XAS and DFT investigation of mononuclear cobalt(III) peroxo complexes: electronic control of the geometric structure in CoO2 versus NiO2 systems

A Co(III)-peroxo-arylboronate complex formed by nucleophilic reaction of a Co(III)-peroxo species

In this work, we report the generation and characterization of two new Co(III)-peroxo complexes 2 and 3. 2 is best described as a mononuclear CoIII-(O2) complex that exhibits an 18O-isotope sensitive OO bond stretching vibration at 845(-49) cm-1, indicating a relatively weak peroxo moiety compared to those of other CoIII-(O2) complexes reported previously. Complex 3 is a CoIII-peroxo-arylboronate species having a rare {CoIIIOOBO} five-membered metallocycle, which is structurally characterized using X-ray crystallography. Investigations of the reaction mechanism using density functional theory calculations show that 2 likely undergoes a nucleophilic attack to an arylboronic acid, which is generated by hydrolysis of the BPh4- anion in wet acetonitrile solution, to first form a CoIII-peroxo-arylboronic acid adduct, followed by the loss of one benzene molecule to generate the five-membered metallocycle. The entire reaction is thermodynamically favorable. Taken together, the conversion of 2 to 3 represents the discovery of a novel nucleophilic reactivity that can be carried out by mononuclear Co(III)-peroxo complexes.

Asymmetric Coordination of Single-Atom Ru Sites Achieves Efficient N(sp3)-H Dehydrogenation Catalysis for Ammonia Oxidation

Ruthenium single-atom catalysts have great potential in ammonia-selective catalytic oxidation (NH3-SCO); however, the stable sp3 hybrid orbital of NH3 molecules makes N(sp3)-H dissociation a challenge for conventional symmetrical metallic oxide catalysts. Herein, we propose a heterogeneous interface reverse atom capture strategy to construct Ru with unique asymmetric Ru1N2O1 coordination. Ru1N2O1/CeO2 exhibits intrinsic low-temperature conversion (T100 at 160 °C) compared to symmetric coordinated Ru-based (280 °C), Ir-based (220 °C), and Pt-based (200 °C) catalysts, and the TOF is 65.4 times that of Ag-based catalysts. The experimental and theoretical studies show that there is a strong d-p orbital interaction between Ru and N atoms, which not only enhances the adsorption of ammonia at the Ru1N2O1 position but also optimizes the electronic configuration of Ru. Furthermore, the affinity of Ru1N2O1/CeO2 to water is significantly weaker than that of conventional catalysts (the binding energy of the Pd3Au1 catalyst is -1.19 eV, but it is -0.39 eV for our material), so it has excellent water resistance. Finally, the N(sp3)-H activation of NH3 requires the assistance of surface reactive oxygen species, but we found that asymmetric Ru1N2O1 can directly activate the N(sp3)-H bond without the involvement of surface reactive oxygen species. This study provides a novel principle for the rational design of the proximal coordination of active sites to achieve its optimal catalytic activity in single-atom catalysis.

Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

The use of supporting ligands that can store either protons or electrons has emerged as a powerful strategy in catalysis. While these strategies are potent individually, natural systems mediate remarkable transformations by combining the storage of both protons and electrons in the secondary coordination sphere. As such, there has been recent interest in using this strategy to enable fundamentally different transformations. Furthermore, outsourcing H-atom or hydrogen storage to ancillary ligands can also enable alternative mechanistic pathways and thereby selectivity. Here, we describe the application of this strategy to facilitate radical reactivity in Co-based hydrogenation catalysis. Metalation of previously reported dihydrazonopyrrole ligands with Co results in paramagnetic complexes, which are best described as having Co(II) oxidation states. These complexes catalytically hydrogenate olefins with low catalyst loadings under mild conditions (1 atm H₂, 23 °C). Mechanistic, spectroscopic, and computational investigations indicate that this system goes through a radical hydrogen-atom transfer (HAT) type pathway that is distinct from classic organometallic mechanisms and is supported by the ability of the ligand to store H₂. These results show how ancillary ligands can facilitate efficient catalysis, and furthermore how classic organometallic mechanisms for catalysis can be altered by the secondary coordination sphere.

Effect of the ring size of TMC ligands in controlling C-H bond activation by metal-superoxo species

Metal-superoxo species play a very important role in many metal-mediated catalytic transformation reactions. Their catalytic reactivity is affected by many factors such as the nature of metal ions and ring size of ligands. Herein, for the first time, we report DFT calculations on the electronic structures of a series of metal-superoxo species (M = V, Cr, Mn, Fe, and Co) with two ring size ligands, i.e., 13-TMC/14-TMC, and a detailed mechanistic study on the C-H bond activation of cyclohexa-1,4-diene followed by the effect of the ring size of ligands. Our DFT results showed that the electron density at the distal oxygen plays an important role in C-H bond activation. By computing the energetics of C-H bond activation and mapping the potential energy surface, it was found that the initial hydrogen abstraction is the rate-determining step with both TMC rings and all the studied metal-superoxo species. The significant electron density at the cyclohex-1,4-diene carbon indicates that the reaction proceeds via the proton-coupled electron transfer mechanism. By mapping the potential energy surfaces, we found that the 13-TMC ligated superoxo with the anti-isomer are more reactive than the 14-TMC superoxo species except for the iron-superoxo species where the 14-TMC ligated superoxo species is more reactive i.e. smaller ring size TMC is more reactive towards C-H bond activation. This is also supported by the structural correlation, i.e., the greater contraction in the smaller ring results in the metal being pushed out of plane along the z-axis, which reduces the steric hindrance. Thus, the ring size can help in designing catalysts with better efficiency for catalytic reactions.

Altering Ligand Fields in Single-Atom Sites through Second-Shell Anion Modulation Boosts the Oxygen Reduction Reaction

Single-atom catalysts based on metal-N₄ moieties and anchored on carbon supports (defined as M-N-C) are promising for oxygen reduction reaction (ORR). Among those, M-N-C catalysts with 4d and 5d transition metal (TM_4d,5d) centers are much more durable and not susceptible to the undesirable Fenton reaction, especially compared with 3d transition metal based ones. However, the ORR activity of these TM_4d,5d-N-C catalysts is still far from satisfactory; thus far, there are few discussions about how to accurately tune the ligand fields of single-atom TM_4d,5d sites in order to improve their catalytic properties. Herein, we leverage single-atom Ru-N-C as a model system and report an S-anion coordination strategy to modulate the catalyst's structure and ORR performance. The S anions are identified to bond with N atoms in the second coordination shell of Ru centers, which allows us to manipulate the electronic configuration of central Ru sites. The S-anion-coordinated Ru-N-C catalyst delivers not only promising ORR activity but also outstanding long-term durability, superior to those of commercial Pt/C and most of the near-term single-atom catalysts. DFT calculations reveal that the high ORR activity is attributed to the lower adsorption energy of ORR intermediates at Ru sites. Metal-air batteries using this catalyst in the cathode side also exhibit fast kinetics and excellent stability.

Graphite Conjugation of a Macrocyclic Cobalt Complex Enhances Nitrite Electroreduction to Ammonia

This work reports on the generation of a graphite-conjugated diimine macrocyclic Co catalyst (GCC-CoDIM) that is assembled at o-quinone edge defects on graphitic carbon electrodes. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirm the existence of a new Co surface species with a coordination environment that is the same as that of the molecular analogue, [Co(DIM)Br₂]⁺. GCC-CoDIM selectively reduces nitrite to ammonium with quantitative Faradaic efficiency and at a rate that approaches enzymatic catalysis. Preliminary mechanistic investigations suggest that the increased rate is accompanied by a change in mechanism from the molecular analogue. These results provide a template for creating macrocycle-based electrocatalysts based on first-row transition metals conjugated to an extreme redox-active ligand.

Docking MOF crystals on graphene support for highly selective electrocatalytic peroxide production

Tailoring the reaction kinetics is the central theme of designer electrocatalysts, which enables the selective conversion of abundant and inert atmospheric species into useful products. Here we show a supporting effect in tuning the electrocatalytic kinetics of oxygen reduction reaction (ORR) from four-electron to two-electron mechanism by docking metalloporphyrin-based metal-organic frameworks (MOFs) crystals on graphene support, leading to highly selective peroxide production with faradaic efficiency as high as 93.4%. A magic angle of 38.1° tilting for the co-facial alignment was uncovered by electron diffraction tomography, which is attributed to the maximization of π-π interaction for mitigating the lattice and symmetry mismatch between MOF and graphene. The facilitated electron migration and oxygen chemisorption could be ascribed to the supportive effect of graphene that disperses of the electron state of the active center, and ultimately regulates rate-determining step.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (synthesis protocols for control samples, morphological and structural characterizations, porosity, electrochemical properties and activities including SEM, TEM, XPS, Raman, AFM investigations) is available in the online version of this article at 10.1007/s12274-021-3382-3.

Single-atom Rh/N-doped carbon electrocatalyst for formic acid oxidation

To meet the requirements of potential applications, it is of great importance to explore new catalysts for formic acid oxidation that have both ultra-high mass activity and CO resistance. Here, we successfully synthesize atomically dispersed Rh on N-doped carbon (SA-Rh/CN) and discover that SA-Rh/CN exhibits promising electrocatalytic properties for formic acid oxidation. The mass activity shows 28- and 67-fold enhancements compared with state-of-the-art Pd/C and Pt/C, respectively, despite the low activity of Rh/C. Interestingly, SA-Rh/CN exhibits greatly enhanced tolerance to CO poisoning, and Rh atoms in SA-Rh/CN resist sintering after long-term testing, resulting in excellent catalytic stability. Density functional theory calculations suggest that the formate route is more favourable on SA-Rh/CN. According to calculations, the high barrier to produce CO, together with the relatively unfavourable binding with CO, contribute to its CO tolerance.

Peroxocobalt(iii) species activates nitriles via a superoxocobalt(ii) diradical state

The dioxygenation of nitriles by [CoIII(TBDAP)(O2)]+ (TBDAP = N,N-di-tert-butyl-2,11-diaza[3.3](2,6)-pyridinophane) is investigated using DFT-calculations. The mechanism proposed previously based on experimental observations, which invoked an outer-sphere cycloaddition, was found to be unreasonable. Instead, calculations suggest that an inner-sphere mechanism involving the cleavage of one of the Co-O bonds assisted by substrate uptake is much more likely. The reactively competent species is a triplet consisting of a Co(ii)-superoxo functionality, which can undergo O-C bond formation and O-O bond cleavage traversing low energy transition states. The role of the structurally rigid TBDAP ligand is to prevent the participation of the pyridyl ligand in the delocalization of the unpaired electron density.

Cobalt-containing zeolitic imidazole frameworks for C–H activation using visible-light redox photocatalysis

Rationalising the photocatalytic activity of different cobalt ZIFs, provides an improved understanding of photocatalytic C–H activation.

The role of photoinduced charge transfer for photocatalysis, photoelectrocatalysis and luminescence sensing in metal–organic frameworks

Understanding PCT taking place within MOFs is crucial for designing porous photo/electrocatalysts and luminescent sensors. Unique features of PCT in MOFs and recent progress along with state-of-the-art characterization methods are discussed in the context of its applications.

Highly Reactive CoIII,IV2(μ-O)2 Diamond Core Complex That Cleaves C-H Bonds

The selective activation of strong sp³ C-H bonds at mild conditions is a key step in many biological and synthetic transformations and an unsolved challenge for synthetic chemists. In nature, soluble methane monooxygenase (sMMO) is one representative example of nonheme dinuclear iron-dependent enzymes that activate strong sp³ C-H bonds by a high-valent diiron(IV) intermediate Q. To date, synthetic model complexes of sMMO-Q have shown limited abilities to oxidize strong C-H bonds. In this work, we generated a high-valent Co^III,IV₂(μ-O)₂ complex 3 supported by a tetradentate tris(2-pyridylmethyl)amine (TPA) ligand via one-electron oxidation of its Co^III₂(μ-O)₂ precursor 2. Characterization of 2 and 3 using X-ray absorption spectroscopy and DFT calculations showed that both species possess a diamond core structure with a short Co···Co distance of 2.78 Å. Furthermore, 3 is an EPR active species showing an S = 1/2 signal with clearly observable hyperfine splittings originated from the coupling of the ⁵⁹Co nuclear spin with the electronic spin. Importantly, 3 is a highly reactive oxidant for sp³ C-H bonds, and an oxygenation reagent. 3 has the highest rate constant (1.5 M^-1 s^-1 at -60 °C) for oxidizing 9,10-dihydroanthracene (DHA) compared to diamond core complexes of other first-row transition metals including Mn, Fe and Cu reported previously. Specifically, 3 is about 4-5 orders of magnitude more reactive than the diiron analogs Fe^III,IV₂(μ-O)₂ and Fe^IV₂(μ-O)₂ supported by TPA and related ligands. These findings shed light on future development of more reactive approaches for C-H bond activation by bioinspired dicobalt complexes.

Effect of Redox Active Ligands on the Electrochemical Properties of Manganese Tricarbonyl Complexes

The synthesis, structural characterization, and electrochemical behavior of the neutral Mn(azpy)(CO)₃(Br) 4 (azpy = 2-phenylazopyridine) complex is reported and compared with its structural analogue Mn(bipy)(CO)₃(Br) 1 (bipy = 2,2'-bipyridine). 4 exhibits reversible two-electron reduction at a mild potential (-0.93 V vs Fc^+/0 in acetonitrile) in contrast to 1, which exhibits two sequential one-electron reductions at -1.68 V and -1.89 V vs Fc^+/0 in acetonitrile. The key electronic structure differences between 1 and 4 that lead to disparate electrochemical properties are investigated using a combination of Mn-K-edge X-ray absorption spectroscopy (XAS), Mn-Kβ X-ray emission spectroscopy (XES), and density functional theory (DFT) on 1, 4, their debrominated analogues, [Mn(L)(CO)₃(CH₃CN)][CF₃SO₃] (L = bipy 2, azpy 5), and two-electron reduced counterparts [Mn(bipy)(CO)₃][K(18-crown-6)] 3 and [Mn(azpy)(CO)₃][Cp₂Co] 6. The results reveal differences in the distribution of electrons about the CO and bidentate ligands (bipy and azpy), particularly upon formation of the highly reduced, formally Mn(-1) species. The data show that the degree of ligand noninnocence and resulting redox-activity in Mn(L)(CO)₃ type complexes impacts not only the reducing power of such systems, but the speciation of the reduced complexes via perturbation of the monomer-dimer equilibrium in the singly reduced Mn(0) state. This study highlights the role of redox-active ligands in tuning the reactivity of metal centers involved in electrocatalytic transformations.

A theoretical investigation into the first-row transition metal–O2 adducts

Effects of both metal center and ligand ring size on the properties of metal–O₂ adducts.

A theoretical study on the oxidation of alkenes to aldehydes catalyzed by ruthenium porphyrins using O2 as the sole oxidant

Density functional theory (DFT) calculations were used to study the ruthenium porphyrin-catalyzed oxidation of styrene to generate an aldehyde. The results indicate that two reactive oxidants, dioxoruthenium and monooxoruthenium-superoxo porphyrins, participate in the catalytic oxidation. In the mechanism, the resultant monooxoruthenium porphyrin acts in the tandem epoxide isomerization (E-I) to selectively yield an aldehyde and generate a dioxoruthenium porphyrin, thereby triggering new oxidation reaction cycles. In this calculation, several key elements responsible for the observed oxidative ability have been established by using Frontier molecular orbital (FMO) theory, natural bond orbital (NBO) analysis, etc., which include the reaction energy, the spin exchange effect, the spin-state conversion process, and the energy level of the lowest unoccupied molecular orbitals (LUMOs) of the reactive oxidants. The comparative oxidative abilities of the ruthenium-oxo/superoxo compounds with different axial ligands are also investigated. The results suggest that the ruthenium-oxo/superoxo species featuring a chlorine axial ligand is more reactive than that substituted with oxygen. This tuneable reactivity can be understood when considering the different electronic characters of the two ligands and the effective atomic number rule (EAN).

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

Terminal cobalt(IV)-oxo (Co^IV-O) species have been implicated as key intermediates in various cobalt-mediated oxidation reactions. Herein we report the photocatalytic generation of a mononuclear non-haem [(13-TMC)Co^IV(O)]²⁺ (2) by irradiating [Co^II(13-TMC)(CF₃SO₃)]⁺ (1) in the presence of [Ru^II(bpy)₃]²⁺, Na₂S₂O₈, and water as an oxygen source. The intermediate 2 was also obtained by reacting 1 with an artificial oxidant (that is, iodosylbenzene) and characterized by various spectroscopic techniques. In particular, the resonance Raman spectrum of 2 reveals a diatomic Co-O vibration band at 770 cm^-1, which provides the conclusive evidence for the presence of a terminal Co-O bond. In reactivity studies, 2 was shown to be a competent oxidant in an intermetal oxygen atom transfer, C-H bond activation and olefin epoxidation reactions. The present results lend strong credence to the intermediacy of Co^IV-O species in cobalt-catalysed oxidation of organic substrates as well as in the catalytic oxidation of water that evolves molecular oxygen.

A mononuclear nonheme cobalt(iii)–hydroperoxide complex with an amphoteric reactivity in electrophilic and nucleophilic oxidative reactions

A mononuclear nonheme cobalt(iii)–hydroperoxide complex bearing a tetramethylated cyclam ligand, which is synthesized by the protonation of its corresponding cobalt(iii)–peroxide complex, exhibits an amphoteric reactivity in electrophilic and nucleophilic oxidative reactions.

Theoretical view on a linear end-on manganese–dioxygen complex bearing a calix[4]arene ligand

The Mn–O–O angle of mononuclear manganese(iii)-superoxo complexes supported by zwitterionic calix[4]arene ligands can be modulated via solvent polarity perturbations and/or ligand size adjustment as indicated by DFT calculations.

A family of [Ni8] cages templated by μ6-peroxide from dioxygen activation

[Ni₈] cages templated by η³:η³:μ₆-O₂²⁻ from O₂ activation: the ligand found oxidized within the cages.

Synthesis, characterization, and reactivity of cobalt(III)-oxygen complexes bearing a macrocyclic N-tetramethylated cyclam ligand

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

Stieber SC, DeBeer S, Chirik PJ. Catalytic hydrogenation activity and electronic structure determination of bis(arylimidazol-2-ylidene)pyridine cobalt alkyl and hydride complexes

The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ((iPr)CNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, ((iPr)CNC)CoCH3 is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, ((iPr)CNC)CoH, was accomplished by hydrogenation of ((iPr)CNC)CoCH3. Over the course of 3 h at 22 °C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H2-(iPr)CNC)CoN2. Similar alkyl migration was observed upon treatment of ((iPr)CNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.

Mononuclear nickel(II)-superoxo and nickel(III)-peroxo complexes bearing a common macrocyclic TMC ligand

Mononuclear metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are generated as key intermediates in the catalytic cycles of dioxygen activation by heme and non-heme metalloenzymes. We have shown recently that the geometric and electronic structure of the Ni-O2 core in [Ni(n-TMC)(O2)]+ (n = 12 and 14) varies depending on the ring size of the supporting TMC ligand. In this study, mononuclear Ni(II)-superoxo and Ni(III)-peroxo complexes bearing a common macrocylic 13-TMC ligand, such as [NiII(13-TMC)(O2)]+ and [NiIII(13-TMC)(O2)]+, were synthesized in the reaction of [NiII(13-TMC)(CH3CN)]2+ and H2O2 in the presence of tetramethylammonium hydroxide (TMAH) and triethylamine (TEA), respectively. The Ni(II)-superoxo and Ni(III)-peroxo complexes bearing the common 13-TMC ligand were successfully characterized by various spectroscopic methods, X-ray crystallography, and DFT calculations. Based on the combined experimental and theoretical studies, we conclude that the superoxo ligand in [NiII(13-TMC)(O2)]+ is bound in an end-on fashion to the nickel(II) center, whereas the peroxo ligand in [NiIII(13-TMC)(O2)]+ is bound in a side-on fashion to the nickel(III) center. Reactivity studies performed with the Ni(II)-superoxo and Ni(III)-peroxo complexes toward organic substrates reveal that the former possesses an electrophilic character, whereas the latter is an active oxidant in nucleophilic reaction.

Mononuclear metal-O2 complexes bearing macrocyclic N-tetramethylated cyclam ligands

Metalloenzymes activate dioxygen to carry out a variety of biological reactions, including the biotransformation of naturally occurring molecules, oxidative metabolism of xenobiotics, and oxidative phosphorylation. The dioxygen activation at the catalytic sites of the enzymes occurs through several steps, such as the binding of O(2) at a reduced metal center, the generation of metal-superoxo and -peroxo species, and the O-O bond cleavage of metal-hydroperoxo complexes to form high-valent metal-oxo oxidants. Because these mononuclear metal-dioxygen (M-O(2)) adducts are implicated as key intermediates in dioxygen activation reactions catalyzed by metalloenzymes, studies of the structural and spectroscopic properties and reactivities of synthetic biomimetic analogues of these species have aided our understanding of their biological chemistry. One particularly versatile class of biomimetic coordination complexes for studying dioxygen activation by metal complexes is M-O(2) complexes bearing the macrocyclic N-tetramethylated cyclam (TMC) ligand. This Account describes the synthesis, structural and spectroscopic characterization, and reactivity studies of M-O(2) complexes bearing tetraazamacrocyclic n-TMC ligands, where M ═ Cr, Mn, Fe, Co, and Ni and n = 12, 13, and 14, based on recent results from our laboratory. We have used various spectroscopic techniques, including resonance Raman and X-ray absorption spectroscopy, and density functional theory (DFT) calculations to characterize several novel metal-O(2) complexes. Notably, X-ray crystal structures had shown that these complexes are end-on metal-superoxo and side-on metal-peroxo species. The metal ions and the ring size of the macrocyclic TMC ligands control the geometric and electronic structures of the metal-O(2) complexes, resulting in the end-on metal-superoxo versus side-on metal-peroxo structures. Reactivity studies performed with the isolated metal-superoxo complexes reveal that they can conduct electrophilic reactions such as oxygen atom transfer and C-H bond activation of organic substrates. The metal-peroxo complexes are active oxidants in nucleophilic reactions, such as aldehyde deformylation. We also demonstrate a complete intermolecular O(2)-transfer from metal(III)-peroxo complexes to a Mn(II) complex. The results presented in this Account show the significance of metal ions and supporting ligands in tuning the geometric and electronic structures and reactivities of the metal-O(2) intermediates that are relevant in biology and in biomimetic reactions.

X-ray absorption near-edge spectroscopy in bioinorganic chemistry: Application to M-O2 systems

Metal K-edge X-ray absorption spectroscopy (XAS) has been extensively applied to bioinorganic chemistry to obtain geometric structure information on metalloprotein and biomimetic model complex active sites by analyzing the higher energy extended X-ray absorption fine structure (EXAFS) region of the spectrum. In recent years, focus has been on developing methodologies to interpret the lower energy K-pre-edge and rising-edge regions (XANES) and using it for electronic structure determination in complex bioinorganic systems. In this review, the evolution and progress of 3d-transition metal K-pre-edge and rising-edge methodology development is presented with particular focus on applications to bioinorganic systems. Applications to biomimetic transition metal-O₂ intermediates (M = Fe, Co, Ni and Cu) are reviewed, which demonstrate the power of the method as an electronic structure determination technique and its impact in understanding the role of supporting ligands in tuning the electronic configuration of transition metal-O₂ systems.