A high-performance watermelon skin ion-solvating membrane for electrochemical CO2 reduction

Ion-solvating membranes have been gaining increasing attention as core components of electrochemical energy conversion and storage devices. However, the development of ion-solvating membranes with low ion resistance and high ion selectivity still poses challenges. In order to propose an effective strategy for high-performance ion-solvating membranes, this study conducted a comprehensive investigation on watermelon skin membranes through a combination of experimental research and molecular dynamics simulation. The micropores and continuous hydrogen-bonding networks constructed by the synergistic effect of cellulose fiber and pectin enable the hypodermis of watermelon skin membranes to have a high ion conductivity of 282.3 mS cm-1 (room temperature, saturated with 1 M KOH). The negatively charged groups and hydroxyl groups on the microporous channels increase the formate penetration resistance of watermelon skin membranes in contrast to commercially available membranes, and this is crucial for CO2 electroreduction. Therefore, the confinement of proton donors and negatively charged groups within three-dimensional microporous polymers gives inspiration for the design of high-performance ion-solvating membranes.

Constructing Co4(SO4)4 Clusters within Metal-Organic Frameworks for Efficient Oxygen Electrocatalysis

Multinuclear metal clusters are ideal candidates to catalyze small molecule activation reactions involving the transfer of multiple electrons. However, synthesizing active metal clusters is a big challenge. Herein, on constructing an unparalleled Co4(SO4)4 cluster within porphyrin-based metal-organic frameworks (MOFs) and the electrocatalytic features of such Co4(SO4)4 clusters for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is reported. The reaction of CoII sulfate and metal complexes of tetrakis(4-pyridyl)porphyrin under solvothermal conditions afforded Co4-M-MOFs (M═Co, Cu, and Zn). Crystallographic studies revealed that these Co4-M-MOFs have the same framework structure, having the Co4(SO4)4 clusters connected by metalloporphyrin units through Co─Npyridyl bonds. In the Co4(SO4)4 cluster, the four CoII ions are chemically and symmetrically equivalent and are each coordinated with four sulfate O atoms to give a distorted cube-like structure. Electrocatalytic studies showed that these Co4-M-MOFs are all active for electrocatalytic OER and ORR. Importantly, by regulating the activity of the metalloporphyrin units, it is confirmed that the Co4(SO4)4 cluster is active for oxygen electrocatalysis. With the use of Co porphyrins as connecting units, Co4-Co-MOF displays the highest electrocatalytic activity in this series of MOFs by showing a 10 mA cm-2 OER current density at 357 mV overpotential and an ORR half-wave potential at 0.83 V versus reversible hydrogen electrode (RHE). Theoretical studies revealed the synergistic effect of two proximal Co atoms in the Co4(SO4)4 cluster in OER by facilitating the formation of O─O bonds. This work is of fundamental significance to present the construction of Co4(SO4)4 clusters in framework structures for oxygen electrocatalysis and to demonstrate the cooperation between two proximal Co atoms in such clusters during the O─O bond formation process.

Unravelling Mn4Ca cluster vibrations in the S1, S2 and S3 states of the Kok-Joliot cycle of photosystem II

Vibrational spectroscopy serves as a powerful tool for characterizing intermediate states within the Kok-Joliot cycle. In this study, we employ a QM/MM molecular dynamics framework to calculate the room temperature infrared absorption spectra of the S1, S2, and S3 states via the Fourier transform of the dipole time auto-correlation function. To better analyze the computational data and assign spectral peaks, we introduce an approach based on dipole-dipole correlation function of cluster moieties of the reaction center. Our analysis reveals variation in the infrared signature of the Mn4Ca cluster along the Kok-Joliot cycle, attributed to its increasing symmetry and rigidity resulting from the rising oxidation state of the Mn ions. Furthermore, we successfully assign the debated contributions in the frequency range around 600 cm-1. This computational methodology provides valuable insights for deciphering experimental infrared spectra and understanding the water oxidation process in both biological and artificial systems.

Metal-Metal Redox Exchange to Produce Heterometallic Manganese-Cobalt Oxo Cubanes via a "Dangler" Intermediate

Pendent metals bound to heterocubanes are components of well-known active sites in enzymes that mediate difficult chemical transformations. Investigations into the specific role of these metal ions, sometimes referred to as "danglers", have been hindered by a paucity of rational synthetic routes to appropriate model structures. To generate pendent metal ions bonded to an oxo cubane through a carboxylate bridge, the cubane Co4(μ3-O)4(OAc)4(t-Bupy)4 (OAc = acetate, t-Bupy = 4-tert-butylpyridine) was exposed to various metal acetate complexes. Reaction with Cu(OAc)2 gave the structurally characterized (by X-ray diffraction) dicopper dangler Cu2Co4(μ4-O)2(μ3-O)2(OAc)6(Cl)2(t-Bupy)4. In contrast, the analogous reaction with Mn(OAc)2 produced the MnIV-containing cubane cation [MnCo3(μ3-O)4(OAc)4(t-Bupy)4]+ by way of a metal-metal exchange that gives Co(OAc)2 and [CoIII(μ-OH)(OAc)]n oligomers as byproducts. Additionally, reaction of the formally CoIV cubane complex [Co4(μ3-O)4(OAc)4(t-Bupy)4][PF6] with Mn(OAc)2 gave the corresponding Mn-containing cubane in 80% yield. A mechanistic examination of the related metal-metal exchange reaction between Co4(μ3-O)4(OBz)4(py)4 (OBz = benzoate) and [Mn(acac)2(py)2][PF6] by ultraviolet-visible (UV-vis) spectroscopy provided support for a process involving rate-determining association of the reactants and electron transfer through a μ-oxo bridge in the adduct intermediate. The rates of exchange correlate with the donor strength of the cubane pyridine and benzoate ligand substituents; more electron-donating pyridine ligands accelerate metal-metal exchange, while both electron-donating and -withdrawing benzoate ligands can accelerate exchange. These experiments suggest that the basicity of the cubane oxo ligands promotes metal-metal exchange reactivity. The redox potentials of the Mn and cubane starting materials and isotopic labeling studies suggest an inner-sphere electron-transfer mechanism in a dangler intermediate.

Fresh Impetus in the Chemistry of Calcium Peroxides

Redox-inactive metal ions are essential in modulating the reactivity of various oxygen-containing metal complexes and metalloenzymes, including photosystem II (PSII). The heart of this unique membrane-protein complex comprises the Mn4CaO5 cluster, in which the Ca2+ ion acts as a critical cofactor in the splitting of water in PSII. However, there is still a lack of studies involving Ca-based reactive oxygen species (ROS) systems, and the exact nature of the interaction between the Ca2+ center and ROS in PSII still generates intense debate. Here, harnessing a novel Ca-TEMPO complex supported by the β-diketiminate ligand to control the activation of O2, we report the isolation and structural characterization of hitherto elusive Ca peroxides, a homometallic Ca hydroperoxide and a heterometallic Ca/K peroxide. Our studies indicate that the presence of K+ cations is a key factor controlling the outcome of the oxygenation reaction of the model Ca-TEMPO complex. Combining experimental observations with computational investigations, we also propose a mechanistic rationalization for the reaction outcomes. The designed approach demonstrates metal-TEMPO complexes as a versatile platform for O2 activation and advances the understanding of Ca/ROS systems.

Adaptive water oxidation catalysis on a carboxylate-sulfonate ligand with low onset potential

A water oxidation catalyst Ru-bcs (bcs = 2,2'-bipyridine-6'-carboxylate-6-sulfonate) with a hybrid ligand was reported. Ru-bcs utilizes the electron-donating properties of carboxylate ligands and the on-demand coordination feature of sulfonate ligands to enable a low onset potential of 1.21 V vs. NHE and a high TOF over 1000 s-1 at pH 7. The adaptive chemistry uncovered in this work provides new perspectives for developing molecular catalysts with high efficiency under low driving forces.

Synthetic Mn3Ce2O5-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The photosynthetic oxygen-evolving center (OEC) is a unique Mn4CaO5-cluster that catalyses water splitting into electrons, protons, and dioxygen. Precisely structural and functional mimicking of the OEC is a long-standing challenge and pressingly needed for understanding the structure-function relationship and catalytic mechanism of O-O bond formation. Herein we report two simple and robust artificial Mn3Ce2O5-complexes that display a remarkable structural similarity to the OEC in regarding of the ten-atom core (five metal ions and five oxygen bridges) and the alkyl carboxylate peripheral ligands. This Mn3Ce2O5-cluster can catalyse the water-splitting reaction on the surface of ITO electrode. These results clearly show that cerium can structurally and functionally replace both calcium and manganese in the cluster. Mass spectroscopic measurements demonstrate that the oxide bridges in the cluster are exchangeable and can be rapidly replaced by the isotopic oxygen of H2 18O in acetonitrile solution, which supports that the oxide bridge(s) may serve as the active site for the formation of O-O bond during the water-splitting reaction. These results would contribute to our understanding of the structure-reactivity relationship of both natural and artificial clusters and shed new light on the development of efficient water-splitting catalysts in artificial photosynthesis.

Identification, Characterization, and Electronic Structures of Interconvertible Cobalt-Oxygen TAML Intermediates

The reaction of Li[(TAML)CoIII]·3H2O (TAML = tetraamido macrocyclic tetraanionic ligand) with iodosylbenzene at 253 K in acetone in the presence of redox-innocent metal ions (Sc(OTf)3 and Y(OTf)3) or triflic acid affords a blue species 1, which is converted reversibly to a green species 2 upon cooling to 193 K. The electronic structures of 1 and 2 have been determined by combining advanced spectroscopic techniques (X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS), and magnetic circular dichroism (MCD)) with ab initio theoretical studies. Complex 1 is best represented as an S = 1/2 [(Sol)(TAML•+)CoIII---OH(LA)]- species (LA = Lewis/Brønsted acid and Sol = solvent), where an S = 1 Co(III) center is antiferromagnetically coupled to S = 1/2 TAML•+, which represents a one-electron oxidized TAML ligand. In contrast, complex 2, also with an S = 1/2 ground state, is found to be multiconfigurational with contributions of both the resonance forms [(H-TAML)CoIV═O(LA)]- and [(H-TAML•+)CoIII═O(LA)]-; H-TAML and H-TAML•+ represent the protonated forms of TAML and TAML•+ ligands, respectively. Thus, the interconversion of 1 and 2 is associated with a LA-associated tautomerization event, whereby H+ shifts from the terminal -OH group to TAML•+ with the concomitant formation of a terminal cobalt-oxo species possessing both singlet (SCo = 0) Co(III) and doublet (SCo = 1/2) Co(IV) characters. The reactivities of 1 and 2 at different temperatures have been investigated in oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions to compare the activation enthalpies and entropies of 1 and 2.

Bioinspired Water Preorganization in Confined Space for Efficient Water Oxidation Catalysis in Metallosupramolecular Ruthenium Architectures

ConspectusNature has established a sustainable way to maintain aerobic life on earth by inventing one of the most sophisticated biological processes, namely, natural photosynthesis, which delivers us with organic matter and molecular oxygen derived from the two abundant resources sunlight and water. The thermodynamically demanding photosynthetic water splitting is catalyzed by the oxygen-evolving complex in photosystem II (OEC-PSII), which comprises a distorted tetramanganese-calcium cluster (CaMn4O5) as catalytic core. As an ubiquitous concept for fine-tuning and regulating the reactivity of the active site of metalloenzymes, the surrounding protein domain creates a sophisticated environment that promotes substrate preorganization through secondary, noncovalent interactions such as hydrogen bonding or electrostatic interactions. Based on the high-resolution X-ray structure of PSII, several water channels were identified near the active site, which are filled with extensive hydrogen-bonding networks of preorganized water molecules, connecting the OEC with the protein surface. As an integral part of the outer coordination sphere of natural metalloenzymes, these channels control the substrate and product delivery, carefully regulate the proton flow by promoting pivotal proton-coupled electron transfer processes, and simultaneously stabilize short-lived oxidized intermediates, thus highlighting the importance of an ordered water network for the remarkable efficiency of the natural OEC.Transferring this concept from nature to the engineering of artificial metal catalysts for fuel production has fostered the fascinating field of metallosupramolecular chemistry by generating defined cavities that conceptually mimic enzymatic pockets. However, the application of supramolecular approaches to generate artificial water oxidation catalysts remained scarce prior to our initial reports, since such molecular design strategies for efficient activation of substrate water molecules in confined nanoenvironments were lacking. In this Account, we describe our research efforts on combining the state-of-the art Ru(bda) catalytic framework with structurally programmed ditopic ligands to guide the water oxidation process in defined metallosupramolecular assemblies in spatial proximity. We will elucidate the governing factors that control the quality of hydrogen-bonding water networks in multinuclear cavities of varying sizes and geometries to obtain high-performance, state-of-the-art water oxidation catalysts. Pushing the boundaries of artificial catalyst design, embedding a single catalytic Ru center into a well-defined molecular pocket enabled sophisticated water preorganization in front of the active site through an encoded basic recognition site, resulting in high catalytic rates comparable to those of the natural counterpart OEC-PSII.To fully explore their potential for solar fuel devices, the suitability of our metallosupramolecular assemblies was demonstrated under (electro)chemical and photocatalytic water oxidation conditions. In addition, testing the limits of structural diversity allowed the fabrication of self-assembled linear coordination oligomers as novel photocatalytic materials and long-range ordered covalent organic framework (COF) materials as recyclable and long-term stable solid-state materials for future applications.

Advances in Naked Metal Clusters for Catalysis

The properties of sub-nano metal clusters are governed by quantum confinement and their large surface-to-bulk ratios, atomically precise compositions and geometric/electronic structures. Advances in metal clusters lead to new opportunities in diverse aspects of sciences including chemo-sensing, bio-imaging, photochemistry, and catalysis. Naked metal clusters having synergic multiple active sites and coordinative unsaturation and tunable stability/activity enable researchers to design atomically precise metal catalysts with tailored catalysis for different reactions. Here we summarize the progress of ligand-free naked metal clusters for catalytic applications. It is anticipated that this review helps to better understand the chemistry of small metal clusters and facilitates the design and development of new catalysts for potential applications.

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

Electric-field-assisted proton coupling enhanced oxygen evolution reaction

The discovery of Mn-Ca complex in photosystem II stimulates research of manganese-based catalysts for oxygen evolution reaction (OER). However, conventional chemical strategies face challenges in regulating the four electron-proton processes of OER. Herein, we investigate alpha-manganese dioxide (α-MnO2) with typical MnIV-O-MnIII-HxO motifs as a model for adjusting proton coupling. We reveal that pre-equilibrium proton-coupled redox transition provides an adjustable energy profile for OER, paving the way for in-situ enhancing proton coupling through a new "reagent"- external electric field. Based on the α-MnO2 single-nanowire device, gate voltage induces a 4-fold increase in OER current density at 1.7 V versus reversible hydrogen electrode. Moreover, the proof-of-principle external electric field-assisted flow cell for water splitting demonstrates a 34% increase in current density and a 44.7 mW/cm² increase in net output power. These findings indicate an in-depth understanding of the role of proton-incorporated redox transition and develop practical approach for high-efficiency electrocatalysis.

Coordination Synergy between Iridium Photosensitizers and Metal Nanoclusters Leading to Enhanced CO2 Cycloaddition under Mild Conditions

The achievement of photocatalytic CO2 and epoxide cycloaddition under mild conditions such as room temperature and atmospheric pressure is important for green chemistry, which can be achieved by developing coordination synergies between catalysts and photosensitizers. In this context, we exploit the use of coordinate bonds to connect pyridine-appended iridium photosensitizers and catalysts for CO2 cycloaddition, which is systematically demonstrated by 1H nuclear magnetic resonance titration and X-ray photoelectron spectroscopic measurements. It is shown that the hybrid Ir(Cltpy)2/Mn2Cd4 photocatalytic system with coordination synergy exhibits excellent catalytic performance (yield ≈ 98.2%), which is 3.75 times higher than that of the comparative Ir(Cltpy-Ph)2/Mn2Cd4 system without coordination synergy (yield ≈ 26.2%), under mild conditions. The coordination between the Mn2Cd4 catalyst and the Ir(Cltpy)2 photosensitizer enhances the light absorption and photoresponse properties of the Mn2Cd4 catalyst. This has been confirmed through transient photocurrent, electrochemical impedance, and electron paramagnetic tests. Consequently, the efficiency of cycloaddition was enhanced by utilizing mild conditions.

Evolution in the Design of Water Oxidation Catalysts with Transition-Metals: A Perspective on Biological, Molecular, Supramolecular, and Hybrid Approaches

Increased demand for a carbon-neutral sustainable energy scheme augmented by climatic threats motivates the design and exploration of novel approaches that reserve intermittent solar energy in the form of chemical bonds in molecules and materials. In this context, inspired by biological processes, artificial photosynthesis has garnered significant attention as a promising solution to convert solar power into chemical fuels from abundantly found H2O. Among the two redox half-reactions in artificial photosynthesis, the four-electron oxidation of water according to 2H2O → O2 + 4H+ + 4e- comprises the major bottleneck and is a severe impediment toward sustainable energy production. As such, devising new catalytic platforms, with traditional concepts of molecular, materials and biological catalysis and capable of integrating the functional architectures of the natural oxygen-evolving complex in photosystem II would certainly be a value-addition toward this objective. In this review, we discuss the progress in construction of ideal water oxidation catalysts (WOCs), starting with the ingenuity of the biological design with earth-abundant transition metal ions, which then diverges into molecular, supramolecular and hybrid approaches, blurring any existing chemical or conceptual boundaries. We focus on the geometric, electronic, and mechanistic understanding of state-of-the-art homogeneous transition-metal containing molecular WOCs and summarize the limiting factors such as choice of ligands and predominance of environmentally unrewarding and expensive noble-metals, necessity of high-valency on metal, thermodynamic instability of intermediates, and reversibility of reactions that create challenges in construction of robust and efficient water oxidation catalyst. We highlight how judicious heterogenization of atom-efficient molecular WOCs in supramolecular and hybrid approaches put forth promising avenues to alleviate the existing problems in molecular catalysis, albeit retaining their fascinating intrinsic reactivities. Taken together, our overview is expected to provide guiding principles on opportunities, challenges, and crucial factors for designing novel water oxidation catalysts based on a synergy between conventional and contemporary methodologies that will incite the expansion of the domain of artificial photosynthesis.

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids

Metalloenzymes with heteromultimetallic active sites perform chemical reactions that control several biogeochemical cycles. Transformations catalyzed by such enzymes include dioxygen generation and reduction, dinitrogen reduction, and carbon dioxide reduction-instrumental transformations for progress in the context of artificial photosynthesis and sustainable fertilizer production. While the roles of the respective metals are of interest in all these enzymatic transformations, they share a common factor in the transfer of one or multiple redox equivalents. In light of this feature, it is surprising to find that incorporation of redox-inactive metals into the active site of such an enzyme is critical to its function. To illustrate, the presence of a redox-inactive Ca2+ center is crucial in the Oxygen Evolving Complex, and yet particularly intriguing given that the transformation catalyzed by this cluster is a redox process involving four electrons. Therefore, the effects of redox inactive metals on redox processes-electron transfer, oxygen- and hydrogen-atom transfer, and O-O bond cleavage and formation reactions-mediated by transition metals have been studied extensively. Significant effects of redox inactive metals have been observed on these redox transformations; linear free energy correlations between Lewis acidity and the redox properties of synthetic model complexes are observed for several reactions. In this Perspective, these effects and their relevance to multielectron processes will be discussed.

Bioinspired Polyoxo-titanium Cluster for Greatly Enhanced Solar-Driven CO2 Reduction

Developing artificial enzymes with excellent catalytic activities and uncovering the structural and chemical determinants remain a grand challenge. Discrete titanium-oxo clusters with well-defined coordination environments at the atomic level can mimic the pivotal catalytic center of natural enzymes and optimize the charge-transfer kinetics. Herein, we report the precise structural tailoring of a self-assembled tetrahedral Ti4Mn3-cluster for photocatalytic CO2 reduction and realize the selective evolution of CO over specific sites. Experiments and theoretical simulation demonstrate that the high catalytic performance of the Ti4Mn3-cluster should be related to the synergy between active Mn sites and the surrounding functional microenvironment. The reduced energy barrier of the CO2 photoreduction reaction and moderate adsorption strength of CO* are beneficial for the high selective evolution of CO. This work provides a molecular scale accurate structural model to give insight into artificial enzyme for CO2 photoreduction.

Co-Co Dinuclear Active Sites Dispersed on Zirconium-doped Heterostructured Co9 S8 /Co3 O4 for High-current-density and Durable Acidic Oxygen Evolution

Developing cost-effective and sustainable acidic water oxidation catalysts requires significant advances in material design and in-depth mechanism understanding for proton exchange membrane water electrolysis. Herein, we developed a single atom regulatory strategy to construct Co-Co dinuclear active sites (DASs) catalysts that atomically dispersed zirconium doped Co9 S8 /Co3 O4 heterostructure. The X-ray absorption fine structure elucidated the incorporation of Zr greatly facilitated the generation of Co-Co DASs layer with stretching of cobalt oxygen bond and S-Co-O heterogeneous grain boundaries interfaces, engineering attractive activity of significantly reduced overpotential of 75 mV at 10 mA cm-2 , a breakthrough of 500 mA cm-2 high current density, and water splitting stability of 500 hours in acid, making it one of the best-performing acid-stable OER non-noble metal materials. The optimized catalyst with interatomic Co-Co distance (ca. 2.80 Å) followed oxo-oxo coupling mechanism that involved obvious oxygen bridges on dinuclear Co sites (1,090 cm-1 ), confirmed by in situ SR-FTIR, XAFS and theoretical simulations. Furthermore, a major breakthrough of 120,000 mA g-1 high mass current density using the first reported noble metal-free cobalt anode catalyst of Co-Co DASs/ZCC in PEM-WE at 2.14 V was recorded.

Self-supported P-doped NiFe2O4 micro-sheet arrays for the efficient conversion of nitrite to ammonia

The nitrite reduction reaction (NO2-RR) is an important process for eliminating toxic nitrites from water while simultaneously producing high-value ammonia under ambient conditions. For the aim to improve the NO2-RR efficiency, we designed a new synthetic strategy to prepare a phosphorus-doped three-dimensional NiFe2O4 catalyst loaded onto a nickel foam in-situ and evaluated its performance for the reduction of NO2- to NH3. The catalyst achieved a high Faradaic efficiency (FE) of 95.39%, and an ammonia (NH3) yield rate of 34788.51 µg h-1 cm-2 at - 0.45 V vs. RHE. A high NH3 yield rate and FE were maintained after 16 cycles at - 0.35 V vs. RHE in an alkaline electrolyte. This study provides a new direction for the rational design of highly stable electrocatalysts for the conversion of NO2- to NH3.

Photo-Activating Biomimetic Polyoxomolybdate for Boosting Oxygen Evolution in Neutral Electrolytes

Inspired by the metal-oxo cluster structural feature and charge separation behaviour of the oxygen evolving center (OEC) in photosystem II (PS-II) under photoirradiation, a new crystalline photochromic polyoxomolybdate, MV2 [β-Mo8 O26 ] (1, MV=methyl viologen cation), is designed as a biomimetic oxygen evolution reaction (OER) catalyst in neutral electrolytes. After photoinduced electron transfer (PIET) with colour change from colourless to grey, it remains in an ultra-stable charge-separated state over a year under ambient conditions. The observed overpotential at 10 mA ⋅ cm-2 and Tafel slope decrease by 49 mV and 62.8 mV ⋅ dec-1 after coloration, respectively. The outstanding OER performance of the coloured state in neutral electrolytes even outperforms the commercial RuO2 benchmark. Experimental and theoretical studies show that oxygen holes within polyanions after irradiation serve as sites for enhancing direct O-O coupling, thus effectively promoting OER. This is the first successful application of electron-transfer photochromism to realize OER activity gain.

Transition from Sequential to Concerted Proton-Coupled Electron Transfer of Water Oxidation on Semiconductor Photoanodes

Accelerating proton transfer has been demonstrated as key to boosting water oxidation on semiconductor photoanodes. Herein, we study proton-coupled electron transfer (PCET) of water oxidation on five typical photoanodes [i.e., α-Fe2O3, BiVO4, TiO2, plasmonic Au/TiO2, and nickel-iron oxyhydroxide (Ni1-xFexOOH)-modified silicon (Si)] by combining the rate law analysis of H2O molecules with the H/D kinetic isotope effect (KIE) and operando spectroscopic studies. An unexpected and universal half-order kinetics is observed for the rate law analysis of H2O, referring to a sequential proton-electron transfer pathway, which is the rate-limiting factor that causes the sluggish water oxidation performance. Surface modification of the Ni1-xFexOOH electrocatalyst is observed to break this limitation and exhibits a normal first-order kinetics accompanied by much enhanced H/D KIE values, facilitating the turnover frequency of water oxidation by 1 order of magnitude. It is the first time that Ni1-xFexOOH is found to be a PCET modulator. The rate law analysis illustrates an effective strategy for modulating PCET kinetics of water oxidation on semiconductor surfaces.

Reversible O-O Bond Scission and O2 Evolution at MOF-Supported Tetramanganese Clusters

The scission of the O-O bond in O₂ during respiration and the formation of the O-O bond during photosynthesis are the engines of aerobic life. Likewise, the reduction of O₂ and the oxidation of reduced oxygen species to form O₂ are indispensable components for emerging renewable technologies, including energy storage and conversion, yet discrete molecule-like systems that promote these fundamental reactions are rare. Herein, we report a square-planar tetramanganese cluster formed by self-assembly within a metal-organic framework that reversibly reduces O₂ by four electrons, facilitating the interconversion between molecular O₂ and metal-oxo species. The tetranuclear cluster spontaneously cleaves the O-O bond of O₂ at room temperature to generate a tetramanganese-bis(μ₂-oxo) species, which, in turn, is competent for O-O bond reformation and O₂ evolution at elevated temperatures, enabled by the head-to-head orientation of two oxo species. This study demonstrates the viability of four-electron interconversion between molecular O₂ and metal-oxo species and highlights the importance of site isolation for achieving multi-electron chemistry at polynuclear metal clusters.

Coordination Number in High-Spin-Low-Spin Equilibrium in Cluster Models of the S2 State of the Oxygen Evolving Complex

The S2 state of the Oxygen Evolving Complex (OEC) of Photosystem II (PSII) shows high-spin (HS) and low-spin (LS) EPR signals attributed to distinct structures based on computation. Five-coordinate MnIII centers are proposed in these species but are absent in available spectroscopic model complexes. Herein, we report the synthesis, crystal structure, electrochemistry, SQUID magnetometry, and EPR spectroscopy of a MnIIIMnIV3O4 cuboidal complex featuring five-coordinate MnIII. This cluster displays a spin ground state of S = 5/2, while conversion to a six-coordinate Mn upon treatment with water results in a spin state change to S = 1/2. These results demonstrate that coordination number, without dramatic changes within the Mn4O4 core, has a substantial effect on spectroscopy.

Does Serial Femtosecond Crystallography Depict State-Specific Catalytic Intermediates of the Oxygen-Evolving Complex?

Recent advances in serial femtosecond crystallography (SFX) of photosystem II (PSII), enabled by X-ray free electron lasers (XFEL), provided the first geometric models of distinct intermediates in the catalytic S-state cycle of the oxygen-evolving complex (OEC). These models are obtained by flash-advancing the OEC from the dark-stable state (S₁) to more oxidized intermediates (S₂ and S₃), eventually cycling back to the most reduced S₀. However, the interpretation of these models is controversial because geometric parameters within the Mn₄CaO₅ cluster of the OEC do not exactly match those expected from coordination chemistry for the spectroscopically verified manganese oxidation states of the distinct S-state intermediates. Here we focus on the first catalytic transition, S₁ → S₂, which represents a one-electron oxidation of the OEC. Combining geometric and electronic structure criteria, including a novel effective oxidation state approach, we analyze existing 1-flash (1F) SFX-XFEL crystallographic models that should depict the S₂ state of the OEC. We show that the 1F/S₂ equivalence is not obvious, because the Mn oxidation states and total unpaired electron counts encoded in these models are not fully consistent with those of a pure S₂ state and with the nature of the S₁ → S₂ transition. Furthermore, the oxidation state definition in two-flashed (2F) structural models is practically impossible to elucidate. Our results advise caution in the extraction of electronic structure information solely from the literal interpretation of crystallographic models and call for re-evaluation of structural and mechanistic interpretations that presume exact correspondence of such models to specific catalytic intermediates of the OEC.

Novel Mn4-Co2 nanocluster@photosensitizers/SalenCo(iii) catalyze the copolymerization of carbon dioxide and propylene oxide

In general, transition metals (TMs) often facilitate highly efficient catalysis. Herein, we synthesized a series of nanocluster composite catalysts by combining with photosensitizers and SalenCo(iii) for the first time and studied the catalytic copolymerization of CO2 and propylene oxide (PO). Systematic experiments have shown that the selectivity of copolymerization products can be improved by the nanocluster composite catalysts, and their synergistic effects significantly improved the photocatalytic performance of carbon dioxide copolymerization. At specific wavelengths, I@S1 can achieve a TON of 536.4, which is 2.26 times that of I@S2. Interestingly, in the photocatalytic products of I@R2, CPC reached 37.1%. These findings provide a new idea for the study of TM nanocluster@photosensitizers for carbon dioxide photocatalysis, and may provide guidance for exploring low cost and highly efficient carbon dioxide emission reduction photocatalysts.

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO₂ reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Regeneration and Degradation in a Biomimetic Polyoxometalate Water Oxidation Catalyst

Complete understanding of catalytic cycles is required to advance the design of water oxidation catalysts, but it is difficult to attain, due to the complex factors governing their reactivity and stability. In this study, we investigate the regeneration and degradation pathways of the highly active biomimetic water oxidation catalyst [Mn³⁺ ₂Mn⁴⁺ ₂V₄O₁₇(OAc)₃]^3-, thereby completing its catalytic cycle. Beginning with the deactivated species [Mn³⁺ ₄V₄O₁₇(OAc)₂]^4- left over after O₂ evolution, we scrutinize a network of reaction intermediates belonging to two alternative water oxidation cycles. We find that catalyst regeneration to the activated species [Mn⁴⁺ ₄V₄O₁₇(OAc)₂(OH)(H₂O)]^- proceeds via oxidation of each Mn center, with one water ligand being bound during the first oxidation step and a second water ligand being bound and deprotonated during the final oxidation step. ΔΔG values for this last oxidation are consistent with previous experimental results, while regeneration within an alternative catalytic cycle was found to be thermodynamically unfavorable. Extensive in silico sampling of catalyst structures also revealed two degradation processes: cubane opening and ligand dissociation, both of which have low barriers at highly reduced states of the catalyst due to the presence of Jahn-Teller effects. These mechanistic insights are expected to spur the development of more efficient and stable Mn cubane water oxidation catalysts.

Decoupled Artificial Photosynthesis

Natural photosynthesis (NP) generates oxygen and carbohydrates from water and CO₂ utilizing solar energy to nourish lives and balance CO₂ levels. Following nature, artificial photosynthesis (AP), typically, overall water or CO₂ splitting, produces fuels and chemicals from renewable energy. However, hydrogen evolution or CO₂ reduction is inherently coupled with kinetically sluggish water oxidation, lowering efficiencies and raising safety concerns. Decoupled systems have thus emerged. In this review, we elaborate how decoupled artificial photosynthesis (DAP) evolves from NP and AP and unveil their distinct photoelectrochemical mechanisms in energy capture, transduction and conversion. Advances of AP and DAP are summarized in terms of photochemical (PC), photoelectrochemical (PEC), and photovoltaic-electrochemical (PV-EC) catalysis based on material and device design. The energy transduction process of DAP is emphasized. Challenges and perspectives on future researches are also presented.

MnIV4O4 Model of the S3 Intermediate of the Oxygen-Evolving Complex: Effect of the Dianionic Disiloxide Ligand

Synthetic complexes provide useful models to study the interplay between the structure and spectroscopy of the different S_n-state intermediates of the oxygen-evolving complex (OEC) of photosystem II (PSII). Complexes containing the Mn^IV₄ core corresponding to the S₃ state, the last observable intermediate prior to dioxygen formation, remain very rare. Toward the development of synthetic strategies to stabilize highly oxidized tetranuclear complexes, ligands with increased anion charge were pursued. Herein, we report the synthesis, electrochemistry, SQUID magnetometry, and electron paramagnetic resonance spectroscopy of a stable Mn^IV₄O₄ cuboidal complex supported by a disiloxide ligand. The substitution of an anionic acetate or amidate ligand with a dianionic disiloxide ligand shifts the reduction potential of the Mn^IIIMn^IV₃/Mn^IV₄ redox couple by up to ∼760 mV, improving stability. The S = 3 spin ground state of the siloxide-ligated Mn^IV₄O₄ complex matches the acetate and amidate variants, in corroboration with the Mn^IV₄ assignment of the S₃ state of the OEC.

Solar utilization beyond photosynthesis

Natural photosynthesis is an efficient biochemical process which converts solar energy into energy-rich carbohydrates. By understanding the key photoelectrochemical processes and mechanisms that underpin natural photosynthesis, advanced solar utilization technologies have been developed that may be used to provide sustainable energy to help address climate change. The processes of light harvesting, catalysis and energy storage in natural photosynthesis have inspired photovoltaics, photoelectrocatalysis and photo-rechargeable battery technologies. In this Review, we describe how advanced solar utilization technologies have drawn inspiration from natural photosynthesis, to find sustainable solutions to the challenges faced by modern society. We summarize the uses of advanced solar utilization technologies, such as converting solar energy to electrical and chemical energy, electrochemical storage and conversion, and associated thermal tandem technologies. Both the foundational mechanisms and typical materials and devices are reported. Finally, potential future solar utilization technologies are presented that may mimic, and even outperform, natural photosynthesis.

Heterometallic bond activation enabled by unsymmetrical ligand scaffolds: bridging the opposites

Heterobi- and multimetallic complexes providing close proximity between several metal centers serve as active species in artificial and enzymatic catalysis, and in model systems, showing unique modes of metal-metal cooperative bond activation. Through the rational design of well-defined, unsymmetrical ligand scaffolds, we create a convenient approach to support the assembly of heterometallic species in a well-defined and site-specific manner, preventing them from scrambling and dissociation. In this perspective, we will outline general strategies for the design of unsymmetrical ligands to support heterobi- and multimetallic complexes that show reactivity in various types of heterometallic cooperative bond activation.

Self-Assembled Nanoscale Manganese Oxides Enhance Carbon Capture by Diatoms

Continuous CO₂ emissions from human activities increase atmospheric CO₂ concentrations and affect global climate change. The carbon storage capacity of the ocean is 20-fold higher than that of the land, and diatoms contribute to approximately 40% of carbon capture in the ocean. Manganese (Mn) is a major driver of marine phytoplankton growth and the marine carbon pump. Here, we discovered self-assembled manganese oxides (MnO_x) for CO₂ fixation in a diatom-based biohybrid system. MnO_x shared key features (e.g., di-μ-oxo-bridged Mn-Mn) with the Mn₄CaO₅ cluster of the biological catalyst in photosystem II and promoted photosynthesis and carbon capture by diatoms/MnO_x. The CO₂ capture capacity of diatoms/MnO_x was 1.5-fold higher than that of diatoms alone. Diatoms/MnO_x easily allocated carbon into proteins and lipids instead of carbohydrates. Metabolomics showed that the contents of several metabolites (e.g., lysine and inositol) were positively associated with increased CO₂ capture. Diatoms/MnO_x upregulated six genes encoding photosynthesis core proteins and a key rate-limiting enzyme (Rubisco, ribulose 1,5-bisphosphate carboxylase-oxygenase) in the Calvin-Benson-Bassham carbon assimilation cycle, revealing the link between MnO_x and photosynthesis. These findings provide a route for offsetting anthropogenic CO₂ emissions and inspiration for self-assembled biohybrid systems for carbon capture by marine phytoplankton.

Advancement of renewable energy technologies via artificial and microalgae photosynthesis

There has been an urgent need to tackle global climate change and replace conventional fuels with alternatives from sustainable sources. This has led to the emergence of bioenergy sources like biofuels and biohydrogen extracted from microalgae biomass. Microalgae takes up carbon dioxide and absorbs sunlight, as part of its photosynthesis process, for growth and producing useful compounds for renewable energy. While, the developments in artificial photosynthesis to a chemical process that biomimics the natural photosynthesis process to fix CO₂ in the air. However, the artificial photosynthesis technology is still being investigated for its implementation in large scale production. Microalgae photosynthesis can provide the same advantages as artificial photosynthesis, along with the prospect of having final microalgae products suitable for various application. There are significant potential to adapt either microalgae photosynthesis or artificial photosynthesis to reduce the CO₂ in the climate and contribute to a cleaner and green cultivation method.

Mechanistic Insights into the Charge Transfer Dynamics of Photocatalytic Water Oxidation at the Lipid Bilayer-Water Interface

Photosystem II, the natural water-oxidizing system, is a large protein complex embedded in a phospholipid membrane. A much simpler system for photocatalytic water oxidation consists of liposomes functionalized with amphiphilic ruthenium(II)-tris-bipyridine photosensitizer (PS) and 6,6'-dicarboxylato-2,2'-bipyridine-ruthenium(II) catalysts (Cat) with a water-soluble sacrificial electron acceptor (Na₂S₂O₈). However, the effect of embedding this photocatalytic system in liposome membranes on the mechanism of photocatalytic water oxidation was not well understood. Here, several phenomena have been identified by spectroscopic tools, which explain the drastically different kinetics of water photo-oxidizing liposomes, compared with analogous homogeneous systems. First, the oxidative quenching of photoexcited PS* by S₂O₈^2- at the liposome surface occurs solely via static quenching, while dynamic quenching is observed for the homogeneous system. Moreover, the charge separation efficiency after the quenching reaction is much smaller than unity, in contrast to the quantitative generation of PS⁺ in homogeneous solution. In parallel, the high local concentration of the membrane-bound PS induces self-quenching at 10:1-40:1 molar lipid-PS ratios. Finally, while the hole transfer from PS⁺ to catalyst is rather fast in homogeneous solution (k_obs > 1 × 10⁴ s^-1 at [catalyst] > 50 μM), in liposomes at pH = 4, the reaction is rather slow (k_obs ≈ 17 s^-1 for 5 μM catalyst in 100 μM DMPC lipid). Overall, the better understanding of these productive and unproductive pathways explains what limits the rate of photocatalytic water oxidation in liposomal vs homogeneous systems, which is required for future optimization of light-driven catalysis within self-assembled lipid interfaces.

Ultrasonic Clusterization Process to Prepare [(NNCO)6Co4Cl2] as a Novel Double-Open-Co4O6 Cubane Cluster: SXRD Interactions, DFT, Physicochemical, Thermal Behaviors, and Biomimicking of Catecholase Activity

A novel double-open-cubane (NNCO)₆Co₄Cl₂ cluster with a Co₄O₆ core was made available under aqua-ultrasonic open atmosphere conditions for the first time. The ultrasonic clusterization of the (3,5-dimethyl-1H-pyrazol-1-yl)methanol (NNCOH) ligand with CoCl₂·6H₂O salts in ethanol yielded a high-purity and high-yield cluster product. Energy-dispersive X-ray (EDX), Fourier transform infrared (FT-IR), and ultraviolet (UV)-visible techniques were used to elucidate the clusterization process. The double-open-Co₄O₆ cubane structure of the (NNCO)₆Co₄Cl₂ cluster was solved by synchrotron single-crystal X-ray diffraction (SXRD) and supported by density functional theory (DFT) optimization and thermogravimetric/differential TG (TG/DTG) measurements; moreover, the DFT structural parameters correlated with the ones determined by SXRD. Molecular electrostatic potential (MEP), Mulliken atomic charge/natural population analysis (MAC/NPA), highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO), density of states (DOS), and GRD quantum analyses were computed at the DFT/B3LYP/6-311G(d,p) theory level. The thermal behavior of the cluster was characterized to support the formation of the Co₄O₆ core as a stable final product. The catalytic property of the (NNCO)₆Co₄Cl₂ cluster was predestined for the oxidation process of 3,5-DTBC diol (3,5-di-tert-butylbenzene-1,2-diol) to 3,5-DTBQ dione (3,5-di-tert-butylcyclohexa-3,5-diene-1,2-dione).

Candidate for Catalyst during Water-Oxidation Reaction in the Presence of Manganese Compounds, from Nanosized Particles to Impurities: Sleep with One Eye Open

Water-oxidation reaction (WOR) catalysts are critical for energy conversion. WOR is a four-electron oxidation and sluggish reaction. WOR needs a high thermodynamic driving force; it is also a kinetically slow reaction. Different compounds have been used for WOR; among these compounds, Mn materials have proven to be interesting because Mn is low-cost and also nontoxic, at least compared to many transition metals. Naturally, it has also been used in the biological water-oxidizing complex (WOC). Indeed, WOR has occurred on a huge scale in natural photosynthesis.For a long time, efforts have been made to design and synthesize various ligands and generate Mn compounds toward WOR catalysts. However, the addition or removal of electrons inside Mn compounds during harsh WOR conditions can lead to the formation or the breakage of bonds and result in the conversion of a precatalyst to a catalyst.Here, our findings on the conversion of Mn compounds to catalysts during WOR are presented. Many Mn compounds have been claimed to be catalysts for WOR in the presence of various chemical oxidants or under electrophotochemical conditions. Currently, the advances in characterization techniques and different spectroscopic methods have enabled a better understanding of catalysts. Different conversions such as that of the Mn complex to Mn oxide and Mn salts to Mn oxide during WOR have been explained. Indeed, the morphology and size of the Mn oxide formed depend on several factors such as the origin compounds, pH, ligands, and conditions. Thus, different Mn compounds show different activities toward WOR. The biomimetic models with Mn-Ca clusters are also decomposed during WOR. On the other hand, stable Mn complexes such as Mn phthalocyanines, which are very stable in the absence of potential, are easily decomposed during WOR. It is noted that for many of these Mn compounds, two steps result in the formation of Mn oxide during WOR: (i) Mn(II) or (III) leaching into the electrolyte and (ii) deposition of the leached Mn ions into the solution.Considering these steps, it can be seen that challenges remain in the area of Mn compounds, given the fact that even in the catalytic cycle at low oxidation numbers no Mn(II) or (III) should be leached to the electrolyte.

Spin-Gated Selectivity of the Water Oxidation Reaction Mediated by Free Pentameric CaxMn5-xO5+ Clusters

We report on the first preparation of isolated ligand-free CaMn₄O₅⁺ gas-phase clusters, as well as other pentameric Ca_xMn_5-xO₅⁺ (x = 0-4) clusters with varying Ca contents, which serve as molecular models of the natural CaMn₄O₅ inorganic cluster in photosystem II. Ion trap reactivity studies with D₂O and H₂¹⁸O reveal a pronounced cluster composition-dependent ability to mediate the oxidation of water to hydrogen peroxide. First-principles density functional theory simulations elucidate the mechanism of water oxidation, proceeding via formation of a terminal oxyl radical followed by oxyl/hydroxy (O/OH) coupling. The critical coupling reaction step entails a single electron transfer from the oxyl radical to the accommodating cluster core with a concurrent O/OH coupling forming an adsorbed OOH intermediate group. The spin-conserving electron transfer step takes place when the spin of the transferred electron is aligned with the spins of the d-electrons of the Mn atoms in the cuboidal high-spin cluster isomer. The d-electrons provide a ferromagnetically ordered environment that facilitates the spin-gated selective electron transfer process, resulting in parallel-spin-exchange stabilization and a lowered transition state barrier for the coupling reaction involving the frontier orbitals of the oxyl and hydroxy reactant intermediates.

Water Oxidation Catalyzed by a Bioinspired Tetranuclear Manganese Complex: Mechanistic Study and Prediction

Density functional theory calculations were utilized to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex a [Mn^III ₃ Mn^IV O₃ (CH₃ COO)₃ (A-α-SiW₉ O₃₄ )]^6- . Theoretical results indicated that catalytic active species 1 (Mn₄ ^{III,III,IV,IV} ) was formed after O₂ formation in the first turnover. From 1, three sequential proton-coupled electron transfer (PCET) oxidations led to the Mn^IV -oxyl radical 4 (Mn₄ ^IV,IV,IV,IV -O⋅). Importantly, 4 had an unusual butterfly-shaped Mn₂ O₂ core for the two substrate-coordinated Mn sites, which facilitated O-O bond formation via direct coupling of the oxyl radical and the adjacent Mn^IV -coordinated hydroxide to produce the hydroperoxide intermediate Int1 (Mn₄ ^III,IV,IV,IV -OOH). This step had an overall energy barrier of 24.9 kcal mol^-1 . Subsequent PCET oxidation of Int1 to Int2 (Mn₄ ^III,IV,IV,IV -O₂ ⋅) enabled the O₂ release in a facile process. Furthermore, apart from the Si-centered complex, computational study suggested that tetramanganese polyoxometalates with Ge, P, and S could also catalyze the water oxidation process, where those bearing P and S likely present higher activities.

Microenvironment Regulation of {Co4IIO4} Cubane for Syngas Photosynthesis

It is a great challenging task for selectivity control of both CO₂ photoreduction and water splitting to produce syngas via precise microenvironment regulation. Herein, a series of UiO-type Eu-MOFs (Eu-bpdc, Eu-bpydc, Ru_x-Eu-bpdc, and Ru_x-Eu-bpydc) with different surrounding confined spaces were designed and synthesized. These photosensitizing Ru_x-Eu-MOFs were used as the molecular platform to encapsulate the [Co^II₄(dpy{OH}O)₄(OAc)₂(H₂O)₂]²⁺ (Co₄) cubane cluster for constructing Co₄@Ru_x-Eu-MOF (x = 0.1, 0.2, and 0.4) heterogeneous photocatalysts for efficient CO₂ photoreduction and water splitting. The H₂ and CO yields can reach 446.6 and 459.8 μmol·g^-1, respectively, in 10 h with Co₄@Ru_0.1-Eu-bpdc as the catalyst, and their total yield can be dramatically improved to 2500 μmol·g^-1 with the ratio of CO/H₂ ranging from 1:1 to 1:2 via changing the photosensitizer content in the confined space. By increasing the N content around the cubane, the photocatalytic performance drops sharply in Co₄@Ru_0.1-Eu-bpydc, but with an enhanced proportion of CO in the final products. In the homogeneous system, the Co₄ cubane was surrounding with Ru photosensitizers via week interactions, which can drive water splitting into H₂ with >99% selectivity. Comprehensive structure-function analysis highlights the important role of microenvironment regulation in the selectivity control via constructing homogeneous and heterogeneous photocatalytic systems. This work provides a new insight for engineering a catalytic microenvironment of the cubane cluster for selectivity control of CO₂ photoreduction and water splitting.

The Role of Redox-Inactive Metals in Modulating the Redox Potential of the Mn4CaO4 Model Complex

Photosynthetic oxygen-evolving center (OEC), the "engine of life", is a unique Mn₄CaO₅ cluster catalyzing the water oxidation. The role of redox-inactive component Ca²⁺, which can only be functionally replaced by Sr²⁺ in a biological environment, has been under debate for a long time. Recently, its modulating effect on the redox potential of native OEC and artificial structural OEC model complex has received great attention, and linear relationship between the potential and the Lewis acidity of the redox-inactive metal has been proposed for the MMn₃O₄ model complex. In this work, the modulating effect has been studied in detail using the Mn₄CaO₄ model complex, which is the closest structural model to OEC to date and has a similar redox potential at the S₁-S₂ transition. We found the redox-inactive metal only has a weak modulating effect on the potential, which is comparable in strength to that of the ligand environments. Meanwhile, the net charge of the complex, which could be changed along with the redox-inactive metal, has a high impact on the potential and can be unified by protonation, deprotonation, or ligand modification. Although the modulating effect of the redox-inactive metal is not very strong, the linear relationship between the potential and the Lewis acidity is still valid for Mn₄MO₄ complexes. Our results of strong modulating effects for net charge and weak modulating effects for redox-inactive metal fit with the previous experimental observations on Mn₄MO₄ (M = Ca²⁺, Y³⁺, and Gd³⁺) model complexes, and suggest that Ca²⁺ can be structurally and electrochemically replaced with other metal cations, together with proper ligand modifications.

Mimicking the Oxygen-Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosystem II (PSII) of oxygenic photosynthetic organisms is a unique heterometallic-oxide Mn₄CaO₅-cluster that catalyzes water splitting into electrons, protons, and molecular oxygen through a five-state cycle (S_n, n = 0 ~ 4). It serves as the blueprint for the developing of the man-made water-splitting catalysts to generate solar fuel in artificial photosynthesis. Understanding the structure-function relationship of this natural catalyst is a great challenge and a long-standing issue, which is severely restricted by the lack of a precise chemical model for this heterometallic-oxide cluster. However, it is a great challenge for chemists to precisely mimic the OEC in a laboratory. Recently, significant advances have been achieved and a series of artificial Mn₄XO₄-clusters (X = Ca/Y/Gd) have been reported, which closely mimic both the geometric structure and the electronic structure, as well as the redox property of the OEC. These new advances provide a structurally well-defined molecular platform to study the structure-function relationship of the OEC and shed new light on the design of efficient catalysts for the water-splitting reaction in artificial photosynthesis.