Sc3+ (or HClO4) Activation of a Nonheme FeIII-OOH Intermediate for the Rapid Hydroxylation of Cyclohexane and Benzene

Proton-assisted activation of a MnIII-OOH for aromatic C-H hydroxylation through a putative [MnVO] species

Adding HClO4 to [(BnTPEN)MnIII-OO]+ in MeOH generates a short-lived MnIII-OOH species, which converts to a putative MnVO species. The potent MnVO species in MeCN oxidizes the pendant phenyl ring of the ligand in an intramolecular fashion. The addition of benzene causes the formation of (BnTPEN)MnIII-phenolate. These findings suggest that high valent Mn species have the potential to catalyze challenging aromatic hydroxylation reactions.

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids

Metalloenzymes with heteromultimetallic active sites perform chemical reactions that control several biogeochemical cycles. Transformations catalyzed by such enzymes include dioxygen generation and reduction, dinitrogen reduction, and carbon dioxide reduction-instrumental transformations for progress in the context of artificial photosynthesis and sustainable fertilizer production. While the roles of the respective metals are of interest in all these enzymatic transformations, they share a common factor in the transfer of one or multiple redox equivalents. In light of this feature, it is surprising to find that incorporation of redox-inactive metals into the active site of such an enzyme is critical to its function. To illustrate, the presence of a redox-inactive Ca2+ center is crucial in the Oxygen Evolving Complex, and yet particularly intriguing given that the transformation catalyzed by this cluster is a redox process involving four electrons. Therefore, the effects of redox inactive metals on redox processes-electron transfer, oxygen- and hydrogen-atom transfer, and O-O bond cleavage and formation reactions-mediated by transition metals have been studied extensively. Significant effects of redox inactive metals have been observed on these redox transformations; linear free energy correlations between Lewis acidity and the redox properties of synthetic model complexes are observed for several reactions. In this Perspective, these effects and their relevance to multielectron processes will be discussed.

Catalytic oxidation properties of an acid-resistant cross-bridged cyclen Fe(II) complex. Influence of the rigid donor backbone and protonation on the reactivity

The catalytic properties of an iron complex bearing a pentadentate cross-bridged ligand backbone are reported. With H₂O₂ as an oxidant, it displays moderate conversions in epoxidation and alkane hydroxylation and satisfactory ones in aromatic hydroxylation. Upon addition of an acid to the reaction medium, a significant enhancement in aromatic and alkene oxidation is observed. Spectroscopic analyses showed that accumulation of the expected Fe^III(OOH) intermediate is limited under these conditions, unless an acid is added to the mixture. This is ascribed to the inertness induced by the cross-bridged ligand backbone, which is partly reduced under acidic conditions.

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.

Effect of Brшnsted Acid on the Reactivity and Selectivity of the Oxoiron(V) Intermediates in C-H and C=C Oxidation Reactions

The effect of HClO4 on the reactivity and selectivity of the catalyst systems 1,2/H2O2/AcOH, based on nonheme iron complexes of the PDP families, [(Me2OMePDP)FeIII(μ-OH)2FeIII(MeOMe2PDP)](OTf)4 (1) and [(NMe2PDP)FeIII(μ-OH)2FeIII(NMe2PDP](OTf)4 (2), toward oxidation of benzylideneacetone (bna), adamantane (ada), and (3aR)-(+)-sclareolide (S) has been studied. Adding HClO4 (2–10 equiv. vs. Fe) has been found to result in the simultaneous improvement of the observed catalytic efficiency (i.e., product yields) and the oxidation regio- or enantioselectivity. At the same time, HClO4 causes a threefold increase of the second-order rate constant for the reaction of the key oxygen-transferring intermediate [(Me2OMePDP)FeV=O(OAc)]2+ (1a), with cyclohexane at −70 °C. The effect of strong Brønsted acid on the catalytic reactivity is discussed in terms of the reversible protonation of the Fe=O moiety of the parent perferryl intermediates.

Incorporation of Cation Affects the Redox Reactivity of Fe-NNN Complexes on C-H Oxidation

Cations such as Lewis acids have been shown to enhance the catalytic activity of high-valent Fe-oxygen intermediates. Herein, we present a pyridine diamine ethylene glycol macrocycle, which can form Zn(II)- or Fe(III)-complex with the NNN site, while allowing redox-inactive cations to bind to the ethylene glycol moiety. The addition of alkali, alkali earth, and lanthanum ions resulted in positive shifts to the Fe(III/II) redox potential. Calculation of dissociation constants showed the tightest binding with a Ba²⁺ ion. Density functional theory calculations were used to elucidate the effects of redox inactive cations toward the electronic structures of Fe complexes. Although the Fe-NNN complexes, both in the absence and presence of cations, can catalyze C-H oxidation of 9,10-dihydroanthracene, to give anthracene [hydrogen atom transfer (HAT) product], anthrone, and anthraquinone [oxygen atom transfer (OAT) products], highest overall activity and OAT/HAT product ratios were obtained in the presence of dications, that is, Ba²⁺ and Mg²⁺, respectively.

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.

Deeper Understanding of Mononuclear Manganese(IV)-Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)-Hydroxide Complex

Binding of Lewis acidic metal ions and Brønsted acid at the metal-oxo group of high-valent metal-oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal-oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)-oxo complex binding triflic acid, {[(dpaq)Mn^IV(O)]-HOTf}⁺ (1-HOTf). First, 1-HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn-O stretching frequencies of 1-HOTf (e.g., ν_Mn-O at ∼793 cm^-1) and 1-Mⁿ⁺ (Mⁿ⁺ = Ca²⁺, Zn²⁺, Lu³⁺, Al³⁺, or Sc³⁺) and the Lewis acidities of H⁺ and Mⁿ⁺ ions, suggesting that H⁺ and Mⁿ⁺ bind at the metal-oxo moiety of [(dpaq)Mn^IV(O)]⁺. Interestingly, a single-crystal structure of 1-HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)-oxo complex but a Mn(IV)-hydroxide complex, [(dpaq)Mn^IV(OH)](OTf)₂ (4), with a Mn-O bond distance of 1.8043(19) Å and a Mn-O stretch at 660 cm^-1. More interestingly, 4 reverted to 1-HOTf upon dissolution, demonstrating that 1-HOTf and 4 are interconvertible depending on the physical states, such as 1-HOTf in solution and 4 in isolated solid. The reactivity of 1-HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1-Mⁿ⁺ complexes; an interesting correlation between the Mn-O stretching frequencies of 1-HOTf and 1-Mⁿ⁺ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

Characterization and chemical reactivity of room-temperature-stable MnIII-alkylperoxo complexes

While alkylperoxomanganese(iii) (MnIII-OOR) intermediates are proposed in the catalytic cycles of several manganese-dependent enzymes, their characterization has proven to be a challenge due to their inherent thermal instability. Fundamental understanding of the structural and electronic properties of these important intermediates is limited to a series of complexes with thiolate-containing N4S- ligands. These well-characterized complexes are metastable yet unreactive in the direct oxidation of organic substrates. Because the stability and reactivity of MnIII-OOR complexes are likely to be highly dependent on their local coordination environment, we have generated two new MnIII-OOR complexes using a new amide-containing N5 - ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H6Medpaq) ligand, we generated the [MnIII(OO t Bu)(6Medpaq)]OTf and [MnIII(OOCm)(6Medpaq)]OTf complexes through reaction of their MnII or MnIII precursors with t BuOOH and CmOOH, respectively. Both of the new MnIII-OOR complexes are stable at room-temperature (t 1/2 = 5 and 8 days, respectively, at 298 K in CH3CN) and capable of reacting directly with phosphine substrates. The stability of these MnIII-OOR adducts render them amenable for detailed characterization, including by X-ray crystallography for [MnIII(OOCm)(6Medpaq)]OTf. Thermal decomposition studies support a decay pathway of the MnIII-OOR complexes by O-O bond homolysis. In contrast, direct reaction of [MnIII(OOCm)(6Medpaq)]+ with PPh3 provided evidence of heterolytic cleavage of the O-O bond. These studies reveal that both the stability and chemical reactivity of MnIII-OOR complexes can be tuned by the local coordination sphere.

Rapid Iron(III)-Fluoride-Mediated Hydrogen Atom Transfer

We anticipate high-valent metal-fluoride species will be highly effective hydrogen atom transfer (HAT) oxidants because of the magnitude of the H-F bond (in the product) that drives HAT oxidation. We prepared a dimeric Fe^III (F)-F-Fe^III (F) complex (1) by reacting [Fe^II (NCCH₃ )₂ (TPA)](ClO₄ )₂ (TPA=tris(2-pyridylmethyl)amine) with difluoro(phenyl)-λ³ -iodane (difluoroiodobenzene). 1 was a sluggish oxidant, however, it was readily activated by reaction with Lewis or Brønsted acids to yield a monomeric [Fe^III (TPA)(F)(X)]⁺ complex (2) where X=F/OTf. 1 and 2 were characterized using NMR, EPR, UV/Vis, and FT-IR spectroscopies and mass spectrometry. 2 was a remarkably reactive Fe^III reagent for oxidative C-H activation, demonstrating reaction rates for hydrocarbon HAT comparable to the most reactive Fe^III and Fe^IV oxidants.

Formation of cobalt-oxygen intermediates by dioxygen activation at a mononuclear nonheme cobalt(ii) center

A mononuclear nonheme cobalt(ii) complex, [(TMG3tren)CoII(OTf)](OTf) (1), activates dioxygen in the presence of hydrogen atom donor substrates, such as tetrahydrofuran and cyclohexene, resulting in the generation of a cobalt(ii)-alkylperoxide intermediate (2), which then converts to the previously reported cobalt(iv)-oxo complex, [(TMG3tren)CoIV(O)]2+-(Sc(OTf)3)n (3), in >90% yield upon addition of a redox-inactive metal ion, Sc(OTf)3. Intermediates 2 and 3 represent the cobalt analogues of the proposed iron(ii)-alkylperoxide precursor that converts to an iron(iv)-oxo intermediate via O-O bond heterolysis in pterin-dependent nonheme iron oxygenases. In reactivity studies, 2 shows an amphoteric reactivity in electrophilic and nucleophilic reactions, whereas 3 is an electrophilic oxidant. To the best of our knowledge, the present study reports the first example showing the generation of cobalt-oxygen intermediates by activating dioxygen at a cobalt(ii) center and the reactivities of the cobalt-oxygen intermediates in oxidation reaction.

Enhancing Chemo- and Stereoselectivity in C-H Bond Oxygenation with H2O2 by Nonheme High-Spin Iron Catalysts: The Role of Lewis Acid and Multimetal Centers

Spin states of iron often direct the selectivity in oxidation catalysis by iron complexes using hydrogen peroxide (H₂O₂) on an oxidant. While low-spin iron(III) hydroperoxides display stereoselective C-H bond hydroxylation, the reactions are nonstereoselective with high-spin iron(II) catalysts. The catalytic studies with a series of high-spin iron(II) complexes of N4 ligands with H₂O₂ and Sc³⁺ reported here reveal that the Lewis acid promotes catalytic C-H bond hydroxylation with high chemo- and stereoselectivity. This reactivity pattern is observed with iron(II) complexes containing two cis-labile sites. The enhanced selectivity for C-H bond hydroxylation catalyzed by the high-spin iron(II) complexes in the presence of Sc³⁺ parallels that of the low-spin iron catalysts. Furthermore, the introduction of multimetal centers enhances the activity and selectivity of the iron catalyst. The study provides insights into the development of peroxide-dependent bioinspired catalysts for the selective oxygenation of C-H bonds without the restriction of using iron complexes of strong-field ligands.

Insight into the Selective Methylene Oxidation Catalyzed by Mn(CF3-PDP)(SbF6)2/H2O2/CH2ClCO2H) System: A DFT Mechanistic Study

DFT study was employed to gain insight into methylene oxidation catalyzed by Mn(CF₃-PDP)(NCMe)₂ (SbF₆)₂/H₂O₂/HOAc^Cl(OAC^Cl ═OC(O)CH₂Cl). The active catalyst was characterized to be [Mn](O)OAc^Cl ([Mn]═Mn(CF₃-PDP)²⁺) which is generated via a sequence from [Mn] to [Mn]OH to [Mn]OAc^Cl to [Mn]OOH. With the active catalyst, the methylene group is sequentially oxidized to an alcohol and then to a carbonyl group via rebound mechanism. The mechanism explains the observed site selectivity.

Insight into the chemoselective aromatic vs. side-chain hydroxylation of alkylaromatics with H2O2 catalyzed by a non-heme imine-based iron complex

Side-chain/ring oxygenated product ratio increases upon decreasing the benzylic bond dissociation energy in the oxidation of alkylaromatics with H₂O₂ catalyzed by an imine-based iron complex.

Degradation pathways of a highly active iron(iii) tetra-NHC epoxidation catalyst

Elucidation of different decomposition pathways of a highly active tetradentate iron–NHC epoxidation catalyst reveals direct carbene oxidation to be the decisive cause of catalyst degradation.

Rational design and synthesis of hollow Fe-N/C electrocatalysts for enhanced oxygen reduction reaction

Herein, we report a one-step synthesis of a hollow Fe-N/C catalyst via a hard-templating strategy, in which FeNx sites are well dispersed on the carbon sphere. In particular, we found an optimal iron ratio on the catalyst surface for an enhanced alkaline oxygen reduction reaction (ORR). The catalyst with a high specific surface area of 311.71 m2 g-1 exposes abundant electroactive sites that facilitate the adsorption of oxygen intermediates, thus exhibiting superior ORR activity in alkaline solution.

Deciphering the origin of million-fold reactivity observed for the open core diiron [HO-FeIII-O-FeIV[double bond, length as m-dash]O]2+ species towards C-H bond activation: role of spin-states, spin-coupling, and spin-cooperation

High-valent metal-oxo species have been characterised as key intermediates in both heme and non-heme enzymes that are found to perform efficient aliphatic hydroxylation, epoxidation, halogenation, and dehydrogenation reactions. Several biomimetic model complexes have been synthesised over the years to mimic both the structure and function of metalloenzymes. The diamond-core [Fe2(μ-O)2] is one of the celebrated models in this context as this has been proposed as the catalytically active species in soluble methane monooxygenase enzymes (sMMO), which perform the challenging chemical conversion of methane to methanol at ease. In this context, a report of open core [HO(L)FeIII-O-FeIV(O)(L)]2+ (1) gains attention as this activates C-H bonds a million-fold faster compared to the diamond-core structure and has the dual catalytic ability to perform hydroxylation as well as desaturation with organic substrates. In this study, we have employed density functional methods to probe the origin of the very high reactivity observed for this complex and also to shed light on how this complex performs efficient hydroxylation and desaturation of alkanes. By modelling fifteen possible spin-states for 1 that could potentially participate in the reaction mechanism, our calculations reveal a doublet ground state for 1 arising from antiferromagnetic coupling between the quartet FeIV centre and the sextet FeIII centre, which regulates the reactivity of this species. The unusual stabilisation of the high-spin ground state for FeIV[double bond, length as m-dash]O is due to the strong overlap of with the orbital, reducing the antibonding interactions via spin-cooperation. The electronic structure features computed for 1 are consistent with experiments offering confidence in the methodology chosen. Further, we have probed various mechanistic pathways for the C-H bond activation as well as -OH rebound/desaturation of alkanes. An extremely small barrier height computed for the first hydrogen atom abstraction by the terminal FeIV[double bond, length as m-dash]O unit was found to be responsible for the million-fold activation observed in the experiments. The barrier height computed for -OH rebound by the FeIII-OH unit is also smaller suggesting a facile hydroxylation of organic substrates by 1. A strong spin-cooperation between the two iron centres also reduces the barrier for second hydrogen atom abstraction, thus making the desaturation pathway competitive. Both the spin-state as well as spin-coupling between the two metal centres play a crucial role in dictating the reactivity for species 1. By exploring various mechanistic pathways, our study unveils the fact that the bridged μ-oxo group is a poor electrophile for both C-H activation as well for -OH rebound. As more and more evidence is gathered in recent years for the open core geometry of sMMO enzymes, the idea of enhancing the reactivity via an open-core motif has far-reaching consequences.

Catalytic oxidation of cycloalkanes by porphyrin cobalt(II) through efficient utilization of oxidation intermediates

The catalytic oxidation of cycloalkanes using molecular oxygen employing porphyrin cobalt(II) as catalyst was enhanced through use of cycloalkyl hydroperoxides, which are the primary intermediates in oxidation of cycloalkanes, as additional oxidants to further oxidize cycloalkanes in the presence of porphyrin copper(II), especially for cyclohexane, for which the selectivity was enhanced from 88.6 to 97.2% to the KA oil; at the same time, the conversion of cyclohexane was enhanced from 3.88 to 4.41%. The enhanced efficiency and selectivity were mainly attributed to the avoided autoxidation of cycloalkanes and efficient utilization of oxidation intermediate cycloalkyl hydroperoxides as additional oxidants instead of conventional thermal decomposition. In addition to cyclohexane, the protocol presented in this research is also very applicable in the oxidation of other cycloalkanes such as cyclooctane, cycloheptane and cyclopentane, and can serve as a applicable and efficient strategy to boost the conversion and selectivity simultaneously in oxidation of alkanes. This work also is a very important reference for the extensive application of metalloporphyrins in catalysis chemistry.

Enhanced Redox Reactivity of a Nonheme Iron(V)-Oxo Complex Binding Proton

Acid effects on the chemical properties of metal-oxygen intermediates have attracted much attention recently, such as the enhanced reactivity of high-valent metal(IV)-oxo species by binding proton(s) or Lewis acidic metal ion(s) in redox reactions. Herein, we report for the first time the proton effects of an iron(V)-oxo complex bearing a negatively charged tetraamido macrocyclic ligand (TAML) in oxygen atom transfer (OAT) and electron-transfer (ET) reactions. First, we synthesized and characterized a mononuclear nonheme Fe(V)-oxo TAML complex (1) and its protonated iron(V)-oxo complexes binding two and three protons, which are denoted as 2 and 3, respectively. The protons were found to bind to the TAML ligand of the Fe(V)-oxo species based on spectroscopic characterization, such as resonance Raman, extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR) measurements, along with density functional theory (DFT) calculations. The two-protons binding constant of 1 to produce 2 and the third protonation constant of 2 to produce 3 were determined to be 8.0(7) × 10⁸ M^-2 and 10(1) M^-1, respectively. The reactivities of the proton-bound iron(V)-oxo complexes were investigated in OAT and ET reactions, showing a dramatic increase in the rate of sulfoxidation of thioanisole derivatives, such as 10⁷ times increase in reactivity when the oxidation of p-CN-thioanisole by 1 was performed in the presence of HOTf (i.e., 200 mM). The one-electron reduction potential of 2 (E_red vs SCE = 0.97 V) was significantly shifted to the positive direction, compared to that of 1 (E_red vs SCE = 0.33 V). Upon further addition of a proton to a solution of 2, a more positive shift of the E_red value was observed with a slope of 47 mV/log([HOTf]). The sulfoxidation of thioanisole derivatives by 2 was shown to proceed via ET from thioanisoles to 2 or direct OAT from 2 to thioanisoles, depending on the ET driving force.

Bio-inspired Nonheme Iron Oxidation Catalysis: Involvement of Oxoiron(V) Oxidants in Cleaving Strong C-H Bonds

Nonheme iron enzymes generate powerful and versatile oxidants that perform a wide range of oxidation reactions, including the functionalization of inert C-H bonds, which is a major challenge for chemists. The oxidative abilities of these enzymes have inspired bioinorganic chemists to design synthetic models to mimic their ability to perform some of the most difficult oxidation reactions and study the mechanisms of such transformations. Iron-oxygen intermediates like iron(III)-hydroperoxo and high-valent iron-oxo species have been trapped and identified in investigations of these bio-inspired catalytic systems, with the latter proposed to be the active oxidant for most of these systems. In this Review, we highlight the recent spectroscopic and mechanistic advances that have shed light on the various pathways that can be accessed by bio-inspired nonheme iron systems to form the high-valent iron-oxo intermediates.

Sc3+-Promoted O-O Bond Cleavage of a (μ-1,2-Peroxo)diiron(III) Species Formed from an Iron(II) Precursor and O2 to Generate a Complex with an FeIV2(μ-O)2 Core

Soluble methane monooxygenase (sMMO) carries out methane oxidation at 4 °C and under ambient pressure in a catalytic cycle involving the formation of a peroxodiiron(III) intermediate (P) from the oxygenation of the diiron(II) enzyme and its subsequent conversion to Q, the diiron(IV) oxidant that hydroxylates methane. Synthetic diiron(IV) complexes that can serve as models for Q are rare and have not been generated by a reaction sequence analogous to that of sMMO. In this work, we show that [Fe^II(Me₃NTB)(CH₃CN)](CF₃SO₃)₂ (Me₃NTB = tris((1-methyl-1H-benzo[d]imidazol-2-yl)methyl)amine) (1) reacts with O₂ in the presence of base, generating a (μ-1,2-peroxo)diiron(III) adduct with a low O-O stretching frequency of 825 cm^-1 and a short Fe···Fe distance of 3.07 Å. Even more interesting is the observation that the peroxodiiron(III) complex undergoes O-O bond cleavage upon treatment with the Lewis acid Sc³⁺ and transforms into a bis(μ-oxo)diiron(IV) complex, thus providing a synthetic precedent for the analogous conversion of P to Q in the catalytic cycle of sMMO.

Pushing the limits of activity and stability: the effects of Lewis acids on non-heme iron–NHC epoxidation catalysts

Tetradentate iron–NHC complexes exhibit unprecedented activity (TOF: 410 000 h⁻¹) in the epoxidation of cis-cyclooctene by addition of Lewis acids.

Proton-promoted disproportionation of iron(v)-imido TAML to iron(v)-imido TAML cation radical and iron(iv) TAML

An iron(v)-imido TAML complex is disproportionated to give an iron(v)-imido TAML cation radical and an iron(iv) TAML upon addition of acids.

Lewis versus Brønsted Acid Activation of a Mn(IV) Catalyst for Alkene Oxidation

Lewis acid (LA) activation by coordination to metal oxido species has emerged as a new strategy in catalytic oxidations. Despite the many reports of enhancement of performance in oxidation catalysis, direct evidence for LA-catalyst interactions under catalytically relevant conditions is lacking. Here, we show, using the oxidation of alkenes with H₂O₂ and the catalyst [Mn₂(μ-O)₃(tmtacn)₂](PF₆)₂ (1), that Lewis acids commonly used to enhance catalytic activity, e.g., Sc(OTf)₃, in fact undergo hydrolysis with adventitious water to release a strong Brønsted acid. The formation of Brønsted acids in situ is demonstrated using a combination of resonance Raman, UV/vis absorption spectroscopy, cyclic voltammetry, isotope labeling, and DFT calculations. The involvement of Brønsted acids in LA enhanced systems shown here holds implications for the conclusions reached in regard to the relevance of direct LA-metal oxido interactions under catalytic conditions.

Lewis acid activation of oxidation catalysts is proposed to be through binding of the Lewis acids to metal-oxo species. The activity of the catalyst [Mn₂(μ-O)₃(tmtacn)₂](PF₆)₂ in the oxidation of alkenes with H₂O₂ appears to correlate with the strength of the Lewis acid used for its activation. We show that the correlation arises from the relative propensity of the Lewis acids to generate Brønsted acids in situ.

Highly Selective and Catalytic Oxygenations of C-H and C=C Bonds by a Mononuclear Nonheme High-Spin Iron(III)-Alkylperoxo Species

The reactivity of a mononuclear high-spin iron(III)-alkylperoxo intermediate [Fe^III (t-BuL^Urea )(OOCm)(OH₂ )]²⁺ (2), generated from [Fe^II (t-BuL^Urea )(H₂ O)(OTf)](OTf) (1) [t-BuL^Urea =1,1'-(((pyridin-2-ylmethyl)azanediyl)bis(ethane-2,1-diyl))bis(3-(tert-butyl)urea), OTf=trifluoromethanesulfonate] with cumyl hydroperoxide (CmOOH), toward the C-H and C=C bonds of hydrocarbons is reported. 2 oxygenates the strong C-H bonds of aliphatic substrates with high chemo- and stereoselectivity in the presence of 2,6-lutidine. While 2 itself is a sluggish oxidant, 2,6-lutidine assists the heterolytic O-O bond cleavage of the metal-bound alkylperoxo, giving rise to a reactive metal-based oxidant. The roles of the urea groups on the supporting ligand, and of the base, in directing the selective and catalytic oxygenation of hydrocarbon substrates by 2 are discussed.

Activation of a Non-Heme FeIII -OOH by a Second FeIII to Hydroxylate Strong C-H Bonds: Possible Implications for Soluble Methane Monooxygenase

Non-heme iron oxygenases contain either monoiron or diiron active sites, and the role of the second iron in the latter enzymes is a topic of particular interest, especially for soluble methane monooxygenase (sMMO). Herein we report the activation of a non-heme FeIII -OOH intermediate in a synthetic monoiron system using FeIII (OTf)3 to form a high-valent oxidant capable of effecting cyclohexane and benzene hydroxylation within seconds at -40 °C. Our results show that the second iron acts as a Lewis acid to activate the iron-hydroperoxo intermediate, leading to the formation of a powerful FeV =O oxidant-a possible role for the second iron in sMMO.

Oxidizing Ability of a Dioxygen-Activating Nonheme Iron(II)-Benzilate Complex Immobilized on Gold Nanoparticles

An iron(II)-benzilate complex [(TPASH)Fe^II(benzilate)]ClO₄@C₈Au (2) (TPASH = 11-((6-((bis(pyridin-2-ylmethyl)amino)methyl)pyridin-2-yl)methoxy)undecane-1-thiol) immobilized on octanethiol stabilized gold nanoparticles (C₈Au) of core diameter less than 5 nm has been prepared to evaluate its reactivity toward O₂-dependent oxidations compared to a nonimmobilized complex [(TPA-O-Allyl)Fe^II(benzilate)]ClO₄ (1a) (TPA-O-Allyl = N-((6-(allyloxymethyl)pyridin-2-yl)methyl)(pyridin-2-yl)- N-(pyridin-2-ylmethyl)methanamine). X-ray crystal structure of the nonimmobilized complex 1a reveals a six-coordinate iron(II) center in which the TPA-O-Allyl acts as a pentadentate ligand and the benzilate anion binds in monodentate fashion. Both the complexes (1a and 2) react with dioxygen under ambient conditions to form benzophenone as the sole product through decarboxylation of the coordinated benzilate. Interception studies reveal that a nucleophilic iron-oxygen intermediate is formed in the decarboxylation reaction. The oxidants from both the complexes are able to carry out oxo atom transfer reactions. The immobilized complex 2 not only performs faster decarboxylation but also exhibits enhanced reactivity in oxo atom transfer to sulfides. Importantly, the immobilized complex 2, unlike 1a, displays catalytic turnovers in sulfide oxidation. However, the complexes are not efficient to carry out cis-dihydroxylation of alkenes. Although the immobilized complex yields a slightly higher amount of cis-diol from 1-octene, restricted access of dioxygen and substrates at the coordinatively saturated metal centers of the complexes likely makes the resulting iron-oxygen species less active in oxygen atom transfer to alkenes. The results implicate that surface immobilized nonheme iron complexes containing accessible coordination sites would exhibit better reactivity in O₂-dependent oxygenation reactions.

Iron-Catalyzed Intramolecular Amination of Aliphatic C-H Bonds of Sulfamate Esters with High Reactivity and Chemoselectivity

It is challenging to develop simple and low cost catalytic systems while maintaining high reactivity and selectivity. An iron-catalyzed intramolecular C-H amination of sulfamate esters using simple and cheap ligands is reported with general substrate scope (31 examples, up to 95% yield). The addition of second ligand, bipyridine, is able to accelerate the reaction and increase the yield. The ready availability of these iron catalysts provides a promising approach to selective introduction of nitrogen into hydrocarbon feedstock.

Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer Reactivity of Heterometallic Mn3MO4 Cubane and Fe3MO(OH) and Mn3MO(OH) Clusters

The modulation of the reactivity of metal oxo species by redox inactive metals has attracted much interest due to the observation of redox inactive metal effects on processes involving electron transfer both in nature (the oxygen-evolving complex of Photosystem II) and in heterogeneous catalysis (mixed-metal oxides). Studies of small-molecule models of these systems have revealed numerous instances of effects of redox inactive metals on electron- and group-transfer reactivity. However, the heterometallic species directly involved in these transformations have rarely been structurally characterized and are often generated in situ. We have previously reported the preparation and structural characterization of multiple series of heterometallic clusters based on Mn₃ and Fe₃ cores and described the effects of Lewis acidity of the heterometal incorporated in these complexes on cluster reduction potential. To determine the effects of Lewis acidity of redox inactive metals on group transfer reactivity in structurally well-defined complexes, we studied [Mn₃MO₄], [Mn₃MO(OH)], and [Fe₃MO(OH)] clusters in oxygen atom transfer (OAT) reactions with phosphine substrates. The qualitative rate of OAT correlates with the Lewis acidity of the redox inactive metal, confirming that Lewis acidic metal centers can affect the chemical reactivity of metal oxo species by modulating cluster electronics.

Heme and Nonheme High-Valent Iron and Manganese Oxo Cores in Biological and Abiological Oxidation Reactions

Utilization of O₂ as an abundant and environmentally benign oxidant is of great interest in the design of bioinspired synthetic catalytic oxidation systems. Metalloenzymes activate O₂ by employing earth-abundant metals and exhibit diverse reactivities in oxidation reactions, including epoxidation of olefins, functionalization of alkane C-H bonds, arene hydroxylation, and syn-dihydroxylation of arenes. Metal-oxo species are proposed as reactive intermediates in these reactions. A number of biomimetic metal-oxo complexes have been synthesized in recent years by activating O₂ or using artificial oxidants at iron and manganese centers supported on heme or nonheme-type ligand environments. Detailed reactivity studies together with spectroscopy and theory have helped us understand how the reactivities of these metal-oxygen intermediates are controlled by the electronic and steric properties of the metal centers. These studies have provided important insights into biological reactions, which have contributed to the design of biologically inspired oxidation catalysts containing earth-abundant metals like iron and manganese. In this Outlook article, we survey a few examples of these advances with particular emphasis in each case on the interplay of catalyst design and our understanding of metalloenzyme structure and function.

Heterogeneity of polyoxometalates by confining within ordered mesopores: toward efficient oxidation of benzene to phenol

Polyoxometalates@ordered mesoporous polymers have been prepared by a mechanochemical coordination assembly with tannin as the renewable precursor.