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.     

Modeling Heme Peroxidase: Heme Saddling Facilitates Reactions with Hyperperoxides To Form High-Valent FeIV-Oxo Species

Saddle-shaped hemes have been discovered in the structures of most peroxidases. How such a macrocycle deformation affects the reaction of Fe^III hemes with hydrogen peroxide (H₂O₂) to form high-valent Fe-oxo species remains uncertain. Through examination of the ESI-MS spectra, absorption changes and ¹H NMR chemical shifts, we investigated the reactions of two Fe^III porphyrins with different degrees of saddling deformation, namely Fe^III(OETPP)ClO₄ ( 1^OE ) and Fe^III(OMTPP)ClO₄ ( 1^OM ), with tert-butyl hydroperoxide (tBuOOH) in CH₂Cl₂ at −40 °C, which quickly resulted in O−O bond homolysis from a highly unstable Fe^III-alkylperoxo intermediate, Fe^III-O(H)OR ( 2 ) into Fe^IV-oxo porphyrins ( 3 ). Insight into the reaction mechanism was obtained from [tBuOOH]-dependent kinetics. At −40 °C, the reaction of 1^OE with tBuOOH exhibited an equilibrium constant (K_a=362.3 M⁻¹) and rate constant (k=1.87×10⁻² s^M−>1) for the homolytic cleavage of the 2 O−O bond that were 2.1 and 1.4 times higher, respectively, than those exhibited by 1^OM (K_a=171.8 M⁻¹ and k=1.36×10⁻² s⁻¹). DFT calculations indicated that an Fe^III porphyrin with greater saddling deformation can achieve a higher HOMO ([Fe(d ,d )-porphyrin(a_2u)]) to strengthen the orbital interaction with the LUMO (O−O bond σ*) to facilitate O−O cleavage.     

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, [Fe^III(OOH)(L₅²)]²⁺ (L₅²=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 [Fe^II(OH)(L₅²)]⁺ 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 Fe^II(OOH) intermediate generated upon reduction of the Fe^III(OOH) starting complex. One plausible mechanistic scenario is that this disproportionation reaction follows an O−O heterolytic cleavage pathway via a Fe^IV-oxo species.     

Synthesis and reactivity of metal-containing monomers

Acid and neutral Co^II, Cu^II, Ni^II, Zn^II, Fe^II, and Fe^III maleates, fumarates, and itaconates were obtained and characterized. The methods for their synthesis were optimized, and the valence state and coordination of metals were studied. Co^II and Fe^II hydrogen maleates, Co^II maleate, and Co^II fumarate were examined by X-ray diffraction analysis. The ligands based on unsaturated dicarboxylic acids can be mono-, bi-, and tetradentate, which results in the formation of acid salts, chain and three-dimensional coordination polymers, whose double bond is not involved in the coordination. The strong antiferromagnetic exchange (μ_elf=1.41 and 0.34 μ_B at 290 and 80 K, respectively) was detected in Cu^II itaconate. Based on the data of Mössbauer spectroscopy, the partial reduction of Fe^III to Fe^II during the synthesis of Fe^III maleate was shown to occur: δFe=0.43 and 1.27 mm s^?1, ΔE _Q=0.57 and 3.13 mm s^?1 and Γ=0.37 and 0.28 mm s^?1 atT=298 K for Fe^III and Fe^II, respectively.     

Beneficial Effect of Acetic Acid on the Formation of FeIII(OOH) Species and on the Catalytic Activity of Bioinspired Nonheme FeII Complexes.

Three new amine/pyridine Fe^II complexes bearing pentadentate ligand with one, two or three electron enriched 4-methoxy-3,5-dimethylpyridine were used as catalysts for the oxidation of small organic molecules by hydrogen peroxide. The distribution of products formed suggests that these ligands are not enough electron donating to promote the O−O heterolytic cleavage of the oxidant in order to generate selective Fe^V(O) species. Using acetic acid in the reaction mixtures results in a significant increase of the efficiency of these catalytic systems. Our investigations show that the use of AcOH leads to the protonation/dissociation of a pyridyl moiety and the formation of (N₄)Fe^II(OAc)(OH) species. These complexes readily react with excess hydrogen peroxide to yield (N₄)Fe^III(OAc)(OOH) intermediates. These latter intermediates are proposed to evolve into (N₄)Fe^IV(OAc)(O), which are more efficient than the usual (N₄)Fe^IV(O) and (N₅)Fe^IV(O).     

Observation of a CuII2(μ‐1,2‐peroxo)/CuIII2(μ‐oxo)2 Equilibrium and its Implications for Copper–Dioxygen Reactivity

Synthesis of small‐molecule Cu₂O₂ adducts has provided insight into the related biological systems and their reactivity patterns including the interconversion of the Cu^II₂(μ‐η²:η²‐peroxo) and Cu^III₂(μ‐oxo)₂ isomers. In this study, absorption spectroscopy, kinetics, and resonance Raman data show that the oxygenated product of [(BQPA)Cu^I]⁺ initially yields an “end‐on peroxo” species, that subsequently converts to the thermodynamically more stable “bis‐μ‐oxo” isomer (K_eq=3.2 at ?90 °C). Calibration of density functional theory calculations to these experimental data suggest that the electrophilic reactivity previously ascribed to end‐on peroxo species is in fact a result of an accessible bis‐μ‐oxo isomer, an electrophilic Cu₂O₂ isomer in contrast to the nucleophilic reactivity of binuclear Cu^II end‐on peroxo species. This study is the first report of the interconversion of an end‐on peroxo to bis‐μ‐oxo species in transition metal‐dioxygen chemistry.     

Highly Efficient FeIII-initiated Self-cycled Fenton System in Piezo-catalytic Process for Organic Pollutants Degradation

Piezo-catalytic self-Fenton (PSF) system has been emerging as a promising technique for wastewater treatment, while the competing O₂ reductive hydrogen peroxide (H₂O₂) production and Fe^III reduction seriously limited the reaction kinetics. Here, we develop a two-electron water oxidative H₂O₂ production (WOR−H₂O₂) coupled with Fe^III reduction over a Fe^III/BiOIO₃ piezo-catalyst for highly efficient PSF. It is found that the presence of Fe^III can simultaneously initiate the WOR−H₂O₂ and reduction of Fe^III to Fe^II, thereby enabling a rapid reaction kinetics towards subsequent Fenton reaction of H₂O₂/Fe^II. The Fe^III initiating PSF system offers exceptional self-recyclable degradation of pollutants with a degradation rate constant for sulfamethoxazole (SMZ) over 3.5 times as that of the classic Fe^II-PSF system. This study offers a new perspective for constructing efficient PSF systems and shatters the preconceived notion of Fe^III in the Fenton reaction.     

How Ligand Geometry Affects the Reactivity of Co(II) Cyclam Complexes

Cobalt complexes are extensively studied as bioinspired models for non-heme oxygenases as they facilitate both the stabilization and characterization of metal-oxygen intermediates. As an analog to the well-known Co(cyclam) complex Co{N₄} (cyclam=1,4,8,11-tetraazacyclotetradecane), the Co^II complex Co{i-N₄} with the isomeric isocyclam ligand (isocyclam=1,4,7,11-tetraazacyclotetradecane) was synthesized and characterized. Despite the identical N₄ donor set of both complexes, Co{i-N₄} enables the 2e⁻/2H⁺ reduction of O₂ with a lower overpotential (η_eff of 385 mV vs. 540 mV for Co{N₄} ), albeit with a diminished turnover frequency. Characterization of the intermediates formed upon O₂ activation of Co{i-N₄} reveals a structurally identified stable μ-peroxo Co^III dimer as the main product. A superoxo Co^III species is also formed as a minor product, as indicated by EPR spectroscopy. In further reactivity studies, the electrophilicity of these in situ generated Co−O₂ species was demonstrated by the oxidation of the O−H bond of TEMPO−H (2,2,6,6-tetramethylpiperidin-1-ol) via a H atom abstraction process. Unlike the known Co(cyclam), Co{i-N₄} can be employed in oxygen atom transfer reactions oxidizing triphenylphosphine to the corresponding phosphine oxide highlighting the impact of geometrical modifications of the ligand while preserving the ring size and donor atom set on the reactivity of biomimetic oxygen activating complexes.     

Electron Transfer in the Cs⊂{Mn4Fe4} Cubic Switch: A Soluble Molecular Model of the MnFe Prussian-Blue Analogues

A mixed-valence {Mn^II₃Mn^IIIFe^II₂Fe^III₂} cyanide-bridged molecular cube hosting a caesium cation, Cs⊂{Mn₄Fe₄}, was synthesized and structurally characterized by X-ray diffraction. Cyclic-voltammetry measurements show that its electronic state can be switched between five different redox states, which results in a remarkable electrochromic effect. Magnetic measurements on fresh samples point to the occurrence of a spin-state change near room temperature, which could be ascribed to a metal-to-metal electron transfer converting the {Fe^II−CN−Mn^III} pair into a {Fe^III−CN−Mn^II} pair. This feature was only previously observed in the polymeric MnFe Prussian-blue analogues (PBAs). Moreover, this novel switchable molecule proved to be soluble and stable in organic solvents, paving the way for its integration into advanced materials.     

Electrochemical study of a nonheme Fe(ii) complex in the presence of dioxygen. Insights into the reductive activation of O2 at Fe(ii) centers

Recent efforts to model the reactivity of iron oxygenases have led to the generation of nonheme Fe^III(OOH) and Fe^IV(O) intermediates from Fe^II complexes and O₂ but using different cofactors. This diversity emphasizes the rich chemistry of nonheme Fe(ii) complexes with dioxygen. We report an original mechanistic study of the reaction of [(TPEN)Fe^II]²⁺ with O₂ carried out by cyclic voltammetry. From this Fe^II precursor, reaction intermediates such as [(TPEN)Fe^IV(O)]²⁺, [(TPEN)Fe^III(OOH)]²⁺ and [(TPEN)Fe^III(OO)]⁺ have been chemically generated in high yield, and characterized electrochemically. These electrochemical data have been used to analyse and perform simulation of the cyclic voltammograms of [(TPEN)Fe^II]²⁺ in the presence of O₂. Thus, several important mechanistic informations on this reaction have been obtained. An unfavourable chemical equilibrium between O₂ and the Fe^II complex occurs that leads to the Fe^III-peroxo complex upon reduction, similarly to heme enzymes such as P450. However, unlike in heme systems, further reduction of this latter intermediate does not result in O–O bond cleavage.     

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)]⁺-Mⁿ⁺ (Mⁿ⁺=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^.−-Mⁿ⁺ complexes. In contrast, the logarithm of the rate constants of the [(TMC)Fe^III(O₂)]⁺-Mⁿ⁺ 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)]⁺-Mⁿ⁺ complexes in electron-transfer, electrophilic, and nucleophilic reactions.     

A Structural and Functional Model for the Tris-Histidine Motif in Cysteine Dioxygenase

The iron(II) complexes [Fe(L)(MeCN)₃](SO₃CF₃)₂ (L are two derivatives of tris(2-pyridyl)-based ligands) have been synthesized as models for cysteine dioxygenase (CDO). The molecular structure of one of the complexes exhibits octahedral coordination geometry and the Fe−N_py bond lengths [1.953(4)–1.972(4) Å] are similar to those in the Cys-bound Fe^II-CDO; Fe−N_His: 1.893–2.199 Å. The iron(II) centers of the model complexes exhibit relatively high Fe^III/II redox potentials (E_1/2=0.988–1.380 V vs. ferrocene/ferrocenium electrode, Fc/Fc⁺), within the range for O₂ activation and typical for the corresponding nonheme iron enzymes. The reaction of in situ generated [Fe(L)(MeCN)(SPh)]⁺ with excess O₂ in acetonitrile (MeCN) yields selectively the doubly oxygenated phenylsulfinic acid product. Isotopic labeling studies using ¹⁸O₂ confirm the incorporation of both oxygen atoms of O₂ into the product. Kinetic and preliminary DFT studies reveal the involvement of an Fe^III peroxido intermediate with a rhombic S= Fe^III center (687–696 nm; g≈2.46–2.48, 2.13–2.15, 1.92–1.94), similar to the spectroscopic signature of the low-spin Cys-bound Fe^IIICDO (650 nm, g≈2.47, 2.29, 1.90). The proposed Fe^III peroxido intermediates have been trapped, and the O−O stretching frequencies are in the expected range (approximately 920 and 820 cm⁻¹ for the alkyl- and hydroperoxido species, respectively). The model complexes have a structure similar to that of the enzyme and structural aspects as well as the reactivity are discussed.     

cis-Dihydroxylation of Olefins by a Non-Heme Iron Catalyst: A Functional Model for Rieske Dioxygenases

The first iron complex capable of olefin cis -dihydroxylation in combination with H₂O₂ provides a functional model for Rieske dioxygenases. Mechanistic studies on the model reaction suggest the participation of an Fe^III(η²-OOH) intermediate, with the oxygen atoms coming exclusively from H₂O₂ (see reaction scheme; L denotes a tris(6-methyl-2-pyridylmethyl)amine ligand, Solv=solvent). The similarities between the model and the enzymes strengthen the proposal that an Fe^III–peroxo intermediate is involved in the enzymatic reactions.     

FeIII and FeII Phosphasalen Complexes: Synthesis,Characterization, and Catalytic Application for 2-Naphthol Oxidative Coupling

We report on the synthesis and characterization of three iron(III) phosphasalen complexes, [Fe^III(Psalen)(X)] differing in the nature of the counter-anion/exogenous ligand (X⁻=Cl⁻, NO₃⁻, OTf⁻), as well as the neutral iron(II) analogue, [Fe^II(Psalen)] . Phosphasalen (Psalen) differs from salen by the presence of iminophosphorane (P=N) functions in place of the imines. All the complexes were characterized by single-crystal X-ray diffraction, UV/Vis, EPR, and cyclic voltammetry. The [Fe^II(Psalen)] complex was shown to remain tetracoordinated even in coordinating solvent but surprisingly exhibits a magnetic moment in line with a Fe^II high-spin ground state. For the Fe^III complexes, the higher lability of triflate anion compared to nitrate was demonstrated. As they exhibit lower reduction potentials compared to their salen analogues, these complexes were tested for the coupling of 2-naphthol using O₂ from air as oxidant. In order to shed light on this reaction, the interaction between 2-naphthol and the Fe^III(Psalen) complexes was studied by cyclic voltammetry as well as UV/Vis spectroscopy.     

Transition metal complexes derived from N-isonicotinamido-N′-benzoylthiocarbamide (H2IBTC)

Summary The synthesis and characterization of Cr^III, Mn^II, Fe^III, Co^II, Ni^II, Cu^II, Zn^II, Cd^II and UO _inf2 ^sup2+ complexes of N-isonicotinamido-_N-benzoylthiocarbamide (H₂IBTC) are reported. I.r. spectral data show that the ligand behaves in a bidentate, tridentate and/or tetradentate manner. Different stereochemistries are proposed for Cr^III, Mn^II, Fe^III, Co^II, Ni^II and Cu^II complexes on the basis of spectral and magnetic studies. The i.r. data indicate that the carbonyl oxygen of the benzoyl moiety is the backbone of chelation in most complexes.     

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.     

Ambidenticity of a tridentate oxygen-nitrogen donor towards bivalent and trivalent transition metal ions

Summary N-salicylidene anthranilamide (H₂SAA) and its Cr^III, Mn^II, Fe^III, Co^II, Ni^II and Cu^II complexes were prepared and characterized by physicochemical and spectroscopic data. H₂SAA enolizes to give a dibasic ONO donor set in the divalent metal complexes. It also binds to the trivalent metal ions in a nonenolized form using a monobasic ONN donor set. Co^II is oxidized to Co^III during complexation. Octahedral geometries are proposed for Cr^III, Mn^II, Fe^III and Co^III complexes, while square planar geometries are suggested for the Ni^II and Cu^II complexes. Phenoxide bridging in the Cr^III and Fe^III complexes and enoxide bridging in the Ni^II and Cu^II complexes is proposed.     

An Artificial Enzyme Made by Covalent Grafting of an FeII Complex into β‐Lactoglobulin: Molecular Chemistry,Oxidation Catalysis,and Reaction‐Intermediate Monitoring in a Protein

An artificial metalloenzyme based on the covalent grafting of a nonheme Fe^II polyazadentate complex into bovine β‐lactoglobulin has been prepared and characterized by using various spectroscopic techniques. Attachment of the Fe^II catalyst to the protein scaffold is shown to occur specifically at Cys121. In addition, spectrophotometric titration with cyanide ions based on the spin‐state conversion of the initial high spin (S=2) Fe^II complex into a low spin (S=0) one allows qualitative and quantitative characterization of the metal center’s first coordination sphere. This biohybrid catalyst activates hydrogen peroxide to oxidize thioanisole into phenylmethylsulfoxide as the sole product with an enantiomeric excess of up to 20 %. Investigation of the reaction between the biohybrid system and H₂O₂ reveals the generation of a high spin (S=5/2) Fe^III(η²‐O₂) intermediate, which is proposed to be responsible for the catalytic sulfoxidation of the substrate.     

Heterotrimetallic Coordination Polymers: {CuIILnIIIFeIII} Chains and {NiIILnIIIFeIII} Layers: Synthesis,Crystal Structures,and Magnetic Properties

The use of the [Fe^III(AA)(CN)₄]^? complex anion as metalloligand towards the preformed [Cu^II(valpn)Ln^III]³⁺ or [Ni^II(valpn)Ln^III]³⁺ heterometallic complex cations (AA=2,2′‐bipyridine (bipy) and 1,10‐phenathroline (phen); H₂valpn=1,3‐propanediyl‐bis(2‐iminomethylene‐6‐methoxyphenol)) allowed the preparation of two families of heterotrimetallic complexes: three isostructural 1D coordination polymers of general formula {[Cu^II(valpn)Ln^III(H₂O)₃(μ‐NC)₂Fe^III(phen)(CN)₂ {(μ‐NC)Fe^III(phen)(CN)₃}]NO₃ ? 7 H₂O}_n (Ln=Gd ( 1 ), Tb ( 2 ), and Dy ( 3 )) and the trinuclear complex [Cu^II(valpn)La^III(OH₂)₃(O₂NO)(μ‐NC)Fe^III(phen)(CN)₃] ? NO₃ ? H₂O ? CH₃CN ( 4 ) were obtained with the [Cu^II(valpn)Ln^III]³⁺ assembling unit, whereas three isostructural heterotrimetallic 2D networks, {[Ni^II(valpn)Ln^III(ONO₂)₂(H₂O)(μ‐NC)₃Fe^III(bipy)(CN)] ? 2 H₂O ? 2 CH₃CN}_n (Ln=Gd ( 5 ), Tb ( 6 ), and Dy ( 7 )) resulted with the related [Ni^II(valpn)Ln^III]³⁺ precursor. The crystal structure of compound 4 consists of discrete heterotrimetallic complex cations, [Cu^II(valpn)La^III(OH₂)₃(O₂NO)(μ‐NC)Fe^III(phen)(CN)₃]⁺, nitrate counterions, and non‐coordinate water and acetonitrile molecules. The heteroleptic {Fe^III(bipy)(CN)₄} moiety in 5 – 7 acts as a tris‐monodentate ligand towards three {Ni^II(valpn)Ln^III} binuclear nodes leading to heterotrimetallic 2D networks. The ferromagnetic interaction through the diphenoxo bridge in the Cu^II?Ln^III ( 1 – 3 ) and Ni^II?Ln^III ( 5 – 7 ) units, as well as through the single cyanide bridge between the Fe^III and either Ni^II ( 5 – 7 ) or Cu^II ( 4 ) account for the overall ferromagnetic behavior observed in 1 – 7 . DFT‐type calculations were performed to substantiate the magnetic interactions in 1 , 4 , and 5 . Interestingly, compound 6 exhibits slow relaxation of the magnetization with maxima of the out‐of‐phase ac signals below 4.0 K in the lack of a dc field, the values of the pre‐exponential factor (τ_o) and energy barrier (E_a) through the Arrhenius equation being 2.0×10^?12 s and 29.1 cm^?1, respectively. In the case of 7 , the ferromagnetic interactions through the double phenoxo (Ni^II–Dy^III) and single cyanide (Fe^III–Ni^II) pathways are masked by the depopulation of the Stark levels of the Dy^III ion, this feature most likely accounting for the continuous decrease of χ_M T upon cooling observed for this last compound.     

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.