Oxo Species Research Articles

(Invited) Structure and Reactivity of Ruthenium-Substituted Prussian-Blue-Type Oxo-Cyanide Bridged Networks: Prospects for Electrocatalysis and Analytical Determinations

Electrochemical deposition of crosslinked oxo-cyanoruthenate, Ru-O/CN-O, from a mixture of RuCl3 and K4Ru(CN)6 is known to yield a film on glassy carbon that promotes oxidations by a combination of the electron and oxygen transfers. The initial suggestion that the film could be a simple ruthenium analogue of Prussian Blue, in which the Ru ions reach higher then III oxidation state, was later clarified using Auger electron spectroscopy and X-ray photoelectron spectroscopy. It was postulated that the electrochemical deposition process yielded the -Ru-O-Ru- network with cyano-groups crosslinks, Ru-O/CN-O, where the ruthenium could exist in mixed oxidation states. Such mixed-valent conditions, facilitated the overall charge distribution and conductivity of the catalytic material. In addition, contrary to iron ions in Prussian Blue, ruthenium can be readily oxidized to higher oxidation states to form catalytically active ruthenium(IV,III) oxo species that include Ru3O2 6+ or Ru2O5 + groups immobilized within the cyanide-bridged network. Furthermore, at pH = 2, the hexacyanoruthenate anions tended to transformation to dinuclear anionic species, [(CN)5-RuIICN-RuIII-(CN)5]6 −. Strong electrostatic attractive interactions as well as the polynuclear character of deposits led to the overall stabilization of the system. Films of Ru-O/CN-O were demonstratedto have low ohmic resistances as a result of effective charge transport involving hydrogen and/or potassium ions.Numerous applications of Ru-O/CN-O, which have been reported or recently demonstrated, will be considered during presentation. The oxidation of alcohols, inorganic and organic sulfur compounds, glucose, hydrazine, aldehydes, nitrosamines, insulin, and tetracycline are representative examples. From the electrochemical analysis point of view, as it is typical with electrode reactions involving mediated electron transfers, the signal-to-background ratios with amperometry under potentiostatic conditions are preferred over potentiodynamic methodology for analytical applications. By the former approach, detection limits of nmol dm− 3 in an injection are achieved. For example, a linear calibration curve has been obtained over the range 8–200 ng insulin in a 7.5 µL injection into a flow system with measurement of current under potentiostatic conditions . Analogous to mixed metal oxides as modifiers, the efficacy of this catalyst should be related to the combination of fast electron displacements and specific oxygen transfers. The system’s resistance to interfacial passivation should be mentioned here. Finally, the carefully designed multicomponent systems provide a route to improving electrocatalytic efficiency over that at single-component materials because of electronic, geometric or stability factors. Combination of Ru-O/CN-O with silica sol-gels as solid electrolytes expands the scope of electrochemical applications and adapts electrocatalysis to chemical analysis of complex samples.

Engineering electronic and geometric structures of Fe, N-doped carbon polyhedrons toward organic contaminant oxidation

A universal gas diffusion strategy was developed to derive highly efficient PMS-activation catalysts comprising isolated Fe atoms coordinated with N species embedded in carbon polyhedrons (Fe-NC). Their electronic and geometric structures were modulated by altering the precursor ratio and calcination temperature. Benefiting from the maximized atomic utilization, the obtained catalysts exhibited superior catalytic performances, outperforming almost all previously reported metal materials. The enhanced activity is likely due to an optimal particle size and Fe content, favorable Fe-Nx active sites, and conductive carbon materials. Scavenging and probing experiments indicated that surface Fe-NC and the defective edges with oxygen functional groups were the active sites for PMS activation to produce surface-bonding active complexes (Fe-NC-PMS) and high-valent iron oxo species (FeV = O) for organic degradation, unlike the reported radicals and singlet oxygen mediated degradation. Overall, this work provides a universal gas diffusion strategy to design dimension-controlled Fe-NC catalysts for the elimination of organic pollutants.

Preparation of Tripodal Vanadium Oxo-Organometallic Species Supported on Silica, [(≡SiO)3V(═O)], for Selective Nonoxidative Dehydrogenation of Propane

In this work, a strategy based on the grafting of a vanadium molecular complex, [V(═O)(OtBu)3], (1), bearing tert-butoxide ligands, on a partially dehydroxylated silica, followed by a thermal treatment has been used for the preparation of supported tripodal V oxo complex, [(≡SiO)3V(═O)], active species, for important chemical processes. The molecular complex reacts with a silica partially dehydroxylated at 200 °C (SiO2–200) to lead to bipodal [(≡SiO)2V(═O)(OtBu)] (2) as major surface species, while monopodal surface species [(≡SiO)V(═O)(OtBu)2] (3) are obtained on SiO2–700. These materials were thermally treated to form active supported tripodal vanadium oxo species by releasing isobutene via a β-H transfer, leading to V–OH intermediate. The latter species lead to the targeted tripodal oxo species [(≡SiO)3V(═O)] after restructuration of the support and will be represented as materials (2a) and (3a). All of the supported vanadium complexes were characterized by diffuse reflectance infrared Fourier transform (DRIFT), UV–vis, solid-state NMR, and X-ray absorption near edge structure (XANES) spectroscopies. In particular, the structure of isolated vanadium oxo tripodal species, [(≡SiO)3V(═O)], was confirmed by 51V NMR with a major peak at −670 ppm, as reported in the literature. The catalytic property of (2a) and (3a) was evaluated for the direct propane dehydrogenation and showed promising performances that can be solely assigned to the isolated vanadium tripodal species.

Dioxygen Activation and Nδ,Nε-Dihydroxylation Mechanism Involved in the Formation of N-Nitrosourea Pharmacophore in Streptozotocin Catalyzed by Nonheme Diiron Enzyme SznF.

SznF is a nonheme diiron-dependent enzyme that catalyzes the critical N-nitrosation involved in the formation of the N-nitrosourea moiety in the pancreatic cancer drug streptozotocin. The N-nitrosation contains two successive N-hydroxylation and N-nitrosation steps, which are carried out by two separate active sites, namely, the central domain and cupin domain. Recently, the crystal structure of SznF was obtained, and the central domain was proved to contain a diiron cofactor to catalyze the N-hydroxylation. In this work, to gain insights into the O2 activation and the successive N-hydroxylation mechanism, on the basis of the high-resolution crystal structure, the enzyme-substrate complex models were constructed, and a series of combined QM/MM calculations were performed. Based on our calculations, the activation of O2 starts from the diiron(II,III)-superoxo (S) to generate the diiron(IV)-oxo species (Q) via a diiron(III,III)-peroxo (P)-like transition state or unstable intermediate (P'), and species P' can be described as a hybridization of diiron(IV)-oxo species and diiron(III,III)-peroxo (P) owing to the long distances of Fe1-Fe2 (4.22 Å) and O1-O2 (1.89 Å), which is different from those of other nonheme diiron enzymes. In the following hydroxylation of Nδ and Nε, the Nδ-hydroxylation was confirmed to occur first, agreeing with the experimental observations. Because the diiron(IV)-oxo species (Q) is responsible for hydroxylation, the reaction follows the H-abstraction/OH rebound mechanism, and the first abstraction occurs on the Nδ-H rather than Nε-H, which may be attributed to the different orientation of Fe(IV)-oxo relative to N-H as well as the bond dissociation enthalpies of two N-H bonds. The hydroxylation of N-methyl-L-arginine does not employ the diiron(III,III)-hydroperoxo (P″) to trigger the electrophilic attack of the guanidine to directly form the N-O bond, as previously suggested. In addition, our calculations also revealed that the direct attack of the Fe(IV)═O unit to the Nδ of the substrate corresponds to a higher barrier than that in the H-abstraction/OH rebound mechanism. These results may provide useful information for understanding the formation of the di-hydroxylation intermediate involved in the biosynthesis of N-nitrosation.

Kinetics of chromium(V)-oxo and chromium(IV)-oxo porphyrins: Reactivity and mechanism for sulfoxidation reactions

In this work, chromium(IV)-oxo porphyrins [CrIV(Por)(O)] (2) (Por = porphyrin) were produced either by oxidation of [CrIII(Por)Cl] (1) with iodobenzene diacetate or visible light photolysis of porphyrin‑chromium(III) chlorates. Subsequent oxidation of 2 with silver perchlorate gave chromium(V)-oxo porphyrins [CrV(Por)(O)](ClO4) (3) in three porphyrin ligands, including 5,10,15,20-tetramesitylporphyrin(TMP, a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin(TDFPP, b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP, c). Complexes 2 and 3 reacted with thioanisoles to produce the corresponding sulfoxides, and their kinetics of sulfoxidation reactions with a series of aryl methyl sulfides(thioanisoles) were studied in organic solutions. Chromium(V)-oxo porphyrins are several orders of magnitudes more reactive than chromium(IV)-oxo species, and representative second-order rate constants (kox) for the oxidation of thioansole are (0.40 ± 0.01) M−1 s−1 (3a), and (2.82 ± 0.20) × 102 M−1 s−1 (3b), and (2.20 ± 0.01) × 103 M−1 s−1 (3c). The order of reactivity for 2 and 3 follows TPFPP > TDFPP > TMP, in agreement with the electrophilic nature of metal-oxo complexes. Hammett analyses indicate significant charge transfer in the transition states for oxidation of para-substituted thioanisoles by [CrV(Por)(O)]+. The ρ+ constants are −1.69 for 3a, −2.63 for 3b, and − 2.89 for 3c, respectively, mirror values found previously for related metal-oxo species. A mechanism involving the electrophilic attack of the CrV-oxo at sulfides to form a sulfur cation intermediate in the rate-determining step is suggested. Competition studies with chromium(III) porphyrin chloride and PhI(OAc)2 gave relative rate constants for oxidations of competing thioanisoles that closely match ratios of absolute rate constants from chromium(V)-oxo species, which are true oxidants under catalytic conditions.

Generation of ruthenium(IV)-oxo species and their utilization in oxidation of different substrates

Generation of ruthenium(IV)-oxo species and their utilization in oxidation of different substrates

An iron chlorophyll derivative for enhanced degradation of bisphenol A: New insight into the generation mechanism of high-valent iron oxo species

The pressure to environmental concern from toxic precursors of metalloporphyrin synthesis and the inadequate understanding of metalloporphyrin with β-site substituents on activation of peroxides have impeded the full potential of metalloporphyrin for the oxidation of recalcitrant organic compounds. Here, we report a “green” iron(III) chlorophyll derived from naturally abundant chlorophyll. It shows remarkable degradation efficiency of bisphenol A in the presence of hydrogen peroxide, with a second-order rate constant of 87.3 ± 1.1 L·mmol−1·min−1, which is 7-fold faster than the degradation rate of the Fenton oxidation system. It also demonstrates a wide pH tolerance range of 4–12 and is barely affected by concomitant inorganic anions or natural organic matter. Different from the generation of oxygen-centered radicals, including •OH and •O2−, in the Fenton system, a high-valent iron (Ⅳ) -oxo chlorin π-cation radical is formed with an atypical pathway of homolytic O O bond cleavage via a proton-coupled electron transfer process and is responsible for the degradation of bisphenol A, which is confirmed with spectroscopic studies and theoretical analysis. This abundant “green” catalyst could be beneficial for wastewater treatment by addressing the limitation of a narrow acidic pH range and the susceptibility to scavenging of nonselective radicals in conventional Fenton reactions.

How Do the Axial and Equatorial Ligands Modulate the Reactivity of a Metal-Bound Terminal Oxidant? An Answer from the Hypochlorite Adduct of Iron(III) Porphyrin

Terminal oxidant adducts of transition-metal complexes have been known as not only the precursors of high-valent oxo species but also the reactive compounds that can catalyze various oxygenation reactions. While there have been various reports on the terminal oxidant adducts of peroxides, peracids, and iodosylarenes, the report of the hypochlorite adduct of metal complexes has been limited. We succeed in the preparation and characterization of bis-hypochlorite adducts of iron(III) porphyrin complexes having various electron-withdrawing substituents. The spectroscopic (ultraviolet–visible absorption, nuclear magnetic resonance, electron paramagnetic resonance, and extended X-ray absorption fine structure) studies and density functional theory calculations indicate that as the electron-donor effect of the porphyrin and axial ligands becomes stronger the Fe–OCl and O–Cl bonds of the iron-bound hypochlorite become weaker and thus the hypochlorite complex becomes more unstable. Furthermore, the kinetic analyses of the hypochlorite complexes for the epoxidation, chlorination, sulfoxidation, and hydrogen abstraction reactions indicate that as the Fe–OCl and O–Cl bonds become weaker the iron-bound hypochlorite becomes more reactive. These results allow us to answer the question how the axial and equatorial ligands control the reactivity of the iron-bound hypochlorite. The key is the electron migration from the fully filled OCl π*-orbitals to the vacant Fe 3dz2- and 4p-orbitals, which is essential for making the Fe–OCl coordination bond. An increase in the electron-donor effect of the ligand decreases the electron migration from the OCl π*-orbitals because of an increase in the energies of the Fe 3dz2- and 4p-orbitals. This change makes the electron density in the OCl π*-orbitals high, the Fe–O and O–Cl bonds weak, and the hypochlorite complex reactive. This would be a common mechanism for controlling the reactivity of other terminal oxidant adducts of transition-metal complexes.

Heterolytic O-O Bond Cleavage Upon Single Electron Transfer to a Nonheme Fe(III)-OOH Complex.

The one-electron reduction of the nonheme iron(III)-hydroperoxo complex, [FeIII (OOH)(L5 2 )]2+ (L5 2 =N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine), carried out at -70 °C results in the release of dioxygen and in the formation of [FeII (OH)(L5 2 )]+ following a bimolecular process. This reaction can be performed either with cobaltocene as chemical reductant, or electrochemically. These experimental observations are consistent with the disproportionation of the hydroperoxo group in the putative FeII (OOH) intermediate generated upon reduction of the FeIII (OOH) starting complex. One plausible mechanistic scenario is that this disproportionation reaction follows an O-O heterolytic cleavage pathway via a FeIV -oxo species.

Application of Mixed-Metal-Oxides As Active Supports for Dispersed Metal Centers: Enhancement of Electrocatalytic Reduction of Carbon Dioxide

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity.For example, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic copper electrocatalytic nanoparticles (generated from Cu2O) over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the reduction of oxygen or carbon dioxide have been enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Pt or Cu) and the metal (W, Zr) oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the specific interactions (via the surface hydroxyl groups) with the reaction intermediates (e.g. CO adsorbates). Mechanistic studies involving chronoamperometric, voltammetric and gas-diffusion electrode measurements are pursued, in addition to spectroscopic and XPS measurements. Results of comparative measurements, involving single components only, will be provided as well. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627.

Enhancement of Activity Low-Pt-Content O2-Reduction Catalysts through Formation of Hybrid Systems with Sub-Stoichiometric Cerium Oxide Nanostructures

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, under such condition, O2-reduction reaction (ORR) often proceeds at a less positive potential, and produces higher amounts of undesirable H2O2-intermediate. Recent studies have demonstrated that the rational design of a catalytic system is based on nanostructures of platinum, or carbon-supported platinum (Pt/C), and metal oxide (MOx). By combining the catalytic properties of both components, such hybrid Pt-MOx catalysts are capable of tuning of the activity and durability of Pt. Due to strong interactions of MOx with Pt catalytic nanoparticles leading to the improved activity and durability during ORR, certain nanostructured metal oxides are considered as co-catalysts or active components of supports. The existence of specific interactions between MOx and noble metal (Pt) nanoparticles should improve the stability and activity of the metal catalytic sites due to modification of the Pt electronic structure and diminishing of the oxo (OH) species adsorption on Pt surface, thus promoting centers for the adsorption of oxygen and the cleavage of O=O bonds. The interactions mentioned above would also facilitate dispersion of Pt, inhibit their detachment and further aggregation, and, consequently, prevent or decrease their degradation during the fuel cell operation [1].The catalytic activity of substoichiometric ceria, CeOx (where x < 2) additive results from its unique structure, presence of oxygen vacancies, and other features resulting from the 4f-electronic configuration of cerium. While the oxygen defects are likely to serve as active oxygen adsorption sites, the mixed-valent CeIII/CeIV redox sites should permit the electron shuffling within the lattice of oxygen vacancies and enhancing the ORR activity. The formation of oxygen-defects is accompanied by the localization of electrons left behind in Ce 4f states, thus leading to the formation of CeIII species capable of elongating and reducing the O–O bond strength of the adsorbed O2 molecule; consequently, by increasing the relative ratio of CeIII-to-CeIV, the ORR electrocatalytic activity can be improved [1]. Therefore, in the present study, we consider the intentionally reduced CeOx nanostructured components through subjecting them to high-temperature pretreatment in the presence of argon gas admixed with hydrogen. It is reasonable to expect that, the increased population of CeIII sites on the surfaces of the pretreated ceria particles would stabilize the neighboring active Pt-metal centers, exhibit reductive interactions toward Pt-oxo species, and to improve durability of the catalytic materials. Thus the boundary formed between the Pt-metal and CeOx-metal oxide should facilitate inhibition of the Pt oxide formation by the CeOx layer. Furthermore, our recent studies clearly show that, in the presence of ceria, the oxidative degradation of carbon carriers is also largely decreased. Finally, cerium oxide is known to act as the oxidative scavenger for free radicals such as hydroxyl (HO•) and hydroperoxyl (HOO•), which, once generated, would otherwise lead to the formation of undesirable hydrogen peroxide. Sub-stoichiometric CeOx is capable of rapidly switching between CeIII and CeIV oxidation states, thus inducing the decomposition of both radicals and peroxides. The above observation seems to be very helpful when it comes to designing highly active and durable ORR catalysts containing low platinum loadings.[1] A.Kostuch, I.A. Rutkowska, B. Dembinska, A. Wadas, E. Negro, K. Vezzù, V. Di Noto, P.J. Kulesza, Molecules 26 (2021) 5147. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627) and under auspices of the European Union EIT Raw Materials ALPE 19247 Project (Specific Grant Agreement No. EIT/RAW MATERIALS/SGA2020/1).

Reactivity of Tantalum/Iridium and Hafnium/Iridium Alkyl Hydrides with Alkyl Lithium Reagents: Nucleophilic Addition, Alpha-H Abstraction, or Hydride Deprotonation?

Cp*IrH4 reacts with Hf(Np)4 (Np = CH2C(CH3)3) to yield the heterobimetallic complex [Hf(Np)3(μ-H)3IrCp*], 2. Treatment of 2 with neopentyl lithium (NpLi) results in the nucleophilic addition of a neopentyl moiety onto the Hf center, restoring Hf(Np)4 and displacing the Cp*IrH3– iridate fragment. By contrast, treatment of the tantalum/iridium analogue [Ta(Np)3IrH2(Cp*)], 1, with NpLi triggers the α-hydrogen abstraction at a Ta–neopentyl moiety and leads to the formation of the alkylidene -ate complex [Li][{Ta(CHtBu)(Np)2}{IrH2Cp*}], 4. The benzylidene analogue [K][{Ta(CHPh)(Np)2}{IrH2Cp*}], 5, is obtained upon reacting 1 with benzyl potassium. An unusual vinyl oxo species, [{Li(THF)3}Ta(O)(Np)2{μ-C2H3}IrH(Cp*)], 6, is formed when the reaction of 1 with NpLi is carried out in tetrahydrofuran (THF) due to THF fragmentation.

Hidden Complexity in the Mechanism of the Autoreduction of Myoglobin Compound II.

The non-native oxidation of horse heart myoglobin with hydrogen peroxide produces compound II which autoreduces by utilizing an internal oxidation site. Here, we utilize full UV–visible time-dependent kinetics with global kinetic singular value decomposition analysis to explore the mechanism and uncover more detail about the high-valent heme spectral features. By varying the hydrogen peroxide and myoglobin concentration, we were able to uncover more detailed spectra of myoglobin compound II and the autoreduction rate under several different pH conditions. The compound II spectra demonstrate pH-dependent features with an inflection point around pH 5.7 ± 0.1. The rate of autoreduction of compound II, k2, increases with lower pH with a half-power proton dependence and no indication of a pKa > 3.9 ± 0.2, indicating that the autoreduction is still dependent on the protonation of the ferryl oxo species. The k2 also demonstrates both hydrogen peroxide and myoglobin dependency. At myoglobin concentrations greater than 6.6 μM, the k2 is myoglobin-independent, but for lower concentrations, a pH-sensitive concentration dependence is seen.

Study the activation mechanism of peroxymonosulfate in iron copper systems for trichloroethane degradation

The quick development of novel water treatment techniques evaluates the mechanisms as a challenge in various systems. The participation of radical and non-radical mechanisms in the degradation of trichloroethane (TCA) in peroxymonosulfate (PMS) activated systems was demonstrated using synthetic catalysts. Various analytical characterizations represented the prominent structure and morphology of CuO and FeS2 particles. Under various circumstances, the CuO/rGO/PMS and FeS2/PMS catalysts demonstrated the coexistence of mechanisms. When compared to CuO/rGO/PMS, the FeS2/PMS system had the highest TCA removal effectiveness. The remediation performance was largely dependent on a variety of factors. The TCA degradation increased with increasing acidity in the FeS2/PMS system, whereas CuO/rGO favored basic conditions. The involvement of radical (SO4●−, OH●, and O2●−) and non-radical oxo (1O2) species has been confirmed by the chemical probe, quenching, and EPR experiment. The scavenging test identified the generation of 1O2 species in all systems that played a critical role in the studied processes. The presence of both radical and non-radical pathways in TCA degradation can be useful in the design and use of PMS activated in groundwater treatment.

Cover Picture: Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO2 Catalysts (Angew. Chem. Int. Ed. 23/2022)

Operando spectroscopy captures the dynamic behavior of ceria-supported Pd single atoms, clusters, and nanoparticles during CO oxidation. In their Research Article (e202200434), Emiel J. M. Hensen et al. elucidate the catalytic role of different Pd species using a combination of in situ spectroscopy with transient and steady-state kinetic analysis. Highly dispersed Pd–oxo species and interfacial Pd sites catalyze low-temperature CO oxidation, while metallic Pd contributes to the activity at elevated temperature.

Mechanism of Methyldehydrofosmidomycin Maturation: Use Olefination to Enable Chain Elongation.

Utilization of mononuclear iron- and 2-oxoglutarate-dependent (Fe/2OG) enzymes to enable C-H bond functionalization is a widely used strategy to diversify the structural complexity of natural products. Besides those well-studied reactions including hydroxylation, epoxidation, and halogenation, in the biosynthetic pathway of dehydrofosmidomycin, an Fe/2OG enzyme is reported to catalyze desaturation, alkyl chain elongation, along with demethylation in which trimethyl-2-aminoethylphosphonate is converted into methyldehydrofosmidomycin. How this transformation takes place is largely unknown. Herein, we characterized the reactive species, revealed the structure of the reaction intermediate, and used mechanistic probes to investigate the reaction pathway and mechanism. These results led to the elucidation of a two-step process in which the first reaction employs a long-lived Fe(IV)-oxo species to trigger C═C bond installation. During the second reaction, the olefin installed in situ enables C-C bond formation that is accompanied with a C-N bond cleavage and hydroxylation to furnish the alkyl chain elongation and demethylation. This work expands the reaction repertoire of Fe/2OG enzymes by introducing a new pathway to the known C-C bond formation mechanisms utilized by metalloenzymes.

In Silico simulation of Cytochrome P450-Mediated metabolism of aromatic amines: A case study of N-Hydroxylation

Aromatic amines, the widely used raw materials in industry, cause long-term exposure to human bodies. They can be metabolized by cytochrome P450 enzymes to form active electrophilic compounds, which will potentially react with nucleophilic DNA to exert carcinogenesis. The short lifetime and versatility of the oxidant (a high-valent iron (IV)-oxo species, compound I) of P450 enzymes prompts us to use theoretical methods to investigate the metabolism of aromatic amines. In this work, the density functional theory (DFT) has been employed to simulate the hydroxylation metabolism through H-abstraction and to calculate the activation energy of this reaction for 28 aromatic amines. The results indicate that the steric effects, inductive effects and conjugative effects greatly contribute to the metabolism activity of the chemicals. The further correlation reveals that the dissociation energy of -NH2 (BDEN-H) can successfully predict the time-consuming calculated activation energy (R2 for aromatic and heteroaromatic amines are 0.93 and 0.86, respectively), so BDEN-H can be taken as a key parameter to characterize the relative stability of aromatic amines in P450 enzymes and further to quickly assess their potential toxicity. The validation results prove such relationship has good statistical performance (qcv2 for aromatic and heteroaromatic amines are 0.95 and 0.90, respectively) and can be used to other aromatic amines in the application domain, greatly reducing computational cost and providing useful support for experimental research.

Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides.

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

Cluster Model Study into the Catalytic Mechanism of α-Ketoglutarate Biodegradation by the Ethylene-Forming Enzyme Reveals Structural Differences with Nonheme Iron Hydroxylases

Ethylene is an important signaling molecule in plants that triggers the growth of leaves, flowers, and fruits. One of the enzymes involved in the biosynthesis of ethylene is the ethylene-forming enzyme (EFE), which is an usual nonheme iron enzyme that biodegrades α-ketoglutarate into three CO2 molecules and ethylene. As the detailed mechanism of EFE in the biosynthesis of ethylene remains controversial and particularly the function of the co-substrate l-arginine, we decided to pursue a density functional theory study on the possible pathways of the enzyme leading to ethylene biosynthesis and test many possible pathways and mechanisms. A large active site cluster model of 322 atoms was created, which contains all the features of the first- and second-coordination sphere of the active site and substrate (α-ketoglutarate) binding pockets. The calculations identify a persuccinate intermediate that triggers a bifurcation pathway in the enzyme and either react with a molecule of CO2 to form a carbonate or forms a high-valent iron(IV)-oxo species through heterolytic dioxygen bond cleavage. Our studies show that both the bifurcation pathways converge to the same intermediate again and can lead to ethylene products, although the two pathways have different kinetics. Interestingly, our studies also show that the iron(IV)-oxo itself can form a carbonate and ethylene but through much higher barriers. As a matter of fact, these barriers are higher in energy than the typical aliphatic hydroxylation barriers and may not be competitive with arginine hydroxylation. Inclusion of the l-arginine co-substrate into the model leads to minor changes in the structure and fold, but its charge and dipole moment does not seem to affect the first stage of the catalytic cycle. Moreover, the key activation barriers seem less affected by the inclusion of l-arginine into the model. We, therefore, believe that the role of l-arginine is to lock α-ketoglutarate and its products into a tight binding pocket to enable its degradation and to prevent early release of CO2. Our studies show that due to the distinct differences in α-ketoglutarate positioning between different arginine activating nonheme iron dioxygenases in the co-substrate binding pocket and its tighter binding in EFE, we predict that the release of CO2 is prevented in the first stage of the oxygen activation mechanism. This enables attack of CO2 on a persuccinate complex to form carbonate products, leading to ethylene formation. This work gives suggestions on the engineering of EFE into a hydroxylase or improving the ethylene biosynthesis.

Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts.

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si–OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.