Mechanistic Investigations of the Reaction of an Iron(III) Octa-Anionic Porphyrin Complex with Hydrogen Peroxide and the Catalyzed Oxidation of Diammonium-2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)

Peroxymonosulfate activated by natural porphyrin derivatives for rapid degradation of organic pollutants via singlet oxygen and high-valent iron-oxo species

The activation of peroxymonosulfate (PMS) by sodium ferric chlorophyllin (SFC), a natural porphyrin derivative extracted from chlorophyll-rich substances, was systematically investigated for facile degradation of bisphenol A (BPA). SFC/PMS is capable of degrading 97.5% of BPA in the first 10 min with the initial BPA concentration of 20 mg/L and pH = 3, whereas conventional Fe²⁺/PMS could only remove 22.6% of BPA under identical conditions. It demonstrates a prominent flexibility to a broad pH range of 3-11 with complete pollutant degradation. A remarkable tolerance toward concomitant high concentration of inorganic anions (100 mM) was also observed, among which (bi)carbonates can even accelerate the degradation. The nonradical oxidation species, including high-valent iron-oxo porphyrin species and ¹O₂, are identified as dominant species. Particularly, the generation and participation of ¹O₂ in the reaction is evidenced by experimental and theoretical methods, which is vastly different from the previous study. The specific activation mechanism is unveiled by density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. The results shed light on effective PMS activation by iron (III) porphyrin and the proposed natural porphyrin derivative would be a promising candidate for efficient abatement of recalcitrant pollutants toward complicated aqueous media in wastewater treatment.

Generation of high-valent iron-oxo porphyrin cation radicals on hemin loaded carbon nanotubes for efficient degradation of sulfathiazole

Hemin has attracted considerable interest as an efficient catalyst recently, however, its direct application is inefficient due to severe molecular aggregation. Immobilizing hemin on various supports is a feasible approach to address this issue. In this work, a CNTs-hemin catalyst was prepared by loading hemin onto multiwalled carbon nanotubes (CNTs) through ball milling. Compared with hemin, CNTs-hemin demonstrates remarkably enhanced performance in the peroxymonosulfate system, with a 650-fold improvement of apparent rate constant, reaching 97.8% degradation of sulfathiazole in 5 min. High-valent iron-oxo porphyrin cation ((Porp)^+•Fe^IV=O) radicals are proposed as the dominant reactive species in the CNTs-hemin/peroxymonosulfate system instead of sulfate radicals (SO₄^•-), hydroxyl radicals (•OH), superoxide radicals (O₂^•-) and singlet oxygen (¹O₂). More in-depth mechanisms reveal that the strong electron transfer between CNTs and hemin promotes the generation of (Porp)^+•Fe^IV=O radicals through a heterolysis pathway. This research enriches the understanding of the catalytic mechanism of supported biomimetic catalysts for PMS activation and provides a perspective on the role of support materials for catalytic activity.

Cooperation of multiple active species generated in hydrogen peroxide activation by iron porphyrin for phenolic pollutants degradation

The narrow acid pH range and the nonselectivity of the dominant •OH limit the Fenton systems to remediate the organic wastewater. Inspired by the role of heme in physiological processes, we employed iron porphyrin as a novel homogeneous catalyst to address this issue. Multiple active species are identified during the activation of H₂O₂, including high-valent iron porphyrin ((por)Fe(IV)) species ((por)Fe(IV)-OH, (por)^+•Fe(IV)=O) and oxygen-centered radicals (•OH, HO₂•/•O₂^-), as well as atomic hydrogen (*H) and carbon-centered radicals. With the cooperation of these active species, the degradation of pollutants could be resistant to the interference of concomitant ions and proceed over a wide pH range. This cooperative behavior is further verified by intermediates identified from bisphenol A degradation. Specifically, the presence of *H could facilitate the cleavage of the C-C bond and the addition of unsaturated or aromatic molecules. (Por)^+•Fe(IV)=O could hydroxylate substrates with an oxygen rebound mechanism. Hydrogen atom abstraction of contaminants could be performed by (por)Fe(IV)-OH to form desaturated products by attacking oxygen-centered radicals. The ecotoxicity of bisphenol A could be significantly decreased through degradation. This study would provide a new approach to wastewater treatment and shed light on the interaction between metalloporphyrin and peroxide in an aqueous solution.

Heterogeneous catalysis with encapsulated haem and other synthetic porphyrins: Harnessing the power of porphyrins for oxidation reactions

Encapsulated metalloporphyrins have been widely studied for their use as efficient heterogeneous catalysts, inspired by the known catalytic activity of porphyrins in haemoproteins. The oxidation of organic substrates by haemoproteins is one of the well-known roles of these proteins, in which the haem (ferriprotoporphyrin IX = FePPIX) cofactor is the centre of reactivity. While these porphyrins are highly efficient catalysts in the protein environment, once removed, they quickly lose their reactivity. It is for this reason that they have garnered much interest in the field of heterogeneous catalysis of oxidation reactions. This review details current research in the field, focusing on the application of encapsulated haem, and other synthetic metalloporphyrins, applied to oxidation reactions.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Trace Oxygen Sensitive Material Based on Two Porphyrin Derivatives in a Heterodimeric Complex

The successful preparation of a novel dimer complex formed between 5,10,15,20-tetrakis(3,4-dimethoxyphenyl)-porphyrin Fe(III) chloride and (5,10,15,20-tetraphenylporphinato) dichlorophosphorus(V) chloride using the well-known reactivity of the P-X bond is reported. The obtained complex was characterized by UV-vis, Fourier transform infrared spectroscopy (FT-IR), fluorescence, ¹H-NMR, ¹³C-NMR, and ³¹P-NMR spectroscopic techniques and also by additional Heteronuclear Single Quantum Coherence (HSQC) and Heteronuclear Multiple Bond Correlation (HMBC) experiments in order to correctly assign the NMR signals. Scanning electron microscopy (SEM) and EDX quantifications completed the characterizations. This novel porphyrin dimer complex demonstrated fluorescence sensing of H₂O₂ in water for low oxygen concentrations in the range of 40-90 µM proving medical relevance for early diagnosis of diseases such as Alzheimer's, Parkinson's, Huntington's, and even cancer because higher concentrations of H₂O₂ than 50 μM are consideredcytotoxic for life. Due to its optical properties, this novel metalloporphyrin-porphyrin based complex is expected to show PDT and bactericidal activity under visible-light irradiation.

Catalytic effects of silver plasmonic nanoparticles on the redox reaction leading to ABTS˙+formation studied using UV-visible and Raman spectroscopy

The strong effect of silver plasmonic nanoparticles on ABTS leading to the formation of ABTS radical cation is investigated by UV-visible, Raman and SERS spectroscopy.

The reactivity of the active metal oxo and hydroxo intermediates and their implications in oxidations

The relationships of active metal oxo and hydroxo moieties have been summarized with their implications for biological and chemical oxidations.

Mechanistic insight into peroxo-shunt formation of biomimetic models for compound II, their reactivity toward organic substrates, and the influence of N-methylimidazole axial ligation

High-valent iron-oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)Fe(III)(OH) porphyrin ([meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H2O2 with (TMPS)Fe(III)(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)Fe(IV)=O(OH) point to the overall associative nature of the process. A pH-dependence study on the formation of (TMPS)Fe(IV)=O(OH) revealed a very high reactivity of OOH(-) toward (TMPS)Fe(III)(OH) in comparison to H2O2. The influence of N-methylimidazole (N-MeIm) ligation on both the formation of iron(IV)-oxo species and their oxidising properties in the reactions with 4-methoxybenzyl alcohol or 4-methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)Fe(III)(H2O)(N-MeIm) is highly reactive toward H2O2 to form the iron(IV)-oxo species, (TMPS)Fe(IV)=O(N-MeIm). The latter species can also be formed in the reaction of (TMPS)Fe(III)(N-MeIm)2 with H2O2 or in the direct reaction of (TMPS)Fe(IV)=O(OH) with N-MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)Fe(IV)=O(OH) and (TMPS)Fe(IV)=O(N-MeIm) do not display a pronounced effect of the N-MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH(-) substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH(-) or N-MeIm) in the trans position to the oxo group in the iron(IV)-oxo species does not significantly affect the activation barriers calculated for C-H dehydrogenation of the selected organic substrates.

A nanosized metal-organic framework of Fe-MIL-88NH₂ as a novel peroxidase mimic used for colorimetric detection of glucose

In this paper, a nanosized porous metal-organic framework, Fe-MIL-88NH₂, was facilely prepared with a uniform octahedral shape by the addition of acetic acid, and for the first time was demonstrated to possess intrinsic peroxidase-like activity. Kinetic analysis and electron spin resonance measurements indicated that the catalytic behavior was consistent with typical Michaelis-Menten kinetics and follows a ping-pong mechanism. As a novel peroxidase mimic material, Fe-MIL-88NH₂ shows the advantages of high catalytic efficiency, ultrahigh stability and high biocompatibility in aqueous medium compared with natural enzymes and other peroxidase nanomimetics. Here, Fe-MIL-88NH₂ was used to quickly catalyze oxidation of the peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to produce a colored product, which provided a simple, sensitive and selective method for the colorimetric detection of glucose. Glucose could be linearly detected in the range from 2.0 × 10⁻⁶ to 3.0 × 10⁻⁴ M with a detection limit of 4.8 × 10⁻⁷ M, and the color variation for glucose response was also obvious by visual observation at concentrations as low as 2.0 × 10⁻⁶ M. More importantly, the colorimetric method could be successfully applied to the determination of glucose in diluted serum samples.

Fe(III) complex of biuret-amide based macrocyclic ligand as peroxidase enzyme mimic

An Fe(III) complex of a biuret-amide based macrocyclic ligand that exhibits both excellent reactivity for the activation of H(2)O(2) and high stability, especially at low pH and high ionic strength, is reported.