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Artificial photosynthesis strives to convert the energy of sunlight into sustainable solar fuels. However, systems with light-driven water oxidation reaction at pH = 1 are rare. Broadly used [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) photosensitizer is insufficient to drive most water oxidation catalysts (WOCs) in acid, while Fe2O3 is not stable at low pH. Here, we present Fe-based metal organic framework (MOF) WOC photoelectrocatalysts active at pH=1. Fe-MIL-126 and Fe MOF-dcbpy were formed with 4,4'-biphenyl dicarboxylate (bpdc), 2,2'-bipyridine-5,5'-dicarboxylic acid (dcbpy) linkers and their mixtures. The dcbpy linker allows integration of metal-based catalysts. Fe-based MOFs were doped with Ru-based precursors to achieve MOFs bearing [Ru(bpy)(dcbpy)(H2O)2]2+ WOC. Materials were analyzed with XRD, SEM, FT-IR, resonance Raman, X-ray absorption spectroscopy, fs optical pump probe, EPR, diffuse reflectance and electric conductivity measurements and were modeled by band structure calculations. We show that under reaction conditions, FeIII and RuIII oxidation states are present, indicating rate-limiting electron transfer in MOF. Fe3O nodes emerge as photosensitizers able to drive prolonged O2 evolution in acid. Further developments are possible via MOF's linker modification for enhanced light absorption, electrical conductivity, reduced MOF solubility in acid, Ru-ligand modification for faster WOC catalysis, or WOC substitution to 3d metal-based systems. Our findings give further insight for development of light-driven water splitting systems based on Earth-abundant metals.

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 3rd generation synchrotron source. In particular, we report the development of a von Hamos spectrometer for the collection of Fe Kβ emission optimized for analysis of dilute biological samples. We further showcase its application in crystals of the immunosuppressive heme-dependent enzyme indoleamine 2,3-dioxygenase. Spectra from protein crystals in different states were compared with relevant reference compounds. Complementary density functional calculations assessing covalency support our spectroscopic analysis and identify active site conformations that correlate to high- and low-spin states. These experiments validate the suitability of an X-ray emission approach for determining spin states of previously uncharacterized metalloenzyme reaction intermediates.

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 thirteen homogeneous Ru−based catalysts—some, the most active catalysts studied: the Ru(tpy−R)(QC) and Ru(tpy−R)(4−pic)2 complexes, where tpy is 2,2’;6’,2”terpyridine, QC is 8−quinolinecarboxylate and 4−pic is 4−picoline. Typical relationships studied among heterogenous catalysts cannot be applied to homogeneous catalysts. The selected group of structurally similar catalysts with impressive catalytic activity deserves closer computational and statistical analysis of multiple reaction step energetics correlating with measured catalytic activity. We report general methods of LFESR analysis yield insufficiently robust relationships between descriptor variables. However, volcano−plot−based analysis grounded in Sabatier’s principle reveals ideal relative energies of the RuIV = O and RuIV−OH intermediates and optimal changes in free energies of water nucleophilic attack on RuV = O. A narrow range of RuIV−OH to RuV = O redox potentials corresponding with the highest catalytic activities suggests facile access to the catalytically competent high−valent RuV = O state, often inaccessible from RuIV = O. Our work incorporates experimental oxygen evolution rates into approaches of LFESR and Sabatier−principle−based analysis, identifying a narrow yet fertile energetic landscape to bountiful oxygen evolution activity, leading to future rational design.

We report the synthesis, characterization, and electron-transfer (ET) oxidation reactivity of a chromium(V)–oxo TAML cation radical complex binding Sc3+ ion, {[CrV(O)(TAML•+)]-Sc3+}3+ (2-Sc). Its p...

Ru-based coordination compounds have important applications as photosensitizers and catalysts. [RuII(bpy)2(bpyNO)]2+ (bpy = 2,2′-bipyridine and bpyNO = 2,2′-bipyridine-N-oxide) was reported to be e...

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). In situ X-ray absorption studies revealed the formation of Ir-V=O intermediate under OER conditions with an Ir-O distance of 1.83 angstrom. Modeling of the reaction mechanism indicates that Ir-V=O is likely a catalyst resting state, which is subsequently oxidized to Ir-VI enabling fast water nucleophilic attack and oxygen evolution. We anticipate that the applied strategy can be instrumental in preparing and studying a broad range of atomically dispersed transition metal catalysts on conductive oxides for (photo) electrochemical applications., ACS Central Science, 6 (7), ISSN:2374-7951

Hypervalent FeV =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 FeV species are exceedingly rare and, so far, were produced by the oxidation of Fe complexes with peroxy acids or H2 O2 : reactions that entail breaking of the O-O bond to form a FeV =O fragment. The key FeV =O species proposed to initiate the O-O bond formation in water oxidation reactions remained undetected, presumably due to their high reactivity. Here, we achieved freeze quench trapping of six coordinated [FeV =O,(OH)(Pytacn)]2+ (Pytacn=1-(2'-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane) (2) generated during catalytic water oxidation. X-ray absorption spectroscopy (XAS) confirmed the FeV oxidation state and the presence of a FeV =O bond at ≈1.60 A. Combined EPR and DFT methods indicate that 2 contains a S=3/2 FeV center. 2 is the first spectroscopically characterized high spin oxo-FeV complex and constitutes a paradigmatic example of the FeV =O(OH) species proposed to be responsible for catalytic water oxidation reactions.

The realization of artificial photosynthesis requires the design of fast and durable water oxidation catalysts that can be incorporated into future sunlight-to-chemical-fuels assemblies. Here we re...

Catalytic water oxidation is an important process for the development of future clean energy solutions and energy storage. Despite the significant number of reports on active catalysts, systematic control of the catalytic activity remains elusive. Here, we continue to explore descriptors which can be correlated with catalytic activity. [Ru(tpy)(pic)(2)(H(2)O)](NO(3))(2), 1, and [Ru(EtO-tpy)(pic)(2)(H(2)O)](NO(3))(2), 2, (where tpy = 2,2′:6′,2″-terpyridine, EtO-tpy = 4′-(ethoxy)-2,2′:6′,2″-terpyridine, pic = 4-picoline) have been synthesized and characterized by NMR, UV-vis, electrochemical analysis, EPR, resonance Raman, and X-ray absorption spectroscopy. Addition of the ethoxy group increases catalytic activity in chemically driven and photocatalytic water oxidation. Thus, the effect of the electron-donating group known for the [Ru(tpy)(bpy)(H(2)O)](2+) family is transferable to architectures with tpy ligand trans to the Ru-oxo unit. Under catalytic conditions, 2 displays new spectroscopic signals tentatively assigned to a peroxo intermediate. Reaction pathways were analyzed using DFT. Currently 2 is one of the most active catalysts functioning via a water nucleophilic attack mechanism.

A mononuclear non-heme iron(III)-peroxo complex, [Fe(III)(O2)(13-TMC)]+ (1), was synthesized and characterized spectroscopically; the characterization with electron paramagnetic resonance, Mossbauer, X-ray absorption, and resonance Raman spectroscopies and mass spectrometry supported a high-spin S = 5/2 Fe(III) species binding an O2 unit. A notable observation was an unusually high νO-O at ∼1000 cm-1 for the peroxo ligand. With regard to reactivity, 1 showed electrophilic reactivity in H atom abstraction (HAA) and O atom transfer (OAT) reactions. In the HAT reaction, a kinetic isotope effect (KIE) value of 5.8 was obtained in the oxidation of 9,10-dihydroanthracene. In the OAT reaction, a negative ρ value of -0.61 in the Hammett plot was determined in the oxidation of p-X-substituted thioanisoles. Another interesting observation was the electrophilic reactivity of 1 in the oxidation of benzaldehyde derivatives, such as a negative ρ value of -0.77 in the Hammett plot and a KIE value of 2.2. To the best of our knowledge, the present study reports the first example of a mononuclear non-heme iron(III)-peroxo complex with an unusually high νO-O value and unprecedented electrophilic reactivity in oxidation reactions.

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)(H2O)]2+ (1; bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine) is representative of the largest known class of water oxidation catalysts. We undertook an extensive spectroscopic analysis of the prototypical 1 water oxidation catalyst and its fastest known analog [Ru(EtO-tpy)(bpy)(H2O)]2+ (2), capable of 10 times faster water oxidation, to investigate the mechanism of action and factors controlling catalytic activity. EPR and resonance Raman spectroscopy did not detect the proposed [RuV═O] intermediate in 1 and 2 but indicated the possible formation of N-oxides. A lag phase was observed prior to O2 evolution, suggesting catalyst modification before the onset of catalysis. The reactive intermediate [Ru(tpy)(bpy-NO)(H2O)]2+ (1-NO; bpy-NO = 2,2'-bipyridine-N-oxide) proposed by combined spectroscopic and DFT analysis was de novo synthesized and demonstrated 100-fold greater catalytic activity than 1. Thus, in situ transient formation of small amounts of the Ru complex with N-oxide ligands can significantly activate single-site Ru-based catalysts. Furthermore, the rate of O2 evolution was found to correlate with the redox potential of the ligand. This observation might assist with rational design of new catalysts.

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 Mn4Ca 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 S2 to S3 transition. Here we demonstrate that early binding of the substrate oxygen to the five-coordinate Mn1 center in the S2 state is likely responsible for the S2 high-spin EPR signal. Substrate binding in the Mn1-OH form explains the prevalence of the high-spin S2 state at higher pH and its low-temperature conversion into the S3 state. The given interpretation was confirmed by X-ray absorption spectroscopic measurements, DFT, and broken symmetry DFT calculations of structures and magnetic properties. Structural, electronic, and spectroscopic properties of the high-spin S2 state model are provided and compared with the available S3 state models. New interpretation of the high-spin S2 state opens opportunity for analysis of factors controlling the oxygen substrate binding in PS II.

Artificial photosynthesis could promise cheap and abundant energy but requires the discovery of new water oxidation catalysts. A RuV = O 7-coordinate intermediate is implicated in the reactivity of the fastest water oxidation catalysts. Previously we reported in situ characterization of the 7-coordinate [RuV = O(L)2(bda)]+ stabilized on the electrode. Here we use ligand protection to stabilize [RuV = O(pic)2(dpp)]3+. We report the transformation of [RuII(pic)2(dpp)]2+ (dpp = 2,9-di-(pyrid-2′-yl)-1,10-phenanthroline, pic = 4-picoline) water oxidation catalyst using a combination of electron paramagnetic resonance (EPR), X-ray diffraction, resonance Raman (RR) and density functional theory (DFT). We observe the generation of [RuV = O(pic)2(dpp)]3+ and its chemical reactivity in solution. Lag phase in the oxygen evolution by [RuII(pic)2(dpp)]2+ is due to catalyst activation via an oxygen atom transfer from [RuV = O(pic)2(dpp)]3+ to the polypyridine dpp ligand and formation of [RuIII(pic)2(dpp-NO,NO)]3+. Detailed information regarding catalyst activation during the reaction will enable the design of more active and stable catalysts.

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. Here, we present a detailed fabrication protocol for von Hamos X-ray optics and give details for a 3D-printed spectrometer design. Additionally, we outline an automated, externally triggered liquid sample delivery system that can be used to repeatedly deliver nanoliter droplets onto a plastic substrate for measurement. These systems are both low cost, efficient and easy to recreate or modify depending on the application. A low cost multiple X-ray analyzer system enables measurement of dilute samples, whereas the sample delivery limits sample loss and replaces spent sample with fresh sample in the same position. While both systems can be used in a wide range of applications, the design addresses several challenges associated specifically with time-resolved XES (TRXES). As an example application, we show results from TRXES measurements of photosystem II, a dilute, photoactive protein.

Catalytic water oxidation is a required process for clean energy production based on the concept of artificial photosynthesis. Here, we provide in situ spectroscopic and computational analysis for the closest known photosystem II analog, [Co(4)O(4)](n+) ([Co(4)O(4)Py(4)Ac(4)](0), Py = pyridine and Ac = CH(3)COO(−)), which catalyzes electrochemical water oxidation. In situ extended X-ray absorption fine structure detects an ultrashort, Co(IV)=O (~1.67 Å) moiety, a crucial intermediate for O–O bond formation. Density function theory analyses show that the intermediate has two Co(IV) centers and a Co(IV)=O unit of strong radicaloid character sufficient to support a Co(IV)=O + H(2)O = Co–OOH + H(+) transition, where the carboxyl ligand accepts the proton and the bridging oxygen stabilizes the peroxide via hydrogen bonding. The proposed water nucleophilic attack mechanism accounts for all prior spectroscopic evidence on the Co(4)O(4)(4+) core. Our results are important for the design and development of efficient water oxidation catalysts, which contribute to the ultimate goal of clean energy from artificial photosynthesis.

Development of economically viable artificial photosynthesis requires use of 3d metal-based catalysts. Water oxidation by [Co4O4]n+ cubane mimics water splitting by CaMn4O5 cluster in Nature but the exact mechanism of O-O bond formation is presently unknown. We demonstrate first in situ detection CoIV=O (~ 1.67 Å) moiety formed upon activation of [Co4O4Py4Ac4]0 (Py = pyridine and Ac = CH3COO−) towards O-O bond formation. Combined spectroscopic and DFT analyses show that the intermediate active in O-O bond formation has two CoIV centers and at least one CoIV=O unit of strong radicaloid character that participates in O-O bond formation via water nucleophilic attack. The multimetallic structure of the cubane provides unique stabilization for CoIV=O + H2O = Co-OOH + H+ transition with the carboxyl accepting the proton and the bridging oxygen stabilizing the peroxide via hydrogen bonding. Results are important for development of oxygen evolution catalysts based on Earth-abundant 3d elements.

Ru-based coordination compounds have important applications as photosensitizers and catalysts. [Ru

Hypervalent Fe

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, oxygen evolution reaction (OER). In situ X-ray absorption studies revealed the formation of IrV=O intermediate under OER conditions with an Ir–O distance of 1.83 Å. Modelling of the reaction mechanism indicates that Ir(V)=O is likely a catalyst resting state, which is subsequently oxidized to Ir(VI) enabling fast water nucleophilic attack and oxygen evolution. We anticipate that the applied strategy can be instrumental in preparing and studying a broad range of atomically dispersed transition metal catalysts on conductive oxides for (photo)electrochemical applications.

Light-driven water oxidation is a fundamental reaction in the biosphere. The Mn4Ca cluster of photosystem II cycles through five redox states termed S0-S4, after which oxygen is evolved. Critically, the timing of O-O bond formation within the Kok cycle remains unknown. By combining recent crystallographic, spectroscopic, and DFT results, we demonstrate an atomistic S3 state model with the possibility of a low barrier to O-O bond formation prior to the final oxidation step. Furthermore, the associated one electron oxidized S4 state does not provide more advantages in terms of spin alignment or the energy of O-O bond formation. We propose that a high energy peroxide isoform of the S3 state can preferentially be oxidized by Tyr zox in the course of final electron transfer leading to O2 evolution. Such a mechanism may explain the peculiar kinetic behavior of O2 evolution as well as serve as an evolutionary adaptation to avoid release of the harmful peroxides.

Investigation of chemical water oxidation was conducted on [Ru(tpy)(dcbpy)(OH2 )]2+ (tpy=2,2':6',2''-terpyridine, dcbpy=5,5'-dicarboxy-2,2'-bipyridine)-doped UiO-67 metal-organic framework (MOF). The MOF catalyst exhibited a single-site reaction pathway with kinetic behavior similar to that of a homogeneous Ru complex. The reaction was first order with respect to both the concentration of the Ru catalyst and ceric ammonium nitrate (CAN), with kcat =3(±2)×10-3 m-1 s-1 in HNO3 (pH 0.5). The common degradation pathways of ligand dissociation and dimerization were precluded by MOF incorporation, which led to sustained catalysis and greater reusability as opposed to the molecular catalyst in homogeneous solution. Lastly, at the same loading (ca. 97 nmol mg-1 ), samples of different particle sizes generated the same amount of oxygen (ca. 100 nmol), indicative of in-MOF reactivity. The results suggest that the rate of redox-hopping charge transport is sufficient to promote chemistry throughout the MOF particulates.

Improvement of the oxygen evolution reaction (OER) is a challenging step toward the development of sustainable energy technologies. Enhancing the OER rate and efficiency relies on understanding the water oxidation mechanism, which entails the characterization of the reaction intermediates. Very active Ru-bda type (bda is 2,2′-bipyridine-6,6′-dicarboxylate) molecular OER catalysts are proposed to operate via a transient 7-coordinate RuV═O intermediate, which so far has never been detected due to its high reactivity. Here we prepare and characterize a well-defined supported Ru(bda) catalyst on porous indium tin oxide (ITO) electrode. Site isolation of the catalyst molecules on the electrode surface allows trapping of the key 7-coordinate RuV═O intermediate at potentials above 1.34 V vs NHE at pH 1, which is characterized by electron paramagnetic resonance and in situ X-ray absorption spectroscopies. The in situ extended X-ray absorption fine structure analysis shows a Ru═O bond distance of 1.75 ± 0.02 A, co...

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. Here we show that the purified Arabidopsis and Phaeodactylum CSK and the cyanobacterial CSK homologue, Histidine kinase 2 (Hik2), are iron-sulfur proteins. The Fe-S cluster of CSK is further revealed to be a high potential redox-responsive [3Fe-4S] center. CSK responds to redox agents with reduced plastoquinone suppressing its autokinase activity. Redox changes within the CSK iron-sulfur cluster translate into conformational changes in the protein fold. These results provide key insights into redox signal perception and propagation by the CSK-based chloroplast two-component system., Ibrahim et al. show that the Arabidopsis and Phaeodactylum Chloroplast Sensor Kinase (CSK) and the cyanobacterial CSK homologue, Histidine kinase 2, are iron sulfur proteins. This study provides insights into how the CSK-based chloroplast two-component system perceives and propagates the plastoquinone redox signals.

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)(H

A mononuclear nonheme manganese(IV)-oxo complex binding the Ce4+ ion, [(dpaq)MnIV (O)]+ -Ce4+ (1-Ce4+ ), was synthesized by reacting [(dpaq)MnIII (OH)]+ (2) with cerium ammonium nitrate (CAN). 1-Ce4+ 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 A with a resonance Raman band at 675 cm-1 . Electron-transfer and oxygen atom transfer reactivities of 1-Ce4+ were found to be greater than those of MnIV (O) intermediates binding redox-inactive metal ions (1-Mn+ ). This study reports the first example of a redox-active Ce4+ ion-bound MnIV -oxo complex and its spectroscopic characterization and chemical properties.

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. A Mn$_{4}$Ca 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. These experiments reveal that the oxygen evolution step, initiated by three sequential laser flashes, is accompanied by rapid (within 50 $\mu$s) changes to the Mn K$\beta$ XES spectrum. However, no oxidation of the Mn$_{4}$Ca core above the all Mn$^{\text{IV}}$ state was detected to precede O-O bond formation. A new mechanism featuring Mn$^{\text{IV}}$=O formation in the S$_{3}$ state is proposed to explain the spectroscopic results. This chemical formulation is consistent with the unique reactivity of the S$_{3}$ state and explains facilitation of the following S$_{3}$ to S$_{0}$ transition, resolving in part the kinetic limitations associated with O-O bond formation. In the proposed mechanism, O-O bond formation precedes transfer of the final (4$^{\text{th}}$) electron from the Mn$_{4}$Ca cluster, in agreement with experiment., Comment: 49 pages, includes main text (pgs. 1-16) and supplementary material (pgs. 17-49), 13 figures (4 main text, and 9 supplementary), 12 tables (2 main text, 10 supplementary)

Water oxidation, a key component in artificial photosynthesis, requires high overpotentials and exhibits slow reaction kinetics that necessitates the use of stable and efficient heterogeneous water-oxidation catalysts (WOCs). Here, we report the synthesis of UiO-67 metal–organic framework (MOF) thin films doped with [Ru(tpy)(dcbpy)OH2]2+ (tpy=2,2′:6′,2′′-terpyridine, dcbpy=5,5′-dicarboxy-2,2′-bipyridine) on conducting surfaces and their propensity for electrochemical water oxidation. The electrocatalyst oxidized water with a turnover frequency (TOF) of (0.2±0.1) s−1 at 1.71 V versus the normal hydrogen electrode (NHE) in buffered solution (pH∼7) and exhibited structural and electrochemical stability. The electroactive sites were distributed throughout the MOF thin film on the basis of scan-ratedependent voltammetry studies. This work demonstrates a promising way to immobilize large concentrations of electroactive WOCs into a highly robust MOF scaffold and paves the way for future photoelectrochemical water-splitting systems.

The realization of artificial photosynthesis carries the promise of cheap and abundant energy, however, significant advances in the rational design of water oxidation catalysts are required. Detailed information on the structure of the catalyst under reaction conditions and mechanisms of O–O bond formation should be obtained. Here, we used a combination of electron paramagnetic resonance (EPR), stopped flow freeze quench on a millisecond–second time scale, X-ray absorption (XAS), resonance Raman (RR) spectroscopy, and density functional theory (DFT) to follow the dynamics of the Ru-based single site catalyst, [RuII(NPM)(4-pic)2(H2O)]2+ (NPM = 4-t-butyl-2,6-di(1′,8′-naphthyrid-2′-yl)pyridine, pic = 4-picoline), under the water oxidation conditions. We report a unique EPR signal with g-tensor, gx = 2.30, gy = 2.18, and gz = 1.83 which allowed us to observe fast dynamics of oxygen atom transfer from the RuIV═O oxo species to the uncoordinated nitrogen of the NPM ligand. In few seconds, the NPM ligand modific...

X-ray emission (XES) spectroscopy is an attractive technique for analysis of the electronic structure of molecules, materials, and metalloproteins. However, a better understanding of XES results is required. Using a combination of experiment and ground-state density functional theory analysis, we rationalize differences in the X-ray emission spectra of multinuclear Mn complexes. Model compounds, including dinuclear [Mn2O2L′4](ClO4)3 (L′= 2,2′-bipyridyl, [1]) and two examples from the Mn4O4L6 “cubane” family of model compounds (L = (p-R-C6H4)PO2−, R = OCH3 [2], CH3 [3] ), were compared with the Oxygen Evolving Complex of Photosystem II. Our analysis shows that changes in the structure of the Mn complexes, resulting in changes to the spin polarization, can introduce significant spectral shifts in compounds of the same formal redox state. The implications of changes in spin polarization for understanding photosynthetic water-splitting catalysis is discussed.

The journal of physical chemistry letters 10(3), 441 - 446 (2019). doi:10.1021/acs.jpclett.8b03595, X-ray free electron lasers (XFELs) provide ultrashort intense X-ray pulses suitable to probe electron dynamics but can also induce a multitude of nonlinear excitation processes. These affect spectroscopic measurements and interpretation, particularly for upcoming brighter XFELs. Here we identify and discuss the limits to observing classical spectroscopy, where only one photon is absorbed per atom for a Mn$^{2+}$ in a light element (O, C, H) environment. X-ray emission spectroscopy (XES) with different incident photon energies, pulse intensities, and pulse durations is presented. A rate equation model based on sequential ionization and relaxation events is used to calculate populations of multiply ionized states during a single pulse and to explain the observed X-ray induced spectral lines shifts. This model provides easy estimation of spectral shifts, which is essential for experimental designs at XFELs and illustrates that shorter X-ray pulses will not overcome sequential ionization but can reduce electron cascade effects., Published by ACS, Washington, DC

Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)MnIV(O)]+-M n+ (1-Mn+, M n+ = Ca2+, Mg2+, Zn2+, Lu3+, Y3+, Al3+, and Sc3+) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)MnIII(OH)]+, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M n+). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M n+ on the reactivity of 1-M n+ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-M n+, such as 1-Sc3+ ≈ 1-Al3+ > 1-Y3+ > 1-Lu3+ > 1-Zn2+ > 1-Mg2+ > 1-Ca2+, follows the Lewis acidity of M n+ bound to the Mn-O moiety; that is, the stronger the Lewis acidity of M n+, the higher the reactivity of 1-M n+ becomes. In sharp contrast, the reactivity of 1-M n+ in the HAT reaction was reversed, giving the reactivity order 1-Ca2+ > 1-Mg2+ > 1-Zn2+ > 1-Lu3+> 1-Y3+> 1-Al3+ ≈ 1-Sc3+; that is, the higher is Lewis acidity of M n+, the lower the reactivity of 1-M n+ in the HAT reaction. The latter result implies that the Lewis acidity of M n+ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-M n+; cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C-H bond activation reactions.

Artificial photosynthesis carries promise to deliver abundant clean energy for the needs of a growing population. Deep mechanistic understanding is required to achieve rational design of fast and durable water oxidation catalysts. Here we provided first evidence for a new mechanism of the O—O bond formation via radical coupling of the oxidized metal═oxo of radicaloid character (RuIV═O) and ligand based radical ([ligand-NO]+• cation radical). O—O bond formation is facilitated via spin alignment and takes place via a virtually barrier less pathway inside the single metal complex. In situ reactive intermediate conversion was monitored by mass spectrometry, resonance Raman (RR) and EPR. Computational analysis have shown that the formation of [ligand-NO]+• happens at a lower overpotential than the formation of the [RuV═O(ligand)]3+ intermediate. Overall, the presented paradigm for O—O bond formation opens new opportunities for rational catalyst design.

In the search for rational design of improved water oxidation catalysts, enhanced catalytic activities were reported for single site Ru catalysis with Ru-iodide coordination. As these complexes are not initially capable of proton coupled electron transfer (PCET) and Ru O formation, a proposal was put forward on the generation of catalytically active 7-coordinate Ru species. We tested this hypothesis by EPR and X-ray spectroscopy and found that [Ru II (bpy)(tpy)I] + only serves as a precursor for the formation of [Ru IV (bpy)(tpy) O] 2+ . Upon oxidation with excess of Ce IV the Ru I bond quickly dissociates with the formation of [Ru III (bpy)(tpy)H 2 O] 3+ and [Ru IV (bpy)(tpy) O] 2+ complexes. The catalytic steady state was composed of 95% [Ru IV (bpy)(tpy) O] 2+ species. Thus, introducing the Ru I bond into initial catalysts does not serve to improve catalyst design. This manuscript also shows how EXAFS can directly probe transition metal–halogen interaction for in situ catalysis.

The functional structure of the Oxygen Evolving Complex, the Mn4Ca cluster of Photosystem II, a critical component of natural photosynthesis, was analyzed at room temperature by EXAFS. An experimental improvement in the form of a spinning sample holder allowed us to efficiently counteract the severe X-ray damage sensitivity of Photosystem II to obtain high quality data subsequently used for a systematic evaluation of atomistic S1 state models. We investigated the accuracy of models collected by both conventional and femtosecond XRD at 1.9 and 1.95 Å resolution, respectively, as well as DFT-based models, to determine the structure most representative of experimental data. Additionally, room temperature EXAFS results do not show a visible reduction in the intensity of the k-space oscillations, supporting a rigid structure of the Mn4Ca cluster at room temperature.

Improvement of the oxygen evolution reaction (OER) is a challenging step toward the development of sustainable energy technologies. Enhancing the OER rate and efficiency relies on understanding the water oxidation mechanism, which entails the characterization of the reaction intermediates. Very active Ru-bda type (bda is 2,2'-bipyridine-6,6'-dicarboxylate) molecular OER catalysts are proposed to operate via a transient 7-coordinate Ru

Investigation of chemical water oxidation was conducted on [Ru(tpy)(dcbpy)(OH

Modern chemistry's grand challenge is to significantly improve catalysts for water splitting. Further progress requires detailed spectroscopic and computational characterization of catalytic mechanisms. We analyzed one of the most studied homogeneous single-site Ru catalysts, [Ru(II)(bpy)(tpy)H2O](2+) (where bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine). Our results reveal that the [Ru(V)(bpy)(tpy)═O](3+) intermediate, reportedly detected in catalytic mixtures as a rate-limiting intermediate in water activation, is not present as such. Using a combination of electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy, we demonstrate that 95% of the Ru complex in the catalytic steady state is of the form [Ru(IV)(bpy)(tpy)═O](2+). [Ru(V)(bpy)(tpy)═O](3+) was not observed, and according to density functional theory (DFT) analysis, it might be thermodynamically inaccessible at our experimental conditions. A reaction product with unique EPR spectrum was detected in reaction mixtures at about 5% and assigned to Ru(III)-peroxo species with (-OOH or -OO- ligands). We also analyzed the [Ru(II)(bpy)(tpy)Cl](+) catalyst precursor and confirmed that this molecule is not a catalyst and its oxidation past Ru(III) state is impeded by a lack of proton-coupled electron transfer. Ru-Cl exchange with water is required to form active catalysts with the Ru-H2O fragment. [Ru(II)(bpy)(tpy)H2O](2+) is the simplest representative of a larger class of water oxidation catalysts with neutral, nitrogen containing heterocycles. We expect this class of catalysts to work mechanistically in a similar fashion via [Ru(IV)(bpy)(tpy)═O](2+) intermediate unless more electronegative (oxygen containing) ligands are introduced in the Ru coordination sphere, allowing the formation of more oxidized Ru(V) intermediate.

Photosynthesis, a process catalysed by plants, algae and cyanobacteria converts sunlight to energy thus sustaining all higher life on Earth. Two large membrane protein complexes, photosystem I and II (PSI and PSII), act in series to catalyse the light-driven reactions in photosynthesis. PSII catalyses the light-driven water splitting process, which maintains the Earth’s oxygenic atmosphere1. In this process, the oxygen-evolving complex (OEC) of PSII cycles through five states, S0 to S4, in which four electrons are sequentially extracted from the OEC in four light-driven charge-separation events. Here we describe time resolved experiments on PSII nano/microcrystals from Thermosynechococcus elongatus performed with the recently developed2 technique of serial femtosecond crystallography. Structures have been determined from PSII in the dark S1 state and after double laser excitation (putative S3 state) at 5 and 5.5 Å resolution, respectively. The results provide evidence that PSII undergoes significant conformational changes at the electron acceptor side and at the Mn4CaO5 core of the OEC. These include an elongation of the metal cluster, accompanied by changes in the protein environment, which could allow for binding of the second substrate water molecule between the more distant protruding Mn (referred to as the ‘dangler’ Mn) and the Mn3CaOx cubane in the S2 to S3 transition, as predicted by spectroscopic and computational studies3, 4. This work shows the great potential for time-resolved serial femtosecond crystallography for investigation of catalytic processes in biomolecules.

(Bacterio)Chlorophyll ((B)Chl) molecules play a major role in photosynthetic light-harvesting proteins, and the knowledge of their triplet state energies is essential to understand the mechanisms of photodamage and photoprotection, as the triplet excitation energy of (B)Chl molecules can readily generate highly reactive singlet oxygen. The triplet state energies of 10 natural chlorophyll (Chl a, b, c2, d) and bacteriochlorophyll (BChl a, b, c, d, e, g) molecules and one bacteriopheophytin (BPheo g) have been directly determined via their phosphorescence spectra. Phosphorescence of four molecules (Chl c2, BChl e and g, BPheo g) was characterized for the first time. Additionally, the relative phosphorescence to fluorescence quantum yield for each molecule was determined. The measurements were performed at 77K using solvents providing a six-coordinate environment of the Mg(2+) ion, which allows direct comparison of these (B)Chls. Density functional calculations of the triplet state energies show good correlation with the experimentally determined energies. The correlation determined computationally was used to predict the triplet energies of three additional (B)Chl molecules: Chl c1, Chl f, and BChl f.

Density functional theory (DFT) is now widely used for analysis of the electronic structure and reactivities of transition metal complexes. However, large variability in how well different combinations of exchange-correlation potentials/basis sets reproduce real molecular geometries and electronic configurations remains a problem. Experimental X-ray absorption near-edge structure (XANES) spectra directly reflect the electronic structure of transition metal complexes. Combined analysis of theoretical calculations and experimental data is highly beneficial for DFT validation as well as for understanding limitations of the DFT. Ruthenium-based molecular water oxidation catalyst cis,cis-[(bpy)2(H2O)RuIIIORuIVO(OH)(bpy)2]4+ is a complex coordination compound with two Ru centers in different oxidation states bound by μ-oxo bridge. Multiple DFT calculations of this catalyst in different oxidation states have been reported previously but it was never clear whether DFT is truly capable of describing its geometry a...

It is well-known that biological samples undergo X-ray-induced degradation. One of the fastest occurring X-ray-induced processes involves redox modifications (reduction or oxidation) of redox-active cofactors in proteins. Here we analyze room-temperature data on the photoreduction of Mn ions in the oxygen-evolving complex (OEC) of photosystem II, one of the most radiation damage-sensitive proteins and a key constituent of natural photosynthesis in plants, green algae, and cyanobacteria. Time-resolved X-ray emission spectroscopy with wavelength-dispersive detection was used to collect data on the progression of X-ray-induced damage. A kinetic model was developed to fit experimental results, and the rate constant for the reduction of OEC Mn(III) and Mn(IV) ions by solvated electrons was determined. From this model, the possible kinetics of X-ray-induced damage at a variety of experimental conditions, such as different rates of dose deposition as well as different excitation wavelengths, can be inferred. We observed a trend of increasing dosage threshold prior to the onset of X-ray-induced damage with increasing rates of dose deposition. This trend suggests that experimentation with higher rates of dose deposition is beneficial for measurements of biological samples sensitive to radiation damage, particularly at pink beam and X-ray free electron laser sources.