Mononuclear Manganese–Peroxo and Bis(μ‐oxo)dimanganese Complexes Bearing a Common N‐Methylated Macrocyclic Ligand

Mononuclear Mn^III–peroxo and dinuclear bis(μ‐oxo)Mn^III₂ complexes that bear a common macrocyclic ligand were synthesized by controlling the concentration of the starting Mn^II complex in the reaction of H₂O₂ (i.e., a Mn^III–peroxo complex at a low concentration (≤1 mM ) and a bis(μ‐oxo)Mn^III₂ complex at a high concentration (≥30 mM )). These intermediates were successfully characterized by various physicochemical methods such as UV–visible spectroscopy, ESI‐MS, resonance Raman, and X‐ray analysis. The structural and spectroscopic characterization combined with density functional theory (DFT) calculations demonstrated unambiguously that the peroxo ligand is bound in a side‐on fashion in the Mn^III–peroxo complex and the Mn₂O₂ diamond core is in the bis(μ‐oxo)Mn^III₂ complex. The reactivity of these intermediates was investigated in electrophilic and nucleophilic reactions, in which only the Mn^III–peroxo complex showed a nucleophilic reactivity in the deformylation of aldehydes.     

Hydrogen‐Atom Abstraction Reactions by Manganese(V)– and Manganese(IV)–Oxo Porphyrin Complexes in Aqueous Solution

High‐valent manganese(IV or V)–oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated Mn^V– and Mn^IV–oxo porphyrins in basic aqueous solution and investigated their reactivities in C? H bond activation of hydrocarbons. We now report that Mn^V– and Mn^IV–oxo porphyrins are capable of activating C? H bonds of alkylaromatics, with the reactivity order of Mn^V–oxo>Mn^IV–oxo; the reactivity of a Mn^V–oxo complex is 150 times greater than that of a Mn^IV–oxo complex in the oxidation of xanthene. The C? H bond activation of alkylaromatics by the Mn^V– and Mn^IV–oxo porphyrins is proposed to occur through a hydrogen‐atom abstraction, based on the observations of a good linear correlation between the reaction rates and the C? H bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of Mn^IV–oxo porphyrins to Mn^V–oxo and Mn^III porphyrins is not a feasible pathway in basic aqueous solution and that Mn^IV–oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The C? H bond activation of alkylaromatics by Mn^V– and Mn^IV–oxo species proceeds through a one‐electron process, in which a Mn^IV–‐oxo porphyrin is formed as a product in the C? H bond activation by a Mn^V–oxo porphyrin, followed by a further reaction of the Mn^IV–oxo porphyrin with substrates that results in the formation of a Mn^III porphyrin complex. This result is in contrast to the oxidation of sulfides by the Mn^V–oxo porphyrin, in which the oxidation of thioanisole by the Mn^V–oxo complex produces the starting Mn^III porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the Mn^V–oxo species occurs by means of a two‐electron oxidation process. In contrast, a Mn^IV–oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.     

Influence of Ligand Architecture on Oxidation Reactions by High‐Valent Nonheme Manganese Oxo Complexes Using Water as a Source of Oxygen

Mononuclear nonheme Mn^IV?O complexes with two isomers of a bispidine ligand have been synthesized and characterized by various spectroscopies and density functional theory (DFT). The Mn^IV?O complexes show reactivity in oxidation reactions (hydrogen‐atom abstraction and sulfoxidation). Interestingly, one of the isomers (L¹) is significantly more reactive than the other (L²), while in the corresponding Fe^IV?O based oxidation reactions the L²‐based system was previously found to be more reactive than the L¹‐based catalyst. This inversion of reactivities is discussed on the basis of DFT and molecular mechanics (MM) model calculations, which indicate that the order of reactivities are primarily due to a switch of reaction channels (σ versus π) and concomitant steric effects.     

Demonstration of the Heterolytic OO Bond Cleavage of Putative Nonheme Iron(II)OOH(R) Complexes for Fenton and Enzymatic Reactions

One‐electron reduction of mononuclear nonheme iron(III) hydroperoxo (Fe^III? OOH) and iron(III) alkylperoxo (Fe^III? OOR) complexes by ferrocene (Fc) derivatives resulted in the formation of the corresponding iron(IV) oxo complexes. The conversion rates were dependent on the concentration and oxidation potentials of the electron donors, thus indicating that the reduction of the iron(III) (hydro/alkyl)peroxo complexes to their one‐electron reduced iron(II) (hydro/alkyl)peroxo species is the rate‐determining step, followed by the heterolytic O? O bond cleavage of the putative iron(II) (hydro/alkyl)peroxo species to give the iron(IV) oxo complexes. Product analysis supported the heterolytic O? O bond‐cleavage mechanism. The present results provide the first example showing the one‐electron reduction of iron(III) (hydro/alkyl)peroxo complexes and the heterolytic O? O bond cleavage of iron(II) (hydro/alkyl)peroxo species to form iron(IV) oxo intermediates which occur in nonheme iron enzymatic and Fenton reactions.     

Iridium(III)‐Catalyzed Intermolecular C(sp3)−H Insertion Reaction of Quinoid Carbene: A Radical Mechanism

Described herein is an Ir^III/porphyrin‐catalyzed intermolecular C(sp³)?H insertion reaction of a quinoid carbene (QC). The reaction was designed by harnessing the hydrogen‐atom transfer (HAT) reactivity of a metal‐QC species with aliphatic substrates followed by a radical rebound process to afford C?H arylation products. This methodology is efficient for the arylation of activated hydrocarbons such as 1,4‐cyclohexadienes (down to 40 min reaction time, up to 99 % yield, up to 1.0 g scale). It features unique regioselectivity, which is mainly governed by steric effects, as the insertion into primary C?H bonds is favored over secondary and/or tertiary C?H bonds in the substituted cyclohexene substrates. Mechanistic studies revealed a radical mechanism for the reaction.     

Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)‐Oxo Complexes

Manganese(IV)‐oxo complexes are often invoked as intermediates in Mn‐catalyzed C−H bond activation reactions. While many synthetic Mn^IV‐oxo species are mild oxidants, other members of this class can attack strong C−H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of Mn^IV‐oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the Mn^IV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K‐edge X‐ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen‐atom and oxygen‐atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these Mn^IV‐oxo complexes, the rate enhancements are correlated with both 1) the energy of a low‐lying ⁴E excited state, which has been postulated to be involved in a two‐state reactivity model, and 2) the Mn^III/IV reduction potentials.     

Synthesis,Characterization, and Reactivity of Cobalt(III)–Oxygen Complexes Bearing a Macrocyclic N‐Tetramethylated Cyclam Ligand

Mononuclear metal–dioxygen species are key intermediates that are frequently observed in the catalytic cycles of dioxygen activation by metalloenzymes and their biomimetic compounds. In this work, a side‐on cobalt(III)–peroxo complex bearing a macrocyclic N‐tetramethylated cyclam (TMC) ligand, [Co^III(15‐TMC)(O₂)]⁺, was synthesized and characterized with various spectroscopic methods. Upon protonation, this cobalt(III)–peroxo complex was cleanly converted into an end‐on cobalt(III)–hydroperoxo complex, [Co^III(15‐TMC)(OOH)]²⁺. The cobalt(III)–hydroperoxo complex was further converted to [Co^III(15‐TMC‐CH₂‐O)]²⁺ by hydroxylation of a methyl group of the 15‐TMC ligand. Kinetic studies and ¹⁸O‐labeling experiments proposed that the aliphatic hydroxylation occurred via a Co^IV–oxo (or Co^III–oxyl) species, which was formed by O? O bond homolysis of the cobalt(III)–hydroperoxo complex. In conclusion, we have shown the synthesis, structural and spectroscopic characterization, and reactivities of mononuclear cobalt complexes with peroxo, hydroperoxo, and oxo ligands.     

Rhodium(III)‐Catalyzed Intramolecular Redox‐Neutral Annulation of Tethered Alkynes: Formal Total Synthesis of (±)‐Goniomitine

A Rh^III‐catalyzed intramolecular redox‐neutral atom‐economic annulation of a tethered alkyne has been developed to efficiently construct 2‐amidealkyl indoles with completely reversed regioselectivity by a C?H activation pathway. Furthermore, using the Rh^III‐catalyzed C?H activation/annulation as a key step, a one‐pot synthesis of pyrido[1,2‐a]indoles has also been developed and applied to a highly efficient formal total synthesis of (±)‐goniomitine.     

Mimicking Elementary Reactions of Manganese Lipoxygenase Using Mn-hydroxo and Mn-alkylperoxo Complexes

Manganese lipoxygenase (MnLOX) is an enzyme that converts polyunsaturated fatty acids to alkyl hydroperoxides. In proposed mechanisms for this enzyme, the transfer of a hydrogen atom from a substrate C-H bond to an active-site Mn^III-hydroxo center initiates substrate oxidation. In some proposed mechanisms, the active-site Mn^III-hydroxo complex is regenerated by the reaction of a Mn^III-alkylperoxo intermediate with water by a ligand substitution reaction. In a recent study, we described a pair of Mn^III-hydroxo and Mn^III-alkylperoxo complexes supported by the same amide-containing pentadentate ligand (^6Medpaq). In this present work, we describe the reaction of the Mn^III-hydroxo unit in C-H and O-H bond oxidation processes, thus mimicking one of the elementary reactions of the MnLOX enzyme. An analysis of kinetic data shows that the Mn^III-hydroxo complex [Mn^III(OH)(^6Medpaq)]⁺ oxidizes TEMPOH (2,2′-6,6′-tetramethylpiperidine-1-ol) faster than the majority of previously reported Mn^III-hydroxo complexes. Using a combination of cyclic voltammetry and electronic structure computations, we demonstrate that the weak Mn^III-N(pyridine) bonds lead to a higher Mn^III/II reduction potential, increasing the driving force for substrate oxidation reactions and accounting for the faster reaction rate. In addition, we demonstrate that the Mn^III-alkylperoxo complex [Mn^III(OO^tBu)(^6Medpaq)]⁺ reacts with water to obtain the corresponding Mn^III-hydroxo species, thus mimicking the ligand substitution step proposed for MnLOX.     

Demonstration of the Heterolytic O?O Bond Cleavage of Putative Nonheme Iron(II)?OOH(R) Complexes for Fenton and Enzymatic Reactions

One‐electron reduction of mononuclear nonheme iron(III) hydroperoxo (Fe^III OOH) and iron(III) alkylperoxo (Fe^III OOR) complexes by ferrocene (Fc) derivatives resulted in the formation of the corresponding iron(IV) oxo complexes. The conversion rates were dependent on the concentration and oxidation potentials of the electron donors, thus indicating that the reduction of the iron(III) (hydro/alkyl)peroxo complexes to their one‐electron reduced iron(II) (hydro/alkyl)peroxo species is the rate‐determining step, followed by the heterolytic O O bond cleavage of the putative iron(II) (hydro/alkyl)peroxo species to give the iron(IV) oxo complexes. Product analysis supported the heterolytic O O bond‐cleavage mechanism. The present results provide the first example showing the one‐electron reduction of iron(III) (hydro/alkyl)peroxo complexes and the heterolytic O O bond cleavage of iron(II) (hydro/alkyl)peroxo species to form iron(IV) oxo intermediates which occur in nonheme iron enzymatic and Fenton reactions.     

Rh‐Catalyzed Direct Amination of Unactivated C(sp3)−H bond with Anthranils Under Mild Conditions

C?N Bond formation is of great significance due to the ubiquity of nitrogen‐containing compounds. Here, a mild and efficient Rh^III‐catalyzed C(sp³)?H aryl amination reaction is reported. Anthranil is employed as the nitrogen source with 100 % atom efficiency. This C?H amination reaction exhibits broad substrate scope without using any external oxidants. Mechanistic studies including rhodacycle intermediates, H–D exchange, kinetic isotope effect (KIE) experiments, and in situ IR are presented.     

Dramatic Influence of an Anionic Donor on the Oxygen‐Atom Transfer Reactivity of a MnV–Oxo Complex

Addition of an anionic donor to an Mn^V(O) porphyrinoid complex causes a dramatic increase in 2‐electron oxygen‐atom‐transfer (OAT) chemistry. The 6‐coordinate [Mn^V(O)(TBP₈Cz)(CN)]^? was generated from addition of Bu₄N⁺CN^? to the 5‐coordinate Mn^V(O) precursor. The cyanide‐ligated complex was characterized for the first time by Mn K‐edge X‐ray absorption spectroscopy (XAS) and gives Mn?O=1.53 Å, Mn?CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN^? complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e^?‐reduced Mn^III(CN)^? complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000‐fold versus the same reaction for the parent 5‐coordinate complex. An Eyring analysis gives ΔH^≠=14 kcal mol^?1, ΔS^≠=?10 cal mol^?1 K^?1. Computational studies fully support the structures, spin states, and relative reactivity of the 5‐ and 6‐coordinate Mn^V(O) complexes.     

Characterization and chemical reactivity of room-temperature-stable MnIII–alkylperoxo complexes

While alkylperoxomanganese(iii) (Mn^III–OOR) intermediates are proposed in the catalytic cycles of several manganese-dependent enzymes, their characterization has proven to be a challenge due to their inherent thermal instability. Fundamental understanding of the structural and electronic properties of these important intermediates is limited to a series of complexes with thiolate-containing N₄S⁻ ligands. These well-characterized complexes are metastable yet unreactive in the direct oxidation of organic substrates. Because the stability and reactivity of Mn^III–OOR complexes are likely to be highly dependent on their local coordination environment, we have generated two new Mn^III–OOR complexes using a new amide-containing N₅⁻ ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H^6Medpaq) ligand, we generated the [Mn^III(OO^tBu)(^6Medpaq)]OTf and [Mn^III(OOCm)(^6Medpaq)]OTf complexes through reaction of their Mn^II or Mn^III precursors with ^tBuOOH and CmOOH, respectively. Both of the new Mn^III–OOR complexes are stable at room-temperature (t_1/2 = 5 and 8 days, respectively, at 298 K in CH₃CN) and capable of reacting directly with phosphine substrates. The stability of these Mn^III–OOR adducts render them amenable for detailed characterization, including by X-ray crystallography for [Mn^III(OOCm)(^6Medpaq)]OTf. Thermal decomposition studies support a decay pathway of the Mn^III–OOR complexes by O–O bond homolysis. In contrast, direct reaction of [Mn^III(OOCm)(^6Medpaq)]⁺ with PPh₃ provided evidence of heterolytic cleavage of the O–O bond. These studies reveal that both the stability and chemical reactivity of Mn^III–OOR complexes can be tuned by the local coordination sphere.

A pair of room-temperature-stable Mn^III–alkylperoxo complexes were characterized and shown to oxidize PPh₃. Thermal decomposition studies provide evidence of both homolysis and heterolysis of the Mn^III–alkylperoxo O–O bond.     

Polynuclear Complexes with Alkoxo and Phenoxo Bridges from In Situ Generated Hydroxy‐Rich Schiff Base Ligands: Syntheses,Structures, and Magnetic Properties

Complexes of new Schiff base ligands generated in situ from the reaction of 1‐aminoglycerol, aldehydes, and metal ions are reported. [Cu₄(HL¹)₄] ( 1 ) and [Ni₄O(HL¹)₃(H₂O)₃)] ? 6 H₂O ? DMF ? DMSO ( 2 ) have M₄O₄ cubane cores, with the L/M molar ratios of 4:4 and 3:4, respectively. [Mn^III₃Mn^IINaOCl₄(HL¹)₃] ? 3 M eCN ( 3 ) has a unique pentanuclear trigonal propeller‐shaped Mn^III₃Mn^IINa core structure, and the coordination assemblies are linked by hydrogen bonds to afford a 3D channel structure. [Cu₂(HL²)₂] ( 4 ) has a bis(μ₂‐alkoxo)‐bridged Cu₂O₂ core, with the binuclear species linked by hydrogen bonds to afford a 1D double‐chain. [Ni₇(OH)₂(OCH₃)₄(H₂L³)₂(MeOH)₂(H₂O)₂]‐ (ClO₄)₂ ? 10 H₂O ( 5 ) has a heptanuclear structure containing heptadentate di‐Schiff base ligands, with the nickel(II) ions bridged by phenoxo, alkoxo, hydroxo, and methoxo groups to afford a very rare face‐sharing hexadruple defective cubane core with a Ni@Ni₆ arrangement. The lattice water molecules are linked by hydrogen bonds to form helical chains, which are further hydrogen‐bonded to the coordination moieties to afford a 2D network. Variable temperature magnetic susceptibility measurements and nonlinear data‐fitting revealed that the “2+4” type of cubane complex 1 shows medium intradimeric ferromagnetic interactions and weak interdimeric ferromagnetic interactions. For complexes 2 and 5 , coexistent ferro‐ and antiferromagnetic couplings afford a non‐zero spin ground state. However, compound 3 shows antiferromagnetic interactions between Mn^III and Mn^II, and ferromagnetic interactions between the Mn^III centers, resulting in a global antiferromagnetic behavior. In conclusion, the reaction of 1‐aminoglycerol with aldehydes and metal salts afforded polynuclear complexes with a rich structural diversity and remarkable magnetic behavior.     

Tuning a Single Ligand System to Stabilize Multiple Spin States of Manganese: A First Example of a Hydrazone‐Based Manganese(III) Spin‐Crossover Complex

A series of bis‐chelate pseudo‐octahedral mononuclear coordination complexes of manganese with the chromophore [MnN₄O₂]ⁿ⁺ (n=0, 1) have been generated in all three principal oxidation states of this transition‐metal center under ambient conditions by utilizing a readily tunable, versatile phenolic pyridylhydrazone ligand system (i.e., H₂(3,5‐R¹,R²)‐L; L=ligand). Strategic combinations of the nature and position of a variety of substituent groups afforded selective, spontaneous stabilization of multiple spin states of the manganese center, which, upon close crystallographic scrutiny, appears to be in part due to the occurrence or absence of hydrogen‐bonding interactions that involve the phenolate/phenolic oxygen atom. The divalent complexes are isolable in two forms, namely, molecular [Mn^II{H(3,5‐R¹,R²)‐L}₂] and ionic [Mn^II{H₂(3,5‐R¹,R²)‐L}{H(3,5‐R¹,R²)‐L}]ClO₄, with the latter complex converting easily into the former complex on deprotonation. Accessibility of the higher‐valent states is achievable only when the phenolate oxygen atom is sterically hindered from participation in hydrogen bonding. The [Mn^III{H(3,5‐tBu₂)‐L}₂]ClO₄ complex is the first example of a hydrazone‐based Mn^III complex to exhibit spin crossover. Formation of the tetravalent complexes [Mn^IV{(3,5‐R¹,R²)‐L}₂] (R¹=tBu, R²=H; R¹=R²=tBu) necessitates base‐assisted abstraction of the hydrazinic proton.     

Selective C−H Halogenation with a Highly Fluorinated Manganese Porphyrin

The selective C?H functionalization of aliphatic molecules remains a challenge in organic synthesis. While radical chain halogenation reactions provide efficient access to many halogenated molecules, the use of typical protocols for the selective halogenation of electron‐deficient and strained aliphatic molecules is rare. Herein, we report selective C?H chlorination and fluorination reactions promoted by an electron‐deficient manganese pentafluorophenyl porphyrin catalyst, Mn(TPFPP)Cl. This catalyst displays superior properties for the aliphatic halogenation of recalcitrant, electron‐deficient, and strained substrates with unique regio‐ and stereoselectivity. UV/Vis analysis during the course of the reaction indicated that an oxo‐Mn^V species is responsible for hydrogen‐atom abstraction. The observed stereoselectivity results from steric interactions between the bulky porphyrin ligand and the intermediate substrate radical in the halogen rebound step.     

Pulse Radiolysis and Ultra‐High‐Performance Liquid Chromatography/High‐Resolution Mass Spectrometry Studies on the Reactions of the Carbonate Radical with Vitamin B12 Derivatives

The reactions of the carbonate radical anion (CO₃ ^{. ?}) with vitamin B₁₂ derivatives were studied by pulse radiolysis. The carbonate radical anion directly oxidizes the metal center of cob(II)alamin quantitively to give hydroxycobalamin, with a bimolecular rate constant of 2.0×10⁹ M ^?1 s^?1. The reaction of CO₃ ^{. ?} with hydroxycobalamin proceeds in two steps. The second‐order rate constant for the first reaction is 4.3×10⁸ M ^?1 s^?1. The rate of the second reaction is independent of the hydroxycobalamin concentration and is approximately 3.0×10³ s^?1. Evidence for formation of corrinoid complexes differing from cobalamin by the abstraction of two or four hydrogen atoms from the corrin macrocycle and lactone ring formation has been obtained by ultra‐high‐performance liquid chromatography/high‐resolution mass spectrometry (UHPLC/HRMS). A mechanism is proposed in which abstraction of a hydrogen atom by CO₃ ^{. ?} from a carbon atom not involved in the π conjugation system of the corrin occurs in the first step, resulting in formation of a Co^III C‐centered radical that undergoes rapid intramolecular electron transfer to form the corresponding Co^II carbocation complex for about 50 % of these complexes. Subsequent competing pathways lead to formation of corrinoid complexes with two fewer hydrogen atoms and lactone derivatives of B₁₂. Our results demonstrate the potential of UHPLC combined with HRMS in the separation and identification of tetrapyrrole macrocycles with minor modifications from their parent molecule.     

Investigation and Comparison of the Mechanistic Steps in the [(Cp*MCl2)2] (Cp*=C5Me5; M=Rh,Ir)‐Catalyzed Oxidative Annulation of Isoquinolones with Alkynes

The mechanism of the [(Cp*MCl₂)₂] (M=Rh, Ir)‐catalyzed oxidative annulation reaction of isoquinolones with alkynes was investigated in detail. In the first acetate‐assisted C? H‐activation process (cyclometalated step) and the subsequent mono‐alkyne insertion into the M? C bonds of the cyclometalated compounds, both Rh and Ir complexes participated well. However, the desired final products, dibenzo[a,g]quinolizin‐8‐one derivatives, were only formed in high yield when the Rh species participated in the final oxidative coupling of the C? N bond. Moreover, a Rh^I sandwich intermediate was isolated during this transformation. The iridium complexes were found to be inactive in the oxidative coupling processes. All of the relevant intermediates were fully characterized and determined by single‐crystal X‐ray diffraction analysis. Based on this mechanistic study, a Rh^III→Rh^I→Rh^III catalytic cycle was proposed for this reaction.     

Lewis Acid Coupled Electron Transfer of Metal–Oxygen Intermediates

Redox‐inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal–oxygen intermediates, such as metal–oxo and metal–peroxo complexes. The mechanisms of the oxidative C?H bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)–oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)₃ have been unified as rate‐determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)₃) by iron(III)–oxo complexes. All logarithms of the observed second‐order rate constants of Lewis acid‐promoted oxidative C?H bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)–oxo complexes exhibit remarkably unified correlations with the driving forces of proton‐coupled electron transfer (PCET) and metal ion‐coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)₃ to the metal–oxo moiety has been confirmed for Mn^IV–oxo complexes. The enhancement of the electron‐transfer reactivity of metal–oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox‐inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)–peroxo complexes, resulting in acceleration of the electron‐transfer reduction but deceleration of the electron‐transfer oxidation. Such a control on the reactivity of metal–oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca²⁺ in the oxidation of water to dioxygen by the oxygen‐evolving complex in photosystem II.     

Robust and efficient amide-based nonheme manganese(III) hydrocarbon oxidation catalysts: substrate and solvent effects on involvement and partition of multiple active oxidants

Two new mononuclear nonheme manganese(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Mn(bpc)Cl(H₂O)] ( 1 ) and [Mn(Me₂bpb)Cl(H₂O)] ? CH₃OH ( 2 ), were prepared and characterized. Complex 2 has also been characterized by X‐ray crystallography. Magnetic measurements revealed that the complexes are high spin (S=5/2) Mn^III species with typical magnetic moments of 4.76 and 4.95 μ_B, respectively. These nonheme Mn^III complexes efficiently catalyzed olefin epoxidation and alcohol oxidation upon treatment with MCPBA under mild experimental conditions. Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn^V?O, Mn^IV?O, and Mn^III? OO(O)CR. Evidence for this approach was derived from reactivity and Hammett studies, KIE (k_H/k_D) values, H₂¹⁸O‐exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. In addition, it has been proposed that the participation of Mn^V?O, Mn^IV?O, and Mn^III? OOR could be controlled by changing the substrate concentration, and that partitioning between heterolysis and homolysis of the O? O bond of a Mn‐acylperoxo intermediate (Mn? OOC(O)R) might be significantly affected by the nature of solvent, and that the O? O bond of the Mn? OOC(O)R might proceed predominantly by heterolytic cleavage in protic solvent. Therefore, a discrete Mn^V?O intermediate appeared to be the dominant reactive species in protic solvents. Furthermore, we have observed close similarities between these nonheme Mn^III complex systems and Mn(saloph) catalysts previously reported, suggesting that this simultaneous operation of the three active oxidants might prevail in all the manganese‐catalyzed olefin epoxidations, including Mn(salen), Mn(nonheme), and even Mn(porphyrin) complexes. This mechanism provides the greatest congruity with related oxidation reactions by using certain Mn complexes as catalysts.