Structural and spectroscopic characterization of metastable thiolate-ligated manganese(III)-alkylperoxo species

Investigating Ligand Sphere Perturbations on MnIII-Alkylperoxo Complexes

Manganese catalysts that activate hydrogen peroxide carry out several different hydrocarbon oxidation reactions with high stereoselectivity. The commonly proposed mechanism for these reactions involves a key manganese(III)-hydroperoxo intermediate, which decays via O-O bond heterolysis to generate a Mn(V)-oxo species that institutes substrate oxidation. Due to the scarcity of characterized MnIII-hydroperoxo complexes, MnIII-alkylperoxo complexes are employed to understand factors that affect the mechanism of the O-O cleavage. Herein, we report a series of novel complexes, including two room-temperature-stable MnIII-alkylperoxo species, supported by a new amide-containing pentadentate ligand (6Medpaq5NO2). We use a combination of spectroscopic methods and density functional theory computations to probe the effects of the electronic changes in the ligand sphere trans to the hydroxo and alkylperoxo units to thermal stability and reactivity. The structural characterizations for both MnII(OTf)(6Medpaq5NO2) and [MnIII(OH)(6Medpaq5NO2)](OTf) were obtained via single-crystal X-ray crystallography. A perturbation to the ligand sphere allowed for a marked increase in reactivity towards an organic substrate, a modest change in the distribution of the O-O cleavage products from homolytic and heterolytic pathways, and little change in thermal stability.

Mechanistic Insights into Amphoteric Reactivity of an Iron-Bispidine Complex

The reactivity of FeIII -alkylperoxido complexes has remained a riddle to inorganic chemists owing to their thermal instability and impotency towards organic substrates. These iron-oxygen adducts have been known as sluggish oxidants towards oxidative electrophilic and nucleophilic reactions. Herein, we report the synthesis and spectroscopic characterization of a relatively stable mononuclear high-spin FeIII -alkylperoxido complex supported by an engineered bispidine framework. Against the notion, this FeIII -alkylperoxido complex serves as a rare example of versatile reactivity in both electrophilic and nucleophilic reactions. Detailed mechanistic studies and computational calculations reveal a novel reaction mechanism, where a putative superoxido intermediate orchestrates the amphoteric property of the oxidant. The design of the backbone is pivotal to convey stability and reactivity to alkylperoxido and superoxido intermediates. Contrary to the well-known O-O bond cleavage that generates an FeIV -oxido species, the FeIII -alkylperoxido complex reported here undergoes O-C bond scission to generate a superoxido moiety that is responsible for the amphiphilic reactivity.

Reaction landscape of a mononuclear MnIII-hydroxo complex with hydrogen peroxide

Peroxomanganese species have been proposed as key intermediates in the catalytic cycles of both manganese enzymes and synthetic catalysts. However, many of these intermediates have yet to be observed. Here, we report the formation of a series of intermediates, each generated from the reaction of the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq2Me)]+ with hydrogen peroxide under slightly different conditions. By changing the acidity of the reaction mixture and/or the quantity of hydrogen peroxide added, we are able to control which intermediate forms. Using a combination of electronic absorption, 1H NMR, EPR, and X-ray absorption spectroscopies, as well as density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations, we formulate these intermediates as the bis(μ-oxo)dimanganese(III,IV) complex [MnIIIMnIV(μ-O)2(dpaq2Me)2]+, the MnIII-hydroperoxo complex [MnIII(OOH)(dpaq2Me)]+, and the MnIII-peroxo complex [MnIII(O2)(dpaq2Me)]. The formation of the MnIII-hydroperoxo species from the reaction of a MnIII-hydroxo complex with hydrogen peroxide mimics an elementary reaction proposed for many synthetic manganese catalysts that activate hydrogen peroxide.

Electronic structure contributions to O-O bond cleavage reactions for MnIII-alkylperoxo complexes

Synthetic manganese catalysts that activate hydrogen peroxide perform a variety of hydrocarbon oxidation reactions. The most commonly proposed mechanism for these catalysts involves the generation of a manganese(III)-hydroperoxo intermediate that decays via heterolytic O-O bond cleavage to generate a Mn(V)-oxo species that initiates substrate oxidation. Due to the paucity of well-defined MnIII-hydroperoxo complexes, MnIII-alkylperoxo complexes are often employed to understand the factors that affect the O-O cleavage reaction. Herein, we examine the decay pathways of the MnIII-alkylperoxo complexes [MnIII(OOtBu)(6Medpaq)]+ and [MnIII(OOtBu)(N4S)]+, which have distinct coordination environments (N5- and N4S-, respectively). Through the use of density functional theory (DFT) calculations and comparisons with published experimental data, we are able to rationalize the differences in the decay pathways of these complexes. For the [MnIII(OOtBu)(N4S)]+ system, O-O homolysis proceeds via a two-state mechanism that involves a crossing from the quintet reactant to a triplet state. A high energy singlet state discourages O-O heterolysis for this complex. In contrast, while quintet-triplet crossing is unfavorable for [MnIII(OOtBu)(6Medpaq)]+, a relatively low-energy single state accounts for the observation of both O-O homolysis and heterolysis products for this complex. The origins of these differences in decay pathways are linked to variations in the electronic structures of the MnIII-alkylperoxo complexes.

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.

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

While alkylperoxomanganese(iii) (MnIII-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 MnIII-OOR complexes are likely to be highly dependent on their local coordination environment, we have generated two new MnIII-OOR complexes using a new amide-containing N5 - ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H6Medpaq) ligand, we generated the [MnIII(OO t Bu)(6Medpaq)]OTf and [MnIII(OOCm)(6Medpaq)]OTf complexes through reaction of their MnII or MnIII precursors with t BuOOH and CmOOH, respectively. Both of the new MnIII-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 MnIII-OOR adducts render them amenable for detailed characterization, including by X-ray crystallography for [MnIII(OOCm)(6Medpaq)]OTf. Thermal decomposition studies support a decay pathway of the MnIII-OOR complexes by O-O bond homolysis. In contrast, direct reaction of [MnIII(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 MnIII-OOR complexes can be tuned by the local coordination sphere.

Geometrical variations of two manganese(II) complexes with closely related quinoline-based tripodal ligands

Structural analyses of the compounds di-μ-acetato-κ4 O:O'-bis-{[2-meth-oxy-N,N-bis-(quinolin-2-ylmeth-yl)ethanamine-κ4 N,N',N'',O]manganese(II)} bis-(tetra-phen-yl-borate) di-chloro-methane 1.45-solvate, [Mn2(C23O2)2(C23H23N3O)2](C24H20B)·1.45CH2Cl2 or [Mn(DQMEA)(μ-OAc)2Mn(DQMEA)](BPh4)2·1.45CH2Cl2 or [1](BPh4)2·1.45CH2Cl2, and (acetato-κO)[2-hy-droxy-N,N-bis(quinolin-2-ylmeth-yl)ethanamine-κ4 N,N',N'',O](methanol-κO)manganese(II) tetra-phenyl-borate methanol monosolvate, [Mn(CH3COO)(C22H21N3O)(CH3OH)](C24H20B)·CH3OH or [Mn(DQEA)(OAc)(CH3OH)]BPh4·CH3OH or [2]BPh4·CH3OH, by single-crystal X-ray diffraction reveal distinct differences in the geometry of coordination of the tripodal DQEA and DQMEA ligands to MnII ions. In the asymmetric unit, compound [1](BPh4)2·(CH2Cl2)1.45 crystallizes as a dimer in which each manganese(II) center is coordinated by the central amine nitro-gen, the nitro-gen atom of each quinoline group, and the meth-oxy-oxygen of the tetra-dentate DQMEA ligand, and two bridging-acetate oxygen atoms. The symmetric MnII centers have a distorted, octa-hedral geometry in which the quinoline nitro-gen atoms are trans to each other resulting in co-planarity of the quinoline rings. For each MnII center, a coordinated acetate oxygen participates in C-H⋯O hydrogen-bonding inter-actions with the two quinolyl moieties, further stabilizing the trans structure. Within the crystal, weak π-π stacking inter-actions and inter-molecular cation-anion inter-actions stabilize the crystal packing. In the asymmetric unit, compound [2]BPh4·CH3OH crystallizes as a monomer in which the manganese(II) ion is coordinated to the central nitro-gen, the nitro-gen atom of each quinoline group, and the alcohol oxygen of the tetra-dentate DQEA ligand, an oxygen atom of OAc, and the oxygen atom of a methanol ligand. The geometry of the MnII center in [2]BPh4·CH3OH is also a distorted octa-hedron, but the quinoline nitro-gen atoms are cis to each other in this structure. Hydrogen bonding between the acetate oxygen atoms and hydroxyl (O-H⋯O) and quinolyl (C-H⋯O and N-H⋯O) moieties of the DQEA ligand stabilize the complex in this cis configuration. Within the crystal, dimerization of complexes occurs by the formation of a pair of inter-molecular O3-H3⋯O2 hydrogen bonds between the coordinated hydroxyl oxygen of the DQEA ligand of one complex and an acetate oxygen of another. Additional hydrogen-bonding and inter-molecular cation-anion inter-actions contribute to the crystal packing.

Room Temperature Aerobic Peroxidation of Organic Substrates Catalyzed by Cobalt(III) Alkylperoxo Complexes

Room temperature aerobic oxidation of hydrocarbons is highly desirable and remains a great challenge. Here we report a series of highly electrophilic cobalt(III) alkylperoxo complexes, Co^III(qpy)OOR supported by a planar tetradentate quaterpyridine ligand that can directly abstract H atoms from hydrocarbons (R'H) at ambient conditions (Co^III(qpy)OOR + R'H → Co^II(qpy) + R'^• + ROOH). The resulting alkyl radical (R'^•) reacts rapidly with O₂ to form alkylperoxy radical (R'OO^•), which is efficiently scavenged by Co^II(qpy) to give Co^III(qpy)OOR' (Co^II(qpy) + R'OO^• → Co^III(qpy)OOR'). This unique reactivity enables Co^III(qpy)OOR to function as efficient catalysts for aerobic peroxidation of hydrocarbons (R'H + O₂ → R'OOH) under 1 atm air and at room temperature.

Influence of Thiolate versus Alkoxide Ligands on the Stability of Crystallographically Characterized Mn(III)-Alkylperoxo Complexes

The work described herein demonstrates the exquisite control that the inner coordination sphere of metalloenzymes and transition-metal complexes can have on reactivity. We report one of few crystallographically characterized Mn-peroxo complexes and show that the tight correlations between metrical and spectroscopic parameters, established previously by our group for thiolate-ligated RS-Mn(III)-OOR complexes, can be extended to include an alkoxide-ligated RO-Mn(III)-OOR complex. We show that the alkoxide-ligated RO-Mn(III)-OOR complex is an order of magnitude more stable (t_1/2^298 K = 6730 s, k_obs^298 K = 1.03 × 10^-4 s^-1) than its thiolate-ligated RS-Mn(III)-OOR derivative (t_1/2^293 K = 249 s, k₁^293 K = 2.78 × 10^-3 s^-1). Electronic structure calculations provide insight regarding these differences in stability. The highest occupied orbital of the thiolate-ligated derivative possesses significant sulfur character and π-backdonation from the thiolate competes with π-backdonation from the peroxo π*(O-O). DFT-calculated Mulliken charges show that the Mn ion Lewis acidity of alkoxide-ligated RO-Mn(III)-OOR (+0.451) is greater than that of thiolate-ligated RS-Mn(III)-OOR (+0.306), thereby facilitating π-backdonation from the antibonding peroxo π*(O-O) orbital and increasing its stability. This helps to explain why the photosynthetic oxygen-evolving Mn complex, which catalyzes O-O bond formation as opposed to cleavage, incorporates O- and/or N-ligands as opposed to ^cysS-ligands.

Mechanistic insights into dioxygen activation by a manganese corrole complex: a broken-symmetry DFT study

The Mn–oxygen species have been implicated as key intermediates in various Mn-mediated oxidation reactions. However, artificial oxidants were often used for the synthesis of the Mn–oxygen intermediates. Remarkably, the Mn(v)–oxo and Mn(iv)–peroxo species have been observed in the activation of O₂ by Mn(iii) corroles in the presence of base (OH⁻) and hydrogen donors. In this work, density functional theory methods were used to get insight into the mechanism of dioxygen activation and formation of Mn(v)–oxo. The results demonstrated that the dioxygen cannot bind to Mn without the axial OH⁻ ligand. Upon the addition of the axial OH⁻ ligand, the dioxygen can bind to Mn in an end-on fashion to give the Mn(iv)–superoxo species. The hydrogen atom transfer from the hydrogen donor (substrate) to the Mn(iv)–superoxo species is the rate-limiting step, having a high reaction barrier and a large endothermicity. Subsequently, the O–C bond formation is concerted with an electron transfer from the substrate radical to the Mn and a proton transfer from the hydroperoxo moiety to the nearby N atom of the corrole ring, generating an alkylperoxo Mn(iii) complex. The alkylperoxo O–O bond cleavage affords a Mn(v)–oxo complex and a hydroxylated substrate. This novel mechanism for the Mn(v)–oxo formation via an alkylperoxo Mn(iii) intermediate gives insight into the O–O bond activation by manganese complexes.

DFT calculations revealed a novel mechanism for the formation of Mn(v)–oxo in the dioxygen activation by a Mn(iii) corrole complex involving a Mn(iii)–alkylperoxo intermediate.

Comparison of two MnIVMnIV-bis-μ-oxo complexes {[MnIV(N4(6-Me-DPEN))]2(μ-O)2}2+ and {[MnIV(N4(6-Me-DPPN))]2(μ-O)2}2

The addition of tert-butyl hydro-peroxide ( t BuOOH) to two structurally related MnII complexes containing N,N-bis-(6-methyl-2-pyridyl-meth-yl)ethane-1,2-di-amine (6-Me-DPEN) and N,N-bis-(6-methyl-2-pyridyl-meth-yl)propane-1,2-di-amine (6-Me-DPPN) results in the formation of high-valent bis-oxo complexes, namely di-μ-oxido-bis-{[N,N-bis-(6-methyl-2-pyridylmeth-yl)ethane-1,2-di-amine]-manganese(II)}(Mn-Mn) bis-(tetra-phenyl-borate) dihydrate, [Mn(C16H22N4)2O2](C24H20B)2·2H2O or {[MnIV(N4(6-Me-DPEN))]2(μ-O)2}(2BPh4)(2H2O) (1) and di-μ-oxido-bis-{[N,N-bis-(6-methyl-2-pyridylmeth-yl)propane-1,3-di-amine]-manganese(II)}(Mn-Mn) bis-(tetra-phenyl-borate) diethyl ether disolvate, [Mn(C17H24N4)2O2](C24H20B)2·2C4H10O or {[MnIV(N4(6-MeDPPN))]2(μ-O)2}(2BPh4)(2Et2O) (2). Complexes 1 and 2 both contain the 'diamond core' motif found previously in a number of iron, copper, and manganese high-valent bis-oxo compounds. The flexibility in the propyl linker in the ligand scaffold of 2, as compared to that of the ethyl linker in 1, results in more elongated Mn-N bonds, as one would expect. The Mn-Mn distances and Mn-O bond lengths support an MnIV oxidation state assignment for the Mn ions in both 1 and 2. The angles around the Mn centers are consistent with the local pseudo-octa-hedral geometry.

Mechanistic dichotomies in redox reactions of mononuclear metal–oxygen intermediates

This review article focuses on various mechanistic dichotomies in redox reactions of metal–oxygen intermediates with the emphasis on understanding and controlling their redox reactivity from experimental and theoretical points of view.

How Metal Ion Lewis Acidity and Steric Properties Influence the Barrier to Dioxygen Binding, Peroxo O-O Bond Cleavage, and Reactivity

Herein we quantitatively investigate how metal ion Lewis acidity and steric properties influence the kinetics and thermodynamics of dioxygen binding versus release from structurally analogous Mn-O₂ complexes, as well as the barrier to Mn peroxo O-O bond cleavage, and the reactivity of Mn oxo intermediates. Previously we demonstrated that the steric and electronic properties of Mn^III-OOR complexes containing N-heterocyclic (N^Ar) ligand scaffolds can have a dramatic influence on alkylperoxo O-O bond lengths and the barrier to alkylperoxo O-O bond cleavage. Herein, we examine the dioxygen reactivity of a new Mn^II complex containing a more electron-rich, less sterically demanding N^Ar ligand scaffold, and compare it with previously reported Mn^II complexes. Dioxygen binding is shown to be reversible with complexes containing the more electron-rich metal ions. The kinetic barrier to O₂ binding and peroxo O-O bond cleavage is shown to correlate with redox potentials, as well as the steric properties of the supporting N^Ar ligands. The reaction landscape for the dioxygen chemistry of the more electron-rich complexes is shown to be relatively flat. A total of four intermediates, including a superoxo and peroxo species, are observed with the most electron-rich complex. Two new intermediates are shown to form following the peroxo, which are capable of cleaving strong X-H bonds. In the absence of a sacrificial H atom donor, solvent, or ligand, serves as a source of H atoms. With TEMPOH as sacrificial H atom donor, a deuterium isotope effect is observed (k_H/k_D = 3.5), implicating a hydrogen atom transfer (HAT) mechanism. With 1,4-cyclohexadiene, 0.5 equiv of benzene is produced prior to the formation of an EPR detected Mn^IIIMn^IV bimetallic species, and 0.5 equiv after its formation.

Geometric and electronic structure of a crystallographically characterized thiolate-ligated binuclear peroxo-bridged cobalt(III) complex

In order to shed light on metal-dependent mechanisms for O-O bond cleavage, and its microscopic reverse, we compare herein the electronic and geometric structures of O2-derived binuclear Co(III)- and Mn(III)-peroxo compounds. Binuclear metal peroxo complexes are proposed to form as intermediates during Mn-promoted photosynthetic H2O oxidation, and a Co-containing artificial leaf inspired by nature's photosynthetic H2O oxidation catalyst. Crystallographic characterization of an extremely activated peroxo is made possible by working with substitution-inert, low-spin Co(III). Density functional theory (DFT) calculations show that the frontier orbitals of the Co(III)-peroxo compound differ noticeably from the analogous Mn(III)-peroxo compound. The highest occupied molecular orbital (HOMO) associated with the Co(III)-peroxo is more localized on the peroxo in an antibonding π*(O-O) orbital, whereas the HOMO of the structurally analogous Mn(III)-peroxo is delocalized over both the metal d-orbitals and peroxo π*(O-O) orbital. With low-spin d6 Co(III), filled t2g orbitals prevent π-back-donation from the doubly occupied antibonding π*(O-O) orbital onto the metal ion. This is not the case with high-spin d4 Mn(III), since these orbitals are half-filled. This weakens the peroxo O-O bond of the former relative to the latter.

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

The iron(II) complexes [Fe(L)(MeCN)₃ ](SO₃ CF₃ )₂ (L are two derivatives of tris(2-pyridyl)-based ligands) have been synthesized as models for cysteine dioxygenase (CDO). The molecular structure of one of the complexes exhibits octahedral coordination geometry and the Fe-N_py bond lengths [1.953(4)-1.972(4) Å] are similar to those in the Cys-bound Fe^II -CDO; Fe-N_His : 1.893-2.199 Å. The iron(II) centers of the model complexes exhibit relatively high Fe^III/II redox potentials (E_1/2 =0.988-1.380 V vs. ferrocene/ferrocenium electrode, Fc/Fc⁺ ), within the range for O₂ activation and typical for the corresponding nonheme iron enzymes. The reaction of in situ generated [Fe(L)(MeCN)(SPh)]⁺ with excess O₂ in acetonitrile (MeCN) yields selectively the doubly oxygenated phenylsulfinic acid product. Isotopic labeling studies using ¹⁸ O₂ confirm the incorporation of both oxygen atoms of O₂ into the product. Kinetic and preliminary DFT studies reveal the involvement of an Fe^III peroxido intermediate with a rhombic S= 1 / 2 Fe^III center (687-696 nm; g≈2.46-2.48, 2.13-2.15, 1.92-1.94), similar to the spectroscopic signature of the low-spin Cys-bound Fe^III CDO (650 nm, g≈2.47, 2.29, 1.90). The proposed Fe^III peroxido intermediates have been trapped, and the O-O stretching frequencies are in the expected range (approximately 920 and 820 cm^-1 for the alkyl- and hydroperoxido species, respectively). The model complexes have a structure similar to that of the enzyme and structural aspects as well as the reactivity are discussed.

Hydrogen by Deuterium Substitution in an Aldehyde Tunes the Regioselectivity by a Nonheme Manganese(III)-Peroxo Complex

Mononuclear nonheme Mn^III -peroxo complexes are important intermediates in biology, and take part in oxygen activation by photosystem II. Herein, we present work on two isomeric biomimetic side-on Mn^III -peroxo intermediates with bispidine ligand system and reactivity patterns with aldehydes. The complexes are characterized with UV/Vis and mass spectrometric techniques and reaction rates with cyclohexane carboxaldehyde (CCA) are measured. The reaction gives an unusual regioselectivity switch from aliphatic to aldehyde hydrogen atom abstraction upon deuteration of the substrate, leading to the corresponding carboxylic acid product for the latter, while the former gives a deformylation reaction. Mechanistic details are established from kinetic isotope effect studies and density functional theory calculations. Thus, replacement of C-H by C-D raises the hydrogen atom abstraction barriers and enables a regioselectivity switch to a competitive pathway that is slightly higher in energy.

One step phenol synthesis from benzene catalysed by nickel(ii) complexes

Nickel(ii)complexes of N₄-ligands are reported as efficient catalysts for direct benzene hydroxylation via bis(μ-oxo)dinickel(iii) intermediate species. The exclusive phenol formation is achieved with a yield of 41%.

MnIII-Peroxo adduct supported by a new tetradentate ligand shows acid-sensitive aldehyde deformylation reactivity

The new tetradentate L7BQ ligand (L7BQ = 1,4-di(quinoline-8-yl)-1,4-diazepane) has been synthesized and shown to support MnII and MnIII-peroxo complexes. X-ray crystallography of the [MnII(L7BQ)(OTf)2] complex shows a monomeric MnII center with the L7BQ ligand providing four donor nitrogen atoms in the equatorial field, with two triflate ions bound in the axial positions. When this species is treated with H2O2 and Et3N at -40 °C, a MnIII-peroxo adduct, [MnIII(O2)(L7BQ)]+ is formed. The formation of this new intermediate is supported by a variety of spectroscopic techniques, including electronic absorption, Mn K-edge X-ray absorption and electron paramagnetic resonance methods. Evaluation of extended X-ray absorption fine structure data for [MnIII(O2)(L7BQ)]+ resolved Mn-O bond distances of 1.85 Å, which are on the short end of those previously reported for crystallographically characterized MnIII-peroxo adducts. An analysis of the X-ray pre-edge region of [MnIII(O2)(L7BQ)]+ revealed a large pre-edge area of 20.8 units. Time-dependent density functional theory computations indicate that the pre-edge intensity is due to Mn 4p-3d mixing caused by geometric distortions from centrosymmetry induced by both the peroxo and L7BQ ligands. The reactivity of [MnIII(O2)(L7BQ)]+ towards aldehydes was assessed through reaction with cyclohexanecarboxaldehyde and 2-phenylpropionaldehyde. From these experiments, it was determined that [MnIII(O2)(L7BQ)]+ only reacts with aldehydes in the presence of acid. Specifically, the addition of cyclohexanecarboxylic acid to [MnIII(O2)(L7BQ)]+ converts the MnIII-peroxo adduct to a new intermediate that could be responsible for the observed aldehyde deformylation activity. These observations underscore the challenges in identifying the reactive metal species in aldehyde deformylation reactions.

Keto-Enol Tautomerization Triggers an Electrophilic Aldehyde Deformylation Reaction by a Nonheme Manganese(III)-Peroxo Complex

Oxygen atom transfer by high-valent enzymatic intermediates remains an enigma in chemical catalysis. In particular, manganese is an important first-row metal involved in key biochemical processes, including the biosynthesis of molecular oxygen (through the photosystem II complex) and biodegradation of toxic superoxide to hydrogen peroxide by superoxide dismutase. Biomimetic models of these biological systems have been developed to gain understanding on the structure and properties of short-lived intermediates but also with the aim to create environmentally benign oxidants. In this work, we report a combined spectroscopy, kinetics and computational study on aldehyde deformylation by two side-on manganese(III)-peroxo complexes with bispidine ligands. Both manganese(III)-peroxo complexes are characterized by UV-vis and mass spectrometry techniques, and their reactivity patterns with aldehydes was investigated. We find a novel mechanism for the reaction that is initiated by a hydrogen atom abstraction reaction, which enables a keto-enol tautomerization in the substrate. This is an essential step in the mechanism that makes an electrophilic attack on the olefin bond possible as the attack on the aldehyde carbonyl is too high in energy. Kinetics studies determine a large kinetic isotope effect for the replacement of the transferring hydrogen atom by deuterium, while replacing the transferring hydrogen atom by a methyl group makes the substrate inactive and hence confirm the hypothesized mechanism. Our new mechanism is confirmed with density functional theory modeling on the full mechanism and rationalized through valence bond and thermochemical cycles. Our unprecedented new mechanism may have relevance to biological and biomimetic chemistry processes in general and gives insight into the reactivity patterns of metal-peroxo and metal-hydroperoxo intermediates in general.

Metal-Assisted Oxo Atom Addition to an Fe(III) Thiolate

Cysteinate oxygenation is intimately tied to the function of both cysteine dioxygenases (CDOs) and nitrile hydratases (NHases), and yet the mechanisms by which sulfurs are oxidized by these enzymes are unknown, in part because intermediates have yet to be observed. Herein, we report a five-coordinate bis-thiolate ligated Fe(III) complex, [Fe^III(S₂^Me2N₃(Pr,Pr))]⁺ (2), that reacts with oxo atom donors (PhIO, IBX-ester, and H₂O₂) to afford a rare example of a singly oxygenated sulfenate, [Fe^III(η²-S^Me2O)(S^Me2)N₃(Pr,Pr)]⁺ (5), resembling both a proposed intermediate in the CDO catalytic cycle and the essential NHase Fe-S(O)^Cys114 proposed to be intimately involved in nitrile hydrolysis. Comparison of the reactivity of 2 with that of a more electron-rich, crystallographically characterized derivative, [Fe^IIIS₂^Me2N^MeN₂^amide(Pr,Pr)]^- (8), shows that oxo atom donor reactivity correlates with the metal ion's ability to bind exogenous ligands. Density functional theory calculations suggest that the mechanism of S-oxygenation does not proceed via direct attack at the thiolate sulfurs; the average spin-density on the thiolate sulfurs is approximately the same for 2 and 8, and Mulliken charges on the sulfurs of 8 are roughly twice those of 2, implying that 8 should be more susceptible to sulfur oxidation. Carboxamide-ligated 8 is shown to be unreactive towards oxo atom donors, in contrast to imine-ligated 2. Azide (N₃^-) is shown to inhibit sulfur oxidation with 2, and a green intermediate is observed, which then slowly converts to sulfenate-ligated 5. This suggests that the mechanism of sulfur oxidation involves initial coordination of the oxo atom donor to the metal ion. Whether the green intermediate is an oxo atom donor adduct, Fe-O═I-Ph, or an Fe(V)═O remains to be determined.

Tuning the Relative Stability and Reactivity of Manganese Dioxygen and Peroxo Intermediates via Systematic Ligand Modification

Many fundamental processes of life depend on the chemical energy stored in the O–O bond of dioxygen (O2), the majority of which is derived from photosynthetic H2O oxidation. Key steps in these processes involve Mn-, Fe-, or Cu-promoted formation or cleavage of O–O and O–H bonds, the mechanisms of which are not fully understood, especially with Mn. Metal–peroxo and high-valent metal–oxo species are proposed to be involved as intermediates. The metal ion properties that favor O–O and O–H bond formation versus cleavage have yet to be systematically explored. Herein we examine the O2 reactivity of a series of structurally related Mn(II) complexes and show that several metastable intermediates are observed, the relative stabilities of which depend on subtle differences in ligand architecture. We show that in contrast to Fe and Cu complexes, O2 binds irreversibly to Mn(II). By crystallizing an entire series of the first reported examples of Mn(III)–OOR peroxos as well as an O2-derived binuclear trans-μ-1,2-bridged Mn(III)–peroxo with varying degrees of O–O bond activation, we demonstrate that there are distinct correlations between spectroscopic, structural, and reactivity properties. Rate-limiting O–O bond cleavage is shown to afford a reactive species capable of abstracting H atoms from 2,4-tBu2-PhOH or 1,4-cyclohexadiene, depending on the ligand substituents. The weakly coordinated N-heterocycle Mn···Npy,quino distance is shown to correlate with the peroxo O–O bond length and modulate the π overlap between the filled πv*(O–O) and Mn dxz orbitals. We also show that there is a strong correlation between the peroxo → Mn charge transfer (CT) band and the peroxo O–O bond length. The energy difference between the CT bands associated with the peroxos possessing the shortest and longest O–O bonds shows that these distances are spectroscopically distinguishable. We show that we can use this spectroscopic parameter to estimate the O–O bond length, and thus the degree of O–O bond activation, in intermediates for which there is no crystal structure, as long as the ligand environment is approximately the same.

Geometric and electronic structure of a peroxomanganese(III) complex supported by a scorpionate ligand

A monomeric Mn(II) complex has been prepared with the facially-coordinating Tp(Ph2) ligand, (Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate). The X-ray crystal structure shows three coordinating solvent molecules resulting in a six-coordinate complex with Mn-ligand bond lengths that are consistent with a high-spin Mn(II) ion. Treatment of this Mn(II) complex with excess KO2 at room temperature resulted in the formation of a Mn(III)-O2 complex that is stable for several days at ambient conditions, allowing for the determination of the X-ray crystal structure of this intermediate. The electronic structure of this peroxomanganese(III) adduct was examined by using electronic absorption, electron paramagnetic resonance (EPR), low-temperature magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD spectroscopies. Density functional theory (DFT), time-dependent (TD)-DFT, and multireference ab initio CASSCF/NEVPT2 calculations were used to assign the electronic transitions and further investigate the electronic structure of the peroxomanganese(III) species. The lowest ligand-field transition in the electronic absorption spectrum of the Mn(III)-O2 complex exhibits a blue shift in energy compared to other previously characterized peroxomanganese(III) complexes that results from a large axial bond elongation, reducing the metal-ligand covalency and stabilizing the σ-antibonding Mn dz(2) MO that is the donor MO for this transition.

Spectroscopic characterization and reactivity studies of a mononuclear nonheme Mn(III)-hydroperoxo complex

We report the first example of a mononuclear nonheme manganese(III)-hydroperoxo complex derived from protonation of an isolated manganese(III)-peroxo complex bearing an N-tetramethylated cyclam (TMC) ligand, [Mn(III)(TMC)(OOH)](2+). The Mn(III)-hydroperoxo intermediate is characterized with various spectroscopic methods as well as with density functional theory (DFT) calculations, showing the binding of a hydroperoxide ligand in an end-on fashion. The Mn(III)-hydroperoxo species is a competent oxidant in oxygen atom transfer (OAT) reactions, such as the oxidation of sulfides. The electrophilic character of the Mn(III)-hydroperoxo complex is demonstrated unambiguously in the sulfoxidation of para-substituted thioanisoles.

Manganese(ii) complexes of tetradentate 4N ligands with diazepane backbones for catalytic olefin epoxidation: effect of nucleophilicity of peroxo complexes on reactivity

Mononuclear Mn(ii) complexes of linear 4N ligands have been studied as catalysts for deformylation of aldehydes and epoxidation (H₂O₂) of unfunctionalised olefins to understand the role of ligand stereoelectronic factors.

Characterization of metastable intermediates formed in the reaction between a Mn(II) complex and dioxygen, including a crystallographic structure of a binuclear Mn(III)-peroxo species

Transition-metal peroxos have been implicated as key intermediates in a variety of critical biological processes involving O2. Because of their highly reactive nature, very few metal-peroxos have been characterized. The dioxygen chemistry of manganese remains largely unexplored despite the proposed involvement of a Mn-peroxo, either as a precursor to, or derived from, O2, in both photosynthetic H2O oxidation and DNA biosynthesis. These are arguably two of the most fundamental processes of life. Neither of these biological intermediates has been observed. Herein we describe the dioxygen chemistry of coordinatively unsaturated [Mn(II)(S(Me2)N4(6-Me-DPEN))] (+) (1), and the characterization of intermediates formed en route to a binuclear mono-oxo-bridged Mn(III) product {[Mn(III)(S(Me2)N4(6-Me-DPEN)]2(μ-O)}(2+) (2), the oxo atom of which is derived from (18)O2. At low-temperatures, a dioxygen intermediate, [Mn(S(Me2)N4(6-Me-DPEN))(O2)](+) (4), is observed (by stopped-flow) to rapidly and irreversibly form in this reaction (k1(-10 °C) = 3780 ± 180 M(-1) s(-1), ΔH1(++) = 26.4 ± 1.7 kJ mol(-1), ΔS1(++) = -75.6 ± 6.8 J mol(-1) K(-1)) and then convert more slowly (k2(-10 °C) = 417 ± 3.2 M(-1) s(-1), ΔH2(++) = 47.1 ± 1.4 kJ mol(-1), ΔS2(++) = -15.0 ± 5.7 J mol(-1) K(-1)) to a species 3 with isotopically sensitive stretches at νO-O(Δ(18)O) = 819(47) cm(-1), kO-O = 3.02 mdyn/Å, and νMn-O(Δ(18)O) = 611(25) cm(-1) consistent with a peroxo. Intermediate 3 releases approximately 0.5 equiv of H2O2 per Mn ion upon protonation, and the rate of conversion of 4 to 3 is dependent on [Mn(II)] concentration, consistent with a binuclear Mn(O2(2-)) Mn peroxo. This was verified by X-ray crystallography, where the peroxo of {[Mn(III)(S(Me2)N4(6-Me-DPEN)]2(trans-μ-1,2-O2)}(2+) (3) is shown to be bridging between two Mn(III) ions in an end-on trans-μ-1,2-fashion. This represents the first characterized example of a binuclear Mn(III)-peroxo, and a rare case in which more than one intermediate is observed en route to a binuclear μ-oxo-bridged product derived from O2. Vibrational and metrical parameters for binuclear Mn-peroxo 3 are compared with those of related binuclear Fe- and Cu-peroxo compounds.

Correlation between structural, spectroscopic, and reactivity properties within a series of structurally analogous metastable manganese(III)-alkylperoxo complexes

Manganese-peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn(III)-OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn(III)-OOR compounds extend the number of known end-on Mn(III)-(η(1)-peroxos) to six. The ligand backbone is shown to alter the metal-ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O-O and Mn···N(py,quin) distances), spectroscopic (E(πv*(O-O) → Mn CT band), ν(O-O)), and kinetic (ΔH(‡) and ΔS(‡)) parameters for these complexes provide compelling evidence for rate-limiting O-O bond cleavage. Products identified in the final reaction mixtures of Mn(III)-OOR decay are consistent with homolytic O-O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O-O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O-O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo πv*(O-O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function.

Characterization and dioxygen reactivity of a new series of coordinatively unsaturated thiolate-ligated manganese(II) complexes

The synthesis, structural, and spectroscopic characterization of four new coordinatively unsaturated mononuclear thiolate-ligated manganese(II) complexes ([Mn(II)(S(Me2)N(4)(6-Me-DPEN))](BF(4)) (1), [Mn(II)(S(Me2)N(4)(6-Me-DPPN))](BPh(4))·MeCN (3), [Mn(II)(S(Me2)N(4)(2-QuinoPN))](PF(6))·MeCN·Et(2)O (4), and [Mn(II)(S(Me2)N(4)(6-H-DPEN)(MeOH)](BPh(4)) (5)) is described, along with their magnetic, redox, and reactivity properties. These complexes are structurally related to recently reported [Mn(II)(S(Me2)N(4)(2-QuinoEN))](PF(6)) (2) (Coggins, M. K.; Kovacs, J. A. J. Am. Chem. Soc.2011, 133, 12470). Dioxygen addition to complexes 1-5 is shown to result in the formation of five new rare examples of Mn(III) dimers containing a single, unsupported oxo bridge: [Mn(III)(S(Me2)N(4)(6-Me-DPEN)](2)-(μ-O)(BF(4))(2)·2MeOH (6), [Mn(III)(S(Me2)N(4)(QuinoEN)](2)-(μ-O)(PF(6))(2)·Et(2)O (7), [Mn(III)(S(Me2)N(4)(6-Me-DPPN)](2)-(μ-O)(BPh(4))(2) (8), [Mn(III)(S(Me2)N(4)(QuinoPN)](2)-(μ-O)(BPh(4))(2) (9), and [Mn(III)(S(Me2)N(4)(6-H-DPEN)](2)-(μ-O)(PF(6))(2)·2MeCN (10). Labeling studies show that the oxo atom is derived from (18)O(2). Ligand modifications, involving either the insertion of a methylene into the backbone or the placement of an ortho substituent on the N-heterocyclic amine, are shown to noticeably modulate the magnetic and reactivity properties. Fits to solid-state magnetic susceptibility data show that the Mn(III) ions of μ-oxo dimers 6-10 are moderately antiferromagnetically coupled, with coupling constants (2J) that fall within the expected range. Metastable intermediates, which ultimately convert to μ-oxo bridged 6 and 7, are observed in low-temperature reactions between 1 and 2 and dioxygen. Complexes 3-5, on the other hand, do not form observable intermediates, thus illustrating the effect that relatively minor ligand modifications have upon the stability of metastable dioxygen-derived species.