentadentate nitrogen donor ligands have been isolated and characterized to study their dioxygen reactivity. Single crystal X-ray structures reveal a mononuclear six-coordinate iron(II) centre in each case, where benzilate binds to the iron center in monodentate mode via one carboxylate oxygen. Introduction of methyl groups in the 6-positions of the pyridine rings makes the N4Py Me2 and N4Py Me4 ligands weaker field compared to the parent N4Py ligand. All the complexes ( 1-3 ) react with dioxygen to decarboxylate the coordinated benzilate to benzophenone quantitatively. The decarboxylation takes lesser time for the complex of more sterically hindered ligand and follows the order 3 > 2 > 1 . The complexes display oxygen atom transfer reactivity to thioanisole and also exhibit hydrogen atom transfer reactions with substrates containing weak C-H bonds. Based on interception studies with external substrates, labelling experiment and Hammett analysis, a nucleophilic iron(II)-hydroperoxo species is proposed to form upon two-electron reductive activation of dioxygen by each iron(II)-benzilate complex. The nucleophilic oxidants are converted to the corresponding electrophilic iron(IV)-oxo oxidant upon treatment with a protic acid. The high-spin iron(II)-benzilate complex with the weakest ligand field results in the formation of a more reactive iron-oxygen oxidant.

Encapsulation of Flavin Cofactor within a Manganese Porphyrin-Based Metal-Organic Polyhedron for Reductive Dioxygen Activation.

Encapsulation of flavin mononucleotide (FMN) in a porphyrinatomanganese(III)-based cubic cage allowed the fast reduction of manganese(III) porphyrin in the presence of nicotinamide adenine dinucleotide (NADH). This supramolecular system was capable of efficiently activating dioxygen and catalyzing the oxidation of benzyl alcohol. Control experiments suggested that the close proximity between FMN and manganese(III) porphyrins forced by the host-guest interaction might benefit the electron-transfer process from the FMN cofactor to the metal centers.

Magnetite Fe3O4 Has no Intrinsic Peroxidase Activity, and Is Probably not Involved in Alzheimer's Oxidative Stress

AbstractDespite stated in some highly cited articles, magnetite is devoided of peroxidase activity. In fact, this very stable mixed valence FeIIO⋅FeIII2O3 complex is not catalytically competent to oxidize standard peroxidase substrates, especially at the biologically relevant pH value of 7.4. In addition, magnetite whose deleterious redox activity has been suspected in Alzheimer's disease brain damages, does not significantly interact with amyloid peptide Aβ in vitro, and is not able to induce, either in the presence or absence of Aβ, the reductive activation of dioxygen, the first step of an oxidative stress. In fact, this highly insoluble mineral iron derivative is probably not involved in the oxidative damage of brain neurons of patients with AD.

Magnetite Fe3 O4 Has no Intrinsic Peroxidase Activity, and Is Probably not Involved in Alzheimer's Oxidative Stress.

Despite stated in some highly cited articles, magnetite is devoided of peroxidase activity. In fact, this very stable mixed valence FeII O⋅FeIII2 O3 complex is not catalytically competent to oxidize standard peroxidase substrates, especially at the biologically relevant pH value of 7.4. In addition, magnetite whose deleterious redox activity has been suspected in Alzheimer's disease brain damages, does not significantly interact with amyloid peptide Aβ in vitro, and is not able to induce, either in the presence or absence of Aβ, the reductive activation of dioxygen, the first step of an oxidative stress. In fact, this highly insoluble mineral iron derivative is probably not involved in the oxidative damage of brain neurons of patients with AD.

Iron-Catalyzed Wacker-Type Oxidation

Compared with the widespread use of Pd-catalyzed Wacker-type oxidation of olefins, iron catalysis for this transformation is almost virgin territory. Our work on an iron-catalyzed Wacker-type oxidation through reductive activation of dioxygen is discussed here. This novel single-electron-transfer process not only addresses the issues of the Pd-catalyzed two-electron Wacker-type oxidation, but also possesses unprecedented functional-group tolerance and chemoselectivity. Importantly, the catalytic system uses ambient air as the sole oxidant, and it permits late-stage oxidations of complex molecules.

DNA Cleavage through Reductive Dioxygen Activation by Iron-Bleomycin Mimics with Carboxamido Ligation: Correlation between DNA Cleavage Efficacy and Redox Potential

DNA cleavage by a series of nonheme iron complexes supported by pentadentate carboxamido ligands, dpaqR (R = OMe, H, Cl, and NO2), was evaluated in the presence of a reducing agent under air and in the presence of hydrogen peroxide. Cleavage efficiency clearly increased as redox potential became more positive. These results provide an opportunity for the designing of Bleomycin mimics with higher activity.

Activation of dioxygen by copper metalloproteins and insights from model complexes.

Nature uses dioxygen as a key oxidant in the transformation of biomolecules. Among the enzymes that are utilized for these reactions are copper-containing metalloenzymes, which are responsible for important biological functions such as the regulation of neurotransmitters, dioxygen transport, and cellular respiration. Enzymatic and model system studies work in tandem in order to gain an understanding of the fundamental reductive activation of dioxygen by copper complexes. This review covers the most recent advancements in the structures, spectroscopy, and reaction mechanisms for dioxygen-activating copper proteins and relevant synthetic models thereof. An emphasis has also been placed on cofactor biogenesis, a fundamentally important process whereby biomolecules are post-translationally modified by the pro-enzyme active site to generate cofactors which are essential for the catalytic enzymatic reaction. Significant questions remaining in copper-ion-mediated O2-activation in copper proteins are addressed.

Dioxygen Activation by a Hexagonal SrMnO3 Perovskite Catalyst for Aerobic Liquid‐Phase Oxidation

AbstractHeterogeneous catalysts, which allow a reductive activation of dioxygen (O2) under mild reaction conditions, are promising candidates for highly efficient aerobic oxidation. An effective hexagonal SrMnO3 (SMO) perovskite catalyst for liquid‐phase selective oxidation with O2 was successfully synthesized by the polymerized complex method. The activity of SMO for the aerobic oxidation of alcohols was higher than those of typical manganese oxide‐based catalysts and was heterogeneous, so that the recovered catalyst could be reused without heat treatment under oxidative conditions while keeping its high catalytic performance. The reversible reaction on the surface Mn species on SMO with O2 leads to the formation of Mn–superoxo species, which catalyze the selective oxidative transformation of various types of organic substrates into the desired oxygenated or dehydrogenated products.

Hydroxylation versus Halogenation of Aliphatic C−H Bonds by a Dioxygen‐Derived Iron–Oxygen Oxidant: Functional Mimicking of Iron Halogenases

AbstractAn iron–oxygen intermediate species generated in situ in the reductive activation of dioxygen by an iron(II)–benzilate complex of a monoanionic facial N3 ligand, promoted the halogenation of aliphatic C−H bonds in the presence of a protic acid and a halide anion. An electrophilic iron(IV)–oxo oxidant with a coordinated halide is proposed as the active oxidant. The halogenation reaction with dioxygen and the iron complex mimics the activity of non‐heme iron halogenases.

Hydroxylation versus Halogenation of Aliphatic C-H Bonds by a Dioxygen-Derived Iron-Oxygen Oxidant: Functional Mimicking of Iron Halogenases.

An iron-oxygen intermediate species generated in situ in the reductive activation of dioxygen by an iron(II)-benzilate complex of a monoanionic facial N3 ligand, promoted the halogenation of aliphatic C-H bonds in the presence of a protic acid and a halide anion. An electrophilic iron(IV)-oxo oxidant with a coordinated halide is proposed as the active oxidant. The halogenation reaction with dioxygen and the iron complex mimics the activity of non-heme iron halogenases.

Bio-inspired electron-delivering system for reductive activation of dioxygen at metal centres towards artificial flavoenzymes.

Development of artificial systems, capable of delivering electrons to metal-based catalysts for the reductive activation of dioxygen, has been proven very difficult for decades, constituting a major scientific lock for the elaboration of environmentally friendly oxidation processes. Here we demonstrate that the incorporation of a flavin mononucleotide (FMN) in a water-soluble polymer, bearing a locally hydrophobic microenvironment, allows the efficient reduction of the FMN by NADH. This supramolecular entity is then capable of catalysing a very fast single-electron reduction of manganese(III) porphyrin by splitting the electron pair issued from NADH. This is fully reminiscent of the activity of natural reductases such as the cytochrome P450 reductases with kinetic parameters, which are three orders of magnitude faster compared with other artificial systems. Finally, we show as a proof of concept that the reduced manganese porphyrin activates dioxygen and catalyses the oxidation of organic substrates in water.

ChemInform Abstract: Electron Transfer and Catalysis with High‐Valent Metal‐Oxo Complexes

AbstractReview: 103 refs.

Electron transfer and catalysis with high-valent metal-oxo complexes.

High-valent metal-oxo complexes are produced by reductive activation of dioxygen via reduction of metal complexes with reductants and dioxygen. Photoinduced electron transfer from substrates to metal complexes with dioxygen also leads to the generation of high-valent metal-oxo complexes that can oxygenate substrates. In such a case metal complexes act as a photocatalyst to oxygenate substrates with dioxygen. High-valent metal-oxo complexes are also produced by proton-coupled electron-transfer oxidation of metal complexes by one-electron oxidants with water, oxygenating substrates to regenerate metal complexes. In such a case metal complexes act as a catalyst for electron-transfer oxygenation of substrates by one-electron oxidants with water that acts as an oxygen source. The one-electron oxidants which can oxidize metal complexes can be replaced by much weaker oxidants by a combination of redox photocatalysts and metal complexes. Thus, photocatalytic oxygenation of substrates proceeds via photoinduced electron transfer from a photocatalyst to reductants followed by proton-coupled electron transfer oxidation of metal complexes with the oxidized photocatalyst to produce high-valent metal-oxo complexes that oxygenate substrates. Thermal and photoinduced electron-transfer catalytic reactions of high-valent metal-oxo complexes for oxygenation of substrates using water or dioxygen as an oxygen source are summarized in this perspective.

Dioxygen Activation at Monovalent Nickel

The monovalent oxidation state of nickel has received a growing amount of attention in recent years, in part due to its suggested catalytic role in a number of metalloprotein-mediated transformations. In coordination chemistry, nickel(I) is suitable for reductive activation of dioxygen, provided ligands are used that stabilize this less common oxidation state against disproportionation reactions. Two distinct molecular systems have been explored, which have provided access to new nickel-dioxygen structure types, namely, monomeric side-on and end-on superoxo and trans-micro-1,2-peroxo-dinickel complexes. The geometric and electronic structures of the complexes have been established by advanced spectroscopic methods, including resonance Raman and X-ray absorption spectroscopies, and augmented by density functional theory analyses.

Oxidative and Reductive Degradation of Mixed Contaminants by Bifunctional Aluminum

Transformation of various contaminants including carbon tetrachloride (CT), methyl tert-butyl ether (MTBE), trichloroethylene (TCE), and bis(2-chloroethyl) ether (BCEE) using bifunctional aluminum was examined in batch reactors. Reductive degradation was observed only in reactions with CT while MTBE, TCE, and BCEE underwent oxidative pathways. In a batch reactor containing both CT and MTBE, oxidation of MTBE and reduction of CT by bifunctional aluminum took place simultaneously in the presence of oxygen. The MTBE was degraded to tert-butyl formate, tert-butyl alcohol, acetone, methyl acetate, and isobutene while the reduction of CT produced chloroform and dichloromethane. This indicates that bifunctional aluminum has a dual functionality of decomposing both oxidatively and reductively degradable contaminants together. Aluminum metal serves as a reductant while oxygen acts as an oxidant. Oxidizing capacity of bifunctional aluminum, resulted from reductive activation of dioxygen ( O2 ) , is dependent on bot...

Spin Transition during H2O2 Formation in the Oxidative Half-Reaction of Copper Amine Oxidases

Dioxygen reduction in the oxidative half-reaction of copper amine oxidases (CAOs) has been studied quantum chemically using the hybrid density functional theory (B3LYP). The reductive activation of dioxygen is a spin-forbidden process for which substantial kinetic O-18 (but no deuterium) isotope effects have been found experimentally. The proposed mechanism was divided into three steps, and the last step was studied for two different potential energy surfaces: the quartet and the doublet surfaces. It is suggested that dioxygen reduction occurs through a spin transition that is induced by the exchange interaction between the unpaired spins of the Cu(II) ion and the O2- anion. The step involving this spin transition is suggested to be rate-limiting, which gives a rationalization for the puzzling experimental results when copper is substituted for other metals. The spin transition is triggered by the calculated vibronic perturbation of 5.4 (kcal/mol) Å-1, which leads to a very fast rate of 8 × 1010 s-1 for the spin transition. However, since the spin transition occurs at a calculated energy that is 18−20 kcal/mol higher than that of the reactant, this step could still be rate-limiting. The difference in the O−O bond distance between the resting state (free dioxygen) and the point of the spin transition provides an explanation for the oxygen isotope effect.

Reductive Activation of Dioxygen by a Myoglobin Reconstituted with a Flavohemin

We successfully converted myoglobin, an oxygen-storage hemoprotein, into an oxygen-activating hemoprotein like cytochrome P450s by replacing the native hemin with the artificially created flavohemin. The reconstituted myoglobin, rMb(1), was chacterized by ESI-TOF-mass, UV-vis, and fluorescence spectra. The 1H NMR spectrum of cyanomet rMb(1) indicates that two hemin conformers are present in a ratio of 1:1. Upon the addition of NADH to the buffer solution of rMb(1) in the presence of SOD and catalase, the oxymyoglobin was rapidly formed. As compared with the formation of the oxygenated native myoglobin in the presence of 10-N-(acetylaminoethyl)isoalloxazine, the rate constant of the oxyheme formation in rMb(1) is 6 times larger. This is because the flavin covalently linked to the terminal heme propionate functions as an effective mediator of an electron transfer from NADH to the hemin in rMb(1). Furthermore, rMb(1) shows the deformylation activity, when 2-phenylpropionaldehyde (2-PPA) was employed as a substrate. This result indicates that the oxyheme is reductively activated to Fe(III)-peroxoanion (Fe(III)-O22-). The result in this report is the first example of the activation of dioxygen by myoglobin. This study shows the utility of the replacement of the native hemin with a chemically modified one for the functionalization of myoglobin.