Co(salophen)-Catalyzed Aerobic Oxidation of p-Hydroquinone: Mechanism and Implications for Aerobic Oxidation Catalysis

Atroposelective synthesis of biaxial bridged eight-membered terphenyls via a Co/SPDO-catalyzed aerobic oxidative coupling/desymmetrization of phenols

Bridged chiral biaryls are axially chiral compounds with a medium-sized ring connecting the two arenes. Compared with plentiful methods for the enantioselective synthesis of biaryl compounds, synthetic approaches for this subclass of bridged atropisomers are limited. Here we show an atroposelective synthesis of 1,3-diaxial bridged eight-membered terphenyl atropisomers through an Co/SPDO (spirocyclic pyrrolidine oxazoline)-catalyzed aerobic oxidative coupling/desymmetrization reaction of prochiral phenols. This catalytic desymmetric process is enabled by combination of an earth-abundant Co(OAc)2 and a unique SPDO ligand in the presence of DABCO (1,4-diaza[2.2.2]bicyclooctane). An array of diaxial bridged terphenyls embedded in an azocane can be accessed in high yields (up to 99%) with excellent enantio- (>99% ee) and diastereoselectivities (>20:1 dr).

Appended Lewis Acids Enable Dioxygen Reactivity and Catalytic Oxidations with Ni(II)

We disclose a suite of Ni(II) complexes featuring secondary sphere Lewis acids of varied Lewis acidity and tether lengths. Several of these complexes feature atypical behavior of Ni(II): reactivity with O2 that occurs only in the presence of a tethered Lewis acid. In situ UV-vis spectroscopy revealed that, although adducts are stable at -40 °C, complexes containing 9-borabicyclo[3.3.1]nonane (9-BBN) Lewis acids underwent irreversible oxidative deborylation when warmed to room temperature. We computationally and experimentally identified that oxidative instability of appended 9-BBN moieties can be mitigated using weaker Lewis acids such as pinacolborane (BPin). These insights enabled the realization of catalytic reactions: hydrogen atom abstraction from phenols and room temperature oxygen atom transfer to PPh3.

Experimental and Theoretical Investigation of the Mechanism of the Reduction of O2 from Air to O22- by VIVO2+-N,N,N-Amidate Compounds and Their Potential Use in Fuel Cells

The two-electron reductive activation of O2 to O22- is of particular interest to the scientific community mainly due to the use of peroxides as green oxidants and in powerful fuel cells. Despite of the great importance of vanadium(IV) species to activate the two-electron reductive activation of O2, the mechanism is still unclear. Reaction of VIVO2+ species with the tridentate-planar N,N,N-carboxamide (ΗL) ligands in solution (CH3OH:H2O) under atmospheric O2, at room temperature, resulted in the quick formation of [VV(═O)(η2-O2)(κ3-L)(H2O)] and cis-[VV(═O)2(κ3-L)] compounds. Oxidation of the VIVO2+ complexes with the sterically hindered tridentate-planar N,N,N-carboxamide ligands by atmospheric O2 gave only cis-[VV(═O)2(κ3-L)] compounds. The mechanism of formation of [VV(═O)(η2-O2)(κ3-L)(H2O)] (I) and cis-[VV(═O)2(κ3-L)] (II) complexes vs time, from the interaction of [VIV(═O)(κ3-L)(Η2Ο)2]+ with atmospheric O2, was investigated with 51V, 1H NMR, UV-vis, cw-X-band EPR, and 18O2 labeling IR and resonance Raman spectroscopies revealing the formation of a stable intermediate (Id). EPR, MS, and theoretical calculations of the mechanism of the formation of I and II revealed a pathway, through a binuclear [VIV(═O)(κ3-L)(H2O)(η1,η1-O2)VIV(═O)(κ3-L)(H2O)]2+ intermediate. The results from cw-EPR, 1H NMR spectroscopies, cyclic voltammetry, and the reactivity of the complexes [VIV(═O)(κ3-L)(Η2Ο)2]+ toward O2 reduction fit better to an intermediate with a binuclear nature. Dynamic experiments in combination with computational calculations were undertaken to fully elucidate the mechanism of the O2 reduction to O22- by [VIV(═O)(κ3-L)(Η2Ο)2]+. The galvanic cell {Zn|VIII,VII||Id, [VIVO(κ3-L)(H2O)2]+|O2|C(s)} was manufactured, demonstrating the important applicability of this new chemistry to Zn|H2O2 fuel cells technology generating H2O2 in situ from the atmospheric O2.

Heterogeneous M-N-C Catalysts for Aerobic Oxidation Reactions: Lessons from Oxygen Reduction Electrocatalysts

Nonprecious metal heterogeneous catalysts composed of first-row transition metals incorporated into nitrogen-doped carbon matrices (M-N-Cs) have been studied for decades as leading alternatives to Pt for the electrocatalytic O2 reduction reaction (ORR). More recently, similar M-N-C catalysts have been shown to catalyze the aerobic oxidation of organic molecules. This Focus Review highlights mechanistic similarities and distinctions between these two reaction classes and then surveys the aerobic oxidation reactions catalyzed by M-N-Cs. As the active-site structures and kinetic properties of M-N-C aerobic oxidation catalysts have not been extensively studied, the array of tools and methods used to characterize ORR catalysts are presented with the goal of supporting further advances in the field of aerobic oxidation.

Redox Mediators in Homogeneous Co-electrocatalysis

Homogeneous electrocatalysis has been well studied over the past several decades for the conversion of small molecules to useful products for green energy applications or as chemical feedstocks. However, in order for these catalyst systems to be used in industrial applications, their activity and stability must be improved. In naturally occurring enzymes, redox equivalents (electrons, often in a concerted manner with protons) are delivered to enzyme active sites by small molecules known as redox mediators (RMs). Inspired by this, co-electrocatalytic systems with homogeneous catalysts and RMs have been developed for the conversion of alcohols, nitrogen, unsaturated organic substrates, oxygen, and carbon dioxide. In these systems, the RMs have been shown to both increase the activity of the catalyst and shift selectivity to more desired products by altering catalytic cycles and/or avoiding high-energy intermediates. However, the area is currently underdeveloped and requires additional fundamental advancements in order to become a more general strategy. Here, we summarize the recent examples of homogeneous co-electrocatalysis and discuss possible future directions for the field.

The regioselective Wacker oxidation of internal allylamines: synthesis of functionalized and challenging β-amino ketones

A convenient and general protocol for the palladium-catalysed oxidation of internal allylamine derivatives to β-amino ketones is reported. The transformation occurs at room temperature and shows a wide substrate scope as well as high functional group and N-protecting group tolerance. We also describe potential applications of the method, e.g., the synthesis of bioactive molecules or simple transformations of selected β-amino ketones into other interesting building blocks.

Colorimetric detection of total antioxidants in green tea with oxidase-mimetic CoOOH nanorings

Green tea is a popular beverage and is widely consumed due to its taste and antioxidative polyphenols. Herein, a smartphone-based colorimetric reader using cobalt oxyhydroxide (CoOOH) nanorings has been successfully applied to detect antioxidants in green tea with high reliability and robustness. By exploiting the oxidase-mimicking activity, the as-synthesized CoOOH nanorings replaces natural enzymes to directly catalyze oxidate colorless 3,3 ´ ,5,5 ´ -tetramethylbenzidine (TMB), while antioxidants can disintegrate CoOOH, leading to an antioxidant concentration-dependent color change. Benefiting from the CoOOH nanorings-based colorimetric strategy, a smartphone-assistant nanosensor was devised for portable and visual detection of antioxidants in green tea. The proposed method can be extended to visual detection of a diverse range of diseases by responding to their specific antioxidant, and thus provide a pivotal disease toolbox that is compatible for development at the point-of-care.

Aerobic Oxidation Reactivity of Well-Defined Cobalt(II) and Cobalt(III) Aminophenol Complexes

This paper describes the synthesis and reactivity studies of three cobalt complexes bearing aminophenol-derived ligands without nitrogen substitution: Co^II(^tBu2APH)₂(^tBu2AP)₂ (1), Co^III₂(^tBu2APH)₂(^tBu2AP)₂(μ-^tBu2BAP)₂ (2), and Co^III(^tBu2AP)₃ (3), where ^tBu2APH = 2-amino-4,6-di-tert-butylphenol, ^tBu2AP = 2-amino-4,6-di-tert-butylphenolate, and μ-^tBu2BAP = bridging 2-amido-4,6-di-tert-butylphenolate. Stoichiometric reactivity studies of these well-defined complexes demonstrate the catalytic competency of both cobalt(II) and cobalt(III) complexes in the aerobic oxidative cyclization of ^tBu2APH with tert-butylisonitrile. Reactions with O₂ reveal the aerobic oxidation of the cobalt(II) complex 1 to generate the cobalt(III) species 2 and 3. UV-visible time-course studies and electron paramagnetic resonance spectroscopy indicate that this oxidation proceeds through a ligand-based radical intermediate. These studies represent the first example of well-defined cobalt aminophenol complexes that participate in catalytic aerobic oxidation reactions and highlight a key role for a ligand radical in the oxidation sequence.

Chemical and Electrochemical O2 Reduction on Earth-Abundant M-N-C Catalysts and Implications for Mediated Electrolysis

M-N-C catalysts, incorporating non-precious-metal ions (e.g. M = Fe, Co) within a nitrogen-doped carbon support, have been the focus of broad interest for electrochemical O₂ reduction and aerobic oxidation reactions. The present study explores the mechanistic relationship between the O₂ reduction mechanism under electrochemical and chemical conditions. Chemical O₂ reduction is investigated via the aerobic oxidation of a hydroquinone, in which the O-H bonds supply the protons and electrons needed for O₂ reduction to water. Mechanistic studies have been conducted to elucidate whether the M-N-C catalyst couples two independent half-reactions (IHR), similar to electrode-mediated processes, or mediates a direct inner-sphere reaction (ISR) between O₂ and the organic molecule. Kinetic data support the latter ISR pathway. This conclusion is reinforced by rate/potential correlations that reveal significantly different Tafel slopes, implicating different mechanisms for chemical and electrochemical O₂ reduction.

Mediated Inner-Sphere Electron Transfer Induces Homogeneous Reduction of CO2 via Through-Space Electronic Conjugation

The electrocatalytic reduction of CO₂ is an appealing method for converting renewable energy sources into value-added chemical feedstocks. We report a co-electrocatalytic system for the reduction of CO₂ to CO comprised of a molecular Cr complex and dibenzothiophene-5,5-dioxide (DBTD) as a redox mediator, which achieves high activity (TOF=1.51-2.84×10⁵  s^-1 ) and quantitative selectivity. Under aprotic or protic conditions, DBTD produces a co-electrocatalytic response with 1 by coordinating trans to the site of CO₂ binding and mediating electron transfer from the electrode with quantitative efficiency for CO. This assembly is reliant on through-space electronic conjugation between the π frameworks of DBTD and the bpy fragment of the catalyst ligand, with contributions from dispersive interactions and weak sulfone coordination.

Inverse Potential Scaling in Co-Electrocatalytic Activity for CO2 Reduction Through Redox Mediator Tuning and Catalyst Design

Electrocatalytic CO2 reduction is an attractive strategy to mitigate the continuous rise in atmospheric CO2 concentrations and generate value-added chemical products. A possible strategy to increase the activity of molecular...

Four electron selective O2 reduction by a tetranuclear vanadium(IV/V)/hydroquinonate catalyst: application in the operation of Zn–air batteries

A tetranuclear vanadium(IV/V) hydroquinonate electrocatalyst for oxygen reduction through proton-coupled electron transfer. The complex enhances the current and power of Zn–air batteries.

Computational investigation into intramolecular hydrogen bonding controlling the isomer formation and pKa of octahedral nickel (II) proton reduction catalysts

This work demonstrates the impact of intramolecular hydrogen bonding (H-bonding) on the calculated pKa of octahedral tris-(pyridinethiolato)nickel (II), [Ni(PyS)3]-, proton reduction catalysts. Density Functional Theory (DFT) calculations on a [Ni(PyS)3]-...

Propargylation of CoQ0 through the Redox Chain Reaction

An efficient catalytic propargylation of CoQ0 is described by employing the cooperative effect of Sc(OTf)₃ and Hantzsch ester. It is suggested to work through the redox chain reaction, which involves hydroquinone and dimeric propargylic moiety intermediates. A broad range of propargylic alcohols can be converted into the appropriate derivatives of CoQ0 containing triple bonds in good to excellent yields. The mechanism of the given transformation is also discussed.

Mechanistic investigation of the aerobic oxidation of 2-pyridylacetate coordinated to a Ru(II) polypyridyl complex

A new ruthenium polypyridyl complex, [Ru(bpy)₂(acpy)]⁺ (acpy = 2-pyridylacetate, bpy = 2,2'-bipyridine), was synthesized and fully characterized. Distinct from the previously reported analog, [Ru(bpy)₂(pic)]⁺ (pic = 2-pyridylcarboxylate), the new complex is unstable under aerobic conditions and undergoes oxidation to yield the corresponding α-keto-2-pyridyl-acetate (acpyoxi) coordinated to the Ru^II center. The reaction is one of the few examples of C-H activation at mild conditions using O₂ as the primary oxidant and can provide mechanistic insights with important implications for catalysis. Theoretical and experimental investigations of this aerobic oxidative transformation indicate that it takes place in two steps, first producing the α-hydroxo-2-pyridyl-acetate analog and then the final product. The observed rate constant for the first oxidation was in the order of 10^-2 h^-1. The reaction is hindered in the presence of coordinating solvents indicating the role of the metal center in the process. Theoretical calculations at the M06-L level of theory were performed for multiple reaction pathways in order to gain insights into the most probable mechanism. Our results indicate that O₂ binding to [Ru(bpy)₂(acpy)]⁺ is favored by the relative instability of the six-ring chelate formed by the acpy ligand and the resulting Ru^III-OO˙^- superoxo is stabilized by the carboxylate group in the coordination sphere. C-H activation by this species involves high activation free energies (ΔG^‡ = 41.1 kcal mol^-1), thus the formation of a diruthenium μ-peroxo intermediate, [(Ru^III(bpy)₂(O-acpy))₂O₂]²⁺via interaction of a second [Ru(bpy)₂(acpy)]⁺ was examined as an alternative pathway. The dimer yields two Ru^IVO centers with a low ΔG^‡ of 2.3 kcal mol^-1. The resulting Ru^IVO species can activate C-H bonds in acpy (ΔG^‡ = 23.1 kcal mol^-1) to produce the coordinated α-hydroxo-2-pyridylacetate. Further oxidation of this intermediate leads to the α-keto-2-pyridyl-acetate product. The findings provide new insights into the mechanism of C-H activation catalyzed by transition-metal complexes using O₂ as the sole oxygen source.

Recent Progress of Electrochemical Production of Hydrogen Peroxide by Two-Electron Oxygen Reduction Reaction

Shifting electrochemical oxygen reduction reaction (ORR) via two-electron pathway becomes increasingly crucial as an alternative/green method for hydrogen peroxide (H2 O2 ) generation. Here, the development of 2e- ORR catalysts in recent years is reviewed, in aspects of reaction mechanism exploration, types of high-performance catalysts, factors to influence catalytic performance, and potential applications of 2e- ORR. Based on the previous theoretical and experimental studies, the underlying 2e- ORR catalytic mechanism is firstly unveiled, in aspect of reaction pathway, thermodynamic free energy diagram, limiting potential, and volcano plots. Then, various types of efficient catalysts for producing H2 O2 via 2e- ORR pathway are summarized. Additionally, the catalytic active sites and factors to influence catalysts' performance, such as electronic structure, carbon defect, functional groups (O, N, B, S, F etc.), synergistic effect, and others (pH, pore structure, steric hindrance effect, etc.) are discussed. The H2 O2 electrogeneration via 2e- ORR also has various potential applications in wastewater treatment, disinfection, organics degradation, and energy storage. Finally, potential future directions and prospects in 2e- ORR catalysts for electrochemically producing H2 O2 are examined. These insights may help develop highly active/selective 2e- ORR catalysts and shape the potential application of this electrochemical H2 O2 producing method.

Non-covalent assembly of proton donors and p-benzoquinone anions for co-electrocatalytic reduction of dioxygen

The two-electron and two-proton p-hydroquinone/p-benzoquinone (H₂Q/BQ) redox couple has mechanistic parallels to the function of ubiquinone in the electron transport chain. This proton-dependent redox behavior has shown applicability in catalytic aerobic oxidation reactions, redox flow batteries, and co-electrocatalytic oxygen reduction. Under nominally aprotic conditions in non-aqueous solvents, BQ can be reduced by up to two electrons in separate electrochemically reversible reactions. With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH)_m]˙⁻. The solvation shell created by these interactions can mediate a second one-electron reduction coupled to proton transfer at more positive potentials ([BQ(AH)_m]˙⁻ + nAH + e⁻ ⇌ [HQ(AH)_(m+n)−1(A)]²⁻), resulting in an overall two electron reduction at a single potential at intermediate acid concentrations. Here we show that hydrogen-bonded adducts of reduced quinones and the proton donor 2,2,2-trifluoroethanol (TFEOH) can mediate the transfer of electrons to a Mn-based complex during the electrocatalytic reduction of dioxygen (O₂). The Mn electrocatalyst is selective for H₂O₂ with only TFEOH and O₂ present, however, with BQ present under sufficient concentrations of TFEOH, an electrogenerated [H₂Q(AH)₃(A)₂]²⁻ adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% H₂O in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded quinone anions can function in an analogous co-electrocatalytic manner to H₂Q.

Non-covalent interactions between reduced p-benzoquinone species and weak acids stabilize intermediates which can switch dioxygen reduction selectivity from H₂O₂ to H₂O for a molecular Mn catalyst.

Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc)2-Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst

Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O₂) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd⁰ during catalytic turnover. We previously showed that Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O₂ in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)₂ catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)₂ catalyst systems under three different oxidation conditions: (1) O₂ alone, (2) O₂ with cocatalytic BQ, and (3) O₂ with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to synergistic roles of O₂ and BQ, both of which are capable of oxidizing Pd⁰ to Pd^II The reaction of O₂ generates H₂O₂ as a byproduct, which can oxidize hydroquinone to quinone in the presence of Pd^II NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance.

Iron(II)-Catalyzed Aerobic Biomimetic Oxidation of Amines using a Hybrid Hydroquinone/Cobalt Catalyst as Electron Transfer Mediator

Herein we report the first Fe^II‐catalyzed aerobic biomimetic oxidation of amines. This oxidation reaction involves several electron transfer steps and is inspired by biological oxidation in the respiratory chain. The electron transfer from the amine to molecular oxygen is aided by two coupled catalytic redox systems, which lower the energy barrier and improve the selectivity of the oxidation reaction. An iron hydrogen transfer complex was utilized as the substrate‐selective dehydrogenation catalyst along with a bifunctional hydroquinone/cobalt Schiff base complex as a hybrid electron transfer mediator. Various primary and secondary amines were oxidized in air to their corresponding aldimines or ketimines in good to excellent yield.

Oxygen Reduction by Iron Porphyrins with Covalently Attached Pendent Phenol and Quinol

Phenols and quinols participate in both proton transfer and electron transfer processes in nature either in distinct elementary steps or in a concerted fashion. Recent investigations using synthetic heme/Cu models and iron porphyrins have indicated that phenols/quinols can react with both ferric superoxide and ferric peroxide intermediates formed during O₂ reduction through a proton coupled electron transfer (PCET) process as well as via hydrogen atom transfer (HAT). Oxygen reduction by iron porphyrins bearing covalently attached pendant phenol and quinol groups is investigated. The data show that both of these can electrochemically reduce O₂ selectively by 4e^-/4H⁺ to H₂O with very similar rates. However, the mechanism of the reaction, investigated both using heterogeneous electrochemistry and by trapping intermediates in organic solutions, can be either PCET or HAT and is governed by the thermodynamics of these intermediates involved. The results suggest that, while the reduction of the Fe^III-O₂̇^- species to Fe^III-OOH proceeds via PCET when a pendant phenol is present, it follows a HAT pathway with a pendant quinol. In the absence of the hydroxyl group the O₂ reduction proceeds via an electron transfer followed by proton transfer to the Fe^III-O₂̇^- species. The hydrogen bonding from the pendant phenol group to Fe^III-O₂̇^- and Fe^III-OOH species provides a unique advantage to the PCET process by lowering the inner-sphere reorganization energy by limiting the elongation of the O-O bond upon reduction.

Efficient Palladium-Catalyzed Aerobic Oxidative Carbocyclization to Seven-Membered Heterocycles

The use of molecular oxygen in palladium-catalyzed oxidation reactions is highly widespread in organic chemistry. However, the direct reoxidation of palladium by O₂ is often kinetically unfavored, thus leading the deactivation of the palladium catalyst during the catalytic cycle. In the present work, we report a highly selective palladium-catalyzed carbocyclization of bisallenes to seven-membered heterocycles under atmospheric pressure of O₂ . The use of a homogenous hybrid catalyst (Co(salophen)-HQ, HQ=hydroquinone) significantly promotes efficient electron transfer between the palladium catalyst and O₂ through a low-energy pathway. This aerobic oxidative transformation shows broad substrate scope and functional group compatibility and allowed the preparation of O-containing seven-membered rings in good yields in most cases.

Oxidation of Ammonia with Molecular Complexes

Oxidation of ammonia by molecular complexes is a burgeoning area of research, with critical scientific challenges that must be addressed. A fundamental understanding of individual reaction steps is needed, particularly for cleavage of N-H bonds and formation of N-N bonds. This Perspective evaluates the challenges of designing molecular catalysts for oxidation of ammonia and highlights recent key contributions to realizing the goals of viable energy storage and retrieval based on the N-H bonds of ammonia in a carbon-free energy cycle.

Oxygen Reduction Assisted by the Concert of Redox Activity and Proton Relay in a Cu(II) Complex

A copper complex, [Cu(dpaq)](ClO₄) (1), of a monoanionic pentadentate amidate ligand (dpaq) has been isolated and characterized to study its efficacy toward electrocatalytic reduction of oxygen in neutral aqueous medium. The Cu(II) mononuclear complex, poised in a distorted trigonal bipyramidal structure, reduces oxygen at an onset potential of 0.50 V vs RHE. Kinetics study by hydrodynamic voltammetry and chronoamperometry suggests a stepwise mechanism for sequential reduction of O₂ to H₂O₂ to H₂O at a single-site Cu-catalyst. The foot-of-the-wave analysis records a turnover frequency of 5.65 × 10² s^-1. At pH 7.0, complex 1 undergoes a quasi-reversible mixed metal-ligand-based reduction and triggers the reduction of dioxygen to water. Electrochemical studies in tandem with quantum chemical investigation, conducted at different redox states, portray the active participation of ligand in completing the process of proton-coupled electron transfer internally. The protonated carboxamido moiety acts as a proton relay, while the quinoline-based orbital supplies the necessary redox equivalent for the conversion of complex 1 to Cu(II)-hydroperoxo species. Thus, a suitable combination of redox non-innocence and proton shuttling functionality in the ligand makes it an effective electron-proton-transfer mediator and subsequently assists the process of oxygen reduction.

Stable Non-Covalent Co(Salphen)-Based Polymeric Catalyst for Highly Efficient and Selective Oxidation of 2,3,6-Trimethylphenol

Developing highly efficient catalyst systems for phenol-quinone transformation is of great significance in the chemical/biological industries. Herein, we reported a novel heterogenous catalytic system based on Co(Salphen) supramolecular polymers (CSP), which delivered an excellent catalytic performance in the oxidation of 2,3,6-trimethylphenol (TMP) under mild conditions. The CSP were constructed through a simple self-assembled process between BiCo(Salphen) complex and 4,4-dipyridine. By applying BiCo-BiPy1:1 CSP as the catalyst, 2,3,5-trimethyl-1,4-benzoquinone (TMBQ) could be obtained with an excellent conversion (>99%) and selectivity over 99% under mild reaction conditions (30 °C, 0.1 MPa). In addition, it can be recycled at least five times without substantial decline in catalytic activities (conversion and selectivity), suggesting its excellent stability and recyclability. This work may provide guidance on designing and building valuable catalysts for environmentally friendly and cost-effective oxidation reactions.

Mediated Fuel Cells: Soluble Redox Mediators and Their Applications to Electrochemical Reduction of O2 and Oxidation of H2, Alcohols, Biomass, and Complex Fuels

Mediated fuel cells are electrochemical devices that produce power in a manner similar to that of conventional proton exchange membrane fuel cells (PEMFCs). They differ from PEMFCs in their use of redox mediators dissolved in liquid electrolyte to conduct oxidation of the fuel or reduction of the oxidant, typically O2, in bulk solution. The mediators transport electrons (and often protons) between the electrode and the catalysts or chemical reagents in solution. This strategy can help overcome many of the challenges associated with conventional fuel cells, including managing complex multiphase reactions (as in O2 reduction) or the use of challenging or heterogeneous fuels, such as hydrocarbons, polyols, and biomass. Mediators are also commonly used in enzymatic fuel cells, where direct electron transfer from the electrode to the enzymatic active site can be slow. This review provides a comprehensive survey of historical and recent mediated fuel cell efforts, including applications using chemical and enzymatic catalysts.

Efficient Stereoselective Carbocyclization to cis-1,4-Disubstituted Heterocycles Enabled by Dual Pd/Electron Transfer Mediator (ETM) Catalysis

An efficient Pd/ETM (ETM = electron transfer mediator)-cocatalyzed stereoselective oxidative carbocyclization of dienallenes under aerobic oxidation conditions has been developed to afford six-membered heterocycles. The use of a bifunctional cobalt complex [Co(salophen)-HQ] as hybrid ETM gave a faster aerobic oxidation than the use of separated ETMs, indicating that intramolecular electron transfer between the hydroquinone unit and the oxidized metal macrocycle occurs. In this way, a class of important cis-1,4-disubstituted six-membered heterocycles, including dihydropyran and tetrahydropyridine derivatives were obtained in high diastereoselectivity with good functional group compatibility. The experimental and computational (DFT) studies reveal that the pendent olefin does not only act as an indispensable element for the initial allene attack involving allenic C(sp³)–H bond cleavage, but it also induces a face-selective reaction of the olefin of the allylic group, leading to a highly diastereoselective formation of the product. Finally, the deuterium kinetic isotope effects measured suggest that the initial allenic C(sp³)–H bond cleavage is the rate-limiting step, which was supported by DFT calculations.

Diversion of Catalytic C-N Bond Formation to Catalytic Oxidation of NH3 through Modification of the Hydrogen Atom Abstractor

We report that (TMP)Ru(NH₃)₂ (TMP = tetramesitylporphryin) is a molecular catalyst for oxidation of ammonia to dinitrogen. An aryloxy radical, tri-tert-butylphenoxyl (ArO·), abstracts H atoms from a bound ammonia ligand of (TMP)Ru(NH₃)₂, leading to the discovery of a new catalytic C-N coupling to the para position of ArO· to form 4-amino-2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one. Modification of the aryloxy radical to 2,6-di-tert-butyl-4-tritylphenoxyl radical, which contains a trityl group at the para position, prevents C-N coupling and diverts the reaction to catalytic oxidation of NH₃ to give N₂. We achieved 125 ± 5 turnovers at 22 °C for oxidation of NH₃, the highest turnover number (TON) reported to date for a molecular catalyst.

Deactivation of Co-Schiff base catalysts in the oxidation of para-substituted lignin models for the production of benzoquinones

Those features which enhance the reactivity of Co-Schiff base oxidation catalysts can also contribute to their demise.

Fast-synthesis and catalytic property of heterogeneous Co-MOF catalysts for the epoxidation of α-pinene with air

A method for synthesizing Co-MOF by rapidly rotating hydrothermal crystallization is proposed. When the rotation speed was 150 rpm, only 2 h was needed to synthesize Co-MOF-150-2 with high catalytic activity and stability.

Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least +0.77 V vs. Fc⁺ /Fc. The green recyclability by catalytic re-oxidation with dioxygen is also shown. Finally, a direct comparison indicates that GFAs are real alternatives to toxic halo- or cyano-substituted benzoquinones.

Preparation of Co-Mo-O ultrathin nanosheets with outstanding catalytic performance in aerobic oxidative desulfurization

In this study, ultrathin α-Co(OH)₂ nanosheets intercalated with molybdate were successfully synthesized using a one-step coprecipitation method, and the derived Co-Mo-O mixed metal oxides demonstrated outstanding catalytic performance in the aerobic oxidative desulfurization (AODS) of fuels using molecular oxygen (O₂) in air as the oxidizing agent.

Inner-Sphere Oxygen Activation Promoting Outer-Sphere Nucleophilic Attack on Olefins

Alkoxylation and hydroxylation reactions of 1,5-cyclooctadiene (cod) in an iridium complex with alcohols and water promoted by the reduction of oxygen to hydrogen peroxide are described. The exo configuration of the OH/OR groups in the products agrees with nucleophilic attack at the external face of the olefin as the key step. The reactions also require the presence of a coordinating protic acid (such as picolinic acid (Hpic)) and involve the participation of a cationic diolefin iridium(III) complex, [Ir(cod)(pic)₂ ]⁺ , which has been isolated. Independently, this cation is also involved in easy alkoxy group exchange reactions, which are very unusual for organic ethers. DFT studies on the mechanism of olefin alkoxylation mediated by oxygen show a low-energy proton-coupled electron-transfer step connecting a superoxide-iridium(II) complex with hydroperoxide-iridium(III) intermediates, rather than peroxide complexes. Accordingly, a more complex reaction, with up to four different products, occurred upon reacting the diolefin-peroxide iridium(III) complex with Hpic. Moreover, such hydroperoxide intermediates are the origin of the regio- and stereoselectivity of the hydroxylation/alkoxylation reactions. If this protocol is applied to the diolefin-rhodium(I) complex [Rh(pic)(cod)], free alkyl ethers ORC₈ H₁₁ (R=Me, Et) resulted, and the reaction is enantioselective if a chiral amino acid, such as l-proline, is used instead of Hpic.

Brønsted Acid Scaling Relationships Enable Control Over Product Selectivity from O2 Reduction with a Mononuclear Cobalt Porphyrin Catalyst

The selective reduction of O₂, typically with the goal of forming H₂O, represents a long-standing challenge in the field of catalysis. Macrocyclic transition-metal complexes, and cobalt porphyrins in particular, have been the focus of extensive study as catalysts for this reaction. Here, we show that the mononuclear Co-tetraarylporphyrin complex, Co(por^OMe) (por^OMe = meso-tetra(4-methoxyphenyl)porphyrin), catalyzes either 2e^-/2H⁺ or 4e^-/4H⁺ reduction of O₂ with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O₂ reduction to H₂O₂ or H₂O in DMF are determined and exhibit a Nernstian dependence on the acid pK _a, while the Co^III/II redox potential is independent of the acid pK _a. The reaction product, H₂O or H₂O₂, is defined by the relationship between the thermodynamic potential for O₂ reduction to H₂O₂ and the Co^III/II redox potential: selective H₂O₂ formation is observed when the Co^III/II potential is below the O₂/H₂O₂ potential, while H₂O formation is observed when the Co^III/II potential is above the O₂/H₂O₂ potential. Mechanistic studies reveal that the reactions generating H₂O₂ and H₂O exhibit different rate laws and catalyst resting states, and these differences are manifested as different slopes in linear free energy correlations between the log(rate) versus pK _a and log(rate) versus effective overpotential for the reactions. This work shows how scaling relationships may be used to control product selectivity, and it provides a mechanistic basis for the pursuit of molecular catalysts that achieve low overpotential reduction of O₂ to H₂O.

One-Step Template-Free Fabrication of Ultrathin Mixed-Valence Polyoxovanadate-Incorporated Metal-Organic Framework Nanosheets for Highly Efficient Selective Oxidation Catalysis in Air

Polyoxometalate (POM)-based metal-organic frameworks (MOFs) with nanostructure represent a class of promising heterogeneous nanocatalysts. As yet, direct one-step controllable synthesis of pure nanoscale POM-MOFs catalysts is an extremely huge challenge owing to highly complicated synthetic conditions. Herein, for the first time, we fabricated ultrathin (∼5 nm) mixed-valence {V₁₆} clusters-incorporated metal-organic framework nanosheets [Ni(4,4'-bpy)₂]₂ [V₇^IVV₉^VO₃₈Cl]·(4,4'-bpy)·6H₂O (NENU-MV-1a) via one-step template-free strategy and successfully achieved one-step removal of all impurities from the multicomponent complex system. The obtained NENU-MV-1a nanosheets possess dramatically different physiochemical properties from bulk crystal, including larger lateral area, and more active sites originated from their nanostructures. As a proof-of-concept application, NENU-MV-1a was applied in olefin epoxidation in air and exhibited more excellent catalytic activity (95% conversion) than the bulk crystal (35%). In addition, detailed catalytic mechanism studies revealed the structure-property correlations of NENU-MV-1a and proposed V^IV-V^V synergistic catalytic effect. Our investigations are of great significance for the development of more active and/or selective mixed-valence metal-oxygen cluster-based MOF nanocatalysts.

Activating a Peroxo Ligand for C-O Bond Formation

Dioxygen activation for effective C-O bond formation in the coordination sphere of a metal is a long-standing challenge in chemistry for which the design of catalysts for oxygenations is slowed down by the complicated, and sometimes poorly understood, mechanistic panorama. In this context, olefin-peroxide complexes could be valuable models for the study of such reactions. Herein, we showcase the isolation of rare "Ir(cod)(peroxide)" complexes (cod=1,5-cyclooctadiene) from reactions with oxygen, and then the activation of the peroxide ligand for O-O bond cleavage and C-O bond formation by transfer of a hydrogen atom through proton transfer/electron transfer reactions to give 2-iradaoxetane complexes and water. 2,4,6-Trimethylphenol, 1,4-hydroquinone, and 1,4-cyclohexadiene were used as hydrogen atom donors. These reactions can be key steps in the oxy-functionalization of olefins with oxygen, and they constitute a novel mechanistic pathway for iridium, whose full reaction profile is supported by DFT calculations.

Dioxygen Reduction to Hydrogen Peroxide by a Molecular Mn Complex: Mechanistic Divergence between Homogeneous and Heterogeneous Reductants

The selective electrocatalytic reduction of dioxygen (O₂) to hydrogen peroxide (H₂O₂) could be an alternative to the anthraquinone process used industrially, as well as enable the on-demand production of a useful chemical oxidant, obviating the need for long-term storage. There are challenges associated with this, since the two-proton/two-electron reduction of H₂O₂ to two equivalents of water (H₂O) or disproportionation to O₂ and H₂O can be competing reactions. Recently, we reported a Mn(III) Schiff base-type complex, Mn(^tbudhbpy)Cl, where 6,6'-di(3,5-di- tert-butyl-2-phenolate)-2,2'-bipyridine = [^tbudhbpy]^2-, which is active for the electrocatalytic reduction of O₂ to H₂O₂ (ca. 80% selectivity). The less-than-quantitative selectivity could be attributed in part to a thermal disproportionation reaction of H₂O₂ to O₂ and H₂O. To understand the mechanism in greater detail, spectrochemical stopped-flow and electrochemical techniques were employed to examine the catalytic rate law and kinetic reaction parameters. Under electrochemical conditions, the catalyst produces H₂O₂ by an ECCEC mechanism with appreciable rates down to overpotentials of 20 mV and exhibits a catalytic response with a strong dependence on proton donor p K_a. Mechanistic studies suggest that under spectrochemical conditions, where the homogeneous reductant decamethylferrocene (Cp*₂Fe) is used, H₂O₂ is instead produced via a disproportionation pathway, which does not show a strong acid dependence. These results demonstrate that differences in mechanistic pathways can occur for homogeneous catalysts in redox processes, dependent on whether an electrode or homogeneous reductant is used.

Replacement of Stoichiometric DDQ with a Low Potential o-Quinone Catalyst Enabling Aerobic Dehydrogenation of Tertiary Indolines in Pharmaceutical Intermediates

A transition-metal/quinone complex, [Ru(phd)₃]²⁺ (phd = 1,10-phenanthroline-5,6-dione), is shown to be effective for aerobic dehydrogenation of 3° indolines to the corresponding indoles. The results show how low potential quinones may be tailored to provide a catalytic alternative to stoichiometric DDQ, due to their ability to mediate efficient substrate dehydrogenation while also being compatible with facile reoxidation by O₂. The utility of the method is demonstrated in the synthesis of key intermediates to pharmaceutically important molecules.