Oxygen Insertion Reactions Research Articles

Ultrathin RhCu bimetallenes for the selective electro-oxidation of glycerol

The selective electro-oxidation reaction represents an efficient and environmentally friendly pathway to produce value-added chemicals from glycerol. Herein, we introduce a high-temperature cyanogel-reduction method to synthesize ultrathin RhCu bimetallenes (RhCu-BMLs) with varying Rh/Cu molar ratios and investigate their catalytic activity and selectivity for the glycerol electro-oxidation reaction (GEOR). Among these catalysts, the as-prepared Rh3Cu1-BMLs exhibit outstanding mass activity (579.3 A gRh−1) for the GEOR under alkaline conditions, marking a 2.32-fold enhancement in mass activity (249.6 A gRh−1) relative to single-component Rh metallens (Rh-MLs). Meanwhile, both experimental characterizations and theoretical calculations reveal that Cu atoms can attenuate OH− adsorption and block the oxygen insertion reaction for the generation of tartaric acid due to the electronic effect. Consequently, Rh3Cu1-BMLs achieve a remarkable 78.7 % selectivity for generating glyceric acid during GEOR. This work provides an important reference for fabricating bimetallic Rh-based catalysts and advancing the selectivity and activity in GEOR.

Mechanistic Studies on Oxygen-Insertion into Osmium(III)–Carbon Bond via High-Valent Oxido-Osmium(V) Complex

Abstract An osmium(III)-metalacycle complex consisting of 2-phenylpyridine is converted to the 2-phenoxidopyridine-osmium(III) complex upon treatment with a tertiary amine N-oxide, where a high-valent oxido-osmium(V) complex is proposed as a key reactive intermediate (H. Sugimoto et al. Organometallics, 2021, 40, 102–106). To gain insight into the reaction mechanism, substituent effects of 2-phenylpyridine moiety on the oxygen atom insertion reaction are examined using a series of 2-(4-X-phenyl)pyridines. All the cyclometalated complexes react with the tertiary amine N-oxide to produce the phenoxido-osmium(III) complexes. Kinetic analyses are conducted on the adduct formation process and the oxygen insertion reaction. In the former process, the binding constant of the amine N-oxide to the osmium(III) center is not influenced by electron withdrawing nature of the substituent X, but decrease of the reaction rate is observed in the latter process as electron withdrawing nature of X increases. Furthermore, the electronic effects of the amine N-oxides are examined using p-substituted N,N-dimethylaniline N-oxide (p-Y-C6H4N(O)(Me)2) to find that reaction rates increase as the electron withdrawing nature of Y increases. These results are consistent with the proposed mechanism in which the C–O bond formation involves conversion of the osmium(III)-N-oxide adduct to its oxido-osmium(V) species as the active oxidant.

Oxidation of chlortetracycline and its isomers by Botrytis aclada laccase in the absence of mediators: pH dependence and identification of transformation products by LC-MS.

Tetracyclines are antibiotics considered emerging pollutants and currently, wastewater treatment plants are not able to remove them efficiently. Laccases are promising enzymes for bioremediation because they can oxidize a wide variety of substrates. The aim of this study was to evaluate the Botrytis aclada laccase for the oxidation of chlortetracycline and its isomers in the absence of a mediator molecule, at a pH range between 3.0 to 7.0, and to characterize the transformation products by LC-MS. Chlortetracycline and three isomers were detected in both, controls and reaction mixtures at 0h and in controls after 48h of incubation but in different proportions depending on pH. An additional isomer was also detected, but only in the presence of BaLac. Based on the transformation products identified in the enzymatic reactions and information from literature, we assembled a network of transformation pathways starting from chlortetracycline and its isomers. The spectrometric analysis of the products indicated the probable occurrence of oxygen insertion, dehydrogenation, demethylation and deamination reactions. Four new products were identified, and we also described a novel transformation product without the chloro group. We observed that increasing pH led to higher diversity of main products. This is the first study using the laccase from fungi Botrytis aclada to oxidate chlortetracycline and its isomers and it can be considered as an ecological alternative to be used in bioremediation processes such as wastewater.

Ozonative epoxidation of ethylene: A novel process for production of ethylene oxide

Here we report a novel reaction for ethylene oxide production using catalytic reaction of ethylene with ozone at room temperature. A key aspect of this path is boosting selective oxygen coverage on the surface of the catalyst which enables achieving high ethylene oxide selectivity and maintaining high conversion. Comparing the structural changes of the catalyst before and after the reaction suggested that the process allows facile generation of the catalytic active sites and selective oxygen species on the Ag/Al2O3 catalyst. With this novel process and under kinetically controlled regime, single-pass ethylene conversion of 98% and selectivity of 56% were achieved at room temperature. The low apparent activation energy of 22.1 kJmol-1 indicated the feasibility and high rate of this reaction at room temperature. This approach is believed to deliver a new class of processes for other oxygen insertion reactions.

Discovery and Characterization of a Baeyer-Villiger Monooxygenase using Sequence Similarity Network Analysis.

Baeyer-Villiger monooxygenases(BVMOs) are important flavin-dependent enzymes which perform oxygen insertion reactions leading to valuable products. As reported in many studies, BVMOs are usually unstable during application, preventing a wider usage in biocatalysis. Here, we discovered a novel NADPH-dependent BVMO which originates fromHalopolyspora algeriensisusing sequence similarity networks (SSNs). The enzyme is stable at temperatures between 10°C to 30°C up to five days after the purification, and yields the normal ester product. In this study,the substrate scope was investigated for a broad range of aliphatic ketones and the enzyme was biochemically characterized to identify optimum reaction conditions. The best substrate (86% conversion) was 2-dodecanone using purified enzyme. This novel BVMO could potentially be applied as part of an enzymatic cascade or in bioprocesses which utilize aliphatic alkanes as feedstock.

Monitoring Light‐Driven Oxygen Insertion Reactions into Metal Carbon Bonds by LED‐NMR Spectroscopy

AbstractThe facile light‐driven insertion reaction of oxygen into metal carbon bonds of the BPI (1,3‐bis(2‐pyridylimino)isoindole) complexes [Pt(BPI)Me] and [Pd(BPI)Me] has been investigated by LED‐NMR in CDCl3. The initial insertion reaction leads to peroxo complexes [Pt(BPI)OOMe] and [Pd(BPI)OOMe], which undergo further reactions over time. Spectra were recorded at 1 minute time intervals, which enabled the tracking of the methyl substituent, which eventually generates formaldehyde (and methanediol) and methanol in almost equal proportions. Degradation of the solvent CDCl3 to phosgene and DCl in the presence of oxygen and light leads to several side reactions. DCl reacts with [M(BPI)Me] and [M(BPI)OOMe] to form [M(BPI)Cl], whereas phosgene reacts with in situ generated methanol to chloro methylformate and dimethyl carbonate.

Photolytic Activation of Late-Transition-Metal–Carbon Bonds and Their Reactivity toward Oxygen

The photolytic activation of palladium(II) and platinum(II) complexes [M(BPI)(R)] (R = alkyl, aryl) featuring the 1,3-bis(2-pyridylimino)isoindole (BPI) ligand has been investigated in various solvents. In the absence of oxygen, the formation of chloro complexes [M(BPI)Cl] is observed in chlorinated solvents, most likely due to the photolytic degradation of the solvent and formation of HCl. The reactivity of the complexes toward oxygen has been studied both experimentally and computationally. Excitation by UV irradiation (365 nm) of the metal complexes [Pt(BPI)Me] and [Pd(BPI)Me] leads to distortion of the square-planar coordination geometry in the excited triplet state and a change in the electronic structure of the complexes that allows the interaction with oxygen. TD-DFT computational studies suggest that, in the case of palladium, the Pd(III) superoxide intermediate [Pd(BPI)(κ1-O2)Me] is formed and, in the case of platinum, the Pt(IV) peroxide intermediate [Pt(BPI)(κ2-O2)Me]. For alkyl complexes where metal–carbon bonds are sufficiently weak, the photoactivation leads to the insertion of oxygen into the metal–carbon bond to generate alkylperoxo complexes: for example [Pd(BPI)OOMe], which has been isolated and structurally characterized. For stronger M–C(aryl) bonds, the reaction of [Pt(BPI)Ph] with O2 and light results in a Pt(IV) complex, tentatively assigned as the peroxo complex [Pt(BPI)(κ2-O2)Ph], which in chlorinated solvents reacts further to give [Pt(BPI)Cl2Ph], which has been isolated and characterized by scXRD. In addition to the facilitation of oxygen insertion reactions, UV irradiation can also affect the reactivity of other components in the reaction mixture, such as the solvent or other reaction products, which can result in further reactions. Labeling studies using [Pt(BPI)(CD3)] in chloroform have shown that photolytic reactions with oxygen involve degradation of the solvent.

Scopariusicide C, a novel cyclobutene-containing meroditerpenoid from artificially cultivated Isodon scoparius

Scopariusicide C (1), a novel ent-clerodane-based meroditerpenoid was isolated from artificially cultivated Isodon scoparius. An oxygen insertion reaction proposed to occur in its biosynthesis makes its scaffold different from those meroterpenoids previously isolated from the same species. The structure of 1 was determined by analysis of spectroscopic data, quantum chemical calculations and use of residual dipolar coupling (RDC). 1 showed moderate inhibitory effects against T and B lymphocyte proliferation and good anti-MRSA activity.

Engineering of catalytic sites of Pdx-Ce1-xO2-δ for dehydrogenation, oxygen insertion and reverse water gas shift reactions during methane combustion

Methane, a highly abundant greenhouse gas with a high global warming potential, is a relatively clean fuel due to its high H/C ratio. This study has taken a unique approach to discover key fundamentals during the solution combustion synthesis of Pd0.03Ce0.97O2-δ catalysts characterised using several physicochemical techniques for catalytic CH4 combustion. Oxygen vacancies were found to be key for the catalysts containing all PdOx substituted in the ceria lattice. On the other hand, catalysts containing both large surface supported PdO that is transformable to PdHx (in situ during the methane combustion reaction) and substituted PdOx were found to provide superior catalytic performance (complete CH4 conversion with 100% selectivity to CO2) with long life under dry and wet conditions. The discussed bifunctionality of the catalyst resulted in achieving a high TOF, high conversion and stability at elevated temperatures. Therefore, this study introduces a new strategy to improve the activity of combustion catalysts.

Selective Oxyfunctionalisation Reactions Driven by Sulfite Oxidase‐Catalysed In Situ Generation of H2O2

AbstractH2O2 can be accepted by several peroxygenases as a clean oxidant, able to supply both the necessary electrons and oxygen atom at the same time. The biocatalysts, in turn, are able to catalyse an array of interesting oxygen insertion reactions at enantio‐ and regio‐selectivities hard to attain with classical chemical methods. The sensitivity of most peroxygenases towards H2O2, however, requires this oxidant to be generated in situ. Here, we suggest the application of (modified) sulfite oxidases to couple the oxidation of sulfites to the reduction of oxygen. This enables us to use calcium sulfite, an industrial waste product from scrubbing flue gases, as an electron donor to reduce oxygen. This will supply the required peroxide in a controlled manner and enables us to perform these challenging reactions at the expense of simple salts.

Discovery and Mechanistic Study of a Totally Organic C(aryl)-C(alkyl)Oxygen Insertion Reaction.

We report an unprecedented photochemical oxygen insertion reaction into an aromatic quinone methide. Insertion happens specifically within a C(aryl)-C(alkyl) bond, whereas the quinone methide moiety remains intact itself. Detailed mechanistic studies, supported by DFT calculations, support a pathway in which the p-QM plays a pivotal activating role.

Discovery of Hydroxylase Activity for PqqB Provides a Missing Link in the Pyrroloquinoline Quinone Biosynthetic Pathway.

Understanding the biosynthesis of cofactors is fundamental to the life sciences, yet to date a few important pathways remain unresolved. One example is the redox cofactor pyrroloquinoline quinone (PQQ), which is critical for C1 metabolism in many microorganisms, a disproportionate number of which are opportunistic human pathogens. While the initial and final steps of PQQ biosynthesis, involving PqqD/E and PqqC, have been elucidated, the precise nature and order of the remaining transformations in the pathway are unknown. Here we show evidence that the remaining essential biosynthetic enzyme PqqB is an iron-dependent hydroxylase catalyzing oxygen-insertion reactions that are proposed to produce the quinone moiety of the mature PQQ cofactor. The demonstrated reactions of PqqB are unprecedented within the metallo β-lactamase protein family and expand the catalytic repertoire of nonheme iron hydroxylases. These new findings also generate a nearly complete description of the PQQ biosynthetic pathway.

Influence of Conserved Structural Elements of the Proximal Pocket in HEME-Thiolate Enzymes on Oxygen Insertion Reactions

In many heme-thiolate enzymes, the chemical transformations of substrates occur in a binding site located on the distal side of the heme. The opposite (proximal) side of the heme is characterized by conserved structural elements that vary subtly from enzyme to enzyme. These structural elements include hydrogen bonds to the proximal sulfur and the dipole moment of an alpha helix that terminates in the proximal cysteine ligand. We present research on two heme-thiolate enzymes, chloroperoxidase and cytochrome P450CAM, that shows that these structural elements have significant influence on a wide range of reactivities (formation of compound I, epoxidation and allylic hydroxylation of alkene substrates, regioselectivity and even enantiospecificity). The fine-tuning of these structural motifs can select among closely related modes of reactivity.

"Two-Story" Calix[6]arene-Based Zinc and Copper Complexes: Structure, Properties, and O2 Binding.

A new "two-story" calix[6]arene-based ligand was synthesized, and its coordination chemistry was explored. It presents a tren cap connected to the calixarene small rim through three amido spacers. X-ray diffraction studies of its metal complexes revealed a six-coordinate ZnII complex with all of the carbonyl groups of the amido arms bound and a five-coordinate CuII complex with only one amido arm bound. These dicationic complexes were poorly responsive toward exogenous neutral donors, but the amido arms were readily displaced by small anions or deprotonated with a base to give the corresponding monocationic complexes. Cyclic voltammetry in various solvents showed a reversible wave for the CuII/CuI couple at very negative potentials, denoting an electron-rich environment. The reversibility of the system was attributed to the amido arms, which can coordinate the metal center in both its +II and +I redox states. The reversibility was lost upon anion binding to Cu. Upon exposure of the CuI complex to O2 at low temperature, a green species was obtained with a UV-vis signature typical of an end-on superoxide CuII complex. Such a species was proposed to be responsible for oxygen insertion reactions onto the ligand according to the unusual and selective four-electron oxidative pathway previously described with a "one-story" calix[6]tren ligand.

Oxygen Insertion Reactions within the One-Dimensional Channels of Phases Related to FeSb2O4.

The structure of the mineral schafarzikite, FeSb2O4, has one-dimensional channels with walls comprising Sb3+ cations; the channels are separated by edge-linked FeO6 octahedra that form infinite chains parallel to the channels. Although this structure provides interest with respect to the magnetic and electrical properties associated with the chains and the possibility of chemistry that could occur within the channels, materials in this structural class have received very little attention. Here we show, for the first time, that heating selected phases in oxygen-rich atmospheres can result in relatively large oxygen uptakes (up to ∼2% by mass) at low temperatures (ca. 350 °C) while retaining the parent structure. Using a variety of structural and spectroscopic techniques, it is shown that oxygen is inserted into the channels to provide a structure with the potential to show high one-dimensional oxide ion conductivity. This is the first report of oxygen-excess phases derived from this structure. The oxygen insertion is accompanied not only by oxidation of Fe2+ to Fe3+ within the octahedral chains but also Sb3+ to Sb5+ in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb5+ ion (which is very rare in an oxide environment), two interstitial O2- ions, and two 4-coordinate Sb3+ ions is suggested and is consistent with all experimental observations. To the best of our knowledge, this is the first example of an oxidation process where the local energetics of the product dictate that simultaneous oxidation of two different cations must occur. This reaction, together with a wide range of cation substitutions that are possible on the transition metal sites, presents opportunities to explore the schafarzikite structure more extensively for a range of catalytic and electrocatalytic applications.

Combined EC-NMR and In Situ FTIR Spectroscopic Studies of Glycerol Electrooxidation on Pt/C, PtRu/C, and PtRh/C

Glycerol, a byproduct of biodiesel production, is an industrial waste because of its excess yield. Electrooxidation of glycerol is a promising way to utilize glycerol—through harvesting electric energy as fuels in a fuel cell or hydrogen as sacrificial agent in electrolysis cell—while generating valuable chemicals. Here, we report a detailed mechanistic study of the glycerol electrooxidation reaction (GOR) on a series of Pt/C, PtxRuy/C, and PtxRhy/C nanocatalysts synthesized by NaBH4 reduction. The EC cyclic voltammetry characterization indicates that alloying Ru with Pt greatly enhanced the GOR activity, especially at low potential, but not as much with alloying Rh, as compared with Pt/C. In situ FTIR and 13C NMR spectroscopies were used to investigate the GOR mechanism at a molecular level. The results demonstrate that the selectivity of products depends on the type of catalysts and the oxidation potential. Although both PtRu/C and PtRh/C could accelerate the oxygen insertion reactions that led to highe...

Photocatalytic C–H Activation of Hydrocarbons over VO@g-C3N4

A highly selective and sustainable method has been developed for the oxidation of methyl arenes and their analogues. The VO@g-C3N4 catalyst is very efficient in the C–H activation and oxygen insertion reaction resulting in formation of the corresponding carbonyl compounds and phenols.

Irrelevance of Carbon Monoxide Poisoning in the Methanol Oxidation Reaction on a PtRu Electrocatalyst.

Based on detailed in situ attenuated total-reflection-surface-enhanced IR reflection absorption spectroscopy (ATR-SEIRAS) studies of the methanol oxidation reaction (MOR) on Ru/Pt thin film and commercial Johnson-Matthey PtRu/C, a revised MOR enhancement mechanism is proposed in which CO on Pt sites is irrelevant but instead Pt-Ru boundary sites catalyze the oxygen insertion reaction that leads to the formation of formate and enhances the direct reaction pathway.

Irrelevance of Carbon Monoxide Poisoning in the Methanol Oxidation Reaction on a PtRu Electrocatalyst

AbstractBased on detailed in situ attenuated total‐reflection–surface‐enhanced IR reflection absorption spectroscopy (ATR‐SEIRAS) studies of the methanol oxidation reaction (MOR) on Ru/Pt thin film and commercial Johnson–Matthey PtRu/C, a revised MOR enhancement mechanism is proposed in which CO on Pt sites is irrelevant but instead Pt‐Ru boundary sites catalyze the oxygen insertion reaction that leads to the formation of formate and enhances the direct reaction pathway.

Oxygen insertion into metal carbon bonds: formation of methylperoxo Pd(II) and Pt(II) complexes via photogenerated dinuclear intermediates.

Platinum(II) and palladium(II) complexes [M(CH3)(L)]SbF6 with substituted terpyridine ligands L undergo light-driven oxygen insertion reactions into metal methyl bonds resulting in methylperoxo complexes [M(OOCH3)(L)]SbF6. The oxygen insertion reactions occur readily for complexes with methyl ligands that are activated due to steric interaction with substituents (NH2, NHMe or CH3) at the 6,6″-positions on the terpyridine ligand. All complexes exhibit attractive intermolecular π···π or M···M interactions in the solid state and in solution, which lead to excited triplet dinuclear M-M complexes upon irradiation. A mechanism is proposed whereby a dinuclear intermediate is generated upon irradiation that has a weakened M-C bond in the excited state, resulting in the observed oxygen insertion reactions.