Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation

Mechanistic elucidation of O2 production from tBuOOH in water using the Mn(II) catalyst [Mn2(mcbpen)2(H2O)2]2+: a DFT study

This study employs density functional theory at the SMD/B3LYP-D3/6-311+G(2d,p),def2-TZVPP//SMD/B3LYP-D3/6-31G(d),SDD level of theory to explore the mechanistic details of O2 generation from tBuOOH, using H218O as the solvent, in the presence of the Mn(II) catalyst [Mn2(mcbpen)2(H2O)2]2+. Since this chemistry was reported to occur through the reaction of Mn(III)(μ-O)Mn(IV)-O˙ with water, we first revaluated this proposal and found that it occurs with an activation barrier greater than 36 kcal mol-1, ruling out the functioning of such a dimer as the active catalyst. Experimental evidence has shown that the oxidation of [Mn2(mcbpen)2(H2O)2]2+ by tBuOOH in H218O produces the Mn(IV) species [Mn(18O)(mcbpen)]+. Our investigations revealed a plausible mechanism for this observation in which [Mn (18O)(mcbpen)]+ acts as the active catalyst, generating the tert-butyl peroxyl radical (tBuOO˙) through its reaction with tBuOOH. In this proposed mechanism, the O-O bond is formed through the interaction of tBuOO˙ with another [Mn(18O)(mcbpen)]+, finally leading to the formation of the 16O18O product. Our findings underscore the pivotal role of [Mn(18O)(mcbpen)]+ in both generating the active species tBuOO˙ and consuming it to produce 16O18O. With activation barriers as low as about 9 kcal mol-1, these elementary steps highlight the feasibility of our proposed mechanism. Moreover, this mechanism elucidates why, experimentally, one of the oxygen atoms in the released O2 comes from water, while the other originates from tBuOOH. This research broadens our understanding of high oxidation state manganese chemistry, setting the stage for the development of more efficient Mn-based catalysts, aimed at improving processes in both renewable energy and synthetic chemistry.

Time-Resolved Mechanistic Depiction of Photoinduced CO2 Reduction Catalysis on a Urea-Modified Iron Porphyrin

The development of functional artificial photosynthetic devices relies on the understanding of mechanistic aspects involved in specialized photocatalysts. Modified iron porphyrins have long been explored as efficient catalysts for the light-induced reduction of carbon dioxide (CO2) towards solar fuels. In spite of the advancements in homogeneous catalysis, the development of the next generation of catalysts requires a complete understanding of the fundamental photoinduced processes taking place prior to and after activation of the substrate by the catalyst. In this work, we employ a state-of-the-art nanosecond optical transient absorption spectroscopic setup with a double excitation capability to induce charge accumulation and trigger the reduction of CO2 to carbon monoxide (CO). Our biomimetic system is composed of a urea-modified iron(III) tetraphenylporphyrin (UrFeIII) catalyst, the prototypical [Ru(bpy)3]2+ (bpy=2,2'-bipyridine) used as a photosensitizer, and sodium ascorbate as an electron donor. Under inert atmosphere, we show that two electrons can be successively accumulated on the catalyst as the fates of the photogenerated UrFeII and UrFeI reduced species are tracked. In the presence of CO2, the catalytic cycle is kick-started providing further evidence on CO2 activation by the UrFe catalyst in its formal FeI oxidation state.

Nitrogen-Rich Covalent Organic Polymer for Metal-Free Tandem Catalysis and Postmetalation-Actuated High-Performance Water Oxidation

Development of high-performing catalytic materials for selective and mild chemical transformations through adhering to the principles of sustainability remains a central focus in modern chemistry. Herein, we report the template-free assembly of a thermochemically robust covalent organic polymer (COP: 1) from 2,2'-bipyridine-5,5'-dicarbonyl dichloride and 2,4,6-tris(4-aminophenyl)triazine as [2 + 3] structural motifs. The two-dimensional (2D) layered architecture contains carboxamide functionality, delocalized π-cloud, and free pyridyl-N site-decked pores. Such trifunctionalization benefits this polymeric network exhibiting tandem alcohol oxidation-Knoevenagel condensation. In contrast to common metal-based catalysts, 1 represents a one of a kind metal-free alcohol oxidation reaction via extended π-cloud delocalization-mediated free radical pathway, as comprehensively supported from diverse control experiments. In addition to reasonable recyclability and broad substrate scope, the mild reaction condition underscores its applicability in benign synthesis of valuable product benzylidene malononitrile. Integration of 2,2'-bipyridyl units in this 2D COP favors anchoring non-noble metal ions to devise 1-M (M: Ni2+/ Co2+) that demonstrate outstanding electrochemical oxygen evolution reaction in alkaline media with high chronoamperometric stability. Electrochemical parameters of both 1-Co and 1-Ni outperform some benchmark, commercial, as well as a majority of contemporary OER catalysts. Specifically, the overpotential and Tafel slope (280 mV, 58 mV/dec) for 1-Ni is better than 1-Co (360 mV, 78 mV/dec) because of increased charge accumulation as well as a higher number of active sites compared to the former. In addition, the turnover frequency of 1-Ni is found to be 6 times higher than that of 1-Co and ranks among top-tier water oxidation catalysts. The results provide valuable insights in the field of metal-free tandem catalysis as well as promising electrochemical water splitting at the interface of task-specific functionality fuelling in polymeric organic networks.

Heterobimetallic Synergism in Triple-Redox 2D Framework for Largely Boosted Water Oxidation and Flanked Carboxylic-Acid-Triggered Unconventional Tandem Catalysis

A fish-bone-shaped and thermochemically stable 2D metal-organic framework (MOF) with multimodal active center-decked pore-wall is devised. Redox-active [Co2(COO)4] node and thiazolo[5,4-d]thiazole functionalization benefit this mixed-ligand MOF exhibiting electrochemical water oxidation with 375 mV overpotential at 10 mA cm-2 current density and 78 mV per dec Tafel slope in alkaline medium. Pair of oppositely oriented carboxylic acids aids postmetalation with transition metal ions to engineer heterobimetallic materials. Notably, overpotential of Ni2+ grafted triple-redox composite reduces to 270 mV with twofold declined Tafel slope than the parent MOF, ranking among the best-reported values, and outperforming majority of related catalysts. Significantly, turnover frequency and charge transfer resistance display 35.5 and 1.4-fold upsurge, respectively, with much uplifted chronopotentiometric stability and increase active surface area owing to synergistic Co(II)-Ni(II) coupling. The simultaneous presence of ─COOH and nitrogen-rich moieties renders this hydrogen-bonded MOF as acid-base synergistic catalyst for recyclable deacetalization-Knoevenagel reaction with >99% product yield under solvent-free mild condition. Besides control experiments, unique role of ─COOH as hydrogen-bond donor site in substrate activation is validated from comparing the performances of molecular-shearing approach-derived structurally similar unfunctionalized MOF, and the heterobimetallic composite. To the best of tandem Knoevenagel condensation, larger-sized acetal exhibits poor yield of α,β-unsaturated dicyanides, and demonstrates pore-fitting-mediated size-selectivity.

Pyrazine-Coordinated Dinuclear and Mononuclear Ruthenium Complexes Formed via the Conversion of the Triply Chlorido-Bridged Diruthenium(II) Complex

The pyrazine-coordinated dinuclear and mononuclear ruthenium complexes were synthesized through the framework conversion reactions of the triply chlorido-bridged diruthenium(II) complex [{RuII,II(bbpma)}2(μ-Cl)3]+ (bbpma; benzylbis(2-pyridylmethyl)amine, [1]+) in the presence of pyrazine, which could function as the simple molecular multinucleation ligand of metal compounds. A reduction reaction of fac-[RuIIICl3(bbpma)] with zinc in the presence of hydrochloric acid afforded [1]+ in solution, and the following addition of pyrazine (1 equiv) in the solution led to the formation of a singly pyrazine (pz)-bridged diruthenium complex, [{RuII,II(μ-Cl2ZnCl2)(bbpma)}2(μ-pz)] ([2(II,II)(ZnCl2)2]). The stoichiometric two-electron oxidation of [2(II,II)(ZnCl2)2] was successfully proceeded, and a Ru(III)-Ru(III) species, [{RuIII,IIICl2(bbpma)}2(μ-pz)](PF6)2 ([III,III](PF6)2), was isolated. The reaction of [1]+ with excess amounts of pyrazine without hydrochloric acid afforded mononuclear Ru(III) and Ru(II) complexes containing one or two pyrazine, fac-[RuClm(pz)3-m(bbpma)]+ (m = 2; [3]+; m = 1; [4]+). The details of the electrochemical and spectroscopic properties of [1]+-[4]+ in organic and aqueous solutions were discussed.

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.

Promoting Water Oxidation by Proton Acceptable Groups Surrounding Catalyst on Electrode Surface

Large-scale hydrogen production through water splitting represents an optimal approach for storing sustainable but intermittent energy sources. However, water oxidation, a complex and sluggish reaction, poses a significant bottleneck for water splitting efficiency. The impact of outer chemical environments on the reaction kinetics of water oxidation catalytic centers remains unexplored. Herein, chemical environment impacts were integrated by featuring methylpyridinium cation group (Py+) around the classic Ru(bpy)(tpy) (bpy=2,2'-bipyridine, tpy=2,2' : 6',2''-terpyridine) water oxidation catalyst on the electrode surface via electrochemical co-polymerization. The presence of Py+ groups could significantly enhance the turnover frequencies of Ru(bpy)(tpy), surpassing the performance of typical proton acceptors such as pyridine and benzoic acid anchored around the catalyst. Mechanistic investigations reveal that the flexible internal proton acceptor anions induced by Py+ around Ru(bpy)(tpy) are more effective than conventionally anchored proton acceptors, which promoted the rate-determining proton transfer process and enhanced the rate of water nucleophilic attack during O-O bond formation. This study may provide a novel perspective on achieving efficient water oxidation systems by integrating cations into the outer chemical environments of catalytic centers.

Promoting Oxygen Evolution Electrocatalysis by Coordination Engineering in Cobalt Phosphate

Understanding the structure-activity correlation is an important prerequisite for the rational design of high-efficiency electrocatalysts at the atomic level. However, the effect of coordination environment on electrocatalytic oxygen evolution reaction (OER) remains enigmatic. In this work, the regulation of proton transfer involved in water oxidation by coordination engineering based on Co3(PO4)2 and CoHPO4 is reported. The HPO4 2- anion has intermediate pKa value between Co(II)-H2O and Co(III)-H2O to be served as an appealing proton-coupled electron transfer (PCET) induction group. From theoretical calculations, the pH-dependent OER properties, deuterium kinetic isotope effects, operando electrochemical impedance spectroscopy (EIS) and Raman studies, the CoHPO4 catalyst beneficially reduces the energy barrier of proton hopping and modulates the formation energy of high-valent Co species, thereby enhancing OER activity. This work demonstrates a promising strategy that involves tuning the local coordination environment to optimize PCET steps and electrocatalytic activities for electrochemical applications. In addition, the designed system offers a motif to understand the structure-efficiency relationship from those amino-acid residue with proton buffer ability in natural photosynthesis.

Photochemistry of Ru(II) Triazole Complexes with 6-Membered Chelate Ligands: Detection and Reactivity of Ligand-Loss Intermediates

Photochemical ligand release from metal complexes may be exploited in the development of novel photoactivated chemotherapy agents for the treatment of cancer and other diseases. Highly intriguing photochemical behavior is reported for two ruthenium(II) complexes bearing conformationally flexible 1,2,3-triazole-based ligands incorporating a methylene spacer to form 6-membered chelate rings. [Ru(bpy)2(pictz)]2+ (1) and [Ru(bpy)2(btzm)]2+ (2) (bpy = 2,2'-bipyridyl; pictz = 1-(picolyl)-4-phenyl-1,2,3-triazole; btzm = bis(4-phenyl-1,2,3-triazol-4-yl)methane) exhibit coordination by the triazole ring through the less basic N2 atom as a consequence of chelation and readily undergo photochemical release of the pictz and btzm ligands (ϕ = 0.079 and 0.091, respectively) in acetonitrile solution to form cis-[Ru(bpy)2(NCMe)2]2+ (3) in both cases. Ligand-loss intermediates of the form [Ru(bpy)2(κ1-pictz or κ1-btzm)(NCCD3)]2+ are detected by 1H NMR spectroscopy and mass spectrometry. Photolysis of 1 yields three ligand-loss intermediates with monodentate pictz ligands, two of which form through simple decoordination of either the pyridine or triazole donor with subsequent solvent coordination (4-tz(N2) and 4-py, respectively). The third intermediate, shown to be able to form photochemically directly from 1, arises through linkage isomerism in which the monodentate pictz ligand is coordinated by the triazole N3 atom (4-tz(N3)) with a comparable ligand-loss intermediate with an N3-bound κ1-btzm ligand also observed for 2.

Catalyst self-assembly accelerates bimetallic light-driven electrocatalytic H2 evolution in water

Hydrogen evolution is an important fuel-generating reaction that has been subject to mechanistic debate about the roles of monometallic and bimetallic pathways. The molecular iridium catalysts in this study undergo photoelectrochemical dihydrogen (H2) evolution via a bimolecular mechanism, providing an opportunity to understand the factors that promote bimetallic H-H coupling. Covalently tethered diiridium catalysts evolve H2 from neutral water faster than monometallic catalysts, even at lower overpotential. The unexpected origin of this improvement is non-covalent supramolecular self-assembly into nanoscale aggregates that efficiently harvest light and form H-H bonds. Monometallic catalysts containing long-chain alkane substituents leverage the self-assembly to evolve H2 from neutral water at low overpotential and with rates close to the expected maximum for this light-driven water splitting reaction. Design parameters for holding multiple catalytic sites in close proximity and tuning catalyst microenvironments emerge from this work.

Elucidating the catalytic mechanisms of O2 generation by [Mn2(μ-O)2(terpy)2(OH2)2]3+ using DFT calculations: a focus on ClO- as oxidant

The experimentally reported Mn(IV)Mn(III) complex [Mn2(μ-O)2(terpy)2(OH2)2]3+ has been observed catalyzing O2 generation with oxidants like ClO- and HSO5-. Previous mechanistic studies primarily focused on O2 generation with HSO5-, concluding that Mn(IV)Mn(III) acts as a catalyst, generating a Mn(IV)Mn(IV)-oxyl species as a key intermediate responsible for O-O bond formation. This computational study employs DFT calculations to investigate whether the catalytic generation of O2 using ClO- follows the same mechanism previously identified with HSO5- as the oxidant, or if it proceeds through an alternate pathway. To this end, we explored multiple pathways using ClO- as the oxidant. Interestingly, our findings confirm that in the case of ClO- as the oxidant, similar to what was observed with HSO5-, the Mn(IV)Mn(IV)-oxyl species indeed plays a crucial role in driving the catalytic evolution of O2 with the potential formation of the binuclear complexes Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH during the reaction. These complexes are reactive in producing O2, with activation free energies of 15.9 and 14.3 kcal mol-1, respectively. However, our calculations revealed that the Mn(IV)Mn(IV)-oxyl complex is significantly more reactive in producing O2 than Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH, with a lower free energy barrier of 8.1 kcal mol-1. Consequently, even though Mn(IV)Mn(IV)-oxyl is predicted to be present in much lower concentrations than Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH, it emerges as the species acting as the active catalyst for catalytic O2 generation. This study enhances our knowledge of high oxidation state (+3 and +4) manganese chemistry, highlighting its key role in catalysis and paving the way for more efficient Mn-based catalysts with broad applications.

Cobalt-Based Nanoscale Material: An Emerging Electrocatalyst for Hydrogen Production

Modern civilization has been highly suffering from energy crisis and environmental pollutions. These two burning issues are directly and indirectly created from fossil fuel consumption and uncontrolled industrialization. The above critical issue can be solved through the proper utilization of green energy sources where no greenhouse gases will be generated upon burning of such materials. Hydrogen is the most eligible candidate for this purpose. Among various methods of hydrogen generation, electrocatalytic process is one of the most efficient methods because of easy handling and high efficiency. In these aspects Co-based nanomaterials are considered to be extremely significant as they can be utilized as efficient, recyclable and ideal catalytic system. In this article a series of Co-based nano-electrocatalysts has been discussed with proper structure-property relationship and their medium dependency. Therefore, such type of stimulating summary on recently reported electrocatalysts and their activity may be helpful for scientists of the corresponding field as well as for broader research communities. This can be inspiration for materials researchers to fabricate active catalysts for the production of hydrogen gas in room temperature.

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.

Effect of Anchoring Dynamics on Proton-Coupled Electron Transfer in the Ru(bda) Coordination Oligomer on a Graphitic Surface

The oligomeric ruthenium-based water oxidation catalyst, Ru(bda), is known to be experimentally anchored on graphitic surfaces through CH-π stacking interactions between the auxiliary bda ([2,2'-bipyridine]-6,6'-dicarboxylate) ligand bonded to ruthenium and the hexagonal rings of the surface. This anchoring provides control over their molecular coverage and enables efficient catalysis of water oxidation to dioxygen. The oligomeric nature of the molecule offers multiple anchoring sites at the surface, greatly enhancing the overall stability of the hybrid catalyst-graphitic surface anode through dynamic bonding. However, the impact of this dynamic anchoring on the overall catalytic mechanism is still a topic of debate. In this study, a crucial proton-coupled electron transfer event in the catalytic cycle is investigated using DFT-based molecular dynamics simulations plus metadynamics. The CH-π stacking anchoring plays a critical role not only in stabilizing this hybrid system but also in facilitating the proton-coupled electron transfer event with possible vibronic couplings between the anchoring bonds motion and charge fluctuations at the catalyst - graphitic surface interface. Furthermore, this computational investigation displays the presence of a quartet spin state intermediate that can lead to the experimentally observed and thermodynamically more stable doublet spin state.

Inhibiting Photo-Oxidation and Enhancing Visible-Light-Driven Photocatalytic Water Oxidation over Covalent Organic Frameworks Through the Coordination of Cobalt with Bipyridine

Photocatalytic water splitting using covalent organic frameworks (COFs) is a promising approach for harnessing solar energy. However, challenges such as slow kinetic dynamics in the photocatalytic oxygen evolution reaction (OER) and COFs' self-oxidation hinder its progress. In this study, an enamine-based COF coordinated is introduced with cobalt dichloride, CoCl2 (CoCl2-TpBPy). The coordination of cobalt ions with bipyridines in CoCl2-TpBPy enhances charge-carrier separation and migration, leading to effective photocatalytic OER. Under visible light irradiation, CoCl2-TpBPy achieves a notable OER rate of up to 1 mmol·g-1·h-1, surpassing the reported organic semiconductor analogs. Additionally, CoCl2-TpBPy shows minimal nitrogen evolution compared to TpBPy and ethanol-treated TpBPy (E-TpBPy), indicating cobalt plays a pivotal role in improving charge utilization and minimizing photo-oxidation. In situ X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) analyses revealed that Co(IV) species are key to the high OER efficiency. This work highlights Co(IV) species in the efficient OER and inhibiting photo-oxidation of CoCl2-TpBPy.

Photoelectrochemical Cells with a Pyridine-Anchored Bilayer Photoanode for Water Splitting

A dye-sensitized photoanode is prepared by coassembling a Ru complex photosensitizer and a Ru water oxidation catalyst (WOC) on a TiO2 substrate, in which the WOC molecules are immobilized in a layer-by-layer fashion through metal-pyridine coordination with the aid of a bifunctional anchoring and bridging molecule containing multiple pyridine groups. Under visible-light irradiation, an anodic photocurrent of around 200 μA/cm2 has been achieved with O2 and H2 being generated at the photoanode and Pt counter electrode, respectively. The pyridine anchoring strategy provides a simple method to prepare photoelectrodes for applications in photoelectrochemical cells.

Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(μ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

Improvement of electrocatalytic water oxidation activity of novel copper complex by modulating the axial coordination of phosphate on metal center

The structure of organic ligand scaffolds of copper complexes critically affects their electrocatalytic properties toward water oxidation, which is widely regarded as the bottleneck of overall water splitting. Herein, two novel mononuclear Cu complexes, [Cu(dmabpy)](ClO4)2 (1, dmabpy = 6,6'-bis(dimethylaminomethyl)-2,2'-bipyridine) and [Cu(mabpy)](ClO4)2 (2, mabpy = 6,6'-bis(methylaminomethyl)-2,2'-bipyridine), with four-coordinated distorted planar quadrilateral geometry were synthesized and explored as efficient catalysts for electrochemical oxygen evolution in phosphate buffer solution. Interestingly, complex 1 with a tertiary amine group catalyzes water oxidation with lower onset overpotential and better catalytic performance, while complex 2 containing a secondary amine fragment displays much lower catalytic activity under identical conditions. The water oxidation catalytic mechanism of the two complexes is proposed based on the electrochemical test results. Experimental methods indicate that phosphate coordinated on the Cu center of the two complexes inhibits their reaction with substrate water molecules, resulting in lower activity toward water oxidation. Electrochemical tests reveal that the structure of the coordinated nitrogen atom improves the catalytic performance of the Cu complexes by modulating the coordination of phosphate on the Cu center, indicating that a minor alteration of the coordinating nitrogen atom of the ligand has a detrimental effect on the catalytic performance of electrochemical WOCs based on transition metal complexes.

Supramolecular catalyst with [FeCl4] unit boosting photoelectrochemical seawater splitting via water nucleophilic attack pathway

Propelled by the structure of water oxidation co-catalysts in natural photosynthesis, molecular co-catalysts have long been believed to possess the developable potential in artificial photosynthesis. However, the interfacial complexity between light absorber and molecular co-catalyst limits its structural stability and charge transfer efficiency. To overcome the challenge, a supramolecular scaffold with the [FeCl4] catalytic units is reported, which undergo a water-nucleophilic attack of the water oxidation reaction, while the supramolecular matrix can be in-situ grown on the surface of photoelectrode through a simple chemical polymerization to be a strongly coupled interface. A well-defined BiVO4 photoanode hybridized with [FeCl4] units in polythiophene reaches 4.72 mA cm-2 at 1.23 VRHE, which also exhibits great stability for photoelectrochemical seawater splitting due to the restraint on chlorine evolution reaction by [FeCl4] units and polythiophene. This work provides a novel solution to the challenge of the interface charge transfer of molecular co-catalyst hybridized photoelectrode.

An artificial metalloenzyme that can oxidize water photocatalytically: design, synthesis, and characterization

In nature, light-driven water oxidation (WO) catalysis is performed by photosystem II via the delicate interplay of different cofactors positioned in its protein scaffold. Artificial systems for homogeneous photocatalytic WO are based on small molecules that often have limited solubility in aqueous solutions. In this work, we alleviated this issue and present a cobalt-based WO-catalyst containing artificial metalloenzyme (ArM) that is active in light-driven, homogeneous WO catalysis in neutral-pH aqueous solutions. A haem-containing electron transfer protein, cytochrome B5 (CB5), served to host a first-row transition-metal-based WO catalyst, CoSalen (CoIISalen, where H2Salen = N,N'-bis(salicylidene)ethylenediamine), thus producing an ArM capable of driving photocatalytic WO. The CoSalen ArM formed a water-soluble pre-catalyst in the presence of [Ru(bpy)3](ClO4)2 as photosensitizer and Na2S2O8 as the sacrificial electron acceptor, with photocatalytic activity similar to that of free CoSalen. During photocatalysis, the CoSalen-protein interactions were destabilized, and the protein partially unfolded. Rather than forming tens of nanometer sized CoOx nanoparticles as free CoSalen does under photocatalytic WO conditions, the CB5 : CoSalen ArM showed limited protein cross-linking and remained soluble. We conclude that a weak, dynamic interaction between a soluble cobalt species and apoCB5 was formed, which generated a catalytically active adduct during photocatalysis. A detailed analysis was performed on protein stability and decomposition processes during the harsh oxidizing reaction conditions of WO, which will serve for the future design of WO ArMs with improved activity and stability.

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.

Pivotal Role of Geometry Regulation on O-O Bond Formation Mechanism of Bimetallic Water Oxidation Catalysts

In this study, we highlight the impact of catalyst geometry on the formation of O-O bonds in Cu2 and Fe2 catalysts. A series of Cu2 complexes with diverse linkers are designed as electrocatalysts for water oxidation. Interestingly, the catalytic performance of these Cu2 complexes is enhanced as their molecular skeletons become more rigid, which contrasts with the behavior observed in our previous investigation with Fe2 analogs. Moreover, mechanistic studies reveal that the reactivity of the bridging O atom results in distinct pathways for O-O bond formation in Cu2 and Fe2 catalysts. In Cu2 systems, the coupling takes place between a terminal CuIII -OH and a bridging μ-O⋅ radical. Whereas in Fe2 systems, it involves the coupling of two terminal Fe-oxo entities. Furthermore, an in-depth structure-activity analysis uncovers the spatial geometric prerequisites for the coupling of the terminal OH with the bridging μ-O⋅ radical, ultimately leading to the O-O bond formation. Overall, this study emphasizes the critical role of precisely adjusting the spatial geometry of catalysts to align with the O-O bonding pathway.

Small Organic Molecular Electrocatalysts for Fuels Production

In recent years, heterocyclic organic compounds have been explored as molecular electrocatalysts in relevant reactions for energy conversion and storage. Merging mimetics of biological systems that perform hydride transfer with rational synthetic chemical design has opened many opportunities for organic molecules to be tuned at the atomic level conferring them interesting reactivities. These molecular electrocatalysts represent an alternative to traditional metallic materials and metal complexes employed for water oxidation, hydrogen production, and carbon dioxide reduction. This minireview describes recent reports concerning design, catalytic activity and the mechanism of synthetic molecular electrocatalysts towards solar fuels production.

Identification of Crucial Photosensitizing Factors to Promote CO2 -to-CO Conversion

The sensitizing ability of a catalytic system is closely related to the visible-light absorption ability, excited-state lifetime, redox potential, and electron-transfer rate of photosensitizers (PSs), however it remains a great challenge to concurrently mediate these factors to boost CO2 photoreduction. Herein, a series of Ir(III)-based PSs (Ir-1-Ir-6) were prepared as molecular platforms to understand the interplay of these factors and identify the primary factors for efficient CO2 photoreduction. Among them, less efficient visible-light absorption capacity results in lower CO yields of Ir-1, Ir-2 or Ir-4. Ir-3 shows the most efficient photocatalytic activity among these mononuclear PSs due to some comprehensive parameters. Although the Kobs of Ir-3 is ≈10 times higher than that of Ir-5, the CO yield of Ir-3 is slightly higher than that of Ir-5 due to the compensation of Ir-5's strong visible-light-absorbing ability. Ir-6 exhibits excellent photocatalytic performance due to the strong visible-light absorption ability, comparable thermodynamic driving force, and electron transfer rate among these PSs. Remarkably, the CO2 photoreduction to CO with Ir-6 can achieve 91.5 μmol, over 54 times higher than Ir-1, and the optimized TONC-1 can reach up to 28160. Various photophysical properties of the PSs were concurrently adjusted by fine ligand modification to promote CO2 photoreduction.

O-O bond formation via radical coupling in a dinuclear iron water oxidation catalyst with high catalytic activity

The use of iron-based catalysts for the water oxidation reaction is highly attractive due to the high abundance of iron. While many molecular catalysts have been made, most show limited activity and short lifetimes. An exception with higher activity was presented by Thummel and co-workers in 2015. Herein we present a study on the feasibility of the coupling of two O centered radicals originating from the two subunits of the dinuclear catalyst. The reaction pathway includes the oxidation to the active species FeIV-O-FeIV but avoids further high potential oxidations which previous mechanistic proposals have relied on.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Coupling between two Ru(bda) catalysts bridged by a trans-dicyano complex

We have prepared two trimetallic complexes [{Ru(bda)(DMSO)(μ-CN)}2Ru(L)4] (with bda = 2,2'-bipyridine-6,6'-dicarboxylate) where two {Ru(bda)} centers are bridged by a cyanide complex of the trans-Ru(L)4CN2 family (with L = pyridine and 4-tert-butylpyridine). The complex [{Ru(bda)(DMSO)(μ-CN)}2Ru(py)4] is fully soluble in aqueous solution and is a catalyst for the oxidation of water both chemically, using Ce(IV) at pH = 1 as the terminal oxidant, and electrochemically. Both reactions are first order in the complex and the resting state of the catalyst is the [RuVRuIII(py)4RuIV]2+ redox state. Electrochemical and spectroelectrochemical studies together with (TD)DFT calculations show that the coupling between the Ru(bda) fragments for the [RuIIIRuII(py)4RuIII]2+ and [RuIVRuII(py)4RuIV]2+ redox states is very weak, but significant for the [RuVRuII(py)4RuIV]2+ ion due to the orientation of the orbitals involved. This coupling affects the reactivity of the [RuVRuII(py)4RuIV]2+ redox state, making it a much slower catalyst towards the water oxidation reaction than [RuVRuIII(py)4RuIV]2+.

Metal Complexes for Dye-Sensitized Photoelectrochemical Cells (DSPECs)

Since Mallouk's earliest contribution, dye-sensitized photoelectrochemical cells (DSPECs) have emerged as a promising class of photoelectrochemical devices capable of storing solar light into chemical bonds. This review primarily focuses on metal complexes outlining stabilization strategies and applications. The ubiquity and safety of water have made its splitting an extensively studied reaction; here, we present some examples from the outset to recent advancements. Additionally, alternative oxidative pathways like HX splitting and organic reactions mediated by a redox shuttle are discussed.

Soluble Gd6Cu24 clusters: effective molecular electrocatalysts for water oxidation

The water oxidation half reaction in water splitting for hydrogen production is extremely rate-limiting. This study reports the synthesis of two heterometallic clusters (Gd6Cu24-IM and Gd6Cu24-AC) for application as efficient water oxidation catalysts. Interestingly, the maximum turnover frequency of Gd6Cu24-IM in an NaAc solution of a weak acid (pH 6) was 319 s-1. The trimetallic catalytic site, H2O-GdIIICuII2-H2O, underwent two consecutive two-electron two-proton coupled transfer processes to form high-valent GdIII-O-O-CuIII2 intermediates. Furthermore, the O-O bond was formed via intramolecular interactions between the CuIII and GdIII centers. The results of this study revealed that synergistic catalytic water oxidation between polymetallic sites can be an effective strategy for regulating O-O bond formation.

Structure and reactivity of a seven-coordinate ruthenium acylperoxo complex

The use of metal-acylperoxo complexes as oxidants has been little explored. Herein we report the synthesis and characterization of the first seven-coordinate Ru-acylperoxo complex, [RuIV(bdpm)(pic)2(mCPBA)]+ (H2bdpm = [2,2'-bipyridine]-6,6'-diylbis(diphenylmethanol); pic = 4-picoline; HmCPBA = m-chloroperbenzoic acid). This complex is a highly reactive oxidant for C-H bond activation and O-atom transfer reactions.

Cluster size dependent coordination of formate to free manganese oxide clusters

The interaction of free manganese oxide clusters, MnxOy+ (x = 1-9, y = 0-12), with formic acid was studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy together with calculations using density functional theory (DFT). Clusters containing only one Mn atom, such as MnO2+ and MnO4+, bind formic acid as an intact molecule in both the cis- and trans-configuration. In contrast, all clusters containing two or more manganese atoms deprotonate the acid's hydroxyl group. The coordination of the resulting formate group is strongly cluster-size-dependent according to supporting DFT calculations for selected model systems. For Mn2O2+ the co-existence of two isomers with the formate bound in a bidentate bridging and chelating configurations, respectively, is found, whereas for Mn2O4+ the bidentate chelating configuration is preferred. In contrast, the bidentate bridging structure is energetically considerably more favorable for Mn4O4+. This binding motif stabilizes the 2D ring structure of the core of the Mn4O4+ cluster with respect to the 3D cubic geometry of the Mn4O4+ cluster core.

A Polymer-Derived Co(Fe)Ox Oxygen Evolution Catalyst Benefiting from the Oxidative Dehydrogenative Coupling of Cobalt Porphyrins

Thin films of cobalt porphyrin conjugated polymers bearing different substituents are prepared by oxidative chemical vapor deposition (oCVD) and investigated as heterogeneous electrocatalysts for the oxygen evolution reaction (OER). Interestingly, the electrocatalytic activity originates from polymer-derived, highly transparent Co(Fe)Ox species formed under operational alkaline conditions. Structural, compositional, electrical, and electrochemical characterizations reveal that the newly formed active catalyst greatly benefited from both the polymeric conformation of the porphyrin-based thin film and the inclusion of the iron-based species originating from the oCVD reaction. High-resolution mass spectrometry analyses combined with density functional theory (DFT) calculations showed that a close relationship exists between the porphyrin substituent, the extension of the π-conjugated system cobalt porphyrin conjugated polymer, and the dynamics of the polymer conversion leading to catalytically active Co(Fe)Ox species. This work evidences the precatalytic role of cobalt porphyrin conjugated polymers and uncovers the benefit of extended π-conjugation of the molecular matrix and iron inclusion on the formation and performance of the true active catalyst.

Reticular framework materials for photocatalytic organic reactions

Photocatalytic organic reactions, harvesting solar energy to produce high value-added organic chemicals, have attracted increasing attention as a sustainable approach to address the global energy crisis and environmental issues. Reticular framework materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are widely considered as promising candidates for photocatalysis owing to their high crystallinity, tailorable pore environment and extensive structural diversity. Although the design and synthesis of MOFs and COFs have been intensively developed in the last 20 years, their applications in photocatalytic organic transformations are still in the preliminary stage, making their systematic summary necessary. Thus, this review aims to provide a comprehensive understanding and useful guidelines for the exploration of suitable MOF and COF photocatalysts towards appropriate photocatalytic organic reactions. The commonly used reactions are categorized to facilitate the identification of suitable reaction types. From a practical viewpoint, the fundamentals of experimental design, including active species, performance evaluation and external reaction conditions, are discussed in detail for easy experimentation. Furthermore, the latest advances in photocatalytic organic reactions of MOFs and COFs, including their composites, are comprehensively summarized according to the actual active sites, together with the discussion of their structure-property relationship. We believe that this study will be helpful for researchers to design novel reticular framework photocatalysts for various organic synthetic applications.

Theoretical elucidation of the structure, bonding, and reactivity of the CaMn4Ox clusters in the whole Kok cycle for water oxidation embedded in the oxygen evolving center of photosystem II. New molecular and quantum insights into the mechanism of the O-O bond formation

This paper reviews our historical developments of broken-symmetry (BS) and beyond BS methods that are applicable for theoretical investigations of metalloenzymes such as OEC in PSII. The BS hybrid DFT (HDFT) calculations starting from high-resolution (HR) XRD structure in the most stable S1 state have been performed to elucidate structure and bonding of whole possible intermediates of the CaMn4Ox cluster (1) in the Si (i = 0 ~ 4) states of the Kok cycle. The large-scale HDFT/MM computations starting from HR XRD have been performed to elucidate biomolecular system structures which are crucial for examination of possible water inlet and proton release pathways for water oxidation in OEC of PSII. DLPNO CCSD(T0) computations have been performed for elucidation of scope and reliability of relative energies among the intermediates by HDFT. These computations combined with EXAFS, XRD, XFEL, and EPR experimental results have elucidated the structure, bonding, and reactivity of the key intermediates, which are indispensable for understanding and explanation of the mechanism of water oxidation in OEC of PSII. Interplay between theory and experiments have elucidated important roles of four degrees of freedom, spin, charge, orbital, and nuclear motion for understanding and explanation of the chemical reactivity of 1 embedded in protein matrix, indicating the participations of the Ca(H2O)n ion and tyrosine(Yz)-O radical as a one-electron acceptor for the O-O bond formation. The Ca-assisted Yz-coupled O-O bond formation mechanisms for water oxidation are consistent with recent XES and very recent time-resolved SFX XFEL and FTIR results.

Engineering a molecular ruthenium catalyst into three-dimensional metal covalent organic frameworks for efficient water oxidation

The water oxidation reaction plays an important role in clean energy conversion, utilization, and storage, but mimicking the oxygen-evolving complex of photosystem II for designing active and stable water oxidation catalysts (WOCs) is still an appealing challenge. Here, we innovatively engineered a molecular ruthenium WOC as a metal complex building unit to construct a series of three-dimensional metal covalent organic frameworks (3D MCOFs) for realizing efficient oxidation catalysis. The resultant MCOFs possessed rare 3D interlocking structures with inclined interpenetration of two-dimensional covalent rhombic nets, and the Ru sites were periodically arranged in the crystalline porous frameworks. Impressively, these MCOFs showed excellent performance towards water oxidation (the O2 evolution rate is as high as 2830 nmol g-1 s-1) via the water nucleophilic attack pathway. Besides, the MCOFs were also reactive for oxidizing organic substrates. This work highlights the potential of MCOFs as a designable platform in integrating molecular catalysts for various applications.

Comprehensive Insights into the Family of Atomically Thin 2D-Materials for Diverse Photocatalytic Applications

2D materials with their fascinating physiochemical, structural, and electronic properties have attracted researchers and have been used for a variety of applications such as electrocatalysis, photocatalysis, energy storage, magnetoresistance, and sensing. In recent times, 2D materials have gained great momentum in the spectrum of photocatalytic applications such as pollutant degradation, water splitting, CO2 reduction, NH3 production, microbial disinfection, and heavy metal reduction, thanks to their superior properties including visible light responsive band gap, improved charge separation and electron mobility, suppressed charge recombination and high surface reactive sites, and thus enhance the photocatalytic properties rationally as compared to 3D and other low-dimensional materials. In this context, this review spot-lights the family of various 2D materials, their properties and their 2D structure-induced photocatalytic mechanisms while giving an overview on their synthesis methods along with a detailed discussion on their diverse photocatalytic applications. Furthermore, the challenges and the future opportunities are also presented related to the future developments and advancements of 2D materials for the large-scale real-time photocatalytic applications.

Theoretical investigation of solvent and oxidation/deprotonation effects on the electronic structure of a mononuclear Ru-aqua-polypyridine complex in aqueous solution

Mononuclear polypyridine ruthenium (Ru) complexes can catalyze various reactions, including water splitting, and can also serve as photosensitizers in solar cells. Despite recent progress in their synthesis, accurately modeling their physicochemical properties, particularly in solution, remains challenging. Herein, we conduct a theoretical investigation of the structural and electronic properties of a mononuclear Ru-aqua polypyridine complex in aqueous solution, considering five of its possible oxidation/protonation states species: [RuII(H2O)(py)(bpy)2]2+, [RuII(OH)(py)(bpy)2]+, [RuIII(H2O)(py)(bpy)2]3+, [RuIII(OH)(py)(bpy)2]2+ and [RuIV(O)(py)(bpy)2]2+, where py = pyridine and bpy = 2,2'-bipyridine. At first, we investigate the impact of proton-coupled and non-coupled electron transfer reactions on the geometry and electronic structure of the complexes in vacuum and in solution, using an implicit solvent model. Then, using a sequential multiscale approach that combines quantum mechanics and molecular mechanics (S-QM/MM), we examine the explicit solvent effects on the electronic excitations of the complexes, and compare them with the experimental results. The complexes were synthesized, and their absorption spectra measured in aqueous solution. To accurately describe the QM interactions between the metal center and the aqueous ligand in the MM simulations, we developed new force field parameters for the Ru atom. We analyze the solvent structure around the complexes and account for its explicit influence on the polarization and electronic excitations of the complexes. Notably, accounting for the explicit solvent polarization effects of the first solvation shells is essential to correctly describe the energy of the electronic transitions, and the explicit treatment of the hydrogen bonds at the QM level in the excitation calculations improves the accuracy of the description of the metal-to-ligand charge-transfer bands. Transition density matrix analysis is used to characterize all electronic transitions in the visible and ultraviolet ranges according to their charge-transfer (CT) character. This study elucidates the electronic structure of those ruthenium polypyridyl complexes in aqueous solution and underscores the importance of precisely describing solvent effects, which can be achieved employing the S-QM/MM method.

In situ assembly of nickel-based ultrathin catalyst film for water oxidation

A nickel-based ultrathin catalyst film is assembled in situ from a solution of Ni(OAc)2 and a Schiff-base ligand L. The resulting ultrathin catalyst film shows a low overpotential of 330 mV, a steady current of 7 mA cm-2 for water oxidation over 10 h.

Tracking an FeV (O) Intermediate for Water Oxidation in Water

High-valent iron-oxo species are appealing for conducting O-O bond formation for water oxidation reactions. However, their high reactivity poses a great challenge to the dissection of their chemical transformations. Herein, we introduce an electron-rich and oxidation-resistant ligand, 2-[(2,2'-bipyridin)-6-yl]propan-2-ol to stabilize such fleeting intermediates. Advanced spectroscopies and electrochemical studies demonstrate a high-valent FeV (O) species formation in water. Combining kinetic and oxygen isotope labelling experiments and organic reactions indicates that the FeV (O) species is responsible for O-O bond formation via water nucleophilic attack under the real catalytic water oxidation conditions.

CdS/TiO2 nano hybrid heterostructured materials for superior hydrogen production and gas sensor applications

In efforts to minimize environmental pollution and carbon-based gas emissions, photocatalytic hydrogen production and sensing applications at ambient temperature are important. This research reports on the development of new 0D/1D materials based on TiO₂ nanoparticles grown onto CdS hetersturctured nanorods via two-stage facile synthesis. The titanate nanoparticles when loaded onto CdS surfaces at an optimized concentration (20 mM), exhibited superior photocatalytic hydrogen production (21.4 mmol/h/g_cat). The optimized nanohybrid was recycled for 6 cycles up to 4 h, indicating its excellent stabity for a prolonged period. Also, the photoelectrochemical water oxidation in alkaline medium was investigated to offer the optimized CRT-2 composite with 1.91 mA/cm²@0.8 V vs. RHE (0 V vs. Ag/AgCl) that was used for effective room-temperature NO₂ gas detection exhibiting a higher response (69.16%) to NO₂ (100 ppm) at room temperature at the lowest detection limit of ∼118 ppb than the pristine counterparts. Further, NO₂ gas sensing performance of CRT-2 sensor was increased using UV light (365 nm) activation energy. Under the UV light, the sensor exhibited a remarkable gas sensing response quick response/recovery times (68/74), excellent long-term cycling stability, and significant selectivity to NO₂ gas. Due to high porosity and surface area values of CdS (5.3), TiO₂ (35.5), and CRT-2 (71.5 m²/g), excellent photocatalytic H₂ production and gas sensing of CRT-2 is ascribed to morphology, synergistic effect, improved charge generation, and separation. Overall, 1D/0D CdS@TiO₂ is proved to be an efficient material for hydrogen production and gas detection.

Iridium-Complexed Dipyridyl-Pyridazine Organosilica as a Catalyst for Water Oxidation

The heterogenization of metal-complex catalysts to be applied in water oxidation reactions is a currently growing field of great scientific impact for the development of energy conversion devices simulating the natural photosynthesis process. The attachment of IrCp*Cl complexes to the dipyridyl-pyridazine N-chelating sites on the surface of SBA-15 promotes the formation of metal bipyridine-like complexes, which can act as catalytic sites in the oxidation of water to dioxygen, the key half-reaction of artificial photosynthetic systems. The efficiency of the heterogeneous catalyst, Ir@NdppzSBA, in cerium(IV)-driven water oxidation was thoroughly evaluated, achieving high catalytic activity even at a long reaction time. The reusability and stability were also examined after three reaction cycles, with a slight loss of activity. A comparison with an analogous homogeneous iridium catalyst revealed the enhanced durability and performance of the heterogeneous system based on the Ir@NdppzSBA catalyst due to the stability of the SBA-15 structure as well as the isolated metal active sites. Thereby, this new versatile synthesis route for the preparation of water oxidation catalysts opens a new avenue for the construction of alternative heterogeneous catalytic systems with high surface area, ease of functionalization, and facile separation to improve the efficiency in the water oxidation reaction.

A tube-like Pd@coordination polymer with enhanced solar light harvesting for boosting photocatalytic H2 production in a wide pH range and seawater

Coordination polymers (CPs) have emerged as promising candidates for photocatalytic H₂ production owing to their structural tailorability and functional diversity. However, the development of CPs with high energy transfer efficiency for highly efficient photocatalytic H₂ production in a wide pH range still faces many challenges. Here we constructed a novel tube-like Pd(ii) coordination polymer with well-distributed Pd nanoparticles (denoted as Pd/Pd(ii)CPs) based on the coordination assembly of rhodamine 6G and Pd(ii) ions and further photo-reduction under visible light irradiation. Both the Br^- ion and double solvent play a key role in forming the hollow superstructures. The resulting tube-like Pd/Pd(ii)CPs exhibit high stability in aqueous solution with the pH range from 3 to 14 due to the high Gibbs free energies of protonation and deprotonation, which provides the feasibility of photocatalytic hydrogen generation in a wide pH range. Electromagnetic field calculations showed that the tube-like Pd/Pd(ii)CPs have a good confinement effect on light. Therefore, the H₂ evolution rate could reach 112.3 mmol h^-1 g^-1 at pH 13 under visible light irradiation, which is far superior to those of reported coordination polymer-based photocatalysts. Moreover, such Pd/Pd(ii)CPs could also reach a H₂ production rate of 37.8 mmol h^-1 g^-1 in seawater under visible light with low optical density (40 mW cm^-2) close to morning or cloudy sunlight. The above unique characteristics make the Pd/Pd(ii)CPs possess great potential for practical applications.

Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers

Generating sustainable fuel from sunlight plays an important role in meeting the energy demands of the modern age. Herein, we report two-coordinate carbene-metal-amide (cMa, M = Cu(I) and Au(I)) complexes that can be used as sensitizers to promote the light-driven reduction of water to hydrogen. The cMa complexes studied here absorb visible photons (ε_vis > 10³ M^-1 cm^-1), maintain long excited-state lifetimes (τ ∼ 0.2-1 μs), and perform stable photoinduced charge transfer to a target substrate with high photoreducing potential (E^+/* up to -2.33 V vs Fc^+/0 based on a Rehm-Weller analysis). We pair these coinage metal complexes with a cobalt-glyoxime electrocatalyst to photocatalytically generate hydrogen and compare the performance of the copper- and gold-based cMa complexes. We also find that the two-coordinate complexes herein can perform photodriven hydrogen production from water without the addition of the cobalt-glyoxime electrocatalyst. In this "catalyst-free" system, the cMa sensitizer partially decomposes to give metal nanoparticles that catalyze water reduction. This work identifies two-coordinate coinage metal complexes as promising abundant metal, solar fuel photosensitizers that offer exceptional tunability and photoredox properties.

Computational Analysis of Chemical Reactions Using a Variational Quantum Eigensolver Algorithm without Specifying Spin Multiplicity

The analysis of a chemical reaction along the ground-state potential energy surface in conjunction with an unknown spin state is challenging because electronic states must be separately computed several times using different spin multiplicities to find the lowest energy state. However, in principle, the ground state could be obtained with just a single calculation using a quantum computer without specifying the spin multiplicity in advance. In the present work, ground-state potential energy curves for PtCO were calculated as a proof-of-concept using a variational quantum eigensolver (VQE) algorithm. This system exhibits a singlet-triplet crossover as a consequence of the interaction between Pt and CO. VQE calculations using a statevector simulator were found to converge to a singlet state in the bonding region, while a triplet state was obtained at the dissociation limit. Calculations performed using an actual quantum device provided potential energies within ±2 kcal/mol of the simulated energies after error mitigation techniques were adopted. The spin multiplicities in the bonding and dissociation regions could be clearly distinguished even in the case of a small number of shots. The results of this study suggest that quantum computing can be a powerful tool for the analysis of the chemical reactions of systems for which the spin multiplicity of the ground state and variations in this parameter are not known in advance.

Fe(III) Docking-Activated Sites in Layered Birnessite for Efficient Water Oxidation

Non-noble metal catalysts for promoting the sluggish kinetics of oxygen evolution reaction (OER) are essential to efficient water splitting for sustainable hydrogen production. Birnessite has a local atomic structure similar to that of an oxygen-evolving complex in photosystem II, while the catalytic activity of birnessite is far from satisfactory. Herein, we report a novel Fe-Birnessite (Fe-Bir) catalyst obtained by controlled Fe(III) intercalation- and docking-induced layer reconstruction. The reconstruction dramatically lowers the OER overpotential to 240 mV at 10 mA/cm² and the Tafel slope to 33 mV/dec, making Fe-Bir the best of all the reported Bir-based catalysts, even on par with the best transition-metal-based OER catalysts. Experimental characterizations and molecular dynamics simulations elucidate that the catalyst features active Fe(III)-O-Mn(III) centers interfaced with ordered water molecules between neighboring layers, which lower reorganization energy and accelerate electron transfer. DFT calculations and kinetic measurements show non-concerted PCET steps conforming to a new OER mechanism, wherein the neighboring Fe(III) and Mn(III) synergistically co-adsorb OH* and O* intermediates with a substantially reduced O-O coupling activation energy. This work highlights the importance of elaborately engineering the confined interlayer environment of birnessite and more generally, layered materials, for efficient energy conversion catalysis.

Dye-induced photoluminescence quenching of quantum dots: role of excited state lifetime and confinement of charge carriers

We investigate the role of quantum confinement and photoluminescence (PL) lifetime of photoexcited charge carriers in semiconductor core/shell quantum dots (QDs) via PL quenching due to surface modification. Surface modification is controlled by varying the number of dye molecules adsorbed onto the QD shell surface forming QD-dye nanoassemblies. We selected CuInS₂/ZnS (CIS) and InP/ZnS (InP) core/shell QDs exhibiting relatively weak (664 meV) and strong (1194 meV) confinement potentials for the conduction band electron. Moreover, the difference in the emission mechanism gives rise to a long and short excited state lifetime of CIS (ca. 290 ns) and InP (ca. 37 ns) QDs. Dye molecules of different ionic characters (rhodamine 575: zwitterionic and rhodamine 560: cationic) are used as quenchers. A detailed analysis of Stern-Volmer data shows that (i) quenching is generally more pronounced in CIS-dye assemblies as compared to InP-dye assemblies, (ii) dynamic quenching is dominating in all QD-dye assemblies with only a minor contribution from static quenching and (iii) the cationic dye shows a stronger interaction with the QD shell surface than the zwitterionic dye. Observations (i) and (ii) can be explained by the differences in the amplitude of the electronic component of the exciton wavefunction near the dye binding sites in both QDs, which results in the breaking up of the electron-hole pair and favors charge trapping. Observation (iii) can be attributed to the variations in electrostatic interactions between the negatively charged QD shell surface and the cationic and zwitterionic dyes, with the former exhibiting a stronger interaction. Moreover, the long lifetime of CIS QDs facilitates us to easily probe different time scales of the trapping processes and thus differentiate the origins of static and dynamic quenching components that appear in the Stern-Volmer analysis.

Structure and Reactivity of a Seven-Coordinate Ruthenium Iodosylbenzene Complex

Seven-coordinate (CN7) ruthenium-oxo species have attracted much attention as highly reactive intermediates in both organic and water oxidation. Apart from metal-oxo, other metal-oxidant adducts, such as metal-iodosylarenes, have also recently emerged as active oxidants. We reported herein the first example of a CN7 Ru-iodosylbenzene complex, [Ru^IV(bdpm)(pic)₂(O)I(Cl)Ph]⁺ (H₂bdpm = [2,2'-bipyridine]-6,6'-diylbis(diphenylmethanol); pic = 4-picoline). The X-ray crystal structure of this complex shows that it adopts a distorted pentagonal bipyramidal geometry with Ru-O(I) and O-I distances of 2.0451(39) and 1.9946(40) Å, respectively. This complex is highly reactive, and it readily undergoes O-atom transfer (OAT) and C-H bond activation reactions with various organic substrates. This work should provide insights for the development of new highly reactive oxidizing agents based on CN7 geometry.

Investigation of quinone reduction by microalgae using fluorescence - do "lake" and "puddle" mechanisms matter?

Photosynthesis is a fundamental process used by Nature to convert solar energy into chemical energy. For the last twenty years, many solutions have been explored to provide electrical power from the photosynthetic chain. In this context, the coupling between microalgae and exogenous quinones is an encouraging strategy because of the capability of quinones to be reduced by the photosynthetic chain. The ability of a quinone to be a good or bad electron acceptor can be evaluated by fluorescence measurements. Fluorescence analyses are thus a convenient tool helping to define a diverting parameter for some quinones. However, this parameter is implicitly designed on the basis of a particular light capture mechanism by algae. In this paper, we propose to revisit previous fluorescence experimental data by considering the two possible mechanisms (lake vs. puddle) and discussing their implication on the conclusions of the analysis. In particular, we show that the maximum extraction efficiency depends on the mechanism (in the case of 2,6-dichlorobenzoquinone - 2,6-DCBQ, (0.45 ± 0.02) vs (0.61 ± 0.03) for lake and puddle mechanisms respectively) but that the trends for different quinones remain correlated to the redox potentials independently of the mechanism.

Crystallographic evidence for the stereoselective substitution of equatorial pyridyl ligands in ruthenium(III) complexes

Mononuclear Ru complexes catalyze dioxygen formation via water splitting; therefore, a detailed investigation into their water-oxidation process is necessary. In this study, we synthesized a series of Ru(III) complexes containing a dianionic tridentate ligand with three pyridine groups (one coordinated to Ru while the other two are "free") and investigated their substitution reactions in a water/acetonitrile mixture. Among the monodentate pyridyl ligands, the one at the equatorial position was crystallographically proven to be selectively substituted. Therefore, our results experimentally demonstrate the proposed coordination geometry for an intermediate during water oxidation over Ru complexes.