Dichotomous hydrogen atom transfer vs proton-coupled electron transfer during activation of X-H bonds (X = C, N, O) by nonheme iron-oxo complexes of variable basicity

Copper-oxygen adducts: new trends in characterization and properties towards C-H activation

This review summarizes the latest discoveries in the field of C-H activation by copper monoxygenases and more particularly by their bioinspired systems. This work first describes the recent background on copper-containing enzymes along with additional interpretations about the nature of the active copper-oxygen intermediates. It then focuses on relevant examples of bioinorganic synthetic copper-oxygen intermediates according to their nuclearity (mono to polynuclear). This includes a detailed description of the spectroscopic features of these adducts as well as their reactivity towards the oxidation of recalcitrant Csp3 -H bonds. The last part is devoted to the significant expansion of heterogeneous catalytic systems based on copper-oxygen cores (i.e. within zeolite frameworks).

Radical ligand transfer: mechanism and reactivity governed by three-component thermodynamics

Here, we demonstrate that the relationship between reactivity and thermodynamics in radical ligand transfer chemistry can be understood if this chemistry is dissected as concerted ion-electron transfer (cIET). Namely, we investigate radical ligand transfer reactions from the perspective of thermodynamic contributions to the reaction barrier: the diagonal effect of the free energy of the reaction, and the off-diagonal effect resulting from asynchronicity and frustration, which we originally derived from the thermodynamic cycle for concerted proton-electron transfer (cPET). This study on the OH transfer reaction shows that the three-component thermodynamic model goes beyond cPET chemistry, successfully capturing the changes in radical ligand transfer reactivity in a series of model FeIII-OH⋯(diflouro)cyclohexadienyl systems. We also reveal the decisive role of the off-diagonal thermodynamics in determining the reaction mechanism. Two possible OH transfer mechanisms, in which electron transfer is coupled with either OH- and OH+ transfer, are associated with two competing thermodynamic cycles. Consequently, the operative mechanism is dictated by the cycle yielding a more favorable off-diagonal effect on the barrier. In line with this thermodynamic link to the mechanism, the transferred OH group in OH-/electron transfer retains its anionic character and slightly changes its volume in going from the reactant to the transition state. In contrast, OH+/electron transfer develops an electron deficiency on OH, which is evidenced by an increase in charge and a simultaneous decrease in volume. In addition, the observations in the study suggest that an OH+/electron transfer reaction can be classified as an adiabatic radical transfer, and the OH-/electron transfer reaction as a less adiabatic ion-coupled electron transfer.

Axial Ligation Impedes Proton-Coupled Electron-Transfer Reactivity of a Synthetic Compound-I Analogue

The nature of the axial ligand in high-valent iron-oxo heme enzyme intermediates and related synthetic catalysts is a critical structural element for controlling proton-coupled electron-transfer (PCET) reactivity of these species. Herein, we describe the generation and characterization of three new 6-coordinate, iron(IV)-oxo porphyrinoid-π-cation-radical complexes and report their PCET reactivity together with a previously published 5-coordinate analogue, FeIV(O)(TBP8Cz+•) (TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato3-) (2) (Cho, K. A high-valent iron-oxo corrolazine activates C-H bonds via hydrogen-atom transfer. J. Am. Chem. Soc. 2012, 134, 7392-7399). The new complexes FeIV(O)(TBP8Cz+•)(L) (L = 1-methyl imidazole (1-MeIm) (4a), 4-dimethylaminopyridine (DMAP) (4b), cyanide (CN-)(4c)) can be generated from either oxidation of the ferric precursors or by addition of L to the Compound-I (Cpd-I) analogue at low temperatures. These complexes were characterized by UV-vis, electron paramagnetic resonance (EPR), and Mössbauer spectroscopies, and cryospray ionization mass spectrometry (CSI-MS). Kinetic studies using 4-OMe-TEMPOH as a test substrate indicate that coordination of a sixth axial ligand dramatically lowers the PCET reactivity of the Cpd-I analogue (rates up to 7000 times slower). Extensive density functional theory (DFT) calculations together with the experimental data show that the trend in reactivity with the axial ligands does not correlate with the thermodynamic driving force for these reactions or the calculated strengths of the O-H bonds being formed in the FeIV(O-H) products, pointing to non-Bell-Evans-Polanyi behavior. However, the PCET reactivity does follow a trend with the bracketed reduction potential of Cpd-I analogues and calculated electron affinities. The combined data suggest a concerted mechanism (a concerted proton electron transfer (CPET)) and an asynchronous movement of the electron/proton pair in the transition state.

A density functional theory analysis of the C-H activation reactivity of iron(IV)-oxo complexes with an 'O' substituted tetramethylcyclam macrocycle

In this article, we present a meticulous computational study to foresee the effect of an oxygen-rich macrocycle on the reactivity for C-H activation. For this study, a widely studied nonheme Fe(IV)O molecule with a TMC (1,4,8,11-tetramethyl 1,4,8,11-tetraazacyclotetradecane) macrocycle that is equatorially attached to four nitrogen atoms (designated as N4) and acetonitrile as an axial ligand has been taken into account. For the goal of hetero-substitution, step-by-step replacement of the N4 framework with O atoms, i.e., N4, N3O1, N2O2, N1O3, and O4 systems, has been considered, and dihydroanthracene (DHA) has been used as the substrate. In order to neutralise the system and prevent the self-interaction error in DFT, triflate counterions have also been included in the calculations. The study of the energetics of these C-H bond activation reactions and the potential energy surfaces mapped therefore reveal that the initial hydrogen abstraction, which is the rate-determining step, follows the two-state reactivity (TSR) pattern, which means that the originally excited quintet state falls lower in the transition state and the product. The reaction follows the hydrogen atom transfer (HAT) mechanism, as indicated by the spin density studies. The results revealed a fascinating reactivity order, in which the reactivity increases with the enrichment of the oxygen atom in the equatorial position, namely the order follows N4 < N3O1 < N2O2 < N1O3 < O4. The impacts of oxygen substitution on quantum mechanical tunneling and the H/D kinetic isotope effect have also been investigated. When analysing the causes of this reactivity pattern, a number of variables have been identified, including the reactant-like transition structure, spin density distribution, distortion energy, and energies of the electron acceptor orbital, i.e., the energy of the LUMO (σ*z2), which validate the obtained outcome. Our results also show very good agreement with earlier combined experimental and theoretical studies considering TMC and TMCO-type complexes. The DFT predictions reported here will undoubtedly encourage experimental research in this biomimetic field, as they provide an alternative with higher reactivity in which heteroatoms can be substituted for the traditional nitrogen atom.

Computational Study Into the Oxidative Ring-Closure Mechanism During the Biosynthesis of Deoxypodophyllotoxin

The nonheme iron dioxygenase deoxypodophyllotoxin synthase performs an oxidative ring-closure reaction as part of natural product synthesis in plants. How the enzyme enables the oxidative ring-closure reaction of (-)-yatein and avoids substrate hydroxylation remains unknown. To gain insight into the reaction mechanism and understand the details of the pathways leading to products and by-products we performed a comprehensive computational study. The work shows that substrate is bound tightly into the substrate binding pocket with the C7'-H bond closest to the iron(IV)-oxo species. The reaction proceeds through a radical mechanism starting with hydrogen atom abstraction from the C7'-H position followed by ring-closure and a final hydrogen transfer to form iron(II)-water and deoxypodophyllotoxin. Alternative mechanisms including substrate hydroxylation and an electron transfer pathway were explored but found to be higher in energy. The mechanism is guided by electrostatic perturbations of charged residues in the second-coordination sphere that prevent alternative pathways.

Enhanced Reactivity through Equatorial Sulfur Coordination in Nonheme Iron(IV)-Oxo Complexes: Insights from Experiment and Theory

Sulfur ligation in metalloenzymes often gives the active site unique properties, whether it is the axial cysteinate ligand in the cytochrome P450s or the equatorial sulfur/thiol ligation in nonheme iron enzymes. To understand sulfur ligation to iron complexes and how it affects the structural, spectroscopic, and intrinsic properties of the active species and the catalysis of substrates, we pursued a systematic study and compared sulfur with amine-ligated iron(IV)-oxo complexes. We synthesized and characterized a biomimetic N4S-ligated iron(IV)-oxo complex and compared the obtained results with an analogous N5-ligated iron(IV)-oxo complex. Our work shows that the amine for sulfur replacement in the equatorial ligand framework leads to a rate enhancement for oxygen atom and hydrogen atom transfer reactions. Moreover, the sulfur-ligated iron(IV)-oxo complex reacts through a different reaction mechanism as compared to the N5-ligated iron(IV)-oxo complex, where the former reacts through hydride transfer with the latter reacting via radical pathways. We show that the reactivity differences are caused by a dramatic change in redox potential between the two complexes. Our studies highlight the importance of implementing a sulfur ligand into the equatorial ligand framework of nonheme iron(IV)-oxo complexes and how it affects the physicochemical properties of the oxidant and its reactivity.

C-H Bond Activation by Iridium(III) and Iridium(IV) Oxo Complexes

Oxidation of an iridium(III) oxo precursor enabled the structural, spectroscopic, and quantum-chemical characterization of the first well-defined iridium(IV) oxo complex. Side-by-side examination of the proton-coupled electron transfer thermochemistry revealed similar driving forces for the isostructural oxo complexes in two redox states due to compensating contributions from H+ and e- transfer. However, C-H activation of dihydroanthracene revealed significant hydrogen tunneling for the distinctly more basic iridium(III) oxo complex. Our findings complement the growing body of data that relate tunneling to ground state properties as predictors for the selectivity of C-H bond activation.

C-H bond activation by high-valent iron/cobalt-oxo complexes: a quantum chemical modeling approach

High-valent metal-oxo species serve as key intermediates in the activation of inert C-H bonds. Here, we present a comprehensive DFT analysis of the parameters that have been proposed as influencing factors in modeled high-valent metal-oxo mediated C-H activation reactions. Our approach involves utilizing DFT calculations to explore the electronic structures of modeled FeIVO (species 1) and CoIVO ↔ CoIII-O˙ (species 2), scrutinizing their capacity to predict improved catalytic activity. DFT and DLPNO-CCSD(T) calculations predict that the iron-oxo species possesses a triplet as the ground state, while the cobalt-oxo has a doublet as the ground state. Furthermore, we have investigated the mechanistic pathways for the first C-H bond activation, as well as the desaturation of the alkanes. The mechanism was determined to be a two-step process, wherein the first hydrogen atom abstraction (HAA) represents the rate-limiting step, involving the proton-coupled electron transfer (PCET) process. However, we found that the second HAA step is highly exothermic for both species. Our calculations suggest that the iron-oxo species (Fe-O = 1.672 Å) exhibit relatively sluggish behavior compared to the cobalt-oxo species (Co-O = 1.854 Å) in C-H bond activation, attributed to a weak metal-oxygen bond. MO, NBO, and deformation energy analysis reveal the importance of weakening the M-O bond in the cobalt species, thereby reducing the overall barrier to the reaction. This catalyst was found to have a C-H activation barrier relatively smaller than that previously reported in the literature.

Electronic Structure and Reactivity of Mononuclear Nonheme Iron-Peroxo Complexes as a Biomimetic Model of Rieske Oxygenases: Ring Size Effects of Macrocyclic Ligands

We report the macrocyclic ring size-electronic structure-electrophilic reactivity correlation of mononuclear nonheme iron(III)-peroxo complexes bearing N-tetramethylated cyclam analogues (n-TMC), [FeIII(O2)(12-TMC)]+ (1), [FeIII(O2)(13-TMC)]+ (2), and [FeIII(O2)(14-TMC)]+ (3), as a model study of Rieske oxygenases. The Fe(III)-peroxo complexes show the same δ and pseudo-σ bonds between iron and the peroxo ligand. However, the strength of these interactions varies depending on the ring size of the n-TMC ligands; the overall Fe-O bond strength and the strength of the Fe-O2 δ bond increase gradually as the ring size of the n-TMC ligands becomes smaller, such as from 14-TMC to 13-TMC to 12-TMC. MCD spectroscopy plays a key role in assigning the characteristic low-energy δ → δ* LMCT band, which provides direct insight into the strength of the Fe-O2 δ bond and which, in turn, is correlated with the superoxo character of the iron-peroxo group. In oxidation reactions, reactivities of 1-3 toward hydrocarbon C-H bond activation are compared, revealing the reactivity order of 1 > 2 > 3; the [FeIII(O2)(n-TMC)]+ complex with a smaller n-TMC ring size, 12-TMC, is much more reactive than that with a larger n-TMC ring size, 14-TMC. DFT analysis shows that the Fe(III)-peroxo complex is not reactive toward C-H bonds, but it is the end-on Fe(II)-superoxo valence tautomer that is responsible for the observed reactivity. The hydrogen atom abstraction (HAA) reactivity of these intermediates is correlated with the overall donicity of the n-TMC ligand, which modulates the energy of the singly occupied π* superoxo frontier orbital that serves as the electron acceptor in the HAA reaction. The implications of these results for the mechanism of Rieske oxygenases are further discussed.

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.

Neutral Phenolic Contaminants Are Not Necessarily More Resistant to Permanganate Oxidation Than Their Dissociated Counterparts: Importance of Proton-Coupled Electron Transfer

Despite decades of research on phenols oxidation by permanganate, there are still considerable uncertainties regarding the mechanisms accounting for the unexpected parabolic pH-dependent oxidation rate. Herein, the pH effect on phenols oxidation was reinvestigated experimentally and theoretically by highlighting the previously unappreciated proton transfer. The results revealed that the oxidation of protonated phenols occurred via proton-coupled electron transfer (PCET) pathways, which can switch from ETPT (electron transfer followed by proton transfer) to CEPT (concerted electron-proton transfer) or PTET (proton transfer followed by electron transfer) with an increase in pH. A PCET-based model was thus established, and it could fit the kinetic data of phenols oxidation by permanganate well. In contrast with what was previously thought, both the simulating results and the density functional theory calculation indicated the rate of CEPT reaction of protonated phenols with OH- as the proton acceptor was much higher than that of deprotonated phenols, which could account for the pH-rate profiles for phenols oxidation. Analysis of the quantitative structure-activity relationships among the modeled rate constants, Hammett constants, and pKa values of phenols further supports the idea that the oxidation of protonated phenols is dominated by PCET. This study improves our understanding of permanganate oxidation and suggests a new pattern of reactivity that may be applicable to other systems.

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.

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.

Stacking of a Cofacially Stacked Iron Phthalocyanine Dimer on Graphite Achieved High Catalytic CH4 Oxidation Activity Comparable to That of pMMO

Numerous biomimetic molecular catalysts inspired by methane monooxygenases (MMOs) that utilize iron or copper-oxo species as key intermediates have been developed. However, the catalytic methane oxidation activities of biomimetic molecule-based catalysts are still much lower than those of MMOs. Herein, we report that the close stacking of a μ-nitrido-bridged iron phthalocyanine dimer onto a graphite surface is effective in achieving high catalytic methane oxidation activity. The activity is almost 50 times higher than that of other potent molecule-based methane oxidation catalysts and comparable to those of certain MMOs, in an aqueous solution containing H₂O₂. It was demonstrated that the graphite-supported μ-nitrido-bridged iron phthalocyanine dimer oxidized methane, even at room temperature. Electrochemical investigation and density functional theory calculations suggested that the stacking of the catalyst onto graphite induced partial charge transfer from the reactive oxo species of the μ-nitrido-bridged iron phthalocyanine dimer and significantly lowered the singly occupied molecular orbital level, thereby facilitating electron transfer from methane to the catalyst in the proton-coupled electron-transfer process. The cofacially stacked structure is advantageous for stable adhesion of the catalyst molecule on the graphite surface in the oxidative reaction condition and for preventing decreases in the oxo-basicity and generation rate of the terminal iron-oxo species. We also demonstrated that the graphite-supported catalyst exhibited appreciably enhanced activity under photoirradiation owing to the photothermal effect.

Brønsted Acids Promote Olefin Oxidations by Bioinspired Nonheme CoIII(PhIO)(OH) Complexes: A Role for Low-Barrier Hydrogen Bonds

Introduction of Brønsted acids into biomimetic nonheme reactions promotes the oxidative ability of metal-oxygen complexes significantly. However, the molecular machinery of the promoted effects is missing. Herein, a comprehensive investigation of styrene oxidation by a cobalt(III)-iodosylbenzene complex, [(TQA)Co^III(OIPh)(OH)]²⁺ (1, TQA = tris(2-quinolylmethyl)amine), in the presence and absence of triflic acid (HOTf) was performed using density functional theory calculations. Results revealed for the first time that there is a low-barrier hydrogen bond (LBHB) between HOTf and the hydroxyl ligand of 1, which forms two valence-resonance structures [(TQA)Co^III(OIPh)(HO^---HOTf)]²⁺ (1_LBHB) and [(TQA)Co^III(OIPh)(H₂O--OTf^-)]²⁺ (1'_LBHB). Due to the oxo-wall, these complexes (1_LBHB and 1'_LBHB) cannot convert to high-valent cobalt-oxyl species. Instead, styrene oxidation by these oxidants (1_LBHB and 1'_LBHB) shows novel spin-state selectivity, i.e., on the ground closed-shell singlet state, styrene is oxidized to an epoxide, whereas on the excited triplet and quintet states, an aldehyde product, phenylacetaldehyde, is formed. The preferred pathway is styrene oxidation by 1'_LBHB, which is initiated by a rate-limiting bond-formation-coupled electron transfer process with an energy barrier of 12.2 kcal mol^-1. The nascent PhIO-styrene-radical-cation intermediate undergoes an intramolecular rearrangement to produce an aldehyde. The halogen bond between the OH^-/H₂O ligand and the iodine of PhIO modulates the activity of the cobalt-iodosylarene complexes 1_LBHB and 1'_LBHB. These new mechanistic findings enrich our knowledge of nonheme chemistry and hypervalent iodine chemistry and will play a positive role in the rational design of new catalysts.

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.

Theoretical investigation on the elusive structure-activity relationship of bioinspired high-valence nickel-halogen complexes in oxidative fluorination reactions

Very recently, bioinspired high-valence metal-halogen complexes have been proven to be competent oxidants in the C-H bond activation and heteroatom dihalogenation reactions. However, the structure-activity relationship of such active species and the reaction mechanisms of oxidations mediated by these oxidants are still elusive. In this study, density functional theory (DFT) calculations were performed to systematically study the oxidizing ability of the high-valence Ni^III-X (X = F and Cl) complexes Et₄N[Ni^III(Cl/F)(L)], (1_Cl/F, Et = ethyl, L = N,N'-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide), such as the reaction mechanism of fluorination of 1,4-cyclohexadiene (CHD) by 1_F in the presence of AgF and the reaction mechanism of difluorination of triphenyl phosphine (PPh₃) by 1_F. All calculated results fit well with the experiments and present new mechanistic findings. The C-H bond activation by the high-valence nickel(III)-halogen complexes was found to proceed via a hydrogen-atom transfer (HAT) mechanism by analysis of the molecular orbitals of the transition states. C-H bond activation by 1_F takes a Ni-F-H angle of ca. 180°, whereas that by 1_Cl takes an angle of ca. 120° on the transition states. These results indicate that the exchange-enhanced reactivity is responsible for the dramatic oxidative difference between these two oxidants. The role of AgF in C-H fluorination of CHD by 1_F is proposed to act as a Lewis acid adduct, AgF-binding Ni(III)-fluorine complex 1_F-Ag-F, which acts both as an oxidant in C-H bond activation and as a fluorine donor in the fluorination step. A cooperative oxidation mechanism involving two 1_F oxidants was proposed for the difluorination of PPh₃ by 1_F. These theoretical findings will enrich the knowledge of high-valence metal-halogen chemistry and play a positive role in the rational design of new catalysts.

C(sp3)-H cyanation by a formal copper(iii) cyanide complex

High-valent metal oxo complexes are prototypical intermediates for the activation and hydroxylation of alkyl C-H bonds. Substituting the oxo ligand with other functional groups offers the opportunity for additional C-H functionalization beyond C-O bond formation. However, few species aside from metal oxo complexes have been reported to both activate and functionalize alkyl C-H bonds. We herein report the first example of an isolated copper(iii) cyanide complex (LCu^IIICN) and its C-H cyanation reactivity. We found that the redox potential (E _ox) of substrates, instead of C-H bond dissociation energy, is a key determinant of the rate of PCET, suggesting an oxidative asynchronous CPET or ETPT mechanism. Among substrates with the same BDEs, those with low redox potentials transfer H atoms up to a million-fold faster. Capitalizing on this mechanistic insight, we found that LCu^IIICN is highly selective for cyanation of amines, which is predisposed to oxidative asynchronous or stepwise transfer of H⁺/e^-. Our study demonstrates that the asynchronous effect of PCET is an appealing tool for controlling the selectivity of C-H functionalization.

Theoretical study on hydrogen transfer in the dissociation of dimethyl disulfide radical cations

Hydrogen transfer (HT) is of crucial importance in biochemistry and atmospheric chemistry. Here, HT processes involved in the dissociation reaction of dimethyl disulfide radical cations (DMDS˙⁺, CH₃SSCH₃˙⁺) are investigated using quantum chemical calculations. Four HTs from the C to S atom and one HT from the S to S atom are observed and the most probable paths are proposed in the dissociation channel from DMDS˙⁺ to CH_nS⁺ (n = 2-4). The mechanisms of all these five HTs are described as hydrogen atom transfer (HAT) and four of them are accompanied by electron transfer (ET). Considering the catalytic effect of water molecules existing in organisms and the atmosphere, five HT processes in the dissociation of the [DMDS + H₂O]˙⁺ complex are further explored, which show lower free energy barriers. With the participation of water molecules acting as a base, two HTs from the C to the S atom, which have the largest decrease in energy barriers, are characterized as concerted proton-coupled electron transfer (cPCET). These results can be extended to understand the mechanism of the HT process during the dissociation of disulfide and help provide a strategy to design a rare cPCET mechanism for the activation of the C-H bond.

Comparative oxidative ability of mononuclear and dinuclear high-valent iron-oxo species towards the activation of methane: does the axial/bridge atom modulate the reactivity?

Over the years, mononuclear Fe^IVO species have been extensively studied, but the presence of dinuclear Fe^IVO species in soluble methane monooxygenase (sMMO) has inspired the development of biomimic models that could activate inert substrates such as methane. There are some successful attempts; particularly the [(Por)(m-CBA) Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- species has been reported to activate methane and yield decent catalytic turnover numbers and therefore regarded as the closest to the sMMO enzyme functional model, as no mononuclear Fe^IVO analogues could achieve this feat. In this work, we have studied a series of mono and dinuclear models using DFT and ab initio DLPNO-CCSD(T) calculations to probe the importance of nuclearity in enhancing the reactivity. We have probed the catalytic activities of four complexes: [(HO)Fe^IV(O)(Por)]^- (1), [(HO)Fe^IV(O)(Por˙⁺)] (2), μ-oxo dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-O)Fe^IV(O) (Por˙⁺)]^- (3) and N-bridged dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- (4) towards the activation of methane. Additionally, calculations were performed on the mononuclear models [(X)Fe^IV(O)(Por˙⁺)]ⁿ {X = N 4a (n = -2), NH 4b (n = -1) and NH₂4c (n = 0)} to understand the role of nuclearity in the reactivity. DFT calculations performed on species 1-4 suggest an interesting variation among them, with species 1-3 possessing an intermediate spin (S = 1) as a ground state and species 4 possessing a high-spin (S = 2) as a ground state. Furthermore, the two Fe^IV centres in species 3 and 4 are antiferromagnetically coupled, yielding a singlet state with a distinct difference in their electronic structure. On the other hand, species 2 exhibits a ferromagnetic coupling between the Fe^IV and the Por˙⁺ moiety. Our calculations suggest that the higher barriers for the C-H bond activation of methane and the rebound step for species 1 and 3 are very high in energy, rendering them unreactive towards methane, while species 2 and 4 have lower barriers, suggesting their reactivity towards methane. Studies on the system reveal that model 4a has multiple FeN bonds facilitating greater reactivity, whereas the other two models have longer Fe-N bonds and less radical character with steeper barriers. Strong electronic cooperativity is found to be facilitated by the bridging nitride atom, and this cooperativity is suppressed by substituents such as oxygen, rendering them inactive. Thus, our study unravels that apart from enhancing the nuclearity, bridging atoms that facilitate strong cooperation between the metals are required to activate very inert substrates such as methane, and our results are broadly in agreement with earlier experimental findings.

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.

Mactanamide and lariciresinol as radical scavengers and Fe2+ ion chelators - A DFT study

A DFT based kinetic study of OOH radical scavenging potency of mactanamide (MA) and lariciresinol (LA), two natural polyphenols, indicates their nearly equal potential via the proton coupled electron transfer (PCET) mechanism in lipid media. Contribution of C-H bond breaking to this potency is negligible compared to O-H bond breaking, contrary to recent claims. The predicted potency of both compounds is not sufficient to protect biological molecules from oxidative damage in lipid media. In aqueous media, the scavenging potency of MA and LA phenoxide anions via the single electron transfer (SET) mechanism is much higher and may contribute to the protection of lipids, proteins, and DNA from OOH radical damage. Also, MA and LA have the potential to chelate catalytic Fe2+ ions, thus suppressing the formation of dangerous OH radicals via Fenton-type reactions. The monoanionic species of MA and LA show stronger monodentate chelating ability with Fe2+ ion compared to its neutral form. The dianionic specie LA2- exhibited the highest chelation ability with Fe2+ ion via bidentate 1:2 coordination. However, direct radical scavenging and metal chelation could be rarely operative in vivo because MA and LA presumably achieve very low concentrations in systemic circulation.

Lanthanide Complexes Containing a Terminal Ln═O Oxo Bond: Revealing Higher Stability of Tetravalent Praseodymium versus Terbium

We report on the reactivity of gas-phase lanthanide-oxide nitrate complexes, [Ln(O)(NO₃)₃]^- (denoted LnO²⁺), produced via elimination of NO₂^• from trivalent [Ln^III(NO₃)₄]^- (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln^III-O^• oxyl, a Ln^IV═O oxo, or an intermediate Ln^III/IV oxyl/oxo bond, depending on the accessibility of the tetravalent Ln^IV state. Hydrogen atom abstraction reactivity of the LnO²⁺ complexes to form unambiguously trivalent [Ln^III(OH)(NO₃)₃]^- reveals the nature of the oxide bond. The result of slower reactivity of PrO²⁺ versus TbO²⁺ is considered to indicate higher stability of the tetravalent praseodymium-oxo, Pr^IV═O, versus Tb^IV═O. This is the first report of Pr^IV as more stable than Tb^IV, which is discussed with respect to ionization potentials, standard electrode potentials, atomic promotion energies, and oxo bond covalency via 4f- and/or 5d-orbital participation.

Hydrogen Abstraction by Alkoxyl Radicals: Computational Studies of Thermodynamic and Polarity Effects on Reactivities and Selectivities

Density functional theory calculations (ωB97X-D) are reported for the reactions of methoxy, tert-butoxy, trichloroethoxy, and trifluoroethoxy radicals with a series of 26 C-H bonds in different environments characteristic of a variety of hydrocarbons and substituted derivatives. The variations in activation barriers are analyzed with modified Evans-Polanyi treatments to account for polarity and unsaturation effects. The treatments by Roberts and Steel and by Mayer have inspired the development of a simple treatment involving the thermodynamics of reactions, the difference between the reactant radical and product radical electronegativities, and the absence or presence of α-unsaturation. The three-parameter equation (ΔH^⧧ = 0.52ΔH_rxn(1 - d) - 0.35Δχ_AB² + 10.0, where d = 0.44 when there is α-unsaturation to the reacting C-H bond), correlates well with quantum mechanically computed barriers and shows the quantitative importance of the thermodynamics of reactions (dictated by the reactant and the product bond dissociation energies) and polar effects.

A mechanistic switch in C−H bond activation by elusive FeV(O)(TAML) reaction intermediate: A theoretical study

The divergent behavior of C−H bond oxidations of aliphatic substrates compared to those of aromatic substrates shown in Gupta's experiment was mechanistically studied herein by means of density functional theory calculations. Our calculations reveal that such difference is caused by different reaction mechanisms between two kinds of substrates (the aliphatic cyclohexane, 2,3-dimethylbutane and the aromatic toluene, ethylbenzene and cumene). For the aliphatic substrates, C−H oxidation by the oxidant FeV(O)(TAML) is a hydrogen atom transfer process; whereas for the aromatic substrates, C−H oxidation is a proton-coupled electron transfer (PCET) process with a proton transfer character on the transition state, that is, a proton-coupled electron transfer process holding a proton transfer-like transition state (PCET(PT)). This difference is caused by the strong π− π interactions between the tetra-anionic TAML ring and the phenyl ring of the aromatic substrates, which has a “pull” effect to make the electron transfer from substrates to the Fe=O moiety inefficient.

The chameleon-like nature of elusive cobalt-oxygen intermediates in C-H bond activation reactions

High-valence metal-oxo (M-O, M = Fe, Mn, etc.) species are well-known reaction intermediates that are responsible for a wide range of pivotal oxygenation reactions and water oxidation reactions in metalloenzymes. Although extensive efforts have been devoted to synthesizing and identifying such complexes in biomimetic studies, the structure-function relationship and related reaction mechanisms of these reaction intermediates remain elusive, especially for the cobalt-oxygen species. In the present manuscript, the calculated results demonstrate that the tetraamido macrocycle ligated cobalt complex, Co(O)(TAML) (1), behaves like a chameleon: the electronic structure varies from a cobalt(III)-oxyl species to a cobalt(IV)-oxo species when a Lewis acid Sc³⁺ salt coordinates or an acidic hydrocarbon attacks 1. The dichotomous correlation between the reaction rates of C-H bond activation by 1 and the bond dissociation energy (BDE) vs. the acidity (pK_a) was rationalized for the first time by different reaction mechanisms: for normal C-H bond activation, the Co(III)-oxyl species directly activates the C-H bond via a hydrogen atom transfer (HAT) mechanism, whereas for acidic C-H bond activation, the Co(III)-oxyl species evolves to a Co(IV)-oxo species to increase the basicity of the oxygen to activate the acidic C-H bond, via a novel PCET(PT) mechanism (proton-coupled electron transfer with a PT(proton-transfer)-like transition state). These theoretical findings will enrich the knowledge of biomimetic metal-oxygen chemistry.

Multiple Proton-Coupled Electron Transfers at a Tricopper Cluster: Modeling the Reductive Regeneration Process in Multicopper Oxidases

Metal clusters in enzymes carry out the life-sustaining reactions by accumulating multiple redox equivalents in a narrow potential range. This redox potential leveling effect commonly observed in Nature has yet to be reproduced with synthetic metal clusters. Herein, we employ a fully encapsulated synthetic tricopper complex to model the three-electron two-proton reductive regeneration of fully reduced trinuclear copper cluster Cu^ICu^ICu^I(μ₂-OH₂) (FR) from native intermediate Cu^IICu^IICu^II(μ₃-O) (NI) in multicopper oxidases (MCOs). The tricopper cluster can access four oxidation states (I,I,I to II,II,II) and four protonation states ([Cu₃(μ₃-O)]LH, [Cu₃(μ₃-OH)]L, [Cu₃(μ₃-OH)]LH, and [Cu₃(μ₃-OH₂)]L, where LH denotes the protonated ligand), allowing mechanistic investigation of proton-coupled electron transfer (PCET) relevant to MCOs. Seven tricopper complexes with discrete oxidation and protonation states were characterized with spectroscopy or X-ray single-crystal diffraction. A stepwise electron transfer-proton transfer (ET-PT) mechanism is established for the reduction of Cu^IICu^IICu^II(μ₃-O)LH to Cu^IICu^IICu^I(μ₃-OH)L, while a stepwise PT-ET mechanism is determined for the reduction of Cu^IICu^ICu^I(μ₃-OH)LH to Cu^ICu^ICu^I(μ₂-OH₂)L. The switch-over from ET-PT to PT-ET mechanism showcases that the tricopper complex can adopt different PCET mechanisms to circumvent high-barrier proton transfer steps. Overall, three-electron two-proton reduction occurs within a narrow potential range of 170 mV, exemplifying the redox potential leveling effect of secondary proton relays in delivering multiple redox equivalents at metal clusters.

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...

Computational Insights into Different Regioselectivities in Ir-Porphyrin-Catalyzed C–H Insertion Reaction of Quinoid Carbene

The mechanisms and regioselectivities of Ir-porphyrin-catalyzed C–H insertion reaction of quinoid carbene (QC) were investigated with density functional theory (DFT) calculations. The competing catalytic cycles were identified as the hydrogen-atom...

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy)₃³⁺-derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (ΔG⁰_ET and ΔG⁰_PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG⁰_ET and ΔG⁰_PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron–proton transfer (CEPT) to an electron transfer limited (ETPT_lim) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPT_lim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis.

Origin and Regioselectivity of Direct Hydrogen Atom Transfer Mechanism of C(sp3)-H Arylation by [W10O32]4-/Ni Metallaphotoredox Catalysis

Polyoxometalates (POMs) have a broad array of applied platforms with well-characterized catalysis including photocatalysis to achieve aliphatic C(sp³)-H bond functionalization. However, the reaction mechanism of POMs in organic transformation remains unknown due to the complexity of POM structures. Here, a challenging [W₁₀O₃₂]^4-/Ni metallaphotoredox-catalyzed C(sp³)-H arylation of alkane has been investigated by density functional theory (DFT) calculations. The calculation revealed that the superficial active center located in bridged oxygen of *[W₁₀O₃₂]^4- is responsible for the abstraction of a foreign hydrogen atom and the activation of a C(sp³)-H bond. Furthermore, we discussed this activated process using the direct activation model of the C(sp³)-H σ-bond to deepen our mechanistic understanding of POM mediated C-H bond activation via the hydrogen atom transfer (HAT) pathway. Specifically, comparing three common mechanisms for nickel catalysis inducing by Ni⁰, Ni^I, and Ni^II to construct a C-C bond, the nickel catalytic cycle induced by the Ni^I active catalyst is profitable in kinetics and thermodynamics. Finally, a radical mechanism merging the ([W₁₀O₃₂]^4--*[W₁₀O₃₂]^4--[HW₁₀O₃₂]^4--[W₁₀O₃₂]^4-) decatungstate reductive quenching cycle, ([HW₁₀O₃₂]^4--[H₂W₁₀O₃₂]^4--[HW₁₀O₃₂]^4-) electron relay, and (Ni^I-Ni^II-Ni^I-Ni^III-Ni^I) nickel catalytic cycle is proposed to be favorable. We hope that this work would provide a better understanding of the unique catalytic activity of decatungstate anions for the direct functionalization of the C(sp³)-H bond.

Methane Activation by the Heteronuclear Cluster [TiAlO4]+: Direct Hydrogen Abstraction by a Nonradical Oxygen

The gas-phase reactions of [TiAlO₄]⁺ with methane have been explored by using FT-ICR mass spectrometry complemented by quantum chemical calculations. Interestingly, the [TiAlO₄]⁺ ions can activate two methane molecules continuously. Moreover, in contrast to the previous reports on gas-phase methane activation by metal oxide clusters, in which hydrogen-atom transfer and/or proton-coupled electron transfer prevail, a hydride transfer process dominates the [TiAlO₄]⁺/CH₄ system. The associated electronic origins have been discussed, and such a terminal metal-oxo active center as addressed in the [TiAlO₄]⁺ cluster has proven to be promising in the construction of efficient catalysts concerning methane conversion.

On the origin of reactivity variation upon sequential ligation: the [Re(Cl)x]+/CH4 (x = 1-3) couples

The potential of [ReCl_x]⁺ (x = 1-3) in activating methane has been explored by using a combination of gas-phase experiments and high-level quantum calculations. When the number of Cl ligands increases, the reactivity towards methane activation varies accordingly. While [ReCl_x]⁺ (x = 1-2) are able to dehydrogenate methane by a three-state reactivity scenario, [ReCl₃]⁺ shows inertness towards methane at ambient conditions. Furthermore, the product ion [ClRe(H)CH]⁺ of the [ReCl]⁺/CH₄ couple could continue to activate methane and liberate molecular dihydrogen but another product ion [Cl₂ReCH₂]⁺ is unreactive with methane. Obviously, the nature and the number of ligands make a difference to the reactivity towards methane activation. The associated reaction mechanism and the electron origins for the rather different reactivities are discussed in detail. Finally and more importantly, instructive information concerning the rational design of Re-catalysts for methane conversion is obtained.

C-H oxidation in fluorenyl benzoates does not proceed through a stepwise pathway: revisiting asynchronous proton-coupled electron transfer

2-Fluorenyl benzoates were recently shown to undergo C–H bond oxidation through intramolecular proton transfer coupled with electron transfer to an external oxidant. Kinetic analysis revealed unusual rate-driving force relationships. Our analysis indicated a mechanism of multi-site concerted proton–electron transfer (MS-CPET) for all of these reactions. More recently, an alternative interpretation of the kinetic data was proposed to explain the unusual rate-driving force relationships, invoking a crossover from CPET to a stepwise mechanism with an initial intramolecular proton transfer (PT) (Costentin, Savéant, Chem. Sci., 2020, 11, 1006). Here, we show that this proposed alternative pathway is untenable based on prior and new experimental assessments of the intramolecular PT equilibrium constant and rates. Measurement of the fluorenyl 9-C–H pK_a, H/D exchange experiments, and kinetic modelling with COPASI eliminate the possibility of a stepwise mechanism for C–H oxidation in the fluorenyl benzoate series. Implications for asynchronous (imbalanced) MS-CPET mechanisms are discussed with respect to classical Marcus theory and the quantum-mechanical treatment of concerted proton–electron transfer.

2-Fluorenyl benzoates were recently shown to undergo C–H bond oxidation through intramolecular proton transfer coupled with electron transfer to an external oxidant.

C-H Bond Cleavage by Bioinspired Nonheme Metal Complexes

The functionalization of C-H bonds is one of the most challenging transformations in synthetic chemistry. In biology, these processes are well-known and are achieved with a variety of metalloenzymes, many of which contain a single metal center within their active sites. The most well studied are those with Fe centers, and the emerging experimental data show that high-valent iron oxido species are the intermediates responsible for cleaving the C-H bond. This Forum Article describes the state of this field with an emphasis on nonheme Fe enzymes and current experimental results that provide insights into the properties that make these species capable of C-H bond cleavage. These parameters are also briefly considered in regard to manganese oxido complexes and Cu-containing metalloenzymes. Synthetic iron oxido complexes are discussed to highlight their utility as spectroscopic and mechanistic probes and reagents for C-H bond functionalization. Avenues for future research are also examined.

Synthesis of a series of M(II) (M = Mn, Fe, Co) chloride complexes with both inter- and intra-ligand hydrogen bonding interactions

Hydrogen bonding networks are vital for metallo-enzymes to function; however, modeling these systems is non-trivial. We report the synthesis of metal chloride (M = Mn, Fe, Co) complexes with intra- and inter-ligand hydrogen bonding interactions. The intra-ligand hydrogen bonds are shown to have a profound effect on the geometry of the metal center.

Combining Structural with Functional Model Properties in Iron Synthetic Analogue Complexes for the Active Site in Rabbit Lipoxygenase

Iron complexes that model the structural and functional properties of the active iron site in rabbit lipoxygenase are described. The ligand sphere of the mononuclear pseudo-octahedral cis-(carboxylato)(hydroxo)iron(III) complex, which is completed by a tetraazamacrocyclic ligand, reproduces the first coordination shell of the active site in the enzyme. In addition, two corresponding iron(II) complexes are presented that differ in the coordination of a water molecule. In their structural and electronic properties, both the (hydroxo)iron(III) and the (aqua)iron(II) complex reflect well the only two essential states found in the enzymatic mechanism of peroxidation of polyunsaturated fatty acids. Furthermore, the ferric complex is shown to undergo hydrogen atom abstraction reactions with O-H and C-H bonds of suitable substrates, and the bond dissociation free energy of the coordinated water ligand of the ferrous complex is determined to be 72.4 kcal·mol^-1. Theoretical investigations of the reactivity support a concerted proton-coupled electron transfer mechanism in close analogy to the initial step in the enzymatic mechanism. The propensity of the (hydroxo)iron(III) complex to undergo H atom abstraction reactions is the basis for its catalytic function in the aerobic peroxidation of 2,4,6-tri(tert-butyl)phenol and its role as a radical initiator in the reaction of dihydroanthracene with oxygen.

Mechanistic Insights into the Oxygen Atom Transfer Reactions by Nonheme Manganese Complex: A Computational Case Study on the Comparative Oxidative Ability of Manganese-Hydroperoxo vs High-Valent MnIV═O and MnIV-OH Intermediates

Understanding the comparative oxidative abilities of high-valent metal-oxo/hydroxo/hydroperoxo species holds the key to robust biomimic catalysts that perform desired organic transformations with very high selectivity and efficiency. The comparative oxidative abilities of popular high-valent iron-oxo and manganese-oxo species are often counterintuitive, for example, oxygen atom transfer (OAT) reaction by [(Me₂EBC)Mn^IV-OOH]³⁺, [(Me₂EBC)Mn^IV-OH]³⁺, and [(Me₂EBC)Mn^IV═O]²⁺ (Me₂EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) shows extremely high reactivity for Mn^IV-OOH species and no reactivity for Mn^IV-OH and Mn^IV═O species toward alkyl/aromatic sulfides. Using a combination of density functional theory (DFT) and ab initio domain-based local pair natural orbital coupled-cluster with single, double, and perturbative triples excitation (DLPNO-CCSD(T)) and complete-active space self-consistent field/N-electron valence perturbation theory second order (CASSCF/NEVPT2) calculations, here, we have explored the electronic structures and sulfoxidation mechanism of these species. Our calculations unveil that Mn^IV-OOH reacts through distal oxygen atom with the substrate via electron transfer (ET) mechanism with a very small kinetic barrier (16.5 kJ/mol), placing this species at the top among the best-known catalysts for such transformations. The Mn^IV-OH and Mn^IV═O species have a much larger barrier. The mechanism has also been found to switch from ET in the former to concerted in the latter, rendering both unreactive under the tested experimental conditions. Intrinsic differences in the electronic structures, such as the presence and absence of the multiconfigurational character coupled with the steric effects, are responsible for such variations observed. This comparative oxidative ability that runs contrary to the popular iron-oxo/hydroperoxo reactivity will have larger mechanistic implications in understanding the reactivity of biomimic catalysts and the underlying mechanisms in PSII.

Surface electronic states mediate concerted electron and proton transfer at metal nanoscale interfaces for catalytic hydride reduction of -NO2 to -NH2

Concerted electron and proton transfer is a key step for the reversible conversion of molecular hydrogen in both heterogeneous nanocatalysis and metalloenzyme catalysis. However, its activation mechanism involving electron and proton transfer kinetics remains elusive. With the most widely used catalytic hydride reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction, we evaluate the catalytic activity of noble metal nanoparticles (NPs) trapped in porous silica in aqueous NaBH4 solution. By virtue of a novel combination of catalyst design, reaction kinetics, isotope labeling, and multiple spectroscopic techniques, the real catalytic site for the conversion of -NO2 to -NH2 is identified to be the water-hydroxyl transition metal complex, which could further react with NaBH4 to form a new triangular configuration metal complex of H3B-water-hydroxyl with dynamic features. It yields an ensemble of surface electronic states (SESs) though space overlapping of p orbitals of one B and several O atoms (including the O atoms of 4-NP), which could act as an alternative channel for concerted electron and proton transfer. This work highlights the critical role of the conceptual SESs model in heterogeneous catalysis to tune the chemical reactivity and also sheds light on the intricate working of the [FeFe]-hydrogenases.

Mechanism-Based Inactivation of Cytochrome P450 Enzymes: Computational Insights

Mechanism-based inactivation (MBI) refers to the metabolic bioactivation of a xenobiotic by cytochrome P450s to a highly reactive intermediate which subsequently binds to the enzyme and leads to the quasi-irreversible or irreversible inhibition. Xenobiotics, mainly drugs with specific functional units, are the major sources of MBI. Two possible consequences of MBI by medicinal compounds are drug-drug interaction and severe toxicity that are observed and highlighted by clinical experiments. Today almost all of these latent functional groups (e.g., thiophene, furan, alkylamines, etc.) are known, and their features and mechanisms of action, owing to the vast experimental and theoretical studies, are determined. In the past decade, molecular modeling techniques, mostly density functional theory, have revealed the most feasible mechanism that a drug undergoes by P450 enzymes to generate a highly reactive intermediate. In this review, we provide a comprehensive and detailed picture of computational advances toward the elucidation of the activation mechanisms of various known groups with MBI activity. To this aim, we briefly describe the computational concepts to carry out and analyze the mechanistic investigations, and then, we summarize the studies on compounds with known inhibition activity including thiophene, furan, alkylamines, terminal acetylene, etc. This study can be reference literature for both theoretical and experimental (bio)chemists in several different fields including rational drug design, the process of toxicity prevention, and the discovery of novel inhibitors and catalysts.

Tuning the H-Atom Transfer Reactivity of Iron(IV)-Oxo Complexes as Probed by Infrared Photodissociation Spectroscopy

Reactivities of non-heme iron(IV)-oxo complexes are mostly controlled by the ligands. Complexes with tetradentate ligands such as [(TPA)FeO]2+ (TPA=tris(2-pyridylmethyl)amine) belong to the most reactive ones. Here, we show a fine-tuning of the reactivity of [(TPA)FeO]2+ by an additional ligand X (X=CH3 CN, CF3 SO3- , ArI, and ArIO; ArI=2-(t BuSO2 )C6 H4 I) attached in solution and reveal a thus far unknown role of the ArIO oxidant. The HAT reactivity of [(TPA)FeO(X)]+/2+ decreases in the order of X: ArIO > MeCN > ArI ≈ TfO- . Hence, ArIO is not just a mere oxidant of the iron(II) complex, but it can also increase the reactivity of the iron(IV)-oxo complex as a labile ligand. The detected HAT reactivities of the [(TPA)FeO(X)]+/2+ complexes correlate with the Fe=O and FeO-H stretching vibrations of the reactants and the respective products as determined by infrared photodissociation spectroscopy. Hence, the most reactive [(TPA)FeO(ArIO)]2+ adduct in the series has the weakest Fe=O bond and forms the strongest FeO-H bond in the HAT reaction.

A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic Promiscuity in C-H Bond Oxidation Reactions

S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 FeIV =O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral FeIV =O complex 2 supported by the sterically demanding 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane ligand. Complex 2 is a very potent oxidant in hydrogen atom abstraction (HAA) reactions with large non-classical deuterium kinetic isotope effects, suggesting hydrogen tunneling contributions. For sterically encumbered substrates, direct HAA is impeded and an alternative oxidative asynchronous proton-coupled electron transfer mechanism prevails, which is unique within the nonheme oxoiron community. The high reactivity and the similar spectroscopic parameters make 2 one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD-J).

Hydrogen Atom Transfer from Benzyl Alcohols to N-Oxyl Radicals. Reactivity Parameters

A model for predicting the rate constants of hydrogen atom transfer (HAT) from the α-C-H bond of p-substituted benzyl alcohols to N-oxyl radicals was proposed. To quantify the factors governing the reactivity of both N-oxyl radicals and benzyl alcohols, multivariate regression analysis was performed using various combinations of reactivity parameters. The analysis was based on a 2D array of 35 HAT reactions, the rate constants of which span 4 orders of magnitude. The proposed polyparameter equation approximates the experimental rate constants of reactions with high accuracy using three independent parameters: Brown and Okamoto's substituent constants σ⁺ in alcohol molecules and the spin population on O and N atoms in the N-O^• fragment of N-oxyl radicals [calculated by DFT/B3LYP/6-31G(d,p)]. The rate constants of HAT reactions from p-substituted benzyl alcohols to a series of aryl-substituted phthalimide-N-oxyl radicals containing either electron-withdrawing or electron-donating substituents (4-Cl, 4-HOOC, 4-CH₃O), quinolinimide-N-oxyl, benzotriazole-N-oxyl, and violuric acid radicals were experimentally determined at 30 °C in acetonitrile.