On The Nature of FeIV=Oaq in Aqueous Media: A DFT analysis

Fe^IV=O_aq is a key intermediate in many advanced oxidation processes and probably in biological systems. It is usually referred to as Fe^IV=O²⁺. The pKa's of Fe^IV=O_aq as derived by DFT are: pKa1=2.37 M06 L/6-311++G(d,p) (SDD for Fe) and pKa2=7.79 M06 L/6-311++G(d,p) (SDD for Fe). This means that in neutral solutions, Fe^IV=O_aq is a mixture of (H₂O)₄(OH)Fe^IV=O⁺ and (H₂O)₂(OH)₂Fe^IV=O. The oxidation potential of Fe^IV=O_aq in an acidic solution, E⁰{(H₂O)₅Fe^IV=O²⁺/Fe^III(H₂O)₆³⁺, pH 0.0} is calculated with and without a second solvation sphere and the recommended value is between 2.86 V (B3LYP/Def2-TZVP, with a second solvation sphere) and 2.23 V (M06 L/Def2-TZVP without a second solvation sphere). This means that Fe^IV=O_aq is the strongest oxidizing agent formed in systems involving Fe^VIO₄²⁻ even in neutral media.     

ESR study on the reactions of iron carbonyls with nitro and nitrosoparaffines. A mechanism of the reductive carbonylation of nitro compounds

The interaction of Fe_n(CO)_m, (n and m equal 1 and 5, 2 and 9, 3 and 12, respectively) with 2-methyl-2-nitrosopropane and sodium salts of nitromethane and nitrocyclohexane was studied. The initial stages of the process, following the activating complex-formation, involves redox disproportionation to give rise to the radical Fe(I) carbonyl complexes and radical anions Fe₂(CO)₈⁻_. (I) Fe₃(CO)₁₁⁻_. (II), Fe₄(CO)₁₃⁻_. (III) and Fe₃(CO)₁₂⁻_. (IV). Also, radical anions I–IV are formed in the interaction of salts of carbonyl ferrate anions Na₂Fe(CO)₄·1.5 diox and PPN₂[Fe_n(CO)_m−1] (where PPN = (PPh₃)₂N⁺), with nitro- and nitroso-tert-butane.Radical anions I–III act as catalytically active species in the coordination sphere of which the nitro compounds undergo a successive deoxygenation to nitrene radical complexes with their subsequent carbonylation to isocyanates. A scheme of the reductive carbonylation is proposed.     

Study on catalytic mechanisms of Fe3O4-rGOx in three typical advanced oxidation processes for tetracycline hydrochloride degradation

This study explored the catalytic mechanism and performance impacted by the materials ratio of Fe₃O₄-GO_x composites in three typical advanced oxidation processes (AOPs) of O₃, peroxodisulfate (PDS) and photo-Fenton processes for tetracycline hydrochloride (TCH) degradation. The ratio of GO in the Fe₃O₄-GO_x composites exhibited different trends of degradation capacity in each AOPs based on different mechanisms. Fe₃O₄-rGO_20wt% exhibited the optimum catalytic performance which enhanced the ozone decomposition efficiency from 33.48% (ozone alone) to 51.83% with the major reactive oxygen species (ROS) of O₂^??. In PDS and photo-Fenton processes, Fe₃O₄-rGO_5wt% had the highest catalytic performance in PDS and H₂O₂ decomposition for SO₄^??, and ^?OH generation, respectively. Compared with using PDS alone, PDS decomposition rate and TCH degradation rate could be increased by 5.97 and 1.73 times under Fe₃O₄-rGO_5wt% catalysis. In the photo-Fenton system, Fe₃O₄-rGO_5wt% with the best catalyst performance in H₂O₂ decomposition, and TCH degradation rate increased by 2.02 times compared with blank group. Meantime, the catalytic mechanisms in those systems of that the ROS produced by conversion between Fe²⁺/Fe³⁺ were also analyzed.     

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.     

Catalytic Hydroxylation of Benzene to Phenol by Dioxygen with an NADH Analogue

Hydroxylation of benzene by molecular oxygen (O₂) occurs efficiently with 10‐methyl‐9,10‐dihydroacridine (AcrH₂) as an NADH analogue in the presence of a catalytic amount of Fe(ClO₄)₃ or Fe(ClO₄)₂ with excess trifluoroacetic acid in a solvent mixture of benzene and acetonitrile (1:1 v/v) to produce phenol, 10‐methylacridinium ion and hydrogen peroxide (H₂O₂) at 298 K. The catalytic oxidation of benzene by O₂ with AcrH₂ in the presence of a catalytic amount of Fe(ClO₄)₃ is started by the formation of H₂O₂ from AcrH₂, O₂, and H⁺. Hydroperoxyl radical (HO₂^.) is produced from H₂O₂ with the redox pair of Fe³⁺/Fe²⁺ by a Fenton type reaction. The rate‐determining step in the initiation is the proton‐coupled electron transfer from Fe²⁺ to H₂O₂ to produce HO^. and H₂O. HO^. abstracts hydrogen rapidly from H₂O₂ to produce HO₂^. and H₂O. The Fe³⁺ produced was reduced back to Fe²⁺ by H₂O₂. HO₂^. reacts with benzene to produce the radical adduct, which abstracts hydrogen from AcrH₂ to give the corresponding hydroperoxide, accompanied by generation of acridinyl radical (AcrH^.) to constitute the radical chain reaction. Hydroperoxyl radical (HO₂^.), which was detected by using the spin trap method with EPR analysis, acts as a chain carrier for the two radical chain pathways: one is the benzene hydroxylation with O₂ and the second is oxidation of an NADH analogue with O₂ to produce H₂O₂.     

Extreme Differences in the Interactions of `Bare' Fe+ and Fe2+ with Hydrogen Peroxide: Fenton Chemistry in the Gas Phase

The interaction of bare iron mono‐ and dications with hydrogen peroxide in the gas phase is studied by ab initio calculations employing the B3LYP/6‐311+G* level of theory. For the monocation, the quartet and sextet coordination complexes Fe(H₂O₂) are high‐energy isomers that easily interconvert to the more stable iron dihydroxide monocation Fe(OH) and hydrated iron oxide (H₂O)FeO⁺ (quartet) or dissociate into FeOH⁺+OH^. (sextet). On the dication surface, however, the order of stabilities is reversed in that Fe(H₂O₂)²⁺ (quintet) corresponds to the most stable doubly charged species, while the formal Fe^IV compounds Fe(OH) and (H₂O)FeO²⁺ are higher in energy.     

Influence of γ-radiation on the reactivity of montmorillonite towards H2O2

Compacted and water saturated bentonite will be used as an engineered barrier in deep geological repositories for radioactive waste in many countries. Due to the high dose rate of ionizing radiation outside the canisters holding the nuclear waste, radiolysis of the interlayer and pore water in the compacted bentonite is unavoidable. Upon reaction with the oxidizing and reducing species formed by water radiolysis (OH^•, e⁻_(aq), H^•, H₂O₂, H₂, HO₂^•, H₃O⁺), the overall redox properties in the bentonite barrier may change. In this study the influence of γ-radiation on the structural Fe(II)/Fe_Tot ratio in montmorillonite and its reactivity towards hydrogen peroxide (H₂O₂) was investigated in parallel experiments. The results show that under anoxic conditions the structural Fe(II)/Fe_Tot ratio of dispersed Montmorillonite increased from ≤3 to 25-30% after γ-doses comparable to repository conditions. Furthermore, a strong correlation between the structural Fe(II)/Fe_Tot ratio and the H₂O₂ decomposition rate in montmorillonite dispersions was found. This correlation was further verified in experiments with consecutive H₂O₂ additions, since the structural Fe(II)/Fe_Tot ratio was seen to decrease concordantly. This work shows that the structural iron in montmorillonite could be a sink for one of the major oxidants formed upon water radiolysis in the bentonite barrier, H₂O₂.     

Synthesis and redox characteristics of iron complexes with triphenylsubstituted corrols in the presence of argon and oxygen

para-Substituted iron meso-triphenylcorrole derivatives [Fe(ms-p-R-Ph)₃Cor] containing electron- donating (R = OMе) and electron-drawing (R = NO₂) groups in phenyl rings are synthesized and characterized by ¹H NMR, electronic absorption spectroscopy, and mass spectrometry. The effect of the nature of functional groups within iron complexes on the redox processes involving these complexes in water–alkaline solutions is analyzed. Electronic transitions in the ligand (E_red/ox = 0.820–0.850 V) and the metal (E_red/ox =–0.005 to–0.190 and–0.790 to–0.870 V for the Fe⁴⁺ ? Fe³⁺ and Fe³⁺ ? Fe²⁺ transitions, respectively) were found in the cyclic voltammograms. Iron in the synthesized complexes I–IV under the conditions under study exists in the +4 oxidation state. The activity of iron complexes in electroreduction of molecular oxygen significantly depends on the nature of a substituent, increases in the series: Fe(ms-p-NO₂Ph)₃Cor (II) < Fe(ms-p-MeOPh)₃Cor (I) < Fe(β-Br)₈(ms-Ph)₃Cor (IV) < Fe(ms-Ph)₃Cor (II) and is caused by the fact that low-energy redox electron transitions occur in the molecules. The electrocatalytic activity of iron corroles is much higher than that of metal porphyrins with a similar structure.     

Visible light induced efficient activation of persulfate by a carbon quantum dots (CQDs) modified γ-Fe2O3 catalyst

In this study, a carbon quantum dots modified maghemite catalyst (CQDs@γ-Fe₂O₃) has been synthesized by a one-step solvothermal method for efficient persulfate (PDS) activation under visible light irradiation. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectroscopy (UV–vis DRS) characterization indicated that the formation of heterojunction structure between CQDs and γ-Fe₂O₃ effectively reduced the catalyst band gap (E_g), favoring the separation rate of electrons and holes, leading to remarkable efficient sulfamethoxazole (SMX) degradation as compared to the dark-CQDs@γ-Fe₂O₃/PDS and vis-γ-Fe₂O₃/PDS systems. The evolution of dissolved irons also demonstrated that CQDs could accelerate the in-situ reduction of surface-bounded Fe³⁺. Electron paramagnetic resonance (EPR) and radical scavenging experiments demonstrated that both OH and SO₄⁻ were generated in the reaction system, while OH was relatively more dominant than SO₄⁻ for SMX degradation. Finally, the reaction mechanism in the vis-CQDs@γ-Fe₂O₃/PDS system was proposed involving an effective and accelerated heterogeneous-homogeneous iron cycle. CQDs would enrich the photo-generated electrons from γ-Fe₂O₃, causing efficient interfacial generation of surface-bond Fe²⁺ and reduction of adsorbed Fe³⁺. This visible light induced iron cycle would eventually lead to effective activation of PDS as well as the efficient degradation of SMX.     

Generation of FeIV=O and its Contribution to Fenton-Like Reactions on a Single-Atom Iron−N−C Catalyst

Generating Fe^IV=O on single-atom catalysts by Fenton-like reaction has been established for water treatment; however, the Fe^IV=O generation pathway and oxidation behavior remain obscure. Employing an Fe−N−C catalyst with a typical Fe−N₄ moiety to activate peroxymonosulfate (PMS), we demonstrate that generating Fe^IV=O is mediated by an Fe−N−C−PMS* complex—a well-recognized nonradical species for induction of electron-transfer oxidation—and we determined that adjacent Fe sites with a specific Fe₁−Fe₁ distance are required. After the Fe atoms with an Fe₁-Fe₁ distance <4 Å are PMS-saturated, Fe−N−C−PMS* formed on Fe sites with an Fe₁-Fe₁ distance of 4–5 Å can coordinate with the adjacent Fe^II−N₄, forming an inter-complex with enhanced charge transfer to produce Fe^IV=O. Fe^IV=O enables the Fenton-like system to efficiently oxidize various pollutants in a substrate-specific, pH-tolerant, and sustainable manner, where its prominent contribution manifests for pollutants with higher one-electron oxidation potential.     

Two new cyano-bridged Cu(II)-Fe(II) complexes: Synthesis and crystal structures

Two new cyano-bridged Cu(II)-Fe(II) binuclear complexes, [Cu(L¹)Fe(CN)₅(NO)] (I) [L¹ = 1,3,6,8,11,14-hexaazatricyclo[12.2.1.1^8,11]octadecane and [Cu(L²)Fe(CN)₅(NO)] · 2H₂O (II) L² = 1,3,6,9,11,14-hexaazatricyclo[12.2.1.1^6,9]octadecane, have been assembled and structurally characterized by spectroscopy and X-ray crystallography. Complex I crystallizes in the monoclinic crystalline system of space group P2₁/c, while complex II crystallizes in the monoclinic crystalline system of space group P2₁/n. These two complexes assume a binuclear structure in which the Fe²⁺ ion is in an octahedron environment and the Cu²⁺ ion is in a square-prism geometry environment.     

Ferric ion-ascorbic acid complex catalyzed calcium peroxide for organic wastewater treatment: Optimized by response surface method

Hydrogen peroxide (H₂O₂) disproportionation, iron precipitation, and narrow pH range are the drawbacks of traditional Fenton process. To surmount these barriers, we proposed a ferric ion (Fe³⁺)-ascorbic acid (AA) complex catalyzed calcium peroxide (CaO₂) Fenton-like system to remove organic dyes in water. This collaborative Fe³⁺/AA/CaO₂ system presented an obvious improvement in the methyl orange (MO) decolorization, and also effectively eliminated other dyes. Response surface method was employed to optimize the running parameters for this coupling process. Under the optimized arguments (2.76 mmol/L Fe³⁺, 0.68 mmol/L AA, and 4 mmol/L CaO₂), the MO removal achieved 98.90% after 15 min at pH 6.50, which was close to the computed outcome of 99.30%. Furthermore, this Fenton-like system could perform well in a wide range of pH (3–11), and enhance the H₂O₂ decomposition and Fe ions recycle. The scavenger experiment result indicated that hydroxyl radical, superoxide anion free radical, and singlet oxygen were acted on the dye elimination. Moreover, electron spin resonance analysis corroborated that the existences of these active species in the Fe³⁺/AA/CaO₂ system. This study could advance the development of Fenton-like technique in organic effluent disposal.     

Non-Heme-Iron-Mediated Selective Halogenation of Unactivated Carbon−Hydrogen Bonds

Oxidation of the iron(II) precursor [(L¹)Fe^IICl₂], where L¹ is a tetradentate bispidine, with soluble iodosylbenzene (^sPhIO) leads to the extremely reactive ferryl oxidant [(L¹)(Cl)Fe^IV=O]⁺ with a cis disposition of the chlorido and oxido coligands, as observed in non-heme halogenase enzymes. Experimental data indicate that, with cyclohexane as substrate, there is selective formation of chlorocyclohexane, the halogenation being initiated by C−H abstraction and the result of a rebound of the ensuing radical to an iron-bound Cl⁻. The time-resolved formation of the halogenation product indicates that this primarily results from ^sPhIO oxidation of an initially formed oxido-bridged diiron(III) resting state. The high yield of up to >70 % (stoichiometric reaction) as well as the differing reactivities of free Fe²⁺ and Fe³⁺ in comparison with [(L¹)Fe^IICl₂] indicate a high complex stability of the bispidine-iron complexes. DFT analysis shows that, due to a large driving force and small triplet-quintet gap, [(L¹)(Cl)Fe^IV=O]⁺ is the most reactive small-molecule halogenase model, that the Fe^III/radical rebound intermediate has a relatively long lifetime (as supported by experimentally observed cage escape), and that this intermediate has, as observed experimentally, a lower energy barrier to the halogenation than the hydroxylation product; this is shown to primarily be due to steric effects.     

A Stable Homoleptic Organometallic Iron(IV) Complex

A homoleptic organometallic Fe^IV complex that is stable in both solution and in the solid state at ambient conditions has been synthesized and isolated as [Fe(phtmeimb)₂](PF₆)₂ (phtmeimb=[phenyl(tris(3-methylimidazolin-2-ylidene))borate]⁻). This Fe^IV N-heterocyclic carbene (NHC) complex was characterized by ¹H NMR, HR-MS, elemental analysis, scXRD analysis, electrochemistry, Mößbauer spectroscopy, and magnetic susceptibility. The two latter techniques unequivocally demonstrate that [Fe(phtmeimb)₂](PF₆)₂ is a triplet Fe^IV low-spin S=1 complex in the ground state, in agreement with quantum chemical calculations. The electronic absorption spectrum of [Fe(phtmeimb)₂](PF₆)₂ in acetonitrile shows an intense absorption band in the red and near IR, due to LMCT (ligand-to-metal charge transfer) excitation. For the first time the excited state dynamics of a Fe^IV complex was studied and revealed a ≈0.8 ps lifetime of the ³LMCT excited state of [Fe(phtmeimb)₂](PF₆)₂ in acetonitrile.     

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.     

Molecular structure of some [1.1 ]ferrocenylruthenocenophanium (n = 1,2 polyiodides salts

Oxidation of [1.1]ferrocenylruthenocenophane with a large excess and 1.5 equivalents of iodine gives dicationic iodo[1.1]ferrocenylruthenocenophanium²⁺I₃⁻ · 0.5I₂2 (1) and monocationic [1.1]ferrocenylruthenocenophanium⁺I₃⁻ (2) salts respectively. The structures of 1 and 2 were analyzed by single-crystal X-ray diffraction studies. The crystal form of 1 is monoclinic space group C2/c, A = 21.35](5), B = 20.594(5), C = 17.397(4) Å, β = 124.17(1)°, Z = 8, and the final R = 0.068 and R_w = 0.070. The cation formulated as [Fe^III(C₅H₄CH₂C₅H₄)₂Ru^IVI]²⁺ exists in a syn-conformation as in the cases of the neutral compound. The distance between the Ru^IV and Fe^II is 4.656(4) Å, which is much shorter than the value of the neutral compound (4.792(2) Å), and the bond angle of I---Ru^IV,Fe^III is 81.26°. The dihedral angle between the two η⁵-C₅H₄ (fulvenide) rings on the Ru^IV moiety is 37.56° due to the Ru^IV---I bond (2.758(3) Å). These two rings of Fe^III and Ru^IV moieties are essentially eclipsed. The unit cell has three kinds of I₃⁻ (I_3a⁻, I_3b⁻ and I_3c⁻) and one I₂, and the formula of 1 is given as [Fe^III(C₅H₄CH₂CSH₄)₂Ru^IVI]²⁺I₃⁻ · 0.5(I₃⁻)₂ · 0.5I₂. The crystal of 2 formulated as [Fe^III(C₅H₄CH₂C₅H₄)₂Ru^II]⁺I₃⁻ is triclinic space group

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, and the final R = 0.067 and R_w = 0.068. The unit cell has two independent molecules (unit A and B); i.e. two kinds of distance between the Ru^II and Fe^III, are observed; one (A) is 4.615(3) and the other (B) is 4.647(3) α. The two η⁵-C₅H₄ rings of both Fe^III and Ru^II are essentially staggered and the dihedral angles between the rings of FcH and RcH moieties are less than 5.8°. Typical ferrocenium-type broad singlet ⁵⁷Fe-Mössbauer lines are observed for both salts (1, 2) at all temperatures.     

Hydroxylation of Aromatics with the Help of a Non‐Haem FeOOH: A Mechanistic Study under Single‐Turnover and Catalytic Conditions

Ferric–hydroperoxo complexes have been identified as intermediates in the catalytic cycle of biological oxidants, but their role as key oxidants is still a matter of debate. Among the numerous synthetic low‐spin Fe^III(OOH) complexes characterized to date, [(L₅²)Fe(OOH)]²⁺ is the only one that has been isolated in the solid state at low temperature, which has provided a unique opportunity for inspecting its oxidizing properties under single‐turnover conditions. In this report we show that [(L₅²)Fe(OOH)]²⁺ decays in the presence of aromatic substrates, such as anisole and benzene in acetonitrile, with first‐order kinetics. In addition, the phenol products are formed from the aromatic substrates with similar first‐order rate constants. Combining the kinetic data obtained at different temperatures and under different single‐turnover experimental conditions with experiments performed under catalytic conditions by using the substrate [1,3,5‐D₃]benzene, which showed normal kinetic isotope effects (KIE>1) and a notable hydride shift (NIH shift), has allowed us to clarify the role played by Fe^III(OOH) in aromatic oxidation. Several lines of experimental evidence in support of the previously postulated mechanism for the formation of two caged Fe^IV(O) and OH ^. species from the Fe^III(OOH) complex have been obtained for the first time. After homolytic O? O cleavage, a caged pair of oxidants [Fe^IVO+HO ^. ] is generated that act in unison to hydroxylate the aromatic ring: HO ^. attacks the ring to give a hydroxycyclohexadienyl radical, which is further oxidized by Fe^IVO to give a cationic intermediate that gives rise to a NIH shift upon ketonization before the final re‐aromatization step. Spin‐trapping experiments in the presence of 5,5‐dimethyl‐1‐pyrroline N‐oxide and GC‐MS analyses of the intermediate products further support the proposed mechanism.     

Efficient Fe(III)/Fe(II) cycling triggered by MoO2 in Fenton reaction for the degradation of dye molecules and the reduction of Cr(VI)

There is a relatively low efficiency of Fe(III)/Fe(II) conversion cycle and H₂O₂ decomposition (<30%) in conventional Fenton process, which further results in a low production efficiency of OH and seriously restricts the application of Fenton. Herein, we report that the commercial MoO₂ can be used as the cocatalyst in Fenton process to dramatically accelerate the oxidation of Lissamine rhodamine B (L-RhB), where the efficiency of Fe(III)/Fe(II) cycling is greatly enhanced in the Fenton reaction meanwhile. And the L-RhB solution could be degraded nearly 100% in 1 min in the MoO₂ cocatalytic Fenton system under the optimal reaction condition, which is apparently better than that of the conventional Fenton system (～50%). Different from the conventional Fenton reaction where the OH plays an important role in the oxidation process, it shows that ¹O₂ contributes most in the MoO₂ cocatalytic Fenton reaction. However, it is found that the exposed Mo⁴⁺ active sites on the surface of MoO₂ powders can greatly promote the rate-limiting step of Fe³⁺/Fe²⁺ cycle conversion, thus minimizing the dosage of H₂O₂ (0.400 mmol/L) and Fe²⁺ (0.105 mmol/L). Interestingly, the MoO₂ cocatalytic Fenton system also exhibits a good ability for reducing Cr(VI) ions, where the reduction ability for Cr(VI) reaches almost 100% within 2 h. In short, this work shows a new discovery for MoO₂ cocatalytic advanced oxidation processes (AOPs), which devotes a lot to the practical water remediation application.     

Hydroxylation of Aromatics by H2O2 Catalyzed by Mononuclear Non-heme Iron Complexes: Role of Triazole Hemilability in Substrate-Induced Bifurcation of the H2O2 Activation Mechanism

Rieske dioxygenases are metalloenzymes capable of achieving cis-dihydroxylation of aromatics under mild conditions using O₂ and a source of electrons. The intermediate responsible for this reactivity is proposed to be a cis-Fe^V(O)(OH) moiety. Molecular models allow the generation of a Fe^III(OOH) species with H₂O₂, to yield a Fe^V(O)(OH) species with tetradentate ligands, or {Fe^IV(O); OH^.} pairs with pentadentate ones. We have designed a new pentadentate ligand, mtL₄², bearing a labile triazole, to generate an “in-between” situation. Two iron complexes, [(mtL₄²)FeCl](PF₆) and [(mtL₄²)Fe(OTf)₂]), were obtained and their reactivity towards aromatic substrates was studied in the presence of H₂O₂. Spectroscopic and kinetic studies reflect that triazole is bound at the Fe^II state, but decoordinates in the Fe^III(OOH). The resulting [(mtL₄²)Fe^III(OOH)(MeCN)]²⁺ then lies on a bifurcated decay pathway (end-on homolytic vs. side-on heterolytic) depending on the addition of aromatic substrate: in the absence of substrate, it is proposed to follow a side-on pathway leading to a putative (N₄)Fe^V(O)(OH), while in the presence of aromatics it switches to an end-on homolytic pathway yielding a {(N₅)Fe^IV(O); OH^.} reactive species, through recoordination of triazole. This switch significantly impacts the reaction regioselectivity.     

Fe6.67(PO4)5.35(HPO4)0.65 and Fe6.23(PO4)4.45(HPO4)1.55: two new mixed‐valence iron phosphates

Two new mixed‐valence iron phosphates, namely heptairon pentaphosphate hydrogen phosphate, Fe_6.67(PO₄)_5.35(HPO₄)_0.65, and heptairon tetraphosphate bis(hydrogen phosphate), Fe_6.23(PO₄)_4.45(HPO₄)_1.55, have been synthesized hydrothermally at 973 K and 0.1 GPa. The structures are similar to that of Fe^II₃Fe^III₄(PO₄)₆ and are characterized by infinite chains of Fe polyhedra parallel to the [101] direction. These chains are formed by the Fe1O₆ and Fe2O₆ octahedra, alternating with the Fe4O₅ distorted pentagonal bipyramids, according to the stacking sequence ...Fe1–Fe1–Fe4–Fe2–Fe2.... The Fe3O₆ octahedra and PO₄ tetrahedra connect the chains together. Fe^II is localized on the Fe3 and Fe4 sites, whereas Fe^III is found in the Fe1 and Fe2 sites, according to bond‐valence calculations. Refined site occupancies indicate the presence of vacancies on the Fe4 site, explained by the substitution mechanism Fe^II + 2(PO₄³⁻) = vacancies + 2(HPO₄²⁻).