Reply to Comment on Stabilization of Low‐Valent Iron(I) in a High‐Valent Vanadium(V) Oxide Cluster

The authors of the Communication “Stabilization of Low‐Valent Iron(I) in a High‐Valent Vanadium(V) Oxide Cluster” reply to a Comment by Dr. Sproules, who offered an alternative interpretation of the metal oxidation states in the two electron reduced iron vanadate (NH₂Me₂)[(FeCl)V₁₂O₃₂Cl]^4?.     

A Molecular Placeholder Strategy To Access a Family of Transition‐Metal‐Functionalized Vanadium Oxide Clusters

Systematic access to metal‐functionalized polyoxometalates has thus far been limited to lacunary tungsten oxide and molybdenum oxide clusters. The first controlled, stepwise bottom‐up assembly route to metal‐functionalized molecular vanadium oxides is now presented. A di‐vacant vanadate cluster with two metal binding sites, (DMA)₂[V₁₂O₃₂Cl]^3? (DMA=dimethylammonium) is formed spontaneously in solution and characterized by single‐crystal X‐ray diffraction, ESI mass spectrometry, ⁵¹V NMR spectroscopy, and elemental analyses. In the cluster, the metal binding sites are selectively blocked by hydrogen‐bonded DMA placeholder cations. Reaction of the cluster with transition metals TM (Fe³⁺, Co²⁺, Cu²⁺, Zn²⁺) gives access to mono‐functionalized vanadate clusters (DMA)[{TM(L)}V₁₂O₃₂Cl]^n? (L=ligand). Metal binding is accomplished by significant distortions of the vanadium oxide framework reminiscent of a pincer movement. Cluster stability under technologically relevant conditions in the solid‐state and solution is demonstrated.     

Low‐Spin Pseudotetrahedral Iron(I) Sites in Fe2(μ‐S) Complexes

Fe^I centers in iron–sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron–sulfur clusters. A series of three diiron [(L₃Fe)₂(μ‐S)] complexes that were isolated and characterized in the low‐valent oxidation states Fe^II? S? Fe^II, Fe^II? S? Fe^I, and Fe^I? S? Fe^I is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe₂(μ‐S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low‐spin S=1/2 states upon reduction from Fe^II to Fe^I. The possibility of accessing low‐spin, pseudotetrahedral Fe^I sites compatible with S^2? as a ligand was previously unknown.     

Electronic Modulation of the SOMO–HOMO Energy Gap in Iron(III) Complexes towards Unimolecular Current Rectification

Amphiphilic five‐coordinate iron(III) complexes with {N₂O₂Cl} and {N₂O₃} coordination spheres are studied to elucidate the roles of electronic structure on the mechanisms for current rectification. The presence of an apical chlorido or phenolato ligand plays a crucial role, and the [Fe^III{N₂O₂Cl}] species supports an asymmetric mechanism while its [Fe^III{N₂O₃}] counterpart seems to allow for unimolecular mechanism. The effects of electron‐donating and electron‐withdrawing substituents in the ligand frameworks are also considered.     

Nonheme Iron Mediated Oxidation of Light Alkanes with Oxone: Characterization of Reactive Oxoiron(IV) Ligand Cation Radical Intermediates by Spectroscopic Studies and DFT Calculations

The oxidation of light alkanes that is catalyzed by heme and nonheme iron enzymes is widely proposed to involve highly reactive {Fe^V?O} species or {Fe^IV?O} ligand cation radicals. The identification of these high‐valent iron species and the development of an iron‐catalyzed oxidation of light alkanes under mild conditions are of vital importance. Herein, a combination of tridentate and bidentate ligands was used for the generation of highly reactive nonheme {Fe?O} species. A method that employs [Fe^III(Me₃tacn)(Cl‐acac)Cl]⁺ as a catalyst in the presence of oxone was developed for the oxidation of hydrocarbons, including cyclohexane, propane, and ethane (Me₃tacn=1,4,7‐trimethyl‐1,4,7‐triazacyclononane; Cl‐acac=3‐chloro‐acetylacetonate). The complex [Fe^III(Tp)₂]⁺ and oxone enabled stoichiometric oxidation of propane and ethane. ESI‐MS, EPR and UV/Vis spectroscopy, ¹⁸O labeling experiments, and DFT studies point to [Fe^IV(Me₃tacn)({Cl‐acac}^.+)(O)]²⁺ as the catalytically active species.     

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.     

Binuclear,High‐Valent Nickel Complexes: Ni−Ni Bonds in Aryl–Halogen Bond Formation

Metal–metal bonds play a vital role in stabilizing key intermediates in bond‐formation reactions. We report that binuclear benzo[h ]quinoline‐ligated Ni^II complexes, upon oxidation, undergo reductive elimination to form carbon–halogen bonds. A mixed‐valent Ni(2.5+)–Ni(2.5+) intermediate is isolated. Further oxidation to Ni^III, however, is required to trigger reductive elimination. The binuclear Ni^III–Ni^III intermediate lacks a Ni−Ni bond. Each Ni^III undergoes separate, but fast reductive elimination, giving rise to Ni^I species. The reactivity of these binuclear Ni complexes highlights the fundamental difference between Ni and Pd in mediating bond‐formation processes.     

Binuclear,High‐Valent Nickel Complexes: Ni−Ni Bonds in Aryl–Halogen Bond Formation

Metal–metal bonds play a vital role in stabilizing key intermediates in bond‐formation reactions. We report that binuclear benzo[h ]quinoline‐ligated Ni^II complexes, upon oxidation, undergo reductive elimination to form carbon–halogen bonds. A mixed‐valent Ni(2.5+)–Ni(2.5+) intermediate is isolated. Further oxidation to Ni^III, however, is required to trigger reductive elimination. The binuclear Ni^III–Ni^III intermediate lacks a Ni−Ni bond. Each Ni^III undergoes separate, but fast reductive elimination, giving rise to Ni^I species. The reactivity of these binuclear Ni complexes highlights the fundamental difference between Ni and Pd in mediating bond‐formation processes.     

One‐Pot Assembly of a New Mixed‐Valent Aggregate Based on Inorganic Polyoxometalate Ligands

Employing a “one‐pot” synthesis strategy, the reaction of Na₂WO₄·2H₂O, Na₂HAsO₄·7H₂O, FeCl₃·6H₂O, various Ln³⁺ ions, and hexamethylenetetramine (HMTA) in aqueous solutions with pH values ranging from 5.5 to 6.5 results in the isolation of polytungstoarsenate‐based iron aggregates, ‐K₈Na₁₄[HMTA]₄[(Fe^III₃Fe^II_0.25(OH)₃)(AsO₄)(AsW₉O₃₄)]₄·24H₂O ( 1 ) (HMTA = hexamethylenetetraamine). The polyoxoanion of 1 contains a mixed‐valent {Fe^III₁₂Fe^II(μ₃‐OH)₁₂(μ₄‐AsO₄)₄} cluster surrounded by four [B‐α‐AsW₉O₃₄]^9? units. It is the first polytungstatoarsenate‐based mixed‐valent {Fe^III₁₂Fe^II} aggregate and the largest iron cluster based on [AsW₉O₃₄]^9? ligands. The compound was characterized by elemental analyses, IR, UV/Vis absorption, and diffuse‐reflectance UV/Vis spectroscopy, TG analyses, XRPD, XPS and gel‐filtration chromatography. The electrochemical and electrocatalytical properties were also investigated. Crystal data for 1 , orthorhombic, Fddd, a = 28.156(6) Å, b = 36.003(7) Å, c = 42.126(8) Å, α = 90°, β = 90°, γ = 90°, Z = 8.     

Activation of a Non‐Heme FeIII‐OOH by a Second FeIII to Hydroxylate Strong C−H Bonds: Possible Implications for Soluble Methane Monooxygenase

Non‐heme iron oxygenases contain either monoiron or diiron active sites, and the role of the second iron in the latter enzymes is a topic of particular interest, especially for soluble methane monooxygenase (sMMO). Herein we report the activation of a non‐heme Fe^III‐OOH intermediate in a synthetic monoiron system using Fe^III(OTf)₃ to form a high‐valent oxidant capable of effecting cyclohexane and benzene hydroxylation within seconds at ?40 °C. Our results show that the second iron acts as a Lewis acid to activate the iron–hydroperoxo intermediate, leading to the formation of a powerful Fe^V=O oxidant—a possible role for the second iron in sMMO.     

A Chromium(III)‐Superoxo Complex as a Three‐Electron Oxidant with a Large Tunneling Effect in Multi‐Electron Oxidation of NADH Analogues

Metal‐superoxo species are involved in a variety of enzymatic oxidation reactions, and multi‐electron oxidation of substrates is frequently observed in those enzymatic reactions. A Cr^III‐superoxo complex, [Cr^III(O₂)(TMC)(Cl)]⁺ ( 1 ; TMC=1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane), is described that acts as a novel three‐electron oxidant in the oxidation of dihydronicotinamide adenine dinucleotide (NADH) analogues. In the reactions of 1 with NADH analogues, a Cr^IV‐oxo complex, [Cr^IV(O)(TMC)(Cl)]⁺ ( 2 ), is formed by a heterolytic O−O bond cleavage of a putative Cr^II‐hydroperoxo complex, [Cr^II(OOH)(TMC)(Cl)], which is generated by hydride transfer from NADH analogues to 1 . The comparison of the reactivity of NADH analogues with 1 and p ‐chloranil (Cl₄Q) indicates that oxidation of NADH analogues by 1 proceeds by proton‐coupled electron transfer with a very large tunneling effect (for example, with a kinetic isotope effect of 470 at 233 K), followed by rapid electron transfer.     

A Redox‐Induced Spin‐State Cascade in a Mixed‐Valent Fe3(μ3‐O) Triangle

One‐electron reduction of a pyrazolate‐bridged triangular Fe₃(μ₃‐O) core induces a cascade wherein all three metal centers switch from high‐spin Fe³⁺ to low‐spin Fe^2.66+. This hypothesis is supported by spectroscopic data (¹H‐NMR, UV‐vis‐NIR, infra‐red, ⁵⁷Fe‐Mössbauer, EPR), X‐ray crystallographic characterization of the cluster in both oxidation states and also density functional theory. The reduction induces substantial contraction in all bond lengths around the metal centers, along with diagnostic shifts in the spectroscopic parameters. This is, to the best of our knowledge, the first example of a one‐electron redox event causing concerted change in multiple iron centers.     

Direct Observation of Primary C−H Bond Oxidation by an Oxido‐Iron(IV) Porphyrin π‐Radical Cation Complex in a Fluorinated Carbon Solvent

Oxido‐iron(IV) porphyrin π‐radical cation species are involved in a variety of heme‐containing enzymes and have characteristic oxidation states consisting of a high‐valent iron center and a π‐conjugated macrocyclic ligand. However, the short lifetime of the complex has hampered detailed reactivity studies. Reported herein is a remarkable increase in the lifetime (80 s at 10 °C) of Fe^IV(TMP^+.)(O)(Cl) ( 2 ; TMP=5,10,15,20‐tetramesitylporphyrin dianion), produced by the oxidation of Fe^III(TMP)(Cl) ( 1 ) by ozone in α,α,α‐trifluorotoluene (TFT). The lifetime is 720 times longer compared to that of the currently most stable species reported to date. The increase in the lifetime improves the reaction efficiency of 2 toward inert alkane substrates, and allowed observation of the reaction of 2 with a primary C?H bond (BDE_C‐H=ca. 100 kcal mol^?1) directly. Activation parameters for cyclohexane hydroxylation were also obtained.     

Axial Ligand and Spin‐State Influence on the Formation and Reactivity of Hydroperoxo–Iron(III) Porphyrin Complexes

The present study focuses on the formation and reactivity of hydroperoxo–iron(III) porphyrin complexes formed in the [Fe^III(tpfpp)X]/H₂O₂/HOO^? system (TPFPP=5,10,15,20‐tetrakis(pentafluorophenyl)‐21H,23H‐porphyrin; X=Cl^? or CF₃SO₃^?) in acetonitrile under basic conditions at ?15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high‐spin [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] could be observed with the application of a low‐temperature rapid‐scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O? O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo‐ to heterolytic O? O bond cleavage is observed for high‐ [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron‐rich porphyrin ligands, electron‐deficient [Fe^III(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe^III(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)–oxo porphyrin π‐cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.     

A Room‐Temperature Verwey‐type Transition in Iron Oxide,Fe5O6

Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next‐generation nanoelectronic devices. To date, vanadium dioxide (VO₂) is the only known simple transition‐metal oxide that demonstrates a near‐room‐temperature metal–insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed‐valent iron oxide with an unconventional Fe₅O₆ stoichiometry. Near 275 K, Fe₅O₆ undergoes a Verwey‐type charge‐ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe?Fe chemical bonds. This unique feature highlights Fe₅O₆ as a promising candidate for the use in innovative applications. We established that the minimal Fe?Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge‐ordering phenomena in transition‐metal oxides in general.     

Diverse Ligand‐Functionalized Mixed‐Valent Hexamanganese Sandwiched Silicotungstates with Single‐Molecule Magnet Behavior

Under hydrothermal conditions, replacement of the water molecules in the [Mn^III₄Mn^II₂O₄(H₂O)₄]⁸⁺ cluster of mixed‐valent Mn₆ sandwiched silicotungstate [(B‐α‐SiW₉O₃₄)₂Mn^III₄Mn^II₂O₄(H₂O)₄]^12? ( 1 a ) with organic N ligands led to the isolation of five organic–inorganic hybrid, Mn₆‐substituted polyoxometalates (POMs) 2 – 6 . They were all structurally characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis, diffuse‐reflectance spectroscopy, and powder and single‐crystal X‐ray diffraction. Compounds 2 – 6 represent the first series of mixed‐valent {Mn^III₄Mn^II₂O₄(H₂O)_4?n(L)_n} sandwiched POMs covalently functionalized by organic ligands. The preparation of 1 – 6 not only indicates that the double‐cubane {Mn^III₄Mn^II₂O₄(H₂O)_4?n(L)_n} clusters are very stable fragments in both conventional aqueous solution and hydrothermal systems and that organic functionalization of the [Mn^III₄Mn^II₂O₄(H₂O)₄]⁸⁺ cluster by substitution reactions is feasible, but also demonstrates that hydrothermal environments can promote and facilitate the occurrence of this substitution reaction. This work confirms that hydrothermal synthesis is effective for making novel mixed‐valent POMs substituted with transition‐metal (TM) clusters by combining lacunary Keggin precursors with TM cations and tunable organic ligands. Furthermore, magnetic measurements reveal that 3 and 6 exhibit single‐molecule magnet behavior.     

A Low‐Valent Iron Imido Heterocubane Cluster: Reversible Electron Transfer and Catalysis of Selective C–C Couplings

Enzymes and cofactors with iron–sulfur heterocubane core structures, [Fe₄S₄], are often found in nature as electron transfer reagents in fundamental catalytic transformations. An artificial heterocubane with a [Fe₄N₄] core is reported that can reversibly store up to four electrons at very negative potentials. The neutral [Fe₄N₄] and the singly reduced low‐valent [Fe₄N₄]^? heterocubanes were isolated and fully characterized. The low‐valent species bears one unpaired electron, which is localized predominantly at one iron center in the electronic ground state but fluctuates with increasing temperatures. The electrons stored or released by the [Fe₄N₄]/[Fe₄N₄]^? redox couple can be used in reductive or oxidative C? C couplings and even allow catalytic one‐pot reactions, which show a remarkably enhanced selectivity in the presence of the [Fe₄N₄] heterocubanes.     

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.     

Amine‐Templated One‐Dimensional Metal Sulfates Including a Mixed‐Valent Fe Compound with a Half‐Kagome Structure

Organically templated metal sulfates are relatively new. Six amine‐templated transition‐metal sulfates with different types of chain structures, including a novel iron sulfate with a chain structure corresponding to one half of the kagome structure, were synthesized by hydro/solvothermal methods. Amongst the one‐dimensional metal sulfates, [C₁₀N₂H₁₀][Zn(SO₄)Cl₂] ( 1 ) is the simplest, being formed by corner‐linked ZnO₂Cl₂ and SO₄ tetrahedra. [C₆N₂H₁₈][Mn(SO₄)₂(H₂O)₂] ( 2 ) and [C₂N₂H₁₀][Ni(SO₄)₂(H₂O)₂] ( 3 ) have ladder structures comprising four‐membered rings formed by SO₄ tetrahedra and metal–oxygen octahedra, just as in the mineral kröhnkite. [C₄N₂H₁₂][V^III(OH)(SO₄)₂]?H₂O ( 4 ) and [C₄N₂H₁₂][VF₃(SO₄)] ( 5 ) exhibit chain topologies of the minerals tancoite and butlerite, respectively. The structure of [C₄N₂H₁₂][H₃O][Fe^IIIFe^II F₆(SO₄)] ( 6 ) is noteworthy in that it corresponds to half of the hexagonal kagome structure. It exhibits ferrimagnetic properties at low temperatures and the absence of frustration, unlike the mixed‐valent iron sulfate with the full kagome structure.     

Electrochemical and Spectroscopic Behaviour of Bis(2‐mercaptopyridine‐N‐oxide)oxovanadium(IV)

The complex [VO(MPO)₂] (MPO = deprotonated 2‐mercaptopyridine N‐oxide) was synthesized and characterized by IR spectroscopy. Its electrochemical behaviour was investigated by cyclic voltammetry in different organic solvents. The V^IV/V^V and V^IV/V^III couples could be identified. The nature of the electroactive species is strongly dependent on the solvent. The results are discussed in terms of a reaction mechanism describing the characteristics of the electron transfer processes and the involved chemical reactions, and the stability of the complex in each solvent was also determined. The electronic spectra of the investigated solutions gave additional support to the proposed mechanisms.