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Model of the Oxygen Evolving Complex Which Is Highly Predisposed to O–O Bond Formation

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_<i>z</i>^ox 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

Model of the Oxygen Evolving Complex Which Is Highly Predisposed to O–O Bond Formation

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_<i>z</i>^ox 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

Model of the Oxygen Evolving Complex Which Is Highly Predisposed to O–O Bond Formation

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_<i>z</i>^ox 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

Model of the Oxygen Evolving Complex Which Is Highly Predisposed to O–O Bond Formation

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_<i>z</i>^ox 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

Spectroscopic Analysis of Catalytic Water Oxidation by [Ru^II(bpy)(tpy)H₂O]²⁺ Suggests That Ru^VO Is Not a Rate-Limiting Intermediate

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

Triplet Excited State Energies and Phosphorescence Spectra of (Bacterio)Chlorophylls

(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 <i>a</i>, <i>b</i>, <i>c</i>₂, <i>d</i>) and bacteriochlorophyll (BChl <i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>, <i>e</i>, <i>g</i>) molecules and one bacteriopheophytin (BPheo <i>g</i>) have been directly determined via their phosphorescence spectra. Phosphorescence of four molecules (Chl <i>c</i>₂, BChl <i>e</i> and <i>g</i>, BPheo <i>g</i>) 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 Mg²⁺ 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 <i>c</i>₁, Chl <i>f</i>, and BChl <i>f</i>

The Key Ru^V=O Intermediate of Site-Isolated Mononuclear Water Oxidation Catalyst Detected by <i>in Situ</i> X‑ray Absorption Spectroscopy

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^V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 Ru^VO intermediate at potentials above 1.34 V vs NHE at pH 1, which is characterized by electron paramagnetic resonance and <i>in situ</i> X-ray absorption spectroscopies. The <i>in situ</i> extended X-ray absorption fine structure analysis shows a RuO bond distance of 1.75 ± 0.02 Å, consistent with computational results. Electrochemical studies and density functional theory calculations suggest that the water nucleophilic attack on the surface-bound Ru^VO intermediate (O–O bond formation) is the rate limiting step for OER catalysis at low pH

Uncovering the Role of Oxygen Atom Transfer in Ru-Based Catalytic Water Oxidation

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, [Ru^II(NPM)­(4-pic)₂(H₂O)]²⁺ (NPM = 4-<i>t</i>-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, g_<i>x</i> = 2.30, g_<i>y</i> = 2.18, and g_<i>z</i> = 1.83 which allowed us to observe fast dynamics of oxygen atom transfer from the Ru^IVO oxo species to the uncoordinated nitrogen of the NPM ligand. In few seconds, the NPM ligand modification results in [Ru^III(NPM-NO)­(4-pic)₂(H₂O)]³⁺ and [Ru^III(NPM-NO,NO)­(4-pic)₂]³⁺ complexes. A proposed [Ru^V(NPM)­(4-pic)₂O]³⁺ intermediate was not detected under the tested conditions. We demonstrate that while the proximal base might be beneficial in O–O bond formation via nucleophilic water attack on an oxo species as shown by DFT, the noncoordinating nitrogen is impractical as a base in water oxidation catalysts due to its facile conversion to the N–O group. This study opens new horizons for understanding the real structure of Ru catalysts under water oxidation conditions and points toward the need to further investigate the role of the N–O ligand in promoting water oxidation catalysis

Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis

Catalytic O₂ evolution with <i>cis</i>,<i>cis</i>-[(bpy)₂(H₂O)­Ru^IIIORu^III(OH₂)­(bpy)₂]⁴⁺ (bpy is 2,2-bipyridine), the so-called blue dimer, the first designed water oxidation catalyst, was monitored by UV–vis, EPR, and X-ray absorption spectroscopy (XAS) with ms time resolution. Two processes were identified, one of which occurs on a time scale of 100 ms to a few seconds and results in oxidation of the catalyst with the formation of an intermediate, here termed [3,4]′. A slower process occurring on the time scale of minutes results in the decay of this intermediate and O₂ evolution. Spectroscopic data suggest that within the fast process there is a short-lived transient intermediate, which is a precursor of [3,4]′. When excess oxidant was used, a highly oxidized form of the blue dimer [4,5] was spectroscopically resolved within the time frame of the fast process. Its structure and electronic state were confirmed by EPR and XAS. As reported earlier, the [3,4]′ intermediate likely results from reaction of [4,5] with water. While it is generated under strongly oxidizing conditions, it does not display oxidation of the Ru centers past [3,4] according to EPR and XAS. EXAFS analysis demonstrates a considerably modified ligand environment in [3,4]′. Raman measurements confirmed the presence of the O–O fragment by detecting a new vibration band in [3,4]′ that undergoes a 46 cm^–1 shift to lower energy upon ¹⁶O/¹⁸O exchange. Under the conditions of the experiment at pH 1, the [3,4]′ intermediate is the catalytic steady state form of the blue dimer catalyst, suggesting that its oxidation is the rate-limiting step