Rate-determining water-assisted O-O bond cleavage of an Fe(III)-OOH intermediate in a bio-inspired nonheme iron-catalyzed oxidation

cis-Dihydroxylation by Synthetic Iron(III)-Peroxo Intermediates and Rieske Dioxygenases: Experimental and Theoretical Approaches Reveal the Key O-O Bond Activation Step

Dioxygen (O2) activation by iron-containing enzymes and biomimetic compounds generates iron-oxygen intermediates, such as iron-superoxo, -peroxo, -hydroperoxo, and -oxo, that mediate oxidative reactions in biological and abiological systems. Among the iron-oxygen intermediates, iron(III)-peroxo species are less frequently implicated as active intermediates in oxidation reactions. In this study, we present the combined experimental and theoretical investigations on cis-dihydroxylation reactions mediated by synthetic mononuclear nonheme iron-peroxo intermediates, demonstrating the importance of supporting ligands and metal centers in activating the peroxo ligand toward the O-O bond homolysis for the cis-dihydroxylation reactions. We found a significant ring size effect of the TMC ligand in [FeIII(O2)(n-TMC)]+ (TMC = tetramethylated tetraazacycloalkane; n = 12, 13, and 14) on the cis-dihydroxylation reactivity order: [FeIII(O2)(12-TMC)]+ > [FeIII(O2)(13-TMC)]+ > [FeIII(O2)(14-TMC)]+. Additionally, we found that only [FeIII(O2)(n-TMC)]+, but not other metal-peroxo complexes such as [MIII(O2)(n-TMC)]+ (M = Mn, Co, and Ni), is reactive for the cis-dihydroxylation of olefins. Using density functional theory (DFT) calculations, we revealed that electron transfer from the Fe dxz orbital to the peroxo σ*(O-O) orbital facilitates the O-O bond homolysis, with the O-O bond cleavage barrier well correlated with the energy gap between the frontier molecular orbitals of dxz and σ*(O-O). Further computational studies showed that the reactivity of the synthetic [FeIII(O2)(12-TMC)]+ complex is comparable to that of Rieske dioxygenases in cis-dihydroxylation, providing compelling evidence of the potential involvement of Fe(III)-peroxo species in Rieske dioxygenases. Thus, the present results significantly advance our understanding of the cis-dihydroxylation mechanisms by Rieske dioxygenases and synthetic nonheme iron-peroxo models.

Dual-functional heterogeneous Fenton catalyst Cu/Ti co-doped Fe3O4@FeOOH for cyanide-containing wastewater treatment: Preparation, performance and mechanism

The dual-functional heterogeneous Fenton catalyst Cu/Ti co-doped iron-based Fenton catalyst (Cu/Ti -Fe3O4@FeOOH, FCT) were successfully prepared by precipitation oxidation method and characterized by XRD, XPS and XAFS. The prepared Cu/Ti co-doped Fe3O4@FeOOH nanoparticles consisted of goethite nanorods and magnetite rod octahedral particles, with Cu and Ti replacing Fe in the catalyst crystal structure, leading to the formation of the goethite structure. The heterogeneous Fenton catalyst FCT exhibited excellent degradation activity for cyanide in wastewater and showed different reaction mechanisms at varying pH levels. When treating 100 mL of 12 mg L-1 NaCN solution, complete degradation occurred within 40 min at 30 °C and pH ranging from 6.5 to 12.5 without external energy. Compared to Fe3O4, FCT shows superior degradation activity for cyanide. The surface Cu(Ⅰ) facilitated the electron transfer and significantly improved the catalytic activity of the catalyst. Additionally, the magnetic properties of the Ti-doped catalyst samples were greatly enhanced compared to the Cu@FeOOH catalyst doped with Cu, making them favorable for recycling and reuse. FCT maintains 100% degradation of cyanogen after three cycles, indicating its excellent stability. Furthermore, electron spin resonance spectroscopy, free radical quenching experiments and fluorescence probe techniques using terephthalic acid (TA) and benzoic acid (BA) confirmed that the presence of •OH and FeⅣ=O reactive species was responsible for the catalysts exhibiting different mechanisms at different pH conditions. Compared with other heterogeneous Fenton catalysts, FCT exhibits intentional degradation activity for cyanide-containing wastewater under different acid-base conditions, which greatly broadened the pH range of the heterogeneous Fenton reaction.

Mechanistic Insights into Amphoteric Reactivity of an Iron-Bispidine Complex

The reactivity of FeIII -alkylperoxido complexes has remained a riddle to inorganic chemists owing to their thermal instability and impotency towards organic substrates. These iron-oxygen adducts have been known as sluggish oxidants towards oxidative electrophilic and nucleophilic reactions. Herein, we report the synthesis and spectroscopic characterization of a relatively stable mononuclear high-spin FeIII -alkylperoxido complex supported by an engineered bispidine framework. Against the notion, this FeIII -alkylperoxido complex serves as a rare example of versatile reactivity in both electrophilic and nucleophilic reactions. Detailed mechanistic studies and computational calculations reveal a novel reaction mechanism, where a putative superoxido intermediate orchestrates the amphoteric property of the oxidant. The design of the backbone is pivotal to convey stability and reactivity to alkylperoxido and superoxido intermediates. Contrary to the well-known O-O bond cleavage that generates an FeIV -oxido species, the FeIII -alkylperoxido complex reported here undergoes O-C bond scission to generate a superoxido moiety that is responsible for the amphiphilic reactivity.

Effect of Redox Potential on Diiron-Mediated Disproportionation of Hydrogen Peroxide

Heme and nonheme dimanganese catalases are widely distributed in living organisms to participate in antioxidant defenses that protect biological systems from oxidative stress. The key step in these processes is the disproportionation of H₂O₂ to O₂ and water, which can be interpreted via two different mechanisms, namely via the formation of high-valent oxoiron(IV) and peroxodimanganese(III) or diiron(III) intermediates. In order to better understand the mechanism of this important process, we have chosen such synthetic model compounds that can be used to map the nature of the catalytically active species and the factors influencing their activities. Our previously reported μ-1,2-peroxo-diiron(III)-containing biomimics are good candidates, as both proposed reactive intermediates (Fe^IVO and Fe^III₂(μ-O₂)) can be derived from them. Based on this, we have investigated and compared five heterobidentate-ligand-containing model systems including the previously reported and fully characterized [Fe^II(L^1-4)₃]²⁺ (L¹ = 2-(2'-pyridyl)-1H-benzimidazole, L² = 2-(2'-pyridyl)-N-methyl-benzimidazole, L³ = 2-(4-thiazolyl)-1H-benzimidazole and L⁴ = 2-(4'-methyl-2'-pyridyl)-1H-benzimidazole) and the novel [Fe^II(L⁵)₃]²⁺ (L⁵ = 2-(1H-1,2,4-triazol-3-yl)-pyridine) precursor complexes with their spectroscopically characterized μ-1,2-peroxo-diiron(III) intermediates. Based on the reaction kinetic measurements and previous computational studies, it can be said that the disproportionation reaction of H₂O₂ can be interpreted through the formation of an electrophilic oxoiron(IV) intermediate that can be derived from the homolysis of the O-O bond of the forming μ-1,2-peroxo-diiron(III) complexes. We also found that the disproportionation rate of the H₂O₂ shows a linear correlation with the Fe^III/Fe^II redox potential (in the range of 804 mV-1039 mV vs. SCE) of the catalysts controlled by the modification of the ligand environment. Furthermore, it is important to note that the two most active catalysts with L³ and L⁵ ligands have a high-spin electronic configuration.

Selective alkane hydroxylation and alkene epoxidation using H2O2 and Fe(II) catalysts electrostatically attached to a fluorinated surface

Fe(II) complexes with pentadentate ligands, including N-heterocyclic carbene moieties, were prepared and electrostatically attached onto the perfluorinated surface of a mesoporous aluminosilicate. The heterogeneous catalysts were applied to the catalytic oxidation of cyclohexane and cyclohexene using H₂O₂ as an oxidant in CH₃CN, demonstrating high performance and selectivity in alkane hydroxylation and cyclohexene epoxidation.

Heterolytic O-O Bond Cleavage Upon Single Electron Transfer to a Nonheme Fe(III)-OOH Complex

The one-electron reduction of the nonheme iron(III)-hydroperoxo complex, [FeIII (OOH)(L5 2 )]2+ (L5 2 =N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine), carried out at -70 °C results in the release of dioxygen and in the formation of [FeII (OH)(L5 2 )]+ following a bimolecular process. This reaction can be performed either with cobaltocene as chemical reductant, or electrochemically. These experimental observations are consistent with the disproportionation of the hydroperoxo group in the putative FeII (OOH) intermediate generated upon reduction of the FeIII (OOH) starting complex. One plausible mechanistic scenario is that this disproportionation reaction follows an O-O heterolytic cleavage pathway via a FeIV -oxo species.

Organometallic 3d transition metal NHC complexes in oxidation catalysis

The development of processes for the selective oxidation of hydrocarbons is a major focus in catalysis research. Making this process simultaneously environmentally friendly is still challenging. 3d transition metals are...

Which is the real oxidant in the competitive ligand self-hydroxylation and substrate oxidation, a biomimetic iron(II)-hydroperoxo species or an oxo-iron(IV)-hydroxy one?

Nonheme iron(II)-hydroperoxo species (FeII-(η2-OOH)) 1 and the concomitant oxo-iron(IV)-hydroxyl one 2 are proposed as the key intermediates of a large class of 2-oxoglutarate dependent dioxygenases (e.g., isopenicillin N synthase). Extensive...

Disproportionation of H2O2 Mediated by Diiron-Peroxo Complexes as Catalase Mimics

Heme iron and nonheme dimanganese catalases protect biological systems against oxidative damage caused by hydrogen peroxide. Rubrerythrins are ferritine-like nonheme diiron proteins, which are structurally and mechanistically distinct from the heme-type catalase but similar to a dimanganese KatB enzyme. In order to gain more insight into the mechanism of this curious enzyme reaction, non-heme structural and functional models were carried out by the use of mononuclear [Fe^II(L_1-4)(solvent)₃](ClO₄)₂ (1-4) (L₁ = 1,3-bis(2-pyridyl-imino)isoindoline, L₂ = 1,3-bis(4'-methyl-2-pyridyl-imino)isoindoline, L₃ = 1,3-bis(4'-Chloro-2-pyridyl-imino)isoindoline, L₄ = 1,3-bis(5'-chloro-2-pyridyl-imino)isoindoline) complexes as catalysts, where the possible reactive intermediates, diiron-perroxo [Fe^III₂(μ-O)(μ-1,2-O₂)(L₁-L₄)₂(Solv)₂]²⁺ (5-8) complexes are known and well-characterized. All the complexes displayed catalase-like activity, which provided clear evidence for the formation of diiron-peroxo species during the catalytic cycle. We also found that the fine-tuning of iron redox states is a critical issue, both the formation rate and the reactivity of the diiron-peroxo species showed linear correlation with the Fe^III/Fe^II redox potentials. Their stability and reactivity towards H₂O₂ was also investigated and based on kinetic and mechanistic studies a plausible mechanism, including a rate-determining hydrogen atom transfer between the H₂O₂ and diiron-peroxo species, was proposed. The present results provide one of the first examples of a nonheme diiron-peroxo complex, which shows a catalase-like 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.

Iron-Catalyzed Highly Enantioselective cis-Dihydroxylation of Trisubstituted Alkenes with Aqueous H2 O2

Reliable methods for enantioselective cis-dihydroxylation of trisubstituted alkenes are scarce. The iron(II) complex cis-α-[Fe^II (2-Me₂ -BQPN)(OTf)₂ ], which bears a tetradentate N₄ ligand (Me₂ -BQPN=(R,R)-N,N'-dimethyl-N,N'-bis(2-methylquinolin-8-yl)-1,2-diphenylethane-1,2-diamine), was prepared and characterized. With this complex as the catalyst, a broad range of trisubstituted electron-deficient alkenes were efficiently oxidized to chiral cis-diols in yields of up to 98 % and up to 99.9 % ee when using hydrogen peroxide (H₂ O₂ ) as oxidant under mild conditions. Experimental studies (including ¹⁸ O-labeling, ESI-MS, NMR, EPR, and UV/Vis analyses) and DFT calculations were performed to gain mechanistic insight, which suggested possible involvement of a chiral cis-Fe^V (O)₂ reaction intermediate as an active oxidant. This cis-[Fe^II (chiral N₄ ligand)]²⁺ /H₂ O₂ method could be a viable green alternative/complement to the existing OsO₄ -based methods for asymmetric alkene dihydroxylation reactions.

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.

Fabrication of iron-dipicolinamide catalyst with Fe-N bonds for enhancing non-radical reactive species under alkaline Fenton process

Iron dipicolinamide (Fedpa), as an efficient Fenton-like catalyst, was fabricated to excite hydrogen peroxide (H₂O₂) for the removal of 2,4-dichlorophenol (2,4-DCP). The unique structures and the electronic properties of Fedpa were contributed to its excellent catalytic performance in alkaline Fenton process. Fe was chelated with dpa by four Fe-N bonds leaved two labile sites, which reduced the oxidation potential of dpa[Fe^III/Fe^II], dpa[Fe^V/Fe^III] or dpa[Fe^IV/Fe^II] to 0.316 V and 1.189 V respectively, and made it easily be bound with H₂O₂ to initiate the reaction. The results showed that 99.5% removal rate of 2,4-DCP (0.58 mM) was achieved by using 0.027 g/L Fedpa and 5.8 mM H₂O₂ in 60 min at pH 9.9. The coordination between Fe and dpa enhanced the catalytic efficiency of Fe^II. The active species generated in Fedpa/H₂O₂ system contained the iron-oxo species (dpaFe^V = O or dpa^IV = O), O₂^- and HO. The iron-oxo species was the main non-radical reactive species for the degradation of 2,4-DCP and some degradation intermediates were detected by GC-QTOF. Furthermore, the influence of factors, such as Fedpa loading, solution pH, temperature and anions (F^-, Cl^-, SO₄^2-, NO₃^- and PO₄^3-) on the catalytic performance of Fedpa were also discussed. This process of complexation between Fe and dpa combined with a green oxidant H₂O₂ presents a new insight for the use of Fenton-like system in the degradation of refractory organics.

Salicylate 5-Hydroxylase: Intermediates in Aromatic Hydroxylation by a Rieske Monooxygenase

Rieske oxygenases (ROs) catalyze a large range of oxidative chemistry. We have shown that cis-dihydrodiol-forming Rieske dioxygenases first react with their aromatic substrates via an active site nonheme Fe(III)-superoxide; electron transfer from the Rieske cluster then completes the product-forming reaction. Alternatively, two-electron-reduced Fe(III)-peroxo or hydroxo-Fe(V)-oxo activated oxygen intermediates are possible and may be utilized by other ROs to expand the catalytic range. Here, the reaction of a Rieske monooxygenase, salicylate 5-hydroxylase, that does not form a cis-dihydrodiol is examined. Single-turnover kinetic studies show fast binding of salicylate and O2. Transfer of the Rieske electron required to form the gentisate product occurs through bonds over ∼12 Å and must also be very fast. However, the observed rate constant for this reaction is much slower than expected and sensitive to substrate type. This suggests that initial reaction with salicylate occurs using the same Fe(III)-superoxo-level intermediate as Rieske dioxygenases and that this reaction limits the observed rate of electron transfer. A transient intermediate (λmax = 700 nm) with an electron paramagnetic resonance (EPR) at g = 4.3 is observed after the product is formed in the active site. The use of 17O2 (I = 5/2) results in hyperfine broadening of the g = 4.3 signal, showing that gentisate binds to the mononuclear iron via its C5-OH in the intermediate. The chromophore and EPR signal allow study of product release in the catalytic cycle. Comparison of the kinetics of single- and multiple-turnover reactions shows that re-reduction of the metal centers accelerates product release ∼300-fold, providing insight into the regulatory mechanism of ROs.

Turning on Visible-Light Photocatalytic C-H Oxidation over Metal-Organic Frameworks by Introducing Metal-to-Cluster Charge Transfer

The tailorable structure and electronic structure of metal-organic frameworks (MOFs) greatly facilitate their modulated light harvesting, redox power, and consequently photocatalysis. Herein, a representative MOF, UiO-66, was furnished by installing Fe³⁺ onto the Zr-oxo clusters, to give Fe-UiO-66, which features extended visible light harvesting, based on metal-to-cluster charge transfer (MCCT). The Fe-UiO-66 with unique electronic structure and strong oxidizing power exhibits visible light-driven water oxidation, which is impossible for pristine UiO-66. More strikingly, under visible irradiation, the generated holes over Fe-UiO-66 are able to exclusively convert H₂O to hydroxide radicals, initiating and driving the activation of stubborn C-H bond, such as toluene oxidation. The electrons reduce O₂ to O₂^•- radicals that further promote the oxidation reaction. The related catalytic mechanism and the structure-activity relationship have been investigated in detail. As far as we know, this is not only an unprecedented report on activating "inert" MOFs for photocatalytic C-H activation but also the first work on extended light harvesting and enhanced photocatalysis for MOFs by introducing an MCCT process.

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water

Using light energy and O₂ for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O₂ by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)₃ ]²⁺ , generating the highly oxidized chromophore and the powerful reductant methyl-viologen radical MV^+. . MV^+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O₂ to O₂ ^.- in an aqueous medium to generate an active iron(III)-(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.

One small step for cytochrome P450 in its catalytic cycle, one giant leap for enzymology

The intermediates operating in the cytochrome P450 catalytic cycle have been investigated for more than half a century, fascinating many enzymologists. Each intermediate has its unique role to carry out diverse oxidations. Natural time course of the catalytic cycle is quite fast, hence, not all of the reactive intermediates could be isolated during physiological catalysis. Different high-valent iron intermediates have been proposed as primary oxidants: the candidates are compound 0 (Cpd 0, [FeOOH][Formula: see text]P450) and compound I (Cpd I, Fe(IV)[Formula: see text]O por[Formula: see text]P450). Among them, the role of Cpd I in hydroxylation is fairly well understood due the discovery of the peroxide shunt. This review endeavors to put the outstanding research efforts conducted to isolate and characterize the intermediates together. In addition to spectral features of each intermediate in the catalytic cycle, the oxidizing powers of Cpd 0 and Cpd I will be discussed along with most recent scientific findings.

Characterized cis-FeV(O)(OH) intermediate mimics enzymatic oxidations in the gas phase

Fe^V(O)(OH) species have long been proposed to play a key role in a wide range of biomimetic and enzymatic oxidations, including as intermediates in arene dihydroxylation catalyzed by Rieske oxygenases. However, the inability to accumulate these intermediates in solution has thus far prevented their spectroscopic and chemical characterization. Thus, we use gas-phase ion spectroscopy and reactivity analysis to characterize the highly reactive [Fe^V(O)(OH)(^5tips3tpa)]²⁺ (3²⁺) complex. The results show that 3²⁺ hydroxylates C–H bonds via a rebound mechanism involving two different ligands at the Fe center and dihydroxylates olefins and arenes. Hence, this study provides a direct evidence of Fe^V(O)(OH) species in non-heme iron catalysis. Furthermore, the reactivity of 3²⁺ accounts for the unique behavior of Rieske oxygenases. The use of gas-phase ion characterization allows us to address issues related to highly reactive intermediates that other methods are unable to solve in the context of catalysis and enzymology.

Fe^V(O)(OH) species have long been thought to play a role in a range of enzymatic oxidations, but their characterization has remained elusive. Here, using gas-phase ion spectroscopy, the authors characterize an Fe^V(O)(OH) species and find that its reactivity mimics that of Rieske oxygenases.

Development of Fe(II) system based on N, N'-dipicolinamide for the oxidative removal of 4-chlorophenol

A novel catalyst system was investigated based on Fe-N, N'-dipicolinamide complex for the degradation of 4-chlorophenol (4-CP) by using hydrogen peroxide as an oxidant under mild alkaline conditions. This complex was stabilized by a ligand that assembles pyridyl and amide groups with a suitable linker. The optimization of the synthesized catalysts was evaluated in terms of the removal efficiency of 4-CP, by using Fe(II) and N, N'-1,2-phenyl-enedipyridine-2-carboxamide with a molar ratio of 1:1. The effects of reaction parameters on the oxidation of 4-CP were investigated by applying the selected catalyst with 4-CP removal rate of 99%. The results indicated that the pH and catalyst concentration could significantly affect the degradation rate of 4-CP. The mineralization level of 4-CP during the reaction was also examined, and almost 62.5% of 4-CP was absolutely mineralized into carbon dioxide and water. The preliminary analysis on the degradation mechanism indicate that the main active species are not hydroxyl radicals, and another kind of active species, called iron-oxo species, were proposed. This study explores a resultful linker between pyridyl and amide and presents a new method to expand the application of pH range of Fenton-like system.

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

[Fe(β-BPMCN)(CH₃CN)₂]²⁺ (1, BPMCN = N,N' -bis(pyridyl-2-methyl)- N,N' -dimethyl- trans-1,2-diaminocyclo-hexane) is a relatively poor catalyst for cyclohexane oxidation by H₂O₂ and cannot perform benzene hydroxylation. However, addition of Sc³⁺ activates the 1/H₂O₂ reaction mixture to be able to hydroxylate cyclohexane and benzene within seconds at -40 °C. A metastable S = 1/2 Fe^III-(η¹-OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon addition of Sc³⁺ at rates independent of [substrate] but linearly dependent on [Sc³⁺]. HClO₄ elicits comparable reactivity as Sc³⁺ at the same concentration. We thus postulate that these additives both facilitate O-O bond heterolysis of 2 to form a common highly electrophilic Fe^V═O oxidant that is comparably reactive to the fastest nonheme high-valent iron-oxo oxidants found to date.

Spectroscopic and DFT Characterization of a Highly Reactive Nonheme FeV-Oxo Intermediate

The reaction of [(PyNMe₃)Fe^II(CF₃SO₃)₂], 1, with excess peracetic acid at -40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe₃ (an electron-enriched version of PyNMe₃) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a Mössbauer isomer shift δ = -0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of 3b(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe₃)Fe^V(O)(OC(O)R)]²⁺ (∼75%) with some Fe^IV(O)(^•OC(O)R) and Fe^III(OOC(O)R) character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1-O2 bond of 0.35. This unusual geometry facilitates reversible O1-O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of Fe^III(OC(O)R) ferric acyl peroxides and Fe^IV(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe-O1 Raman frequency (815 cm^-1) relative to the 903 cm^-1 value predicted for the hypothetical [(MeO-PyNMe₃)Fe^V(O)(NCMe)]³⁺ complex.

Formation of Mono Oxo Molybdenum(IV) PNP Pincer Complexes: Interplay between Water and Molecular Oxygen

The synthesis of cationic mono oxo MoIV PNP pincer complexes of the type [Mo(PNPMe-iPr)(O)X]+ (X = I, Br) from [Mo(PNPMe-iPr)(CO)X2] is described. These compounds are coordinatively unsaturated and feature a strong Mo≡O triple bond. The formation of these complexes proceeds via cationic 14e intermediates [Mo(PNPMe-iPr)(CO)X]+ and requires both molecular oxygen and water. ESI MS measurements with 18O labeled water (H2 18O) and molecular oxygen (18O2) indicates that water plays a crucial role in the formation of the Mo≡O bond. A plausible mechanism based on DFT calculations is provided. The X-ray structure of [Mo(PNPMe-iPr)(O)I]SbF6 is presented.

Acid-Triggered O-O Bond Heterolysis of a Nonheme FeIII (OOH) Species for the Stereospecific Hydroxylation of Strong C-H Bonds

A novel hydroperoxoiron(III) species [Fe^III (OOH)(MeCN)(PyNMe₃ )]²⁺ (3) has been generated by reaction of its ferrous precursor [Fe^II (CF₃ SO₃ )₂ (PyNMe₃ )] (1) with hydrogen peroxide at low temperatures. This species has been characterized by several spectroscopic techniques and cryospray mass spectrometry. Similar to most of the previously described low-spin hydroperoxoiron(III) compounds, 3 behaves as a sluggish oxidant and it is not kinetically competent for breaking weak C-H bonds. However, triflic acid addition to 3 causes its transformation into a much more reactive compound towards organic substrates that is capable of oxidizing unactivated C-H bonds with high stereospecificity. Stopped-flow kinetic analyses and theoretical studies provide a rationale for the observed chemistry, a triflic-acid-assisted heterolytic cleavage of the O-O bond to form a putative strongly oxidizing oxoiron(V) species. This mechanism is reminiscent to that observed in heme systems, where protonation of the hydroperoxo intermediate leads to the formation of the high-valent [(Porph^. )Fe^IV (O)] (Compound I).

Detection of a transient FeV(O)(OH) species involved in olefin oxidation by a bio-inspired non-haem iron catalyst

Here we provide direct evidence for the formation of an Fe^V(O)(OH) species in non-haem iron catalysis using ambient mass spectrometry.

Equilibrating (L)FeIII-OOAc and (L)FeV(O) Species in Hydrocarbon Oxidations by Bio-Inspired Nonheme Iron Catalysts Using H2O2 and AcOH

Inspired by the remarkable chemistry of the family of Rieske oxygenase enzymes, nonheme iron complexes of tetradentate N4 ligands have been developed to catalyze hydrocarbon oxidation reactions using H2O2 in the presence of added carboxylic acids. The observation that the stereo- and enantioselectivity of the oxidation products can be modulated by the electronic and steric properties of the acid implicates an oxidizing species that incorporates the carboxylate moiety. Frozen solutions of these catalytic mixtures generally exhibit EPR signals arising from two S = 1/2 intermediates, a highly anisotropic g2.7 subset (gmax = 2.58 to 2.78 and Δg = 0.85-1.2) that we assign to an FeIII-OOAc species and a less anisotropic g2.07 subset (g = 2.07, 2.01, and 1.96 and Δg ≈ 0.11) we associate with an FeV(O)(OAc) species. Kinetic studies on the reactions of iron complexes supported by the TPA (tris(pyridyl-2-methyl)amine) ligand family with H2O2/AcOH or AcOOH at -40 °C reveal the formation of a visible chromophore at 460 nm, which persists in a steady state phase and then decays exponentially upon depletion of the peroxo oxidant with a rate constant that is substrate independent. Remarkably, the duration of this steady state phase can be modulated by the nature of the substrate and its concentration, which is a rarely observed phenomenon. A numerical simulation of this behavior as a function of substrate type and concentration affords a kinetic model in which the two S = 1/2 intermediates exist in a dynamic equilibrium that is modulated by the electronic properties of the supporting ligands. This notion is supported by EPR studies of the reaction mixtures. Importantly, these studies unambiguously show that the g2.07 species, and not the g2.7 species, is responsible for substrate oxidation in the (L)FeII/H2O2/AcOH catalytic system. Instead the g2.7 species appears to be off-pathway and serves as a reservoir for the g2.07 species. These findings will be helpful not only for the design of regio- and stereospecific nonheme iron oxidation catalysts but also for providing insight into the mechanisms of the remarkably versatile oxidants formed by nature's most potent oxygenases.

Following a Chemical Reaction on the Millisecond Time Scale by Simultaneous X-ray and UV/Vis Spectroscopy

An innovative approach aimed at disclosing the mechanism of chemical reactions occurring in solution on the millisecond time scale is presented. Time-resolved energy dispersive X-ray absorption and UV/vis spectroscopies with millisecond resolution are used simultaneously to directly follow the evolution of both the oxidation state and the local structure of the metal center in an iron complex. Two redox reactions are studied, the former involving the transformation of Fe^II into two subsequent Fe^III species and the latter involving the more complex Fe^II-Fe^III-Fe^IV-Fe^III sequence. The structural modifications occurring around the iron center are correlated to the reaction mechanisms. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase and represents a new powerful tool to characterize short-lived intermediates that are silent to common spectroscopic techniques.

Synthesis, Resolution, and Absolute Configuration of Chiral Tris(2-pyridylmethyl)amine-Based Hemicryptophane Molecular Cages

The synthesis, characterization, and chiroptical properties of a new class of hemicryptophane cages combining a cyclotriveratrylene unit and a tris(2-pyridylmethyl)amine (TPA) moiety are reported. Changing the linkers between these two units allows for the modification of the size and shape of the cavity. The synthesis is straightforward and efficient, providing gram-scale of cage compounds. The racemic mixture of each hemicryptophane host can be readily resolved by chiral HPLC, giving an easy access to the enantiopure molecular cages of which absolute configurations have been assigned by ECD spectroscopy. These new hemicryptophanes are available chemical platforms ready to use for various purposes due to the versatile metal complexation properties of the TPA unit. A Zn(II)@hemicryptophane complex has been obtained and used as a heteroditopic host for the selective recognition of zwitterionic guests.