Î¼-Nitrido Diiron Macrocyclic Platform: Particular Structure for Particular

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μ‑Nitrido Diiron Macrocyclic Platform: Particular Structure for Particular Catalysis Pavel Afanasiev* and Alexander B. Sorokin* Institut de Recherches sur la Catalyse et l’Environnement de Lyon IRCELYON, UMR 5256, CNRS - Université Lyon 1, 2 avenue A. Einstein, 69626 Villeurbanne cedex, France CONSPECTUS: The ultimate objective of bioinspired catalysis is the development of efficient and clean chemical processes. Cytochrome P450 and soluble methane monooxygenase enzymes efficiently catalyze many challenging reactions. Extensive research has been performed to mimic their exciting chemistry, aiming to create efficient chemical catalysts for functionalization of strong C−H bonds. Two current biomimetic approaches are based on (i) mononuclear metal porphyrin-like complexes and (ii) iron and diiron nonheme complexes. However, biomimetic catalysts capable of oxidizing CH4 are still to be created. In the search for powerful oxidizing catalysts, we have recently proposed a new bioinspired strategy using N-bridged diiron phthalocyanine and porphyrin complexes. This platform is particularly suitable for stabilization of Fe(IV)Fe(IV) complexes and can be useful to generate high-valent oxidizing active species. Indeed, the possibility of charge delocalization on two iron centers, two macrocyclic ligands, and the nitrogen bridge makes possible the activation of H2O2 and peracids. The ultrahigh-valent diiron−oxo species (L)FeIV−N−FeIV(L+•)O (L = porphyrin or phthalocyanine) have been prepared at low temperatures and characterized by cryospray MS, UV−vis, EPR, and Mössbauer techniques. The highly electrophilic (L)FeIV−N−FeIV(L+•)O species exhibit remarkable reactivity. In this Account, we describe the catalytic applications of μ-nitrido diiron complexes in the oxidation of methane and benzene, in the transformation of aromatic C−F bonds under oxidative conditions, in oxidative dechlorination, and in the formation of C−C bonds. Importantly, all of these reactions can be performed under mild and clean conditions with high conversions and turnover numbers. μ-Nitrido diiron species retain their binuclear structure during catalysis and show the same mechanistic features (e.g., 18 O labeling, formation of benzene epoxide, and NIH shift in aromatic oxidation) as the enzymes operating via high-valent iron− oxo species. μ-Nitrido diiron complexes can react with perfluorinated aromatics under oxidative conditions, while the strongest oxidizing enzymes cannot. Advanced spectroscopic, labeling, and reactivity studies have confirmed the involvement of high-valent diiron−oxo species in these catalytic reactions. Computational studies have shed light on the origin of the remarkable catalytic properties, distinguishing the Fe−N−Fe scaffold from Fe−C−Fe and Fe−O−Fe analogues. X-ray absorption and emission spectroscopies assisted with DFT calculations allow deeper insight into the electronic structure of these particular complexes. Besides the novel chemistry involved, iron phthalocyanines are cheap and readily available in bulk quantities, suggesting high application potential. A variety of macrocyclic ligands can be used in combination with different transition metals to accommodate M−N−M platform and to tune their electronic and catalytic properties. The structural simplicity and flexibility of μ-nitrido dimers make them promising catalysts for many challenging reactions.

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INTRODUCTION

Complexes mimicking the structural and spectroscopic properties of sMMO have been described, but a diiron non-heme methane oxidation catalyst is still to be developed.4 In the search for catalysts for challenging oxidations, we have proposed a novel approach based on diiron macrocyclic complexes, which combine the structural features of the two most powerful oxidizing enzymes: a diiron site as in sMMO and a porphyrinoid ligand as in P450 (Figure 1).

Extensive research in bioinspired oxidation is motivated by the development of efficient and practical chemical catalysts. Soluble methane monooxygenase (sMMO) and cytochrome P450 are particularly exciting and challenging enzymes to mimic because of their ability to oxidize the strongest C−H bonds. These enzymes activate O2 to form high-valent iron− oxo species, which are powerful oxidants. P450 contains a mononuclear iron porphyrin1 active center, while the diiron non-heme site is responsible for the oxidation of methane by sMMO.2 Native P450 enzymes are not capable of oxidizing methane,3 mild oxidation of which is a fundamental challenge. © XXXX American Chemical Society

Received: October 9, 2015

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DOI: 10.1021/acs.accounts.5b00458 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Chart 1. Structures of μ-Nitrido Diiron Complexes Studied in Catalysis

withdrawing9 substituents. In contrast to their μ-oxo diiron counterparts, μ-nitrido complexes retain their dimeric structure even under acid conditions and during the formation of ultrahigh-valent species. μ-Nitrido diiron complexes contain one unpaired electron and show typical S = 1/2 axial spectra with weak or nondetectable hyperfine splitting from the bridging nitrogen. Their Mössbauer spectra exhibit a single doublet indicating two equivalent iron sites, each with an intermediate +3.5 oxidation state.6 Synchrotron Fe K-edge Xray absorption and emission spectroscopies (XAS and XES) combined with density functional theory (DFT) studies have provided further information on the iron oxidation and spin states. Unlike their mononuclear precursors, μ-nitrido dimers have very strong pre-edge features, suggesting unsymmetrical coordination and a high degree of mixing between the Fe and μ-N orbitals. In contrast to related μ-oxo dimers containing antiferromagnetically coupled high-spin (HS) centers, μ-nitrido Fe(III)Fe(IV) and Fe(IV)Fe(IV) complexes are all low-spin (LS) systems.10 Monooxygenases and biomimetic complexes involve Fe2+ or Fe3+ to form oxo species via two-electron oxidation. Such a process should be more difficult for the Fe+3.5Fe+3.5 species in the already high oxidation state. Nevertheless, spectroscopic studies indicate the formation of diiron−oxo species from peroxo complexes (vide infra). This particular feature of μnitrido complexes makes possible the formation of ultrahighvalent species, in which the effective oxidation state is higher than that in mononuclear oxo complexes (Figure 3). In this Account, we discuss particular features of the electronic structure of μ-nitrido dimers that confer upon them peculiar catalytic properties. Mild oxidation of methane and oxidative defluorination of aromatic C−F bonds are the

Figure 1. Schematic representation of the diiron macrocyclic concept.

We suggested that the binuclear macrocyclic assembly can provide powerful oxidation catalysts. Indeed, upon generation of active oxidizing species, the iron site(s) of P450 and sMMO increase their oxidation state by two redox equivalents. The high-valent species are stabilized by charge delocalization at the iron center and porphyrin ligand of (P+•)FeIVO in P450 and at two Fe(IV) sites in sMMO. Two iron sites and two porphyrinoid macrocycles available for charge delocalization in the diiron scaffold should favor the formation of high-valent iron−oxo species. Such oxidizing species can be formed using H2O2, tBuOOH, or m-chloroperoxybenzoic acid (m-CPBA). From the economic and environmental perspectives, H2O2 is the most attractive oxidant. The iron−peroxo complex (L)Fe− OOH can undergo both homolytic and heterolytic cleavage of the O−O bond, but in mononuclear iron porphyrin complexes the former is a principal route to a sluggish PFeIVO intermediate (Figure 2a).5 Heterolytic cleavage of the O−O bond to form strongly oxidizing PFeIV−O−FeIV(P)O species should be more favorable in PFeIII−O−FeIII(P)OOH because of charge delocalization on two iron sites and two macrocyclic ligands (Figure 2b). The nature of the bridging atom determines the oxidation state of iron and the properties of the complex. The two iron sites of binuclear complexes can form Fe(III)−O− Fe(III), Fe(III)−NFe(IV), or Fe(IV)CFe(IV) units.6 Among these single-atom-bridged complexes, μ-nitrido species (Chart 1) exhibit particularly interesting reactivity.7 Their properties can be tuned using phthalocyanine,6,7 porphyrazine,8 or porphyrin6,7 ligands with electron-donating6,7 or electron-

Figure 2. Two pathways for the generation of high-valent iron−oxo species from mononuclear and binuclear iron−peroxo complexes. B


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unpaired electron located on the porphyrin macrocycle. The labeled 1-57FeO complex was prepared by the addition of mCPBA to a concentrated 1-57Fe solution at −90 °C followed by quenching with liquid nitrogen.15 Importantly, the frozen solution contained only one species. The Mössbauer spectrum was reproduced with one quadrupole doublet (δ = 0.001 mm s−1, ΔEQ1 = +0.752 mm s−1). The spectra recorded under an external magnetic field indicated zero spin density on the FeIV(μ-N)FeIV unit, which behaves as an SFeFe = 0 site weakly coupled to a porphyrin radical.15 Further studies using the heteroleptic complex 4 with different iron sites provided deeper insight into the structural parameters that govern the catalytic activity.16 The diiron platform provides the unique possibility to compare the reactivities of two iron sites in a single molecule, since two isomeric Fe(μ-N)FeO species can be formed using H2O2 or m-CPBA (Figure 5). These transient species were trapped in the quadrupole region of the mass spectrometer and underwent collision-induced dissociation tandem MS/MS fragmentation (CID-MS2). Analysis of the fragmentation patterns from two heteroleptic diiron complexes and DFT studies showed that the hydroperoxo ligand binds on the more electron-rich iron site, followed by formation of corresponding oxo species (pathway A in Figure 5). Therefore, the nature of the macrocyclic ligand is important for the formation of the oxo species and can probably affect their catalytic properties.

Figure 3. Formation of monoiron and diiron high-valent oxo species.

notable examples. We review the detection, characterization, and catalytic properties of the μ-nitrido diiron−oxo species.

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HIGH-VALENT DIIRON−OXO SPECIES AND THEIR SPECTROSCOPIC CHARACTERIZATION Addition of peroxides to solutions of the μ-nitrido dimers leads to the formation of short-lived high-valent diiron−oxo species that can be detected at low temperatures. The first μ-nitrido diiron phthalocyanine oxo species, 2O, was observed by ESIMS using H2O2 or labeled H218O2, indicating retention of the Fe−N−Fe unit upon oxidation.11 The high reactivity of 2O prevented its preparation in pure form for structural characterization. Only FeIV−N−FeIV−OH could be isolated in 70% yield and characterized by Mö ssbauer, electron paramagnetic resonance (EPR), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) techniques.12 The porphyrin ligand seems more suitable for the generation of the oxo species. Monomeric high-valent iron porphyrin oxo species are well-known,13 while the first iron phthalocyanine oxo complex was reported recently.14 When 1 was treated with m-CPBA at −80 °C, a green species was formed with a broad Q band indicative of the porphyrin cation radical (Figure 4).15

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REACTIVITY AND CATALYTIC PROPERTIES To assess the influence of the diiron core structure on the catalytic properties, we prepared mono- and diiron−oxo species supported by the same tetraphenylporphyrin ligand. Their reactivities were evaluated using UV−vis decay kinetics in the presence of ethylbenzene, adamantane, or cyclohexane.15 Comparison of the reactivities of 1O and (TPP+•)FeO under single-turnover conditions showed 1O to be a much stronger oxidant. The binuclear oxo species was 26 and 130 times more reactive than the mononuclear counterpart in the oxidation of ethylbenzene and adamantane, respectively (Figure 6). The stronger the C−H bond is, the larger is the reactivity difference between the two species. Oxidation of cyclohexane, which has a C−H bond dissociation energy (BDEC−H) of 99 kcal/mol, was not observed with (TPP+•)FeO at −60 °C, whereas the diiron−oxo complex exhibited a second-order rate constant k−60°C = 0.079 M−1 s−1, which is among the highest reported rate constants for cyclohexane oxidation by biomimetic complexes.15 The rate constant at 25 °C estimated from the temperature dependence was k25°C ≈ 3.4 M−1 s−1. The value of kH/kD measured from the second-order rate constants for C6H12 and C6D12 oxidation was 3.6, indicating the involvement of C−H bond cleavage in the rate-determining step. These results reveal μ-nitrido diiron complexes to be powerful oxidation catalysts. Oxidation of Methane and Ethane

Figure 4. Formation of 1O from 1 and m-CPBA at −80 °C in CH2Cl2.

The C−H bonds in methane and ethane are particularly strong, with dissociation energies of 104.9 and 101.4 kcal/mol−1, respectively.3 Only the most potent oxidants can activate these alkanes. Even P450 does not catalyze methane oxidation.3 The high rate constant for cyclohexane oxidation by 1O prompted us to attempt the oxidation of methane by 1/mCPBA. Formic acid was obtained in 43.5% yield with a turnover number (TON) of 13.7.15 The efficiency of methane oxidation was increased by using 2 and H2O2.11,17 Isotope labeling studies

Cryospray MS showed a transient signal with m/z = 1366.0 corresponding to the 1O formulation. Addition of cyclohexene to the green complex regenerated the UV−vis spectrum of 1 and produced the epoxide without formation of allylic alcohol and ketone, indicating clean oxo-transfer chemistry. A single narrow EPR signal suggested the S = 1/2 state with the C


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Figure 5. Formation of two isomeric diiron−oxo species on the heteroleptic complex 4 and determination of the position of the oxo group by CIDMS2.

Figure 6. Comparison of the catalytic activities of 1O and (TPP+•)FeO in the oxidation of ethylbenzene and adamantane in 1:1 MeCN/ CH2Cl2 at −60 °C.

does occur with TON = 41 but is masked by solvent oxidation. To avoid solvent oxidation, we performed the reaction in water using SiO2-supported catalyst (20 μmol/g (FePctBu4)2N loading, 185 m2/g). When ∼1:1 13CH4/CH4 substrate was used, the same isotopic compositions of the products were obtained. The 2/SiO2/H2O2 system oxidized methane even at 25 °C, though with a moderate TON of 13. The catalytic activity increased at 50−60 °C (TON = 29). Significant improvement in the catalytic activity was obtained at low pH via peroxide protonation, which facilitates O−O heterolytic cleavage and formation of the oxo species (Figure 8).17 A high turnover number of 223 was obtained in 75 mM H2SO4. Methane was oxidized to HCOOH with 88−97% selectivity in 50% yield based on H2O2. Under acidic conditions the catalyst stability was improved, and it could be used in three successive runs to achieve a total TON of 492. Combined ESI-MS and DFT studies showed that the more electron-rich iron site is more suitable for the formation of the oxo species.16 The same tendency was observed in the catalytic oxidation of methane. With the increase in the electrondonating character of the macrocyclic ligand along the series [FePc(SO2tBu)4]2N < (FePc)2N < [FePc(tBu)4]2N, the TONs strongly increased: 32.1 → 102.9 → 223.4.16 The high selectivity of the 2/H2O2 system in the oxidation of light alkanes to acids may be advantageously used for the direct oxidation of ethane to CH3COOH. Two μ-nitrido diiron phthalocyanines supported on silica, graphite, or Nafion

were performed to evidence the oxidation of methane (Figure 7).

Figure 7. Oxidation of methane using CD3CN solvent and 13C-labeled methane.

Particular attention must be paid to the origin of the products if methane oxidation is performed in an organic solvent because the solvent can be oxidized to give the same products as methane. The origin of C1 products can be determined in labeling experiments with deuterated solvents. Using CD3CN we demonstrated co-oxidation of the solvent (32%) and methane (68%) by analysis of the isotopic composition of the formic acid product.11 Methane oxidation D


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Figure 8. Proton-assisted formation of the active species and influence of the acid concentration on the methane oxidation rate.

Figure 9. Mechanistic features of benzene oxidation by the 2/H2O2 system: incorporation of 18O from H218O2, formation of benzene epoxide, and detection of the NIH shift.

aromatic oxidation as monooxygenase oxo species, suggesting that the same mechanism is involved.20 Using H2O2 instead of tBuOOH changed the oxidation selectivity to the preferential benzylic oxidation of alkyl aromatic compounds.9 Toluene was oxidized to benzoic acid with 83% selectivity and TON = 197. p-Toluic acid was the main product of p-xylene oxidation, and a TON of 587 was attained.9 Thus, depending on the oxidant, μ-nitrido diiron complexes catalyze the oxidation of either benzylic or aromatic cycles.

showed promising performance in ethane oxidation by H2O2 in water at 60 °C.18 Acetic acid was obtained with 71% selectivity and TON = 58. As in other systems,18 formic acid was a side product. The nature of the support and the strength and amount of acidic groups should be further optimized to limit C−C bond cleavage and increase the activity and selectivity. Oxidation of Benzene and Alkyl Aromatics

The oxidation of aromatic compounds is both industrially and biologically important. Biomimetic systems for the mild oxidation of benzene are still rare.19 The 2/H2O2 system oxidizes benzene to phenol and minor amounts of benzoquinone, benzene epoxide, and sym-oxepin oxide (Figure 9).20 A labeling study showed the exclusive incorporation of 18 O atoms from H218O2 into phenol. Of particular importance is the formation of benzene epoxide,20 which was previously observed only with monooxygenases operating via high-valent iron−oxo species.21 Another important feature of bio-oxidation is the migration of the substituent from the hydroxylation site to the adjacent carbon atom, which is called the NIH shift.22 We used benzene-1,3,5-d3 as a mechanistic probe and evidenced the NIH shift performed by 2/H2O2 by analysis of the product isotopic composition (Figure 9). Thus, μ-nitrido high-valent diiron−oxo species exhibit the same features in

Formation of C−C Bonds

Iron complexes in combination with peroxides usually oxidize olefins to epoxides and/or allylic compounds. Unexpectedly, the oxidation of cyclohexene by 2/tBuOOH produced a significant amount of 3,3′-dicyclohexenyl. This C−C bond formation reactivity was extended to hydroacylation of olefins with CH3CHO with a high TON and selectivity for methyl ketones (Figure 10).23 This remarkable reactivity of 2 was observed with tBuOOH and not with H2O2. To rationalize this difference, we have proposed different behaviors of the peroxo complexes formed with tBuOOH and H2O2 (Figure 11). (Pc)FeIV(μ-N)FeIII(Pc)−O−OH undergoes heterolytic cleavage to form strongly oxidizing (Pc)FeIV(μ-N)FeIV(Pc+•)O. In contrast, (Pc)FeIV(μ-N)FeIII(Pc)−O−OtBu undergoes hoE


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3.39(1) Å. The Fe−Cl and Fe−Br lengths were 2.33 and 2.54 Å, respectively. These complexes can oxidize 2-mercaptoethanol to disulfide and hydroquinone to quinone via two-electron processes.24 The formation of Cl−(Pc)FeIV(μ-N)FeIV(Pc+•)− Cl indicates cleavage of the C−Cl bond in CH2Cl2, which is routinely used as a solvent for oxidation reactions. Inspired by the oxidative dechlorination of CH2Cl2, we screened the reactivity of (FePctBu4)2N toward different halogenated compounds. Remarkably, the oxidative dehalogenation could be extended to fluorinated aromatics.

Figure 10. Large-scope hydroacylation of olefins with CH3CHO.

Transformation of Aromatic C−F Bonds under Oxidative Conditions

Because of the high electronegativity of fluorine, the activation of C−F bonds involves electron-rich reagents: low- and zerovalent transition metal complexes, strong reductants, and nucleophiles. Transformation of C−F bonds under oxidative conditions using electron-deficient species is highly challenging. Nevertheless, μ-nitrido diiron complexes can perform the oxidative dehalogenation of fluorinated aromatics including C6F6 (BDEC−F = 154 kcal/mol). Treatment of 2 with 30 equiv of tBuOOH in a C6F6/C6H6 mixture resulted in stoichiometric formation of the complex [F−(Pc)FeIV(μ-N)FeIV(Pc+•)−F], which was characterized by ESI-MS, UV−vis, EPR, 19F NMR, energy-dispersive X-ray (EDX), EXAFS, XANES, and XES techniques (Figure 13).25

Figure 11. Different evolution of the peroxo complexes obtained from t BuOOH and H2O2.

molytic O−O cleavage to produce one-electron-oxidized (Pc)FeIV(μ-N)FeIV(Pc)O and tBuO• radical. Both FeIV(μ-N)FeIVO and tBuO• may generate acyl radicals, which add to the olefin with anti-Markovnikov regioselectivity to form radical A (Figure 12). Hydrogen abstraction from CH3CHO by A is

Figure 12. Proposed mechanism for olefin hydroacylation by the 2/tBuOOH system. Figure 13. Stoichiometric formation of [F−(Pc)Fe IV (μ-N)FeIV(Pc+•)−F] and possible regeneration of the catalyst with H2O2. Ovals stand for tetra-tert-butylphthalocyanine.

inefficient, but this step can be performed by FeIV(μ-N)FeIII− OH to afford the hydroacylation product, thus regenerating FeIV(μ-N)FeIV(Pc)O for the next cycle. Thus, μ-nitrido diiron species perform two key reactions forming a catalytic cycle. tBuOOH can be used in catalytic amount (15 mol % with respect to olefin) just to initiate the reaction. This protocol employing a 0.01 mol % loading of nontoxic iron catalyst represents a clean alternative to traditional coupling methods applying large amounts of expensive Rh and Ru complexes.23

To realize catalytic defluorination, the fluorinated complex must be reduced back to the initial 2. This can be done by replacing tBuOOH with H2O2, which acts as an oxidant to generate oxo species that react with the fluoroaromatics and as a reductant of [F−(Pc)FeIV(μ-N)FeIV(Pc+•)−F] to complete the catalytic cycle. Catalytic defluorination by 2/H2O2 was applied to a large scope of substrates (Figure 14). Perfluorinated aromatics and functionalized polyfluorinated compounds were converted with high TONs. Remarkably, carbon-supported 2 showed a high defluorination efficiency in water: up to 4825 C−F bonds per catalyst molecule were cleaved with release of F− (Figure 15). Total organic carbon determination, 19F NMR spectroscopy, and GC−MS indicated efficient mineralization of C6F5OH in water, resulting in 54% organic carbon loss due to the formation of CO2/CO and 89% defluorination of C−F (Figure 16). This outstanding reactivity might be used to eliminate polyhalogenated pollutants that are resistant to biodegradation and existing remediation methods. The large-scale production and application of fluorinated compounds (e.g., 40% of agrochemicals and 25% of drugs currently used contain C−F

Oxidative Dehalogenation

The 2/tBuOOH system exhibits another unusual reactivity, as it can react with CH2Cl2 and CH2Br2 to form Cl−(Pc)FeIV(μN)FeIV(Pc+•)−Cl and Br−(Pc)FeIV(μ-N)FeIV(Pc+•)−Br complexes.24 This is a rare case of stable Fe(IV) complexes bearing a macrocycle cation radical, and these complexes were isolated in high yields and fully characterized. A narrow EPR signal at g = 2.0012 due to the phthalocyanine cation radical and one Mössbauer doublet (δ = −0.10 mm s−1, ΔEQ1 = 1.64 mm s−1) due to the identical Fe(IV) sites attest to structures isoelectronic with those of high-valent iron−oxo species. The Fe(IV) state was confirmed by characteristic doublet pre-edge peaks in the Fe K-edge XANES spectra at 7115.2 and 7113.4 eV. EXAFS analysis of X−(Pc)FeIV(μ-N)FeIV(Pc+•)−X showed a symmetric Fe−N−Fe fragment with an Fe−Fe distance of F


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Figure 14. Catalytic defluorination by the 2/H2O2 system in CD3CN at 60 °C for 15 h. The catalyst:substrate:oxidant ratio was 1:250:4000. TONs were calculated as moles of F− per mole of catalyst.

system performs benzene oxidation via benzene epoxide with an NIH shift.20 Similarly, epoxide was observed in the oxidation of octafluoronaphthalene.25 Using 1,4-difluorobenzene, we detected NIH-shifted 2,4-difluorophenol (Figure 17).

Figure 15. Catalytic heterogeneous defluorination by the carbonsupported 2/H2O2 system in D2O at 60 °C for 15 h. The catalyst:substrate:oxidant ratio was 1:1000:26000. Figure 17. Detection of fluorine NIH shift in the oxidation of 1,4difluorobenzene.

bonds 26) and their exceptional stability lead to their accumulation in the environment. In contrast to reductive and organometallic approaches demanding anhydrous and inert conditions, the 2/H2O2 system is tolerant to air and water. μNitrido diiron catalysts showed promising results in the degradation of chlorinated phenols and Orange II.27,28 Reactivity studies of the 2/H2O2 system toward C−F, C−Cl, and C−H bonds shed light on the mechanism of defluorination. Four mechanistic hypotheses were considered: free-radical Fenton oxidation, nucleophilic substitution, electrophilic attack, and initial epoxidation of the aromatic cycle. The available data are in agreement with the epoxidation mechanism. The 2/H2O2

A tentative reaction pathway is proposed in Figure 18. The first step could involve epoxidation of a fluoroarene. The fluoroarene epoxide would rearrange to form phenol via a ketone intermediate, giving rise to a rarely observed fluorine NIH shift. Further oxidation of phenol would produce fluoranil, which would undergo ring cleavage via an epoxidation/epoxide hydrolysis/HF elimination sequence to provide difluoromaleic, fluorooxaloacetic, and oxalic acid and finally HF and CO2. The proposed pathway agrees with all of the available data.

Figure 16. Heterogeneous transformation of C6F5OH in deuterated water. G


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Figure 18. Proposed pathway for oxidative defluorination. Identified intermediates and products are shown in red. Reproduced from ref 25. Copyright 2014 American Chemical Society.

However, further experimental and theoretical studies are necessary to gain deeper mechanistic insights.

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COMPUTATIONAL STUDIES The high catalytic activity of μ-nitrido diiron species can be considered as arising from the oxoiron(IV) site, with the remaining part of the complex representing an axial ligand. Biochemical studies have shown that hemoprotein function is modulated by the nature of the axial ligand through a push− pull electronic effect. DFT and spectroscopic studies suggest that the electron-donating thiolate axial ligand in P450 might significantly increase the basicity of the FeO moiety, thus improving the thermodynamics of C−H abstraction.29 Most of the work in this area has focused on monomeric hemes, and only limited research on the μ-nitrido dimers is available. Using extended Huckel theory, Tatsumi and Hoffmann30 correctly predicted the μ-nitrido dimer to have a low-spin ground-state configuration and the corresponding μcarbido dimer to be a closed-shell system. Ghosh et al.31 studied the porphyrin μ-nitrido dimer using a generalized gradient approximation functional and found a doublet configuration for the neutral [(por)Fe]2N and a closed-shell configuration for the one-electron-oxidized [(por)Fe]2N+. Full geometry optimization led to a linear Fe−N−Fe fragment, suggesting strong π bonding along the z axis. SilaghiDumitrescu et al.32 compared the electronic structures of the μ-nitrido and μ-oxo diiron porphyrazine cores, and redox noninnocence of the μ-nitrido bridge was proposed as a key feature to explain its catalytic properties. We recently carried out DFT calculations on a series of Fe− X−Fe (X = C, N, O) phthalocyanine complexes to explain the particular activity of the μ-nitrido dimer.33 The structure with staggered macrocycles approximated by D4d symmetry has the canonical orbital manifold shown in Figure 19. Because of the very short Fe−(μ-N) distance, the antibonding a1 singly occupied molecular orbital (SOMO) (dz2−s−dz2) is raised high above the degenerate a2u orbitals of the macrocycle. According to UB86 calculations and EXAFS fitting, the Fe−N−Fe fragment is linear and symmetric. Within the Fe−X−Fe (X = C, N, O) series, the μ-nitrido dimer has the shortest Fe−(μ-N) distance, the highest bond order, and hence the maximal covalence, although formally the μ-carbido complex should have the highest Fe(μ-C) bond order. The doublet spin state was the ground state in all of the calculations, while higher spin states have much higher energies. Broken-symmetry (BS) calculations suggested that antiferro-

Figure 19. Valent-zone orbital energies of (FePc)2N and selected isosurfaces of canonical orbitals obtained from UBP86 (def2-TVZP, CP(PPP)) calculations. Blue bars correspond to predominantly metallic orbitals and rose bars to macrocycle orbitals.

magnetically coupled HS solutions are unfavorable (in contrast to the μ-oxo dimer, for which BS calculations converge to antiferromagnetically coupled HS configurations).33 These conclusions agree with the crystal structure34 and Mössbauer data.6 The presence of one unpaired electron in the μ-nitrido species was evidenced by magnetic measurements (μeff = 2.24μB at 298 K) and EPR spectra for porphyrin, phthalocyanine, and porphyrazine dimers.6 Spin-selective XANES measurements confirmed low local spin density on the iron atoms in the μ-nitrido dimers.10 The results of geometry optimizations critically depend on the applied functional and basis set. Hybrid functionals and/or insufficient basis sets lead to asymmetric solutions with distinct Fe(III)/Fe(IV) sites and very different Fe−(μ-N) distances,35 while all of the experimental data indicate a symmetric Fe−N− Fe fragment.6 Calculations using the BP86 functional with triple-ζ basis sets (LACV3P+, 6-311+G*, TVZP) accurately predict the geometries and XANES spectra. In contrast, B3LYP is biased toward unsymmetric solutions, though a correct geometry could be recovered using very large basis sets. B3LYP and B3LYP* are known to perform poorly in predicting geometries of dinuclear and tetranuclear Fe complexes.36 In summary, low-spin μ-nitrido dimers possess unusually strong covalent bonding within the Fe−N−Fe fragment. Strong overlap between the dz2 orbital of iron and orbitals of the bridging nitrogen raises the SOMO and makes this molecule H


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Figure 20. Spin densities in dimeric and monomeric oxo species.

which is σ or π for HS and only π in the LS systems.38 The increased Fe(μ-N)FeO reactivity can be associated with the LS pathway and attributed to the low-energy SOMO stabilized by conjugation along the z axis (px,y of O, N and dxz,yz of Fe). The energy of the lowest unoccupied orbital in Fe(μ-N)FeO is −4.92 eV, among the lowest reported38 and much lower than the energy of the SOMO for the corresponding monomer (−4.08 eV). In the recent B3LYP calculations, unsymmetrical geometries were obtained for the Fe+3.5(μ-N)Fe+3.5 porphyrin complexes and their oxo species.35 In such structures the π conjugation along the z axis is broken and the antiferromagnetically coupled HS configuration is favored. However, the experimental data evidence the essentially LS nature of the Fe+3.5(μ-N)Fe+3.5 complexes. The DFT-optimized geometric parameters of Fe(μN)FeO species depend on the functional used (Figure 22).33

prone to one-electron oxidation toward the high-valent FeIVFeIV state. High-Valent Oxo Complexes

The factors determining the reactivity of mononuclear FeIVO species have been the subject of intense research.13 The accessibility of multiple spin states depending on the nature of the ligand was invoked to explain differences in reactivity. Computational studies of monomeric species suggest higher reactivity of the S = 2 FeIVO species compared with the S = 1 counterparts.37 Diiron species do not follow this trend. DFT calculations and experimental data show that FeIV(μ-N)FeIV O species have an S = 1/2 ground state well-separated from the higher spin states. In contrast, monomeric FeIVO species have low-lying excited quartet spin states. Thus, the monomeric (Pc+•)(Cl)FeIVO complex bears two unpaired electrons in the FeO π* orbital that are antiferromagnetically coupled to one electron delocalized in the macrocycle a2 orbital (Figure 20). The addition of an oxygen atom to the μ-nitrido dimer lowers the symmetry to C4v and removes the degeneracy of the energy levels of the two marcocycles. The a2 orbital of the macrocycle bearing FeIVO becomes 0.25 eV higher than the a2 orbital of the second macrocycle (Figure 21). The SOMO of (Pc)Fe(μ-N)Fe(Pc)O is an antibonding orbital with e symmetry and strong π* FeO character that is delocalized over the z-axis fragment. Recently, a descriptor of the FeIVO reactivity was defined as the energy of the LUMO (SOMO),

Figure 22. Geometries of the Fe(μ-N)FeO species optimized using the B3LYP and BP86 functionals.

The B3LYP and OLYP hybrid functionals tend to predict unsymmetrical geometries with a long Fe−(μ-N) bond for the oxygen-bearing iron (2.05−2.1 Å) and a short FeO bond (1.61−1.63 Å). In contrast, BP86 predicts a weaker perturbation of the Fe−N−Fe fragment and a longer FeO bond (1.73 Å). In the unsymmetrical B3LYP structures, π conjugation along the z axis is lost, as they contain short Fe1 O and Fe2(μ-N) bonds separated by a very long Fe1−(μ-N) single bond. As expected, with a decrease in the Fe1−(μ-N) bonding in the B3LYP structures, HS configurations become preferred. BP86 describes the experimental XAS spectra and the geometry of 3 much better than B3LYP.33 We suppose the same to be true for (Pc)Fe(μ-N)Fe(Pc)O. There seems to be no rationale why the addition of an axial oxygen ligand would transform them to the HS state. A correct description of these species is very important for understanding their reactivity. Overall, Fe(μ-N)FeO species seem to have particularly strong π stabilization, resulting in the unusually low SOMO. The relatively long FeO bond bearing considerable spin on the terminal oxygen might have an enhanced reactivity due its

Figure 21. Valent-zone orbital energies of (Pc)Fe(μ-N)Fe(Pc)O and selected isosurfaces of canonical orbitals obtained from UBP86 (def2-TVZP, CP(PPP)) calculations. I


Article

Accounts of Chemical Research Biographies

oxyl radical character. Weaker bonding of the terminal oxygen correlates with its higher reactivity, but the exact details of the reaction with the strong C−H bonds are still to be determined.

Pavel Afanasiev got his Ph.D. in 1989 at Moscow State University. Since 1992 he has worked at the IRCELYON in Villeurbanne, France. His research interests have a wide scope from synthesis of inorganic materials to XAS spectroscopy and DFT calculations.

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CONCLUSION AND OUTLOOK Traditional model catalysts mimic the mononuclear porphyrin active site of cytochrome P450 or the non-heme diiron site of sMMO.1,2,4 We have proposed a binuclear macrocyclic scaffold for the oxidation catalysts, which is not used by nature to construct monooxygenases. Can μ-nitrido complexes be considered as bioinspired? Their common feature with sMMO is the diiron site, but it is in a macrocyclic environment. The iron sites are coordinated to exogenous phthalocyanine ligands or structurally related endogenous porphyrin cores. Most importantly, μ-nitrido dimers are able to form high-valent iron−oxo species similar to enzymes and show the mechanistic features (formation of benzene oxide, NIH shift) typical of enzymes operating via high-valent iron−oxo species. Therefore, μ-nitrido diiron macrocyclic complexes can be considered as bioinspired catalysts. As a result of their particular electronic structure, these complexes are much stronger oxidants and even exhibit reactivity not observed with enzymes, e.g., oxidative defluorination of perfluoroaromatics. Their catalytic properties can be tuned by the appropriate structural modifications. The higher activity of iron sites supported by electron-donating phthalocyanine ligands was evidenced by combined experimental spectroscopic studies and DFT calculations.16 The different reactivities of μ-nitrido diiron complexes in the presence of H2O2 and tBuOOH point to their rich redox chemistry, which can be used for diverse catalytic reactions, including C−C bond formation. Of particular significance is the discovery of oxidative C−F activation, which could have not only fundamental but also practical interest. For a long time, fluorine chemistry has been focused on the syntheses of novel fluorinated molecules. Fluorinated compounds accumulate in the environment because of their large-scale use and resistance to biodegradation. The development of remediation methods for these emerging pollutants is therefore of increasing importance. The oxidative approach seems to be very promising since the catalytic system including the iron catalyst and H2O2 is cheap and nontoxic, tolerant to water and air, and applicable to a large substrate scope. This approach has a great potential for further development. Different macrocyclic ligands (porphyrins, phthalocyanines, porphyrazines, corroles, etc.) can be used to support homometallic or heterometallic M−X−M units with various metals and bridging groups. Moreover, a variety of homoleptic and heteroleptic complexes are potentially available. Structural variation should considerably affect the electronic structure and could therefore be used to tune the catalytic properties to provide access to improved and tailored catalysts for many reactions. We hope that the unusual properties of μ-nitrido diiron complexes will stimulate further studies and open new possibilities in biomimetic catalysis.

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Alexander B. Sorokin obtained his Ph.D. in chemistry from the Institute of Chemical Physics, Chernogolovka, Russia. After postdoctoral work with Dr. Meunier at LCC in Toulouse, he joined the IRCELYON in Villeurbanne in 1997. His research focuses on bioinspired oxidation and phthalocyanine catalytic chemistry.

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ACKNOWLEDGMENTS We thank our co-workers for their valuable contributions to the development of this topic. Research support was provided by the Agence Nationale de Recherche (ANR), France (Grant ANR-08-BLANC-0183-01), CNRS (PICS Project 6295), and Région Rhône-Alpes.

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*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. J


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Î¼-Nitrido Diiron Macrocyclic Platform: Particular Structure for Particular

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