Nonredox Metal Ions Promoted Olefin Epoxidation by Iron(II) Complexes with H2O2: DFT Calculations Reveal Multiple Channels for Oxygen Transfer

Permanganate activation with Mn oxides at different oxidation states: Insight into the surface-promoted electron transfer mechanism

The development of new strategies to improve the removal of organic pollutants with permanganate (KMnO₄) is a hot topic in water treatment. While Mn oxides have been extensively used in Advanced Oxidation Processes through an electron transfer mechanism, the field of KMnO₄ activation remains relatively unexplored. Interestingly, this study has discovered that Mn oxides with high oxidation states including γ-MnOOH, α-Mn₂O₃ and α-MnO₂, exhibited excellent performance to degrade phenols and antibiotics in the presence of KMnO₄. The MnO₄^- species initially formed stable complexes with the surface Mn(III/IV) species and showed an increased oxidation potential and electron transfer reactivity, caused by the electron-withdrawing capacity of the Mn species acting as Lewis acids. Conversely, for MnO and γ-Mn₃O₄ with Mn(II) species, they reacted with KMnO₄ to produce cMnO₂ with very low activity for phenol degradation. The direct electron transfer mechanism in α-MnO₂/KMnO₄ system was further confirmed through the inhibiting effect of acetonitrile and the galvanic oxidation process. Moreover, the adaptability and reusability of α-MnO₂ in complicated waters indicated its potential for application in water treatment. Overall, the findings shed light on the development of Mn-based catalysts for organic pollutants degradation via KMnO₄ activation and understanding of the surface-promoted mechanism.

Brønsted Acids Promote Olefin Oxidations by Bioinspired Nonheme CoIII(PhIO)(OH) Complexes: A Role for Low-Barrier Hydrogen Bonds

Introduction of Brønsted acids into biomimetic nonheme reactions promotes the oxidative ability of metal-oxygen complexes significantly. However, the molecular machinery of the promoted effects is missing. Herein, a comprehensive investigation of styrene oxidation by a cobalt(III)-iodosylbenzene complex, [(TQA)Co^III(OIPh)(OH)]²⁺ (1, TQA = tris(2-quinolylmethyl)amine), in the presence and absence of triflic acid (HOTf) was performed using density functional theory calculations. Results revealed for the first time that there is a low-barrier hydrogen bond (LBHB) between HOTf and the hydroxyl ligand of 1, which forms two valence-resonance structures [(TQA)Co^III(OIPh)(HO^---HOTf)]²⁺ (1_LBHB) and [(TQA)Co^III(OIPh)(H₂O--OTf^-)]²⁺ (1'_LBHB). Due to the oxo-wall, these complexes (1_LBHB and 1'_LBHB) cannot convert to high-valent cobalt-oxyl species. Instead, styrene oxidation by these oxidants (1_LBHB and 1'_LBHB) shows novel spin-state selectivity, i.e., on the ground closed-shell singlet state, styrene is oxidized to an epoxide, whereas on the excited triplet and quintet states, an aldehyde product, phenylacetaldehyde, is formed. The preferred pathway is styrene oxidation by 1'_LBHB, which is initiated by a rate-limiting bond-formation-coupled electron transfer process with an energy barrier of 12.2 kcal mol^-1. The nascent PhIO-styrene-radical-cation intermediate undergoes an intramolecular rearrangement to produce an aldehyde. The halogen bond between the OH^-/H₂O ligand and the iodine of PhIO modulates the activity of the cobalt-iodosylarene complexes 1_LBHB and 1'_LBHB. These new mechanistic findings enrich our knowledge of nonheme chemistry and hypervalent iodine chemistry and will play a positive role in the rational design of new catalysts.

A Contrasting Effect of Acid in Electron Transfer, Oxygen Atom Transfer, and Hydrogen Atom Transfer Reactions of a Nickel(III) Complex

There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [Ni^III(PaPy₃*)]²⁺ (1) in acetone/CH₃CN (v/v 19:1). 1 was synthesized by reacting [Ni^II(PaPy₃*)]⁺ (2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H₂O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H⁺ to the PaPy₃* ligand of 2; the one-electron reduction potential (E_red) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the E_red value resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA^•+ to 2 due to the binding of H⁺ to the PaPy₃* ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions.

Lewis acid improved dioxygen activation by a non-heme iron(II) complex towards tryptophan 2,3-dioxygenase activity for olefin oxygenation

Dioxygen activation and catalysis around ambient temperature is a long-standing challenge in chemistry. Inspired by the significant roles of the hydrogen bond network in dioxygen activation and catalysis by redox enzymes, this work presents a Lewis acid improved dioxygen activation by an Fe^II(BPMEN)(OTf)₂ complex towards tryptophan 2,3-dioxygenase (TDO) activity for 3-methylindole and common olefinic CC  bond oxygenation and cleavage (enzymatic Brønsted acid vs. chemical Lewis acid). It was found that the presence of a Lewis acid such as Sc³⁺ could substantially improve olefinic CC  bond oxygenation and cleavage activity through Fe^II(BPMEN)(OTf)₂ catalyzed dioxygen activation. Notably, a more negative ρ value in the Hammett plot of para-substituted styrene oxygenations was observed in the presence of a stronger Lewis acid, disclosing the enhanced electrophilic oxygenation capability of the putative iron(III) superoxo species through its electrostatic interaction with a stronger Lewis acid. Thereof, this work has demonstrated a new strategy in catalyst design for dioxygen activation and catalysis for olefin oxygenation, a significant process in the chemical industry.

Incorporation of Cation Affects the Redox Reactivity of Fe-NNN Complexes on C-H Oxidation

Cations such as Lewis acids have been shown to enhance the catalytic activity of high-valent Fe-oxygen intermediates. Herein, we present a pyridine diamine ethylene glycol macrocycle, which can form Zn(II)- or Fe(III)-complex with the NNN site, while allowing redox-inactive cations to bind to the ethylene glycol moiety. The addition of alkali, alkali earth, and lanthanum ions resulted in positive shifts to the Fe(III/II) redox potential. Calculation of dissociation constants showed the tightest binding with a Ba²⁺ ion. Density functional theory calculations were used to elucidate the effects of redox inactive cations toward the electronic structures of Fe complexes. Although the Fe-NNN complexes, both in the absence and presence of cations, can catalyze C-H oxidation of 9,10-dihydroanthracene, to give anthracene [hydrogen atom transfer (HAT) product], anthrone, and anthraquinone [oxygen atom transfer (OAT) products], highest overall activity and OAT/HAT product ratios were obtained in the presence of dications, that is, Ba²⁺ and Mg²⁺, respectively.

Acid Catalysis in the Oxidation of Substrates by Mononuclear Manganese(III)-Aqua Complexes

Acids are known to enhance the reactivities of metal-oxygen intermediates, such as metal-oxo, -hydroperoxo, -peroxo, and -superoxo complexes, in biomimetic oxidation reactions. Although metal-aqua (and metal-hydroxo) complexes have been shown to be potent oxidants in oxidation reactions, acid effects on the reactivities of metal-aqua complexes have never been investigated previously. In this study, a mononuclear manganese(III)-aqua complex, [(dpaq^5NO2)Mn^III(OH₂)]²⁺ (1; dpaq^5NO2 = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamidate with an NO₂ substituent at the 5 position), which is relatively stable in the presence of triflic acid (HOTf), is used in the investigation of acid-catalyzed oxidation reactions by metal-aqua complexes. As a result, we report a remarkable acid catalysis in the six-electron oxidation of anthracene by 1 in the presence of HOTf; anthraquinone is formed as the product. In the HOTf-catalyzed six-electron oxidation of anthracene by 1, the rate constant increases linearly with an increase of the HOTf concentration. Combined with the observed one-electron oxidation product, anthracene (derivative) radical cation, and the substitution effect at the 5 position of the dpaq ligand in 1 on the rate constants of the oxidation of anthracene, it is concluded that the oxidation of anthracene occurs via an acid-promoted electron transfer (APET) from anthracene to 1. The dependence of the rate constants of the APET from electron donors, including anthracene derivatives, to 1 on the driving force of electron transfer is also shown to be well fitted by the Marcus equation of outer-sphere electron transfer. To the best of our knowledge, this is the first example showing acid catalysis in the oxidation of substrates by metal(III)-aqua complexes.

Deeper Understanding of Mononuclear Manganese(IV)-Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)-Hydroxide Complex

Binding of Lewis acidic metal ions and Brønsted acid at the metal-oxo group of high-valent metal-oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal-oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)-oxo complex binding triflic acid, {[(dpaq)Mn^IV(O)]-HOTf}⁺ (1-HOTf). First, 1-HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn-O stretching frequencies of 1-HOTf (e.g., ν_Mn-O at ∼793 cm^-1) and 1-Mⁿ⁺ (Mⁿ⁺ = Ca²⁺, Zn²⁺, Lu³⁺, Al³⁺, or Sc³⁺) and the Lewis acidities of H⁺ and Mⁿ⁺ ions, suggesting that H⁺ and Mⁿ⁺ bind at the metal-oxo moiety of [(dpaq)Mn^IV(O)]⁺. Interestingly, a single-crystal structure of 1-HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)-oxo complex but a Mn(IV)-hydroxide complex, [(dpaq)Mn^IV(OH)](OTf)₂ (4), with a Mn-O bond distance of 1.8043(19) Å and a Mn-O stretch at 660 cm^-1. More interestingly, 4 reverted to 1-HOTf upon dissolution, demonstrating that 1-HOTf and 4 are interconvertible depending on the physical states, such as 1-HOTf in solution and 4 in isolated solid. The reactivity of 1-HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1-Mⁿ⁺ complexes; an interesting correlation between the Mn-O stretching frequencies of 1-HOTf and 1-Mⁿ⁺ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

Nanocrystals perovskites photocatalyzed singlet oxygen generation for light-driven organic reactions

Lead halide perovskite nanocrystals were prepared and used as photocatalysts for the in situ ¹O₂ generation to perform hetero Diels-Alder, ene and oxidation reactions with suitable dienes and alkenes. The methodology has been reasonably standardized and made applicable to a variety of olefinic substrates. The scope of the method is finely illustrated by the results in all the tested reactions, which allowed to obtain desymmetrized hydroxy-ketone derivatives, unsaturated ketones and epoxides. Some limitations were also observed especially in the case of the alkene oxidations as well as poor chemoselectivity was somewhere observed. ¹O₂ generated by lead halide perovskite nanocrystals as photocatalyst in organic reactions.

A Density Functional Theory Study on Comparing the Reactivity of [Mn(13-TMC)(OOH)]2+ and [Mn(13-TMC)(O2)]+ for the Sulfoxidation of Thioanisole: Elucidation of Substrate and Non-Redox Metal Ion Effects

The reactivities of [Mn(13-TMC)(OOH)]²⁺ (1) and [Mn(13-TMC)(O₂)]⁺ (2) in the sulfoxidation of thioanisole have been compared using density functional theory methods. The orientation of the 13-TMC ligand and substrate and non-redox metal ion effects have been considered to improve the oxidation efficiency of 1 and 2. In 1, the syn- and anti-orientation of the 13-TMC ligand do not change the coordination of the Mn ion. In contrast, the orientation of the 13-TMC ligand regulates the geometry of 2, wherein the syn-13-TMC ligand exhibits the Mn^III-peroxo (2_hs and 2_ls) species, while the anti-13-TMC shows the Mn^II-superoxo (2'_hs and 2'_ls) species. However, the Mn^II-superoxo species are found to be less stable than the Mn^III-peroxo complexes by around +26.6 kcal/mol. The ground state geometries of 1 and 2 with the syn-13-TMC ligand are found to be more stable in the high- (S = 2) spin states (1_hs and 2_hs) than the low- (S = 1) spin complexes (1_ls and 2_ls), by +15.6 and +25.5 kcal/mol, respectively. The computed mechanistic pathways clearly indicate that the sulfoxidation of thioanisole by 1_hs is kinetically (by +16.6 to +46.1 kcal/mol) and thermodynamically (+14.4 to +56.1 kcal/mol) more preferred than 1_ls, 2_hs, and 2_ls species. This is mainly due to the feasible heterolytic O1-O2 bond cleavage followed by the proton transfer step. In addition, the molecular electrostatic potential analysis indicates that the higher oxidation efficacy of 1_hs than 2_hs is due to the -OOH moiety. The reactivity of 1_hs is further enhanced by incorporating electron donating substituents in thioanisole, wherein the p-NH₂ thioanisole decreases the ΔG^‡ of 1_hs by 28%. Interestingly, the incorporation of non-redox metal ions (Mⁿ⁺ = Sc³⁺, Y³⁺, Mg²⁺, and Zn²⁺) improves the reactivity of 2_hs, wherein the non-redox metal ions tend to bind with the oxygen atoms of 2_hs and subsequently shift the one-electron reduction potential (E⁰_(red) vs SCE) toward the positive side. The positive shift in the E⁰_(red) is more evident in 2_hs-Y³⁺ that significantly decreases the ΔG^‡ of 2_hs by 58.7%, which is in fact lower than the ΔG^‡ of 1_hs by +2.0 kcal/mol. Hence, in the presence of Y³⁺, the reactivity of 2_hs is comparable with 1_hs in the sulfoxidation of thioanisole.

Enhancing Chemo- and Stereoselectivity in C-H Bond Oxygenation with H2O2 by Nonheme High-Spin Iron Catalysts: The Role of Lewis Acid and Multimetal Centers

Spin states of iron often direct the selectivity in oxidation catalysis by iron complexes using hydrogen peroxide (H₂O₂) on an oxidant. While low-spin iron(III) hydroperoxides display stereoselective C-H bond hydroxylation, the reactions are nonstereoselective with high-spin iron(II) catalysts. The catalytic studies with a series of high-spin iron(II) complexes of N4 ligands with H₂O₂ and Sc³⁺ reported here reveal that the Lewis acid promotes catalytic C-H bond hydroxylation with high chemo- and stereoselectivity. This reactivity pattern is observed with iron(II) complexes containing two cis-labile sites. The enhanced selectivity for C-H bond hydroxylation catalyzed by the high-spin iron(II) complexes in the presence of Sc³⁺ parallels that of the low-spin iron catalysts. Furthermore, the introduction of multimetal centers enhances the activity and selectivity of the iron catalyst. The study provides insights into the development of peroxide-dependent bioinspired catalysts for the selective oxygenation of C-H bonds without the restriction of using iron complexes of strong-field ligands.

Degradation pathways of a highly active iron(iii) tetra-NHC epoxidation catalyst

Elucidation of different decomposition pathways of a highly active tetradentate iron–NHC epoxidation catalyst reveals direct carbene oxidation to be the decisive cause of catalyst degradation.

g-C3N4/metal halide perovskite composites as photocatalysts for singlet oxygen generation processes for the preparation of various oxidized synthons

Singlet oxygen generated by g-C₃N₄/metal halide perovskite composites as photocatalysts in organic oxidation reactions.

Enhanced Redox Reactivity of a Nonheme Iron(V)-Oxo Complex Binding Proton

Acid effects on the chemical properties of metal-oxygen intermediates have attracted much attention recently, such as the enhanced reactivity of high-valent metal(IV)-oxo species by binding proton(s) or Lewis acidic metal ion(s) in redox reactions. Herein, we report for the first time the proton effects of an iron(V)-oxo complex bearing a negatively charged tetraamido macrocyclic ligand (TAML) in oxygen atom transfer (OAT) and electron-transfer (ET) reactions. First, we synthesized and characterized a mononuclear nonheme Fe(V)-oxo TAML complex (1) and its protonated iron(V)-oxo complexes binding two and three protons, which are denoted as 2 and 3, respectively. The protons were found to bind to the TAML ligand of the Fe(V)-oxo species based on spectroscopic characterization, such as resonance Raman, extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR) measurements, along with density functional theory (DFT) calculations. The two-protons binding constant of 1 to produce 2 and the third protonation constant of 2 to produce 3 were determined to be 8.0(7) × 10⁸ M^-2 and 10(1) M^-1, respectively. The reactivities of the proton-bound iron(V)-oxo complexes were investigated in OAT and ET reactions, showing a dramatic increase in the rate of sulfoxidation of thioanisole derivatives, such as 10⁷ times increase in reactivity when the oxidation of p-CN-thioanisole by 1 was performed in the presence of HOTf (i.e., 200 mM). The one-electron reduction potential of 2 (E_red vs SCE = 0.97 V) was significantly shifted to the positive direction, compared to that of 1 (E_red vs SCE = 0.33 V). Upon further addition of a proton to a solution of 2, a more positive shift of the E_red value was observed with a slope of 47 mV/log([HOTf]). The sulfoxidation of thioanisole derivatives by 2 was shown to proceed via ET from thioanisoles to 2 or direct OAT from 2 to thioanisoles, depending on the ET driving force.

The effects of nonredox metal ions on the activation of peroxymonosulfate for organic pollutants degradation in aqueous solution with cobalt based catalysts: A new mechanism investigation

Herein, a new peroxymonosulfate (PMS) activation system was proposed employing nonredox metal ions as Lewis acids (LA), which have been widely recognized to play important roles in many biological and chemical oxidations. With Co²⁺ ions as model catalysts, it was found that oxidizing power of PMS was enhanced after binding weak LA such as Ca²⁺ ions, leading to its easier reduction to active radicals and substantial enhancement of dye degradation. The promoting effect of Ca²⁺ was also observed with other cobalt catalysts including CoFe₂O₄ and Co₃O₄. The rate of PMS decomposition in Co²⁺+LA/PMS system was correlated with Lewis acidity; while in the presence of strong LA including La³⁺ and Y³⁺, the dye degradation rate declined. The interactions of LA with PMS were characterized and the detailed mechanism was proposed. The present study provides the first example of the promoting effect of weak LA on PMS activation with cobalt based catalysts.

Pushing the limits of activity and stability: the effects of Lewis acids on non-heme iron–NHC epoxidation catalysts

Tetradentate iron–NHC complexes exhibit unprecedented activity (TOF: 410 000 h⁻¹) in the epoxidation of cis-cyclooctene by addition of Lewis acids.

Lewis acids promoted organic pollutants degradation in aqueous solution with peroxymonosulfate and MnO2: New insights into the activation mechanism

Nonredox metal ions have been widely recognized to be important in a wide range of biological and chemical oxidations as Lewis acids (LA). However, the role of LA in peroxymonosulfate (PMS) activation for wastewater treatment has not been considered until now. This study shows that oxidizing power of PMS can be promoted after binding nonredox metal ions such as Ca²⁺ as LA, leading to the easier reduction of the oxidant to radicals and substantial enhancement of dye degradation by employing manganese oxides OMS-2 as model catalysts. Increased with Lewis acidity of the metal ion, the rate of PMS decomposition enhanced linearly, while the dye degradation rate first increased and then declined due to the formation of a larger amount of dioxygen. The interactions between Ca²⁺ and PMS were further investigated by Raman, cyclic voltammetry and XPS; and the detailed mechanism of PMS activation was proposed. The performance of Ca²⁺+OMS-2/PMS system under different conditions was also studied. The findings indicate the importance of LA in PMS activation reaction and their role must be considered in other transition metal oxides/PMS systems. It will be also helpful to design new and highly active catalysts for the reactions.

Proton-promoted disproportionation of iron(v)-imido TAML to iron(v)-imido TAML cation radical and iron(iv) TAML

An iron(v)-imido TAML complex is disproportionated to give an iron(v)-imido TAML cation radical and an iron(iv) TAML upon addition of acids.

Lewis versus Brønsted Acid Activation of a Mn(IV) Catalyst for Alkene Oxidation

Lewis acid (LA) activation by coordination to metal oxido species has emerged as a new strategy in catalytic oxidations. Despite the many reports of enhancement of performance in oxidation catalysis, direct evidence for LA-catalyst interactions under catalytically relevant conditions is lacking. Here, we show, using the oxidation of alkenes with H₂O₂ and the catalyst [Mn₂(μ-O)₃(tmtacn)₂](PF₆)₂ (1), that Lewis acids commonly used to enhance catalytic activity, e.g., Sc(OTf)₃, in fact undergo hydrolysis with adventitious water to release a strong Brønsted acid. The formation of Brønsted acids in situ is demonstrated using a combination of resonance Raman, UV/vis absorption spectroscopy, cyclic voltammetry, isotope labeling, and DFT calculations. The involvement of Brønsted acids in LA enhanced systems shown here holds implications for the conclusions reached in regard to the relevance of direct LA-metal oxido interactions under catalytic conditions.

Lewis acid activation of oxidation catalysts is proposed to be through binding of the Lewis acids to metal-oxo species. The activity of the catalyst [Mn₂(μ-O)₃(tmtacn)₂](PF₆)₂ in the oxidation of alkenes with H₂O₂ appears to correlate with the strength of the Lewis acid used for its activation. We show that the correlation arises from the relative propensity of the Lewis acids to generate Brønsted acids in situ.

Organic Solvent-Free Olefins and Alcohols (ep)oxidation Using Recoverable Catalysts Based on [PM12O40]3- (M = Mo or W) Ionically Grafted on Amino Functionalized Silica Nanobeads

Catalyzed organic solvent-free (ep)oxidation were achieved using H3PM12O40 (M = Mo or W) complexes ionically grafted on APTES-functionalized nano-silica beads obtained from straightforward method (APTES = aminopropyltriethoxysilane). Those catalysts have been extensively analyzed through morphological studies (Dynamic Light Scattering (DLS), TEM) and several spectroscopic qualitative (IR, multinuclear solid-state NMR) and quantitative (1H and 31P solution NMR) methods. Interesting catalytic results were obtained for the epoxidation of cyclooctene, cyclohexene, limonene and oxidation of cyclohexanol with a lower [POM]/olefin ratio. The catalysts were found to be recyclable and reused during three runs with similar catalytic performances.

Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer Reactivity of Heterometallic Mn3MO4 Cubane and Fe3MO(OH) and Mn3MO(OH) Clusters

The modulation of the reactivity of metal oxo species by redox inactive metals has attracted much interest due to the observation of redox inactive metal effects on processes involving electron transfer both in nature (the oxygen-evolving complex of Photosystem II) and in heterogeneous catalysis (mixed-metal oxides). Studies of small-molecule models of these systems have revealed numerous instances of effects of redox inactive metals on electron- and group-transfer reactivity. However, the heterometallic species directly involved in these transformations have rarely been structurally characterized and are often generated in situ. We have previously reported the preparation and structural characterization of multiple series of heterometallic clusters based on Mn₃ and Fe₃ cores and described the effects of Lewis acidity of the heterometal incorporated in these complexes on cluster reduction potential. To determine the effects of Lewis acidity of redox inactive metals on group transfer reactivity in structurally well-defined complexes, we studied [Mn₃MO₄], [Mn₃MO(OH)], and [Fe₃MO(OH)] clusters in oxygen atom transfer (OAT) reactions with phosphine substrates. The qualitative rate of OAT correlates with the Lewis acidity of the redox inactive metal, confirming that Lewis acidic metal centers can affect the chemical reactivity of metal oxo species by modulating cluster electronics.

Electron Paramagnetic Resonance Signature of Tetragonal Low Spin Iron(V)-Nitrido and -Oxo Complexes Derived from the Electronic Structure Analysis of Heme and Non-Heme Archetypes

Iron(V)-nitrido and -oxo complexes have been proposed as key intermediates in a diverse array of chemical transformations. Herein we present a detailed electronic-structure analysis of [Fe^V(N)(TPP)] (1, TPP^2– = tetraphenylporphyrinato), and [Fe^V(N)(cyclam-ac)]⁺ (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and ⁵⁷Fe Mössbauer spectroscopy coupled with wave function based complete active-space self-consistent field (CASSCF) calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and -oxo complexes, [Fe^V(N)(MePy₂tacn)](PF₆)₂ (3, MePy₂tacn = methyl-N′,N″-bis(2-picolyl)-1,4,7-triazacyclononane), [Fe^V(N){PhB(t-BuIm)₃}]⁺ (4, PhB(^tBuIm)₃^– = phenyltris(3-tert-butylimidazol-2-ylidene)borate), and [Fe^V(O)(TAML)]⁻ (5, TAML^4– = tetraamido macrocyclic ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener, compound I, which contains a ferryl unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and -oxo complexes 1–3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial EPR spectra with g_|| < g_⊥ ≤ 2 were measured for them, and their g_|| and g_⊥ values were found to obey a simple relation of g_⊥² + (2 – g_∥)² = 4. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by its distinct EPR spectrum with g_|| < 2 < g_⊥. Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and -oxo complexes feature electronic structures akin to those found for complexes 1–3 and 5. Therefore, the characteristic EPR signals determined for 1–3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.

Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions

Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)Mn^IV(O)]⁺-M ⁿ⁺ (1-Mⁿ⁺, M ⁿ⁺ = Ca²⁺, Mg²⁺, Zn²⁺, Lu³⁺, Y³⁺, Al³⁺, and Sc³⁺) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)Mn^III(OH)]⁺, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M ⁿ⁺). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M ⁿ⁺ on the reactivity of 1-M ⁿ⁺ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-M ⁿ⁺, such as 1-Sc³⁺ ≈ 1-Al³⁺ > 1-Y³⁺ > 1-Lu³⁺ > 1-Zn²⁺ > 1-Mg²⁺ > 1-Ca²⁺, follows the Lewis acidity of M ⁿ⁺ bound to the Mn-O moiety; that is, the stronger the Lewis acidity of M ⁿ⁺, the higher the reactivity of 1-M ⁿ⁺ becomes. In sharp contrast, the reactivity of 1-M ⁿ⁺ in the HAT reaction was reversed, giving the reactivity order 1-Ca²⁺ > 1-Mg²⁺ > 1-Zn²⁺ > 1-Lu³⁺> 1-Y³⁺> 1-Al³⁺ ≈ 1-Sc³⁺; that is, the higher is Lewis acidity of M ⁿ⁺, the lower the reactivity of 1-M ⁿ⁺ in the HAT reaction. The latter result implies that the Lewis acidity of M ⁿ⁺ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-M ⁿ⁺; cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C-H bond activation reactions.

Sc3+ (or HClO4) Activation of a Nonheme FeIII-OOH Intermediate for the Rapid Hydroxylation of Cyclohexane and Benzene

[Fe(β-BPMCN)(CH₃CN)₂]²⁺ (1, BPMCN = N,N' -bis(pyridyl-2-methyl)- N,N' -dimethyl- trans-1,2-diaminocyclo-hexane) is a relatively poor catalyst for cyclohexane oxidation by H₂O₂ and cannot perform benzene hydroxylation. However, addition of Sc³⁺ activates the 1/H₂O₂ reaction mixture to be able to hydroxylate cyclohexane and benzene within seconds at -40 °C. A metastable S = 1/2 Fe^III-(η¹-OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon addition of Sc³⁺ at rates independent of [substrate] but linearly dependent on [Sc³⁺]. HClO₄ elicits comparable reactivity as Sc³⁺ at the same concentration. We thus postulate that these additives both facilitate O-O bond heterolysis of 2 to form a common highly electrophilic Fe^V═O oxidant that is comparably reactive to the fastest nonheme high-valent iron-oxo oxidants found to date.