Oxidative properties of FeO2+: electronic structure and solvation effects

Unraveling the Geometry-Driven C═C Epoxidation and C-H Hydroxylation Reactivity of Tetra-Coordinated Nonheme Iron(IV)-Oxo Complexes

The electronic structure and reactivity of tetra-coordinated nonheme iron(IV)-oxo complexes have remained unexplored for years. The recent synthesis of a closed-shell iron(IV)-oxo complex [(quinisox)FeIV(O)]+ (1) has set up a platform to understand how such complexes compare with the celebrated open-shell iron-oxo chemistry. Herein, using density functional theory and ab initio calculations, we present an in-depth electronic structure investigation of the C═C epoxidation [oxygen atom transfer (OAT)] and C-H hydroxylation [hydrogen atom transfer (HAT)] reactivity of 1. Using a solvent-coordinated geometry of 1 (1') and other potential tetra-coordinated iron(IV)-oxo complexes bearing rigid ligands (2 and 3), we established the geometric origin of spin-state energetics and reactivity of 1. Complex 1 featuring a strong Fe-O bond exhibits OAT and HAT reactivity in its quintet state. The lowest quintet OAT pathway has a lower barrier by ∼4 kcal/mol than the quintet HAT pathway, corroborating the experimentally observed gas-phase OAT reactivity preference. A conventional HAT reactivity preference for 2 and a comparable OAT and HAT reactivity for 3 are observed. This further supports the geometry-driven reactivity preference for 1. Noncovalent interaction analyses reveal a pronounced π-π interaction between the substrate and ligand in the OAT transition state, rationalizing the origin of the observed reactivity preference for 1.

How Axial Coordination Regulates the Electronic Structure and C-H Amination Reactivity of Fe-Porphyrin-Nitrene?

Detailed electronic structure and its correlation with the intramolecular C-H amination reactivity of Fe-porphyrin-nitrene intermediates bearing different "axial" coordination have been investigated using multiconfigurational complete active space self-consistent field (CASSCF), N-electron valence perturbation theory (NEVPT2), and hybrid density functional theory (DFT-B3LYP) calculations. Three types of "axial" coordination, -OMe/-O(H)Me (1-Sul/2-Sul), -SMe/-S(H)Me (3-Sul/4-Sul), and -NMeIm (MeIm = 3-methyl-imidazole) (5-Sul) mimicking serine, cysteine, and histidine, respectively, along with no axial coordination (6-Sul) have been considered to decipher how the "axial" coordination of different strengths regulates the electronic integrity of the Fe-N core and nitrene-transfer reactivity of Fe-porphyrin-nitrene intermediates. CASSCF-based natural orbitals reveal two distinct classes of electronic structures: Fe-nitrenes (1-Sul and 3-Sul) with relatively stronger axial coordination (-OMe and -SMe) display "imidyl" nature and those (2-Sul, 4-Sul, and 6-Sul) with weaker axial coordination (-O(H)Me, -S(H)Me and no axial coordination) exhibit "imido-like" character. A borderline between the two classes is also observed with NMeIm axial coordination (5-Sul). Axial coordination of different strengths not only regulates the electronic structure but also modulates the Fe-3d orbital energies, as revealed through the d-d transition energies obtained by CASSCF/NEVPT2 calculations. The relatively lower energy of Fe-3dz2 orbital allows easy access to low-lying high-spin quintet states in the cases of weaker "axial" coordination (2-Sul, 4-Sul, and 6-Sul), and the associated hydrogen atom transfer (HAT) reactivity appears to involve two-state triplet-quintet reactivity through minimum energy crossing point (3,5MECP) between the spin states. In stark contrast, Fe-nitrenes with relatively stronger "axial" coordination (1-Sul and 3-Sul) undergo triplet-only HAT reactivity. Overall, this in-depth electronic structure investigation and HAT reactivity evaluation reveal that the weaker axial coordination in Fe-porphyrin-nitrene complexes (2-Sul, 4-Sul, and 6-Sul) can promote more efficient C-H oxidation through the quintet spin state.

Change in the Nature of ZSM-5 Zeolite Depending on the Type of Metal Adsorbent-The Analysis of DOS and Orbitals for Iron Species

Transition-metal-modified zeolites have recently gained the greatest interest among scientists. Ab initio calculations within the density functional theory were used. The exchange and correlation functional was approximated with the Perdew-Burke-Ernzerhof (PBE) functional. Cluster models of ZSM-5 (Al₂Si₁₈O₅₃H₂₆) zeolites were used with Fe particles adsorbed above aluminum. The adsorption of three iron adsorbates inside the pores of the ZSM-5 zeolite-Fe, FeO and FeOH-was carried out with different arrangements of aluminum atoms in the zeolite structure. The DOS diagram and the HOMO, SOMO and LUMO molecular orbitals for these systems were analyzed. It has been shown that depending on the adsorbate and the position of aluminum atoms in the pore structure of the zeolite, the systems can be described as insulators or conductors, which significantly affects their activity. The main aim of the research was to understand the behavior of these types of systems in order to select the most efficient one for a catalytic reaction.

Nitrogen-doped carbon nanotubes enhanced Fenton chemistry: Role of near-free iron(III) for sustainable iron(III)/iron(II) cycles

The sluggish kinetics of Fe(II) recovery strongly impedes the scientific progress of Fenton reaction (Fe(II)/H₂O₂) towards practical application. Here, we propose a novel mechanism that metal-free nitrogen-doped carbon nanotubes (NCNT) can enhance Fenton chemistry with H₂O₂ as electron donors by elevating the oxidation potential of Fe(III). NCNT remarkably promotes the circulation of Fe(III)/Fe(II) to produce hydroxyl radical (^•OH) with excellent stability for multiple usages (more than 10 cycles) in the NCNT/Fe(III)/H₂O₂ system. Although carbonyl on NCNT can act as the electron supplier for Fe(III) reduction, the behavior of NCNT is distinct from common reductants such as hydroxylamine and boron. Electrochemical analysis and density functional theory calculation unveil that nitrogen sites of NCNT can weakly bind with Fe(III) to elevate the oxidation potential of Fe(III) (named near-free Fe(III), primarily FeOH²⁺) at pH ranging from 2.0 to 4.0. Without inputs of external stimulations or electron sacrificers, near-free Fe(III) can promote H₂O₂ induced reduction of Fe(III) to initiate Fenton chain reactions for long-lasting generation of ^•OH. To our delight, it is a common property of N-doped carbon materials (e.g., graphene, carbon nanofibers, and acetylene black), our research thus provides a novel, sustainable, and green strategy for promoting Fenton chemistry.

Anoxic oxidation of As(III) during Fe(II)-induced goethite recrystallization: Evidence and importance of Fe(IV) intermediate

Under anoxic conditions, aqueous Fe(II) (Fe(II)_aq)-induced recrystallization of iron (oxyhydr)oxides changes the speciation and geochemical cycle of trace elements in environments. Oxidation of trace element, i.e., As(III), driven by Fe(II)_aq-iron (oxyhydr)oxides interactions under anoxic condition was observed previously, but the oxidative species and involved mechanisms are remained unknown. In the present study, we explored the formed oxidative intermediates during Fe(II)_aq-induced recrystallization of goethite under anoxic conditions. The methyl phenyl sulfoxide-based probe experiment suggested the featured oxidation by Fe(IV) species in Fe(II)_aq-goethite system. Both the Mössbauer spectra and X-ray absorption near edge structure spectroscopic evidenced the generation and quenching of Fe(IV) intermediate. It was proved that the interfacial electron exchange between Fe(II)_aq and Fe(III) of goethite initiated the generation of Fe(IV). After transferring electrons to goethite, Fe(II)_aq was transformed to labile Fe(III), which was then transformed to Fe(IV) via a proton-coupled electron transfer process. This highly reactive transient Fe(IV) could quickly react with reductive species, i.e. Fe(II) or As(III). Considering the ubiquitous occurrence of Fe(II)-iron (oxyhydr)oxides reactions under anoxic conditions, our findings are expected to provide new insight into the anoxic oxidative transformation processes of matters in non-surface environments on earth.

Catalytic properties of the ferryl ion in the solid state: a computational review

This review summarises the last findings in the emerging field of heterogeneous catalytic oxidation of light alkanes by ferryl species supported on solid-state systems such as the conversion of methane into methanol by FeO-MOF74.

A New Mechanism of the Selective Photodegradation of Antibiotics in the Catalytic System Containing TiO2 and the Inorganic Cations

The mechanism of sulfisoxazole (SFF) selective removal by photocatalysis in the presence of titanium (IV) oxide (TiO2) and iron (III) chloride (FeCl3) was explained and the kinetics and degradation pathways of SFF and other antibiotics were compared. The effects of selected inorganic ions, oxygen conditions, pH, sorption processes and formation of coordination compounds on the photocatalytic process in the presence of TiO2 were also determined. The Fe3+ compounds added to the irradiated sulfonamide (SN) solution underwent surface sorption on TiO2 particles and act as acceptors of excited electrons. Most likely, the SFF degradation is also intensified by organic radicals or cation organic radicals. These radicals can be initially generated by reaction with electron holes, hydroxyl radicals and as a result of electron transfer mediated by iron ions and then participate in propagation processes. The high sensitivity of SFF to decomposition caused by organic radicals is associated with the steric effect and the high bond polarity of the amide substituent.

Unveiling the catalytic potential of the Fe(iv)oxo species for the oxidation of hydrocarbons in the solid state

We have investigated the three steps in the conversion of methane into methanol by Fe(iv)Ooxo species supported in MOF-74. We use ab initio MD and static approximations to predict the reaction barriers using enthalpy ΔH and free energy ΔG.

Regioselective degradation of [beta] 1,3 glucan by ferrous ion and hydrogen peroxide (Fenton oxidation)

Many species use Fe⁺² and H₂O₂ to oxidize a wide variety of compounds to simpler molecules. Both pathogen killing by leukocytes (neutrophils and lymphocytes) and degradation of cellulose by brown rot fungi rely on excretion of Fe⁺² ions and H₂O₂, the Fenton reagent. To elucidate the mechanism of Fenton oxidation of carbohydrates, β1,3 glucan (laminaran), a major fungal wall polysaccharide, was oxidized using a molar ratio of monomer/Fe⁺²/H₂O₂ of 10:1:1 (primarily). We labeled the reaction products and profiled them as fluorescent-labeled molecules in polyacrylamide gels and as hydrophobic-tagged molecules using reverse phase liquid chromatography/mass spectrometry (HPLC/MS). Sub-stoichiometric concentrations of Fe⁺² and H₂O₂ fragmented laminaran into smaller molecules containing carbonyl and carboxylic acid groups visible on fluorescent-labeled carbohydrate polyacrylamide gel electrophoresis. HPLC/MS analysis of glucan fragments showed masses consistent with six classes of molecules: aldoses, dialdoses, uronic acids, hexosuloses, aldonic acids, and hexulosonic acids. The results were consistent with published mechanisms where hydrogen radical (H•) abstraction from a C-H or O-H bond begins a cascade of reactions leading to 1) C-C bond cleavage to produce aldose/dialdose pairs; 2) oxo-group (O = ) addition to produce uronic and aldonic acids; 3) hydroxyl group (HO-) addition to produce gluconolactone and hexosuloses; and 4) hexulosonic acids. Most products resulted from regioselective H• abstractions characteristic of oxidations by ferryl-oxo ion [(FeO)⁺²] or perferryl-oxo ion [(FeO)⁺³] in close contact with specific positions in the glycan. Therefore, oxidations initiated by regioselectively-bound Fe ions were the predominant initiators of polysaccharide degradations.

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.

Density-functional theory models of Fe(iv)O reactivity in metal-organic frameworks: self-interaction error, spin delocalisation and the role of hybrid exchange

We study the reactivity of Fe(iv)O moieties supported by a metal-organic framework (MOF-74) in the oxidation reaction of methane to methanol using all-electron, periodic density-functional theory calculations. We compare results concerning the electronic properties and reactivity obtained using two hybrid (B3LYP and sc-BLYP) and two standard generalised gradient corrected (PBE and BLYP) semi-local density functional approximations. The semi-local functionals are unable to reproduce the expected reaction profiles and yield a qualitatively incorrect representation of the reactivity. Non-local hybrid functionals provide a substantially more reliable description and predict relatively modest (ca. 60 kJ mol^-1) reaction energy barriers for the H-atom abstraction reaction from CH₄ molecules. We examine the origin of these differences and we highlight potential means to overcome the limitations of standard semi-local functionals in reactivity calculations in solid-state systems.

Theoretical Identification of the Factors Governing the Reactivity of C-H Bond Activation by Non-Heme Iron(IV)-Oxo Complexes

Selective functionalization of C-H bonds provides a straightforward approach to a large variety of well-defined derivatives. High-valent mononuclear iron(IV)-oxo complexes are proposed to carry out these C-H activation reactions in enzymes or in biomimetic syntheses. In this Minireview, we aim to highlight the features that delineate the distinct reactivity of non-heme oxo-iron(IV) motifs to cleave strong C-H bonds in hydrocarbons, primarily focusing on the hydrogen atom transfer (HAT) process. We describe how the structural and electronic properties of supporting ligands modulate the oxidative property of the iron(IV)-oxo complexes. Furthermore, we highlight the decisive role played by spin-state in these biomimetic reactions. We also discuss how tunneling and external perturbations like electric field influence the transfer of hydrogen atoms. Lastly, we emphasize how computations could work as a practical guide to sketch and develop synthetic models with greater efficacy.

Why the Reactive Oxygen Species of the Fenton Reaction Switches from Oxoiron(IV) Species to Hydroxyl Radical in Phosphate Buffer Solutions? A Computational Rationale

It has been shown that the major reactive oxygen species (ROS) generated by the aqueous reaction of Fe(II) and H2O2 (i.e., the Fenton reaction) are high-valent oxoiron(IV) species, whereas the hydroxyl radical plays a role only in very acidic conditions. Nevertheless, when the Fenton reaction is conducted in phosphate buffer solutions, the resulting ROS turns into hydroxyl radical even in neutral pH conditions. The present density functional theory (DFT) study discloses the underlying principle for this phenomenon. Static and dynamic DFT calculations indicate that in phosphate buffer solutions, the iron ion is highly coordinated by phosphoric acid anions. Such a coordination environment substantially raises the pK a of coordinated water on Fe(III). As a consequence, the Fe(III)-OH intermediate, resulting from the reductive decomposition of H2O2 by ferrous ion is relatively unstable and will be readily protonated by phosphoric acid ligand or by free proton in solution. These proton-transfer reactions, which become energetically favorable when the number of phosphate coordination goes up to three, prevent the Fe(III)-OH from hydrogen abstraction by nascent •OH to form Fe(IV)=O species. On the basis of this finding, a ligand design strategy toward controlling the nature of ROS produced in the Fenton reaction is put forth. In addition, it is found that while phosphate buffers facilitate •OH radical generation in the Fenton reaction, phosphoric acid anions can act as •OH radical scavengers through hydrogen atom transfer reactions.

Electronic structure and reactivity of Fe(iv)oxo species in metal-organic frameworks

We investigate the potential use of Fe(iv)oxo species supported on a metal-organic framework in the catalytic hydroxylation of methane to produce methanol. We use periodic density-functional theory calculations at the 6-31G**/B3LYP level of theory to study the electronic structure and chemical reactivity in the hydrogen abstraction reaction from methane in the presence of Fe(iv)O(oxo) supported on MOF-74. Our results indicate that the Fe(iv)O moiety in MOF-74 is characterised by a highly reactive (quintet) ground-state, with a distance between Fe(iv) and O(oxo) of 1.601 Å, consistent with other high-spin Fe(iv)O inorganic complexes in the gas phase and in aqueous solution. Similar to the latter systems, the highly electrophilic character (and thus the reactivity) of Fe(iv)O in MOF-74 is determined by the presence of a low-lying anti-bonding virtual orbital (3σ*), which acts as an electron acceptor in the early stages of the hydrogen atom abstraction from methane. We estimate an energy barrier for hydrogen abstraction of 50.77 kJ mol-1, which is comparable to the values estimated in other gas-phase and hydrated Fe(iv)O-based complexes with the ability to oxidise methane. Our findings therefore suggest that metal-organic frameworks can provide suitable supports to develop new solid-state catalysts for organic oxidation reactions.

Strong Inhibition of the Fe3+ + H2O2 Reaction by Ethanol: Evidence against the Free Radical Theory

The effect of ethanol on the catalytic decomposition of H2O2 by Fe3+ was investigated. While expecting a simple competitive oxidation of C2H5OH, far more complex kinetics were encountered experimentally: already minute amounts of C2H5OH (1% of H2O2) had a powerful retardation effect on the disappearance of H2O2. This fact indicated the operation of an intricate mechanism. It excluded the possibility of OH• radicals being the active agents in the oxidation: OH• radicals generated by radiolysis react with C2H5OH with a very high rate constant. The interpretation of the experimental results was based on a mechanism involving iron in a +5 oxidation state (FeO3+) as the active intermediate and its binding in complex structures in which activity is reduced. The question of free radical versus non-radical mechanisms is discussed. The conclusions differ from generally accepted concepts in relation to the Fenton and related reactions.

A computational study of the Fenton reaction in different pH ranges

The mechanism of the Fenton reaction is pH dependent and four distinct reactive species have been identified and found to display quite different oxidation reactivities.

Development of active, nanoparticle, antimicrobial technologies for muscle-based packaging applications

Fresh and processed muscle-based foods are highly perishable food products and packaging plays a crucial role in providing containment so that the full effect of preservation can be achieved through the provision of shelf-life extension. Conventional packaging materials and systems have served the industry well, however, greater demands are being placed upon industrial packaging formats owing to the movement of muscle-based products to increasingly distant markets, as well as increased customer demands for longer product shelf-life and storage capability. Consequently, conventional packaging materials and systems will have to evolve to meet these challenges. This review presents some of the new strategies that have been developed by employing novel nanotechnological concepts which have demonstrated some promise in significantly extending the shelf-life of muscle-based foods by providing commercially-applicable, antimicrobially-active, smart packaging solutions. The primary focus of this paper is applied to subject aspects, such as; material chemistries employed, forming methods utilised, interactions of the packaging functionalities including nanomaterials employed with polymer substrates and how such materials ultimately affect microbes. In order that such materials become industrially feasible, it is important that safe, stable and commercially-viable packaging materials are shown to be producible and effective in order to gain public acceptance, legislative approval and industrial adoption.

C-H Bond Cleavage by Bioinspired Nonheme Oxoiron(IV) Complexes, Including Hydroxylation of n-Butane

The development of efficient and selective hydrocarbon oxidation processes with low environmental impact remains a major challenge of the 21st century because of the strong and apolar nature of the C-H bond. Naturally occurring iron-containing metalloenzymes can, however, selectively functionalize strong C-H bonds on substrates under mild and environmentally benign conditions. The key oxidant in a number of these transformations is postulated to possess an S = 2 Fe(IV)═O unit in a nonheme ligand environment. This oxidant has been trapped and spectroscopically characterized and its reactivity toward C-H bonds demonstrated for several nonheme iron enzyme classes. In order to obtain insight into the structure-activity relationships of these reactive intermediates, over 60 synthetic nonheme Fe(IV)(O) complexes have been prepared in various laboratories and their reactivities investigated. This Forum Article summarizes the current status of efforts in the characterization of the C-H bond cleavage reactivity of synthetic Fe(IV)(O) complexes and provides a snapshot of the current understanding of factors that control this reactivity, such as the properties of the supporting ligands and the spin state of the iron center. In addition, new results on the oxidation of strong C-H bonds such as those of cyclohexane and n-butane by a putative S = 2 synthetic Fe(IV)(O) species that is generated in situ using dioxygen at ambient conditions are presented.

Benchmark study on methanol C-H and O-H bond activation by bare [Fe(IV)O](2+)

We present a high-level computational study on methanol C-H and O-H bond cleavages by bare [Fe(IV)O](2+), as well as benchmarks of various density functional theory (DFT) methods. We considered direct and concerted hydrogen transfer (DHT and CHT) pathways, respectively. The potential energy surfaces were constructed at the CCSD(T)/def2-TZVPP//B3LYP/def2-TZVP level of theory. Mechanistically, (1) the C-H bond cleavage is dominant and the O-H activation only plays minor role on the PESs; (2) the DHT from methyl should be the most practical channel; and (3) electronic structure analysis demonstrates the proton and electron transfer coupling behavior along the reaction coordinates. The solvent effect is evident and plays distinct roles in regulating the two bond activations in different mechanisms during the catalysis. The effect of optimizing the geometries using different density functionals was also studied, showing that it is not meaningful to discuss which DFT method could give the accurate prediction of the geometries, especially for transition structures. Furthermore, the gold-standard CCSD(T) method was used to benchmark 19 different density functionals with different Hartree-Fock exchange fractions. The results revealed that (i) the structural factor plays a minor role in the single point energy (SPE) calculations; (ii) reaction energy prediction is quite challenging for DFT methods; (iii) the mean absolute deviations (MADs) reflect the problematic description of the DFs when dealing with metal oxidation state change, giving a strong correlation on the HF exchange in the DFs. Knowledge from this study should be of great value for computational chemistry, especially for the de novo design of transition metal catalysts.

Ferryl-oxo species produced from Fenton's reagent via a two-step pathway: minimum free-energy path analysis

A mixture of ferrous ions and hydrogen peroxide, known as Fenton's reagent, is an effective oxidant and has been widely used in various industrial applications; however, there is still controversy about what the oxidizing agents are and how they are produced. In this study, we have determined minimum free-energy paths (MFEPs) from Fenton's reagent to possible oxidizing agents such as hydroxyl radicals and ferryl-oxo species by combining ab initio molecular dynamics simulations and an MFEP search method. Along the MFEPs, representative free-energy profiles of the Fenton reaction were elucidated. On the basis of the free-energy profiles, we revealed that the reaction producing ferryl-oxo species from Fenton's reagent is more energetically favorable than that yielding a free hydroxyl radical, by 24.4 kcal mol(-1), which indicates that the ferryl-oxo species is the primary oxidizing agent in reactions of Fenton's reagent. Moreover, we clarified that the ferryl-oxo species is favorably formed via a two-step reaction pathway, which reaches the product through a dihydroxyiron(IV) intermediate. The energetics charting the free-energy profiles provided valuable information for a comprehensive understanding of Fenton reactions. We concluded that a ferryl-oxo species produced from Fenton's reagent serves as the primary oxidizing agent in the Fenton reaction.

Reactivity of compound II: electronic structure analysis of methane hydroxylation by oxoiron(IV) porphyrin complexes

The methane hydroxylation reaction by a Compound II (Cpd II) mimic PorFe(IV)=O and its hydrosulfide-ligated derivative [Por(SH)Fe(IV)=O](-) is investigated by density functional theory (DFT) calculations on the ground triplet and excited quintet spin-state surfaces. On each spin surface both the σ- and π-channels are explored. H-abstraction is invariably the rate-determining step. In the case of PorFe(IV)=O the H-abstraction reaction can proceed either through the classic π-channel or through the nonclassical σ-channel on the triplet surface, but only through the classic σ-mechanism on the quintet surface. The barrier on the quintet σ-pathway is much lower than on the triplet channels so the quintet surface cuts through the triplet surfaces and a two state reactivity (TSR) mechanism with crossover from the triplet to the quintet surface becomes a plausible scenario for C-H bond activation by PorFe(IV)=O. In the case of the hydrosulfide-ligated complex the H-abstraction follows a π-mechanism on the triplet surface: the σ* is too high in energy to make a σ-attack of the substrate favorable. The σ- and π-channels are both feasible on the quintet surface. As the quintet surface lies above the triplet surface in the entrance channel of the oxidative process and is highly destabilized on both the σ- and π-pathways, the reaction can only proceed on the triplet surface. Insights into the electron transfer process accompanying the H-abstraction reaction are achieved through a detailed electronic structure analysis of the transition state species and the reactant complexes en route to the transition state. It is found that the electron transfer from the substrate σ(CH) into the acceptor orbital of the catalyst, the Fe-O σ* or π*, occurs through a rather complex mechanism that is initiated by a two-orbital four-electron interaction between the σ(CH) and the low-lying, oxygen-rich Fe-O σ-bonding and/or Fe-O π-bonding orbitals of the catalyst.

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 N(4) 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(8)Py(2)) 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*(z2) 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.

Hydroxylation catalysis by mononuclear and dinuclear iron oxo catalysts: a methane monooxygenase model system versus the Fenton reagent Fe(IV)O(H2O)5(2+)

Hydroxylation of aliphatic C-H bonds is a chemically and biologically important reaction, which is catalyzed by the oxidoiron group FeO(2+) in both mononuclear (heme and nonheme) and dinuclear complexes. We investigate the similarities and dissimilarities of the action of the FeO(2+) group in these two configurations, using the Fenton-type reagent [FeO(2+) in a water solution, FeO(H(2)O)(5)(2+)] and a model system for the methane monooxygenase (MMO) enzyme as representatives. The high-valent iron oxo intermediate MMOH(Q) (compound Q) is regarded as the active species in methane oxidation. We show that the electronic structure of compound Q can be understood as a dimer of two Fe(IV)O(2+) units. This implies that the insights from the past years in the oxidative action of this ubiquitous moiety in oxidation catalysis can be applied immediately to MMOH(Q). Electronically the dinuclear system is not fundamentally different from the mononuclear system. However, there is an important difference of MMOH(Q) from FeO(H(2)O)(5)(2+): the largest contribution to the transition state (TS) barrier in the case of MMOH(Q) is not the activation strain (which is in this case the energy for the C-H bond lengthening to the TS value), but it is the steric hindrance of the incoming CH(4) with the ligands representing glutamate residues. The importance of the steric factor in the dinuclear system suggests that it may be exploited, through variation in the ligand framework, to build a synthetic oxidation catalyst with the desired selectivity for the methane substrate.

Cu(bipy)2+/TEMPO-catalyzed oxidation of alcohols: radical or nonradical mechanism?

In the oxidation of alcohols with TEMPO as catalyst, the substrate has alternatively been postulated to be oxidized but uncoordinated TEMPO(+) (Semmelhack) or Cu-coordinated TEMPO(•) radical (Sheldon). The reaction with the Cu(bipy)(2+)/TEMPO cocatalyst system has recently been claimed, on the basis of DFT calculations, to not be a radical reaction but to be best viewed as electrophilic attack on the alcohol C-H(α) bond by coordinated TEMPO(+). This mechanism combines elements of the Semmelhack mechanism (oxidation of TEMPO to TEMPO(+)) and the Sheldon proposal ("in the coordination sphere of Cu"). The recent proposal has been challenged on the basis of DFT calculations with a different functional, which were reported to lead to a radical mechanism. We carefully examine the results for the two functionals and conclude from both the calculated energetics and from an electronic structure analysis that the results of the two DFT functionals are consistent and that both lead to the proposed mechanism with TEMPO not acting as radical but as (coordinated) positive ion.

Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C-H bond activation transition state

Oxo-iron(IV) species are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize aliphatic C-H bonds. Ferryl complexes can exist in either quintet or triplet ground states. Density functional theory calculations predict that the quintet oxo-iron(IV) species is more reactive toward C-H bond activation than its corresponding triplet partner, however; the available experimental data on model complexes suggests that both spin multiplicities display comparable reactivities. To clarify this ambiguity, a detailed electronic structure analysis of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed. According to our results, the lengthening of the Fe-oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl-iron(III) species that then perform actual C-H bond activation. The differential reactivity stems from the fact that the two spin states have different requirements for the optimal angle at which the substrate should approach the (FeO)(2+) core because distinct electron acceptor orbitals are employed on the two surfaces. The H-atom abstraction on the quintet surface favors the "σ-pathway" that requires an essentially linear attack; by contrast a "π-channel" is operative on the triplet surface that leads to an ideal attack angle near 90°. However, the latter is not possible due to steric crowding; thus, the attenuated orbital interaction and the unavoidably increased Pauli repulsion result in the lower reactivity of the triplet oxo-iron(IV) complexes.

Electronic structure and spectroscopy of "superoxidized" iron centers in model systems: theoretical and experimental trends

Recent advances in synthetic chemistry have led to the discovery of "superoxidized" iron centers with valencies Fe(v) and Fe(vi) [K. Meyer et al., J. Am. Chem. Soc., 1999, 121, 4859-4876; J. F. Berry et al., Science, 2006, 312, 1937-1941; F. T. de Oliveira et al., Science, 2007, 315, 835-838.]. Furthermore, in recent years a number of high-valent Fe(iv) species have been found as reaction intermediates in metalloenzymes and have also been characterized in model systems [C. Krebs et al., Acc. Chem. Res., 2007, 40, 484-492; L. Que, Jr, Acc. Chem. Res., 2007, 40, 493-500.]. These species are almost invariably stabilized by a highly basic ligand X(n-) which is either O(2-) or N(3-). The differences in structure and bonding between oxo- and nitrido species as a function of oxidation state and their consequences on the observable spectroscopic properties have never been carefully assessed. Hence, fundamental differences between high-valent iron complexes having either Fe=O or Fe=N multiple bonds have been probed computationally in this work in a series of hypothetical trans-[FeO(NH(3))(4)OH](+/2+/3+) (1-3) and trans-[FeN(NH(3))(4)OH](0/+/2+) (4-6) complexes. All computational properties are permeated by the intrinsically more covalent character of the Fe=N multiple bond as compared to the Fe=O bond. This difference is likely due to differences in Z* between N and O that allow for better orbital overlap to occur in the case of the Fe=N multiple bond. Spin-state energetics were addressed using elaborate multireference ab initio computations that show that all species 1-6 have an intrinsic preference for the low-spin state, except in the case of 1 in which S=1 and S=2 states are very close in energy. In addition to Mössbauer parameters, g-tensors, zero-field splitting and iron hyperfine couplings, X-ray absorption Fe K pre-edge spectra have been simulated using time-dependent DFT methods for the first time for a series of compounds spanning the high-valent states +4, +5, and +6 for iron. A remarkably good correlation of these simulated pre-edge features with experimental data on isolated high-valent intermediates has been found, allowing us to assign the main pre-edge features to excitations into the empty Fe d(z(2)) orbital, which is able to mix with Fe 4p(z), allowing an efficient mechanism for the intensification of pre-edge features.