Fast and Long‐Lasting Iron(III) Reduction by Boron Toward Green and Accelerated Fenton Chemistry

Generation of hydroxyl radicals in the Fenton system (Fe^II/H₂O₂) is seriously limited by the sluggish kinetics of Fe^III reduction and fast Fe^III precipitation. Here, boron crystals (C‐Boron) remarkably accelerate the Fe^III/Fe^II circulation in Fenton‐like systems (C‐Boron/Fe^III/H₂O₂) to produce a myriad of hydroxyl radicals with excellent efficiencies in oxidative degradation of various pollutants. The surface B?B bonds and interfacial suboxide boron in the surface B₁₂ icosahedra are the active sites to donate electrons to promote fast Fe^III reduction to Fe^II and further enhance hydroxyl radical production via Fenton chemistry. The C‐Boron/Fe^III/H₂O₂ system outperforms the benchmark Fenton (Fe^II/H₂O₂) and Fe^III‐based sulfate radical systems. The reactivity and stability of crystalline boron is much higher than the popular molecular reducing agents, nanocarbons, and other metal/metal‐free nanomaterials.     

Intramolecular Oxidative O‐Demethylation of an Oxoferryl Porphyrin Complexed with a Per‐O‐methylated β‐Cyclodextrin Dimer

The intramolecular oxidation of ROCH₃ to ROCH₂OH, where the latter compound spontaneously decomposed to ROH and HCHO, was observed during the reaction of the supramolecular complex (met‐hemoCD3) with cumene hydroperoxide in aqueous solution. Met‐hemoCD3 is composed of meso‐tetrakis(4‐sulfonatophenyl)porphinatoiron(III) (Fe^IIITPPS) and a per‐O‐methylated β‐cyclodextrin dimer having an ‐OCH₂PyCH₂O‐ linker (Py=pyridine‐3,5‐diyl). The O=Fe^IVTPPS complex was formed by the reaction of met‐hemoCD3 with cumene hydroperoxide, and isolated by gel‐filtration chromatography. Although the isolated O=Fe^IVTPPS complex in the cyclodextrin cage was stable in aqueous solution at 25 °C, it was gradually converted to Fe^IITPPS (t_1/2=7.6 h). This conversion was accompanied by oxidative O‐demethylation of an OCH₃ group in the cyclodextrin dimer. The results indicated that hydrogen abstraction by O=Fe^IVTPPS from ROCH₃ yields HO‐Fe^IIITPPS and ROCH₂^.. This was followed by radical coupling to afford Fe^IITPPS and ROCH₂OH. The hemiacetal (ROCH₂OH) immediately decomposed to ROH and HCHO. This study revealed the ability of oxoferryl porphyrin to induce two‐electron oxidation.     

Mössbauer Spectroscopic Investigation of FeII and FeIII 3,4,5‐Trihydroxybenzoates (Gallates) – Proposed Model Compounds for Iron‐Gall Inks

Iron gallates with iron in the oxidation states Fe²⁺ and Fe³⁺ were prepared and studied by Mössbauer spectroscopy, X‐ray diffraction, and IR spectroscopy. Fe^III 3,4,5‐trihydroxybenzoate (gallate) Fe(C₇O₅H₄) · 2H₂O, whose structure was first determined by Wunderlich, was obtained by the reaction of gallic acid and metallic iron or by oxidation of the Fe^II gallate, which was obtained by the reaction of ferrous sulfate with 3,4,5‐trihydroxybezoic acid (gallic acid) under anoxic conditions. Trials to reproduce the hydrothermal preparation method of Feller and Cheetham show that the result depends crucially on the free gas volume in the reaction vessel. If there is no free volume one obtains the same Fe^III gallate as in the other preparation methods. With a large free volume another compound was found to form whose composition and structure could not be determined. It could be specified only by Mössbauer spectroscopy. Fe^III gallate, the Fe^II gallate, and the new phase show magnetic ordering at liquid helium temperature.     

On the Catalytic Mechanism of (S)‐2‐Hydroxypropylphosphonic Acid Epoxidase (HppE): A Hybrid DFT Study

The mechanism of oxidative epoxidation catalyzed by HppE, which is the ultimate step in the biosynthesis of fosfomycin, was studied by using hybrid DFT quantum chemistry methods. An active site model used in the computations was based on the available crystal structure for the HppE‐Fe^II‐(S)‐HPP complex and it comprised first‐shell ligands of iron as well as second‐shell polar groups interacting with the substrates. The reaction energy profiles were constructed for three a priori plausible mechanisms proposed in the literature, and it was found that the most likely scenario for the native substrate, that is, (S)‐HPP, involves generation of the reactive Fe^III? O ^. /Fe^IV?O species, which is responsible for the C? H bond‐cleavage. At the subsequent reaction stage, the OH‐rebound, which would lead to a hydroxylated product, is prevented by a fast protonation of the OH ligand and, as a result, ring closure is the energetically preferred step. For the R enantiomer of the substrate ((R)‐HPP), which is oxidized to a keto product, comparable barrier heights were found for the C? H bond activation by both the Fe^III? O₂ ^. and Fe^IV?O species.     

{Bis­[2‐(2‐oxido‐1‐naphthylmethylidene­amino)­phenyl] disulfide}chloroiron(III)

The crystal structure of the title compound, {bis­[2‐(2‐oxido‐2‐naphthyl­idene­amino)­phenyl] di­sulfide‐κ⁵O,N,S,N′,O′}chloroiron(III), [FeCl(C₃₄H₂₂N₂O₂S₂)], has been determined. The structure consists of monomeric iron(III) complexes with distorted octahedral coordination. The di­sulfide functions as a pentadentate ligand and the Fe^III atom is coordinated through two N, two O and one S atom, and one chloride ion. The distance between the second S atom and the Fe^III atom is a non‐bonding 3.8473 (14) Å.     

The first three‐dimensional FeIII–SrII heterometallic coordination polymer: poly[[diaquatetrakis(μ3‐pyridine‐2,3‐dicarboxylato)diiron(III)strontium(II)] dihydrate]

The title compound, poly[[diaqua‐1κ²O‐tetrakis(μ₃‐pyridine‐2,3‐dicarboxylato)‐2:1:2′κ¹⁰N,O²:O^2′,O³:O^3′;2:1:2′κ⁸O³:O^3′:N,O²‐diiron(III)strontium(II)] dihydrate], {[Fe₂Sr(C₇H₃O₄)₄(H₂O)₂]·2H₂O}_n, which has triclinic (P) symmetry, was prepared by the reaction of pyridine‐2,3‐dicarboxylic acid, SrCl₂·6H₂O and Fe(OAc)₂(OH) (OAc is acetate) in the presence of imidazole in water at 363 K. In the crystal structure, the pyridine‐2,3‐dicarboxylate (pydc²⁻) ligand exhibits μ₃‐η¹,η¹:η¹:η¹ and μ₃‐η¹,η¹:η¹,η¹:η¹ coordination modes, bridging two Fe^III cations and one Sr^II cation. The Sr^II cation, which is located on an inversion centre, is eight‐coordinated by six O atoms of four pydc²⁻ ligands and two water molecules. The coordination geometry of the Sr^II cation can be best described as distorted dodecahedral. The Fe^III cation is six‐coordinated by O and N atoms of four pydc²⁻ ligands in a slightly distorted octahedral geometry. Each Fe^III cation bridges two neighbouring Fe^III cations to form a one‐dimensional [Fe₂(pydc)₄]_n chain. The chains are connected by Sr^II cations to form a three‐dimensional framework. The topology type of this framework is tfj . The structure displays O—H...O and C—H...O hydrogen bonding.     

FerriIridium: A Lysosome‐Targeting Iron(III)‐Activated Iridium(III) Prodrug for Chemotherapy in Gastric Cancer Cells

Reported is the Fe^III‐activated lysosome‐targeting prodrug FerriIridium for gastric cancer theranostics. It contains a meta‐imino catechol group that can selectively bond to, and be oxidized by, free Fe^III inside the cell. Subsequent oxidative rearrangement releases Fe^II and hydrolyses the amine bond under acidic conditions, forming an aminobipyridyl Ir complex and 2‐hydroxybenzoquinone. Thus, Fe^II catalyzes the Fenton reaction, transforming hydrogen peroxide into hydroxyl radicals, the benzoquinone compounds interfere with the respiratory chain, and conversion of the prodrug into the Ir complex leads to an increase in phosphorescence and toxicity. These properties, combined with the high Fe^III content and acidity of cancer cells, make FerriIridium a selective and efficient theranostic agent (IC₅₀=9.22 μm for AGS cells vs. >200 μm for LO2 cells). FerriIridium is the first metal‐based compound that has been developed for chemotherapy using Fe^III to enhance both selectivity and potency.     

FeIII in a low‐spin state in caesium bis[3‐ethoxysalicylaldehyde 4‐methylthiosemicarbazonato(2–)‐κ3O2,N1,S]ferrate(III) methanol monosolvate

The synthesis and crystal structure (at 100 K) of the title compound, Cs[Fe(C₁₁H₁₃N₃O₂S₂)₂]·CH₃OH, is reported. The asymmetric unit consists of an octahedral [Fe^III(L)₂]⁻ fragment, where L²⁻ is 3‐ethoxysalicylaldehyde 4‐methylthiosemicarbazonate(2−) {systematic name: [2‐(3‐ethoxy‐2‐oxidobenzylidene)hydrazin‐1‐ylidene](methylamino)methanethiolate}, a caesium cation and a methanol solvent molecule. Each L²⁻ ligand binds through the thiolate S, the imine N and the phenolate O atoms as donors, resulting in an Fe^IIIS₂N₂O₂ chromophore. The O,N,S‐coordinating ligands are orientated in two perpendicular planes, with the O and S atoms in cis positions and the N atoms in trans positions. The Fe^III cation is in the low‐spin state at 100 K.     

Elimination of 4‐chlorophenol in aqueous solution by the Novel Pd/MIL‐101(Cr)‐Hydrogen‐Accelerated catalytic fenton system

Excessive consumption of Fe (II) and massive generation of sludge containing Fe (III) from classic Fenton process remains a major obstacle for its poor recycling of Fe (III) to Fe (II). Therefore, the MHACF‐MIL‐101(Cr) system, by introducing H₂, Pd⁰ and MIL‐101(Cr) into Fenton reaction system, was developed at normal temperature and pressure. In this system, the reduction of Fe^III back to Fe^II by solid catalyst Pd/MIL‐101(Cr) for the storage and activation of H₂, was accelerated significantly by above 10‐fold and 5‐fold controlled with the H₂‐MIL‐101(Cr) system and H₂‐Pd⁰ system, respectively. However, the concentration of Fe (II) generated by the reduction of Fe (III) could not be detected with the only input of H₂ and without the addition of MOFs material. In addition, the apparent consumption of Fe (II) in MHACF‐MIL‐101(Cr) system was half of that in classical Fenton system, while more Fe (II) might be reused infinitely in fact. Accordingly, only trace amount of Fe (II) vs H₂O₂ concentration was needed and hydroxyl radicals through the detection of para‐hydroxybenzoic acid (p‐HBA) as the oxidative product of benzoic acid (BA) by·OH could be continuously generated for the effective degradation of 4‐chlorophenol(4‐CP). The effects of initial pH, concentration of 4‐CP, dosage of Fe²⁺, H₂O₂ and Pd/MIL‐101(Cr) catalyst, Pd content and H₂ flow were investigated, combined with systematic controlled experiments. Moreover, the robustness and morphology change of Pd/MIL‐101(Cr) were thoroughly analyzed. This study enables better understanding of the H₂‐mediated Fenton reaction enhanced by Pd/MIL‐101(Cr) and thus, will shed new light on how to accelerate Fe (III)/Fe (II) redox cycle and develop more efficient Fenton system.     

Multimetal‐Substituted Epsilon‐Iron Oxide ϵ‐Ga0.31Ti0.05Co0.05Fe1.59O3 for Next‐Generation Magnetic Recording Tape in the Big‐Data Era

From the viewpoints of large capacity, long‐term guarantee, and low cost, interest in magnetic recording tapes has undergone a revival as an archive storage media for big data. Herein, we prepared a new series of metal‐substituted ?‐Fe₂O₃, ?‐Ga^III_0.31Ti^IV_0.05Co^II_0.05Fe^III_1.59O₃, nanoparticles with an average size of 18 nm. Ga, Ti, and Co cations tune the magnetic properties of ?‐Fe₂O₃ to the specifications demanded for a magnetic recording tape. The coercive field was tuned to 2.7 kOe by introduction of single‐ion anisotropy on Co^II (S=3/2) along the c‐axis. The saturation magnetization was increased by 44 % with Ga^III (S=0) and Ti^IV (S=0) substitution through the enhancement of positive sublattice magnetizations. The magnetic tape media was fabricated using an actual production line and showed a very sharp signal response and a remarkably high signal‐to‐noise ratio compared to the currently used magnetic tape.     

Caesium bis(5‐bromosalicylaldehyde thiosemicarbazonato‐κ3O,N,S)ferrate(III): supramolecular arrangement of low‐spin FeIII complex anions mediated by Cs+ cations

The synthesis and crystal structure determination (at 293 K) of the title complex, Cs[Fe(C₈H₆BrN₃OS)₂], are reported. The compound is composed of two dianionic O,N,S‐tridentate 5‐bromosalicylaldehyde thiosemicarbazonate(2−) ligands coordinated to an Fe^III cation, displaying a distorted octahedral geometry. The ligands are orientated in two perpendicular planes, with the O‐ and S‐donor atoms in cis positions and the N‐donor atoms in trans positions. The complex displays intermolecular N—H...O and N—H...Br hydrogen bonds, creating R₄⁴(18) rings, which link the Fe^III units in the a and b directions. The Fe^III cation is in the low‐spin state at 293 K.     

Boron Cluster Anions [BnHn]2– (n = 10, 12) in Reactions of Iron(II) and Iron(III) Complexation with 2, 2′‐Bipyridyl and 1, 10‐Phenanthroline

Complexation of Fe^II and Fe^III with azaheterocyclic ligands L (L = phen or bipy) were studied in the presence and in the absence of boron cluster anions [B_nH_n]^2– (n = 10, 12). The reactions were carried out in air at room temperature in organic solvents and/or water. In all the solvents used, well known [FeL₃]An (An = 2Cl^– or SO₄^2–) ferrous complexes were formed from Fe^II salts. Composition of ferric complexes with L ligands depends on the nature of solvent: either dinuclear oxo‐iron(III) chlorides [L₂ClFe^III–O–Fe^IIIL₂Cl]Cl₂ or ferric ferrates(III) [Fe^IIIL₂Cl₂][Fe^IIICl₄], or [Fe^IIIL₂Cl₂][Fe^IIICl₄L] were isolated from Fe^III salts. Introduction of the closo‐borate anions to a Fe³⁺(or Fe²⁺)/L/solv. mixture stabilizes ferrous cationic complexes [FeL₃]²⁺ in all the solvents used: only ferrous [FeL₃][B_nH_n] (n = 10, 12) complexes were isolated from all the reaction mixtures in the presence of boron cluster anions.     

Oxidative Degradation of 2,4‐Dihydroxybenzoic Acid by the Fenton and Photo‐Fenton Processes: Kinetics,Mechanisms, and Evidence for the Substitution of H2O2 by O2

The kinetics and mechanisms of the oxidative degradation of 2,4‐dihydroxybenzoic acid (2,4‐DHBA) by the Fenton and photo‐Fenton processes were investigated in detail by a combination of HPLC, IC, and TOC analyses. The formation of 2,3,4‐trihydroxybenzoic acid (2,3,4‐THBA) at an early oxidation stage shows that hydroxylation of the aromatic ring is the first step of the process. This intermediate was able to reduce Fe^III and to contribute to the recycling of Fe^II. Complete mineralization could only be achieved under irradiation (photo‐Fenton). A detailed study of the dependence of the rate of mineralization on the concentration of H₂O₂ and dissolved O₂ was carried out. It was found that, even at a low initial concentration of H₂O₂, mineralization by the photo‐Fenton process was complete in a relatively short time, provided that the O₂ concentration was high enough, indicating that O₂ may, at least in part, substitute H₂O₂. Channeling reaction pathways toward O₂ rather than H₂O₂ consumption is of particular interest for the technical development of the photo‐Fenton process.     

Two new soluble iron–oxo complexes: [Fe2(μ‐O)(μ‐O2CCF3)2(O2CCF3)2(C10H8N2)2] and [Fe4(μ3‐O)2(μ‐O2CCF3)6(O2CCF3)2(C10H8N2)2]·CF3CO2H

Two new iron–oxo clusters, viz. di‐μ‐tri­fluoro­acetato‐μ‐oxo‐bis­[(2,2′‐bi­pyridine‐κ²N,N′)(tri­fluoro­acetato‐κO)­iron(III)], [Fe₂O(CF₃CO₂)₄(C₁₀H₈N₂)₂], and bis(2,2′‐bi­pyridine)­di‐μ₃‐oxo‐hexa‐μ‐tri­fluoro­acetato‐bis­(tri­fluoro­acetato)­tetrairon(III) tri­fluoro­acetic acid solvate, [Fe₄O₂(CF₃CO₂)₈(C₁₀H₈N₂)₂]·CF₃CO₂H, contain dinuclear and tetranuclear Fe^III cores, respectively. The Fe^III atoms are in distorted octahedral environments in both compounds and are linked by oxide and tri­fluoro­acetate ions. The tri­fluoro­acetate ions are either bridging (bidentate) or coordinated to the Fe^III atoms via one O atom only. The fluorinated peripheries enhance the solubility of these compounds. Formal charges for all the Fe centers were assigned by summing valences of the chemical bonds to the Fe^III atom.     

Tetrakis(5‐tert‐butylpyrazole‐1κN2)tetrachloro‐1κCl,2κ3Cl‐μ‐oxo‐1:2κ2O‐diiron(III)

The title compound, [Fe₂Cl₄O(C₇H₁₂N₂)₄], contains vertex‐sharing distorted tetrahedral [FeOCl₃]^? and octahedral [FeOCl(Hpz^tBu)₄]⁺ moieties (Hpz^tBu is 5‐tert‐­butyl­pyrazole), linked by a bent oxo bridging ligand. The two Fe^III centres are also bridged by intramolecular hydrogen bonds between the pyrazole N—H groups and the O^2? and Cl^? ligands.     

Poly[aquaneodymium(III)‐μ5‐2‐oxido‐5‐sulfonatobenzoato]

The title compound, [Nd(C₇H₃O₆S)(H₂O)]_n or [Nd(SSA)(H₂O)]_n (H₃SSA is 5‐sulfosalicylic acid), was synthesized by the hydrothermal reaction of Nd₂O₃ with H₃SSA in water. The compound forms a three‐dimensional network in which the asymmetric unit contains one Nd^III atom, one SSA ligand and one coordinated water mol­ecule. The central Nd^III ion is eight‐coordinate, bonded to seven O atoms from five different SSA ligands [Nd—O = 2.405 (4)–2.612 (4) Å] and one aqua O atom [Nd—OW = 2.441 (4) Å].     

Fine Control of the Redox Reactivity of a Nonheme Iron(III)–Peroxo Complex by Binding Redox‐Inactive Metal Ions

Redox‐inactive metal ions are one of the most important co‐factors involved in dioxygen activation and formation reactions by metalloenzymes. In this study, we have shown that the logarithm of the rate constants of electron‐transfer and C−H bond activation reactions by nonheme iron(III)–peroxo complexes binding redox‐inactive metal ions, [(TMC)Fe^III(O₂)]⁺‐M^{n +} (M^{n +}=Sc³⁺, Y³⁺, Lu³⁺, and La³⁺), increases linearly with the increase of the Lewis acidity of the redox‐inactive metal ions (ΔE ), which is determined from the g_zz values of EPR spectra of O₂^.−‐M^{n +} complexes. In contrast, the logarithm of the rate constants of the [(TMC)Fe^III(O₂)]⁺‐M^{n +} complexes in nucleophilic reactions with aldehydes decreases linearly as the ΔE value increases. Thus, the Lewis acidity of the redox‐inactive metal ions bound to the mononuclear nonheme iron(III)–peroxo complex modulates the reactivity of the [(TMC)Fe^III(O₂)]⁺‐M^{n +} complexes in electron‐transfer, electrophilic, and nucleophilic reactions.     

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.     

Absolute rate constants for hydrogen abstraction from hydrocarbons by the trichloromethylperoxyl radical

Absolute rate constants have been measured for the reactions of trichloromethylperoxyl radicals with cyclohexane, cyclohexene, and hexamethylbenzene. The CCl₃O₂ radicals were produced by pulse radiolysis of air-saturated CCl₄ solutions containing various amounts of the hydrocarbons. The rate constants were determined by competition with the one-electron oxidation of metalloporphyrins, using the rate of formation of the metalloporphyrin radical cation absorption to monitor the reaction by kinetic spectrophotometry. The rate constants for hydrogen abstraction from cyclohexane, cyclohexene, and hexamethylbenzene were found to be 1 × 10³, 1.0 × 10⁵, and 7.5 × 10⁴ M^?1 s^?1, respectively.     

A Non‐Heme Iron Photocatalyst for Light‐Driven Aerobic Oxidation of Methanol

Non‐heme (L)Fe^III and (L)Fe^III‐O‐Fe^III(L) complexes (L=1,1‐di(pyridin‐2‐yl)‐N,N‐bis(pyridin‐2‐ylmethyl)ethan‐1‐amine) underwent reduction under irradiation to the Fe^II state with concomitant oxidation of methanol to methanal, without the need for a secondary photosensitizer. Spectroscopic and DFT studies support a mechanism in which irradiation results in charge‐transfer excitation of a Fe^III?μ‐O?Fe^III complex to generate [(L)Fe^IV=O]²⁺ (observed transiently during irradiation in acetonitrile), and an equivalent of (L)Fe^II. Under aerobic conditions, irradiation accelerates reoxidation from the Fe^II to the Fe^III state with O₂, thus closing the cycle of methanol oxidation to methanal.