Mechanistic Insight into Concerted Proton-Electron Transfer of a Ru(IV)-Oxo Complex: A Possible Oxidative Asynchronicity

Mechanistic studies of NOx reduction reactions involving copper complexes: encouragement of DFT calculations

The reduction of nitrogen oxides (NOx), which is mainly mediated by metalloenzymes and metal complexes, is a critical process in the nitrogen cycle and environmental remediation. This Frontier article highlights the importance of density functional theory (DFT) calculations to gain mechanistic insights into nitrite (NO2-) and nitric oxide (NO) reduction reactions facilitated by copper complexes by focusing on two key processes: the reduction of NO2- to NO by a monocopper complex, with special emphasis on the concerted proton-electron transfer, and the reduction of NO to N2O by a dicopper complex, which involves N-N bond formation, N2O2 isomerization, and N-O bond cleavage. These findings underscore the utility of DFT calculations in unraveling complicated reaction mechanisms and offer a foundation for future research aimed at improving the reactivity of transition metal complexes in NOx reduction reactions.

The Role of Electron Transfer in Copper-Mediated C(sp2)-H Trifluoromethylation

We report copper(II) and copper(III) trifluoromethyl complexes supported by a pyridinedicarboxamide ligand (L) as a platform for investigating the role of electron transfer in C(sp2)-H trifluoromethylation. While the copper(II) trifluoromethyl complex is unreactive towards (hetero)arenes, the formal copper(III) trifluoromethyl complex performs C(sp2)-H trifluoromethylation of a wide range of (hetero)arenes. Mechanistic studies using the copper(III) trifluoromethyl complex suggest that the mechanism of arene trifluoromethylation is substrate-dependent. When the thermodynamic driving force for electron transfer is high, the reaction proceeds through a previously unidentified single electron transfer (SET) mechanism, where an initial electron transfer occurs between the substrate and oxidant prior to CF3 group transfer. Otherwise, a CF3 radical release/electrophilic aromatic substitution (SEAr) mechanism is followed. These studies provide valuable insights into the role of strong oxidants and potential mechanistic dichotomy in Cu-mediated C(sp2)-H trifluoromethylation.

Basicity of MnIII-Hydroxo Complexes Controls the Thermodynamics of Proton-Coupled Electron Transfer Reactions

Several manganese-dependent enzymes utilize MnIII-hydroxo units in concerted proton-electron transfer (CPET) reactions. We recently demonstrated that hydrogen bonding to the hydroxo ligand in the synthetic [MnIII(OH)(PaPy2N)]+ complex increased rates of CPET reactions compared to the [MnIII(OH)(PaPy2Q)]+ complex that lacks a hydrogen bond. In this work, we determine the effect of hydrogen bonding on the basicity of the hydroxo ligand and evaluate the corresponding effect on CPET reactions. Both [MnIII(OH)(PaPy2Q)]+ and [MnIII(OH)(PaPy2N)]+ react with strong acids to yield MnIII-aqua complexes [MnIII(OH2)(PaPy2Q)]2+ and [MnIII(OH2)(PaPy2N)]2+, for which we determined pKa values of 7.6 and 13.1, respectively. Reactions of the MnIII-aqua complexes with one-electron reductants yielded estimates of reduction potentials, which were combined with pKa values to give O-H bond dissociation free energies (BDFEs) of 77 and 85 kcal mol-1 for the MnII-aqua complexes [MnII(OH2)(PaPy2Q)]+ and [MnII(OH2)(PaPy2N)]+. Using these BDFEs, we performed an analysis of the thermodynamic driving force for phenol oxidation by these complexes and observed the unexpected result that slower rates are associated with more asynchronous CPET. In addition, reactions of acidic phenols with the MnIII-hydroxo complexes show rates that deviate from the thermodynamic trends, consistent with a change in mechanism from CPET to proton transfer.

Mechanistic insights into copper-mediated benzylic C(sp3)-H bond trifluoromethylation

The mechanisms underlying copper-mediated trifluoromethylation of benzylic C(sp3)-H bonds were investigated using density functional theory (DFT) calculations. Two distinct pathways were identified: radical recombination/reductive elimination and single-electron transfer (SET). In the radical recombination/reductive elimination pathway, the CuII species recombines with benzyl radicals to generate a CuIII intermediate, which subsequently undergoes reductive elimination. Conversely, the SET pathway involves single-electron transfer from benzyl radicals to CuII species, forming a cationic benzylic intermediate and CuI species, followed by coupling with a CF3 group coordinated to Cu. DFT calculations revealed that the radical recombination/reductive elimination pathway is favoured for trifluoromethylation of primary and secondary benzylic C(sp3)-H bonds, with the reductive elimination step being rate-determining. In contrast, the SET pathway exhibits preference for trifluoromethylation of tertiary benzylic C(sp3)-H bonds. These mechanistic insights have significant implications for enhancing the selectivity of copper-mediated trifluoromethylation reactions.

Heteromultimetallic Platform for Enhanced C-H Bond Activation: Aluminum-Incorporated Dicopper Complex Mimicking Cu-ZSM-5 Structure and Oxidative Reactivity

Bimetallic complexes have sparked interest across various chemical disciplines, driving advancements in research. Recent advancements in this field have shed light on complex reactions in metalloenzymes and unveiled new chemical transformations. Two primary types of bimetallic platforms have emerged: (1) systems where both metals actively participate in reactivity, and (2) systems where one metal mediates the reaction while the other regulates reactivity. This study introduces a novel multinucleating ligand platform capable of integrating both types of bimetallic systems. To demonstrate the significance of this platform, we synthesized a unique dicopper complex incorporating aluminum in its coordination environment. This complex serves as the first structural model for the active site in copper-based zeolites, highlighting the role of aluminum in hydrogen atom abstraction reactivity. Comparative studies of oxidative C-H bond activation revealed that the inclusion of aluminum significantly alters the thermodynamic driving force (by -11 kcal mol-1) for bond activation and modifies the proton-coupled electron-transfer reaction mechanism, resulting in a 14-fold rate increase. Both computational and experimental data support the high modularity of this multinucleating ligand platform, offering a new approach to fine-tune the reactivity of bimetallic complexes.

Monomeric Fe(III)-Hydroxo and Fe(III)-Aqua Complexes Display Oxidative Asynchronous Hydrogen Atom Abstraction Reactivity

This paper presents the synthesis and characterization of a series of novel monomeric aqua-ligated iron(III) complexes, [FeIII(L5R)(OH2)]2+ (R=OMe, H, Cl, NO2), supported by an amide-containing pentadentate N5 donor ligand, L5R [HL5R=2-(((1-methyl-1H-imidazol-2-yl)methyl)(pyridin-2-yl-methyl)amino)-N-(5-R-quinolin-8-yl)acetamide]. The complexes were characterized by various spectroscopic and analytical techniques, including electrochemistry and magnetic measurements. The Fe(III)-hydroxo complexes, [FeIII(L5R)(OH)]1+, were generated in situ by deprotonating the corresponding aqua complexes in a pH ~7 aqueous medium. In another way, adding one equivalent of a base to a methanolic solution of the Fe(III)-aqua complexes also produced the Fe(III)-hydroxo complexes. The study uses linoleic fatty acid as a substrate to explore the hydrogen atom abstraction (HAA) reactivity of both hydroxo and aqua complexes. The investigation highlights the substitution effect of the L5R ligand on reactivity, revealing a higher rate when an electron-withdrawing group is present. Hammett analyses and(or) determination of the asynchronicity factor (η) suggest an oxidative asynchronous concerted proton-electron transfer (CPET) pathway for the HAA reactions. Aqua complexes exhibited a higher asynchronicity in CPET, resulting in higher reaction rates than their hydroxo analogs. Overall, the work provides insights into the beneficial role of a higher imbalance in electron-transfer-proton-transfer (ET-PT) contributions in HAA reactivity.

Unlocking Efficient Hydrogen Production: Nucleophilic Oxidation Reactions Coupled with Water Splitting

Electrocatalytic water splitting driven by sustainable energy is a clean and promising water-chemical fuel conversion technology for the production of high-purity green hydrogen. However, the sluggish kinetics of anodic oxygen evolution reaction (OER) pose challenges for large-scale hydrogen production, limiting its efficiency and safety. Recently, the anodic OER has been replaced by a nucleophilic oxidation reaction (NOR) with biomass as the substrate and coupled with a hydrogen evolution reaction (HER), which has attracted great interest. Anode NOR offers faster kinetics, generates high-value products, and reduces energy consumption. By coupling NOR with hydrogen evolution reaction, hydrogen production efficiency can be enhanced while yielding high-value oxidation products or degrading pollutants. Therefore, NOR-coupled HER hydrogen production is another new green electrolytic hydrogen production strategy after electrolytic water hydrogen production, which is of great significance for realizing sustainable energy development and global decarbonization. This review explores the potential of nucleophilic oxidation reactions as an alternative to OER and delves into NOR mechanisms, guiding future research in NOR-coupled hydrogen production. It assesses different NOR-coupled production methods, analyzing reaction pathways and catalyst effects. Furthermore, it evaluates the role of electrolyzers in industrialized NOR-coupled hydrogen production and discusses future prospects and challenges. This comprehensive review aims to advance efficient and economical large-scale hydrogen production.

Synthesis, Characterization, and Reactivity of a Highly Oxidative Mononuclear Manganese(IV)-Bis(Fluoro) Complex

Recently, transition-metal terminal nonoxo complexes have shown a remarkable ability to activate and functionalize C-H bonds via proton-coupled electron transfer (PCET). Here we report the first example of a mononuclear manganese(IV) bis(fluoro) complex bearing a tetradentate pyridinophane ligand, [MnIV(TBDAP)(F)2]2+ (3), with an X-ray single crystal structure and physicochemical characterization. The manganese(IV) bis(fluoro) complex has a very high reduction potential of 1.61 V vs SCE, thereby enabling the four-electron oxidation of mesitylene to 3,5-dimethylbenzaldehyde. Kinetic studies, including the kinetic isotope effect and employment of other toluene derivatives, reveal the electron transfer (ET)-driven PCET in the C-H bond activation of mesitylene by 3. This novel metal halide intermediate would be prominently valuable for expanding transition-metal halide chemistry.

High-valent nonheme Fe(IV)O/Ru(IV)O complexes catalyze C-H activation reactivity and hydrogen tunneling: a comparative DFT investigation

A comprehensive density functional theory investigation has been presented towards the comparison of the C-H activation reactivity between high-valent iron-oxo and ruthenium-oxo complexes. A total of four compounds, e.g., [Ru(IV)O(tpy-dcbpy)] (1), [Fe(IV)O(tpy-dcbpy)] (1'), [Ru(IV)O(TMCS)] (2), and [Fe(IV)O(TMCS)] (2'), have been considered for this investigation. The macrocyclic ligand framework tpy(dcbpy) implies tpy = 2,2':6',2''-terpyridine, dcbpy = 5,5'-dicarboxy-2,2'-bipyridine, and TMCS is TMC with an axially tethered -SCH2CH2 group. Compounds 1 and 2' are experimentally synthesized standard complexes with Ru and Fe, whereas compounds 1' and 2 were considered to keep the macrocycle intact when switching the central metal atom. Three reactants including benzyl alcohol, ethyl benzene, and dihydroanthracene were selected as substrates for C-H activation. It is noteworthy to mention that Fe(IV)O complexes exhibit higher reactivity than those of their Ru(IV)O counterparts. Furthermore, regardless of the central metal, the complex featuring a tpy-dcbpy macrocycle demonstrates higher reactivity than that of TMCS. Here, a thorough analysis of the reactivity-controlling characteristics-such as spin state, steric factor, distortion energy, energy of the electron acceptor orbital, and quantum mechanical tunneling-was conducted. Fe(IV)O exhibits the exchanged enhanced two-state-reactivity with the quintet reactive state, whereas Ru(IV)O has only a triplet reactive state. Both the distortion energy and acceptor orbital energy are low in the case of Fe(IV)O supporting its higher reactivity. All the investigated C-H activation processes involve a significant contribution from hydrogen tunneling, which is more pronounced in the case of Ru, although it cannot alter the reactivity pattern. Furthermore, it has also been found that, independent of the central metal, aliphatic hydroxylation is always preferable to aromatic hydroxylation. Overall, this work is successful in establishing and investigating the cause of enzymes' natural preference for Fe over Ru as a cofactor for C-H activation enzymes.

Polar Effects in Hydrogen Atom Transfer Reactions from a Proton-Coupled Electron Transfer (PCET) Perspective: Abstractions from Toluenes

Rate constants for hydrogen atom transfer (HAT) reactions of substituted toluenes with tert-butyl, tert-butoxy, and tert-butylperoxyl radicals are reanalyzed here using the free energies of related proton transfer (PT) and electron transfer (ET) reactions, calculated from an extensive set of compiled or estimated pKa and E° values. The Eyring activation energies ΔGHAT‡ do not correlate with the relatively constant ΔG°HAT, but do correlate close-to-linearly with ΔG°PT and ΔG°ET. The slopes of correlations are similar for the three radicals except that the tBu• barriers shift in the opposite direction from the oxyl radical barriers─a clear example of the qualitative "polar effect" in HAT reactions. When cast quantitatively in free energy terms (ΔGHAT‡ vs ΔG°PT/ET), this effect is very small, only 5-10% of the typical Bell-Evans-Polanyi (BEP) effect of changing ΔG°HAT. This analysis also highlights connections between polar effects and the concepts of "asynchronous" or "imbalanced" HAT reactions in which the PT and ET components of ΔG°HAT contribute differently to the barrier. Finally, these observations are discussed in light of the traditional explanations of polar effects and the potential for a rubric that could predict the extent to which contra-thermodynamic selectivity may be achieved in HAT reactions.

Reconciling Imbalanced and Nonadiabatic Reactivity in Transition Metal-Oxo-Mediated Concerted Proton Electron Transfer (CPET)

Recently, there have been several experimental demonstrations of how the rates of concerted proton electron transfer (CPET) are affected by stepwise thermodynamic parameters of only proton (ΔG°PT) or electron (ΔG°ET) transfer. Semiclassical structure-activity relationships have been invoked to rationalize these linear free energy relationships, but it is not clear how they would manifest in a nonadiabatic reaction. Using density functional theory calculations, we demonstrate how a decrease in ΔG°PT can lead to transition state imbalance in a nonadiabatic framework. We then use these calculations to anchor a theoretical model that reproduces experimental trends with ΔG°PT and ΔG°ET. Our results reconcile predictions from semiclassical transition state theory with models that treat proton transfer quantum mechanically in CPET reactivity, make new predictions about the importance of basicity for uphill CPET reactions, and suggest similar treatments may be possible for other nonadiabatic reactions.

Enhancement of Reactivity of a RuIV-Oxo Complex in Oxygen-Atom-Transfer Catalysis by Hydrogen-Bonding with Amide Moieties in the Second Coordination Sphere

We have synthesized and characterized a RuII-OH2 complex (2), which has a pentadentate ligand with two pivalamide groups as bulky hydrogen-bonding (HB) moieties in the second coordination sphere (SCS). Complex 2 exhibits a coordination equilibrium through the coordination of one of the pivalamide oxygens to the Ru center in water, affording a η6-coordinated complex, 3. A detailed thermodynamic analysis of the coordination equilibrium revealed that the formation of 3 from 2 is entropy-driven owing to the dissociation of the axial aqua ligand in 2. Complex 2 was oxidized by a CeIV salt to produce the corresponding RuIII(OH) complex (5), which was characterized crystallographically. In the crystal structure of 5, hydrogen bonds are formed among the NH groups of the pivalamide moieties and the oxygen atom of the hydroxo ligand. Further 1e--oxidation of 5 yields the corresponding RuIV(O) complex, 6, which has intramolecular HB of the oxo ligand with two amide N-H protons. Additionally, the RuIII(OH) complex, 5, exhibits disproportionation to the corresponding RuIV(O) complex, 6, and a mixture of the RuII complexes, 2 and 3, in an acidic aqueous solution. We investigated the oxidation of a phenol derivative using complex 6 as the active species and clarified the switch of the reaction mechanism from hydrogen-atom transfer at pH 2.5 to electron transfer, followed by proton transfer at pH 1.0. Additionally, the intramolecular HB in 6 exerts enhancing effects on oxygen-atom transfer reactions from 6 to alkenes such as cyclohexene and its water-soluble derivative to afford the corresponding epoxides, relative to the corresponding RuIV(O) complex (6') lacking the HB moieties in the SCS.

Mechanistic Study of Reduction of Nitrite to NO by the Copper(II) Complex: Different Concerted Proton-Electron Transfer Reactivity between Nitrite and Nitro Complexes

The literature contains numerous reports of copper complexes for nitrite (NO2-) reduction. However, details of how protons and electrons arrive and how nitric oxide (NO) is released remain unknown. The influence of the coordination mode of nitrite on reactivity is also under debate. Kundu and co-workers have reported nitrite reduction by a copper(II) complex [J. Am. Chem. Soc. 2020, 142, 1726-1730]. In their report, the copper(II) complex reduced nitrite using a phenol derivative as a reductant, resulting in NO, a hydroxyl copper(II) complex, and the corresponding biphenol. Also, the involvement of proton-coupled electron transfer was proposed by mechanistic studies. Herein, density functional theory calculations were performed to determine a mechanism for reduction of nitrite by a copper(II) complex. As a result of geometry optimization of an initial complex, two possible structures were obtained: Cu-ONO and Cu-NO2. Two possible reaction pathways initiated from Cu-ONO or Cu-NO2 were then considered. The calculation results indicated that the Cu-ONO pathway is energetically favorable. When changes in the electronic structure were considered, both pathways were found to involve concerted proton-electron transfer (CPET). In addition, an intrinsic reaction coordinate analysis revealed that the two pathways were achieved by different types of CPET. Furthermore, an intrinsic bond orbital analysis clearly indicated that, in the Cu-ONO pathway, the chemical events involved proceeded concertedly yet asynchronously.

The role of basicity in selective C-H bond activation by transition metal-oxidos

The development of (bio)catalysts capable of selectively activating strong C-H bonds is a continuing challenge in modern chemistry. In both metalloenzymes and synthetic systems capable of activating C-H bonds, transition metal-oxido intermediates serve as the active species for reactivity whose thermodynamic properties influence the bond strengths they are capable of activating. In this Frontier article, we present current ideas of how the basicity of transition metal-oxidos impacts their reactivity with C-H bonds and present new opportunities within this field. We highlight recent insights into the role basicity plays in the activation process and its influence on mechanism, as well as the important role that secondary coordination sphere effects, such as hydrogen bonds, in tuning the basicity of the metal-oxido species is discussed.

Independent Tuning of the pKa or the E1/2 in a Family of Ruthenium Pyridine-Imidazole Complexes

Two series of RuII(acac)2(py-imH) complexes have been prepared, one with changes to the acac ligands and the other with substitutions to the imidazole. The proton-coupled electron transfer (PCET) thermochemistry of the complexes has been studied in acetonitrile, revealing that the acac substitutions almost exclusively affect the redox potentials of the complex (|ΔE1/2| ≫ |ΔpKa|·0.059 V) while the changes to the imidazole primarily affect its acidity (|ΔpKa|·0.059 V ≫ |ΔE1/2|). This decoupling is supported by DFT calculations, which show that the acac substitutions primarily affect the Ru-centered t2g orbitals, while changes to the py-imH ligand primarily affect the ligand-centered π orbitals. More broadly, the decoupling stems from the physical separation of the electron and proton within the complex and highlights a clear design strategy to separately tune the redox and acid/base properties of H atom donor/acceptor molecules.

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)-Oxo Complexes

Transition metal-oxo complexes are key intermediates in a variety of oxidative transformations, notably C-H bond activation. The relative rate of C-H bond activation mediated by transition metal-oxo complexes is typically predicated on substrate bond dissociation free energy in cases with a concerted proton-electron transfer (CPET). However, recent work has demonstrated that alternative stepwise thermodynamic contributions such as acidity/basicity or redox potentials of the substrate/metal-oxo may dominate in some cases. In this context, we have found basicity-governed concerted activation of C-H bonds with the terminal CoIII-oxo complex PhB(tBuIm)3CoIIIO. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analogous, more basic complex, PhB(AdIm)3CoIIIO, and studied its reactivity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB(tBuIm)3CoIIIO with C-H substrates, and O-H activation of phenol substrates displays mechanistic crossover to stepwise proton transfer-electron transfer (PTET) reactivity. Analysis of the thermodynamics of proton transfer (PT) and electron transfer (ET) reveals a distinct thermodynamic crossing point between concerted and stepwise reactivity. Furthermore, the relative rates of stepwise and concerted reactivity suggest that maximally imbalanced systems provide the fastest CPET rates up to the point of mechanistic crossover, which results in slower product formation.

Ru(IV)-Ru(IV) complexes having the doubly oxido-bridged core with a bridging carbonato or hydrogencarbonato ligand

Ru(IV)-Ru(IV) complexes having the doubly oxido-bridged diamond core with a bridging carbonato or hydrogencarbonato ligand, [{Ru^IV(ebpma)}₂(μ-O)₂(μ-O₂CO(H)_m)]X_n (ebpma; ethylbis(2-pyridylmethyl)amine, m = 0; [IV,IV]X₂ (X = PF₆, ClO₄), m = 1; [IV,IV_1H](ClO₄)₃), were isolated via the oxidation of the corresponding carbonato-bridged Ru(III)-Ru(IV) complex ([III,IV]⁺), and "[IV,IV](ClO₄)₂ and [IV,IV_1H](ClO₄)₃" were structurally characterized. The electrochemical and spectroscopic properties of [IV,IV]²⁺ and [IV,IV_1H]³⁺ were investigated both in organic solvents and aqueous solutions. The reactivity toward organic solvents having (a) methyl group(s) and reactions with organic substrates were studied as well. This should be the first time when systematic comparisons of the Ru(IV)-Ru(IV) species and corresponding Ru(III)-Ru(IV) complexes in the same tridentate ligand system were made.

Copper-Catalyzed Electrochemical C-H Fluorination

We report the systematic development of an electrooxidative methodology that translates stoichiometric C-H fluorination reactivity of an isolable CuIII fluoride complex into a catalytic process. The critical challenges of electrocatalysis with a highly reactive CuIII species were addressed by the judicious selection of electrolyte, F- source, and sacrificial electron acceptor. Catalyst-controlled C-H fluorination occurs with a preference for hydridic C-H bonds with high bond dissociation energies over weaker but less hydridic C-H bonds. The selectivity is driven by an oxidative asynchronous proton-coupled elelctron transfer (PCET) at an electrophilic CuIII-F complex. We further demonstrate that the asynchronicity factor of hydrogen atom transfer η can be used as a guideline to rationalize the selectivity of C-H fluorination.

A Contrasting Effect of Acid in Electron Transfer, Oxygen Atom Transfer, and Hydrogen Atom Transfer Reactions of a Nickel(III) Complex

There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [Ni^III(PaPy₃*)]²⁺ (1) in acetone/CH₃CN (v/v 19:1). 1 was synthesized by reacting [Ni^II(PaPy₃*)]⁺ (2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H₂O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H⁺ to the PaPy₃* ligand of 2; the one-electron reduction potential (E_red) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the E_red value resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA^•+ to 2 due to the binding of H⁺ to the PaPy₃* ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions.

Light-driven oxidation of CH4 to C1 chemicals catalysed by an organometallic Ru complex with O2

CH₄ conversion is one of the most challenging chemical reactions due to its inertness in terms of physical and chemical properties. We have achieved photo-induced C–H bond breaking of CH₄ and successive C–O bond formation to form CH₃OH concomitant with HCHO by an organometallic Ru complex with O₂.

We have achieved aerobic transformation of methane to C₁ chemicals catalysed by a homogeneous organometallic catalyst with light energy input.

Acid Catalysis in the Oxidation of Substrates by Mononuclear Manganese(III)-Aqua Complexes

Acids are known to enhance the reactivities of metal-oxygen intermediates, such as metal-oxo, -hydroperoxo, -peroxo, and -superoxo complexes, in biomimetic oxidation reactions. Although metal-aqua (and metal-hydroxo) complexes have been shown to be potent oxidants in oxidation reactions, acid effects on the reactivities of metal-aqua complexes have never been investigated previously. In this study, a mononuclear manganese(III)-aqua complex, [(dpaq^5NO2)Mn^III(OH₂)]²⁺ (1; dpaq^5NO2 = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamidate with an NO₂ substituent at the 5 position), which is relatively stable in the presence of triflic acid (HOTf), is used in the investigation of acid-catalyzed oxidation reactions by metal-aqua complexes. As a result, we report a remarkable acid catalysis in the six-electron oxidation of anthracene by 1 in the presence of HOTf; anthraquinone is formed as the product. In the HOTf-catalyzed six-electron oxidation of anthracene by 1, the rate constant increases linearly with an increase of the HOTf concentration. Combined with the observed one-electron oxidation product, anthracene (derivative) radical cation, and the substitution effect at the 5 position of the dpaq ligand in 1 on the rate constants of the oxidation of anthracene, it is concluded that the oxidation of anthracene occurs via an acid-promoted electron transfer (APET) from anthracene to 1. The dependence of the rate constants of the APET from electron donors, including anthracene derivatives, to 1 on the driving force of electron transfer is also shown to be well fitted by the Marcus equation of outer-sphere electron transfer. To the best of our knowledge, this is the first example showing acid catalysis in the oxidation of substrates by metal(III)-aqua complexes.

Phytate Coordination-Enhanced Electrocatalytic Activity of Copper for Nitroarene Hydrogenation through Concerted Proton-Coupled Electron Transfer

Coupling acid-electrolyte proton exchange membrane fuel cells for electricity generation and cathodic hydrogenation for valuable chemical production shows great potential in energy and chemical industry. The key for this promising approach is the identification of cathode electrocatalysts with acid resistance, high activity, and low fabrication cost for practical applications. Among various promising cathodic candidates for this integrative approach, the easily available and cheap Cu suffers from low acidic hydrogenation activity due to kinetically arduous proton adsorption/activation. Inspired by the kinetic advantages of the concerted proton-coupled electron transfer (CPET) over the sequential proton-electron transfer process, herein, we use phytate coordination on Cu surface to overcome the kinetic bottleneck for proton adsorption/activation through the CPET pathway in an acidic half-cell setup; this leads to 1 order of magnitude activity enhancement (36.94-fold) for nitrobenzene hydrogenation. Mechanistic analysis confirms that phytate, as proton acceptor, induces the CPET process and overcomes the above kinetic limitations by tuning the d-band center and concentrating protons on the Cu surface. Consequently, the CPET process facilitates the formation of active hydrogen intermediates for efficient cathodic hydrogenation. This work provides a promising approach to integrate electricity generation and chemical production.

Accelerating Anode Reaction with Electro-oxidation of Alcohols over Ru Nanoparticles to Reduce the Potential for Water Splitting

Generating hydrogen by water electrolysis is a promising and sustainable approach to the production of a green energy carrier, but the sluggish kinetics of the oxygen evolution reaction (OER) at anode leads to a high working potential. Replacing OER with electro-oxidation of organics driven at a low potential offers an effective way to accelerate the sluggish anode reaction, and thus increase hydrogen evolution in water-splitting. Herein, we have prepared a Ru nanoparticles on N-doped carbon nanotubes (Ru-NPs@NCNTs) to implement electro-oxidation of benzyl alcohol toward reducing the anodic potential in watersplitting. The potential of the anode reaction is remarkably decreased from 1.76 to 1.19 V vs RHE at a current density of 10 mA cm^-2 with the assistance of a Ru-NPs catalyst. Furthermore, 100% selectivity and 95% yield of valuable benzaldehyde were achieved simultaneously. The Ru-NPs also exhibits good durability and wide applicability to other alcohols. The high performance of Ru-NPs is mainly attributed to the unique horizontal adsorption configuration of benzyl alcohol with surface atoms of the catalyst, shortening the distance between the ^•OH group and Ru atoms, and increasing the activation rate of the ^•OH group. This work presents a feasible strategy to boost water-splitting performance and concurrently produce value-added organics under mild conditions.

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(III)-hydroxo and Mn(III)-aqua complexes

Hydrogen atom transfer (HAT) of metal-oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes have yet to be...

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH_n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H₂), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H₂) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

Semiempirical method for examining asynchronicity in metal-oxido-mediated C-H bond activation

The oxidation of substrates via the cleavage of thermodynamically strong C-H bonds is an essential part of mammalian metabolism. These reactions are predominantly carried out by enzymes that produce high-valent metal-oxido species, which are directly responsible for cleaving the C-H bonds. While much is known about the identity of these transient intermediates, the mechanistic factors that enable metal-oxido species to accomplish such difficult reactions are still incomplete. For synthetic metal-oxido species, C-H bond cleavage is often mechanistically described as synchronous, proton-coupled electron transfer (PCET). However, data have emerged that suggest that the basicity of the M-oxido unit is the key determinant in achieving enzymatic function, thus requiring alternative mechanisms whereby proton transfer (PT) has a more dominant role than electron transfer (ET). To bridge this knowledge gap, the reactivity of a monomeric MnIV-oxido complex with a series of external substrates was studied, resulting in a spread of over 104 in their second-order rate constants that tracked with the acidity of the C-H bonds. Mechanisms that included either synchronous PCET or rate-limiting PT, followed by ET, did not explain our results, which led to a proposed PCET mechanism with asynchronous transition states that are dominated by PT. To support this premise, we report a semiempirical free energy analysis that can predict the relative contributions of PT and ET for a given set of substrates. These findings underscore why the basicity of M-oxido units needs to be considered in C-H functionalization.

Single-atom cobalt-fused biomolecule-derived nitrogen-doped carbon nanosheets for selective oxidation reactions

Non-noble metal single-atom catalysts hold great promise in selective oxidation reactions, although the progress is still unsatisfactory because of the synthesis challenge and the lack of mechanistic interpretations. Herein, we develop a biomolecule-based strategy to synthesize isolated Co single atom site catalysts by one-step pyrolysis of guanosine and Co precursors. Due to the abundant hydrogen bonding and π-π interaction of guanosine, the as-synthesized Co-N-C catalysts present a hierarchical porous two-dimensional (2D) nanostructure with an ultrahigh specific surface area, large pore volume, and high density of cobalt single atoms. Aberration-corrected electron microscopy and X-ray photoelectron spectroscopy reveal that Co species are present as isolated single sites and stabilized by nitrogen-doped carbon nanosheets. These characteristics make Co-GS-900 suitable as an efficient catalyst for selective oxidation of aromatic alkanes. For oxidation of ethylbenzene, Co-GS-900 exhibits a superior performance f with 91% conversion and 98% selectivity of acetophenone.