Facile and Reversible Formation of Iron(III)–Oxo–Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce^IV(NH₄)₂(NO₃)₆ is often used in artificial water oxidation and generally considered to be an outer‐sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe^III‐O‐Ce^IV(OH₂)(NO₃)₄]⁺ ( 3 ), a complex obtained from the reaction of [(N4Py)Fe^II(NCMe)]²⁺ with 2 equiv CAN or [(N4Py)Fe^IV=O]²⁺ ( 2 ) with Ce^III(NO₃)₃ in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S =1 Fe^IV in 2 to S =5/2 in 3 , which is found to be facile despite the formal spin‐forbidden nature of this process. This observation suggests that Fe^IV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.     

Tuning the reactivity of mononuclear nonheme manganese(iv)-oxo complexes by triflic acid

Triflic acid (HOTf)-bound nonheme Mn(iv)-oxo complexes, [(L)Mn^IV(O)]²⁺–(HOTf)₂ (L = N4Py and Bn-TPEN; N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine and Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine), were synthesized by adding HOTf to the solutions of the [(L)Mn^IV(O)]²⁺ complexes and were characterized by various spectroscopies. The one-electron reduction potentials of the Mn^IV(O) complexes exhibited a significant positive shift upon binding of HOTf. The driving force dependences of electron transfer (ET) from electron donors to the Mn^IV(O) and Mn^IV(O)–(HOTf)₂ complexes were examined and evaluated in light of the Marcus theory of ET to determine the reorganization energies of ET. The smaller reorganization energies and much more positive reduction potentials of the [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes resulted in greatly enhanced oxidation capacity towards one-electron reductants and para-X-substituted-thioanisoles. The reactivities of the Mn(iv)-oxo complexes were markedly enhanced by binding of HOTf, such as a 6.4 × 10⁵-fold increase in the oxygen atom transfer (OAT) reaction (i.e., sulfoxidation). Such a remarkable acceleration in the OAT reaction results from the enhancement of ET from para-X-substituted-thioanisoles to the Mn^IV(O) complexes as revealed by the unified ET driving force dependence of the rate constants of OAT and ET reactions of [(L)Mn^IV(O)]²⁺–(HOTf)₂. In contrast, deceleration was observed in the rate of H-atom transfer (HAT) reaction of [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes with 1,4-cyclohexadiene as compared with those of the [(L)Mn^IV(O)]²⁺ complexes. Thus, the binding of two HOTf molecules to the Mn^IV(O) moiety resulted in remarkable acceleration of the ET rate when the ET is thermodynamically feasible. When the ET reaction is highly endergonic, the rate of the HAT reaction is decelerated due to the steric effect of the counter anion of HOTf.     

Comparison of Nonheme Manganese- and Iron-Containing Flavone Synthase Mimics

Heme and nonheme-type flavone synthase enzymes, FS I and FS II are responsible for the synthesis of flavones, which play an important role in various biological processes, and have a wide range of biomedicinal properties including antitumor, antimalarial, and antioxidant activities. To get more insight into the mechanism of this curious enzyme reaction, nonheme structural and functional models were carried out by the use of mononuclear iron, [Fe^II(CDA-BPA*)]²⁺ (6) [CDA-BPA = N,N,N’,N’-tetrakis-(2-pyridylmethyl)-cyclohexanediamine], [Fe^II(CDA-BQA*)]²⁺ (5) [CDA-BQA = N,N,N’,N’-tetrakis-(2-quinolilmethyl)-cyclohexanediamine], [Fe^II(Bn-TPEN)(CH₃CN)]²⁺ (3) [Bn-TPEN = N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane], [Fe^IV(O)(Bn-TPEN)]²⁺ (9), and manganese, [Mn^II(N4Py*)(CH₃CN)]²⁺ (2) [N4Py* = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine)], [Mn^II(Bn-TPEN)(CH₃CN)]²⁺ (4) complexes as catalysts, where the possible reactive intermediates, high-valent Fe^IV(O) and Mn^IV(O) are known and well characterised. The results of the catalytic and stoichiometric reactions showed that the ligand framework and the nature of the metal cofactor significantly influenced the reactivity of the catalyst and its intermediate. Comparing the reactions of [Fe^IV(O)(Bn-TPEN)]²⁺ (9) and [Mn^IV(O)(Bn-TPEN)]²⁺ (10) towards flavanone under the same conditions, a 3.5-fold difference in reaction rate was observed in favor of iron, and this value is three orders of magnitude higher than was observed for the previously published [Fe^IV(O)(N2Py2Q*)]²⁺ [N,N-bis(2-quinolylmethyl)-1,2-di(2-pyridyl)ethylamine] species.     

Facile and Reversible Formation of Iron(III)–Oxo–Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce^IV(NH₄)₂(NO₃)₆ is often used in artificial water oxidation and generally considered to be an outer-sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe^III-O-Ce^IV(OH₂)(NO₃)₄]⁺ ( 3 ), a complex obtained from the reaction of [(N4Py)Fe^II(NCMe)]²⁺ with 2 equiv CAN or [(N4Py)Fe^IV=O]²⁺ ( 2 ) with Ce^III(NO₃)₃ in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S=1 Fe^IV in 2 to S=5/2 in 3 , which is found to be facile despite the formal spin-forbidden nature of this process. This observation suggests that Fe^IV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.     

Switchover of the Mechanism between Electron Transfer and Hydrogen‐Atom Transfer for a Protonated Manganese(IV)–Oxo Complex by Changing Only the Reaction Temperature

Hydroxylation of mesitylene by a nonheme manganese(IV)–oxo complex, [(N4Py)Mn^IV(O)]²⁺ ( 1 ), proceeds via one‐step hydrogen‐atom transfer (HAT) with a large deuterium kinetic isotope effect (KIE) of 3.2(3) at 293 K. In contrast, the same reaction with a triflic acid‐bound manganese(IV)‐oxo complex, [(N4Py)Mn^IV(O)]²⁺‐(HOTf)₂ ( 2 ), proceeds via electron transfer (ET) with no KIE at 293 K. Interestingly, when the reaction temperature is lowered to less than 263 K in the reaction of 2 , however, the mechanism changes again from ET to HAT with a large KIE of 2.9(3). Such a switchover of the reaction mechanism from ET to HAT is shown to occur by changing only temperature in the boundary region between ET and HAT pathways when the driving force of ET from toluene derivatives to 2 is around ?0.5 eV. The present results provide a valuable and general guide to predict a switchover of the reaction mechanism from ET to the others, including HAT.     

Mild and Practical Dirhodium(II)/NHPI-Mediated Allylic and Benzylic Oxidations with Air as the Oxidant

Aerobic allylic and benzylic oxidations catalyzed by dirhodium(II) complexes with N-hydroxyphthalimide (NHPI) are described. The open flask reaction occurs at mild temperature, using air as the oxidant. Mechanistic studies suggest that dirhodium(II) complexes axially coordinate with NHPI to activate the O−H bond in NHPI and decrease the bond-dissociation energy (BDE).     

Structural Characterization of a Series of N5-Ligated MnIV-Oxo Species

Analysis of extended X-ray absorption fine structure (EXAFS) data for the Mn^IV-oxo complexes [Mn^IV(O)(^DMMN4py)]²⁺, [Mn^IV(O)(2pyN2B)]²⁺, and [Mn^IV(O)(2pyN2Q)]²⁺ (^DMMN4py=N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; 2pyN2B=(N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, and 2pyN2Q=N,N-bis(2-pyridyl)-N,N-bis(2-quinolylmethyl)methanamine) afforded Mn=O and Mn−N bond lengths. The Mn=O distances for [Mn^IV(O)(^DMMN4py)]²⁺ and [Mn^IV(O)(2pyN2B)]²⁺ are 1.72 and 1.70 Å, respectively. In contrast, the Mn=O distance for [Mn^IV(O)(2pyN2Q)]²⁺ was significantly longer (1.76 Å). We attribute this long distance to sample heterogeneity, which is reasonable given the reduced stability of [Mn^IV(O)(2pyN2Q)]²⁺. The Mn=O distances for [Mn^IV(O)(^DMMN4py)]²⁺ and [Mn^IV(O)(2pyN2B)]²⁺ could only be well-reproduced using DFT-derived models that included strong hydrogen-bonds between second-sphere solvent 2,2,2-trifluoroethanol molecules and the oxo ligand. These results suggest an important role for the 2,2,2-trifluoroethanol solvent in stabilizing Mn^IV-oxo adducts. The DFT methods were extended to investigate the structure of the putative [Mn^IV(O)(N4py)]²⁺⋅(HOTf)₂ adduct. These computations suggest that a Mn^IV-hydroxo species is most consistent with the available experimental data.     

Enantioselective Copper(I)/Chiral Phosphoric Acid Catalyzed Intramolecular Amination of Allylic and Benzylic C−H Bonds

Radical-involved enantioselective oxidative C−H bond functionalization by a hydrogen-atom transfer (HAT) process has emerged as a promising method for accessing functionally diverse enantioenriched products, while asymmetric C(sp³)−H bond amination remains a formidable challenge. To address this problem, described herein is a dual Cu^I/chiral phosphoric acid (CPA) catalytic system for radical-involved enantioselective intramolecular C(sp³)−H amination of not only allylic positions but also benzylic positions with broad substrate scope. The use of 4-methoxy-NHPI (NHPI=N-hydroxyphthalimide) as a stable and chemoselective HAT mediator precursor is crucial for the fulfillment of this transformation. Preliminary mechanistic studies indicate that a crucial allylic or benzylic radical intermediate resulting from a HAT process is involved.     

Beneficial Effect of Acetic Acid on the Formation of FeIII(OOH) Species and on the Catalytic Activity of Bioinspired Nonheme FeII Complexes.

Three new amine/pyridine Fe^II complexes bearing pentadentate ligand with one, two or three electron enriched 4-methoxy-3,5-dimethylpyridine were used as catalysts for the oxidation of small organic molecules by hydrogen peroxide. The distribution of products formed suggests that these ligands are not enough electron donating to promote the O−O heterolytic cleavage of the oxidant in order to generate selective Fe^V(O) species. Using acetic acid in the reaction mixtures results in a significant increase of the efficiency of these catalytic systems. Our investigations show that the use of AcOH leads to the protonation/dissociation of a pyridyl moiety and the formation of (N₄)Fe^II(OAc)(OH) species. These complexes readily react with excess hydrogen peroxide to yield (N₄)Fe^III(OAc)(OOH) intermediates. These latter intermediates are proposed to evolve into (N₄)Fe^IV(OAc)(O), which are more efficient than the usual (N₄)Fe^IV(O) and (N₅)Fe^IV(O).     

Generation of FeIV=O and its Contribution to Fenton-Like Reactions on a Single-Atom Iron−N−C Catalyst

Generating Fe^IV=O on single-atom catalysts by Fenton-like reaction has been established for water treatment; however, the Fe^IV=O generation pathway and oxidation behavior remain obscure. Employing an Fe−N−C catalyst with a typical Fe−N₄ moiety to activate peroxymonosulfate (PMS), we demonstrate that generating Fe^IV=O is mediated by an Fe−N−C−PMS* complex—a well-recognized nonradical species for induction of electron-transfer oxidation—and we determined that adjacent Fe sites with a specific Fe₁−Fe₁ distance are required. After the Fe atoms with an Fe₁-Fe₁ distance <4 Å are PMS-saturated, Fe−N−C−PMS* formed on Fe sites with an Fe₁-Fe₁ distance of 4–5 Å can coordinate with the adjacent Fe^II−N₄, forming an inter-complex with enhanced charge transfer to produce Fe^IV=O. Fe^IV=O enables the Fenton-like system to efficiently oxidize various pollutants in a substrate-specific, pH-tolerant, and sustainable manner, where its prominent contribution manifests for pollutants with higher one-electron oxidation potential.     

Mechanistic Insight into the Nitric Oxide Dioxygenation Reaction of Nonheme Iron(III)–Superoxo and Manganese(IV)–Peroxo Complexes

Reactions of nonheme Fe^III–superoxo and Mn^IV–peroxo complexes bearing a common tetraamido macrocyclic ligand (TAML), namely [(TAML)Fe^III(O₂)]^2? and [(TAML)Mn^IV(O₂)]^2?, with nitric oxide (NO) afford the Fe^III–NO₃ complex [(TAML)Fe^III(NO₃)]^2? and the Mn^V–oxo complex [(TAML)Mn^V(O)]^? plus NO₂^?, respectively. Mechanistic studies, including density functional theory (DFT) calculations, reveal that M^III–peroxynitrite (M=Fe and Mn) species, generated in the reactions of [(TAML)Fe^III(O₂)]^2? and [(TAML)Mn^IV(O₂)]^2? with NO, are converted into M^IV(O) and ^.NO₂ species through O?O bond homolysis of the peroxynitrite ligand. Then, a rebound of Fe^IV(O) with ^.NO₂ affords [(TAML)Fe^III(NO₃)]^2?, whereas electron transfer from Mn^IV(O) to ^.NO₂ yields [(TAML)Mn^V(O)]^? plus NO₂^?.     

A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3O Macrocyclic Ligand

The sluggish oxidants [Fe^IV(O)(TMC)(CH₃CN)]²⁺ (TMC=1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane) and [Fe^IV(O)(TMCN‐d₁₂)(OTf)]⁺ (TMCN‐d₁₂=1,4,7,11‐tetra(methyl‐d₃)‐1,4,7,11‐tetraazacyclotetradecane) are transformed into the highly reactive oxidant [Fe^IV(O)(TMCO)(OTf)]⁺ ( 1 ; TMCO=4,8,12‐trimethyl‐1‐oxa‐4,8,12‐triazacyclotetradecane) upon replacement of an NMe donor in the TMC and TMCN ligands by an O atom. A rate enhancement of five to six orders of magnitude in both H atom and O atom transfer reactions was observed upon oxygen incorporation into the macrocyclic ligand. This finding was explained in terms of the higher electrophilicity of the iron center and the higher availability of the more reactive S=2 state in 1 . This rationalizes nature's preference for using O‐rich ligand environments for the hydroxylation of strong C−H bonds in enzymatic reactions.     

Intramolecular Gas‐Phase Reactions of Synthetic Nonheme Oxoiron(IV) Ions: Proximity and Spin‐State Reactivity Rules

The intramolecular gas‐phase reactivity of four oxoiron(IV) complexes supported by tetradentate N₄ ligands ( L ) has been studied by means of tandem mass spectrometry measurements in which the gas‐phase ions [Fe^IV(O)( L )(OTf)]⁺ (OTf=trifluoromethanesulfonate) and [Fe^IV(O)( L )]²⁺ were isolated and then allowed to fragment by collision‐induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane (tmc) and N,N′‐bis(2‐pyridylmethyl)‐1,5‐diazacyclooctane ( L ⁸Py₂) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N′‐bis(2‐pyridylmethyl)‐1,2‐diaminoethane (bpmen) and N,N′‐bis(2‐pyridylmethyl)‐1,3‐diaminopropane (bpmpn) showed predominant oxidative N‐dealkylation for the hexacoordinate [Fe^IV(O)( L )(OTf)]⁺ cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe^IV(O)( L )]²⁺ cations. DFT calculations on [Fe^IV(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N‐dealkylation versus dehydrogenation of the diaminoethane linker for the hexa‐ and pentacoordinate ions, respectively, is dictated by the proximity of the target C? H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σ_CH–σ* overlap, all the reactions still proceed preferentially on the quintet (S=2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S=2 reactions for both hexa‐ and pentacoordinate complexes.     

NMR Reveals That a Highly Reactive Nonheme FeIV=O Complex Retains Its Six-Coordinate Geometry and S=1 State in Solution

The [Fe^IV(O)(Me₃NTB)]²⁺ (Me₃NTB=tris[(1-methyl-benzimidazol-2-yl)methyl]amine) complex 1 has been shown by Mössbauer spectroscopy to have an S=1 ground state at 4 K, but is proposed to become an S=2 trigonal-bipyramidal species at higher temperatures based on a DFT model to rationalize its very high C−H bond-cleavage reactivity. In this work, ¹H NMR spectroscopy was used to determine that 1 does not have C₃-symmetry in solution and is not an S=2 species. Our results show that 1 is unique among nonheme Fe^IV=O complexes in retaining its S=1 spin state and high reactivity at 193 K, providing evidence that S=1 Fe^IV=O complexes can be as reactive as their S=2 counterparts. This result emphasizes the need to identify factors besides the ground spin state of the Fe^IV=O center to rationalize nonheme oxoiron(IV) reactivity.     

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, [Fe^III(OOH)(L₅²)]²⁺ (L₅²=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 [Fe^II(OH)(L₅²)]⁺ 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 Fe^II(OOH) intermediate generated upon reduction of the Fe^III(OOH) starting complex. One plausible mechanistic scenario is that this disproportionation reaction follows an O−O heterolytic cleavage pathway via a Fe^IV-oxo species.     

Activation and Deactivation of Benzylic C−H Bonds Guided by Stereoelectronic Effects in Hydrogen Atom Transfer from Amides and Amines to Alkoxyl Radicals

Kinetic and product studies on the reactions of tert-alkoxyl radicals with secondary and tertiary alkanamides bearing benzylic α-C−H bonds, isoindoline, tetrahydroisoquinoline and the corresponding N-acetyl derivatives were carried out. Product studies on the reactions with the tert-butoxyl radical (tBuO⋅) point toward exclusive HAT from the benzylic α-C−H bonds. Comparison of the k_H values measured for reaction with the cumyloxyl radical (CumO⋅) with those obtained previously for the corresponding reactions of N-alkyl- and N,N-dialkylalkanamides, are indicative of a lack of benzylic activation and the operation of steric and stereoelectronic effects. Compared to N-methyl and N-ethyl groups, the presence of N-benzyl groups increases the barrier required to reach the optimal conformation for HAT, where the α-C−H bond to be cleaved is perpendicular to the plane of the amide, precluding concurrent overlap with the phenyl π-system. When the benzylic α-C−H bonds are in a conformation that allows for optimal overlap with both the phenyl π-system and the amide π-system or amine nitrogen lone pair, as in the isoindoline and tetrahydroisoquinoline derivatives, increases in k_H that exceed 2-orders of magnitude were observed, highlighting the strong contribution provided by stereoelectronic activation to these HAT processes.     

Fluorescent oligo(N‐phenylmaleimide)s via aerobic radical telomerization initiated by benzylic hydrocarbons: Catalytic effect of CoII/N‐hydroxyphthalimide pair

Radical oligomerization of N‐phenylmaleimide (NPMI) was performed in benzylic hydrocarbons as the solvent. The thermally induced oligomerization occurred only above 130 °C, with the initiation attributed to autoxidation of benzylic hydrocarbons as well as formation and dissociation of charge‐transfer complexes between benzylic hydrocarbons and maleimides. The end‐group analysis on oligo(N‐ethylmaleimide) prepared under similar conditions confirmed that the chain transfer to benzylic hydrocarbons was the primary fashion in forming oligomeric chains, and radical telomerization underlaid the oligomerization with benzylic hydrocarbons as both the solvent, the initiator and the telogen. Co^II/N‐hydroxyphthalimide (NHPI) pairs could catalyze the telomerization at 110 °C. In such a catalytic process, Co^II‐based oxidative complexes oxidized benzylic hydrocarbons and NHPI into benzylic radicals and phthalimide N‐oxyl (PINO), and benzylic hydrocarbons underwent hydrogen atom transfer (HAT) to PINO. Oligo(NPMI)s were formed via HAT with benzylic hydrocarbons and NHPI. These oligo(NPMI)s exhibited fluorescent properties with excitation at 270–350 nm and 400–550 nm and emission at 530–750 nm. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3846–3857     

Hydroxylation of Aromatics by H2O2 Catalyzed by Mononuclear Non-heme Iron Complexes: Role of Triazole Hemilability in Substrate-Induced Bifurcation of the H2O2 Activation Mechanism

Rieske dioxygenases are metalloenzymes capable of achieving cis-dihydroxylation of aromatics under mild conditions using O₂ and a source of electrons. The intermediate responsible for this reactivity is proposed to be a cis-Fe^V(O)(OH) moiety. Molecular models allow the generation of a Fe^III(OOH) species with H₂O₂, to yield a Fe^V(O)(OH) species with tetradentate ligands, or {Fe^IV(O); OH^.} pairs with pentadentate ones. We have designed a new pentadentate ligand, mtL₄², bearing a labile triazole, to generate an “in-between” situation. Two iron complexes, [(mtL₄²)FeCl](PF₆) and [(mtL₄²)Fe(OTf)₂]), were obtained and their reactivity towards aromatic substrates was studied in the presence of H₂O₂. Spectroscopic and kinetic studies reflect that triazole is bound at the Fe^II state, but decoordinates in the Fe^III(OOH). The resulting [(mtL₄²)Fe^III(OOH)(MeCN)]²⁺ then lies on a bifurcated decay pathway (end-on homolytic vs. side-on heterolytic) depending on the addition of aromatic substrate: in the absence of substrate, it is proposed to follow a side-on pathway leading to a putative (N₄)Fe^V(O)(OH), while in the presence of aromatics it switches to an end-on homolytic pathway yielding a {(N₅)Fe^IV(O); OH^.} reactive species, through recoordination of triazole. This switch significantly impacts the reaction regioselectivity.     

Electrochemical study of a nonheme Fe(ii) complex in the presence of dioxygen. Insights into the reductive activation of O2 at Fe(ii) centers

Recent efforts to model the reactivity of iron oxygenases have led to the generation of nonheme Fe^III(OOH) and Fe^IV(O) intermediates from Fe^II complexes and O₂ but using different cofactors. This diversity emphasizes the rich chemistry of nonheme Fe(ii) complexes with dioxygen. We report an original mechanistic study of the reaction of [(TPEN)Fe^II]²⁺ with O₂ carried out by cyclic voltammetry. From this Fe^II precursor, reaction intermediates such as [(TPEN)Fe^IV(O)]²⁺, [(TPEN)Fe^III(OOH)]²⁺ and [(TPEN)Fe^III(OO)]⁺ have been chemically generated in high yield, and characterized electrochemically. These electrochemical data have been used to analyse and perform simulation of the cyclic voltammograms of [(TPEN)Fe^II]²⁺ in the presence of O₂. Thus, several important mechanistic informations on this reaction have been obtained. An unfavourable chemical equilibrium between O₂ and the Fe^II complex occurs that leads to the Fe^III-peroxo complex upon reduction, similarly to heme enzymes such as P450. However, unlike in heme systems, further reduction of this latter intermediate does not result in O–O bond cleavage.     

Nonheme Iron Imido Complexes Bearing a Non-Innocent Ligand: A Synthetic Chameleon Species in Oxidation Reactions

High-valent iron-imido complexes can perform C−H activation and sulfimidation reactions, but are far less studied than the more ubiquitous iron-oxo species. As case studies, we have looked at a recently published iron(V)-imido ligand π-cation radical complex, which is formally an iron(VI)-imido complex [Fe^V(NTs)(TAML^+.)] ( 1 ; NTs=tosylimido), and an iron(V)-imido complex [Fe^V(NTs)(TAML)]⁻ ( 2 ). Using a theoretical approach, we found that they have multiple energetically close-lying electromers, sometimes even without changing spin states, reminiscent of the so-called Compound I in Cytochrome P450. When studying their reactivity theoretically, it is indeed found that their electronic structures may change to perform efficient oxidations, emulating the multi-spin state reactivity in Fe^IVO systems. This is actually in contrast to the known [Fe^V(O)(TAML)]⁻ species ( 3 ), where the reactions occur only on the ground spin state. We also looked into the whole reaction pathway for the C−H bond activation of 1,4-cyclohexadiene by these intermediates to reproduce the experimentally observed products, including steps that usually attract no interest (neither theoretically nor experimentally) due to their non-rate-limiting status and fast reactivity. A new “clustering non-rebound mechanism” is presented for this C−H activation reaction.