Formation, Characterization, and O–O Bond Activation of a

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Formation, Characterization, and O−O Bond Activation of a Peroxomanganese(III) Complex Supported by a Cross-Clamped Cyclam Ligand Hannah E. Colmer, Anthony W. Howcroft, and Timothy A. Jackson* Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States S Supporting Information *

ABSTRACT: Although there have been reports describing the nucleophilic reactivity of peroxomanganese(III) intermediates, as well as their conversion to high-valent oxobridged dimers, it remains a challenge to activate peroxomanganese(III) species for conversion to high-valent, mononuclear manganese complexes. Herein, we report the generation, characterization, and activation of a peroxomanganese(III) adduct supported by the cross-clamped, macrocyclic Me 2 EBC ligand (4,11-dimethyl-1,4,8,11tetraazabicyclo[6.6.2]hexadecane). This ligand is known to support high-valent, mononuclear MnIV species with welldefined spectroscopic properties, which provides an opportunity to identify mononuclear MnIV products from O−O bond activation of the corresponding MnIII−peroxo adduct. The peroxomanganese(III) intermediate, [MnIII(O2)(Me2EBC)]+, was prepared at low-temperature by the addition of KO2 to [MnII(Cl)2(Me2EBC)] in CH2Cl2, and this complex was characterized by electronic absorption, electron paramagnetic resonance (EPR), and Mn K-edge X-ray absorption (XAS) spectroscopies. The electronic structure of the [MnIII(O2)(Me2EBC)]+ intermediate was examined by density functional theory (DFT) and time-dependent (TD) DFT calculations. Detailed spectroscopic investigations of the decay products of [MnIII(O2)(Me2EBC)]+ revealed the presence of mononuclear MnIII− hydroxo species or a mixture of mononuclear MnIV and MnIII−hydroxo species. The nature of the observed decay products depended on the amount of KO2 used to generate [MnIII(O2)(Me2EBC)]+. The MnIII−hydroxo product was characterized by Mn K-edge XAS, and shifts in the pre-edge transition energies and intensities relative to [MnIII(O2)(Me2EBC)]+ provide a marker for differences in covalency between peroxo and nonperoxo ligands. To the best of our knowledge, this work represents the first observation of a mononuclear MnIV center upon decay of a nonporphyrinoid MnIII−peroxo center.

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peroxo ligand (H3bupa2−, H2bpaa−; for a full list of ligand abbreviations, see ref 34),26,27,34,35 a series of neutral tetradentate aminopyridyl ligands, (L7py2R),34,36 and a series of neutral macrocyclic ligands (TMC).18,23,25,34 With the TMCtype ligands, the rate of deformylation was shown to increase as the electron donating ability of the ligand trans to the peroxo moiety increased within a series of axial ligand substituents.18,23,25 In contrast, a series of L7py2R ligands displayed decreasing rates as a function of increased steric hindrance around the peroxo unit, although the correlation between reactivity and ligand structure was not very strong.34,36 Peroxomanganese(III) species have also been proposed to undergo O−O activation by reaction with acids and other metals to form high-valent species. Groni et al. reported a peroxomanganese(III) species supported by an aminopyridyl, pentadentate ligand (mL52), [MnIII(O2)(mL52)]+, that reacted with an equivalent of [MnIII(OH2)(mL52)]3+ in basic, aqueous

INTRODUCTION Manganese-containing enzymes are capable of a wide variety of biological functions, often involving reaction with dioxygen and its derivatives.1−4 Peroxomanganese(III) adducts5 are proposed as intermediates in many of these reactions, including the detoxification of reactive oxygen species by Mn superoxide dismutase,3,6−10 the conversion of nucleotides to deoxynucleotides by Mn ribonucleotide reductase,11,12 and oxalate degradation by oxalate oxidase13,14 and oxalate decarboxylase.15−17 While there are many examples of synthetic modeling of these active sites and their corresponding reactivity,5,18−32 a description of factors directing the reactivity of specific Mn− oxygen intermediates, especially peroxomanganese(III) adducts, is lacking. One of the most well-known types of reactivity of MnIII− peroxo species is the deformylation of aldehydes, which is driven by the nucleophilicity of the peroxo moiety (Scheme 1a).33 This reactivity has been observed for a wide range of supporting ligand structures, such as anionic tripodal ligands with an intramolecular hydrogen bonding network around the © XXXX American Chemical Society

Received: October 16, 2015

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DOI: 10.1021/acs.inorgchem.5b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Reactivity of Peroxomanganese(III) Complexes, Highlighting the Nucleophilic Reactivity (a−c) and O−O Bond Activation upon Electrochemical (d) or Chemical (e) Reductiona

a

Ligand abbreviations are defined in ref 34.

that have not been amenable to characterization. In fact, although peroxo-bridged dimanganese intermediates are invoked in several decay pathways, there is only one example of a peroxo-bridged dimanganese species, the [MnIII2(trans-μO2)(SMe2N4(6-Me-DPEN))2]2+ complex reported by Kovacs and co-workers.34,40 In this work, we investigate the reactivity of the peroxomanganese(III) species, [MnIII(O2)(Me2EBC)]+, supported by a neutral, cross-clamped macrocyclic ligand (Me2EBC, Scheme 2). This Me2EBC ligand provides a rigid,

solution to form a bis(μ-oxo)dimanganese(III,IV) species, presumably via O−O bond cleavage of an unobserved μperoxodimanganese(III,III) intermediate (Scheme 1b).34,37 Similarly, the pentadentate N4py ligand supported a peroxomanganese(III) species that likely undergoes O−O bond cleavage upon reaction with an equivalent of [MnII(N4py)(OTf)]+, forming a bis(μ-oxo)dimanganese(III,IV) complex (Scheme 1c).34,38 Electrochemical MnIII− peroxo activation was reported for a complex supported by a pentadentate N4O− (phenolato-containing) ligand.34,39 In that study, the strength of the added acid directed the electrochemical reaction. With a weak acid, concerted two-electron reduction and O−O bond cleavage occurred; however, characterization of the products of this reaction was hindered by their instability. With added strong acid, the Mn−O bond was broken and H2O2 was released upon electrochemical reduction (Scheme 1d). Finally, Shook et al. reported O−O bond activation of a peroxomanganese(III) species supported by the H3bupa2− ligand by reaction with electrophilic substrates that serve as H-atom donors, i.e., diphenylhydrazine and hydrazine (Scheme 1e).27,34 Thus, in all cases, activation of the MnIII−peroxo unit is coupled with chemical or electrochemical reduction, with the electrons supplied by a second Mn center, a sacrificial reductant, or an electrode. These synthetic complexes, as well as the enzymatic active sites, are structurally and electronically diverse, and the factors that direct specific reactivity are not well understood. Moreover, in most cases, MnIII−peroxo activation has led to either the formation of multinuclear, high-valent species, or the fleeting formation of mononuclear, high-valent intermediates

Scheme 2. Me2EBC Ligand (Left) and [MnIII(O2)(Me2EBC)]+ (Right)

stable framework that is able to support a range of Mn oxidation states. In particular, this ligand is known to support the high-valent oxidants [MnIV(O)(OH)(Me2EBC)]+ and [MnIV(OH)2(Me2EBC)]2+ that are well-characterized structurally and spectroscopically.41−43 Thus, these MnIV species have established spectroscopic fingerprints with which to compare to intermediates formed upon activation of the [MnIII(O2)B

DOI: 10.1021/acs.inorgchem.5b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Preparation of [MnIV(O)(OH)(Me2EBC)]+. MnIV(OH)2(Me2EBC)]2+ was prepared from [MnII(Cl)2(Me2EBC)] and H2O2 and NH4PF6 in H2O as reported previously.44 A 2 mM solution of [MnIV(OH)2(Me2EBC)]2+ was prepared in a 3:1 CH2Cl2/ acetone mixture and reacted with 1 equiv of NEt3 in CH2Cl2 at −30 °C to form [MnIV(O)(OH)(Me2EBC)]+. Formation of [MnIV(O)(OH)(Me2EBC)]+ was monitored by electronic absorption and EPR spectroscopies. EPR Experiments. A 2 mM sample of [MnIII(O2)(Me2EBC)]+ was prepared by using a precooled pipet to transfer 250 μL of the blue solution in CH2Cl2 at −60 °C to a precooled quartz EPR tube that was quickly flash frozen in liquid N2. A frozen sample of the orange solution (250 μL) was prepared from the decay of a 2 mM solution of [MnIII(O2)(Me2EBC)]+ at 25 °C and transferred to a quartz EPR tube and flash frozen in liquid N2. An EPR sample of the independently prepared 2 mM [MnIV(O)(OH)(Me2EBC)]+ complex (250 μL) in 3:1 CH2Cl2/acetone was prepared at −30 °C and transferred to a precooled quartz EPR tube and flash frozen in liquid N2. An EPR sample of the pink solution was prepared from the decay of a 5 mM solution of [MnIII(O2)(Me2EBC)]+ that was formed with 4 equiv of KO2 and 20 equiv of 18-crown-6 at −60 °C in CH2Cl2. A 250 μL volume of this pink solution was quickly transferred to a precooled quartz EPR tube and flash frozen in liquid N2. An additional sample of the pink solution (250 μL) was prepared from the decay of a 2 mM solution of [MnIII(O2)(Me2EBC)]+ that was prepared with 2 equiv of KO2 and 4 equiv of 18-crown-6 at −60 °C in CH2Cl2 and flash frozen in a precooled quartz EPR tube. EPR samples of [MnII(Cl)2(Me2EBC)] were prepared by transferring 250 μL of 2 mM solutions of [MnII(Cl)2(Me2EBC)] in CH2Cl2 or in 1:10 butyronitrile/CH2Cl2 mixture and flash frozen in liquid N2. EPR spectra were acquired on an X-band (9 GHz) Bruker EMXPlus spectrophotometer with an Oxford ESR900 continuous-flow liquid helium cryostat and an Oxford ITC503 temperature system. Paralleland perpendicular-mode spectra were collected using a Bruker ER4116DM dual-mode cavity. All spectra were collected under nonsaturating conditions at 20 dB (for [MnIII(O2)(Me2EBC)]+ and the orange and pink solutions) and 38 dB for [MnII(Cl)2(Me2EBC)], with frequencies near 9.3918 GHz (parallel mode) or 9.6375 GHz (perpendicular mode) microwave frequency, 0.6 mT modulation amplitude, 100 kHz modulation frequency, and 163 ms time constant. The Matlab-based EPR simulation software, EasySpin, developed by Stoll,45 was used to simulate EPR spectra and define zero-field-splitting parameters. Mn K-Edge X-ray Absorption Spectroscopy (XAS). An XAS sample of [MnIII(O2)(Me2EBC)]+ was prepared from 400 μL of a 15 mM solution in CH2Cl2 that was frozen in a precooled sample cup. XAS data were also collected on a frozen solution of a 20 mM sample in CH2Cl2. XAS data for [MnIII(O2)(Me2EBC)]+ were collected on beamline X3B at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (storage ring conditions, 2.8 GeV, 100−300 mA) using previously reported parameters.42 Contamination of higher harmonic radiation was minimized using a harmonic rejection mirror, manganese foil spectra were simultaneously recorded for internal energy calibration, and the first inflection point of the Kedge energy was assigned to 6539.0 eV. Spectra were measured with 5 eV steps below the edge (6359−6529 eV), 0.3 eV steps in the edge region (6529−6564 eV), and steps corresponding to 0.05 Å−1 above the edge in the EXAFS region. XAS data for a frozen sample of the orange solution were collected at beamline 2-2 at Stanford Synchrotron Radiation Lightsource (SSRL) using similar data collection parameters. EXAFS data reduction and averaging were performed using the program EXAFSPAK.46 The pre-edge background intensity was removed by fitting a Gaussian function to the pre-edge background and then subtracting this function from the whole spectrum. The spectrum was also fit with a three-segment spline with fourth-order polynomial components to remove low-frequency background. EXAFS refinement was carried out on k3χ(k) data, using phase and amplitude functions obtained from FEFF, version 6,47 and structural models of [MnIII(O2)(Me2EBC)]+ and [MnIII(OH)(Cl)(Me2EBC)]+ with DFT-

(Me2EBC)]+ species. Here we describe the characterization of [MnIII(O2)(Me2EBC)]+ by a variety of spectroscopic methods, such as electronic absorption, electron paramagnetic resonance (EPR), and Mn K-edge X-ray absorption (XAS) spectroscopies. We also investigated the decay products of [MnIII(O2)(Me2EBC)]+. When formed in high yields, [MnIII(O2)(Me2EBC)]+ decays slowly to give a mononuclear MnIII product. In contrast, when formed in lower yields (∼60%), or when treated with [MnII(Cl)2(Me2EBC)], the MnIII−peroxo species decays to give a mononuclear MnIV species, among other products. The MnIV adduct is unstable in CH2Cl2 and decays to a mononuclear MnIII complex. Potential pathways for this observed chemistry are proposed.

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MATERIALS AND METHODS

Chemicals and reagents used were ACS reagent grade or better and were obtained through commercial vendors. Electronic absorption experiments were performed on a Cary 50 Bio spectrophotometer (Varian) interfaced with a Unisoku cryostat (USP-203-A). Electrospray ionization mass spectrometry (ESI-MS) data were collected on an LCT Primers MicroMass electrospray ionization time-of-flight instrument. EPR spectra were collected on a Bruker EMXPlus instrument with a dual-mode cavity. Preparation of [MnIII(O2)(Me2EBC)]+. Optimized formation of the blue species [MnIII(O2)(Me2EBC)]+ was achieved by reacting a 2 mM solution of [MnII(Cl)2(Me2EBC)] in CH2Cl2 at −60 °C with a KO2 slurry. The KO2 slurry was prepared under ambient conditions by stirring a solution of 100 equivalents (equiv) of KO2 and 100 equiv of 18-crown-6 in 1 mL of butyronitrile for 10 min. A 100 μL volume (10 equiv of KO2 and 18-crown-6) of the slurry was added via syringe to a cuvette containing a 2 mM solution of [MnII(Cl)2(Me2EBC)] in CH2Cl2 in a cryostat at −60 °C. The addition of the slurry generates a deep blue intermediate with intense features in the visible region that can be monitored by electronic absorption spectroscopy. The use of fewer equivalents of KO2 resulted in a lower yield of the blue intermediate, as assessed by electronic absorption spectroscopy, and a thermal decay reaction to additional intermediates. The thermal decay reaction was optimized to maximize formation of the ensuing intermediates by preparing a slurry with 40 equiv of KO2 and 200 equiv of 18-crown-6. When 100 μL (4 equiv of KO2 and 20 equiv of 18-crown-6) of this less concentrated slurry was added to a 5 mM solution of [MnII(Cl)2(Me2EBC)] in CH2Cl2 at −60 °C, [MnIII(O2)(Me2EBC)]+ was generated in a yield 60% of that observed when using 10 equiv of KO2. Preparation of Room-Temperature Decay Products of [MnIII(O2)(Me2EBC)]+. When [MnIII(O2)(Me2EBC)]+ was prepared using 10 equiv of KO2 in CH2Cl2 at −60 °C, an orange solution was formed as the thermal decay product upon warming the solution of [MnIII(O2)(Me2EBC)]+ to 25 °C. Excess 18-crown-6 was removed from this solution by filtering the solution with a syringe filter. Crystallization at room temperature by ether diffusion into CH2Cl2 yielded orange crystals which were used for subsequent ESI-MS and XAS experiments. This orange solution was also observed as the final thermal decay product of [MnIII(O2)(Me2EBC)]+ prepared with 4 equiv of KO2 in CH2Cl2 at −60 °C. Preparation of Low-Temperature Decay Product of [MnIII(O2)(Me2EBC)]+. When [MnIII(O2)(Me2EBC)]+ was prepared with 4 equiv of KO2 in CH2Cl2 at −60 °C, a pink solution formed as the initial thermal decay product. Under these conditions, the products showed low solubility in CH2Cl2 at −60 °C and rapidly formed a purple precipitate that could not be isolated. When [MnIII(O2)(Me2EBC)]+ was instead prepared using 2 equiv of KO2 in CH2Cl2 at −60 °C, the pink chromophore formed in lower yield and no precipitate was observed. Under these conditions, the pink solution converts to an orange solution over 15 min. The pink solution could also be generated when [MnIII(O2)(Me2EBC)]+, prepared in high yield using 10 equiv of KO2, was treated with 1 equiv of [MnII(Cl)2(Me2EBC)] in CH2Cl2 at −60 °C. C

DOI: 10.1021/acs.inorgchem.5b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry optimized coordinates. For each fit, the parameters r (average distance between Mn and scattering atom) and σ2 (Debye−Waller factor) were optimized, while n, the number of atoms in the shell, was kept fixed. n was varied by integer steps systematically. The goodness of fit (GOF) was evaluated by the parameter F, where F = ∑(χcalcd − χexpt)2/N, and N is the number of data points. The threshold energy, E0, in electronvolts (k = 0 point) was kept at a common, variable value for every shell of a given fit. Density Functional Theory (DFT) Calculations. All DFT computations were performed with the ORCA 3.0.1 software package.48 Models of [MnIII(O2)(Me2EBC)]+ and [MnIII(OH)(Cl)(Me 2EBC)]+ were built using the X-ray coordinates of the [MnII(Cl)2(Me2EBC)] complex43 and replacing both of the chloride ions with a side-on peroxo moiety for [MnIII(O2)(Me2EBC)]+ and one of the chloride ions with a hydroxide ligand for [MnIII(OH)(Cl)(Me2EBC)]+. These structures were optimized using the Becke− Perdew (BP86) functional49,50 and converged to the S = 2 spin state using the SVP (Ahlrichs split valence polarized)51,52 basis sets with SV/J auxiliary basis sets for carbon and hydrogen atoms with the larger TZVP (Ahlrichs triple-ζ valence polarized) and the auxiliary TZV/J basis sets for manganese, nitrogen, oxygen, and chloride atoms. The resolution of identity (RI) approximation53 was used for all geometry and frequency calculations. Optimized geometries were verified as points of minimum energy with frequency calculations to ensure no negative frequencies. Solvation effects were included with COSMO, as implemented in ORCA,54 with acetone (for consistency and comparison with previous study41,42) and dichloromethane. Cartesian coordinates for all DFT-optimized structures are included in Supporting Information (Tables S1 and S2). Electronic transition energies were calculated for [MnIII(O2)(Me2EBC)]+ using the timedependent density functional theory (TD-DFT) method55−57 within the Tamm−Dancoff approximation58,59 using the B3LYP functional60−62 with the SVP basis set on carbon and hydrogen atoms and the larger TZVP basis set on all other atoms. Forty excited states were calculated by including all one-electron transitions within ±3 hartrees of the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energies. TD-DFT-calculated electronic absorption spectra were generated using the calculated transition energies and oscillator strengths, assuming that the electronic transitions are well-described by Gaussian functions with full width at half-maxima (ν1/2) of 2000 cm−1, and that the extinction coefficient (εmax) is related to the oscillator strength ( f) as follows: f = (4.61 × 10−9)(εmaxν1/2). The spin−orbit coupling term (DSOC) for the zero-field-splitting parameters was calculated using the coupledperturbed method, as shown to be appropriate for use with hybrid functionals.63 The spin−spin term (Dss) was calculated with spin density from unrestricted natural orbitals (UNO) to avoid known errors that occur when using spin-unrestricted Kohn−Sham functions to treat this term.64,65 Molecular orbitals (MOs) and electron density difference maps (EDDMs) were visualized with isosurface plots generated using the gOpenMol program66,67 with isodensity values of 0.05 and 0.005 b−3, respectively. Mn K-edge XAS pre-edge data for [MnIII(O2)(Me2EBC)]+ and [MnIII(OH)(Cl)(Me2EBC)]+ were calculated using the TD-DFT method with the B3LYP functional and the def2-TZVP(-f) basis set68 on all atoms and no RI approximation. Scalar relativistic effects were accounted for at the ZORA level, and a dense integration grid (ORCA Grid4) and tight SCF convergence criteria were utilized. Calculated spectra were shifted to higher energy by 32.6 eV, as was determined previously to account for systematic errors in the computational treatment.69 Calculated areas (A) were obtained from the calculated intensity (I) from the correlation A = 6.01(I) + 1.79, as established in a previous study of monomeric MnII and MnIII complexes.69

Figure 1. (A) Formation of blue intermediate, [MnIII(O2 )(Me2EBC)]+, (blue solid trace) from the reaction of 2 mM [MnII(Cl)2(Me2EBC)] with 10 equiv of KO2 and 18-crown-6 in CH2Cl2 at −60 °C. (B) Decay of [MnIII(O2)(Me2EBC)]+ (formed with 10 equiv of KO2 and 2 mM [MnII(Cl)2(Me2EBC)]) to the orange solution (red solid trace) in CH2Cl2 at 0 °C. (C) Conversion of [MnIII(O2)(Me2EBC)]+ (formed with 4 equiv of KO2 and 5 mM [MnII(Cl)2(Me2EBC)]) to the pink solution (black solid trace) in CH2Cl2 at −60 °C. (D) Conversion of the pink solution to the orange solution in CH2Cl2 at −60 °C (formed with 4 equiv of KO2 and 5 mM [MnII(Cl)2(Me2EBC)]).

equiv of KO2 and 18-crown-6 in CH2Cl2 at −60 °C and is stable indefinitely at this temperature. The use of 10 equiv of KO2 is necessary to maximize the formation of the blue intermediate (Figure S1 in the Supporting Information). The superoxide radical is known to decay upon reaction with CH2Cl2,70 decreasing the amount of viable superoxide radical available for reaction with [MnII(Cl)2(Me2EBC)] (Figure S2 in the Supporting Information). The blue intermediate displays electronic absorption features at 650 and 400 nm (ε = 530 and 185 M−1 cm−1, respectively, Figure 1A, blue trace).71 When warmed to 0 °C, the blue solution decays to give an orange solution within ∼3 min (Figure 1B). This orange solution is characterized by weak electronic absorption bands at 410 and 890 nm, which are unchanged even after weeks at room temperature. When [MnII(Cl)2(Me2EBC)] is reacted with less than 10 equiv of KO2, the formation of the blue intermediate is still observed, but it forms in substantially lower yields and decays at −60 °C to give a pink solution. For example, when [MnII(Cl)2(Me2EBC)] is treated with 4 equiv of KO2 and 20 equiv of 18-crown-6, the blue intermediate forms in 60% yield (relative to the amount formed with 10 equiv of KO2) and, at −60 °C, converts over the course of 40 min to the pink solution, which has absorption features at 540 and 930 nm (ε = 66 and 47 M−1 cm−1, respectively; Figure 1C, black solid trace). Under these conditions, precipitation is observed with formation of a purple solid that could not be isolated. When

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RESULTS AND ANALYSIS The electronic absorption spectrum of the colorless [MnII(Cl)2(Me2EBC)] complex shows no distinct features at energies below 400 nm (Figure 1A, black trace). A blue species is formed upon reaction of [MnII(Cl)2(Me2EBC)] with 10 D

DOI: 10.1021/acs.inorgchem.5b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 3. Reactivity Profile of [MnII(Cl)2(Me2EBC)] with 10 and 4 equiv of KO2a

a

The rate of conversion of the blue intermediate to the pink solution, and of the pink solution to the orange solution, depends on the equivalents of KO2 used to form the blue intermediate. Faster decay rates are observed using fewer equivalents of KO2. The 60% yield of the blue intermediate is relative to the amount formed when using 10 equiv of KO2.

the blue intermediate is formed using 2 equiv of KO2, no precipitation is observed. In this case, the pink solution decays within 60 min at −60 °C to an orange solution with electronic absorption signals identical to those of the room-temperature decay product (Figure 1D, red solid trace). This decay reaction occurs within seconds at 25 °C. Thus, the decay of the blue intermediate is dependent on the yield in which it is formed. When the blue intermediate is formed in high yield using 10 equiv of KO2, it is indefinitely stable at −60 °C, but decays at higher temperatures to give an orange solution. In contrast, if the blue intermediate is formed in lower yields, it decays at −60 °C to give a pink solution, which evolves to the orange solution. This observed chemistry is summarized in Scheme 3. The characterization of the blue intermediate and the components of the pink and orange solutions are explored in the following sections. Characterization of the Blue Intermediate by Electronic Absorption and EPR Spectroscopies. The transition energies and extinction coefficients (ε) of the blue intermediate are similar to those observed for peroxomanganese(III) species. For example, MnIII−peroxo adducts supported by the N4py, mL52, and imL52 ligands displayed λmax values of 617, 585, and 542 nm, respectively, with corresponding ε values of 280, 335, and 484 M−1 cm−1.5,21,37,38 All attempts to characterize the blue intermediate by mass spectrometry, which has seen much use in characterization of MnIII−peroxo adducts, proved unsuccessful. Because of the large excess of 18-crown-6 required for optimal formation, only 18-crown-6 was observed in ESI-MS experiments for the blue solution. X-band parallel-mode EPR spectra of the blue intermediate at 5 K showed a six-line signal at geff = 7.96, indicative of a mononuclear MnIII species (Figure 2, black trace). The hyperfine splitting, A = 6.84 mT, is in excellent agreement with that observed for peroxomanganese(III) species (6−7 mT).5 A plot of the product of signal intensity and temperature versus temperature of the geff = 7.96 signal from 5 to 30 K reveals an initial drop and a leveling at higher temperatures (Figure 2, inset). Such temperature-dependent behavior indicates that the ms = ±2 doublet, which gives rise to the geff = 7.96 EPR signal, is lowest in energy (D < 0 cm−1).72 To date, all MnIII−peroxo species have D < 0 cm−1.5 The 5 K data were well simulated with S = 2, g = 1.98, A = 6.8 mT, D = −1.8 cm−1, and E/D = 0.10(2) (Figure 2, red trace), with the goodness of fit largely dependent on the magnitude of E/D.

Figure 2. The 5 K parallel-mode EPR spectrum of the blue intermediate (black trace) and simulation (red trace) with D = −1.8 cm−1 and E/D = 0.10(2). (inset) Plot of signal intensity (at 84.4 mT) times temperature versus temperature (black dots) and fit (red trace) yielding D = −2(1) cm−1.

The D value was refined to −2(1) cm−1 through fits of the variable-temperature EPR data (Figure 2, inset). The 5 K perpendicular-mode EPR spectrum of the blue intermediate shows an axial signal at g = 1.99 that is characteristic of the superoxide radical (Figure S3, black trace, in the Supporting Information). However, when the blue intermediate is formed using 10 equiv of KO2, signals from the [MnII(Cl)2(Me2EBC)] starting material are not observed, suggesting complete consumption of this species. In fact, the only EPR signals observed are from unreacted superoxide and the mononuclear MnIII species. In contrast, when the blue intermediate is formed using less than 10 equiv of KO2, a perpendicular-mode signal is observed at g = 1.98 with A = 8.02 mT, which is associated with a mononuclear MnII species (Figure S3, red trace, in the Supporting Information). As the formation of the blue intermediate in maximum yield required 10 equiv of KO2, we monitored the lifetime of the excess superoxide radical in solution prior to conducting reactivity studies. This was to determine if unreacted superoxide was influencing any further chemistry. The decay of the superoxide radical in CH2Cl2 in the absence of [MnII(Cl)2(Me2EBC)] was monitored over 150 min by 5 K perpendicular-mode EPR experiments (Figure S2 in the Supporting Information). Aliquots of a solution with 10 equiv of KO2 and 10 equiv of 18-crown-6 in CH2Cl2 at −60 °C were taken over time and frozen in EPR tubes. At 150 min, the EPR E

DOI: 10.1021/acs.inorgchem.5b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Fourier transform peaks at 2.03 and 2.35 Å−1 (Figure 4, right). However, these fits resulted in a negative Debye−Waller factor for the shell of N atoms at 2.12 Å and a very large Debye− Waller factor for the shell of O atoms at 1.90 Å, which are physically unreasonable. We note that these fitting parameters were observed to be correlated, so these Debye−Waller factors could be of limited accuracy. In any case, the fit of the single N shell at 2.10 Å (fit 4) is an acceptable average of the four N distances calculated from DFT computations (2.28 Å, vida infra). Finally, a small peak at 3.3 Å was best fit with a shell of 4 C atoms at 2.92 Å and a shell of 6 C atoms at 3.44 Å. Overall, these fitting results are comparable to the recently reported EXAFS characterization of a peroxomanganese(III) species supported by the TMC ligand. In that case, the data were wellfit with a shell of 2 O atoms at 1.88 Å and a shell of 4 N atoms at 2.24 Å, as well as shells corresponding to scatterers from the TMC ligand.73 Taken together, the edge energy and EXAFS fit provide excellent support for our assignment of the blue intermediate as the side-on peroxomanganese(III) species, [MnIII(O2)(Me2EBC)]+. A full examination of the pre-edge region offers useful insight into the electronic structure of the peroxomanganese(III) complex. The pre-edge feature of [MnIII(O2)(Me2EBC)]+ occurs at 6540.8 eV, which lies between the pre-edge peak energies of the MnII and MnIV complexes supported by the Me2EBC ligand42 (Table 2, Figure S4). Analysis of the area of the pre-edge feature supplies information about the geometric environment of the MnIII center. As the environment about the Mn center becomes less centrosymmetric and the amount of 4p character in the 3d manifold increases, μel-forbidden 1s → 3d transitions are able to gain intensity by mixing with μel-allowed 1s → 4p transitions. [MnIII(O2)(Me2EBC)]+ exhibits a preedge area of 9.5 (10−2 eV), which is intermediate between the previously studied Me2EBC complexes. Specifically, the least centrosymmetric [MnIV(O)(OH)(Me2EBC)]+ complex displays a larger area of 14.2 × 10−2 eV and the more centrosymmetric [MnIV(OH)2(Me2EBC)]2+ and [MnII(Cl)2(Me2EBC)] complexes display smaller areas (6.9 and 3.9 10−2 eV, respectively) (Table 2). A detailed analysis of the specific transitions involved in the pre-edge of [MnIII(O2)(Me2EBC)]+, as well as the basis of the large pre-edge intensity, is discussed in the section DFT Computations and Electronic Structure of [MnIII(O2)(Me2EBC)]+. DFT Computations and Electronic Structure of [MnIII(O2)(Me2EBC)]+. DFT calculations were used to describe the electronic structure of [MnIII(O2)(Me2EBC)]+, as well as to provide a basis of comparison for X-ray absorption analysis. Coupled-perturbed (CP) DFT calculations were utilized to calculate the D-tensor orientation for [MnIII(O2)(Me2EBC)]+. The z-axis lies along the elongated Naxial−Mn−Naxial axis, and the y-axis bisects the O−O bond (Figure S5, top left, in the Supporting Information). The calculated Mn−O bond lengths (1.868 Å) are in excellent agreement with the EXAFSdetermined distance of 1.85 Å and with those observed in other peroxomanganese(III) adducts.5 The O−O bond of 1.450 Å is typical of a side-on bound peroxo. Calculated Mn−N bond lengths show two shorter Mn−Nequatorial bonds of 2.132 and 2.133 Å, nearly identical to the Mn−Nequatorial distance in the EXAFS data (2.10 Å). DFT-calculated Mn−Naxial bonds of 2.427 and 2.432 Å are longer than the EXAFS-determined average (2.10 Å). These calculated distances also lie outside the range of Mn−N distances observed for other MnIII−peroxo species (2.2−2.37 Å).5,74 The agreement improves marginally

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