Mn K-edge X-ray Absorption Spectroscopy Research Articles

Spectroscopic Characterization of Mn1+ Low Oxidation State in Prussian Blue-Based Battery Anodes.

Stabilization of ions in exotic oxidation states is beneficial for the development of new materials for green energy technologies. Exotic Mn1+ was proposed to play a role in the function of sodium-based Prussian blue analogues (PBA) batteries, a highly sought-out technology for industrial energy storage. Here, we report the detailed electronic structure characterization of uncharged and charged sodium-based manganese hexacyanomanganate anodes via Mn K-edge X-ray absorption spectroscopy (XAS), Kβ nonresonant X-ray emission (XES), and resonant inelastic X-ray scattering (RIXS). The latter allowed us to obtain site-selective XANES information about two distinct Mn centers. The obtained spectroscopic data represent the first electronic structure characterization of low-spin Mn1+ using hard X-ray RIXS and XES and allowed us to confirm its role in anode reduction. Our experimental approach can be expanded to analysis of analogues with other 3d transition metals broadening the application of exotic ionic states in materials engineering.

X-Ray Absorption Spectroscopy Studies of Ti-Mn Redox Flow Battery to Clarify the Redox Reaction

Redox flow battery (RFB) is a promising candidate of large-scale stationary energy storage which is necessary to utilize renewable energy. For RFBs, the capability to easily increase the capacity by enlarging the electrolyte tank, high charge-discharge cycle performance, and wide flexibility to high- and low-frequency fluctuations are highly advantageous.Nowadays vanadium RFBs are used for practical use, but the material cost of vanadium is problematic to further spread them. An alternative candidate for commercial use is titanium-manganese (Ti-Mn) RFBs [1] whose material cost should be lower. On the other hand, the stability of the electrolyte for positive electrode (catholyte) need to be improved, because precipitates are created by a charge disproportionation reaction of Mn ions in the charged catholyte [2]. The formation of precipitates decreases the energy density and degrades the cyclability. To improve the characteristics, the chemical state and redox reaction of the electrolytes of Ti-Mn RFBs should be clarified by precise analyses.We performed synchrotron radiation X-ray absorption spectroscopy (XAS) for the electrolytes of a Ti-Mn RFB to directly observe the redox reaction [3]. In this study, the same Ti-Mn RFB electrolyte was used for both positive and negative electrodes. The Ti K-edge XAS of the electrolyte for negative electrode (anolyte) revealed gradual reduction reaction of Ti from Ti4+ on the charge process. The Ti L 2,3-edge XAS spectrum for the anolyte at state of charge (SOC) of 80% was mostly attributed to Ti3+ state. The results for Ti are consistent with electrochemical views in previous reports [2]. The Mn K-edge XAS of the catholyte gradually shifted to higher energy-side up to SOC of 50%, indicating a slight oxidation reaction of Mn from Mn2+. However, the Mn L 2,3-edge XAS spectrum for the catholyte solution at SOC of 80% was of Mn2+ state that was not changed from the initial state. Instead, scanning transmission X-ray microscopy (STXM; XAS mapping with a high spatial resolution) at the Mn L 3-edge unveiled that the precipitates mostly consist of Mn4+. Thus, the charge disproportionation reaction of 2Mn3+ -> Mn2+ (solution) + Mn4+ (precipitates) in the charged catholyte was confirmed. In the presentation, the electronic-structure change of the Ti and Mn ions in both catholyte and anolyte will be discussed in detail.

X-ray Absorption Spectroscopy Illustrates the Participation of Oxygen in the Electrochemical Cycling of Li4Mn2O5

A combination of oxygen redox and Mn-based oxides would be the best option for high-energy-density Li-ion batteries crucial for a sustainable society. The disordered rock-salt Li4Mn2O5 was recently reported to display a very large capacity of 460 mAh/g with moderate reversibility. Previous studies proposed the involvement of lattice oxygen redox in such intriguing electrochemical performance, whereas no direct evidence was presented. To clarify the charge compensation mechanism, we systematically investigated the evolution of the electronic structure of both Mn and O upon cycling via Mn/O K-edge X-ray absorption spectroscopy (XAS). Mn K-edge XAS unequivocally demonstrates the participation of Mn redox upon the initial stages of charging, yet changes are arrested at the high potentials, while O continues to evolve according to O K-edge XAS. Upon discharging, both Mn and O are simultaneously reduced, but to states different from pristine. The results highlight the significance of a disordered structure in maintaining the reversible redox chemistry of both transition metals and oxygen to design cathode materials with high energy density.

High manganese redox variability and manganate predominance in temperate soil profiles as determined by X-ray absorption spectroscopy

Manganese speciation is a key to understanding the fate of contaminants, nutrients, and organic matter in soils. To date, quantification of Mn species in bulk soils has been performed mainly by sequential extraction methods and rarely supported by spectroscopic analysis. In order to obtain quantitative information on the Mn species inventory of soils, we investigated 46 soil horizons (<2-mm fraction, 45.1–2,280 mg/kg Mn) of nine typical Central European soils (Cambisols, Chernozems, Luvisols, Podzol, Stagnosol) by chemical Mn analyses and Mn K-edge X-ray absorption spectroscopy, and related speciation results to major soil properties. Amounts of Mn2+, Mn3+, and Mn4+ and the average oxidation state of Mn were evaluated by linear combination fitting (LCF) of X-ray absorption near edge structure (XANES) spectra. Additionally, we used extended X-ray absorption fine structure (EXAFS) spectroscopy to identify and quantify major Mn species. For this, EXAFS spectra of 20 organic and mineral soil samples from five soils (Cambisols, Chernozem, Luvisol) were analyzed by LCF and shell fitting. XANES analyses revealed a high Mn redox variability in organic surface layers, with Mn2+ being most abundant (≤100%, x¯ = 54%), followed by Mn3+ (≤80%, x¯ = 32%) and Mn4+ (≤55%, x¯ = 14%). Mineral soil horizons contained significantly less Mn2+ (≤56%, x¯ = 23%), about equal quantities of Mn3+ (≤68%, x¯ = 31%), and were enriched in Mn4+ (≤89%, x¯ = 46%). EXAFS analyses implied the presence of six major Mn species groups: manganates, organically complexed Mn, Mn(III) oxyhydroxides, silicate-bound Mn, Mn oxides without tunnel- or layer structure, and physisorbed Mn. In litter horizons, Mn was mainly present in organic complexes (58–91%, x¯ = 78%) and as physisorbed Mn (≤15%), but individual horizons also comprised manganates, Mn(III) oxyhydroxides, and silicate-bound Mn. Manganates, likely mixtures of phyllomanganates with hexagonal layer symmetry and tectomanganates, dominated in all mineral soil horizons (37–94%, x¯ = 67%). Correlation analysis showed that manganates dissolve completely during dithionite-citrate and acid ammonium oxalate extractions, and suggested that Mn4+-rich manganates preferentially form under less acidic soil conditions, partly by oxidation of organically complexed Mn(II), and that they are enriched in the soil clay fraction. Mineral soil horizons also contained minor quantities of organically complexed Mn (≤39%, x¯ = 11%), silicate-bound Mn (≤30%, x¯ = 8%), Mn(III) oxyhydroxides (≤37%, x¯ = 7%), Mn oxides without tunnel- or layer structure (≤18%, x¯ = 5%), and physisorbed Mn (≤14%, x¯ <1%). The detection of Mn(III) oxyhydroxides such as feitknechtite (β-MnOOH) or groutite (α-MnOOH) as well as the spinel hausmannite (Mn3O4) in acidic soils is remarkable, since their formation is normally linked to neutral or alkaline pH conditions. Minor contributions of silicate-bound Mn indicate the release of Mn from primary minerals already at early stages of soil formation, and low concentrations of physisorbed Mn suggest that exchangeable Mn is rapidly converted into manganates in oxic soils. The predominance of manganates in mineral soils has far-reaching implications for the functioning of soils and biogeochemical element cycles, as these minerals play an important role in metal binding, plant nutrition, and redox-related processes.

Solid-phase Mn speciation in suspended particles along meltwater-influenced fjords of West Greenland

Manganese (Mn) is an essential micro-nutrient that can limit or, along with iron (Fe), co-limit phytoplankton growth in the ocean. Glacier meltwater is thought to be a key source of trace metals to high latitude coastal systems, but little is known about the nature of Mn delivered to glacially-influenced fjords and adjacent coastal waters. In this work, we combine in-situ dissolved Mn (dMn) measurements of surface waters with Mn K-edge X-ray absorption spectroscopy (XAS) data of suspended particles in four fjords of West Greenland. Data were collected from transects of up to 100 km in fjords with different underlying bedrock geology from 64 to 70°N. We found that dMn concentrations generally decreased conservatively with increasing salinity (from 80 to 120 nM at salinity < 8 to < 40 nM at salinities > 25). Dissolved Fe (dFe) trends in these fjords similarly declined with increasing distance from glacier outflows (declining from > 20 nM to < 8 nM). However, the dMn/dFe ratio increased rapidly likely due to the greater stability of dMn at intermediate salinities (i.e. 10–20) compared to rapid precipitation of dFe across the salinity gradient. The XAS data indicated a widespread presence of Mn(II)-rich suspended particles near fjord surfaces, with structures akin to Mn(II)-bearing phyllosilicates. However, a distinct increase in Mn oxidation state with depth and the predominance of birnessite-like Mn(IV) oxides was observed for suspended particles in a fjord with tertiary basalt geology. The similar dMn behaviour in fjords with different suspended particle Mn speciation (i.e., Mn(II)-bearing phyllosilicates and Mn(IV)-rich birnessite) is consistent with the decoupling of dissolved and particulate Mn and suggests that dMn concentrations on the scale of these fjords are controlled primarily by dilution of a freshwater dMn source rather than exchange between dissolved and particle phases. This work provides new insights into the Mn cycle in high latitude coastal waters, where small changes in the relative availabilities of dMn, dFe and macronutrients may affect the identity of the nutrient(s) proximally limiting primary production.

Development of a MnOOH Mineral Electrocatalyst for Water Splitting by Controlling the Surface Defects of a Naturally Occurring Ore

Abstract A MnOOH-based mineral electrocatalyst for the oxygen evolution reaction was developed using a natural ore that is typically insulating, simply by applying a ball milling treatment. This material catalytically decomposed water molecules to generate oxygen. Mn K-edge X-ray absorption spectroscopy analyses in the bulk and surface sensitive modes indicated that structural distortion at the surface provided the catalytically active sites. The formation of oxygen vacancies on natural ore surfaces is likely to be the key to developing efficient mineral electrocatalysts.

X-Ray Absorption Spectroscopy Study to Elucidate the Redox Reaction Mechanism of Li4Mn2O5 upon Delithiation

Li-ion batteries are one of the most promising energy storage sources because of excellent electrochemical performance and has dominated the field of portable devices for the past 20 years. It has also been widely used in vehicle electrification and grid storage. Thus, the demand of Li-ion batteries with high energy density increased dramatically. The electrochemical behavior of current Li-ion batteries mainly relies on the transition metal oxide cathodes1,2,3. Mn-based cathode materials have become highly desirable owing to the low cost and high earth abundance. Recently, a nanostructured disordered rocksalt-type Li4Mn2O5 was synthesized, which displays the highest reported capacity till date up to 355 mAh/g with relative reversibility as compared to other contemporary Li-Mn-O electrodes4,5. In spite of such intriguing electrochemical performance, the detailed charge compensation mechanism associated with (de)lithiation is still unclear. Previous study predicted that the electrochemistry of Li4Mn2O5 was driven by Oxygen/Mn redox. Further study in Mn via X-ray absorption spectroscopy (XAS) revealed that Mn4+/Mn5+ redox couple contributes a small partial capacity in the redox mechanism, 4,5 indicating the significant involvement of oxygen during cycling. However, the mechanism of oxygen participation in the electrochemistry was not studied. In pursuit of understanding the detailed electrochemical mechanism, in this study, X-ray absorption spectroscopic study was performed in both Mn and O K-edge at various states of charge-discharge cycle. XAS measurements were conducted at different charged and discharged states of Li4Mn2O5 cycled between 4.8V and 1.2V voltage window. The obtained Mn K-edge XAS spectra clearly unravels the change of oxidation state of Mn upon charging and discharging, with a quantitative analysis linking the oxidation state and absorption energy via integral method. As a complementary probe to Mn K-edge XAS, O K-edge XAS unambiguously demonstrates the involvement of oxygen upon cycling and uncovers the dominant contribution of oxygen to the electrochemical performance at high voltage charging. This finding could shed light on the design of Oxygen-involved Mn-based cathodes with high capacity and reversibility.

Regenerated Manganese-Oxide Coated Sands: The Role of Mineral Phase in Organic Contaminant Reactivity.

Manganese oxide-coated sand can oxidize electron-rich organic contaminants, but after extended exposure to contaminated water its reactivity decreases. To assess the potential for regenerating geomedia, we measured the ability of passivated manganese-oxide coated sand to oxidize bisphenol A after treatment with oxidants, acid, or methanol. Among the regenerants studied, KMnO4, HOCl, HOBr, and pH 2 or 3 HCl solutions raised the average oxidation state of the Mn, but only HOCl and HOBr restored the reactivity of passivated geomedia to levels comparable to those of the virgin manganese-oxide coated sand. Treatment with HCl restored about one third of the reactivity of the material, likely due to dissolution of reduced Mn. Mn K-edge X-ray absorption spectroscopy data indicated that the reactive manganese oxide phases present in virgin geomedia and geomedia regenerated with HOCl or HOBr had nanocrystalline cryptomelane-like structures and diminished Mn(III) abundance relative to the passivated geomedia. KMnO4-regenerated geomedia also had less Mn(III), but it exhibited less reactivity with bisphenol A because regeneration produced a structure with characteristics of δ-MnO2. The results imply that manganese oxide reactivity depends on both oxidation state and crystal structure; the most effective chemical regenerants oxidize Mn(III) to Mn(IV) oxides exhibiting nanocrystalline, cryptomelane-like forms.

Vanadium-Substituted Tunnel Structured Silver Hollandite (Ag1.2VxMn8-xO16): Impact on Morphology and Electrochemistry.

A series of tunnel structured V-substituted silver hollandite (Ag1.2VxMn8-xO16, x = 0-1.4) samples is prepared and characterized through a combination of synchrotron X-ray diffraction (XRD), synchrotron X-ray absorption spectroscopy (XAS), laboratory Raman spectroscopy, and electron microscopy measurements. The oxidation states of the individual transition metals are characterized using V and Mn K-edge XAS data indicating the vanadium centers exist as V5+, and the Mn oxidation state decreases with increased V substitution to balance the charge. Scanning transmission electron microscopy of reduced materials shows reduction-displacement of silver metal at high levels of lithiation. In lithium batteries, the V-substituted tunneled manganese oxide materials reveal previously unseen reversible nonaqueous Ag electrochemistry and exhibit up to 2.5× higher Li storage capacity relative to their unsubstituted counterparts. The highest capacity was observed for the Ag1.2(V0.8Mn7.2)O16·0.8H2O material with an intermediate level of V substitution, likely due to a combination of the atomic composition, the morphology of the particle, and the homogeneous distribution of the active material within the electrode structure where factors over multiple length scales contribute to the electrochemistry.

Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0 0 0 1)

We report the influence of Mn dopant on magnetic properties of Zn0.95Mn0.05O (ZMO)/Al2O3(0 0 0 1) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L2,3 edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0 0 0 5) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L3-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (VZn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous VZn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density VZn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.

Characteristics of Fe and Mn bearing precipitates generated by Fe(II) and Mn(II) co-oxidation with O2, MnO4 and HOCl in the presence of groundwater ions

In this work, we combined macroscopic measurements of precipitate aggregation and chemical composition (Mn/Fe solids ratio) with Fe and Mn K-edge X-ray absorption spectroscopy to investigate the solids formed by co-oxidation of Fe(II) and Mn(II) with O2, MnO4, and HOCl in the presence of groundwater ions. In the absence of the strongly sorbing oxyanions, phosphate (P) and silicate (Si), and calcium (Ca), O2 and HOCl produced suspensions that aggregated rapidly, whereas co-oxidation of Fe(II) and Mn(II) by MnO4 generated colloidally stable suspensions. The aggregation of all suspensions decreased in P and Si solutions, but Ca counteracted these oxyanion effects. The speciation of oxidized Fe and Mn in the absence of P and Si also depended on the oxidant, with O2 producing Mn(III)-incorporated lepidocrocite (Mn/Fe = 0.01–0.02 mol/mol), HOCl producing Mn(III)-incorporated hydrous ferric oxide (HFO) (Mn/Fe = 0.08 mol/mol), and MnO4 producing poorly-ordered MnO2 and HFO (Mn/Fe > 0.5 mol/mol). In general, the presence of P and Si decreased the crystallinity of the Fe(III) phase and increased the Mn/Fe solids ratio, which was found by Mn K-edge XAS analysis to be due to an increase in surface-bound Mn(II). By contrast, Ca decreased the Mn/Fe solids ratio and decreased the fraction of Mn(II) associated with the solids, suggesting that Ca and Mn(II) compete for sorption sites. Based on these results, we discuss strategies to optimize the design (i.e. filter bed operation and chemical dosing) of water treatment plants that aim to remove Fe(II) and Mn(II) by co-oxidation.

Polaronic effect in the x-ray absorption spectra of La1−xCaxMnO3 manganites

X-ray absorption spectroscopy (XAS) is performed to study changes in the electronic structures of colossal magnetoresistance (CMR) and charged ordered (CO) La1−xCaxMnO3 manganites with respect to temperature. The pre-edge features in O and Mn K-edge XAS spectra, which are highly sensitive to the local distortion of MnO6 octahedral, exhibit contrasting temperature dependence between CMR and CO samples. The seemingly counter-intuitive XAS temperature dependence can be reconciled in the context of polarons. These results help identify the most relevant orbital states associated with polarons and highlight the crucial role played by polarons in understanding the electronic structures of manganites.

Study of mixed-valence Mn2BO4 using XRD, XPS and XAFS spectroscopies

The valence states and local structure around Mn atoms in mixed-valence Mn2BO4 have been studied by temperature dependent X-ray powder diffraction (XRPD), X-ray photoelectron (XPS) and Mn K-edge X-ray absorption (XAFS) spectroscopies measurements. X-ray absorption near-edge structure (XANES) and XPS have been used to measure the average oxidation state of Mn in bulk and near-surface of the material. The edge position, peak shapes and pre-edge features of Mn K-edge XANES spectra have been discussed. The pronounced temperature dependence of the Debye-Waller (DW) factor corresponding to the MnO coordination shell has been found from the extended x-ray absorption fine structure (EXAFS) analysis and has been associated with variations in the local distortions in MnO6 octahedra and emergence of short-range magnetic correlations at low temperatures. The XRPD measurements have been carried out at 298, 523 and 773 K. The monoclinic symmetry (P21/n) was found to persist up to highest temperature measured. The BVS calculations have revealed large valence difference between two manganese sites that strongly supports the presence of charge ordering up to high temperatures.

Spectroscopic and Structural Characterization of Mn(III)-Alkylperoxo Complexes Supported by Pentadentate Amide-Containing Ligands.

Manganese-alkylperoxo species have been proposed as important intermediates in certain enzymatic pathways and are presumed to play a key role in catalytic substrate oxidation cycles involving manganese catalysts and peroxide oxidants. However, structural and spectroscopic understanding of these intermediates is very limited, with only one series of synthetic MnIII-alkylperoxo complexes having been reported. In the present study, we describe the formation and properties of two new MnIII-alkylperoxo complexes, namely, [MnIII(OO tBu)(dpaq)]+ and [MnIII(OO tBu)(dpaq2Me)]+, which utilize the anionic, amide-containing pentadentate dpaq ligand platform. These complexes were generated by reacting the corresponding MnII precursors with a large excess of tBuOOH at -15 °C in MeCN. In both cases, the corresponding mononuclear MnIII-hydroxo complexes [MnIII(OH)(dpaq)]+ and [MnIII(OH)(dpaq2Me)]+ are observed as intermediates en route to the MnIII-alkylperoxo adducts. These new MnIII-alkylperoxo complexes were characterized by electronic absorption, infrared, and Mn K-edge X-ray absorption spectroscopies. Complementary density functional theory calculations were also performed to gain insight into their bonding and structural properties. Compared to previously reported MnIII-alkylperoxo adducts, the MnIII centers in these complexes exhibit significantly altered primary coordination spheres, with a strongly donating anionic amide nitrogen located trans to the alkylperoxo moiety. This results in MnIII-alkylperoxo bonding that is dominated by σ-interactions between the alkylperoxo πip*(O-O) orbital and the Mn d z2 orbital.

Selective Catalytic Olefin Epoxidation with MnII-Exchanged MOF-5

Partial substitution of ZnII by MnII in Zn4O(terephthalate)3 (MOF-5) leads to a distorted all-oxygen ligand field supporting a single MnII site, whose structure was confirmed by Mn K-edge X-ray absorption spectroscopy. The MnII ion at the MOF-5 node engages in redox chemistry with a variety of oxidants. With tBuSO2PhIO, it produces a putative MnIV-oxo intermediate, which upon further reaction with adventitious hydrogen is trapped as a MnIII–OH species. Most intriguingly, the intermediacy of the high-spin MnIV–oxo species is likely responsible for catalytic activity of the MnII-MOF-5 precatalyst, which in the presence of tBuSO2PhIO catalyzes oxygen atom transfer reactivity to form epoxides from cyclic alkenes with >99% selectivity. These results demonstrate that MOF secondary building units serve as competent platforms for accessing terminal high-valent metal–oxo species that consequently engage in catalytic oxygen atom transfer chemistry owing to the relatively weak ligand fields provided by the SBU.

Mn K-edge X-ray absorption studies of mononuclear Mn(III)-hydroxo complexes.

Mn K-edge X-ray absorption spectroscopy experiments were performed on the solid- and solution-phase samples of [MnII(dpaqR)](OTf) (R=H, Me) and [MnIII(OH)(dpaqR)](OTf). The extended X-ray absorption fine structure (EXAFS) data show distinct differences between the MnII and MnIII-OH complexes, with fits providing metric parameters in excellent agreement with values from X-ray crystallography and density functional theory (DFT) computations. Evaluation of the EXAFS data for solid-phase [MnIII(OH)(dpaq)](OTf) resolved a short Mn-OH bond distance of 1.79Å; however, the short trans-amide nitrogen bond of the supporting ligand precluded the resolution of the Mn-OH bond distance in the corresponding solution-phase sample and for both [MnIII(OH)(dpaqMe)](OTf) samples. The edge energy also increases by approximately 2eV from the MnII to the MnIII-OH complexes. Experimental pre-edge analysis shows the MnII complexes to have pre-edge areas comparable to the MnIII-OH complexes, despite the presence of the relatively short Mn-OH distance. Time-dependent density functional theory (TD-DFT) computations illustrate that Mn 3d-4p mixing, a primary contributor to pre-edge intensities, decreases by~0.3% from the MnII to MnIII-OH complexes, which accounts for the very similar pre-edge areas. Collectively, this work shows that combined EXAFS and XANES analysis has great potential for identification of reactive MnIII-OH intermediates, such as those proposed in enzyme active sites.

Cerium sequestration and accumulation in fractured crystalline bedrock: The role of Mn-Fe (hydr-)oxides and clay minerals

This study focuses on the mechanisms of Ce sequestration and accumulation in the fracture network of the upper kilometer of the granitoid bedrock of the Baltic Shield in southeast Sweden (Laxemar area, Sweden). The material includes 81 specimens of bulk secondary mineral precipitates (“fracture coatings”) collected on fracture walls identified in 17 drill cores, and 66 groundwater samples collected from 21 deep boreholes with equipment designed for retrieval of representative groundwater at controlled depths. The concentrations of Ce in the fracture coatings, although varying considerably (10–90th percentiles: 67–438mgkg−1), were frequently higher than those of the wall rock (10–90th percentiles: 70–118mgkg−1). Linear combination fitting analysis of Ce LIII-edge X-ray absorption near-edge structure (XANES) spectra, obtained for 19 fracture coatings with relatively high Ce concentrations (⩾145mgkg−1) and a wide range of Ce-anomaly values, revealed that Ce(IV) occurs frequently in the upper 10m of the fracture network (Ce(IV)/Cetotal=0.06–1.00 in 8 out of 11 specimens) and is mainly associated with Mn oxides (modeled as Ce oxidatively scavenged by birnessite). These features are in line with the strong oxidative and sorptive capacities of Mn oxide as demonstrated by previous studies, and abundant todorokite and birnessite-like Mn oxides identified in 3 out of 4 specimens analyzed by Mn K-edge X-ray absorption spectroscopy (XAS) in the upper parts of the fracture network (down to 5m). For a specimen with very high Ce concentration (1430mgkg−1) and NASC-normalized Ce anomaly (3.63), the analysis of Ce XANES and Mn XAS data revealed (i) a predominance of Ce oxide in addition to Ce scavenged by Mn oxide; and (ii) a large fraction of poorly-crystalline hexagonal birnessite and aqueous Mn2+, suggesting a recent or on-going oxidation of Mn2+ in this fracture. In addition, the Ce oxide precipitates on this fracture observed by in situ SEM-EDS contained considerable amounts of Mn. These spectroscopic and microscopic features led us to suggest that the remarkable accumulation of Ce(IV) in this fracture is a result of repeated formation and dissolution of Mn oxides, that is, formation of Mn oxide followed by oxidative scavenging of Ce as Ce oxide nanoparticles, which largely remained during the subsequent reductive dissolution of the Mn oxides. In addition, the XANES data indicate that goethite has the capability to oxidize Ce at near-neutral pH under our experimental conditions (goethite reacted with 0.001M Ce for 48h in a glove box with O2 <1ppm). This previously unrecognized Ce oxidation pathway also seems to contribute to a minor extent to the oxidative scavenging of Ce in the fracture network. Trivalent Ce in the fracture coatings, in particular below 2.5m, is mainly sorbed as inner-sphere complexes on clay minerals. Taking into account the facts that Ce in the present groundwater is scarce and modeled to be largely complexed with humic substance, it is argued that the inner-sphere complexes were mainly formed from past (Paleozoic) hydrothermal fluids.

Formation, Characterization, and O-O Bond Activation of a Peroxomanganese(III) Complex Supported by a Cross-Clamped Cyclam Ligand.

Although there have been reports describing the nucleophilic reactivity of peroxomanganese(III) intermediates, as well as their conversion to high-valent oxo-bridged 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 Me2EBC ligand (4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). This ligand is known to support high-valent, mononuclear Mn(IV) species with well-defined spectroscopic properties, which provides an opportunity to identify mononuclear Mn(IV) products from O-O bond activation of the corresponding Mn(III)-peroxo adduct. The peroxomanganese(III) intermediate, [Mn(III)(O2)(Me2EBC)](+), was prepared at low-temperature by the addition of KO2 to [Mn(II)(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 [Mn(III)(O2)(Me2EBC)](+) intermediate was examined by density functional theory (DFT) and time-dependent (TD) DFT calculations. Detailed spectroscopic investigations of the decay products of [Mn(III)(O2)(Me2EBC)](+) revealed the presence of mononuclear Mn(III)-hydroxo species or a mixture of mononuclear Mn(IV) and Mn(III)-hydroxo species. The nature of the observed decay products depended on the amount of KO2 used to generate [Mn(III)(O2)(Me2EBC)](+). The Mn(III)-hydroxo product was characterized by Mn K-edge XAS, and shifts in the pre-edge transition energies and intensities relative to [Mn(III)(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 Mn(IV) center upon decay of a nonporphyrinoid Mn(III)-peroxo center.

ChemInform Abstract: A Balancing Act: Stability Versus Reactivity of Mn(O) Complexes

AbstractReview: 36 refs.

A Balancing Act: Stability versus Reactivity of Mn(O) Complexes.

A large class of heme and non-heme metalloenzymes utilize O2 or its derivatives (e.g., H2O2) to generate high-valent metal-oxo intermediates for performing challenging and selective oxidations. Due to their reactive nature, these intermediates are often short-lived and very difficult to characterize. Synthetic chemists have sought to prepare analogous metal-oxo complexes with ligands that impart enough stability to allow for their characterization and an examination of their inherent reactivity. The challenge in designing these molecules is to achieve a balance between their stability, which should allow for their in situ characterization or isolation, and their reactivity, in which they can still participate in interesting chemical transformations. This Account focuses on our recent efforts to generate and stabilize high-valent manganese-oxo porphyrinoid complexes and tune their reactivity in the oxidation of organic substrates. Dioxygen can be used to generate a high-valent Mn(V)(O) corrolazine (Mn(V)(O)(TBP8Cz)) by irradiation of Mn(III)(TBP8Cz) with visible light in the presence of a C-H substrate. Quantitative formation of the Mn(V)(O) complex occurs with concomitant selective hydroxylation of the benzylic substrate hexamethylbenzene. Addition of a strong H(+) donor converted this light/O2/substrate reaction from a stoichiometric to a catalytic process with modest turnovers. The addition of H(+) likely activates a transient Mn(V)(O) complex to achieve turnover, whereas in the absence of H(+), the Mn(V)(O) complex is an unreactive "dead-end" complex. Addition of anionic donors to the Mn(V)(O) complex also leads to enhanced reactivity, with a large increase in the rate of two-electron oxygen atom transfer (OAT) to thioether substrates. Spectroscopic characterization (Mn K-edge X-ray absorption and resonance Raman spectroscopies) revealed that the anionic donors (X(-)) bind to the Mn(V) ion to form six-coordinate [Mn(V)(O)(X)](-) complexes. An unusual "V-shaped" Hammett plot for the oxidation of para-substituted thioanisole derivatives suggested that six-coordinate [Mn(V)(O)(X)](-) complexes can act as both electrophiles and nucleophiles, depending on the nature of the substrate. Oxidation of the Mn(V)(O) corrolazine resulted in the in situ generation of a Mn(V)(O) π-radical cation complex, [Mn(V)(O)(TBP8Cz(•+))](+), which exhibited more than a 100-fold rate increase in the oxidation of thioethers. The addition of Lewis acids (LA; Zn(II), B(C6F5)3) to the closed-shell, diamagnetic Mn(V)(O)(TBP8Cz) stabilized a paramagnetic valence tautomer Mn(IV)(O)(TBP8Cz(•+))(LA), which was characterized as a second π-radical cation complex by NMR, EPR, UV-vis, and high resolution cold spray ionization MS. The Mn(IV)(O)(TBP8Cz(•+))(LA) complexes are able to abstract H(•) from phenols and exhibit a rate enhancement of up to ∼100-fold over the parent Mn(V)(O) valence tautomer. In contrast, a large decrease in rate is observed for OAT for the Mn(IV)(O)(TBP8Cz(•+))(LA) complexes. The rate enhancement for hydrogen atom transfer (HAT) may derive from the higher redox potential for the π-radical cation complex, while the large rate decrease seen for OAT may come from a decrease in electrophilicity for an Mn(IV)(O) versus Mn(V)(O) complex.