Spectroscopic Signature of Metal-hydroxo and Peroxo Species in K-edge X-ray Absorption Spectra.

Metal-dioxygen species are important intermediates formed during dioxygen activations by metalloenzymes in various biological processes, by catalysts in fuel cells, and prior to O evolution by photosystem II. In this work, we focus on manganese-porphyrin complexes using tetramesitylporphyrin ligand (TMP) to explore changes in Mn K-edge X-ray absorption spectroscopy (XAS) associated with the formation of Mn-hydroxide and Mn-O peroxide species. With limited spectroscopic characterization of these compounds, Mn K X-ray emission spectroscopy (XES), XAS, density functional theory (DFT), and time-dependent DFT (TD-DFT) analysis will enhance our understanding of their complex electronic structure.

Insights from Ca→Sr substitution on the mechanism of O-O bond formation in photosystem II.

In recent years, there has been a steady interest in unraveling the intricate mechanistic details of water oxidation mechanism in photosynthesis. Despite the substantial progress made over several decades, a comprehensive understanding of the precise kinetics underlying O-O bond formation and subsequent evolution remains elusive. However, it is well-established that the oxygen evolving complex (OEC), specifically the CaMnO cluster, plays a crucial role in O-O bond formation, undergoing a series of four oxidative events as it progresses through the S-states of the Kok cycle.

Rational Design of Improved Ru Containing Fe-Based Metal-Organic Framework (MOF) Photoanode for Artificial Photosynthesis.

Metal-Organic Frameworks (MOFs) recently emerged as a new platform for the realization of integrated devices for artificial photosynthesis. However, there remain few demonstrations of rational tuning of such devices for improved performance. Here, a fast molecular water oxidation catalyst working via water nucleophilic attack is integrated into the MOF MIL-142, wherein FeO nodes absorb visible light, leading to charge separation.

Evaluation of Ce-MOFs as Photoanode Materials for the Water Oxidation Reaction: The Effect of Doping with [Ru(bpy)(dcbpy)(HO)] Catalyst.

Artificial photosynthesis stands out as a highly effective method for harnessing sunlight to produce clean and renewable energy. The light-absorbing properties, chemical stability, and high redox activity of Ce-based metal-organic frameworks (MOFs) make them attractive materials for visible-light-driven water splitting. Currently, Ce-based MOFs remain a relatively underexplored system for photocatalytic water oxidation in acidic media.

Spectroscopic Characterization of Mn 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 Mn 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).

Revisit the E2 Domain of Amyloid Precursor Protein: Ferroxidase, Superoxide and Peroxynitrite Scavenging Activities.

Amyloid precursor protein (APP) is the biological precursor of β-amyloids, a known histopathological hallmark associated with Alzheimer's disease (AD). The function of APP is of great interest yet remains elusive. One of the extracellular domains of APP, the E2 domain, has been proposed to possess ferroxidase activity and affect neuronal iron homeostasis.

Computational Analysis of Structure - Activity Relationships in Highly Active Homogeneous Ruthenium-based Water Oxidation Catalysts.

Linear free energy scaling relationships (LFESRs) and regression analysis may predict the catalytic performance of heterogeneous and recently, homogenous water oxidation catalysts (WOCs). This study analyses twelve homogeneous Ru-based catalysts - some, the most active catalysts studied: the Ru(tpy-R)(QC) and Ru(tpy-R)(4-pic) catalysts, where tpy is 2,2:6,2-terpyridine, QC is 8-quinolinecarboxylate and 4-pic is 4-picoline. Typical relationships studied among heterogenous and solid-state catalysts cannot be broadly applied to homogeneous catalysts.

A Manganese Compound I Model with a High Reactivity in the Oxidation of Organic Substrates and Water.

A high-valent manganese(IV)-hydroxo porphyrin π-cation radical complex, [Mn(IV)(OH)(Porp)(X)], was synthesized and characterized spectroscopically. The Mn porphyrin intermediate was highly reactive in alkane hydroxylation and oxygen atom transfer reactions. More importantly, the Mn porphyrin intermediate reacted with water at a fast rate, resulting in the dioxygen evolution.

X-ray Emission Spectroscopy of Single Protein Crystals Yields Insights into Heme Enzyme Intermediates.

Enzyme reactivity is often enhanced by changes in oxidation state, spin state, and metal-ligand covalency of associated metallocofactors. The development of spectroscopic methods for studying these processes coincidentally with structural rearrangements is essential for elucidating metalloenzyme mechanisms. Herein, we demonstrate the feasibility of collecting X-ray emission spectra of metalloenzyme crystals at a third-generation synchrotron source.

Photoexcitation of Fe O Nodes in MOF Drives Water Oxidation at pH=1 When Ru Catalyst Is Present.

Artificial photosynthesis strives to convert the energy of sunlight into sustainable, eco-friendly solar fuels. However, systems with light-driven water oxidation reaction (WOR) at pH=1 are rare. Broadly used [Ru(bpy) ] (bpy=2,2'-bipyridine) photosensitizer has a fixed +1.

A Spectroscopically Observed Iron Nitrosyl Intermediate in the Reduction of Nitrate by a Surface-Conjugated Electrocatalyst.

We report an iron-based graphite-conjugated electrocatalyst (GCC-FeDIM) that combines the well-defined nature of homogeneous molecular electrocatalysts with the robustness of a heterogeneous electrode. A suite of spectroscopic methods, supported by the results of DFT calculations, reveals that the electrode surface is functionalized by high spin ( = 5/2) Fe(III) ions in an FeNCl coordination environment. The chloride ions are hydrolyzed in aqueous solution, with the resulting cyclic voltammogram revealing a Gaussian-shaped wave assigned to 1H/1e reduction of surface Fe(III)-OH surface.

Systematic Influence of Electronic Modification of Ligands on the Catalytic Rate of Water Oxidation by a Single-Site Ru-Based Catalyst.

Catalytic water oxidation is an important process for the development of clean energy solutions and energy storage. Despite the significant number of reports on active catalysts, systematic control of the catalytic activity remains elusive. In this study, descriptors are explored that can be correlated with catalytic activity.

A Mononuclear Non-heme Iron(III)-Peroxo Complex with an Unprecedented High O-O Stretch and Electrophilic Reactivity.

A mononuclear non-heme iron(III)-peroxo complex, [Fe(III)(O)(13-TMC)] (), was synthesized and characterized spectroscopically; the characterization with electron paramagnetic resonance, Mössbauer, X-ray absorption, and resonance Raman spectroscopies and mass spectrometry supported a high-spin = / Fe(III) species binding an O unit. A notable observation was an unusually high ν at ∼1000 cm for the peroxo ligand. With regard to reactivity, showed electrophilic reactivity in H atom abstraction (HAA) and O atom transfer (OAT) reactions.

Do multinuclear 3d metal catalysts achieve O-O bond formation via radical coupling or via water nucleophilic attack? WNA leads the way in [CoO].

Catalytic water oxidation is a required process for clean energy production based on the concept of artificial photosynthesis. Here, we provide spectroscopic and computational analysis for the closest known photosystem II analog, [CoO] ([CoOPyAc], Py = pyridine and Ac = CHCOO), which catalyzes electrochemical water oxidation. extended X-ray absorption fine structure detects an ultrashort, Co=O (~1.

Facile Light-Induced Transformation of [Ru(bpy)(bpyNO)] to [Ru(bpy)].

Ru-based coordination compounds have important applications as photosensitizers and catalysts. [Ru(bpy)(bpyNO)] (bpy = 2,2'-bipyridine and bpyNO = 2,2'-bipyridine--oxide) was reported to be extremely light-sensitive, but its light-induced transformation pathways have not been analyzed. Here, we elucidated a mechanism of the light-induced transformation of [Ru(bpy)(bpyNO)] using UV-vis, EPR, resonance Raman, and NMR spectroscopic techniques.

Atomically Dispersed Iridium on Indium Tin Oxide Efficiently Catalyzes Water Oxidation.

Heterogeneous catalysts in the form of atomically dispersed metals on a support provide the most efficient utilization of the active component, which is especially important for scarce and expensive late transition metals. These catalysts also enable unique opportunities to understand reaction pathways through detailed spectroscopic and computational studies. Here, we demonstrate that atomically dispersed iridium sites on indium tin oxide prepared via surface organometallic chemistry display exemplary catalytic activity in one of the most challenging electrochemical processes, the oxygen evolution reaction (OER).

DIY XES - development of an inexpensive, versatile, and easy to fabricate XES analyzer and sample delivery system.

The application of X-ray emission spectroscopy (XES) has grown substantially with the development of X-ray free electron lasers, third and fourth generation synchrotron sources and high-power benchtop sources. By providing the high X-ray flux required for XES, these sources broaden the availability and application of this method of probing electronic structure. As the number of sources increase, so does the demand for X-ray emission detection and sample delivery systems that are cost effective and customizable.

Characterization of the Fe =O Complex in the Pathway of Water Oxidation.

Hypervalent Fe =O species are implicated in a multitude of oxidative reactions of organic substrates, as well as in catalytic water oxidation, a reaction crucial for artificial photosynthesis. Spectroscopically characterized Fe species are exceedingly rare and, so far, were produced by the oxidation of Fe complexes with peroxy acids or H O : reactions that entail breaking of the O-O bond to form a Fe =O fragment. The key Fe =O species proposed to initiate the O-O bond formation in water oxidation reactions remained undetected, presumably due to their high reactivity.

An evolutionarily conserved iron-sulfur cluster underlies redox sensory function of the Chloroplast Sensor Kinase.

Photosynthetic efficiency depends on equal light energy conversion by two spectrally distinct, serially-connected photosystems. The redox state of the plastoquinone pool, located between the two photosystems, is a key regulatory signal that initiates acclimatory changes in the relative abundance of photosystems. The Chloroplast Sensor Kinase (CSK) links the plastoquinone redox signal with photosystem gene expression but the mechanism by which it monitors the plastoquinone redox state is unclear.

Unraveling the Mechanism of Catalytic Water Oxidation via Synthesis of Reactive Intermediate.

Artificial photosynthesis could promise abundant, carbon-neutral energy, but implementation is currently limited by a lack of control over the multi-electron catalysis of water oxidation. Discoveries of the most active catalysts still rely heavily on serendipity. [Ru(tpy)(bpy)(HO)] (; bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine) is representative of the largest known class of water oxidation catalysts.

A High-Valent Manganese(IV)-Oxo-Cerium(IV) Complex and Its Enhanced Oxidizing Reactivity.

A mononuclear nonheme manganese(IV)-oxo complex binding the Ce ion, [(dpaq)Mn (O)] -Ce (1-Ce ), was synthesized by reacting [(dpaq)Mn (OH)] (2) with cerium ammonium nitrate (CAN). 1-Ce was characterized using various spectroscopic techniques, such as UV/Vis, EPR, CSI-MS, resonance Raman, XANES, and EXAFS, showing an Mn-O bond distance of 1.69 Å with a resonance Raman band at 675 cm .

Early Binding of Substrate Oxygen Is Responsible for a Spectroscopically Distinct S State in Photosystem II.

The biological generation of oxygen by the oxygen-evolving complex (OEC) in photosystem II (PS II) is one of nature's most important reactions. The OEC is a MnCa cluster that has multiple Mn-O-Mn and Mn-O-Ca bridges and binds four water molecules. Previously, binding of an additional oxygen was detected in the S to S transition.

Rapid evolution of the Photosystem II electronic structure during water splitting.

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. A MnCa cluster embedded in the photosystem II protein environment is responsible for the production of atmospheric oxygen. Here, time-resolved x-ray emission spectroscopy (XES) was used to observe the process of oxygen formation in real time.

Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions.

Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)Mn(O)]-M (1-M, M = Ca, Mg, Zn, Lu, Y, Al, and Sc) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)Mn(OH)], with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M ). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M on the reactivity of 1-M in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions.