Electronic design criteria for O-O bond formation via metal-oxo complexes

Manganese-Cobalt Oxido Cubanes Relevant to Manganese-Doped Water Oxidation Catalysts

Incorporation of Mn into an established water oxidation catalyst based on a Co(III)₄O₄ cubane was achieved by a simple and efficient assembly of permanganate, cobalt(II) acetate, and pyridine to form the cubane oxo cluster MnCo₃O₄(OAc)₅py₃ (OAc = acetate, py = pyridine) (1-OAc) in good yield. This allows characterization of electronic and chemical properties for a manganese center in a cobalt oxide environment, and provides a molecular model for Mn-doped cobalt oxides. The electronic properties of the cubane are readily tuned by exchange of the OAc^- ligand for Cl^- (1-Cl), NO₃^- (1-NO₃), and pyridine ([1-py]⁺). EPR spectroscopy, SQUID magnetometry, and DFT calculations thoroughly characterized the valence assignment of the cubane as [Mn^IVCo^III₃]. These cubanes are redox-active, and calculations reveal that the Co ions behave as the reservoir for electrons, but their redox potentials are tuned by the choice of ligand at Mn. This MnCo₃O₄ cubane system represents a new class of easily prepared, versatile, and redox-active oxido clusters that should contribute to an understanding of mixed-metal, Mn-containing oxides.

Fluorocarbene, fluoroolefin, and fluorocarbyne complexes of Rh

The manuscript reports the synthesis, characterization, and analysis of electronic structure in a series of complexes of small perfluorocarbon ligands with the (PNP)Rh fragment (where PNP is a diarylamido/bis(phosphine) pincer ligand). Reactions of (PNP)Rh(TBE) as the source of (PNP)Rh with CHF3 and C2HF5 produced perfluoroalkylidene complexes (PNP)Rh[double bond, length as m-dash]CF2 and (PNP)Rh[double bond, length as m-dash]C(F)(CF3). (PNP)Rh[double bond, length as m-dash]CF2 could also be obtained via the reaction of (PNP)Rh(TBE) with Me3SiCF3/CsF, with an admixture of (PNP)Rh(C2F4), where TBE = tert-butylethylene. Abstraction of fluoride from these neutral (PNP)RhC x F y complexes was successful, although only abstraction from (PNP)Rh[double bond, length as m-dash]CF2 allowed unambiguous identification of the Rh product, [(PNP)Rh[triple bond, length as m-dash]CF]+. DFT computational studies allowed comparison of relative energies of (PNP)Rh(C2F4) and [(PNP)Rh(C2F3)]+ isomers as well as comparisons between the electronic structure of the [double bond, length as m-dash]CF2, C2F4, and [triple bond, length as m-dash]CF+ complexes and their hydrocarbon analogues.

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

Terminal cobalt(IV)-oxo (Co^IV-O) species have been implicated as key intermediates in various cobalt-mediated oxidation reactions. Herein we report the photocatalytic generation of a mononuclear non-haem [(13-TMC)Co^IV(O)]²⁺ (2) by irradiating [Co^II(13-TMC)(CF₃SO₃)]⁺ (1) in the presence of [Ru^II(bpy)₃]²⁺, Na₂S₂O₈, and water as an oxygen source. The intermediate 2 was also obtained by reacting 1 with an artificial oxidant (that is, iodosylbenzene) and characterized by various spectroscopic techniques. In particular, the resonance Raman spectrum of 2 reveals a diatomic Co-O vibration band at 770 cm^-1, which provides the conclusive evidence for the presence of a terminal Co-O bond. In reactivity studies, 2 was shown to be a competent oxidant in an intermetal oxygen atom transfer, C-H bond activation and olefin epoxidation reactions. The present results lend strong credence to the intermediacy of Co^IV-O species in cobalt-catalysed oxidation of organic substrates as well as in the catalytic oxidation of water that evolves molecular oxygen.

Regulation and Plasticity of Catalysis in Enzymes: Insights from Analysis of Mechanochemical Coupling in Myosin

The mechanism of ATP hydrolysis in the myosin motor domain is analyzed using a combination of DFTB3/CHARMM simulations and enhanced sampling techniques. The motor domain is modeled in the pre-powerstroke state, in the post-rigor state, and as a hybrid based on the post-rigor state with a closed nucleotide-binding pocket. The ATP hydrolysis activity is found to depend on the positioning of nearby water molecules, and a network of polar residues facilitates proton transfer and charge redistribution during hydrolysis. Comparison of the observed hydrolysis pathways and the corresponding free energy profiles leads to detailed models for the mechanism of ATP hydrolysis in the pre-powerstroke state and proposes factors that regulate the hydrolysis activity in different conformational states. In the pre-powerstroke state, the scissile P_γ-O_3β bond breaks early in the reaction. Proton transfer from the lytic water to the γ-phosphate through active site residues is an important part of the kinetic bottleneck; several hydrolysis pathways that feature distinct proton transfer routes are found to have similar free energy barriers, suggesting a significant degree of plasticity in the hydrolysis mechanism. Comparison of hydrolysis in the pre-powerstroke state and the closed post-rigor model suggests that optimization of residues beyond the active site for electrostatic stabilization and preorganization is likely important to enzyme design.

Synthesis, crystal structure and water oxidation activity of [Ru(terpy)(bipy)Cl]+ complexes: influence of ancillary ligands on O2 generation

Synthesis of four new complexes [Ru^II(MeMPTP)(bpy)Cl]PF₆ (1), [Ru^II(MeMPTP)(dmbpy)Cl]PF₆ (2), [Ru^II(MeMPTP)(dmdcbpy)Cl]PF₆ (3) and [Ru^II(MeMPTP)(Pic)₂Cl]PF₆ (4) and their catalytic activity for chemical oxidation of water into O₂ generation has been demonstrated.

O–O bond formation in ruthenium-catalyzed water oxidation: single-site nucleophilic attack vs. O–O radical coupling

A review of water oxidation by ruthenium-based molecular catalysts, with emphasis on the mechanism of O–O bond formation.

A nickel-borate nanoarray: a highly active 3D oxygen-evolving catalyst electrode operating in near-neutral water

A nickel-borate nanoarray on carbon cloth (Ni-Bi/CC) derived from NiO/CC shows high activity and durability for water oxidation electrocatalysis in a borate electrolyte (pH 9.2), offering a current density of 10 mA cm⁻² at an overpotential of 470 mV.

Zr-Doped Mesoporous Ta3N5 Microspheres for Efficient Photocatalytic Water Oxidation

Tantalum nitride (Ta₃N₅) has been considered as a promising candidate for photocatalytic water splitting because of its strong visible-light absorbance as far as 600 nm. However, its catalytic activity is often hampered by various intrinsic/extrinsic defects. Here, we prepared a series of Zr-doped mesoporous tantalum nitride (Ta₃N₅) via a template-free method and carried out a detailed investigation of the role of Zr doping upon the photocatalytic performance. Various physicochemical properties including crystal structure, optical absorption, and so on were systematically explored. Our results show that doping Zr into Ta₃N₅ induces an enhancement of oxygen content and a suppression of absorption band around 720 nm, indicating an increase of O_N^• defects and a decrease of V_N^••• defects in the structure. Introduction of Zr significantly boosts the photocatalytic oxygen production of Ta₃N₅. The optimized photocatalytic oxygen production rate approaches 105 μmol h^-1 under visible light illumination (λ ≥ 420 nm), corresponding to an apparent quantum efficiency as high as 3.2%. Photoelectrochemical analysis and DFT calculation reveal that the superior photocatalytic activity of Zr-doped Ta₃N₅ originates from a high level of O_N^• defects' concentration, which contributes to a high electron mobility, and a low level of V_N^••• defects' concentration, which often act as charge recombination centers.

Earth-Abundant Heterogeneous Water Oxidation Catalysts

Water oxidation is a key chemical transformation for the conversion of solar energy into chemical fuels. Our review focuses on recent work on robust earth-abundant heterogeneous catalysts for the oxygen-evolving reaction (OER). We point out that improvements in the performance of OER catalysts will depend critically on the success of work aimed at understanding reaction barriers based on atomic-level mechanisms. We highlight the challenge of obtaining acid-stable OER catalysts, with proposals for elements that could be employed to reach this goal. We suggest that future advances in solar fuels science will be accelerated by the development of new methods for materials synthesis and characterization, along with in-depth investigations of redox mechanisms at catalytic surfaces.

Mononuclear ruthenium polypyridine complexes that catalyze water oxidation

Over the past decade, significant advances have been made in the development of molecular water oxidation catalysts (WOCs) in the context of developing a system that would accomplish artificial photosynthesis. Mononuclear ruthenium complexes with polypyridine ligands have drawn considerable attention in this regard, due to their high catalytic activity and relatively simple structure. In this perspective review, we will discuss mononuclear Ru polypyridine WOCs by organizing them into four groups according to their ligand environments. Each group will be discussed with regard to three fundamental questions: first, how does the catalyst initiate O-O bond formation? Second, which step in the catalytic cycle is rate-determining? Third, how efficient is the catalyst according to the specific descriptors such as turnover frequency? All discussion is based on the high-valent ruthenium intermediates that are proposed in the catalytic cycle according to experimental observation and theoretical simulation. Two fundamental mechanisms are set forth. An acid-base mechanism that involves the attack of a water molecule on the oxo of a high valent Ru 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O species to form the O-O bond. Subsequent steps lead to dissociation of O₂ and rehydration of the metal center. A second mechanism involves the formation of a Ru-O˙ radical species, two of which then couple to form a Ru-O-O-Ru species that can release O₂ afterwards. The acid-base mechanism appears to be more common and mechanistic differences could result from variation directly related to polypyridine ligand structures. Understanding how electronic, steric, and conformational properties can effect catalyst performance will lead to the rational design of more effective WOCs with not only ruthenium but also other transition metals.

Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn(III)(TBP8Cz)] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn(III)(TBP8Cz) to give Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] with protonation constants of 9.0 × 10(6) and 4.7 × 10(3) M(-1), respectively. The protonated sites of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn(III)(OTf)(TBP8Cz(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O2 to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] in the presence of O2 revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn(III)(OTf)(TBP8Cz(H)) reacted with O2 with a diffusion-limited rate constant to produce the putative Mn(IV)(O2(•-))(OTf)(TBP8Cz(H)), whereas the tripseptet excited state of [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] exhibited no reactivity toward O2. In the presence of HOTf, Mn(V)(O)(TBP8Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn(III)(OTf)(TBP8Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d12, suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn(III)(OTf)(TBP8Cz(H)), acts as an efficient photocatalyst for the oxidation of HMB by O2 to the alcohol.

Porous LaCo1-xNixO3-δ Nanostructures as an Efficient Electrocatalyst for Water Oxidation and for a Zinc-Air Battery

Perovskites have emerged as promising earth-abundant alternatives to precious metals for catalyzing the oxygen evolution reaction (OER). Herein, we report the synthesis of a series of porous perovskite nanostructures, LaCo0.97O3-δ, with systematic Ni substitution in Co octahedral sites. Their electrocatalytic activity during the water oxidation reaction was studied in alkaline electrolytes. The electrocatalytic OER activity and stability of the perovskite nanostructure was evaluated using the rotating disk electrode technique. We show that the progressive replacement of Co by Ni in the LaCo0.97O3-δ perovskite structure greatly altered the electrocatalytic activity and that the La(Co0.71Ni0.25)0.96O3-δ composition exhibited the lowest OER overpotential of 324 and 265 mV at 10 mA cm(-2) in 0.1 M KOH and 1 M KOH, respectively. This value was much lower than that of the noble metal catalysts, IrO2, Ru/C, and Pt/C. Furthermore, the La(Co0.71Ni0.25)0.96O3-δ nanostructure showed outstanding electrode stability, with no observable decrease in performance up to 114th cycle in the auxiliary linear sweep voltammetry that lasted for 10 h in chronoamperometry studies. The excellent oxygen evolution activity of the La(Co0.71Ni0.25)0.96O3-δ perovskite nanostructure can be attributed to its intrinsic structure, interconnected particle arrangement, and unique redox characteristics. The enhanced intrinsic electrocatalytic activity of the La(Co0.71Ni0.25)0.96O3-δ catalyst was correlated with several parameters, such as the electrochemical surface area, the roughness factor, and the turnover frequency, with respect to variation in the transition metals of the perovskite structure. Subsequently, La(Co0.71Ni0.25)0.96O3-δ was utilized as the air cathode in a zinc-air battery application.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Co3(OH)2(HPO4)2 as a novel photocatalyst for O2 evolution under visible-light irradiation

Co₃(OH)₂(HPO₄)₂ was proved to be a novel visible-light-driven photocatalyst for O₂ evolution due to the unique characteristics of Co²⁺ octahedra.

Redox flexibility of iron complexes supported by sulfur-based tris(o-methylenethiophenolato)amine relative to its tripodal oxygen-based congener

Tripodal ligands designed to generate a local C₃ symmetry have resulted in novel types of metal complexes that feature unusual bonding and electronic properties.

Capsoplexes: encapsulating complexes via guest recognition

A new dinuclear Ni–NHC complex is able to selectively recognise and self-assemble with guests via tennis-ball like encapsulation, exemplarily demonstrated employing halides.

Synthesis and characterisation of a mesocyclic tripodal triamine ligand

The preparation and characterisation of mesocyclic polydentate amine ligands is described. The novel tridentate ligands will be employed in the stabilisation of highly reactive high-valent oxidants.

Characteristics and reactivity of ruthenium–oxo complexes

In this perspective, we have surveyed the synthetic procedure, characteristics, and reactivity of high-valent ruthenium–oxo complexes.

Structure Sensitivity of the Oxygen Evolution Reaction Catalyzed by Cobalt(II,III) Oxide

Quantum chemical calculations and simulated kinetics were used to examine the structure sensitivity of the oxygen evolution reaction on several surface terminations of Co3O4. Active sites consisting of two adjacent Co(IV) cations connected by bridging oxos were identified on both the (001) and (311) surfaces. Formation of the O-O bond proceeds on these sites by nucleophilic attack of water on a bridging oxo. It was found that the relative turnover frequencies for the different sites are highly dependent on the overpotential, with the dual-Co site on the (311) surface being most active at medium overpotentials (0.46-0.77 V), where O-O bond formation by water addition is rate limiting. A similar dual-Co site on the (001) surface is most active at low overpotentials (<0.46 V), where O2 release is rate limiting, and a single-Co site on the (110) surface is most active at overpotentials that are high enough (>0.77 V) to form a significant concentration of highly reactive terminal Co(V)═O species. Two overpotential-dependent Sabatier relationships were identified based on the Brønsted basicity and redox potential of the active site, explaining the change in the active site with overpotential. The (311) dual-Co site that is most active in the medium overpotential range is consistent with recent experimental observations suggesting that a defect site is responsible for the observed oxygen evolution activity and that a modest concentration of superoxo intermediates is present on the surface. Importantly, we find that it is essential to consider the kinetics of the water addition and O2 release steps rather than only the thermodynamics.

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

Artificial photosynthesis (AP) promises to replace society's dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth's crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide's activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co(III)4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be oxidized to the [Co(IV)Co(III)3] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)4] cubane is regenerated with stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires disproportionation of the [Co(IV)Co(III)3] state to generate an even higher oxidation state, formally [Co(V)Co(III)3] or [Co(IV)2Co(III)2]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems. Although extensive studies have focused on the metal-oxide catalysts, the effect of metal coordination on the catalytic ability remains still elusive. Here we select four cobalt-based phosphate catalysts with various cobalt- and phosphate-group coordination as a platform to better understand the catalytic activity of cobalt-based materials. Although they exhibit various catalytic activities and stabilities during water oxidation, Na2CoP2O7 with distorted cobalt tetrahedral geometry shows high activity comparable to that of amorphous cobalt phosphate under neutral conditions, along with high structural stability. First-principles calculations suggest that the surface reorganization by the pyrophosphate ligand induces a highly distorted tetrahedral geometry, where water molecules can favourably bind, resulting in a low overpotential (∼0.42 eV). Our findings emphasize the importance of local cobalt coordination in the catalysis and suggest the possible effect of polyanions on the water oxidation chemistry.

Oxygen Evolution Catalyzed by Nickel-Iron Oxide Nanocrystals with a Nonequilibrium Phase

Mixed nickel-iron oxides are of great interest as electrocatalysts for the oxygen evolution reaction (OER), the kinetically challenging half-reaction required for the generation of hydrogen gas from water via electrolysis. Previously, we had reported the synthesis of single crystal, soluble nickel-iron oxide nanoparticles over a wide range of nickel:iron compositions, with a metastable cubic rock salt phase ([Ni,Fe]O) that can be isolated despite the low solubility of iron in cubic nickel oxide at ambient temperatures. Here, activity for OER was examined, catalyzed by these [Ni,Fe]O nanoparticles integrated with indium tin oxide (ITO) electrodes. Because the as-prepared [Ni,Fe]O nanoparticles are oleate-capped, the surface ligands needed to be removed to induce adherence to the ITO substrate, and to enable charge transfer and contact with water to enable OER catalysis. Two different approaches were taken to reduce or eliminate the coverage of oleate ligands in these films: UV irradiation (254 nm) and air plasma. UV irradiation proved to lead to better results in terms of stable and OER-active films at pH 13. Kinetic analysis revealed that the Tafel slopes of these nanoparticle [Ni,Fe]O OER electrodes were limited by the electrochemical surface area and were found to be within the range of 30 to 50 mV/decade. Across the four compositions of Ni:Fe studied, from 24:76 to 88:12, the observed overpotential at 10 mA/cm2 for the OER in basic conditions decreased from 0.47 to 0.30 V as the proportion of nickel increased from 24% to 88%.

Bifunctional Perovskite Oxide Catalysts for Oxygen Reduction and Evolution in Alkaline Media

Oxygen electrocatalysis, namely of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), governs the performance of numerous electrochemical energy systems such as reversible fuel cells, metal-air batteries, and water electrolyzers. However, the sluggish kinetics of these two reactions and their dependency on expensive noble metal catalysts (e.g, Pt or Ir) prohibit the sustainable commercialization of these highly innovative and in-demand technologies. Bifunctional perovskite oxides have emerged as a new class of highly efficient non-precious metal catalysts (NPMC) for oxygen electrocatalysis in alkaline media. In this review, we discuss the state-of-the-art understanding of bifunctional properties of perovskites with regards to their OER/ORR activity in alkaline media and review the associated reaction mechanisms on the oxides surface and the related activity descriptors developed in the recent literature. We also summarize the present strategies to modify their electronic structure and to further improve their performance for the ORR/OER through highlighting the new concepts relating to the role of surface redox chemistry and oxygen deficiency of perovskite oxides for the ORR/OER activity. In addition, we provide a brief account of recently developed advanced perovskite-nanocarbon hybrid bifunctional catalysts with much improved performances.

How a [Co(IV) a bond and a half O](2+) fragment oxidizes water: involvement of a biradicaloid [Co(II)-(⋅O⋅)](2+) species in forming the O-O bond

The mechanism of water oxidation performed by a recently discovered cobalt complex [Co(Py5)(OH2)](ClO4)2 (1; Py5=2,6-(bis(bis-2-pyridyl)-methoxymethane)pyridine) was examined using quantum chemical models based on density functional theory. The computer models were first benchmarked against the experimental cyclic voltammetry data to identify the catalytically competent resting state of the catalyst, which was thought to contain a Co(IV) -oxyl complex. The electronic structure calculations suggest that the low-spin doublet state is energetically most favorable, but the catalytically most active species is the intermediate-spin quartet complex that is almost isoenergetic with the doublet state. The electronic structure of the quartet state shows significant spin polarization on the terminal oxygen atom, which is consistent with an intramolecular electron transfer from the oxygen to the metal. Based on the calculated spin densities, the formally [Co(IV) a bond and a half O] can be viewed as a biradicaloid [Co(II)-(⋅O⋅)](2+), that is, a cobalt-oxene moiety. This electronic structure is reminiscent of many other systems where similar electronic patterns were proposed to be responsible for the oxidative reactivity. In this context, this first-row transition-metal system constitutes a logical extension, because the oxyl-radical character is maximized by using the more easily accessible high-spin configurations in which two half-filled Co-dπ orbitals can work in concert to maximize the oxyl-radical character to ultimately afford a new reactive intermediate that can be characterized as carrying a biradicaloid oxene moiety with a formal oxidation state of zero. This conceptual proposal for the catalytically active species provides a plausible rationale for the remarkable oxidative reactivity.

Rapid water oxidation electrocatalysis by a ruthenium complex of the tripodal ligand tris(2-pyridyl)phosphine oxide

The tris(2-pyridyl)phosphine oxide (Py₃PO) complex [Ru(Py₃PO)(bpy)(OH₂)]²⁺ (bpy is 2,2'-bipyridine) is a pH-dependent water oxidation electrocatalyst that accelerates dramatically with increasing pH-up to 780 s^-1 at pH 10 (∼1 V overpotential). Despite retaining the pentakis(pyridine) ligand arrangement common to previously reported catalysts, the tripodal Py₃PO ligand framework supports much faster electrocatalysis. The early stages of the catalytic cycle are proposed to follow the typical pattern of single-site ruthenium catalysts, with two sequential 1H⁺/1e^- proton-coupled electron transfer (PCET) oxidations, but the pH-dependent onset of catalysis and rapid rates are distinguishing features of the present system.

The kinetics and mechanism of photo-assisted Ag(i)-catalysed water oxidation with S2O82−

It is reported that visible light (λ ≥ 400 nm) can improve Ag(i)-catalyzed water oxidation into O₂ with S₂O₈²⁻ remarkably. The photo-assistant results from the absorbance of the AgO⁺ species at 375 nm, promoting the rate-determining step (AgO⁺ + H₂O → Ag⁺ + H₂O₂). A reasonable mechanism has been proposed.

Copper–silver oxide nanowires grown on an alloy electrode as an efficient electrocatalyst for water oxidation

An efficient oxygen-evolving electrode is facilely fabricated based on CuO–Ag₂O nanowires grown in situ on an alloy substrate.