EPR Spectroscopic Detection of the Elusive FeV═O Intermediates in Selective Catalytic Oxofunctionalizations of Hydrocarbons Mediated by Biomimetic Ferric Complexes

Intramolecular C-H Oxidation in Iron(V)-oxo-carboxylato Species Relevant in the γ-Lactonization of Alkyl Carboxylic Acids

High-valent oxoiron species have been invoked as oxidizing agents in a variety of iron-dependent oxygenases. Taking inspiration from nature, selected nonheme iron complexes have been developed as catalysts to elicit C-H oxidation through the mediation of putative oxoiron(V) species, akin to those proposed for Rieske oxygenases. The addition of carboxylic acids in these iron-catalyzed C-H oxidations has proved highly beneficial in terms of product yields and selectivities, suggesting the direct involvement of iron(V)-oxo-carboxylato species. When the carboxylic acid functionality is present in the alkane substrate, it acts as a directing group, enabling the selective intramolecular γ-C-H hydroxylation that eventually affords γ-lactones. While this mechanistic frame is solidly supported by previous mechanistic studies, direct spectroscopic detection of the key iron(V)-oxo-carboxylato intermediate and its competence for engaging in the selective γ-C-H oxidation leading to lactonization have not been accomplished. In this work, we generate a series of well-defined iron(V)-oxo-carboxylato species (2c-2f) differing in the nature of the bound carboxylate ligand. Species 2c-2f are characterized by a set of spectroscopic techniques, including UV-vis spectroscopy, cold-spray ionization mass spectrometry (CSI-MS), and, in selected cases, EPR and Mössbauer spectroscopies. We demonstrate that 2c-2f undergo site-selective γ-lactonization of the carboxylate ligand in a stereoretentive manner, thus unequivocally identifying metal-oxo-carboxylato species as the powerful yet selective C-H cleaving species in catalytic γ-lactonization reactions of carboxylic acids. Reactivity experiments confirm that the intramolecular formation of γ-lactones is in competition with the intermolecular oxidation of external alkanes and olefins. Finally, mechanistic studies, together with DFT calculations, support a mechanism involving a site-selective C-H cleavage in the γ-position of the carboxylate ligand by the oxo moiety, followed by a fast carboxylate rebound, eventually leading to the selective formation of γ-lactones.

Elusive Active Intermediates and Reaction Mechanisms of ortho-/ipso-Hydroxylation of Benzoic Acid by Hydrogen Peroxide Mediated by Bioinspired Iron(II) Catalysts

Aromatic hydroxylation of benzoic acids (BzOH) to salicylates and phenolates is fundamentally interesting in industrial chemistry. However, key mechanistic uncertainties and dichotomies remain after decades of effort. Herein, the elusive mechanism of the competitive ortho-/ipso-hydroxylation of BzOH by H2O2 mediated by a nonheme iron(II) catalyst was comprehensively investigated using density functional theory calculations. Results revealed that the long-postulated FeV(O)(anti-BzO) oxidant is an FeIV(O)(anti-BzO•) species 2 (anti- and syn- are defined by the orientation of the carboxyl oxygen of BzO to the oxo), which rules out the noted two-oxidant mechanism proposed previously. We propose a new mechanism in which, following the formation of an FeV(O)(syn-BzO) species (3) and its electromer FeIV(O)(syn-BzO•) (3'), 3/3' either converts to salicylate and phenolate via intramolecular self-hydroxylation (route A) or acts as an oxidant to oxygenate another BzOH to generate the same products (route B). In route A, the rotation of the BzO group along the C-O bond forms 2, in which the BzO group is orientated by π-π stacking interactions. An electrophilic ipso-addition forms a phenolate by concomitant decarboxylation or an ortho-attack forms a cationic complex, which readily undergoes an NIH shift and a BzOH-assisted proton shift to form a salicylate. In route B, 3 oxidizes an additional BzOH molecule directed by hydrogen bonding and π-π stacking interactions. In both routes, selectivity is determined by the chemical property of the BzO ring. These mechanistic findings provide a clear mechanistic scenario and enrich the knowledge of hydroxylation of aromatic acids.

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.

Reactivity vs. Selectivity of Biomimetic Catalyst Systems of the Fe(PDP) Family through the Nature and Spin State of the Active Iron-Oxygen Species

Catalytic approaches to late-stage creation of new C-O bonds, especially via oxygenation of particular C-H groups in complex organic molecules, provide challenging tools for the synthesis of biologically active compounds and candidate drugs. In the last decade, significant efforts were invested in designing bioinspired iron based catalyst systems, capable of conducting selective oxidations of organic compounds. The key role of the oxygen-transferring high-valent iron-oxygen species in selective oxygenation is now well established; the next logical step would be gaining insight into the factors governing the oxidation chemo- and stereoselectivity, in relation to the peculiarities of their electronic structure, which would allow introducing the desired level of predictability into those catalytic transformations. In this Personal Account we analyze recent data on the reactivity of bioinspired formally oxoiron(V) catalytically active sites toward organic substrates having C=C and C(sp³ )-H groups. While the majority of reported oxoiron(V) active species are low-spin (S=1/2) complexes, the presence of strong electron-donating groups (NR₁ R₂ ) in the ligand backbone favors the high-spin (S=3/2) ground state. Remarkably, the high-spin perferryl species exhibit higher chemo-, regio-, and stereoselectivity in the oxidations than their low-spin counterparts, thus witnessing the significance of these subtle electronic effects for the selectivity of oxidations conducted by bioinspired catalysts of the Fe(PDP) family.

μ-Oxo-bridged diiron(iii) complexes of tripodal 4N ligands as catalysts for alkane hydroxylation reaction using m-CPBA as an oxidant: substrate vs. self hydroxylation

A series of non-heme μ-oxo-bridged dinuclear iron(iii) complexes of the type [Fe₂(μ-O)(L1–L6)₂Cl₂]Cl₂1–6 have been isolated and their catalytic activity towards oxidative transformation of alkanes into alcohols has been studied using m-choloroperbenzoic acid (m-CPBA) as an oxidant. All the complexes were characterized by CHN, electrochemical, and UV-visible spectroscopic techniques. The molecular structures of 2 and 5 have been determined successfully by single crystal X-ray diffraction analysis and both possesses octahedral coordination geometry and each iron atom is coordinated by four nitrogen atoms of the 4N ligand and a bridging oxygen. The sixth position of each octahedron is coordinated by a chloride ion. The (μ-oxo)diiron(iii) core is linear in 2 (Fe–O–Fe, 180.0°), whereas it is non-linear (Fe–O–Fe, 161°) in 5. All the diiron(iii) complexes show quasi-reversible one electron transfer in the cyclic voltammagram and catalyze the hydroxylation of alkanes like cyclohexane, adamantane with m-CPBA as an oxidant. In acetonitrile solution, adding excess m-CPBA to the diiron(iii) complex 2 without chloride ions leads to intramolecular hydroxylation reaction of the oxidant. Interestingly, 2 catalyzes alkane hydroxylation in the presence of chloride ions, but intramolecular hydroxylation in the absence of chloride ions. The observed selectivity for cyclohexane (A/K, 5–7) and adamantane (3°/2°, 9–18) suggests the involvement of high-valent iron–oxo species rather than freely diffusing radicals in the catalytic reaction. Moreover, 4 oxidizes (A/K, 7) cyclohexane very efficiently up to 513 TON while 5 oxidizes adamantane with good selectivity (3°/2°, 18) using m-CPBA as an oxidant. The electronic effects of ligand donors dictate the efficiency and selectivity of catalytic hydroxylation of alkanes.

The ligand stereoelectronic effect of diiron(iii) complexes determines the efficiency and selectivity of catalytic alkane hydroxylation with m-CPBA as an oxidant.

Hydroxylation of Unactivated C(sp3)-H Bonds with m-Chloroperbenzoic Acid Catalyzed by an Iron(III) Complex Supported by a Trianionic Planar Tetradentate Ligand

Hydroxylation of cyclohexane with m-chloroperbenzoic acid was examined in the presence of an iron(III) complex supported by a trianionic planar tetradentate ligand. The present reaction system shows a high turnover number of 2750 with a high product selectivity of alcohol (93%). The turnover frequency was 0.51 s^-1, and the second-order rate constant (k) for the C-H bond activation of cyclohexane was 1.08 M^-1 s^-1, which is one of the highest values among the iron complexes in the oxidation of cyclohexane so far reported. The present catalytic system can be adapted to the hydroxylation of substrates having only primary C-H bonds such as 2,2,3,3-tetramethylbutane as well as gaseous alkanes such as butane, propane, and ethane. The involvement of an iron(III) acyl peroxido complex as the reactive species was suggested by spectroscopic measurements of the reaction solution.

Low-Spin and High-Spin Perferryl Intermediates in Non-Heme Iron Catalyzed Oxidations of Aliphatic C-H Groups

The selectivity patterns of iron catalysts of the Fe(PDP) family in aliphatic C-H oxidation with H₂ O₂ have been studied (PDP=N,N'-bis(pyridine-2-ylmethyl)-2,2'-bipyrrolidine). Cyclohexane, adamantane, 1-bromo-3,7-dimethyloctane, 3,7-dimethyloctyl acetate, (-)-acetoxy-p-menthane, and cis-1,2-dimethylcyclohexane were used as substrates. The studied catalyst systems generate low-spin (S=1/2) oxoiron(V) intermediates or high-spin (S=3/2) oxoiron(V) intermediates, depending on the electron-donating ability of remote substituents at the pyridine rings. The low-spin perferryl intermediates demonstrate lower stability and higher reactivity toward aliphatic C-H groups of cyclohexane than their high-spin congeners, according to the measured self-decay and second-order rate constants k₁ and k₂ . Unexpectedly, there appears to be no uniform correlation between the spin state of the oxoiron(V) intermediates, and the chemo- and regioselectivity of the corresponding catalyst systems in the oxidation of the considered substrates. This contrasts with the asymmetric epoxidations by the same catalyst systems, in which case the epoxidation enantioselectivity increases when passing from the systems featuring the more reactive low-spin perferryl intermediates to those with their less reactive high-spin congeners.

Iron-Based Catalytically Active Complexes in Preparation of Functional Materials

Iron complexes are particularly interesting as catalyst systems over the other transition metals (including noble metals) due to iron’s high natural abundance and mediation in important biological processes, therefore making them non-toxic, cost-effective, and biocompatible. Both homogeneous and heterogeneous catalysis mediated by iron as a transition metal have found applications in many industries, including oxidation, C-C bond formation, hydrocarboxylation and dehydration, hydrogenation and reduction reactions of low molecular weight molecules. These processes provided substrates for industrial-scale use, e.g., switchable materials, sustainable and scalable energy storage technologies, drugs for the treatment of cancer, and high molecular weight polymer materials with a predetermined structure through controlled radical polymerization techniques. This review provides a detailed statement of the utilization of homogeneous and heterogeneous iron-based catalysts for the synthesis of both low and high molecular weight molecules with versatile use, focusing on receiving functional materials with high potential for industrial application.

Iron and manganese oxo complexes, oxo wall and beyond

High-valent metal-oxo species with multiply-bonded M-O groups have been proposed as key intermediates in many biological and abiological catalytic oxidation reactions. These intermediates are implicated as active oxidants in alkane hydroxylation, olefin epoxidation and other oxidation reactions. For example, [Fe^ivO(porphyrinato^•-)]⁺ cofactors bearing π-radical porphyrinato^•- ligands oxidize organic substrates in cytochrome P450 enzymes, which are common to many life forms. Likewise, high-valent Mn-oxo species are active for H₂O oxidation in photosystem II. The chemistry of these native reactive species has inspired chemists to prepare highly oxidized transition-metal complexes as functional mimics. Although many synthetic Fe-O and Mn-O complexes now exist, the analogous oxo complexes of the late transition metals (groups 9-11) are rare. Indeed, late-transition-metal-oxo complexes of tetragonal (fourfold) symmetry should be electronically unstable, a rule commonly referred to as the 'oxo wall'. A few late metal-oxos have been prepared by targeting other symmetries or unusual spin states. These complexes have been studied using spectroscopic and theoretical methods. This Review describes mononuclear non-haem Fe-O and Mn-O species, the nature of the oxo wall and recent advances in the preparation of oxo complexes of Co, Ni and Cu beyond the oxo wall.

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.

Palladium aminopyridine complexes catalyzed selective benzylic C–H oxidations with peracetic acid

Palladium complexes with tripodal ligands of the tpa family efficiently catalyze benzylic C–H oxidation of various substrates with peracetic acid, affording the corresponding ketones in high yields (up to 100%), at <1 mol% catalyst loadings.

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Spectroscopic and Reactivity Comparisons between Nonheme Oxoiron(IV) and Oxoiron(V) Species Bearing the Same Ancillary Ligand

This work directly compares the spectroscopic and reactivity properties of an oxoiron(IV) and an oxoiron(V) complex that are supported by the same neutral tetradentate N-based PyNMe₃ ligand. A complete spectroscopic characterization of the oxoiron(IV) species (2) reveals that this compound exists as a mixture of two isomers. The reactivity of the thermodynamically more stable oxoiron(IV) isomer (2b) is directly compared to that exhibited by the previously reported 1e^--oxidized analogue [Fe^V(O)(OAc)(PyNMe₃)]²⁺ (3). Our data indicates that 2b is 4 to 5 orders of magnitude slower than 3 in hydrogen atom transfer (HAT) from C-H bonds. The origin of this huge difference lies in the strength of the O-H bond formed after HAT by the oxoiron unit, the O-H bond derived from 3 being about 20 kcal·mol^-1 stronger than that from 2b. The estimated bond strength of the Fe^IVO-H bond of 100 kcal·mol^-1 is very close to the reported values for highly active synthetic models of compound I of cytochrome P450. In addition, this comparative study provides direct experimental evidence that the lifetime of the carbon-centered radical that forms after the initial HAT by the high valent oxoiron complex depends on the oxidation state of the nascent Fe-OH complex. Complex 2b generates long-lived carbon-centered radicals that freely diffuse in solution, while 3 generates short-lived caged radicals that rapidly form product C-OH bonds, so only 3 engages in stereoretentive hydroxylation reactions. Thus, the oxidation state of the iron center modulates not only the rate of HAT but also the rate of ligand rebound.

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.

Trapping of a Highly Reactive Oxoiron(IV) Complex in the Catalytic Epoxidation of Olefins by Hydrogen Peroxide

The generation of a nonheme oxoiron(IV) intermediate, [(cyclam)Fe^IV (O)(CH₃ CN)]²⁺ (2; cyclam=1,4,8,11-tetraazacyclotetradecane), is reported in the reactions of [(cyclam)Fe^II ]²⁺ with aqueous hydrogen peroxide (H₂ O₂ ) or a soluble iodosylbenzene (sPhIO) as a rare example of an oxoiron(IV) species that shows a preference for epoxidation over allylic oxidation in the oxidation of cyclohexene. Complex 2 is kinetically and catalytically competent to perform the epoxidation of olefins with high stereo- and regioselectivity. More importantly, 2 is likely to be the reactive intermediate involved in the catalytic epoxidation of olefins by [(cyclam)Fe^II ]²⁺ and H₂ O₂ . In spite of the predominance of the oxoiron(IV) cores in biology, the present study is a rare example of high-yield isolation and spectroscopic characterization of a catalytically relevant oxoiron(IV) intermediate in chemical oxidation reactions.

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.

Highly enantioselective epoxidation of olefins by H2O2 catalyzed by a non-heme Fe(ii) catalyst of a chiral tetradentate ligand

A new chiral Fe(ii)-complex mediates the asymmetric epoxidation of prochiral olefins with good enantioselectivity.

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.

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.

Heteroleptic μ-nitrido diiron complex supported by phthalocyanine and octapropylporphyrazine ligands: Formation of oxo species and their reactivity with fluorinated compounds

The synthesis and reactivity of [Formula: see text]-bridged diiron macrocyclic complexes have been a topic of increasing interest in recent years since the observation of particular catalytic properties of these complexes. Herein, we report a preparation of a novel heteroleptic μ-nitrido diiron complex with unsubstituted phthalocyanine and octapropylporphyrazine macrocycles. This complex reacts with [Formula: see text]-chloroperbenzoic acid to form high-valent diiron oxo species showing strong oxidizing properties. The formation and structure of the transient oxo species was investigated by cryospray collision induced dissociation MS/MS technique. Analysis of fragmentation pattern showed that the attachment of oxo moiety occurred at either iron phthalocyanine or at iron porphyrazine site with slight preference for the phthalocyanine iron site. The catalytic properties of the heteroleptic μ-nitrido diiron complex were evaluated in the oxidative transformation of hexafluorobenzene and perfluoro(allylbenzene).

DFT studies of the substituent effects of dimethylamino on non-heme active oxidizing species: iron(V)-oxo species or iron(IV)-oxo acetate aminopyridine cation radical species?

Through the introduction of dimethylamino (Me₂N) substituent at the pyridine ring of 2-((R)-2-[(R)-1-(pyridine-2-ylmethyl)pyrrolidin-2-yl]pyrrolidin-1-ylmethyl)pyridine (PDP) ligand, the non-heme Fe^II(^Me2NPDP)/H₂O₂/AcOH catalyst system was found to exhibit significant higher catalytic activity and enantioselectivity than the non-substituent one in the asymmetric epoxidation experiments. The mechanistic origin of the remarkable substituent effects in these oxidation reactions has not been well established. To ascertain the potent oxidant and the related reaction mechanism, a detailed DFT calculation was performed. Interestingly, a novel Fe(IV)-oxo ^Me2NPDP cation radical species, [(^Me2NPDP)^+·Fe^IV(O)(OAc)]²⁺ ( ^Me2N 5), with about one spin spreading over the non-heme ^Me2NPDP ligand was formed via a carboxylic-acid-assisted O-O bond heterolysis, which is reminiscent of Compound I (an Fe(IV)(O)(porphyrin cation radical) species) in cytochrome P450 chemistry. ^Me2N 5 is energetically comparable with the cyclic ferric peracetate species ^Me2N 6, while in the pristine Fe(PDP) catalyst system, ^H 6 is more stable than ^H 5. Comparison of the activation energy for the ethylene epoxidation promoted by ^Me2N 5 and ^Me2N 6, ^Me2N 5 is supposed as the true oxidant triggering the epoxidation of olefins. In addition, a systematic research on the substituent effects varied from the electron-donating substituent (dMM, the substituents at sites 3, 4, and 5 of the pyridine ring: methyl, methoxyl, and methyl) to the electron-withdrawing one (CF₃, 2,6-bis(trifluoromethyl)phenyl) on the electronic structure of the reaction intermediates has also been investigated. An alternative cyclic ferric peracetate complex is obtained, indicating that the substituents at the pyridine ring of PDP ligands have significant impacts on the electronic structure of the oxidants.