Does a higher metal oxidation state necessarily imply higher reactivity toward H-atom transfer? A computational study of C-H bond oxidation by high-valent iron-oxo and -nitrido complexes

Influence of Equatorial Co-Ligands on the Reactivity of LFeIIIOIPh

Previous biomimetic studies clearly proved that equatorial ligands significantly influence the redox potential and thus the stability/reactivity of biologically important oxoiron intermediates; however, no such studies were performed on FeIIIOIPh species. In this study, the influence of substituted pyridine co-ligands on the reactivity of iron(III)-iodosylbenzene adduct has been investigated in sulfoxidation and epoxidation reactions. Selective oxidation of thioanisole, cis-cyclooctene, and cis- and trans-stilbene in the presence of a catalytic amount of [FeII(PBI)3](OTf)2 with PhI(OAc)2 provide products in good to excellent yields through an FeIIIOIPh intermediate depending on the co-ligand (4R-Py) used. Several mechanistic studies were performed to gain more insight into the mechanism of oxygen atom transfer (OAT) reactions to support the reactive intermediate and investigate the effect of the equatorial co-ligands. Based on competitive experiments, including a linear free-energy relationship between the relative reaction rates (logkrel) and the σp (4R-Py) parameters, strong evidence has been observed for the electrophilic character of the reactive species. The presence of the [(PBI)2(4R-Py)FeIIIOIPh]3+ intermediates and the effect of the co-ligands was also supported by UV-visible measurements, including the color change from red to green and the hypsochromic shifts in the presence of co-ligands. This is another indication that the title iron(III)-iodosylbenzene adduct is able to oxygenate sulfides and alkenes before it is transformed into the oxoiron form by cleavage of the O-I bond.

Modulating the Energetics of C-H Bond Activation in Methane by Utilizing Metalated Porphyrinic Metal-Organic Frameworks

In recent years, much effort has been directed toward utilizing metal-organic frameworks (MOFs) for activating C-H bonds of light alkanes. The energy demanding steps involved in the catalytic pathway are the formation of metal-oxo species and the subsequent cleavage of the C-H bonds of alkanes. With the intention of exploring the tunability of the activation barriers involved in the catalytic pathway of methane hydroxylation, we have employed density functional theory to model metalated porphyrinic MOFs (MOF-525(M)). We find that the heavier congeners down a particular group have high exothermic oxo-formation enthalpies ΔH_O and hence are associated with low N₂O activation barriers. Independent analyses of activation barriers and structure-activity relationship leads to the conclusion that MOF-525(Ru) and MOF-525(Ir) can act as an effective catalysts for methane hydroxylation. Hence, ΔH_O has been found to act as a guide, in the first place, in choosing the optimum catalyst for methane hydroxylation from a large set of available systems.

New Strategies for Direct Methane-to-Methanol Conversion from Active Learning Exploration of 16 Million Catalysts

Despite decades of effort, no earth-abundant homogeneous catalysts have been discovered that can selectively oxidize methane to methanol. We exploit active learning to simultaneously optimize methane activation and methanol release calculated with machine learning-accelerated density functional theory in a space of 16 M candidate catalysts including novel macrocycles. By constructing macrocycles from fragments inspired by synthesized compounds, we ensure synthetic realism in our computational search. Our large-scale search reveals that low-spin Fe(II) compounds paired with strong-field (e.g., P or S-coordinating) ligands have among the best energetic tradeoffs between hydrogen atom transfer (HAT) and methanol release. This observation contrasts with prior efforts that have focused on high-spin Fe(II) with weak-field ligands. By decoupling equatorial and axial ligand effects, we determine that negatively charged axial ligands are critical for more rapid release of methanol and that higher-valency metals [i.e., M(III) vs M(II)] are likely to be rate-limited by slow methanol release. With full characterization of barrier heights, we confirm that optimizing for HAT does not lead to large oxo formation barriers. Energetic span analysis reveals designs for an intermediate-spin Mn(II) catalyst and a low-spin Fe(II) catalyst that are predicted to have good turnover frequencies. Our active learning approach to optimize two distinct reaction energies with efficient global optimization is expected to be beneficial for the search of large catalyst spaces where no prior designs have been identified and where linear scaling relationships between reaction energies or barriers may be limited or unknown.

C-H Activation by an Iron-Nitrido Bis-Pocket Porphyrin Species

High-valent iron-nitrido species are nitrogen analogues of iron-oxo species which are versatile reagents for C-H oxidation. Nonetheless, C-H activation by iron-nitrido species has been scarcely explored, as this is often hampered by their instability and short lifetime in solutions. Herein, the hydrogen atom transfer (HAT) reactivity of an Fe porphyrin nitrido species (2 c) toward C-H substrates was studied in solutions at room temperature, which was achieved by nanosecond laser flash photolysis (LFP) of its Fe^III -azido precursor (1 c) supported by a bulky bis-pocket porphyrin ligand. C-H bonds with bond dissociation enthalpies (BDEs) of up to ≈84 kcal mol^-1 could be activated, and the second-order rate constants (k₂ ) are on the order of 10² -10⁴  s^-1  m^-1 . The Fe-amido product formed after HAT could further release ammonia upon protonation.

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.

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.

Electronic structure of iron dinitrogen complex [(TPB)FeN2]2−/1−/0: correlation to Mössbauer parameters

Low-valent species of iron are key intermediates in many important biological processes such as the nitrogenase enzymatic catalytic reaction. These species play a major role in activating highly stable N₂ molecules. Thus, there is a clear need to establish the factors which are responsible for the reactivity of the metal–dinitrogen moiety. In this regard, we have investigated the electronic structure of low-valent iron (2−/1−/0) in a [(TPB)FeN₂]^2−/1−/0 complex using density functional theory (DFT). The variation in the oxidation states of iron in the nitrogenase enzyme cycle is associated with the flexibility of Fe→B bonding. Therefore, the flexibility of Fe→B bonding acts as an electron source that sustains the formation of various oxidation states, which is necessary for the key species in dinitrogen activation. AIM calculations are also performed to understand the strength of Fe→B and Fe–N₂ bonds. A detailed interpretation of the contributions to the isomer shift (IS) and quadrupole splitting (ΔE_Q) are discussed. The major contribution to IS comes mainly from the 3s-contribution, which differs depending on the d orbital population due to different shielding. The valence shell contribution also comes from the 4s-orbital. The Fe–N₂ bond distance has a great influence on the Mössbauer parameters, which are associated with the radial distribution, i.e. the shape of the 4s-orbital and the charge density at the nucleus. A linear relationship between IS with Fe–N₂ and ΔE_Q with Fe–N₂ is observed.

We use density functional theory studies to explore the electronic structure, bonding and spectroscopic analysis of a low-valent iron (2−/1−/0) complex [(TPB)FeN₂]^2−/1−/0 and reveled the factor which affects the reactivity of the metal–dinitrogen moiety.

Electron Paramagnetic Resonance Signature of Tetragonal Low Spin Iron(V)-Nitrido and -Oxo Complexes Derived from the Electronic Structure Analysis of Heme and Non-Heme Archetypes

Iron(V)-nitrido and -oxo complexes have been proposed as key intermediates in a diverse array of chemical transformations. Herein we present a detailed electronic-structure analysis of [Fe^V(N)(TPP)] (1, TPP^2– = tetraphenylporphyrinato), and [Fe^V(N)(cyclam-ac)]⁺ (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and ⁵⁷Fe Mössbauer spectroscopy coupled with wave function based complete active-space self-consistent field (CASSCF) calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and -oxo complexes, [Fe^V(N)(MePy₂tacn)](PF₆)₂ (3, MePy₂tacn = methyl-N′,N″-bis(2-picolyl)-1,4,7-triazacyclononane), [Fe^V(N){PhB(t-BuIm)₃}]⁺ (4, PhB(^tBuIm)₃^– = phenyltris(3-tert-butylimidazol-2-ylidene)borate), and [Fe^V(O)(TAML)]⁻ (5, TAML^4– = tetraamido macrocyclic ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener, compound I, which contains a ferryl unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and -oxo complexes 1–3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial EPR spectra with g_|| < g_⊥ ≤ 2 were measured for them, and their g_|| and g_⊥ values were found to obey a simple relation of g_⊥² + (2 – g_∥)² = 4. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by its distinct EPR spectrum with g_|| < 2 < g_⊥. Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and -oxo complexes feature electronic structures akin to those found for complexes 1–3 and 5. Therefore, the characteristic EPR signals determined for 1–3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.

Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins

Oxygenase reactivity toward selective partial oxidation of CH₄ to CH₃OH requires an atomic oxygen-radical bound to metal (M-O^•: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH₄. Because such a reaction is frequently observed in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M-O^• intermediate. However, no experimental data of M-O^• have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O^•, its reactivity for CH₄, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH₄ over Zn^II-O^• in the MFI zeolite. One Zn^II-O^• species does perform H-abstraction from CH₄ at room temperature. The resultant CH₃^• species reacts with the other Zn^II-O^• site to form the Zn^II-OCH₃ species. The H₂O-assisted extraction of surface methoxide yields 29 μmol g^-1 of CH₃OH with a 94% selectivity. The quantum mechanics (QM)/molecular mechanics (MM) calculation determined the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol^-1. On the basis of the findings in gas-phase experiments regarding the CH₄ activation by the free [M-O^•]⁺ species, the remarkable H-abstraction reactivity of the Zn^II-O^• species in zeolites was totally rationalized. Additionally, the experimentally validated QM/MM calculation revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M-O^• bond and thereby enables to create effectively the highly reactive M-O^• bond required for low-temperature activation of CH₄. The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.

What drives the H-abstraction reaction in bio-mimetic oxoiron-bTAML complexes? A computational investigation

Monomeric iron-oxo units have been confirmed as intermediates involved in the C-H bond activation in various metallo-enzymes. Biomimetic oxoiron complexes of the biuret modified tetra-amido macrocyclic ligand (bTAML) have been demonstrated to oxidize a wide variety of unactivated C-H bonds. In the current work, density functional theory (DFT) has been employed to investigate the hydrogen abstraction (HAT) reactivity differences across a series of bTAML complexes. The cause for the differences in the HAT energy barriers has been found to be the relative changes in the energy of the frontier molecular orbitals (FMOs) induced by electronic perturbation.

Mechanistic insights into intramolecular ortho-amination/hydroxylation by nonheme FeIV[double bond, length as m-dash]NTs/FeIV[double bond, length as m-dash]O species: the σ vs. the π channels

Comparative oxidative abilities of nonheme Fe^IV[double bond, length as m-dash]NTs and Fe^IV[double bond, length as m-dash]O species using DFT has been explored. Our calculations reveal that the Fe^IV[double bond, length as m-dash]NTs is found to be a stronger oxidant in two electron transfer reactions and react exclusively via π channels while the Fe^IV[double bond, length as m-dash]O species is found to be a stronger oxidant when the σ-pathway is activated such as in HAT reactions.

High-Spin Iron Imido Complexes Competent for C-H Bond Amination

Reduction of previously reported (ArL)FeCl with potassium graphite furnished a low-spin (S = 1/2) iron complex (ArL)Fe which features an intramolecular η6-arene interaction and can be utilized as an FeI synthon (ArL = 5-mesityl-1,9-(2,4,6-Ph3C6H2)dipyrrin). Treatment of (ArL)Fe with adamantyl azide or mesityl azide led to the formation of the high-spin (S = 5/2), three-coordinate imidos (ArL)Fe(NAd) and (ArL)Fe(NMes), respectively, as determined by EPR, zero-field 57Fe Mössbauer, magnetometry, and single crystal X-ray diffraction. The high-spin iron imidos are reactive with a variety of substrates: (ArL)Fe(NAd) reacts with azide yielding a ferrous tetrazido (ArL)Fe(κ2-N4Ad2), undergoes intermolecular nitrene transfer to phosphine, abstracts H atoms from weak C-H bonds (1,4-cyclohexadiene, 2,4,6-tBu3C6H2OH) to afford ferrous amido product (ArL)Fe(NHAd), and can mediate intermolecular C-H amination of toluene [PhCH3/PhCD3 kH/kD: 15.5(3); PhCH2D kH/kD: 11(1)]. The C-H bond functionalization reactivity is rationalized from a two-step mechanism wherein each step occurs via maximal energy and orbital overlap between the imido fragment and the C-H bond containing substrate.

Nonclassical Single-State Reactivity of an Oxo-Iron(IV) Complex Confined to Triplet Pathways

C-H bond activation mediated by oxo-iron (IV) species represents the key step of many heme and nonheme O₂-activating enzymes. Of crucial interest is the effect of spin state of the Fe^IV(O) unit. Here we report the C-H activation kinetics and corresponding theoretical investigations of an exclusive tetracarbene ligated oxo-iron(IV) complex, [L^NHCFe^IV(O)(MeCN)]²⁺ (1). Kinetic traces using substrates with bond dissociation energies (BDEs) up to 80 kcal mol^-1 show pseudo-first-order behavior and large but temperature-dependent kinetic isotope effects (KIE 32 at -40 °C). When compared with a topologically related oxo-iron(IV) complex bearing an equatorial N-donor ligand, [L^TMCFe^IV(O) (MeCN)]²⁺ (A), the tetracarbene complex 1 is significantly more reactive with second order rate constants k'₂ that are 2-3 orders of magnitude higher. UV-vis experiments in tandem with cryospray mass spectrometry evidence that the reaction occurs via formation of a hydroxo-iron(III) complex (4) after the initial H atom transfer (HAT). An extensive computational study using a wave function based multireference approach, viz. complete active space self-consistent field (CASSCF) followed by N-electron valence perturbation theory up to second order (NEVPT2), provided insight into the HAT trajectories of 1 and A. Calculated free energy barriers for 1 reasonably agree with experimental values. Because the strongly donating equatorial tetracarbene pushes the Fe-d_x²-y² orbital above d_z², 1 features a dramatically large quintet-triplet gap of ∼18 kcal/mol compared to ∼2-3 kcal/mol computed for A. Consequently, the HAT process performed by 1 occurs on the triplet surface only, in contrast to complex A reported to feature two-state-reactivity with contributions from both triplet and quintet states. Despite this, the reactive Fe^IV(O) units in 1 and A undergo the same electronic-structure changes during HAT. Thus, the unique complex 1 represents a pure "triplet-only" ferryl model.

Targeting of High-Valent Iron-TAML Activators at Hydrocarbons and Beyond

TAML activators of peroxides are iron(III) complexes. The ligation by four deprotonated amide nitrogens in macrocyclic motifs is the signature of TAMLs where the macrocyclic structures vary considerably. TAML activators are exceptional functional replicas of the peroxidases and cytochrome P450 oxidizing enzymes. In water, they catalyze peroxide oxidation of a broad spectrum of compounds, many of which are micropollutants, compounds that produce undesired effects at low concentrations-as with the enzymes, peroxide is typically activated with near-quantitative efficiency. In nonaqueous solvents such as organic nitriles, the prototype TAML activator gave the structurally authenticated reactive iron(V)oxo units (Fe^VO), wherein the iron atom is two oxidation equivalents above the Fe^III resting state. The iron(V) state can be achieved through the intermediacy of iron(IV) species, which are usually μ-oxo-bridged dimers (Fe^IVFe^IV), and this allows for the reactivity of this potent reactive intermediate to be studied in stoichiometric processes. The present review is primarily focused at the mechanistic features of the oxidation by Fe^VO of hydrocarbons including cyclohexane. The main topic is preceded by a description of mechanisms of oxidation of thioanisoles by Fe^VO, because the associated studies provide valuable insight into the ability of Fe^VO to oxidize organic molecules. The review is opened by a summary of the interconversions between Fe^III, Fe^IVFe^IV, and Fe^VO species, since this information is crucial for interpreting the kinetic data. The highest reactivity in both reaction classes described belongs to Fe^VO. The resting state Fe^III is unreactive oxidatively. Intermediate reactivity is typically found for Fe^IVFe^IV; therefore, kinetic features for these species in interchange and oxidation processes are also reviewed. Examples of using TAML activators for C-H bond cleavage applied to fine organic synthesis conclude the review.

Spectroscopic and Reactivity Comparisons of a Pair of bTAML Complexes with FeV═O and FeIV═O Units

In this report we compare the geometric and electronic structures and reactivities of [Fe^V(O)]^- and [Fe^IV(O)]^2- species supported by the same ancillary nonheme biuret tetraamido macrocyclic ligand (bTAML). Resonance Raman studies show that the Fe═O vibration of the [Fe^IV(O)]^2- complex 2 is at 798 cm^-1, compared to 862 cm^-1 for the corresponding [Fe^V(O)]^- species 3, a 64 cm^-1 frequency difference reasonably reproduced by density functional theory calculations. These values are, respectively, the lowest and the highest frequencies observed thus far for nonheme high-valent Fe═O complexes. Extended X-ray absorption fine structure analysis of 3 reveals an Fe═O bond length of 1.59 Å, which is 0.05 Å shorter than that found in complex 2. The redox potentials of 2 and 3 are 0.44 V (measured at pH 12) and 1.19 V (measured at pH 7) versus normal hydrogen electrode, respectively, corresponding to the [Fe^IV(O)]^2-/[Fe^III(OH)]^2- and [Fe^V(O)]^-/[Fe^IV(O)]^2- couples. Consistent with its higher potential (even after correcting for the pH difference), 3 oxidizes benzyl alcohol at pH 7 with a second-order rate constant that is 2500-fold bigger than that for 2 at pH 12. Furthermore, 2 exhibits a classical kinteic isotope effect (KIE) of 3 in the oxidation of benzyl alcohol to benzaldehyde versus a nonclassical KIE of 12 for 3, emphasizing the reactivity differences between 2 and 3.

Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes

High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido [{PhB(Im^R)₃}FeN], R = ^tBu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, [{PhB(Im^R)₃}Fe(NR')]⁺, R = Mes, R' = 1-adamantyl (3), ^tBu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, [(TIMEN^Ar)FeN]⁺ (TIMEN^Ar = tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine; Ar = Xyl (xylyl), Mes), were investigated by ⁵⁷Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3d_z² (a₁) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices.

Spectroscopic analyses of 2-oxoglutarate-dependent oxygenases: TauD as a case study

A wide range of spectroscopic approaches have been used to interrogate the mononuclear iron metallocenter in 2-oxoglutarate (2OG)-dependent oxygenases. The results from these spectroscopic studies have provided valuable insights into the structural changes at the active site during substrate binding and catalysis, thus providing critical information that complements investigations of these enzymes by X-ray crystallography, biochemical, and computational approaches. This mini-review highlights taurine hydroxylase (taurine:2OG dioxygenase, TauD) as a case study to illustrate the wealth of knowledge that can be generated by applying a diverse array of spectroscopic investigations to a single enzyme. In particular, electronic absorption, circular dichroism, magnetic circular dichroism, conventional and pulse electron paramagnetic, Mössbauer, X-ray absorption, and resonance Raman methods have been exploited to uncover the properties of the metal site in TauD.

On the Mechanisms of Hydrogen-Atom Transfer from Water to the Heteronuclear Oxide Cluster [Ga2 Mg2 O5 ](.+) : Remarkable Electronic Structure Effects

Mechanistic insight into the homolytic cleavage of the O-H bond of water by the heteronuclear oxide cluster [Ga2 Mg2 O5 ](.+) has been derived from state-of-the-art gas-phase experiments in conjunction with quantum chemical calculations. Three pathways have been identified computationally. In addition to the conventional hydrogen-atom transfer (HAT) to the radical center of a bridging oxygen atom, two mechanistically distinct proton-coupled electron-transfer (PCET) processes have been identified. The energetically most favored path involves initial coordination of the incoming water ligand to a magnesium atom followed by an intramolecular proton transfer to the lone-pair of the bridging oxygen atom. This step, which is accomplished by an electronic reorganization, generates two structurally equivalent OH groups either of which can be liberated, in agreement with labeling experiments.

Magnetic circular dichroism and computational study of mononuclear and dinuclear iron(IV) complexes

High-valent iron(IV)-oxo species are key intermediates in the catalytic cycles of a range of O2-activating iron enzymes. This work presents a detailed study of the electronic structures of mononuclear ([FeIV(O)(L)(NCMe)]2+, 1, L = tris(3,5-dimethyl-4-methoxylpyridyl-2-methyl)amine) and dinuclear ([(L)FeIV(O)(μ-O)FeIV(OH)(L)]3+, 2) iron(IV) complexes using absorption (ABS), magnetic circular dichroism (MCD) spectroscopy and wave-function-based quantum chemical calculations. For complex 1, the experimental MCD spectra at 2-10 K are dominated by a broad positive C-term band between 12000 and 18000 cm-1. As the temperature increases up to ~20 K, this feature is gradually replaced by a derivative-shaped signal. The computed MCD spectra are in excellent agreement with experiment, which reproduce not only the excitation energies and the MCD signs of key transitions but also their temperature-dependent intensity variations. To further corroborate the assignments suggested by the calculations, the individual MCD sign for each transition is independently determined from the corresponding electron donating and accepting orbitals. Thus, unambiguous assignments can be made for the observed transitions in 1. The ABS/MCD data of complex 2 exhibit ten features that are assigned as ligand-field transitions or oxo- or hydroxo-to-metal charge transfer bands, based on MCD/ABS intensity ratios, calculated excitation energies, polarizations, and MCD signs. In comparison with complex 1, the electronic structure of the FeIV=O site is not significantly perturbed by the binding to another iron(IV) center. This may explain the experimental finding that complexes 1 and 2 have similar reactivities toward C-H bond activation and O-atom transfer.

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.