Crystallographic and spectroscopic characterization and reactivities of a mononuclear non-haem iron(III)-superoxo complex

Trapping an Elusive Fe(IV)-Superoxo Intermediate Inside a Self-Assembled Nanocage in Water at Room Temperature

Molecular cavities that mimic natural metalloenzymes have shown the potential to trap elusive reaction intermediates. Here, we demonstrate the formation of a rare yet stable Fe(IV)-superoxo intermediate at room temperature subsequent to dioxygen binding at the Fe(III) site of a (Et4N)2[FeIII(Cl)(bTAML)] complex confined inside the hydrophobic interior of a water-soluble Pd6L412+ nanocage. Using a combination of electron paramagnetic resonance, Mössbauer, Raman/IR vibrational, X-ray absorption, and emission spectroscopies, we demonstrate that the cage-encapsulated complex has a Fe(IV) oxidation state characterized by a stable S = 1/2 spin state and a short Fe-O bond distance of ∼1.70 Å. We find that the O2 reaction in confinement is reversible, while the formed Fe(IV)-superoxo complex readily reacts when presented with substrates having weak C-H bonds, highlighting the lability of the O-O bond. We envision that such optimally trapped high-valent superoxos can show new classes of reactivities catalyzing both oxygen atom transfer and C-H bond activation reactions.

Structures of SenB and SenA enzymes from Variovorax paradoxus provide insights into carbon-selenium bond formation in selenoneine biosynthesis

Selenoneine, an ergothioneine analog, is important for antioxidation and detoxification. SenB and SenA are two crucial enzymes that form carbon-selenium bonds in the selenoneine biosynthetic pathway. To investigate their underlying catalytic mechanisms, we obtained complex structures of SenB with its substrate UDP-N-acetylglucosamine (UDP-GlcNAc) and SenA with N-α-trimethyl histidine (TMH). SenB adopts a type-B glycosyltransferase fold. Structural and functional analysis of the interaction network at the active center provide key information on substrate recognition and suggest a metal-ion-independent, inverting mechanism is utilized for SenB-mediated selenoglycoside formation. Moreover, the complex structure of SenA with TMH and enzymatic activity assays highlight vital residues that control substrate binding and specificity. Based on the conserved structure and substrate-binding pocket of the type I sulfoxide synthase EgtB in the ergothioneine biosynthetic pathway, a similar reaction mechanism was proposed for the formation of C-Se bonds by SenA. The structures provide knowledge on selenoneine synthesis and lay groundwork for further applications of this pathway.

The Unique Role of the Second Coordination Sphere to Unlock and Control Catalysis in Nonheme Fe(II)/2-Oxoglutarate Histone Demethylase KDM2A

Nonheme Fe(II) and 2-oxoglutarate (2OG)-dependent histone lysine demethylases 2A (KDM2A) catalyze the demethylation of the mono- or dimethylated lysine 36 residue in the histone H3 peptide (H3K36me1/me2), which plays a crucial role in epigenetic regulation and can be involved in many cancers. Although the overall catalytic mechanism of KDMs has been studied, how KDM2 catalysis takes place in contrast to other KDMs remains unknown. Understanding such differences is vital for enzyme redesign and can help in enzyme-selective drug design. Herein, we employed molecular dynamics (MD) and combined quantum mechanics/molecular mechanics (QM/MM) to explore the complete catalytic mechanism of KDM2A, including dioxygen diffusion and binding, dioxygen activation, and substrate oxidation. Our study demonstrates that the catalysis of KDM2A is controlled by the conformational change of the second coordination sphere (SCS), specifically by a change in the orientation of Y222, which unlocks the 2OG rearrangement from off-line to in-line mode. The study demonstrates that the variant Y222A makes the 2OG rearrangement more favorable. Furthermore, the study reveals that it is the size of H3K36me3 that prevents the 2OG rearrangement, thus rendering the enzyme inactivity with trimethylated lysine. Calculations show that the SCS and long-range interacting residues that stabilize the HAT transition state in KDM2A differ from those in KDM4A, KDM7B, and KDM6A, thus providing the basics for the enzyme-selective redesign and modulation of KDM2A without influencing other KDMs.

Oxygen Activation in Aromatic Ring Cleaving Salicylate Dioxygenase: Detection of Reaction Intermediates with a Nitro-substituted Substrate Analog

Cupin dioxygenases such as salicylate 1,2-dioxygense (SDO) perform aromatic C-C bond scission via a 3-His motif tethered iron cofactor. Here, transient kinetics measurements are used to monitor the catalytic cycle of SDO by using a nitro-substituted substrate analog, 3-nitrogentisate. Compared to the natural substrate, the nitro group reduces the enzymatic kcat by 500-fold, thereby facilitating the detection and kinetic characterization of reaction intermediates. Sums and products of reciprocal relaxation times derived from kinetic measurements were found to be linearly dependent on O2 concentration, suggesting reversible formation of two distinct intermediates. Dioxygen binding to the metal cofactor takes place with a forward rate of 5.9×103 M-1 s-1: two orders of magnitude slower than other comparable ring-cleaving dioxygenses. Optical chromophore of the first intermediate is distinct from the in situ generated SDO Fe(III)-O2⋅- complex but closer to the enzyme-substrate precursor.

A Nonheme Iron(III) Superoxide Complex Leads to Sulfur Oxygenation

A new alkylthiolate-ligated nonheme iron complex, FeII(BNPAMe2S)Br (1), is reported. Reaction of 1 with O2 at -40 °C, or reaction of the ferric form with O2•- at -80 °C, gives a rare iron(III)-superoxide intermediate, [FeIII(O2)(BNPAMe2S)]+ (2), characterized by UV-vis, 57Fe Mössbauer, ATR-FTIR, EPR, and CSIMS. Metastable 2 then converts to an S-oxygenated FeII(sulfinate) product via a sequential O atom transfer mechanism involving an iron-sulfenate intermediate. These results provide evidence for the feasibility of proposed intermediates in thiol dioxygenases.

Amphoteric reactivity of a putative Cu(II)-mCPBA intermediate

In copper-based enzymes, Cu-hydroperoxo/alkylperoxo species are proposed as key intermediates for their biological activity. A vast amount of literature is available on the functional and structural mimics of enzymatic systems with heme and non-heme ligand frameworks to stabilize high valent metal intermediates, mostly at low temperatures. Herein, we report a reaction between [CuI(NCCH3)4]+ and meta-chloroperoxybenzoic acid (mCPBA) in CH3CN that produces a putative CuII(mCPBA) species (1). 1 was characterized by UV/Vis, resonance Raman, and EPR spectroscopies. 1 can catalyze both electrophilic and nucleophilic reactions, demonstrating its amphoteric behavior. Additionally, 1 can also conduct electron transfer reactions with a weak reducing agent such as diacetyl ferrocene, making it one of the reactive copper-based intermediates. One of the most important aspects of the current work is the easy synthesis of a CuII(mCPBA) adduct with no complicated ligands for stabilization. Over time, 1 decays to form a CuII paddle wheel complex (2) and is found to be unreactive towards substrate oxidation.

Mechanistic Insights into Amphoteric Reactivity of an Iron-Bispidine Complex

The reactivity of FeIII -alkylperoxido complexes has remained a riddle to inorganic chemists owing to their thermal instability and impotency towards organic substrates. These iron-oxygen adducts have been known as sluggish oxidants towards oxidative electrophilic and nucleophilic reactions. Herein, we report the synthesis and spectroscopic characterization of a relatively stable mononuclear high-spin FeIII -alkylperoxido complex supported by an engineered bispidine framework. Against the notion, this FeIII -alkylperoxido complex serves as a rare example of versatile reactivity in both electrophilic and nucleophilic reactions. Detailed mechanistic studies and computational calculations reveal a novel reaction mechanism, where a putative superoxido intermediate orchestrates the amphoteric property of the oxidant. The design of the backbone is pivotal to convey stability and reactivity to alkylperoxido and superoxido intermediates. Contrary to the well-known O-O bond cleavage that generates an FeIV -oxido species, the FeIII -alkylperoxido complex reported here undergoes O-C bond scission to generate a superoxido moiety that is responsible for the amphiphilic reactivity.

Electronic Structure and Reactivity of Mononuclear Nonheme Iron-Peroxo Complexes as a Biomimetic Model of Rieske Oxygenases: Ring Size Effects of Macrocyclic Ligands

We report the macrocyclic ring size-electronic structure-electrophilic reactivity correlation of mononuclear nonheme iron(III)-peroxo complexes bearing N-tetramethylated cyclam analogues (n-TMC), [FeIII(O2)(12-TMC)]+ (1), [FeIII(O2)(13-TMC)]+ (2), and [FeIII(O2)(14-TMC)]+ (3), as a model study of Rieske oxygenases. The Fe(III)-peroxo complexes show the same δ and pseudo-σ bonds between iron and the peroxo ligand. However, the strength of these interactions varies depending on the ring size of the n-TMC ligands; the overall Fe-O bond strength and the strength of the Fe-O2 δ bond increase gradually as the ring size of the n-TMC ligands becomes smaller, such as from 14-TMC to 13-TMC to 12-TMC. MCD spectroscopy plays a key role in assigning the characteristic low-energy δ → δ* LMCT band, which provides direct insight into the strength of the Fe-O2 δ bond and which, in turn, is correlated with the superoxo character of the iron-peroxo group. In oxidation reactions, reactivities of 1-3 toward hydrocarbon C-H bond activation are compared, revealing the reactivity order of 1 > 2 > 3; the [FeIII(O2)(n-TMC)]+ complex with a smaller n-TMC ring size, 12-TMC, is much more reactive than that with a larger n-TMC ring size, 14-TMC. DFT analysis shows that the Fe(III)-peroxo complex is not reactive toward C-H bonds, but it is the end-on Fe(II)-superoxo valence tautomer that is responsible for the observed reactivity. The hydrogen atom abstraction (HAA) reactivity of these intermediates is correlated with the overall donicity of the n-TMC ligand, which modulates the energy of the singly occupied π* superoxo frontier orbital that serves as the electron acceptor in the HAA reaction. The implications of these results for the mechanism of Rieske oxygenases are further discussed.

Theoretical study of the formation of metal-oxo species of the first transition series with the ligand 14-TMC: driving factors of the "Oxo Wall"

Terminal metal-oxo species of the early transition metal series are well known, whereas those for the late transition series are rare, and this is related to the "Oxo Wall". Here, we have undertaken a theoretical study on the formation of metal-oxo species from the metal hydroperoxo species of the 3d series (Cr, Mn, Fe, Co, Ni, and Cu) with the ligand 14-TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) via O⋯O bond cleavage. DFT calculations reveal that the barrier for O⋯O bond cleavage is higher with the late transition metals (Co, Ni, and Cu) than the early transition metals (Cr, Mn, and Fe), and the formed late metal-oxo species are also thermodynamically less stable. The higher barrier may be due to electronic repulsion because of the pairing of d electrons. In the late transition metal series, the electron goes into an antibonding orbital, which decreases the bond order and hence decreases the possibility of metal-oxo formation. Computed structural parameters and spin densities suggest that valence tautomerism occurs in the late transition metal-oxo species which remain as a metal-oxyl. Our findings support the concept of the "Oxo Wall".

Reversible O-O Bond Scission and O2 Evolution at MOF-Supported Tetramanganese Clusters

The scission of the O-O bond in O₂ during respiration and the formation of the O-O bond during photosynthesis are the engines of aerobic life. Likewise, the reduction of O₂ and the oxidation of reduced oxygen species to form O₂ are indispensable components for emerging renewable technologies, including energy storage and conversion, yet discrete molecule-like systems that promote these fundamental reactions are rare. Herein, we report a square-planar tetramanganese cluster formed by self-assembly within a metal-organic framework that reversibly reduces O₂ by four electrons, facilitating the interconversion between molecular O₂ and metal-oxo species. The tetranuclear cluster spontaneously cleaves the O-O bond of O₂ at room temperature to generate a tetramanganese-bis(μ₂-oxo) species, which, in turn, is competent for O-O bond reformation and O₂ evolution at elevated temperatures, enabled by the head-to-head orientation of two oxo species. This study demonstrates the viability of four-electron interconversion between molecular O₂ and metal-oxo species and highlights the importance of site isolation for achieving multi-electron chemistry at polynuclear metal clusters.

Spectroscopic analysis of the mammalian enzyme cysteine dioxygenase

l-Cysteine (Cys) is an essential building block for the synthesis of new proteins and serves as a precursor for several biologically important sulfur-containing molecules, such as coenzyme A, taurine, glutathione, and inorganic sulfate. However, organisms must tightly regulate the concentration of free Cys, as elevated levels of this semi-essential amino acid can be extremely harmful. The non-heme iron enzyme cysteine dioxygenase (CDO) serves to maintain the proper levels of Cys by catalyzing its oxidation to cysteine sulfinic acid. Crystal structures of resting and substrate-bound mammalian CDO revealed two surprising structural motifs in the first and second coordination spheres of the Fe center. The first is the existence of a neutral three histidine (3-His) facial triad that coordinates the Fe ion, as opposed to an anionic 2-His-1-carboxylate facial triad that is typically observed in mononuclear non-heme Fe(II) dioxygenases. The second unusual structural feature exhibited by mammalian CDO is the presence of a covalent crosslink between the sulfur of a Cys residue and an ortho-carbon of a tyrosine residue. Spectroscopic studies of CDO have provided invaluable insights into the roles that these unusual features play with regards to substrate Cys and co-substrate O2 binding and activation. In this chapter, we summarize results obtained from electronic absorption, electron paramagnetic resonance, magnetic circular dichroism, resonance Raman, and Mössbauer spectroscopic studies of mammalian CDO carried out in the last two decades. Pertinent results obtained from complementary computational studies are also briefly summarized.

Flavonol dioxygenase chemistry mediated by a synthetic nickel superoxide

Nature exploits transition metal centers to enhance and tune the oxidizing power of natural oxidants such as O₂ and H₂O₂. The design and interrogation of synthetic metallocomplexes with similar reactivity to metalloproteins provides one strategy for gaining insight into the mechanistic underpinnings of oxygen-activating enzymes such as oxidases, oxygenases, and dioxygenases like Ni-quercetinase (Ni-QueD). Ni-QueD catalyzes the oxidative ring opening of the polyphenol quercetin, a natural product with antioxidant properties. Herein, we report the synthesis and characterization of Ni(13-DOB), a Ni(II) species complexed by an N4-macrocycle that has been characterized by single crystal X-ray crystallography. Ni(13-DOB) forms a Ni-superoxide intermediate (Ni(13-DOB)O₂^•-) upon treatment with H₂O₂ and Et₃N, as verified by resonance Raman spectroscopy. We demonstrate through UV/vis and LCMS that Ni(13-DOB)O₂^•- is capable of the 1-electron oxidation of flavonols, including both 3-hydroxyflavone (3-HF, the simplest flavonol) and quercetin itself. Incorporation of two O-atoms into the flavonol radical via superoxide from Ni(13-DOB)O₂^•- precedes oxidative cleavage of the flavonol scaffold in each case, consistent with quercetinase ring cleavage by Ni-QueD in Streptomyces sp. FLA. Conversion of 3-HF into 2-hydroxybenzoylbenzoic acid was accomplished with catalytic turnover of Ni(13-DOB) at ambient temperature, as confirmed by HPLC timecourses and GCMS analysis of isotopic labeling studies. The Ni(13-DOB)-mediated oxidative cleavage of quercetin to the corresponding biomimetic phenolic ester was also verified through ¹⁸O-isotopic labeling studies. Through the HPLC characterization of both on- and off-pathway products of flavonol dioxygenation by Ni(13-DOB)O₂^•-, the stringent reaction pathway control provided by enzyme active sites is highlighted.

Isolating Fe-O2 Intermediates in Dioxygen Activation by Iron Porphyrin Complexes

Dioxygen (O₂) is an environmentally benign and abundant oxidant whose utilization is of great interest in the design of bioinspired synthetic catalytic oxidation systems to reduce energy consumption. However, it is unfortunate that utilization of O₂ is a significant challenge because of the thermodynamic stability of O₂ in its triplet ground state. Nevertheless, nature is able to overcome the spin state barrier using enzymes, which contain transition metals with unpaired d-electrons facilitating the activation of O₂ by metal coordination. This inspires bioinorganic chemists to synthesize biomimetic small-molecule iron porphyrin complexes to carry out the O₂ activation, wherein Fe-O₂ species have been implicated as the key reactive intermediates. In recent years, a number of Fe-O₂ intermediates have been synthesized by activating O₂ at iron centers supported on porphyrin ligands. In this review, we focus on a few examples of these advances with emphasis in each case on the particular design of iron porphyrin complexes and particular reaction environments to stabilize and isolate metal-O₂ intermediates in dioxygen activation, which will provide clues to elucidate structures of reactive intermediates and mechanistic insights in biological processes.

Electronic Structure and Reactivity of Dioxygen-Derived Aliphatic Thiolate-Ligated Fe-Peroxo and Fe(IV) Oxo Compounds

Herein, we examine the electronic and geometric structural properties of O₂-derived aliphatic thiolate-ligated Fe-peroxo, Fe-hydroxo, and Fe(IV) oxo compounds. The latter cleaves strong C-H bonds (96 kcal mol^-1) on par with the valine C-H bond cleaved by isopencillin N synthase (IPNS). Stopped-flow kinetics studies indicate that the barrier to O₂ binding to [Fe^II(S^Me2N₄(tren))]⁺ (3) is extremely low (E_a = 36(2) kJ mol^-1), as theoretically predicted for IPNS. Dioxygen binding to 3 is shown to be reversible, and a superoxo intermediate, [Fe^III(S^Me2N₄(tren))(O₂)]⁺ (6), forms in the first 25 ms of the reaction at -40 °C prior to the rate-determining (E_a = 46(2) kJ mol^-1) formation of peroxo-bridged [(S^Me2N₄(tren))Fe(III)]₂(μ-O₂)²⁺ (7). A log(k_obs) vs log([Fe]) plot for the formation of 7 is consistent with the second-order dependence on iron, and H₂O₂ assays are consistent with a 2:1 ratio of Fe/H₂O₂. Peroxo 7 is shown to convert to ferric-hydroxo [Fe^III(S^Me2N(tren))(OH)]⁺ (9, g_⊥ = 2.24, g_∥ = 1.96), the identity of which was determined via its independent synthesis. Rates of the conversion 7 → 9 are shown to be dependent on the X-H bond strength of the H-atom donor, with a k_H/k_D = 4 when CD₃OD is used in place of CH₃OH as a solvent. A crystallographically characterized cis thiolate-ligated high-valent iron oxo, [Fe^IV(O)(S^Me2N₄(tren))]⁺ (11), is shown to form en route to hydroxo 9. Electronic structure calculations were shown to be consistent with 11 being an S = 1 Fe(IV)═O with an unusually high ν_Fe-O stretching frequency at 918 cm^-1 in line with the extremely short Fe-O bond (1.603(7) Å).

Effect of the ring size of TMC ligands in controlling C-H bond activation by metal-superoxo species

Metal-superoxo species play a very important role in many metal-mediated catalytic transformation reactions. Their catalytic reactivity is affected by many factors such as the nature of metal ions and ring size of ligands. Herein, for the first time, we report DFT calculations on the electronic structures of a series of metal-superoxo species (M = V, Cr, Mn, Fe, and Co) with two ring size ligands, i.e., 13-TMC/14-TMC, and a detailed mechanistic study on the C-H bond activation of cyclohexa-1,4-diene followed by the effect of the ring size of ligands. Our DFT results showed that the electron density at the distal oxygen plays an important role in C-H bond activation. By computing the energetics of C-H bond activation and mapping the potential energy surface, it was found that the initial hydrogen abstraction is the rate-determining step with both TMC rings and all the studied metal-superoxo species. The significant electron density at the cyclohex-1,4-diene carbon indicates that the reaction proceeds via the proton-coupled electron transfer mechanism. By mapping the potential energy surfaces, we found that the 13-TMC ligated superoxo with the anti-isomer are more reactive than the 14-TMC superoxo species except for the iron-superoxo species where the 14-TMC ligated superoxo species is more reactive i.e. smaller ring size TMC is more reactive towards C-H bond activation. This is also supported by the structural correlation, i.e., the greater contraction in the smaller ring results in the metal being pushed out of plane along the z-axis, which reduces the steric hindrance. Thus, the ring size can help in designing catalysts with better efficiency for catalytic reactions.

An Iron(III) Superoxide Corrole from Iron(II) and Dioxygen

A new structurally characterized ferrous corrole [FeII (ttppc)]- (1) binds one equivalent of dioxygen to form [FeIII (O2-. )(ttppc)]- (2). This complex exhibits a 16/18 O2 -isotope sensitive ν(O-O) stretch at 1128 cm-1 concomitantly with a single ν(Fe-O2 ) at 555 cm-1 , indicating it is an η1 -superoxo ("end-on") iron(III) complex. Complex 2 is the first well characterized Fe-O2 corrole, and mediates the following biologically relevant oxidation reactions: dioxygenation of an indole derivative, and H-atom abstraction from an activated O-H bond.

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(III)-hydroxo and Mn(III)-aqua complexes

Hydrogen atom transfer (HAT) of metal-oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes have yet to be...

Why Intermolecular Nitric Oxide (NO) Transfer? Exploring the Factors and Mechanistic Aspects of NO Transfer Reaction

Small molecule activation & their transfer reactions in biological or catalytic reactions are greatly influenced by the metal-centers and the ligand frameworks. Here, we report the metal-directed nitric oxide (NO)...

Iron Binding in the Ferroxidase Site of Human Mitochondrial Ferritin

Ferritins are nanocage proteins that store iron ions in their central cavity as hydrated ferric oxide biominerals. In mammals, further the L (light) and H (heavy) chains constituting cytoplasmic maxi-ferritins, an additional type of ferritin has been identified, the mitochondrial ferritin (MTF). Human MTF (hMTF) is a functional homopolymeric H-like ferritin performing the ferroxidase activity in its ferroxidase site (FS), in which Fe(II) is oxidized to Fe(III) in the presence of dioxygen. To better investigate its ferroxidase properties, here we performed time-lapse X-ray crystallography analysis of hMTF, providing structural evidence of how iron ions interact with hMTF and of their binding to the FS. Transient iron binding sites, populating the pathway along the cage from the iron entry channel to the catalytic center, were also identified. Furthermore, our kinetic data at variable iron loads indicate that the catalytic iron oxidation reaction occurs via a diferric peroxo intermediate followed by the formation of ferric-oxo species, with significant differences with respect to human H-type ferritin.

A Mononuclear Non-heme Iron(III)-Peroxo Complex with an Unprecedented High O-O Stretch and Electrophilic Reactivity

A mononuclear non-heme iron(III)-peroxo complex, [Fe(III)(O₂)(13-TMC)]⁺ (1), was synthesized and characterized spectroscopically; the characterization with electron paramagnetic resonance, Mössbauer, X-ray absorption, and resonance Raman spectroscopies and mass spectrometry supported a high-spin S = ⁵/₂ Fe(III) species binding an O₂ unit. A notable observation was an unusually high ν_O-O at ∼1000 cm^-1 for the peroxo ligand. With regard to reactivity, 1 showed electrophilic reactivity in H atom abstraction (HAA) and O atom transfer (OAT) reactions. In the HAT reaction, a kinetic isotope effect (KIE) value of 5.8 was obtained in the oxidation of 9,10-dihydroanthracene. In the OAT reaction, a negative ρ value of -0.61 in the Hammett plot was determined in the oxidation of p-X-substituted thioanisoles. Another interesting observation was the electrophilic reactivity of 1 in the oxidation of benzaldehyde derivatives, such as a negative ρ value of -0.77 in the Hammett plot and a KIE value of 2.2. To the best of our knowledge, the present study reports the first example of a mononuclear non-heme iron(III)-peroxo complex with an unusually high ν_O-O value and unprecedented electrophilic reactivity in oxidation reactions.

Structural and Spectroscopic Evidence for a Side-on Fe(III)-Superoxo Complex Featuring Discrete O-O Bond Distances

The O-O bond length is often used as a structural indicator to determine the valence states of bound O₂ ligands in biological metal-dioxygen intermediates and related biomimetic complexes. Here, we report very distinct O-O bond lengths found for three crystallographic forms (1.229(4), 1.330(4), 1.387(2) Å at 100 K) of a side-on iron-dioxygen species. Despite their different O-O bond distances, all forms possess the same electronic structure of Fe(III)-O₂ ^•-, as evidenced by their indistinguishable spectroscopic features. Density functional theory and ab initio calculations, which successfully reproduce spectroscopic parameters, predict a flat potential energy surface of an η²-O₂ motif binding to the iron center regarding the O-O distance. Therefore, the discrete O-O bond lengths observed likely arise from differential intermolecular interactions in the second coordination sphere. The work suggests that the O-O distance is not a reliable benchmark to unequivocally identify the valence state of O₂ ligands for metal-dioxygen species in O₂-utilizing metalloproteins and synthetic complexes.

Identifying Intermediates in Electrocatalytic Water Oxidation with a Manganese Corrole Complex

Water nucleophilic attack (WNA) on high-valent terminal Mn-oxo species is proposed for O-O bond formation in natural and artificial water oxidation. Herein, we report an electrocatalytic water oxidation reaction with Mn^III tris(pentafluorophenyl)corrole (1) in propylene carbonate (PC). O₂ was generated at the Mn^V/IV potential with hydroxide, but a more anodic potential was required to evolve O₂ with only water. With a synthetic Mn^V(O) complex of 1, a second-order rate constant, k₂(OH^-), of 7.4 × 10³ M^-1 s^-1 was determined in the reaction of the Mn^V(O) complex of 1 with hydroxide, whereas its reaction with water occurred much more slowly with a k₂(H₂O) value of 4.4 × 10^-3 M^-1 s^-1. This large reactivity difference of Mn^V(O) with hydroxide and water is consistent with different electrocatalytic behaviors of 1 with these two substrates. Significantly, during the electrolysis of 1 with water, a Mn^IV-peroxo species was identified with various spectroscopic methods, including UV-vis, electron paramagnetic resonance, and infrared spectroscopy. Isotope-labeling experiments confirmed that both O atoms of this peroxo species are derived from water, suggesting the involvement of the WNA mechanism in water oxidation by a Mn complex. Density functional theory calculations suggested that the nucleophilic attack of hydroxide on Mn^V(O) and also WNA to 1e^--oxidized Mn^V(O) are feasibly involved in the catalytic cycles but that direct WNA to Mn^V(O) is not likely to be the main O-O bond formation pathway in the electrocatalytic water oxidation by 1.

Structure and Reactivity of a High-Spin, Nonheme Iron(III)- Superoxo Complex Supported by Phosphinimide Ligands

Nonheme iron oxygenases utilize dioxygen to accomplish challenging chemical oxidations. A further understanding of the Fe-O₂ intermediates implicated in these processes is challenged by their highly transient nature. To that end, we have developed a ligand platform featuring phosphinimide donors intended to stabilize oxidized, high-spin iron complexes. O₂ exposure of single crystals of a three-coordinate Fe(II) complex of this framework allowed for in crystallo trapping of a terminally bound Fe-O₂ complex suitable for XRD characterization. Spectroscopic and computational studies of this species support a high-spin Fe(III) center antiferromagnetically coupled to a superoxide ligand, similar to that proposed for numerous nonheme iron oxygenases. In addition to the apparent stability of this synthetic Fe-O₂ complex, its ability to engage in a range of stoichiometric and catalytic oxidation processes demonstrates that this iron-phosphinimide system is primed for development in modeling oxidizing bioinorganic intermediates and green oxidation chemistry.

Superoxide Oxidation by a Thiolate-Ligated Iron Complex and Anion Inhibition

Superoxide (O₂^•-) is a toxic radical, generated via the adventitious reduction of dioxygen (O₂), which has been implicated in a number of human disease states. Nonheme iron enzymes, superoxide reductase (SOR) and superoxide dismutase (SOD), detoxify O₂^•- via reduction to afford H₂O₂ and disproportionation to afford O₂ and H₂O₂, respectively. The former contains a thiolate in the coordination sphere, which has been proposed to prevent O₂^•- oxidation to O₂. The work described herein shows that, in contrast to this, oxidized thiolate-ligated [Fe^III(S^Me2N₄(tren)(THF)]²⁺ (1^ox-THF) is capable of oxidizing O₂^•- to O₂. Coordinating anions, Cl^- and OAc^-, are shown to inhibit dioxygen evolution, implicating an inner-sphere mechanism. Previously we showed that the reduced thiolate-ligated [Fe^II(S^Me2N₄(tren))]⁺ (1) is capable of reducing O₂^•- via a proton-dependent inner-sphere mechanism involving a transient Fe(III)-OOH intermediate. A transient ferric-superoxo intermediate, [Fe^III(S^Me2N₄(tren))(O₂)]⁺ (3), is detected by electronic absorption spectroscopy at -130 °C in the reaction between 1^ox-THF and KO₂ and shown to evolve O₂ upon slight warming to -115 °C. The DFT calculated O-O (1.306 Å) and Fe-O (1.943 Å) bond lengths of 3 are typical of ferric-superoxo complexes, and the time-dependent DFT calculated electronic absorption spectrum of 3 reproduces the experimental spectrum. The electronic structure of 3 is shown to consist of two antiferromagnetically coupled (J^calc = -180 cm^-1) unpaired electrons, one in a superoxo π*(O-O) orbital and the other in an antibonding π*(Fe(d_yz)-S(p_y)) orbital.

Near-infrared C-term MCD spectroscopy of octahedral uranium(V) complexes

C-term magnetic circular dichroism (MCD) spectroscopy is a powerful method for probing d-d and f-f transitions in paramagnetic metal complexes. However, this technique remains underdeveloped both experimentally and theoretically for studies of U(v) complexes of Oh symmetry, which have been of longstanding interest for probing electronic structure, bonding, and covalency in 5f systems. In this study, C-term NIR MCD of the Laporte forbidden f-f transitions of [UCl6]- and [UF6]- are reported, demonstrating the significant fine structure resolution possible with this technique including for the low energy Γ7 → Γ8 transitions in [UF6]-. The experimental NIR MCD studies were further extended to [U(OC6F5)6]-, [U(CH2SiMe3)6]-, and [U(NC(tBu)(Ph))6]- to evaluate the effects of ligand-type on the f-f MCD fine structure features. Theoretical calculations were conducted to determine the Laporte forbidden f-f transitions and their MCD intensity experimentally observed in the NIR spectra of the U(v) hexahalide complexes, via the inclusion of vibronic coupling, to better understand the underlying spectral fine structure features for these complexes. These spectra and simulations provide an important platform for the application of MCD spectroscopy to this widely studied class of U(v) complexes and identify areas for continued theoretical development.

Weak O2 binding and strong H2O2 binding at the non-heme diiron center of trypanosome alternative oxidase

Alternative oxidase (AOX) catalyzes the four-electron reduction of dioxygen to water as an additional terminal oxidase, and the catalytic reaction is critical for the parasite to survive in its bloodstream form. Recently, the X-ray crystal structure of trypanosome alternative oxidase (TAO) complexed with ferulenol was reported and the molecular structure of the non-heme diiron center was determined. The binding of O₂ was a unique side-on type compared to other iron proteins. In order to characterize the O₂ binding state of TAO, the O₂ binding states were searched at a quantum mechanics/molecular mechanics (QM/MM) theoretical level in the present study. We found that the most stable O₂ binding state is the end-on type, and the binding states of the side-on type are higher in energy. Based on the binding energies and electronic structure analyses, O₂ binds very weakly to the TAO iron center (ΔE =6.7 kcal mol^-1) in the electronic state of Fe(II)…OO, not in the suggested charge transferred state such as the superoxide state (Fe(III)OO· ^-) as seen in hemerythrin. Coordination of other ligands such as water, Cl^-, CN^-, CO, N₃^- and H₂O₂ was also examined, and H₂O₂ was found to bind most strongly to the Fe(II) site by ΔE = 14.0 kcal mol^-1. This was confirmed experimentally through the measurement of ubiquinol oxidase activity of TAO and Cryptosporidium parvum AOX which was found to be inhibited by H₂O₂ in a dose-dependent and reversible manner.

Ambiphilicity of a mononuclear cobalt(III) superoxo complex

Addition of HOTf to a mixture of CoIII(BDPP)(O2˙) (1, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*2Fe produced H2O2 in high yield implying formation of CoIII(BDPP)(OOH) (3), and reaction of Sc(OTf)3 with the same mixture gave a peroxo-bridged CoIII/ScIII5. These findings demonstrate the ambiphilic property of CoIII-superoxo 1.

Enhancing Tris(pyrazolyl)borate-Based Models of Cysteine/Cysteamine Dioxygenases through Steric Effects: Increased Reactivities, Full Product Characterization and Hints to Initial Superoxide Formation

The design of biomimetic model complexes for the cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO) is reported, where the 3-His coordination of the iron ion is simulated by three pyrazole donors of a trispyrazolyl borate ligand (Tp) and protected cysteine and cysteamine represent substrate ligands. It is found that the replacement of phenyl groups-attached at the 3-positions of the pyrazole units in a previous model-by mesityl residues has massive consequences, as the latter arrange to a more spacious reaction pocket. Thus, the reaction with O2 proceeds much faster and afterwards the first structural characterization of an iron(II) η2 -O,O-sulfinate product became possible. If one of the three Tp-mesityl groups is placed in the 5-position, an even larger reaction pocket results, which leads to yet faster rates and accumulation of a reaction intermediate at low temperatures, as shown by UV/Vis and Mössbauer spectroscopy. After comparison with the results of investigations on the cobalt analogues this intermediate is tentatively assigned to an iron(III) superoxide species.

Functional models of nonheme diiron enzymes: reactivity of the μ-oxo-μ-1,2-peroxo-diiron(iii) intermediate in electrophilic and nucleophilic reactions

The reactivity of the previously reported peroxo-adduct [FeIII2(μ-O)(μ-1,2-O2)(IndH)2(solv)2]2+ (1) (IndH = 1,3-bis(2-pyridyl-imino)isoindoline) has been investigated in nucleophilic (e.g., deformylation of alkyl and aryl alkyl aldehydes) and electrophilic (e.g. oxidation of phenols) stoichiometric reactions as biomimics of ribonucleotide reductase (RNR-R2) and aldehyde deformylating oxygenase (ADO) enzymes. Based on detailed kinetic and mechanistic studies, we have found further evidence for the ambiphilic behaviour of the peroxo intermediates proposed for diferric oxidoreductase enzymes.

Homogeneous Electrochemical Water Oxidation at Neutral pH by Water-Soluble NiII Complexes Bearing Redox Non-innocent Tetraamido Macrocyclic Ligands

Water oxidation is the bottleneck reaction in artificial photosynthesis. Exploring highly active and stable molecular water oxidation catalysts (WOCs) is still a great challenge. In this study, a water-soluble Ni^II complex bearing a redox non-innocent tetraamido macrocyclic ligand (TAML) is found to be an efficient electrocatalyst for water oxidation in neutral potassium phosphate buffer. Controlled-potential electrolysis experiments show that it can sustain at a steady current of approximately 0.2 mA cm^-2 for >7 h at 1.75 V versus normal hydrogen electrode (NHE) without the formation of NiO_x . Electrochemical and spectroelectrochemical tests show that the redox-active ligand, as well as HPO₄ ^2- in the buffer, participate in the catalytic cycle. More importantly, catalytically active intermediate [Ni^III (TAML^2- )-O^. ] is formed via several proton-coupled electron transfer processes and reacts with H₂ O with the assistance of base to release molecular oxygen. Thus, the employment of redox non-innocent ligands is a useful strategy for designing effective molecular WOCs.

A Manganese(IV)-Hydroperoxo Intermediate Generated by Protonation of the Corresponding Manganese(III)-Superoxo Complex

Earlier work revealed that metal-superoxo species primarily function as radicals and/or electrophiles. Herein, we present ambiphilicity of a Mn^III-superoxo complex revealed by its proton- and metal-coupled electron-transfer processes. Specifically, a Mn^IV-hydroperoxo intermediate, [Mn(BDP^BrP)(OOH)]⁺ (1, H₂BDP^BrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) was generated by treatment of a Mn^III-superoxo complex, Mn(BDP^BrP)(O₂^•) (2) with trifluoroacetic acid at -120 °C. Detailed insights into the electronic structure of 1 are obtained using resonance Raman and multi-frequency electron paramagnetic resonance spectroscopies coupled with density functional theory calculations. Similarly, the reaction of 2 with scandium(III) triflate was shown to give a Mn(IV)/Sc(III) bridging peroxo species, [Mn(BDP^BrP)(OO)Sc(OTf)_n]^(3-n)+ (4). Furthermore, it is found that deprotonation of 1 quantitatively regenerates 2, and that one-electron oxidation of the corresponding Mn^III-hydroperoxo species, Mn(BDP^BrP)(OOH) (3), also yields 1.

Nitric oxide monooxygenation (NOM) reaction of cobalt-nitrosyl {Co(NO)}8 to CoII-nitrito {CoII(NO2 -)}: base induced hydrogen gas (H2) evolution

Here, we report the nitric oxide monooxygenation (NOM) reactions of a CoIII-nitrosyl complex (1, {Co-NO}8) in the presence of mono-oxygen reactive species, i.e., a base (OH-, tetrabutylammonium hydroxide (TBAOH) or NaOH/15-crown-5), an oxide (O2- or Na2O/15-crown-5) and water (H2O). The reaction of 1 with OH- produces a CoII-nitrito complex {3, (CoII-NO2 -)} and hydrogen gas (H2), via the formation of a putative N-bound Co-nitrous acid intermediate (2, {Co-NOOH}+). The homolytic cleavage of the O-H bond of proposed [Co-NOOH]+ releases H2 via a presumed CoIII-H intermediate. In another reaction, 1 generates CoII-NO2 - when reacted with O2- via an expected CoI-nitro (4) intermediate. However, complex 1 is found to be unreactive towards H2O. Mechanistic investigations using 15N-labeled-15NO and 2H-labeled-NaO2H (NaOD) evidently revealed that the N-atom in CoII-NO2 - and the H-atom in H2 gas are derived from the nitrosyl ligand and OH- moiety, respectively.

Degradation strategies and associated regulatory mechanisms/features for aromatic compound metabolism in bacteria

As a result of anthropogenic activity, large number of recalcitrant aromatic compounds have been released into the environment. Consequently, microbial communities have adapted and evolved to utilize these compounds as sole carbon source, under both aerobic and anaerobic conditions. The constitutive expression of enzymes necessary for metabolism imposes a heavy energy load on the microbe which is overcome by arrangement of degradative genes as operons which are induced by specific inducers. The segmentation of pathways into upper, middle and/or lower operons has allowed microbes to funnel multiple compounds into common key aromatic intermediates which are further metabolized through central carbon pathway. Various proteins belonging to diverse families have evolved to regulate the transcription of individual operons participating in aromatic catabolism. These proteins, complemented with global regulatory mechanisms, carry out the regulation of aromatic compound metabolic pathways in a concerted manner. Additionally, characteristics like chemotaxis, preferential utilization, pathway compartmentalization and biosurfactant production confer an advantage to the microbe, thus making bioremediation of the aromatic pollutants more efficient and effective.

Ligand Taxonomy for Bioinorganic Modeling of Dioxygen-Activating Non-Heme Iron Enzymes

Novel functions emerge from novel structures. To develop efficient catalytic systems for challenging chemical transformations, chemists often seek inspirations from enzymatic catalysis. A large number of iron complexes supported by nitrogen-rich multidentate ligands have thus been developed to mimic oxo-transfer reactivity of dioxygen-activating metalloenzymes. Such efforts have significantly advanced our understanding of the reaction mechanisms by trapping key intermediates and elucidating their geometric and electronic properties. Critical to the success of this biomimetic approach is the design and synthesis of elaborate ligand systems to balance the thermodynamic stability, structural adaptability, and chemical reactivity. In this Concept article, representative design strategies for biomimetic atom-transfer chemistry are discussed from the perspectives of "ligand builders". Emphasis is placed on how the primary coordination sphere is constructed, and how it can be elaborated further by rational design for desired functions.

Heme-FeIII Superoxide, Peroxide and Hydroperoxide Thermodynamic Relationships: FeIII-O2•- Complex H-Atom Abstraction Reactivity

Establishing redox and thermodynamic relationships between metal-ion-bound O2 and its reduced (and protonated) derivatives is critically important for a full understanding of (bio)chemical processes involving dioxygen processing. Here, a ferric heme peroxide complex, [(F8)FeIII-(O22-)]- (P) (F8 = tetrakis(2,6-difluorophenyl)porphyrinate), and a superoxide complex, [(F8)FeIII-(O2•-)] (S), are shown to be redox interconvertible. Using Cr(η-C6H6)2, an equilibrium state where S and P are present is established in tetrahydrofuran (THF) at -80 °C, allowing determination of the reduction potential of S as -1.17 V vs Fc+/0. P could be protonated with 2,6-lutidinium triflate, yielding the low-spin ferric hydroperoxide species, [(F8)FeIII-(OOH)] (HP). Partial conversion of HP back to P using a derivatized phosphazene base gave a P/HP equilibrium mixture, leading to the determination of pKa = 28.8 for HP (THF, -80 °C). With the measured reduction potential and pKa, the O-H bond dissociation free energy (BDFE) of hydroperoxide species HP was calculated to be 73.5 kcal/mol, employing the thermodynamic square scheme and Bordwell relationship. This calculated O-H BDFE of HP, in fact, lines up with an experimental demonstration of the oxidizing ability of S via hydrogen atom transfer (HAT) from TEMPO-H (2,2,6,6-tetramethylpiperdine-N-hydroxide, BDFE = 66.5 kcal/mol in THF), forming the hydroperoxide species HP and TEMPO radical. Kinetic studies carried out with TEMPO-H(D) reveal second-order behavior, kH = 0.5, kD = 0.08 M-1 s-1 (THF, -80 °C); thus, the hydrogen/deuterium kinetic isotope effect (KIE) = 6, consistent with H-atom abstraction by S being the rate-determining step. This appears to be the first case where experimentally derived thermodynamics lead to a ferric heme hydroperoxide OO-H BDFE determination, that FeIII-OOH species being formed via HAT reactivity of the partner ferric heme superoxide complex.

Catalysis by the Non-Heme Iron(II) Histone Demethylase PHF8 Involves Iron Center Rearrangement and Conformational Modulation of Substrate Orientation

PHF8 (KDM7B) is a human non-heme 2-oxoglutarate (2OG) JmjC domain oxygenase that catalyzes the demethylation of the di/mono-Nε-methylated K9 residue of histone H3. Altered PHF8 activity is linked to genetic diseases and cancer; thus, it is an interesting target for epigenetic modulation. We describe the use of combined quantum mechanics/molecular mechanics (QM/MM) and molecular dynamics (MD) simulations to explore the mechanism of PHF8, including dioxygen activation, 2OG binding modes, and substrate demethylation steps. A PHF8 crystal structure manifests the 2OG C-1 carboxylate bound to iron in a nonproductive orientation, i.e., trans to His247. A ferryl-oxo intermediate formed by activating dioxygen bound to the vacant site in this complex would be nonproductive, i.e., "off-line" with respect to reaction with Nε-methylated K9. We show rearrangement of the "off-line" ferryl-oxo intermediate to a productive "in-line" geometry via a solvent exchange reaction (called "ferryl-flip") is energetically unfavorable. The calculations imply that movement of the 2OG C-1 carboxylate prior to dioxygen binding at a five-coordination stage in catalysis proceeds with a low barrier, suggesting that two possible 2OG C-1 carboxylate geometries can coexist at room temperature. We explored alternative mechanisms for hydrogen atom transfer and show that second sphere interactions orient the Nε-methylated lysine in a conformation where hydrogen abstraction from a methyl C-H bond is energetically more favorable than hydrogen abstraction from the N-H bond of the protonated Nε-methyl group. Using multiple HAT reaction path calculations, we demonstrate the crucial role of conformational flexibility in effective hydrogen transfer. Subsequent hydroxylation occurs through a rebound mechanism, which is energetically preferred compared to desaturation, due to second sphere interactions. The overall mechanistic insights reveal the crucial role of iron-center rearrangement, second sphere interactions, and conformational flexibility in PHF8 catalysis and provide knowledge useful for the design of mechanism-based PHF8 inhibitors.

Mechanistic dichotomies in redox reactions of mononuclear metal–oxygen intermediates

This review article focuses on various mechanistic dichotomies in redox reactions of metal–oxygen intermediates with the emphasis on understanding and controlling their redox reactivity from experimental and theoretical points of view.