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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.

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.