Oxygen Bond Formation Research Articles

Rose‐Bengal‐Photocatalyzed Cross‐Dehydrogenative Coupling Reactions under Visible Light

AbstractOver the past few years, organic chemistry has used the organic dye rose bengal‘s ability to absorb visible light. In present‐day research, the innovation of environmentally friendly processes for carbon‐carbon/carbon‐heteroatoms (Nitrogen, Oxygen, Sulphur, and Phosphorus) bond formation has great importance. The photocatalyzed cross‐dehydrogenative coupling (CDC) reactions using rose bengal (RB) is a promising technique for creating carbon‐carbon/carbon‐heteroatom bonds directly from readily available compounds. Our review focuses on the current advancement in rose bengal that uses photocatalyzed carbon‐carbon/carbon‐heteroatom bond‐making reactions to synthesize various important organic molecules via CDC reactions.

Insights into PSII's S3YZ• State: An Electronic and Magnetic Analysis.

Using BS-DFT (broken-symmetry density functional theory), the electronic and magnetic properties of the S3YZ• state of photosystem II were investigated and compared to those of the S3 state. While the O5 oxo-O6 hydroxo species presents little difference between the two states, a previously identified [O5O6]3- exhibits reduced stabilization of the O5-O6 shared spin. This species is shown to have some coupling with the YZ• center through Mn1 and O6. Similarly, a peroxo species is found to exhibit significant exchange couplings between the YZ• center and the Mn cluster through Mn1. Mechanistic changes in O-O bond formation in S3YZ• are highlighted by analysis of IBOs (intrinsic bonding orbitals) showing deviation for Mn1 and O6 centered IBOs. This change in coupling interactions throughout the complex as a result of S3YZ• formation presents implications for the determination of the mechanism spanning the end of the S3 and the start of the S4 states, affecting both electron movement and oxygen bond formation.

Exploring visible light for carbon–nitrogen and carbon–oxygen bond formationvianickel catalysis

In this review, we highlight the recent synthetic methodology development for the formation of C–N and C–O bondsviavisible-light-driven nickel catalysis, with in-depth discussions with reaction designs and mechanistic scenarios.

Selectivity of dehydrogenative silicone–oxygen bond formation in diphenylsilane by base and base‐activated catalysts

AbstractVarious base catalysts and base‐activated catalysts were evaluated for their steric inhibitions and selectivity in silicon–oxygen bond formation between hydroxyl groups and each of the two active hydrogen sites of diphenylsilane. Three hydroxides, six amines, and an organic‐soluble N‐heterocyclic carbene‐copper alkoxide complex (CuIPr‐NHC) were reviewed using in situ FTIR monitoring of reaction progress for the stepwise consumption of both Si–H bonds in diphenylsilane in the presence of water and various alcohols. Hydroxide catalysts with sodium, potassium, and tetramethylammonium (TMA) cations showed strong dependence of cation identity on reaction rate and observable rate order but did not lead to accumulation of a once‐oxidized silane. Amine catalysts exhibited varying levels of selectivity to the once oxidized silane, leading to accumulation of diphenylsilanol and hydride terminated oligomers with relative reaction rates seemingly dependent on the size of amine's alkyl substituents. CuIPr‐NHC showed very limited reactivity to the second dehydrogenation step in comparison to the first, leading to high accumulation of once‐oxidized silane. Leveraging this, the production of diphenyl(monoalkoxy)silanes has been demonstrated with representative primary, secondary, and tertiary alcohols.

Organometallic Ni(ii), Ni(iii), and Ni(iv) complexes relevant to carbon–carbon and carbon–oxygen bond formation reactions

Herein, the pyridinophane tetradentate ligand PyNMe3 is used to isolate and structurally characterize well-defined organometallic Ni(ii) and Ni(iii) complexes bearing the cycloneophyl fragment, an alkyl/aryl C-donor ligand.

Theoretical consideration on phosphorus–oxygen bond formation of sterically crowded triarylphosphines and their radical cations

The phosphorus–oxygen bond formation reactions often observed for the sterically crowded triarylphosphines and their radical cations were studied by DFT calculation. The oxidations of the trimethyl-, triphenyl-, and trimesityl-phosphines to the corresponding phosphine oxides with peroxides are shown to proceed through the SN2 like transition states and the overall influence of the steric hindrance on the activation energy is limited. The reaction of the radical cation of tris(2,4,6-triisopropylphenyl)phosphine with oxygen to give the heterocyclic phosphonium cation is modeled by employing (2,6-diisopropylphenyl)diphenylphosphine and the comparison of the intermediates suggests highly exergonic nature of the reaction.

Theoretical Insights into the Cooperative Catalytic Mechanism of a PW-Containing Keggin Heteropolyacid Anion and Ethanol toward Conversion of Fructose into 5-Ethoxymethylfurfural in Ethanol Solution

Toward the conversion of fructose and ethanol into 5-ethoxymethylfurfural (EMF), the catalytic contribution of typical functional groups in PW-containing Keggin polyoxometalates has been theoretically studied at the M06/LANL2DZ,6-31++G(d,p) level in ethanol solution. The [HPW12O40]2– anion is regarded as the model catalyst for the PW-containing polyoxometalates of both homogeneous and heterogeneous catalysts in ethanol solution, as it possesses basic functional groups, that is, [W–OcH–W], [W═OdH], and [W═Od]. The [HPW12O40]2– anion displays good catalytic activity, which originates from the contained proton H+, that is, both [W–OcH–W] and [W═OdH] catalytically active sites. Herein, over both [W–OcH–W] and [W═OdH], the rate-controlling steps are associated with both C2–OH and C1–H bond cleavages in fructose moiety, respectively. A mechanism of proton-induced C–H bond cleavage is proposed, in which the [HPW12O40]2– anion is responsible for both the C–OH bond cleavage and C–O–C ether–oxygen bond formation, while the ethanol solvent answers for the C–H activation of the protonated intermediates. The appropriate protonation capacity of a protic solvent plays an important role in the conversion of fructose and ethanol into EMF. Alternatively, the [HPW12O40]2– anion displays almost no catalytic activity toward the O–H bond cleavage through [2 + 2] addition, which comes from the very strong Lewis acidity of the W-site.

A Platform for Decarboxylative Couplings via Photoredox Catalysis: Direct Access to Carbocations from Carboxylic Acids for Carbon–Oxygen Bond Formation

A Platform for Decarboxylative Couplings via Photoredox Catalysis: Direct Access to Carbocations from Carboxylic Acids for Carbon–Oxygen Bond Formation

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Summary O–O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O–O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O–O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts.

How ab initio Molecular Dynamics Can Change the Understanding on Transition Metal Catalysed Water Oxidation.

Artificial water splitting is a promising technology that allows the storage of renewable energy in the form of energy-rich compounds. This mini-review showcases how theoretical studies contribute to the under-standing of existing water oxidation catalysts (WOCs) as well as inspiring the development of novel WOCs. In order to understand the chemical complexity of transition metal complexes and their interaction with the solvent environment, the use of sophisticated simulation protocols is necessary. As an illustration, a family of ruthe- nium-based WOCs is presented which were investigated employing a wide range of forefront computational methods with emphasis on ab initiomolecular dynamic based approaches. In those studies a base assisted oxygen-oxygen bond formation was identified as the energetically most favourable reaction mechanism. By examining the role of local environmental effects at ambient temperature and the effect of modifications in the ligand framework, a comprehensible picture of the WOCs can be given, where the latter can serve as a guideline for further experimental and computational studies. In this mini-review, we provide a description of the methods, and the findings of our previous computational studies in compacted form, aimed at scientists with a theoretical as well as experimental background.

Computational mechanistic study on molecular catalysis of water oxidation by cyclam ligand-based iron complex

The iron metal complexes containing cyclam-based macrocyclic ligands are active water oxidation catalysts, which can facilitate oxygen–oxygen bond formation during the water-splitting process. To understand the mechanism of the catalytic process, we explored this process by [Fe(cyclam)Cl2]Cl performing density functional theory-based first-principles calculations. We examined the energetics of the formation of the oxygen–oxygen bond through the investigation of complexes [FeV(cyclam)(O)2]+, and [FeV(cyclam)(OH)(O)]+2. The process of water nucleophilic addition by this FeV–(oxo) complexes was explored in detail. Our computational study confirms the formation of these species, which were reported earlier in experimental conditions. The transition states for various reactions of the catalytic cycle were obtained within the implicit water model. From these calculations, we find that the proton transfers to cis–oxo or hydroxide moiety require high activation energy during the formation of the oxygen–oxygen bond. Our calculations reveal that the oxygen–oxygen bond formation by the transfer of a proton to the explicit water molecule requires more activation free energy than the transfer of a proton to the cis–oxo or hydroxide of the FeV–oxo species. Overall [FeV(cyclam)(O)2]+ and [FeV(cyclam)(OH)(O)]+2 species with one explicit water molecule requires similar free energy. The Mulliken spin density data confirm the formation of the superoxide complexes. The activation free energy for the release of the oxygen molecule is lesser than that of the oxygen–oxygen bond formation. The natural bond orbital analysis for the complexes before and after the formation of the oxygen–oxygen bond formation shows that this bond formation happens through the interaction of antibonding orbital, π*(d2 −2 –2py) of Fe=O moiety, with the σ*–orbital of the hydroxide group of the water molecule.

Oxygen Atom Transfer as an Alternative Pathway for Oxygen-Oxygen Bond Formation.

Fundamental understanding of catalytic mechanisms of water oxidation is a prerequisite for the design and development of efficient and rugged water oxidation catalysts. In this work, a detailed mechanistic study of the water oxidation mechanism of the [RuII(npm)(4-pic)2(H2O)]2+ (npm = 4-t-butyl-2,6-di(1',8'-naphthyrid-2'-yl)-pyridine, pic = 4-picoline) complex, [RuII-OH2]2+, reveals oxygen atom transfer from highly reactive ruthenium oxo intermediates to noncoordinating nitrogen atoms of the ligand as a novel route for oxygen evolution via storage of oxidizing equivalents as N-oxide groups on the ligand framework. Theoretical calculations show that the initial complex, [RuII-OH2]2+, is transformed to a di-N-oxide [RuII-OH2,(-NO)2]2+ complex upon oxidation via facile OAT steps from RuV=O species and that [RuV=O,(-NO)2]3+ represents the most likely reactive species for the critical O-O bond formation. Furthermore, a new stepwise mechanism for oxygen evolution is introduced, which proceeds via coupling of Ru-O and N-O moieties producing a peroxide intermediate, [RuV-OO-N,(-NO)]3+, and can compete with the water nucleophilic attack pathway for the oxygen evolution reaction. In this mechanism, a water molecule is oxidatively activated to an "oxygen atom" which is "stored" at a noncoordinating pyridine. Oxidative activation of a second water molecule, facilitated by coordination expansion of the intermediate N-oxide, generates the second oxygen atom required to produce a dioxygen molecule.

Selenocysteine as a Substrate, an Inhibitor and a Mechanistic Probe for Bacterial and Fungal Iron-Dependent Sulfoxide Synthases.

Sulfoxide synthases are non-heme iron enzymes that participate in the biosynthesis of thiohistidines, such as ergothioneine and ovothiol A. The sulfoxide synthase EgtB from Chloracidobacterium thermophilum (CthEgtB) catalyzes oxidative coupling between the side chains of N-α-trimethyl histidine (TMH) and cysteine (Cys) in a reaction that entails complete reduction of molecular oxygen, carbon-sulfur (C-S) and sulfur-oxygen (S-O) bond formation as well as carbon-hydrogen (C-H) bond cleavage. In this report, we show that CthEgtB and other bacterial sulfoxide synthases cannot efficiently accept selenocysteine (SeCys) as a substrate in place of cysteine. In contrast, the sulfoxide synthase from the filamentous fungus Chaetomium thermophilum (CthEgt1) catalyzes C-S and C-Se bond formation at almost equal efficiency. We discuss evidence suggesting that this functional difference between bacterial and fungal sulfoxide synthases emerges from different modes of oxygen activation.

Facile protic hydration of acetonitrile to protonated acetamide at oxygen mediated by chloroauric acid: insights from experimental and calculations

Chemical transformations such as nitrile hydration or carbon–oxygen bond formation reactions under gentle conditions are important in the pharmaceutical industry because of the presence of potentially delicate functional groups. We present a non-catalytic hydration reaction of acetonitrile to the corresponding protonated acetamide gold(III) salt [CH3(OH)NH2]AuCl4 under ambient conditions in water using chloroauric acid H[AuCl4] for the first time. ATR-FTIR, Raman and 1H and 13C NMR spectroscopic data in addition to X-ray crystallography supported the isolation of protonated acetamide stabilized with [AuCl4]− anion. The protonation of N–C=O fragment of acetamide, O- versus N-protonation aptitude, was validated experimentally and theoretically. The X-ray crystal structure of the acetamide salt [CH3C(OH)NH2]AuCl4 in the triclinic Pī space group suggested the enolic form. However, the reaction of gold(III) trichloride AuCl3 with acetonitrile in water in the absence of a proton source formed the gold(I)/gold(III) salt [Au(CH3CN)2]AuCl4 without hydration as shown in X-ray structure in the monoclinic P21/c space group. Mapping of HOMO–LUMO energy gap using frontier molecular orbital theory and MESP surfaces of OH and NH conformers of acetamide from DFT calculations clearly shows subsequent changes in their profiles with the change in their protonation states. An energy gap of 56.4 kcal/mol in the optimized energies of OH and NH conformers of acetamide along with computed HOMO–LUMO energy difference represents the relative stability of OH conformer compared to NH conformer, thus leading to the conclusion that OH protonation site is more likely to exist in the acetamide structure as compared to the NH protonation state.

Theoretical and computational investigations of geometrical, electronic and spin structures of the CaMn4 OX (X = 5, 6) cluster in the Kok cycle Si (i = 0-3) of oxygen evolving complex of photosystem II.

The optimized geometries of the CaMn4 OX (X = 5, 6) cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) by large-scale quantum mechanics (QM) and molecular mechanics (MM) calculations are compared with recent serial femtosecond crystallography (SFX) results for the Si (i = 0-3) states. The valence states of four Mn ions by the QM/MM calculations are also examined in relation to the experimental results by the X-ray emission spectroscopy (XES) for the Si intermediates. Geometrical and valence structures of right-opened Mn-hydroxide, Mn-oxo and Mn-peroxide intermediates in the S3 state are investigated in detail in relation to recent SFX and XES experiments for the S3 state. Interplay between theory and experiment indicates that the Mn-oxo intermediate is a new possible candidate for the S3 state. Implications of the computational results are discussed in relation to possible mechanisms of the oxygenoxygen bond formation for water oxidation in OEC of PSII.

Unusual Hypochlorous Acid (HClO) Recognition Mechanism Based on Chlorine–Oxygen Bond (Cl−O) Formation

One of the most important endogenous reactive oxygen species, hypochlorous acid (HClO), is involved in numerous pathological and physiological processes. Herein, a near-infrared fluorescence probe (CyHR) was designed and synthesized for ultrafast (within 0.2 s), sensitive (limit of detection=39.44 nm), and selective response to HClO. The reaction mechanism was systematically analyzed by MS, 1 H NMR spectroscopy, HPLC-MS techniques, and theoretical calculations. The results indicated that HClO can be recognized by CyHR, which is based on chlorine-oxygen (Cl-O) bond formation. To the best of our knowledge, this study is the first to find Cl-O bonds among organic aromatic compounds, given that Cl-O bonds are common among inorganics. Through biological experiments, CyHR was successfully applied to image exogenous and endogenous HClO in macrophage cells (RAW 264.7). Thus, CyHR is a promising tool for HClO-related physiological and pathological studies and may provide a means for designing HClO-specific fluorescence probes.

Zeolite X with potassium diformate as a sustained-release antibacterial agent

A reaction of 800 mesh stellerite and a concentration of 15% hydrochloric acid in a solid to liquid ratio of 1:3 was carried out for 2 h at \(80^{\circ }\hbox {C}\). Most of its impurities were removed. It can be used as precursors for the preparation of high quality zeolite. Zeolite X which is octahedral crystal shape as cube, 96% of the degree of crystallinity has been synthesized hydrothermally from \(n(\hbox {SiO}_{2}/\hbox {Al}_{2}\hbox {O}_{3})=3, n(\hbox {Na}_{2}\hbox {O}/\hbox {SiO}_{2})=1.14\) and \(n(\hbox {H}_{2}\hbox {O/Na}_{2}\hbox {O})=37\) by controlling reaction temperature and time. The synthesis included zeolite X as carriers, chitosan as intermediates, a mass ratio of zeolite X:chitosan:potassium diformate \(=\) 3:1:2 in weight for 2 h at \(40^{\circ }\hbox {C}\). X-ray photoelectron spectroscopy analysis showed hydrogen bond formation by the chitosan amino hydrogen, potassium diformate oxygen and zeolite X oxygen or hydrogen bond formation by free hydroxyl group of zeolite X and oxygen of chitosan’s C–O, indicating effective grafting. Zeolite X sustained-release antibacterial agent was prepared with an inhibition rate of 78.16% by the antibacterial.

Theory of chemical bonds in metalloenzymes XXII: a concerted bond-switching mechanism for the oxygen–oxygen bond formation coupled with one electron transfer for water oxidation in the oxygen-evolving complex of photosystem II

ABSTRACTQM(UB3LYP)/MM(AMBER) calculations were performed for the locations of the transition structure (TS) of the oxygen–oxygen (O–O) bond formation in the S4 state of the oxygen-evolving complex (OEC) of photosystem II (PSII). The natural orbital (NO) analysis of the broken-symmetry (BS) solutions was also performed to elucidate the nature of the chemical bonds at TS on the basis of several chemical indices defined by the occupation numbers of NO. The computational results revealed a concerted bond switching (CBS) mechanism for the oxygen–oxygen bond formation coupled with the one-electron transfer (OET) for water oxidation in OEC of PSII. The orbital interaction between the σ-HOMO of the Mn(IV)4–O(5) bond and the π*-LUMO of the Mn(V)1=O(6) bond plays an important role for the concerted O–O bond formation for water oxidation in the CaMn4O6 cluster of OEC of PSII. One electron transfer (OET) from the π-HOMO of the Mn(V)1=O(6) bond to the σ*-LUMO of the Mn(IV)4–O(5) bond occurs for the formation of electron transfer diradical, where the generated anion radical [Mn(IV)4–O(5)]-• part is relaxed to the •Mn(III)4 … O(5)- structure and the cation radical [O(6)=Mn(V)1]+ • part is relaxed to the +O(6)–Mn(IV)1• structure because of the charge-spin separation for the electron-and hole-doped Mn–oxo bonds. Therefore, the local spins are responsible for the one-electron reductions of Mn(IV)4->Mn(III)4 and Mn(V)1->Mn(IV)1. On the other hand, the O(5)- and O(6)+ sites generated undergo the O–O bond formation in the CaMn4O6 cluster. The Ca(II) ion in the cubane- skeleton of the CaMn4O6 cluster assists the above orbital interactions by the lowering of the orbital energy levels of π*-LUMO of Mn(V)1=O(6) and σ*-LUMO of Mn(IV)4–O(5), indicating an important role of its Lewis acidity. Present CBS mechanism for the O–O bond formation coupled with one electron reductions of the high-valent Mn ions is different from the conventional radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems. The proton-coupled electron transfer (PC-OET) mechanism for the O–O bond formation is also touched in relation to the CBS-OET mechanism.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical.

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

Control over Electrochemical Water Oxidation Catalysis by Preorganization of Molecular Ruthenium Catalysts in Self-Assembled Nanospheres.

Oxygen formation through water oxidation catalysis is a key reaction in the context of fuel generation from renewable energies. The number of homogeneous catalysts that catalyze water oxidation at high rate with low overpotential is limited. Ruthenium complexes can be particularly active, especially if they facilitate a dinuclear pathway for oxygen bond formation step. A supramolecular encapsulation strategy is reported that involves preorganization of dilute solutions (10−5 m) of ruthenium complexes to yield high local catalyst concentrations (up to 0.54 m). The preorganization strategy enhances the water oxidation rate by two‐orders of magnitude to 125 s−1, as it facilitates the diffusion‐controlled rate‐limiting dinuclear coupling step. Moreover, it modulates reaction rates, enabling comprehensive elucidation of electrocatalytic reaction mechanisms.