Stoichiometric Aldehyde Deformylation Mediated by Nucleophilic Peroxo-diiron(III) Complex as a Functional Model of Aldehyde Deformylating Oxygenase

Investigating the evolution of reactive species in the CuO-mediated peroxymonosulfate activation process

The utilization of copper oxide (CuO) as a catalyst in the peroxymonosulfate (PMS) activation process holds great promise for effectively degrading aqueous organic pollutants, while the relevant mechanism remains inadequately understood. In this study, we delve into the evolution pathways of reactive species in the CuO/PMS system through a comprehensive series of experimental analyses. Our findings indicate that various reactive species are generated in the CuO/PMS system with the specific sequence, where the decomposition of surface Cu(II)-OOSO3- leads to the formation of surface Cu(III) species, which are responsible for the subsequent generation of HO•. The reactivity of these reactive species and the sequence of their generation explain the distinct oxidation behaviors of pollutants with different values of ionization potential (IP). In addition, singlet oxygen (1O2) may be produced during the PMS activation process, while its involvement in the oxidation of substrates is deemed negligible. This investigation presents a novel perspective, enhancing our comprehension of the mechanism underlying transition metal-mediated PMS activation processes. ENVIRONMENTAL IMPLICATION: The removal of refractory organic contaminations in water constitutes a fundamental concern within the realm of environmental pollution management. Peroxymonosulfate activation induced by transition metal oxides has garnered significant recognition as a promising technological approach for the degradation of aqueous organic contaminants, while the underlying mechanism remains enigmatic. In this study, we systematically investigate the evolution pathways of reactive species in the CuO/peroxymonosulfate system to reveal the mystery of the reaction mechanism between CuO and peroxymonosulfate. The outcomes of our study contribute to enhancing the practical applicability of transition metal-triggered PMS activation processes.

Mechanisms of Sulfoxidation and Epoxidation Mediated by Iron(III)-Iodosylbenzene Adduct: Electron-Transfer vs. Oxygen-Transfer Mechanism

The mechanisms of sulfoxidation and epoxidation mediated by previously synthesized and characterized iron(III)-iodosylbenzene adduct, Fe^III(OIPh) were investigated using para-substituted thioanisole and styrene derivatives as model substrates. Based on detailed kinetic reaction experiments, including the linear free-energy relationships between the relative reaction rates (logk_rel) and the σ_p (4R-PhSMe) with ρ = -0.65 (catalytic) and ρ = -1.13 (stoichiometric), we obtained strong evidence that the stoichiometric and catalytic oxidation of thioanisoles mediated by Fe^III(OIPh) species involves direct oxygen transfer. The small negative slope -2.18 from log k_obs versus E_ox for 4R-PhSMe gives further clear evidence for the direct oxygen atom transfer mechanism. On the contrary, with the linear free-energy relationships between the relative reaction rates (logk_rel) and total substituent effect (TE, 4R-PhCHCH₂) parameters with slope = 0.33 (catalytic) and 2.02 (stoichiometric), the stoichiometric and catalytic epoxidation of styrenes takes place through a nonconcerted electron transfer (ET) mechanism, including the formation of the radicaloid benzylic radical intermediate in the rate-determining step. On the basis of mechanistic studies, we came to the conclusion that the title iron(III)-iodosylbenzene complex is able to oxygenate sulfides and alkenes before it is transformed into the oxo-iron form by cleavage of the O-I bond.

Effect of Redox Potential on Diiron-Mediated Disproportionation of Hydrogen Peroxide

Heme and nonheme dimanganese catalases are widely distributed in living organisms to participate in antioxidant defenses that protect biological systems from oxidative stress. The key step in these processes is the disproportionation of H₂O₂ to O₂ and water, which can be interpreted via two different mechanisms, namely via the formation of high-valent oxoiron(IV) and peroxodimanganese(III) or diiron(III) intermediates. In order to better understand the mechanism of this important process, we have chosen such synthetic model compounds that can be used to map the nature of the catalytically active species and the factors influencing their activities. Our previously reported μ-1,2-peroxo-diiron(III)-containing biomimics are good candidates, as both proposed reactive intermediates (Fe^IVO and Fe^III₂(μ-O₂)) can be derived from them. Based on this, we have investigated and compared five heterobidentate-ligand-containing model systems including the previously reported and fully characterized [Fe^II(L^1-4)₃]²⁺ (L¹ = 2-(2'-pyridyl)-1H-benzimidazole, L² = 2-(2'-pyridyl)-N-methyl-benzimidazole, L³ = 2-(4-thiazolyl)-1H-benzimidazole and L⁴ = 2-(4'-methyl-2'-pyridyl)-1H-benzimidazole) and the novel [Fe^II(L⁵)₃]²⁺ (L⁵ = 2-(1H-1,2,4-triazol-3-yl)-pyridine) precursor complexes with their spectroscopically characterized μ-1,2-peroxo-diiron(III) intermediates. Based on the reaction kinetic measurements and previous computational studies, it can be said that the disproportionation reaction of H₂O₂ can be interpreted through the formation of an electrophilic oxoiron(IV) intermediate that can be derived from the homolysis of the O-O bond of the forming μ-1,2-peroxo-diiron(III) complexes. We also found that the disproportionation rate of the H₂O₂ shows a linear correlation with the Fe^III/Fe^II redox potential (in the range of 804 mV-1039 mV vs. SCE) of the catalysts controlled by the modification of the ligand environment. Furthermore, it is important to note that the two most active catalysts with L³ and L⁵ ligands have a high-spin electronic configuration.

Stoichiometric Alkane and Aldehyde Hydroxylation Reactions Mediated by In Situ Generated Iron(III)-Iodosylbenzene Adduct

Previously synthesized and spectroscopically characterized mononuclear nonheme, low-spin iron(III)-iodosylbenzene complex bearing a bidentate pyridyl-benzimidazole ligands has been investigated in alkane and aldehyde oxidation reactions. The in situ generated Fe(III) iodosylbenzene intermediate is a reactive oxidant capable of activating the benzylic C-H bond of alkane. Its electrophilic character was confirmed by using substituted benzaldehydes and a modified ligand framework containing electron-donating (Me) substituents. Furthermore, the results of kinetic isotope experiments (KIE) using deuterated substrate indicate that the C-H activation can be interpreted through a tunneling-like HAT mechanism. Based on the results of the kinetic measurements and the relatively high KIE values, we can conclude that the activation of the C-H bond mediated by iron(III)-iodosylbenzene adducts is the rate-determining step.

Catalytic and Stoichiometric Baeyer-Villiger Oxidation Mediated by Nonheme Peroxo-Diiron(III), Acylperoxo, and Iodosylbenzene Iron(III) Intermediates

In this paper we describe a detailed mechanistic studies on the [Fe^II(PBO)₂(CF₃SO₃)₂] (1), [Fe^II(PBT)₂(CF₃SO₃)₂] (2), and [Fe^II(PBI)₃](CF₃SO₃)₂ (3)-catalyzed (PBO = 2-(2′-pyridyl)benzoxazole, PBT = 2-(2′-pyridyl)benzthiazole, PBI = 2-(2′-pyridyl)benzimidazole) Baeyer–Villiger oxidation of cycloketones by dioxygen with cooxidation of aldehydes and peroxycarboxylic acids, including the kinetics on the reactivity of (μ-1,2-peroxo)diiron(III), acylperoxo- and iodosylbenzene-iron(III) species as key intermediates.

Spectroscopic Evidence for a Cobalt-Bound Peroxyhemiacetal Intermediate

Aldehyde deformylation reactions by metal dioxygen adducts have been proposed to involve peroxyhemiacetal species as key intermediates. However, direct evidence of such intermediates has not been obtained to date. We report the spectroscopic characterization of a mononuclear cobalt(III)-peroxyhemiacetal complex, [Co(Me₃-TPADP)(O₂CH(O)CH(CH₃)C₆H₅)]⁺ (2), in the reaction of a cobalt(III)-peroxo complex (1) with 2-phenylpropionaldehyde (2-PPA). The formation of 2 is also investigated by isotope labeling experiments and kinetic studies. The conclusion that the peroxyhemiacetalcobalt(III) intermediate is responsible for the aldehyde deformylation is supported by the product analyses. Furthermore, isotopic labeling suggests that the reactivity of the cobalt(III)-peroxo complex depends on the second reactant. The aldehyde inserts between the oxygen atoms of 1, whereas the reaction with acyl chlorides proceeds by a nucleophilic attack. The observation of the peroxyhemiacetal intermediate provides significant insight into the initial step of aldehyde deformylation by metalloenzymes.

Disproportionation of H2O2 Mediated by Diiron-Peroxo Complexes as Catalase Mimics

Heme iron and nonheme dimanganese catalases protect biological systems against oxidative damage caused by hydrogen peroxide. Rubrerythrins are ferritine-like nonheme diiron proteins, which are structurally and mechanistically distinct from the heme-type catalase but similar to a dimanganese KatB enzyme. In order to gain more insight into the mechanism of this curious enzyme reaction, non-heme structural and functional models were carried out by the use of mononuclear [Fe^II(L_1-4)(solvent)₃](ClO₄)₂ (1-4) (L₁ = 1,3-bis(2-pyridyl-imino)isoindoline, L₂ = 1,3-bis(4'-methyl-2-pyridyl-imino)isoindoline, L₃ = 1,3-bis(4'-Chloro-2-pyridyl-imino)isoindoline, L₄ = 1,3-bis(5'-chloro-2-pyridyl-imino)isoindoline) complexes as catalysts, where the possible reactive intermediates, diiron-perroxo [Fe^III₂(μ-O)(μ-1,2-O₂)(L₁-L₄)₂(Solv)₂]²⁺ (5-8) complexes are known and well-characterized. All the complexes displayed catalase-like activity, which provided clear evidence for the formation of diiron-peroxo species during the catalytic cycle. We also found that the fine-tuning of iron redox states is a critical issue, both the formation rate and the reactivity of the diiron-peroxo species showed linear correlation with the Fe^III/Fe^II redox potentials. Their stability and reactivity towards H₂O₂ was also investigated and based on kinetic and mechanistic studies a plausible mechanism, including a rate-determining hydrogen atom transfer between the H₂O₂ and diiron-peroxo species, was proposed. The present results provide one of the first examples of a nonheme diiron-peroxo complex, which shows a catalase-like reaction.

Are Free Radicals the Primary Reactive Species in Co(II)-Mediated Activation of Peroxymonosulfate? New Evidence for the Role of the Co(II)-Peroxymonosulfate Complex

The catalytic activation of peroxymonosulfate (PMS) is under intensive investigation with potentials as an alternative advanced oxidation process (AOP) in wastewater treatment. Among all catalysts examined, Co(II) exhibits the highest reactivity for the activation of PMS, following the conventional Fenton-like mechanism, in which free radicals (i.e., sulfate radicals and hydroxyl radicals) are reckoned as the reactive species. Herein, we report that the primary reactive species (PRS) is proposed to be a Co(II)-PMS complex (Co(II)-OOSO₃^-), while free radicals and Co(III) species act as the secondary reactive species (SRS) that play a minor role in the Co(II)/PMS process. This Co(II)-OOSO₃^- exhibits several intriguing properties including ability to conduct both one-electron-transfer and oxygen-atom-transfer reactions with selected molecules, both nucleophilic and electrophilic in nature, and strongly pH-dependent reactivity. This study provides novel insights into the chemical nature of the Co(II)-catalyzed PMS activation process.

A nonheme peroxo-diiron(III) complex exhibiting both nucleophilic and electrophilic oxidation of organic substrates

The complex [Fe^III₂(μ-O₂)(L₃)₄(S)₂]⁴⁺ (L₃ = 2-(4-thiazolyl)benzimidazole, S = solvent) forms upon reaction of [Fe^II(L₃)₂] with H₂O₂ and is a functional model of peroxo-diiron intermediates invoked during the catalytic cycle of oxidoreductases. The spectroscopic properties of the complex are in line with those of complexes formed with N-donor ligands. [Fe^III₂(μ-O₂)(L₃)₄(S)₂]⁴⁺ shows both nucleophilic (aldehydes) and electrophilic (phenol, N,N-dimethylanilines) oxidative reactivity and unusually also electron transfer oxidation.

A bidentate ligand based iron complex shows nucleophillic and electrophillice reactivity in the oxidation of organic substrates.

Cyanobacterial aldehyde deformylating oxygenase: Structure, function, and potential in biofuels production

The conversion of aldehydes to valuable alkanes via cyanobacterial aldehyde deformylating oxygenase is of great interest. The availability of fossil reserves that keep on decreasing due to human exploitation is worrying, and even more troubling is the combustion emission from the fuel, which contributes to the environmental crisis and health issues. Hence, it is crucial to use a renewable and eco-friendly alternative that yields compound with the closest features as conventional petroleum-based fuel, and that can be used in biofuels production. Cyanobacterial aldehyde deformylating oxygenase (ADO) is a metal-dependent enzyme with an α-helical structure that contains di‑iron at the active site. The substrate enters the active site of every ADO through a hydrophobic channel. This enzyme exhibits catalytic activity toward converting C_n aldehyde to C_n-1 alkane and formate as a co-product. These cyanobacterial enzymes are small and easy to manipulate. Currently, ADOs are broadly studied and engineered for improving their enzymatic activity and substrate specificity for better alkane production. This review provides a summary of recent progress in the study of the structure and function of ADO, structural-based engineering of the enzyme, and highlight its potential in producing biofuels.

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.