ManganeseâOxygen Intermediates in OâO Bond Activation and

Article Cite This: Acc. Chem. Res. 2017, 50, 2706-2717

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Manganese−Oxygen Intermediates in O−O Bond Activation and Hydrogen-Atom Transfer Reactions Derek B. Rice, Allyssa A. Massie, and Timothy A. Jackson* Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States CONSPECTUS: Biological systems capitalize on the redox versatility of manganese to perform reactions involving dioxygen and its derivatives superoxide, hydrogen peroxide, and water. The reactions of manganese enzymes influence both human health and the global energy cycle. Important examples include the detoxification of reactive oxygen species by manganese superoxide dismutase, biosynthesis by manganese ribonucleotide reductase and manganese lipoxygenase, and water splitting by the oxygen-evolving complex of photosystem II. Although these enzymes perform very different reactions and employ structurally distinct active sites, manganese intermediates with peroxo, hydroxo, and oxo ligation are commonly proposed in catalytic mechanisms. These intermediates are also postulated in mechanisms of synthetic manganese oxidation catalysts, which are of interest due to the earth abundance of manganese. In this Account, we describe our recent efforts toward understanding O−O bond activation pathways of MnIII-peroxo adducts and hydrogen-atom transfer reactivity of MnIV-oxo and MnIII-hydroxo complexes. In biological and synthetic catalysts, peroxomanganese intermediates are commonly proposed to decay by either Mn−O or O−O cleavage pathways, although it is often unclear how the local coordination environment influences the decay mechanism. To address this matter, we generated a variety of MnIII-peroxo adducts with varied ligand environments. Using parallel-mode EPR and Mn K-edge X-ray absorption techniques, the decay pathway of one MnIII-peroxo complex bearing a bulky macrocylic ligand was investigated. Unlike many MnIII-peroxo model complexes that decay to oxo-bridged-MnIIIMnIV dimers, decay of this MnIIIperoxo adduct yielded mononuclear MnIII-hydroxo and MnIV-oxo products, potentially resulting from O−O bond activation of the MnIII-peroxo unit. These results highlight the role of ligand sterics in promoting the formation of mononuclear products and mark an important step in designing MnIII-peroxo complexes that convert cleanly to high-valent Mn-oxo species. Although some synthetic MnIV-oxo complexes show great potential for oxidizing substrates with strong C−H bonds, most MnIVoxo species are sluggish oxidants. Both two-state reactivity and thermodynamic arguments have been put forth to explain these observations. To address these issues, we generated a series of MnIV-oxo complexes supported by neutral, pentadentate ligands with systematically perturbed equatorial donation. Kinetic investigations of these complexes revealed a correlation between equatorial ligand-field strength and hydrogen-atom and oxygen-atom transfer reactivity. While this trend can be understood on the basis of the two-state reactivity model, the reactivity trend also correlates with variations in MnIII/IV reduction potential caused by changes in the ligand field. This work demonstrates the dramatic influence simple ligand perturbations can have on reactivity but also illustrates the difficulties in understanding the precise basis for a change in reactivity. In the enzyme manganese lipoxygenase, an active-site MnIII-hydroxo adduct initiates substrate oxidation by abstracting a hydrogen atom from a C−H bond. Precedent for this chemistry from synthetic MnIII-hydroxo centers is rare. To better understand hydrogen-atom transfer by MnIII centers, we developed a pair of MnIII-hydroxo complexes, formed in high yield from dioxygen oxidation of MnII precursors, capable of attacking weak O−H and C−H bonds. Kinetic and computational studies show a delicate interplay between thermodynamic and steric influences in hydrogen-atom transfer reactivity, underscoring the potential of MnIII-hydroxo units as mild oxidants.

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fidelity models of the oxygen-evolving complex (OEC),3 O2activating complexes,4,5 and catalysts for efficient and selective C−H bond oxidation.6 A unifying feature of both the Mndependent enzymes and model systems is the central role of Mn-oxygen species, such as Mn-peroxo, Mn-hydroxo, and Mnoxo adducts, in reactivity.

INTRODUCTION Manganese centers in nature react with dioxygen and its reduced species (O2−, H2O2, H2O) to perform a range of biological functions, including defense against reactive oxygen species, biosynthesis of DNA and organic hydroperoxides, and water splitting.1,2 These enzymes have also served as inspiration for model complexes capable of mimicking biological functions or performing synthetically useful transformations. Recent developments in bioinspired Mn chemistry has led to high© 2017 American Chemical Society

Received: July 10, 2017 Published: October 24, 2017 2706

DOI: 10.1021/acs.accounts.7b00343 Acc. Chem. Res. 2017, 50, 2706−2717

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Accounts of Chemical Research

Figure 1. Active-site structures of peroxomanganese adduct of MnSOD (left) and MnLOX (right) from 3K9S and 5FNO, respectively.

Scheme 1. Reactions of [MnII(N4py)(OTf)]+ upon Treatment with KO2 and H2O2

bonds of cyclohexane, while others react sluggishly with activated C−H bonds. The development of structure−reactivity relationships for HAT reactions by MnIII-hydroxo and MnIVoxo complexes is an area of active research. In the following sections, we describe our efforts toward understanding O−O activation of MnIII-peroxo adducts and HAT reactions of MnIV-oxo and MnIII-hydroxo species. Our approach has been to combine kinetic, spectroscopic, and computational methods to understand how the geometric and electronic structure of a Mn center influences chemical reactivity.

Manganese-peroxo intermediates have been invoked in the catalytic cycles of several enzymes, and a crystal structure of a Mn-peroxo adduct was reported for manganese superoxide dismutase (MnSOD; Figure 1, left).1,7 In MnSOD, a Mnperoxo intermediate is presumed to undergo acid-assisted Mn− O cleavage to yield H2O2 and a MnIII center.1 In contrast, synthetic Mn/H2O2 catalysts feature Mn-peroxo intermediates that are postulated to undergo O−O bond cleavage to yield high-valent Mn-oxo species.6,8 This variation in reactivity has fostered interest in identifying factors controlling O−O versus Mn−O bond cleavage for Mn-peroxo adducts.9 In biological and synthetic systems, Mn-hydroxo and Mn-oxo adducts are involved in hydrogen-atom transfer (HAT) reactions. In manganese lipoxygenase (MnLOX; Figure 1, right), a MnIII-hydroxo abstracts a hydrogen atom from a polyunsaturated fatty acid, leading to substrate dioxygenation.10,11 In Mn ribonucleotide reductase, an oxo-bridgedMnIIIMnIV unit completes cofactor assembly by oxidizing a tyrosine residue to a tyrosyl radical, possibly by a HAT mechanism.12 While synthetic MnIII-hydroxo complexes known to perform HAT reactions are scarce,5,13−15 synthetic MnIV-oxo adducts show a remarkable range of reactivities toward C−H bonds.16−21 Some MnIV-oxo species can attack the strong C−H

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MANGANESE(III)-PEROXO COMPLEXES The first indirect evidence from our lab of O−O bond activation of a MnIII-peroxo species came from studies of [MnIII(O2)(κ4-N4py)]+ (N4py = N,N-bis(2-pyridylmethyl)-Nbis(2-pyridyl)methylamine; Scheme 1).22 In collaboration with Anxolabéhère-Mallart and Dorlet, we used spectroscopic and computational methods to demonstrate that [MnIII(O2)(κ4N4py)]+ featured a side-on (η2) peroxo with N4py bound in an unusual tetradentate fashion, giving a six-coordinate MnIII center.23 The [MnIII(O2)(κ4-N4py)]+ complex was unique among MnIII-peroxo adducts known to date, as it could only be 2707


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Accounts of Chemical Research Scheme 2. Intermediates and Products Observed when [MnII(Cl)2(Me2EBC)] Reacts with KO2a

a

Spectroscopic methods used to support the formulation of the products are listed below each species.

Figure 2. X-band EPR spectra of [MnII(Cl)2(Me2EBC)] (top), [MnIII(O2)(Me2EBC)]+ (center), and the pink solution observed following decay of [MnIII(O2)(Me2EBC)]+ (bottom). Parallel- and perpendicular-mode spectra are on the left and right panels, respectively. Adapted with permission from ref 31. Copyright 2016 American Chemical Society.

generated upon reaction of the MnII compound with O2−.22−24 Commonly, MnIII-peroxo species can be accessed by oxidation with either O2− or H2O2 and base. Treatment of [MnII(N4py)(OTf)]+ with H2O2 led to the formation of the oxo-bridged dimer [MnIIIMnIV(μ-O)2(κ4-N4py)2]3+ (Scheme 1). Crystallographic characterization of [MnIIIMnIV(μ-O)2(κ4-N4py)2]3+ confirmed the tetradentate binding mode of the N4py ligand.22 Evidence of potential O−O bond activation was obtained when we observed that [MnIIIMnIV(μ-O)2(κ4-N4py)2]3+ also forms

when the [Mn III (O 2 )(κ 4 -N4py)] + complex reacts with [MnII(N4py)(OTf)]+ (Scheme 1). We postulated that the oxo bridges in the resulting [MnIIIMnIV(μ-O)2(κ4-N4py)2]3+ complex originate from the peroxo unit of [MnIII(O2)(κ4N4py)]+. Although our observations for [MnIII(O2)(κ4-N4py)]+ were suggestive of O−O bond activation to generate a high-valent species, the resulting [MnIIIMnIV(μ-O)2(κ4-N4py)]3+ complex is very stable and not particularly reactive toward C−H bonds. 2708


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Accounts of Chemical Research In contrast, our lab25 and Nam and co-workers16 separately reported the mononuclear MnIV-oxo analogue, [MnIV(O)(N4py)]2+ that is reactive toward C−H bonds (vide infra). Unlike the [MnIII(O2)(κ4-N4py)]+ complex, [MnIV(O)(N4py)]2+ presumably features the N4py ligand bound in a pentadentate fashion. Thus, conversion of [MnIII(O2)(κ4N4py)]+ to [MnIV(O)(N4py)]2+ would require recoordination of the dangling pyridine ligand. In light of these considerations, we were motivated to demonstrate a MnIII-peroxo to MnIV-oxo conversion using a rigid supporting ligand, thereby suppressing ligand acrobatics. A well-suited system was provided by our collaborator Prof. Daryle Busch. Over 15 years ago, the Busch lab identified the Me 2 EBC ligand (Me 2 EBC = 4,11-dimethyl-1,4,8,11tetraazabicyclo[6.6.2]hexadecane), which is exceptionally rigid and strongly coordinating.26 This cross-clamped cyclam is capable of supporting a MnIV-oxo adduct, which has been wellcharacterized spectroscopically.17,27−30 We envisioned that knowledge of the spectroscopic signatures of this MnIV-oxo complex would permit its identification if formed by O−O bond activation reactions of a MnIII-peroxo adduct. We were able to generate the [MnIII(O2)(Me2EBC)]+ complex by reacting [MnII(Cl)2(Me2EBC)] with O2− in CH2Cl2 at −60 °C and investigated the decay of this complex under various conditions (Scheme 2).31 The identification of [MnIII(O2)(Me2EBC)]+ and its decay products was greatly facilitated by the combined use of perpendicular- and parallelmode X-band electron paramagnetic resonance (EPR) spectroscopy. Although all reactions were also monitored by optical spectroscopy, only through the EPR experiments were we able to unambiguously determine Mn oxidation state changes and fully appreciate the complexity of the chemistry. The perpendicular-mode EPR spectrum of the starting [MnII(Cl)2(Me2EBC)] complex displays broad signals spanning a large field range, not uncommon for a high-spin S = 5/2 MnII center (Figure 2, top). After the reaction with O2−, the perpendicular-mode MnII signal disappears, and a sharp signal due to unreacted O2− appears. In parallel-mode, a six-line signal spanning 60−100 mT is observed. The field position of the six resonances, and the magnitude of the hyperfine splitting, are diagnostic of a mononuclear S = 2 MnIII center. Further support for the [MnIII(O2)(Me2EBC)]+ assignment was provided by analysis of Mn K-edge EXAFS data, which revealed Mn−O and Mn−N scatters at 1.85 and 2.10 Å, respectively. Although over 30 MnIII-peroxo adducts have been reported, only three (including a MnIII-hydroperoxo adduct32) have been characterized by Mn K-edge X-ray absorption spectroscopy (XAS). Considering the widespread use of Mn XAS to characterize biological and biomimetic Mn centers, the paucity of XAS data for MnIII-peroxo centers is somewhat surprising. We attribute the dearth of data to the following: (i) many MnIII-peroxo adducts cannot be generated in the high yields required for XAS experiments; and (ii) MnIII centers can be susceptible to photoreduction upon X-ray irradiation, which we observed in Mn XAS studies of [MnIII(O2)(TpPh2)].33 When formed in high yield, [MnIII(O2)(Me2EBC)]+ decays to give an orange solution, the major component of which is [MnIII(OH)(Cl)(Me2EBC)]+ (Scheme 2). In contrast, the decay of [Mn III (O 2 )(Me 2 EBC)] + in the presence of [MnII(Cl)2(Me2EBC)] gives rise to a pink solution, in which four separate Mn species are detectable by EPR spectroscopy (Figure 2, bottom). The perpendicular-mode spectrum is dominated by a signal centered at g = 2 (ca. 340 mT), due to a

mononuclear MnII decay product, likely representing only a small fraction of total Mn. Signals from unreacted [MnII(Cl)2(Me2EBC)] are also observed near 80 and 260 mT. A new, albeit weak, signal appears at g = 5.26 (ca. 125 mT) that has a field position and hyperfine coupling consistent with a mononuclear MnIV center. These EPR signals, as well as the optical signals associated with the pink solution, are quite similar to those of [MnIV(O)(OH)(Me2EBC)]+.29 The parallelmode EPR spectrum of the pink solution reveals a six-line signal from [MnIII(OH)(Cl)(Me2EBC)]+. Collectively, the data suggest that [Mn I I I (O 2 )(Me 2 EBC)] + reacts with [MnII(Cl)2(Me2EBC)] to give mononuclear MnIII and MnIV products. Presumably the bulk of the Me2EBC ligand disfavors formation of an oxo-bridged-MnIIIMnIV dimer. Unfortunately, the resulting MnIV product is quite unstable in CH2Cl2 and formation of [MnIII(O2)(Me2EBC)]+ in high yields was not possible in other solvents. Nonetheless, these observations suggest that the judicious use of ligand sterics can allow for the formation of MnIV centers from MnIII-peroxo adducts.

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MANGANESE(IV)-OXO COMPLEXES Our observations that the MnIII-peroxo species [MnIII(O2)(κ4N4py)]+ could be converted to the oxo-bridged [MnIIIMnIV(μO)2(κ4-N4py)2]3+ complex (Scheme 1)22 led us to consider routes for generating a terminal MnIV-oxo adduct using N4py. In 2013, our group25 and Nam and co-workers16 independently reported [MnIV(O)(N4py)]2+, which was generated by iodosobenzene (PhIO) oxidation of [MnII(N4py)(OTf)](OTf) in 2,2,2-trifluoroethanol (TFE). The presence of a terminal MnIV-oxo unit in [MnIV(O)(N4py)]2+ was supported by Xband EPR and Mn K-edge XAS data.25 To assess the ability of this MnIV-oxo to attack C−H bonds, we undertook kinetic studies of the reactivity of [MnIV(O)(N4py)]2+ toward hydrocarbons, using the disappearance of a near-IR absorption band of [MnIV(O)(N4py)]2+ (950 nm; 10 500 cm−1) upon the addition of substrate to determine rate constants. From these experiments, we showed that [MnIV(O)(N4py)]2+ was capable of attacking C−H bonds with bond dissociation free energies (BDFEs) < 90 kcal/mol. The second-order rate constant for 9,10-dihydroanthracene (DHA) oxidation (k2 = 3.6 M−1 s−1, 25 °C) was among the highest for nonporphyrinic MnIV-oxo complexes.25 While additional data provided support for a HAT mechanism, the basis for the relatively high reactivity of [MnIV(O)(N4py)]2+ was unclear. Both thermodynamic25 and excited-state reactivity35 arguments have been offered to explain the reactivity. We postulated that the high reactivity of [MnIV(O)(N4py)]2+ is caused by its high MnIII/IV reduction potential (+800 mV versus SCE). In comparison, the [MnIV(O)(OH)(H,MePytacn)]+ complex of Costas and co-workers reacts 100times more slowly with DHA than [MnIV(O)(N4py)]2+ and has a much lower MnIII/IV reduction potential (+50 mV versus SCE).21 However, the thermodynamic driving force for a HAT reaction depends on both the MnIII/IV reduction potential and the basicity of the MnIII-hydroxo product. The importance of basicity is highlighted by comparisons with the [MnIV(O)(H3buea)]− complex of Borovik and co-workers.34 This complex has an exceptionally low MnIII/IV potential of −570 mV versus SCE but shows a DHA oxidation rate only slightly less than that of [MnIV(O)(N4py)]2+ (k2 = 0.1 M−1 s−1 at 20 °C). Thus, a proper understanding of reactivity requires knowledge of the basicity of the MnIII-hydroxo product 2709


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Figure 3. Electron configurations for 4B1 and 4E states (left) and qualitative HAT reaction coordinate based on DFT computations from ref 35 (right).

complexes, which, at present, is lacking for the majority of these species. An alternative description of the enhanced HAT reactivity of [MnIV(O)(N4py)]2+ was offered by Nam and Shaik.35 Their DFT calculations suggested a two-state reactivity model, where a 4E excited state offers a lower-energy transition state for HAT than the 4B1 ground state (Figure 3). Because the 4E excited state differs from the 4B1 ground-state by an e(dxz,dyz) → b1(dx2−y2) excitation (i.e., a MnIV-based d−d transition), we setout to identify the 4E excited state using magnetic circular dichroism (MCD) spectroscopy. Excited states with actual, or near, orbital degeneracy, such as the 4E state in question, appear in an MCD spectrum as a pseudo-A term; a derivative-shaped signal whose intensity increases at low temperature. Electronic absorption and MCD data collected for [MnIV(O)(N4py)]2+ show that the near-IR absorption band at 10 500 cm−1 appears as a pseudo-A term in the corresponding MCD spectrum (Figure 4), allowing us to assign this near-IR feature as the 4B1 → 4E(e(dxz,dyz) → b1(dx2−y2)) transition.36 Electronic structure calculations and a graphical analysis of the pseudo-A term sign lent further support to the assignment. Having identified the 4E excited state through MCD experiments, we next used CASSCF/NEVPT2 computations to determine the influence of MnIV-oxo bond elongation on the energy and composition of this state (Figure 5). This approach follows that employed by Solomon and co-workers in their investigations of the two-state reactivity of FeIV-oxo complexes.37 Our CASSCF/NEVPT2 calculations revealed little relative stabilization of the 4E state along MnIV-oxo elongation (representative of the HAT reaction coordinate in the absence of substrate), although the CASSCF-wave function for this state developed significant MnIII-oxyl character (Figure 5). Thus, if the 4E state were stabilized by interactions with substrate, this state could be a reactive channel for HAT in MnIV-oxo complexes. To further probe the potential involvement of the 4E excited state in the reactivity of [MnIV(O)(N4py)]2+, we used N4py derivatives to perturb the 4E energy. Because the b1(dx2−y2) MO

Figure 4. Electronic absorption (top) and MCD (bottom) spectra of [MnIV(O)(N4py)]2+. Individual Gaussian curves (dashed black traces) and their sums (dashed blue traces) are displayed. The vertical line at 10 500 cm−1 shows correspondence between the near-IR absorption maxima and the center of the MCD pseudo-A term. Reproduced with permission from ref 36. Copyright 2016 American Chemical Society.

is Mn−N4py σ-antibonding, the electron-rich DMMN4py (DMMN4py = N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) should destabilize the 4E state, while the sterically encumbered 2pyN2Q (2pyN2Q = bis(2-pyridyl)-N,N-bis(2 quinolylmethyl)methanamine), which weakens the equatorial field, should stabilize the 4E state (Figure 6, top).38 XRD structures of the MnII complexes show increases in average Mn−N bond lengths, from 2.246 Å for [MnII(DMMN4py)(OTf)](OTf) to 2.279 Å for [MnII(2pyN2Q)(OH2)](OTf)2, consistent with the expected trend in equatorial donor strength. Each of these MnII complexes was converted to corresponding MnIV-oxo species upon reaction with PhIO in TFE, with spectroscopic data supporting the formulation of these species.38 The expected 2710


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Figure 5. CASSCF/NEVPT2 potential energy surfaces for 4B1 and 4E states (left). Configurations contributing to the CASSCF wave functions, with corresponding surface contour plots (right). The red ellipses in the configuration diagrams represent electron-accepting orbitals for HAT. Reproduced with permission from ref 36. Copyright 2016 American Chemical Society.

modulation of the 4E energy on the basis of equatorial field strength could be readily appreciated by comparing the near-IR absorption maxima of these complexes, which decrease in the order [MnIV(O)(DMMN4py)]2+ > [MnIV(O)(N4py)]2+ > [MnIV(O)(2pyN2Q)]2+ (Figure 6, center). Perturbations in the equatorial ligand-field caused by the DMM N4py and 2pyN2Q ligands also led to variations in HAT and oxygen-atom transfer (OAT) rates. Using a range of hydrocarbon substrates similar to those employed in our studies of [MnIV(O)(N4py)]2+, we determined second-order rate constants for reactions with [MnIV(O)(DMMN4py)]2+ and [MnIV(O)(2pyN2Q)]2+. These results are encapsulated in Figure 6 (bottom), which shows log(k2)́ values versus bond dissociation enthalpies for hydrocarbon oxidation by this set of MnIV-oxo complexes. For each substrate, oxidation by [Mn IV (O)( DMM N4py)] 2+ is ∼10-fold slower than for [Mn IV (O)(N4py)] 2+ , whereas oxidation by [Mn IV (O)(2pyN2Q)]2+ is ∼10-fold more rapid than for [MnIV(O)(N4py)]2+. The OAT ability of these complexes, assessed by thioanisole oxidation, followed the same trend, but with more dramatic rate variations. In this case, oxidation by [MnIV(O)(2pyN2Q)]2+ was 150- and 4000-fold more rapid than that for [MnIV(O)(N4py)]2+ and [MnIV(O)(DMMN4py)]2+, respectively. A comparison of log(k2′) for these HAT and OAT reactions versus the 4E energies of these three complexes shows a linear relationship, albeit with some scatter (Figure 7, top). This behavior is consistent with the involvement of the 4E state in these reactions. However, we also observe an excellent correlation between the log(k2′) values and the MnIII/IV reduction potentials (Figure 7, bottom), suggesting that changes in thermodynamic driving force can account for the observed variations in reactivity. Although, these comparisons are currently limited to a small set of complexes with neutral, N5 ligation, they demonstrate how subtle variations in the MnIV-oxo ligand field can modulate HAT and OAT reactions.

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Figure 6. Molecular structures (top), electronic absorption spectra (center), and comparison of log(k2′) versus substrate BDE (bottom) for [MnIV(O)(N4py)]2+, [MnIV(O)(DMMN4py)]2+, and [MnIV(O)(2pyN2Q)]2+. (Xan = xanthene, DHA = 9,10-dihydroanthracene, DPM = diphenylmethane, EtBn = ethylbenzene, Tol = toluene) Adapted with permission from ref 38. Copyright 2017 Wiley.

MANGANESE(III)-HYDROXO In 2014, our lab began exploring the Mn chemistry of the pentadentate, amide-containing dpaq ligand (dpaq = 2[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate). 2711


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Figure 7. Correlation of log(k2′) values for ethylbenzene (left) and thioanisole (right) oxidation as a function of the near-IR absorption maxima (top) and MnIII/IV reduction potentials (bottom) for [MnIV(O)(N4py)]2+ and its derivatives. Adapted with permission from ref 38. Copyright 2017 Wiley.

the second-order rate constant for xanthene oxidation by [MnIII(OH)(PY5)]2+ to 250 equiv of xanthene, we estimate kobs = 8 × 10−3 s−1, which is an order of magnitude faster than that of [MnIII(OH)(dpaq)]+. This difference in reactivity can be understood on the basis of the more positive MnII/III reduction potential of [MnIII(OH)(PY5)]2+ compared to [MnIII(OH)(dpaqH)]+ (+0.14 and −0.73 V, respectively, versus Fc/Fc+ in MeCN). The 870 mV difference in reduction potential might suggest that [MnIII(OH)(PY5)]2+ should be a far better HAT agent than [MnIII(OH)(dpaq)]+. Presumably, the reactivity of [MnIII(OH)(PY5)]2+ is dampened by its lower proton affinity, as assessed experimentally by a pKa of 13 for [MnII(OH2)(PY5)]+. In comparison, DFT computations indicated a far greater basicity for MnII(OH2)(dpaq)] (pKa = 21).15 We next investigated the ability of [MnIII(OH)(dpaq)]+ to attack O−H bonds, focusing on the highly activated bond of TEMPOH (2,2′,6,6′-tetramethylpiperidine-1-ol), as well as the stronger bonds of para-substituted-2,6-di-tert-butylphenols. TEMPOH oxidation by [MnIII(OH)(dpaq)]+ proceeded with a second-order rate constant (k2) of 0.13 M−1 s−1 at 25 °C, which was obtained by fitting the linear increase in kobs with increasing TEMPOH concentration (Figure 9, top). In contrast, kobs values for phenol oxidation showed saturation at higher substrate concentrations, suggestive of a rapid equilibrium prior to a rate-determining step (Figure 9, center). We determined both equilibrium constants (Keq) and firstorder rate constants (k1) for these steps for four parasubstituted-2,6-di-tert-butylphenols. The saturation behavior could arise from several possibilities, the most likely of which are a proton-transfer equilibrium followed by rate-determining

This ligand, developed by Hitomi and co-workers, was shown to support high-valent Fe oxidants and could be modified to tune steric and electronic properties.39 Our interest in dpaq stemmed from its monoanionic nature, which mimicked the net charge of amino-acid-derived ligands of mononuclear Mn enzymes (Figure 1). Our investigations revealed that [MnII(dpaq)](OTf) reacts with O2 in MeCN to generate the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq)]+ in 98% yield.14 Such clean reactivity with O2 is unusual for MnII complexes. XRD characterization of [MnIII(OH)(dpaq)]+ revealed a six-coordinate MnIII center with the hydroxo trans to the amide of dpaq (Figure 8, top-left). The high thermal stability of [MnIII(OH)(dpaq)]+ was unanticipated (t1/2 in MeCN at 25 °C of ∼26 days), as the XRD structure reveals no steric protection of the hydroxo ligand. Inspired by the HAT reactivity of the MnIII-hydroxo unit of MnLOX, we investigated the ability of [MnIII(OH)(dpaq)]+ to react with the activated C−H bond in xanthene.14 Xanthene provides an analogue of the polyunsaturated fatty acid substrates of MnLOX, which contain doubly allylic C−H bonds. At 50 °C, the electronic absorption signals of [MnIII(OH)(dpaq)]+ decayed in the presence of 250 equiv xanthene, yielding a pseudo-first-order rate constant (kobs) of 8 × 10−4 s−1. Although xanthene oxidation is slow, this represented only the second report of a synthetic MnIIIhydroxo unit attacking a C−H bond. The other example was [Mn III (OH)(PY5)] 2+ (PY5 = 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine; Figure 8, bottom-left), which features a neutral pentadentate ligand, and can attack a range of C−H bonds, including those of xanthene.13 By extrapolating 2712


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Figure 8. ORTEP diagrams of [MnIII(OH)(dpaq)]+, [MnIII(OH)(dpaq2Me)]+, [MnIII(OH)(PY5)]2+, and [MnIII(OH)(SMe2N4(tren))]+, with Mn− OH bond distance and selected rate constants. k2 is the second-order rate constant at 25 °C; kobs is the pseudo-first-order rate constant at 50 °C and 312.5 mM xanthene. *Values estimated (see text).

To begin to address these issues, we compared the reactivity of [MnIII(OH)(dpaq)]+ to its MnIII-methoxy analogue, [MnIII(OMe)(dpaq)]+,40 and the MnIII-hydroxo derivative [MnIII(OH)(dpaq2Me)]+, the latter of which contains a methyl-substituted quinoline (Figure 8, top-right).15 Both of these complexes resemble [MnIII(OH)(dpaq)]+ structurally, consisting of six coordinate MnIII−OR units with the OR− ligand trans to an amide. The [MnIII(OH)(dpaq2Me)]+ complex has additional steric bulk near the hydroxo ligand, which we envisioned would disrupt the formation of the phenol-based precursor complex. An additional consequence of this bulk is a longer Mn−N(quinoline) distance in [MnIII(OH)(dpaq2Me)]+ (elongated by 0.11 Å relative to [MnIII(OH)(dpaq)]+). This weakening of quinoline donation is manifested in the more positive reduction potential of [MnIII(OH)(dpaq2Me)]+ than [MnIII(OH)(dpaq)]+ (−0.62 and −0.74 V, respectively, versus Fc/Fc+ in MeCN). The presence of a MnIII-methoxy group in [MnIII(OMe)(dpaq)]+ had little effect on TEMPOH oxidation, with only a 1.6-fold reduction in k2 relative to [MnIII(OH)(dpaq)]+.40 However, the [MnIII(OMe)(dpaq)]+ complex did not show saturation behavior in reactions with phenols. One possibility is the additional steric bulk of the methoxy ligand destabilizes the formation of any precursor complex. Phenol oxidation by [MnIII(OH)(dpaq2Me)]+ also proceeded without saturation behavior, but with a k2 value ∼10-fold greater than that of

electron transfer, or formation of a precursor complex followed by rate-determining HAT. The latter scenario was supported by an H/D kinetic isotope effect for k1 and a linear correlation between log(k1) and phenol BDFE. In addition, there was no correlation between phenol pKa and the measured Keq, which is inconsistent with a proton-transfer equilibrium. These observations led us to propose the formation of H-bonded precursor complex prior to rate-determining HAT (Figure 9, center). The small equilibrium constant measured for most of the substituted phenols (Keq = 8−20) represents a minor stabilization relative to the free reactants. The reactivity of [MnIII(OH)(dpaq)]+ toward O−H bonds can be compared with that of the [MnIII(OH)(SMe2N4(tren))]+ complex of Kovacs and co-workers (Figure 8, bottom-right).5 TEMPOH oxidation by [MnIII(OH)(SMe2N4(tren))]+ proceeds 10 4 -fold more rapidly than [Mn III (OH)(dpaq)] + , but [MnIII(OH)(SMe2N4(tren))]+ is unable to oxidize O−H bonds stronger than that of TEMPOH (BDFE = 66.5 kcal/ mol). Thus, a comparison of the reactivities of these complexes offers a conundrum−the [MnIII(OH)(SMe2N4(tren))]+ complex is far more reactive toward TEMPOH, but the [MnIII(OH)(dpaq)]+ complex is able to attack the stronger O−H bonds of phenols. Such issues underscore the importance of exploring the basis of reactivity for midvalent metal oxidants as a means of better understanding HAT reactions in general. 2713


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Figure 9. Reaction schemes and pseudo-first-order rate constants (kobs) as a function of substrate concentration (−OH, red; −OD, blue) for [MnIII(OH)(dpaq)]+ with TEMPOH (top), 2,4,6-tri-tert-butylphenol (center), and [MnIII(OH)(dpaq2Me)]+ with 2,4,6-tri-tert-butylphenol (bottom).

[MnIII(OMe)(dpaq)]+. The [MnIII(OH)(dpaq2Me)]+ complex was also capable of oxidizing xanthene, with a rate 3-fold smaller than that of [MnIII(OH)(dpaq)]+.15 Given that [MnIII(OH)(dpaq2Me)]+ reacts with xanthene and phenols with rates similar to those of [MnIII(OH)(dpaq)]+, we were surprised when TEMPOH oxidation by [MnIII(OH)(dpaq2Me)]+ showed a dramatic rate enhancement. Remarkably, the k2 value obtained for TEMPOH oxidation by [MnIII(OH)(dpaq2Me)]+ at −35 °C is 30-fold larger than that of [MnIII(OH)(dpaq)]+ at 25 °C. Comparisons of activation parameters indicate a 240-fold rate difference at parity of temperature.15 DFT computations allowed us to understand the basis for the enhanced reactivity of [MnIII(OH)(dpaq2Me)]+ in TEMPOH oxidation and the muted reactivity in phenol and xanthene oxidation. For TEMPOH oxidation by [MnIII(OH)(dpaq)]+ and [MnIII(OH)(dpaq2Me)]+, DFT-computations yielded ΔG‡ values in exceptional agreement with their experimental counterparts (Figure 10, top) and reproduced the 2 kcal/mol lower barrier for the latter complex. The computations also predict a 4 kcal/mol greater driving force for TEMPOH oxidation by [MnIII(OH)(dpaq2Me)]+, consistent with the more positive reduction potential of this complex. The computational results provide strong evidence that the enhanced reactivity of [MnIII(OH)(dpaq2Me)]+ toward TEMPOH arises because of a greater driving force. Why then is this thermodynamic effect not manifested in phenol and xanthene

oxidation? An overlay plot of the DFT-calculated transition states for xanthene oxidation by [MnIII(OH)(dpaq)]+ and [MnIII(OH)(dpaq2Me)]+ (Figure 10, bottom) reveals an answer to this question. These plots show that the steric bulk of the 2Me-quinoline group in [MnIII(OH)(dpaq2Me)]+ requires a significantly different orientation of xanthene than in the case of [MnIII(OH)(dpaq)]+. Importantly, corresponding plots of the transition states for TEMPOH oxidation show virtually identical TEMPOH positions. We propose that steric effects, prominent for xanthene and phenols, dampen the thermodynamic advantage of [MnIII(OH)(dpaq2Me)]+ in HAT reactions. Because of these observations, future work on MnIII-hydroxo complexes will employ ligand modifications more remote from the hydroxo ligand.

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SUMMARY AND CHALLENGES By combining kinetic, spectroscopic, and computational techniques, we and others have been able to provide a new understanding of how Mn centers in biological and synthetic systems promote O−O bond cleavage reactions and oxidize O−H and C−H bonds. In this Account we have focused on the role of ligand sterics in controlling the type of high-valent intermediate formed upon decay of a MnIII-peroxo species and have highlighted the role of EPR spectroscopy in understanding the complexity of these decay processes. We have also demonstrated how combined experimental and computational studies of MnIV-oxo and MnIII-hydroxo adducts with system2714


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through electronic structure computations. Through systematic perturbations of ligand structures (dpaq for MnIII-hydroxo and N4py for MnIV-oxo), we have been able to identify how specific steric and electronic factors can influence both the thermodynamics and kinetics of HAT reactions. A persistent gap in much of this work is knowledge of experimental pKa values for MnIIaqua or MnIII-hydroxo species, which are required for a thorough understanding of thermodynamic driving force. Although we have had success in referencing calculated pKa values using closely related complexes for which experimental values are known, further validation of this approach is required. Future work will focus on evaluating the robustness of structure−reactivity using more diverse ligand architectures. Such studies should yield general principles that can be used to evaluate proposed mechanisms for manganese-dependent enzymes and design more reactive manganese complexes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Timothy A. Jackson: 0000-0002-3529-2715 Notes

The authors declare no competing financial interest. Biographies Derek Rice graduated from Truman State University in 2010 with a B.S. degree in chemistry. He is currently in his fifth year in the Jackson lab at the University of Kansas, where his research is focused on understanding how the ligand structure of manganese complexes influences proton-coupled electron-transfer reactivity.

Figure 10. Top: DFT-calculated reaction coordinate for [MnIII(OH)(dpaq)]+ (red) and [MnIII(OH)(dpaq2Me)]+ (blue) with TEMPOH. Experimental values are in parentheses. All values are in kcal mol−1. Bottom: Overlay of DFT-optimized transition states for xanthene oxidation by [MnIII(OH)(dpaq)]+ and [MnIII(OH)(dpaq2Me)]+. Adapted with permission from ref 15. Copyright 2016 American Chemical Society.

Allyssa Massie graduated from Baker University in 2013 with B.A. degrees in chemistry and Spanish. She is currently a fifth year graduate student in Tim Jackson’s lab at the University of Kansas, where her research project aims at understanding structural and electronic contributions to reactivity for MnIV-oxo complexes. Tim Jackson grew up in Portage, WI. He obtained his B.S. degree in chemistry from St. Cloud State University, where he performed undergraduate research on sol−gel-doped glasses in the lab of Professor Donald Neu. It was during his Ph.D. work in the lab of Professor Thomas Brunold at the University of WisconsinMadison that he gained his love for the rich spectroscopic properties of Mn centers. After completing work as an NIH postdoctoral fellow in the lab of Professor Lawrence Que Jr. at the University of Minnesota, he began his independent career at the University of Kansas in 2007. He is currently an Associate Professor of Chemistry.

atically perturbed ligand environments can provide new insights into trends in HAT reactivity. Although we have described how ligand sterics can be used to promote formation of mononuclear complexes from decay of MnIII-peroxo species, a recurrent obstacle in MnIII-peroxo activation is the formation of multiple products. Only in a limited number of cases does activation of a MnIII-peroxo species yield a single, well-defined product.4 Two recent examples suggest the solution to this dilemma lies in better control of hydrogen-bonding and/or proton delivery. For example, Borovik and co-workers have described a MnIII-peroxo adduct in a hydrogen-bonding cavity that reacts with hydrogenatom donors to give a single MnIII product.4 In addition, an electrochemical investigation of a MnIII-peroxo species revealed partitioning between Mn−O and O−O bond cleavage pathways depending on the strength of added acid.41 In HAT reactions for MnIII-hydroxo and MnIV-oxo units, we have seen that the starting point in understanding trends in reactivity lies in thermodynamic data, such as reduction potentials and, when available, pKa values. This follows the pioneering work of Mayer et al.42 Nonetheless, electronic and steric factors can modulate the kinetic−thermodynamic relationship, and these influences can often be illuminated

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ACKNOWLEDGMENTS

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REFERENCES

We thank all past graduate, postdoctoral, and undergraduate students who have contributed to this work, and we thank our collaborators Elodie Anxolabéhère-Mallart (Université Paris Diderot) and Ebbe Nordlander (Lund University) for their outstanding contributions. Our current research is supported by NSF (CHE-1565661) and DOE (DE-SC0016359).

(1) Sheng, Y.; Abreu, I. A.; Cabelli, D. E.; Maroney, M. J.; Miller, A.F.; Teixeira, M.; Valentine, J. S. Superoxide Dismutases and Superoxide Reductases. Chem. Rev. 2014, 114, 3854−3918.

2715


Article

Accounts of Chemical Research (2) Yano, J.; Yachandra, V. Mn4Ca Cluster in Photosynthesis: Where and How Water is Oxidized to Dioxygen. Chem. Rev. 2014, 114, 4175−4205. (3) Kanady, J. S.; Tsui, E. Y.; Day, M. W.; Agapie, T. A Synthetic Model of the Mn3Ca Subsite of the Oxygen-Evolving Complex in Photosystem II. Science 2011, 333, 733−736. (4) Shook, R. L.; Peterson, S. M.; Greaves, J.; Moore, C.; Rheingold, A. L.; Borovik, A. S. Catalytic Reduction of Dioxygen to Water with a Monomeric Manganese Complex at Room Temperature. J. Am. Chem. Soc. 2011, 133, 5810−5817. (5) Coggins, M. K.; Brines, L. M.; Kovacs, J. A. Synthesis and Structural Characterization of a Series of MnIIIOR Complexes, Including a Water-Soluble MnIIIOH That Promotes Aerobic Hydrogen-Atom Transfer. Inorg. Chem. 2013, 52, 12383−12393. (6) Ottenbacher, R.; Talsi, E.; Bryliakov, K. Direct Selective Oxidative Functionalization of C−H Bonds with H2O2: Mn-Aminopyridine Complexes Challenge the Dominance of Non-Heme Fe Catalysts. Molecules 2016, 21, 1454. (7) Porta, J.; Vahedi-Faridi, A.; Borgstahl, G. E. O. Structural Analysis of Peroxide-Soaked MnSOD Crystals Reveals Side-On Binding of Peroxide to Active-Site Manganese. J. Mol. Biol. 2010, 399, 377−384. (8) Codola, Z.; Lloret-Fillol, J.; Costas, M. Aminopyridine iron and manganese complexes as molecular catalysts for challenging oxidative transformations. Prog. Inorg. Chem. 2014, 59, 447−531. (9) Leto, D. F.; Jackson, T. A. Peroxomanganese complexes as an aid to understanding redox-active manganese enzymes. JBIC, J. Biol. Inorg. Chem. 2014, 19, 1−15. (10) Wennman, A.; Karkehabadi, S.; Oliw, E. H. Kinetic investigation of the rate-limiting step of manganese- and iron-lipoxygenases. Arch. Biochem. Biophys. 2014, 555−556, 9−15. (11) Su, C.; Sahlin, M.; Oliw, E. H. Kinetics of Manganese Lipoxygenase with a Catalytic Mononuclear Redox Center. J. Biol. Chem. 2000, 275, 18830−18835. (12) Cotruvo, J. A.; Stich, T. A.; Britt, R. D.; Stubbe, J. Mechanism of Assembly of the Dimanganese-Tyrosyl Radical Cofactor of Class Ib Ribonucleotide Reductase: Enzymatic Generation of Superoxide Is Required for Tyrosine Oxidation via a Mn(III)Mn(IV) Intermediate. J. Am. Chem. Soc. 2013, 135, 4027−4039. (13) Goldsmith, C. R.; Cole, A. P.; Stack, T. D. P. C−H Activation by a Mononuclear Manganese(III) Hydroxide Complex: Synthesis and Characterization of a Manganese-Lipoxygenase Mimic? J. Am. Chem. Soc. 2005, 127, 9904−9912. (14) Wijeratne, G. B.; Corzine, B.; Day, V. W.; Jackson, T. A. Saturation Kinetics in Phenolic O−H Bond Oxidation by a Mononuclear Mn(III)−OH Complex Derived from Dioxygen. Inorg. Chem. 2014, 53, 7622−7634. (15) Rice, D. B.; Wijeratne, G. B.; Burr, A. D.; Parham, J. D.; Day, V. W.; Jackson, T. A. Steric and Electronic Influence on Proton-Coupled Electron-Transfer Reactivity of a Mononuclear Mn(III)-Hydroxo Complex. Inorg. Chem. 2016, 55, 8110−8120. (16) Chen, J.; Lee, Y.-M.; Davis, K. M.; Wu, X.; Seo, M. S.; Cho, K.B.; Yoon, H.; Park, Y. J.; Fukuzumi, S.; Pushkar, Y. N.; Nam, W. A Mononuclear Non-Heme Manganese(IV)−Oxo Complex Binding Redox-Inactive Metal Ions. J. Am. Chem. Soc. 2013, 135, 6388−6391. (17) Shi, S.; Wang, Y.; Xu, A.; Wang, H.; Zhu, D.; Roy, S. B.; Jackson, T. A.; Busch, D. H.; Yin, G. Distinct Reactivity Differences of Metal Oxo and Its Corresponding Hydroxo Moieties in Oxidations: Implications from a Manganese(IV) Complex Having a Dihydroxide Ligand. Angew. Chem., Int. Ed. 2011, 50, 7321−7324. (18) Baglia, R. A.; Krest, C. M.; Yang, T.; Leeladee, P.; Goldberg, D. P. High-Valent Manganese−Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C−H Bond Cleavage. Inorg. Chem. 2016, 55, 10800−10809. (19) Wu, X.; Seo, M. S.; Davis, K. M.; Lee, Y.-M.; Chen, J.; Cho, K.B.; Pushkar, Y. N.; Nam, W. A Highly Reactive Mononuclear NonHeme Manganese(IV)−Oxo Complex That Can Activate the Strong C−H Bonds of Alkanes. J. Am. Chem. Soc. 2011, 133, 20088−20091. (20) Liu, W.; Groves, J. T. Manganese Catalyzed C−H Halogenation. Acc. Chem. Res. 2015, 48, 1727−1735.

(21) Garcia-Bosch, I.; Company, A.; Cady, C. W.; Styring, S.; Browne, W. R.; Ribas, X.; Costas, M. Evidence for a Precursor Complex in C-H Hydrogen Atom Transfer Reactions Mediated by a Manganese(IV) Oxo Complex. Angew. Chem., Int. Ed. 2011, 50, 5648− 5653. (22) Leto, D. F.; Chattopadhyay, S.; Day, V. W.; Jackson, T. A. Reaction landscape of a pentadentate N5-ligated MnII complex with O2•‑ and H2O2 includes conversion of a peroxomanganese(iii) adduct to a bis(μ-oxo)dimanganese(iii,iv) species. Dalton Transactions 2013, 42, 13014−13025. (23) Geiger, R. A.; Leto, D. F.; Chattopadhyay, S.; Dorlet, P.; Anxolabéhère-Mallart, E.; Jackson, T. A. Geometric and Electronic Structures of Peroxomanganese(III) Complexes Supported by Pentadentate Amino-Pyridine and -Imidazole Ligands. Inorg. Chem. 2011, 50, 10190−10203. (24) El Ghachtouli, S.; Vincent Ching, H. Y.; Lassalle-Kaiser, B.; Guillot, R.; Leto, D. F.; Chattopadhyay, S.; Jackson, T. A.; Dorlet, P.; Anxolabehere-Mallart, E. Electrochemical formation of MnIII-peroxo complexes supported by pentadentate amino pyridine and imidazole ligands. Chem. Commun. 2013, 49, 5696−5698. (25) Leto, D. F.; Ingram, R.; Day, V. W.; Jackson, T. A. Spectroscopic properties and reactivity of a mononuclear oxomanganese(iv) complex. Chem. Commun. 2013, 49, 5378−5380. (26) Hubin, T. J.; McCormick, J. M.; Alcock, N. W.; Busch, D. H. Topologically Constrained Manganese(III) and Iron(III) Complexes of Two Cross-Bridged Tetraazamacrocycles. Inorg. Chem. 2001, 40, 435−444. (27) Yin, G.; McCormick, J. M.; Buchalova, M.; Danby, A. M.; Rodgers, K.; Day, V. W.; Smith, K.; Perkins, C. M.; Kitko, D.; Carter, J. D.; Scheper, W. M.; Busch, D. H. Synthesis, Characterization, and Solution Properties of a Novel Cross-Bridged Cyclam Manganese(IV) Complex Having Two Terminal Hydroxo Ligands. Inorg. Chem. 2006, 45, 8052−8061. (28) Chattopadhyay, S.; Geiger, R. A.; Yin, G.; Busch, D. H.; Jackson, T. A. Oxo- and Hydroxo-Manganese(IV) Species: A Comparative Spectroscopic and Computational Study. Inorg. Chem. 2010, 49, 7530−7535. (29) Leto, D. F.; Massie, A. A.; Colmer, H. E.; Jackson, T. A. X-Band Electron Paramagnetic Resonance Comparison of Mononuclear MnIV-oxo and MnIV-hydroxo Complexes and Quantum Chemical Investigation of MnIV Zero-Field Splitting. Inorg. Chem. 2016, 55, 3272−3282. (30) Leto, D. F.; Jackson, T. A. Mn K-Edge X-ray Absorption Studies of Oxo- and Hydroxo-manganese(IV) Complexes: Experimental and Theoretical Insights into Pre-Edge Properties. Inorg. Chem. 2014, 53, 6179−6194. (31) Colmer, H. E.; Howcroft, A. W.; Jackson, T. A. Formation, Characterization, and O−O Bond Activation of a Peroxomanganese(III) Complex Supported by a Cross-Clamped Cyclam Ligand. Inorg. Chem. 2016, 55, 2055−2069. (32) So, H.; Park, Y. J.; Cho, K.-B.; Lee, Y.-M.; Seo, M. S.; Cho, J.; Sarangi, R.; Nam, W. Sarangi, R.; Nam, W. Spectroscopic Characterization and Reactivity Studies of a Mononuclear Nonheme Mn(III)Hydroperoxo Complex. J. Am. Chem. Soc. 2014, 136, 12229−12232. (33) Colmer, H. E.; Geiger, R. A.; Leto, D. F.; Wijeratne, G. B.; Day, V. W.; Jackson, T. A. Geometric and electronic structure of a peroxomanganese(iii) complex supported by a scorpionate ligand. Dalton Trans. 2014, 43, 17949−17963. (34) Parsell, T. H.; Yang, M.-Y.; Borovik, A. S. C−H Bond Cleavage with Reductants: Re-Investigating the Reactivity of Monomeric MnIII/ IV−Oxo Complexes and the Role of Oxo Ligand Basicity. J. Am. Chem. Soc. 2009, 131, 2762−2763. (35) Cho, K.-B.; Shaik, S.; Nam, W. Theoretical Investigations into C−H Bond Activation Reaction by Nonheme MnIVO Complexes: Multistate Reactivity with No Oxygen Rebound. J. Phys. Chem. Lett. 2012, 3, 2851−2856. (36) Leto, D. F.; Massie, A. A.; Rice, D. B.; Jackson, T. A. Spectroscopic and computational investigations of a mononuclear 2716


Article

Accounts of Chemical Research manganese(IV)-oxo complex reveal electronic structure contributions to reactivity. J. Am. Chem. Soc. 2016, 138, 15413−15424. (37) Srnec, M.; Wong, S. D.; Solomon, E. I. Excited state potential energy surfaces and their interactions in FeIV = O active sites. Dalton Trans. 2014, 43, 17567−17577. (38) Massie, A. A.; Denler, M. C.; Cardoso, L. T.; Walker, A. N.; Hossain, M. K.; Day, V. W.; Nordlander, E.; Jackson, T. A. Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)-Oxo Complexes. Angew. Chem., Int. Ed. 2017, 56, 4178−4182. (39) Hitomi, Y.; Arakawa, K.; Kodera, M. Electronic Tuning of Iron− Oxo-Mediated C-H Activation: Effect of Electron-Donating Ligand on Selectivity. Chem. - Eur. J. 2013, 19, 14697−14701. (40) Wijeratne, G. B.; Day, V. W.; Jackson, T. A. O-H bond oxidation by a monomeric MnIII-OMe complex. Dalton Trans. 2015, 44, 3295− 3306. (41) Ching, H. Y. V.; Anxolabehere-Mallart, E.; Colmer, H. E.; Costentin, C.; Dorlet, P.; Jackson, T. A.; Policar, C.; Robert, M. Electrochemical formation and reactivity of a manganese peroxo complex: acid driven H2O2 generation vs. O-O bond cleavage. Chem. Sci. 2014, 5, 2304−2310. (42) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications. Chem. Rev. 2010, 110, 6961−7001.

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