High-valent Iron-oxo Intermediates Research Articles

Bio-inspired strategy to enhance catalytic oxidative desulfurization by O-bridged diiron perfluorophthalocyanine axially coordinated with 4-mercaptopyridine

Developing efficient and environmentally friendly catalytic system is significant for the removal of sulfide in fuel. O-bridged diiron perfluorophthalocyanine/4-mercaptopyridine (FePcF16-O-FePcF16/4-Mpy) as a novel composite catalyst was designed and prepared by axial coordination. The catalyst characterization was conducted through ultraviolet–visible spectroscopy, fourier transform infrared spectroscopy, X-ray diffractometry, electrospray ionization–mass spectrometry and density-functional theory technology. A high-efficient extraction oxidative desulfurization system is composed of hydrogen peroxide (H2O2), the mixture of ethanol and water and FePcF16-O-FePcF16/4-Mpy. Notably, the coordination of 4-Mpy greatly enhanced the catalytic activity of FePcF16-O-FePcF16, 99.4% of the desulfurization rate was achieved in 20 min at 30 °C. The catalyst system was cycled 8 times without significantly reducing the removal of dibenzothiophene. Electron paramagnetic resonance and gas chromatography–mass spectroscopy proved the high-valent iron-oxo intermediates were the main active substances in the FePcF16-O-FePcF16/4-Mpy/H2O2 system, and the pathway of catalytic oxidation DBT was proposed. This work provided useful insights to select suitable ligands to improve the activity of metal phthalocyanine, and offered a novel idea for the construction of efficient oxidative desulfurization system.

Iron(V)/Iron(IV) species in graphitic carbon nitride-ferrate(VI)-visible light system: Enhanced oxidation of micropollutants

This paper investigated for the first time the visible light-induced oxidation of micropollutants using graphitic carbon nitride (g-C3N4) photocatalyst in the presence of ferrate(VI) (FeVIO42−) under mild alkaline conditions. The studied micropollutants have different molecular structures (i.e., carbamazepine (CBZ), sulfamethoxazole (SMX), sulfadimethoxine (SDM), trimethoprim (TMP), diclofenac (DCF), flumequine (FLU), atenolol (ATL), and caffeine (CAF)). Enhanced oxidation of all investigated micropollutants by the combined g-C3N4-FeVIO42− system was observed compared to g-C3N4 or FeVIO42− alone under visible light irradiation. For example, 5.0 µM CBZ could be almost completely degraded in 5.0 min by the g-C3N4-FeVIO42− system. Comparatively, only ∼ 40% degradation of CBZ occurred by g-C3N4 or FeVIO42− alone in 5.0 min under the same condition. Similar observations were found with other micropollutants, and the magnitude of enhancement varied with the structure of the micropollutants. The degradation experiments with and without oxygen in solution suggested the generation of O2●- and 1O2 in g-C3N4-visible light system. However, with the presence of FeVIO42− in solution, the g-C3N4-visible light system generated high-valent iron-oxo intermediates (FeV/FeIV) as the oxidizing species, evidenced by the selective conversion of methyl phenol oxide (PMSO) to PMSO2, to cause increased oxidation of micropollutants. A plausible mechanism involving reactive species to oxidize micropollutants by the g-C3N4-FeVIO42− system under visible light irradiation was discussed. Oxidized products of CBZ were identified to reveal possible reaction pathways. The results of the g-C3N4-FeVIO42−-visible light system provided a new advanced oxidation process to synergistically degrade micropollutants in water efficiently.

Enhanced ferrate(VI) oxidation of micropollutants in water by carbonaceous materials: Elucidating surface functionality

Metal-free carbonaceous materials have increasingly attracted worldwide attention owing to their unprecedented properties in catalytic elimination of environmental pollutants in water. Nevertheless, the surface functionality-dependent catalytic efficacy and activation mechanism for ferrate(VI) (FeVIO42−), an emerging green water-treatment agent, have remained largely unknown. Here we have examined if different carbonaceous materials (i.e., hydrochar, graphene oxide (GO), reduced graphene oxide, graphite, and fullerene) can activate FeVIO42− and enhance its effectiveness for rapid oxidation of a wide range of micropollutants (i.e., carbamazepine (CBZ), diclofenac, sulfamethoxazole, sulfadimethoxine, trimethoprim, flumequine, atenolol, and caffeine) under mild alkaline conditions. For example, the addition of hydrochar or GO into the reaction system, at pH 9.0, achieved remarkable enhancement of degradation of CBZ as compared to insignificant effect observed for three other carbonaceous materials. The magnitude of enhancement varied with the moieties of micropollutants. The Fourier-transform infrared spectroscopy (FTIR) technique in conjunction with the chemical probe (i.e., methyl phenyl sulfoxide) tests demonstrated chemical interactions of surface CO groups of hydrochar with FeVIO42−, of which high-valent iron-oxo intermediates (i.e., FeIV/FeV) were generated as the oxidizing species for enhanced remediation. Additionally, this enhancement was seen in five successive catalytic runs, indicating high recyclability of hydrochar to oxidize micropollutants by FeVIO42−. These findings suggest that heterogeneous carbonaceous activation of FeVIO42− holds promise for advancing water remediation under mild alkaline reaction conditions.

Bio‐inspired Nonheme Iron Oxidation Catalysis: Involvement of Oxoiron(V) Oxidants in Cleaving Strong C−H Bonds

Nonheme iron enzymes generate powerful and versatile oxidants that perform a wide range of oxidation reactions, including the functionalization of inert C-H bonds, which is a major challenge for chemists. The oxidative abilities of these enzymes have inspired bioinorganic chemists to design synthetic models to mimic their ability to perform some of the most difficult oxidation reactions and study the mechanisms of such transformations. Iron-oxygen intermediates like iron(III)-hydroperoxo and high-valent iron-oxo species have been trapped and identified in investigations of these bio-inspired catalytic systems, with the latter proposed to be the active oxidant for most of these systems. In this Review, we highlight the recent spectroscopic and mechanistic advances that have shed light on the various pathways that can be accessed by bio-inspired nonheme iron systems to form the high-valent iron-oxo intermediates.

Luminescence-Sensing Tb-MOF Nanozyme for the Detection and Degradation of Estrogen Endocrine Disruptors.

Using flexible structures and components of metal-organic framework (MOF) materials, we designed and developed an artificial nanozyme with dual functions of a catalyst and luminescent sensor specifically for the determination and degradation of hormone 17β-estradiol (E2) and its derivatives (E1, E3, and EE2), a class of disruptors with strong effect on the human endocrine system. This nanozyme composed of the luminescent Tb3+ ion, catalytic coenzyme factor hemin, and light-harvesting ligand can be used to both degrade E2 like natural horseradish peroxidase (HRP) and sense E2 as low as 50 pM by its luminescence. The nanozyme catalyzes the decomposition of E2 and its derivatives through a mechanism of active hydroxyl radicals and oxidative high-valent iron-oxo intermediates. The prepared nanozyme is pluripotent, stable, and cheap and can replace the widely used combination of natural enzyme and chromogenic substrate. The present strategy of constructing artificial enzymes directly from functional units provides a new way for the design and development of smart, multifunctional artificial enzymes.

Di- and Tetrairon(III) μ-Oxido Complexes of an N3S-Donor Ligand: Catalyst Precursors for Alkene Oxidations.

The new di- and tetranuclear Fe(III) μ-oxido complexes [Fe4(μ-O)4(PTEBIA)4](CF3SO3)4(CH3CN)2] (1a), [Fe2(μ-O)Cl2(PTEBIA)2](CF3SO3)2 (1b), and [Fe2(μ-O)(HCOO)2(PTEBIA)2](ClO4)2 (MeOH) (2) were prepared from the sulfur-containing ligand (2-((2,4-dimethylphenyl)thio)-N,N-bis ((1-methyl-benzimidazol-2-yl)methyl)ethanamine (PTEBIA). The tetrairon complex 1a features four μ-oxido bridges, while in dinuclear 1b, the sulfur moiety of the ligand occupies one of the six coordination sites of each Fe(III) ion with a long Fe-S distance of 2.814(6) Å. In 2, two Fe(III) centers are bridged by one oxido and two formate units, the latter likely formed by methanol oxidation. Complexes 1a and 1b show broad sulfur-to-iron charge transfer bands around 400–430 nm at room temperature, consistent with mononuclear structures featuring Fe-S interactions. In contrast, acetonitrile solutions of 2 display a sulfur-to-iron charge transfer band only at low temperature (228 K) upon addition of H2O2/CH3COOH, with an absorption maximum at 410 nm. Homogeneous oxidative catalytic activity was observed for 1a and 1b using H2O2 as oxidant, but with low product selectivity. High valent iron-oxo intermediates could not be detected by UV-vis spectroscopy or ESI mass spectrometry. Rather, evidence suggest preferential ligand oxidation, in line with the relatively low selectivity and catalytic activity observed in the reactions.

Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy.

Semiconductor electrodes capable of using solar photons to drive water-splitting reactions, such as haematite (α-Fe2O3), have been the subject of tremendous interest over recent decades. The surface has been found to play a significant role in determining the efficiency of water oxidation with haematite; however, previous works have only allowed hypotheses to be formulated regarding the identity of relevant surface species. Here we investigate the water-oxidation reaction on haematite using infrared spectroscopy under photoelectrochemical (PEC) water-oxidation conditions. A potential- and light-dependent absorption peak at 898 cm(-1) is assigned to a Fe(IV)=O group, which is an intermediate in the PEC water-oxidation reaction. These results provide direct evidence of high-valent iron-oxo intermediates as the product of the first hole-transfer reaction on the haematite surface and represent an important step in establishing the mechanism of PEC water oxidation on semiconductor electrodes.

Oxygen atom transfer mediated by an iron(IV)/iron(II) macrocyclic complex containing pyridine and tertiary amine donors

A new non-heme iron model complex containing a high-spin iron(II) complexed with N-methylated pyridine-containing macrocycle was synthesized and crystallographically characterized. The complex generates peroxo- and high-valent iron-oxo intermediates in reactions with tert-butylhydroperoxide and isopropyl 2-iodoxybenzoate, respectively, allowing to gain insight into the formation and reactivity of enzyme-like intermediates related to biological oxygen activation. The formation and reactivity of these intermediate species were investigated by the stopped-flow methodology. The mechanism of oxygen transfer to organic substrates involving reaction of oxoiron(IV) intermediate was elucidated on the basis of spectroscopic and kinetic data. Incorporation of a pyridine ring into the macrocycle increased the reactivity of the FeIVO intermediates in comparison with polyamine tetraaza macrocyclic complexes: ferryl (FeIVO) species derived from 3 demonstrated electrophilic reactivity in transferring an oxygen atom to substituted triarylphosphines and to olefins (such as cyclooctene). However, iron(III) alkylperoxo intermediate was unreactive with cyclooctene.

Redox Potential and C−H Bond Cleaving Properties of a Nonheme FeIV═O Complex in Aqueous Solution

High-valent iron-oxo intermediates have been identified as the key oxidants in the catalytic cycles of many nonheme enzymes. Among the large number of synthetic Fe(IV)=O complexes characterized to date, [Fe(IV)(O)(N4Py)](2+) (1) exhibits the unique combination of thermodynamic stability, allowing its structural characterization by X-ray crystallography, and oxidative reactivity sufficient to cleave C-H bonds as strong as those in cyclohexane (D(C-H) = 99.3 kcal mol(-1)). However, its redox properties are not yet well understood. In this work, the effect of protons on the redox properties of 1 has been investigated electrochemically in nonaqueous and aqueous solutions. While the cyclic voltammetry of 1 in CH(3)CN is complicated by coupling of several chemical and redox processes, the Fe(IV/III) couple is reversible in aqueous solution with E(1/2) = +0.41 V versus SCE at pH 4 and involves the transfer of one electron and one proton to give the Fe(III)-OH species. This is in fact the first example of reversible electrochemistry to be observed for this family of nonheme oxoiron (IV) complexes. C-H bond oxidations by 1 have been studied in H(2)O and found to have reaction rates that depend on the C-H bond strength but not on the solvent. Furthermore, our electrochemical results have allowed a D(O-H) value of 78(2) kcal mol(-1) to be calculated for the Fe(III)-OH unit derived from 1. Interestingly, although this D(O-H) value is 6-11 kcal mol(-1) lower than those corresponding to oxidants such as [Fe(IV)(O)(TMP)] (TMP = tetramesitylporphinate), [Ru(IV)(O)(bpy)(2)(py)](2+) (bpy = bipyridine, py = pyridine), and the tert-butylperoxyl radical, the oxidation of dihydroanthracene by 1 occurs at a rate comparable to rates for these other oxidants. This comparison suggests that the nonheme N4Py ligand environment confers a kinetic advantage over the others that enhances the C-H bond cleavage ability of 1.

Antimalarial Activity of Artemisinin (Qinghaosu) and Related Trioxanes: Mechanism (s) of Action

Malaria is one of the most deadly infectious diseases known to mankind. The species of Plasmodium that cause malaria can develop resistance to chemotherapeutic drugs that is going in recent times. Artemisinin and its analogs comprise a rapidly expanding chemotherapeutic arsenal in the ongoing worldwide fight against the resurgent threat of malaria. The promise of artemisinin, its semisynthetic derivatives, and its fully synthetic analogs as a new class of nonalkaloidal, fast-acting antimalarials has stimulated extensive synthetic and mechanistic research into their novel mode of action. Artemisinin and its expanding array of structurally related 1,2,4-trioxanes are effective against both simple and complicated malaria. The desire to develop new, more potent agents related to this promising class of drugs has prompted researchers to elucidate the mechanism(s) of action of these relatively new antimalarials. The developing understanding is the mechanism of action in which the endoperoxide linkage is a trigger, activated by iron-induced reduction inside the malaria parasite that releases a cascade of reactive intermediates—cytotoxic free radicals, one or more high-valent iron-oxo intermediates, and electrophilic alkylating agents—that ultimately cause fatal damage to the parasites.

High valent iron oxo intermediates might be involved during activation of ribonucleotide reductase: Single oxygen atom donors generate the tyrosyl radical

The active form of protein B2, the small subunit of ribonucleotide reductase from E. Coli, contains a binuclear non heme iron center and a tyrosyl radical. MetB2 is an inactive form that lacks the radical but retains the Fe(III) center. We earlier proposed that the function of the iron center was to catalyze the one-electron oxidation of the tyrosine residue from metB2 by dioxygen. We now report that incubation of metB2 with single oxygen atom donors, hydrogen peroxide, 3-chloroperoxybenzoic acid, monoperoxophtalate and 2-iodosobenzoate, also results in the formation of the tyrosyl radical, as monitored by UV-visible and EPR spectroscopy. A mechanism of reductive activation of dioxygen by the binuclear non heme iron center involving iron-oxo intermediates is proposed.