{Co4O4} and {CoxNi4–xO4} Cubane Water Oxidation Catalysts as Surface Cut-Outs of Cobalt Oxides

Caught in the Act of Substitution: Interadsorbate Effects on an Atomically Precise Fe/Co/Se Nanocluster

Directing groups guide substitution patterns in organic synthetic schemes, but little is known about pathways to control reactivity patterns, such as regioselectivity, in complex inorganic systems such as bioinorganic cofactors or extended surfaces. Interadsorbate effects are known to encode surface reactivity patterns in inorganic materials, modulating the location and binding strength of ligands. However, owing to limited experimental resolution into complex inorganic structures, there is little opportunity to resolve these effects on the atomic scale. Here, we utilize an atomically precise Fe/Co/Se nanocluster platform, [Fe3(L)2Co6Se8L'6]+ ([1(L)2]+; L = CN t Bu, THF; L' = Ph2PN(-)Tol), in which allosteric interadsorbate effects give rise to pronounced site-differentiation. Using a combination of spectroscopic techniques and single-crystal X-ray diffractometry, we discover that coordination of THF at the ligand-free Fe site in [1(CN t Bu)2]+ sets off a domino effect wherein allosteric through-cluster interactions promote the regioselective dissociation of CN t Bu at a neighboring Fe site. Computational analysis reveals that this active site correlation is a result of delocalized Fe···Se···Co···Se covalent interactions that intertwine edge sites on the same cluster face. This study provides an unprecedented atom-scale glimpse into how interfacial metal-support interactions mediate a collective and regiospecific path for substrate exchange across multiple active sites.

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

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

Heptanuclear Mixed-Valence Co4IIICo3II Molecular Wheel─A Molecular Analogue of Layered Double Hydroxides with Single-Molecule Magnet Behavior and Electrocatalytic Activity for Hydrogen Evolution Reactions

We present a bifunctional heptanuclear cobalt(II)/cobalt(III) molecular complex formulated as [Co7(μ3-OH)4(H2L1)2(HL2)2](NO3)6·6H2O (1) (where H5L1 is 2,2'-(((1E,1'E)-((2-hydroxy-5-methyl-1,3-phenylene)bis(methanylylidene))bis(azanylylidene))bis(propane-1,3-diol)) and H2L2 is 2-amino-1,3-propanediol). Compound 1 has been characterized by single-crystal X-ray diffraction analysis along with other spectral and magnetic measurements. Structural analysis indicates that 1 contains a mixed-valence Co7 cluster where a central Co(II) ion is connected to six different Co centers (four CoIII and two CoII ions) by four μ3-OH groups, giving rise to a planar heptanuclear cluster that resembles a molecular fragment of a layered double hydroxide (LDH). Two triply deprotonated (H2L1)3- ligands form the outer side of the cluster while two singly deprotonated (HL2)- ligands are located at the top and bottom of the central heptanuclear core. Variable temperature magnetic measurements indicate the presence of weak ferromagnetic CoII···CoII interactions (J = 3.53(6) cm-1) within the linear trinuclear CoII cluster. AC susceptibility measurements show that 1 is a field-induced single-molecule magnet (SMM) with τ0 = 8.2(7) × 10-7 s and Ueff = 11.3(4) K. The electrocatalytic hydrogen evolution reaction (HER) activity of 1 in homogeneous phase shows an overpotential of 455 mV, with a Faradaic efficiency of 81% and a TOF of 8.97 × 104 μmol H2 h-1 mol-1.

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.

Atomically Dispersed Pt-Doped Co3 O4 Spinel Nanoparticles Embedded in Polyhedron Frames for Robust Propane Oxidation at Low Temperature

This study adopts a facile and effective in situ encapsulation-oxidation strategy for constructing a coupling catalyst composed of atomically dispersed Pt-doped Co₃ O₄ spinel nanoparticles (NPs) embedded in polyhedron frames (PFs) for robust propane total oxidation. Benefiting from the abundant oxygen vacancies and more highly valent active Co³⁺ species caused by the doping of Pt atoms as well as the confinement effect, the optimized 0.2Pt-Co₃ O₄ NPs/PFs catalyst exhibits excellent propane catalytic activity with low T₉₀ (184 °C), superior apparent reaction rate (21.62×10⁸ (mol g_cat ^-1 s^-1 )), low apparent activation energy (E_a = 17.89 kJ mol^-1 ), high turnover frequency ( 811×10⁷ (mol g_cat ^-1 s^-1 )) as well as good stability. In situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculations indicate that the doping of Pt atoms enhances the oxygen activation ability, and decreases the energy barrier required for CH bond breaking, thus improving the deep oxidation process of the intermediate species. This study opens up new ideas for constructing coupling catalysts from atomic scale with low cost to enhance the activation of oxygen molecules and the deep oxidation of linear short chain alkanes at low temperature.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Coordination-induced bond weakening of water at the surface of an oxygen-deficient polyoxovanadate cluster

Hydrogen-atom (H-atom) transfer at the surface of heterogeneous metal oxides has received significant attention owing to its relevance in energy conversion and storage processes. Here, we present the synthesis and characterization of an organofunctionalized polyoxovanadate cluster, (calix)V6O5(OH2)(OMe)8 (calix = 4-tert-butylcalix[4]arene). Through a series of equilibrium studies, we establish the BDFE(O-H)avg of the aquo ligand as 62.4 ± 0.2 kcal mol-1, indicating substantial bond weaking of water upon coordination to the cluster surface. Subsequent kinetic isotope effect studies and Eyring analysis indicate the mechanism by which the hydrogenation of organic substrates occurs proceeds through a concerted proton-electron transfer from the aquo ligand. Atomistic resolution of surface reactivity presents a novel route of hydrogenation reactivity from metal oxide surfaces through H-atom transfer from surface-bound water molecules.

Microenvironment Regulation of {Co4IIO4} Cubane for Syngas Photosynthesis

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

High quantum yield photochemical water oxidation using a water-soluble cobalt phthalocyanine as a homogenous catalyst

We demonstrated high catalytic activity (TON = 670, TOF_max = 2.7 s^-1) of a water-soluble cobalt phthalocyanine complex (CoPcTS, PcTS = phthalocyaninetetrasulfonate) for visible light-driven photochemical water oxidation and investigated its reaction mechanism by electrochemical and spectroscopic measurements.

Multi-active Site Dynamics on a Molecular Cr/Co/Se Cluster Catalyst

This study uncovers the interconnected reactivity of the three catalytically active sites of an atomically precise nanocluster Cr₃(py)₃Co₆Se₈L₆ (1(py)₃, L = Ph₂PNTol^-, Ph = phenyl, Tol = 4-tolyl). Catalytic and stoichiometric studies into tosyl azide activation and carbodiimide formation enabled the isolation and crystallographic characterization of key catalytically competent metal-imido intermediates, including the tris(imido) cluster 1(NTs)₃, the catalytic resting state 1(NTs)₃(CN^tBu)₃, and the site-differentiated mono(imido) cluster 1(NTs)(CN^tBu)₂. In the stoichiometric regime, nitrene transfer proceeds via a stepwise mechanism, with the three active sites engaging sequentially to produce carbodiimide. Moreover, the chemical state of neighboring active sites was found to regulate the affinity for substrates of an individual Cr-imido edge site, as revealed by comparative structural analysis and CN^tBu binding studies.

Serendipitous formation of oxygen-bridged CuII6M (M = MnII, CoII) double cubanes showing electrocatalytic water oxidation

Hydroxido-bridged CuII6M double-cubane clusters (M = Mn^II, Co^II) supported by D-penicillaminedisulfide were unexpectedly formed by treating a D-penicillaminato CuII2PtII2 complex with MBr₂ in water. The clusters displayed heterogeneous electrocatalytic activities for water oxidation dependent on the central M shared by two Cu^II cubane units.

A tetra Co(II/III) complex with an open cubane Co4O4 core and square-pyramidal Co(II) and octahedral Co(III) centres: bifunctional electrocatalytic activity towards water splitting at neutral pH

The reaction of 2,6-diformyl-4-methylphenol, 4-methoxybenzoylhydrazine and Co(OAc)₂·4H₂O in 1 : 2 : 2 mole ratio in methanol under aerobic conditions produced in 61% yield a tetranuclear complex having the molecular formula [Co^IICo^III(μ-OAc)(μ₃-OH)(μ-L)]₂ where OAc^- and L^3- represent acetate and N',N''-(5-methyl-2-oxido-1,3-phenylene)bis(methan-1-yl-1-ylidene)bis(4-methoxybenzoylhydrazonate), respectively. The elemental analysis and the mass spectrometric data confirmed the molecular formula of the complex. It is electrically non-conducting and paramagnetic. The complex crystallized as acetonitrile solvate. The X-ray structure shows that each Co(II) centre has a distorted square-pyramidal NO₄ coordination sphere, while each Co(III) centre is in a distorted octahedral NO₅ environment. The four metal atoms and the four bridging O-atoms form an open cubane type Co₄O₄ motif. In the crystal lattice, self-assembly of the solvated complex via intermolecular O-H⋯O interaction leads to a two-dimensional network structure. The infrared and electronic spectroscopic features of the complex are consistent with its molecular structure. Cryomagnetic measurements together with theoretical calculations suggest the presence of easy-axis anisotropy for the square-pyramidal Co(II) centres. The complex is redox-active and displays metal centred oxidation and reduction responses on the anodic and cathodic sides, respectively, of the Ag/AgCl electrode. Bifunctional heterogeneous electrocatalytic activity of the complex towards O₂ and H₂ evolution reactions (OER and HER) in neutral aqueous medium has been explored in detail.

Hemicubane topological analogs of the oxygen-evolving complex of photosystem II mediating water-assisted propylene carbonate oxidation

A series of Ca-Mn clusters with the ligand 2-pyridinemethoxide (Py-CH₂O) have been prepared with varying degrees of topological similarity to the biological oxygen-evolving complex. These clusters activate water as a substrate in the oxidative degradation of propylene carbonate, with activity correlated with topological similarity to the OEC, lowering the onset potential of the oxidation by as much as 700 mV.

Electrochemical water oxidation catalyzed by a mononuclear cobalt complex of a pentadentate ligand: the critical effect of the borate anion

A cobalt complex is found as a homogeneous water oxidation electrocatalyst. Electrochemical examinations indicate that the implementation of proton-couple electron transfer process and formation of O–O bond are assisted by borate anion.

Evaluation of Manganese Cubanoid Clusters for Water Oxidation Catalysis: From Well-Defined Molecular Coordination Complexes to Catalytically Active Amorphous Films

With a view to developing multimetallic molecular catalysts that mimic the oxygen-evolving catalyst (OEC) in Nature's photosystem II, the synthesis of various dicubanoid manganese clusters is described and their catalytic activity investigated for water oxidation in basic, aqueous solution. Pyridinemethanol-based ligands are known to support polynuclear and cubanoid structures in manganese coordination chemistry. The chelators 2,6-pyridinedimethanol (H₂ L¹ ) and 6-methyl-2-pyridinemethanol (HL² ) were chosen to yield polynuclear manganese complexes; namely, the tetranuclear defective dicubanes [Mn^II ₂ Mn^III ₂ (HL¹ )₄ (OAc)₄ (OMe)₂ ] and [Mn^II ₂ Mn^III ₂ (HL¹ )₆ (OAc)₂ ] (OAc)₂ ⋅2 H₂ O, as well as the octanuclear-dicubanoid [Mn^II ₆ Mn^III ₂ (L² )₄ (O)₂ (OAc)₁₀ (HOMe/OH₂ )₂ ]⋅3MeOH⋅MeCN. In freshly prepared solutions, polynuclear species were detected by electrospray ionization mass spectrometry, whereas X-band electron paramagnetic resonance studies in dilute, liquid solution suggested the presence of divalent mononuclear Mn species with g values of 2. However, the magnetochemical investigation of the complexes' solutions by the Evans technique confirmed a haphazard combination of manganese coordination complexes, from mononuclear to polynuclear species. Subsequently, the newly synthesized and characterized manganese molecular complexes were employed as precursors to prepare electrode-deposited films in a buffer-free solution to evaluate and compare their stability and catalytic activity for water oxidation electrocatalysis.

Structural Modulation on NiCo2 S4 Nanoarray by N Doping to Enhance 2e-ORR Selectivity for Photothermal AOPs and Zn-O2 Batteries*

As a H₂ O₂ generator, a 2e^- oxygen reduction reaction active electrocatalyst plays an important role in the advanced oxidation process to degrade organic pollutants in sewage. To enhance the tendency of NiCo₂ S₄ towards the 2e^- reduction reaction, N atoms are doped in its structure and replace S^2- . The result implies that this weakens the interaction between NiCo₂ S₄ and OOH*, suppresses O-O bond breaking and enhances H₂ O₂ selectivity. This electrocatalyst also shows photothermal effect. Under photothermal heating, H₂ O₂ produced by the oxidation reduction reaction can decompose and releaseOH, which degrades organic pollutants through the advanced oxidation process. Photothermal effect induced by the advance oxidation process shows obvious advantages over the traditional Fenton reaction, such as wide pH adaptation scope and low secondary pollutant due to its Fe²⁺ free character. With Zn as anode and the electrocatalyst as cathode material, a Zn-O₂ battery is assembled. It achieves electricity generation and photothermal effect induced by the advance oxidation process simultaneously.

Unraveling Nanoscale Cobalt Oxide Catalysts for the Oxygen Evolution Reaction: Maximum Performance, Minimum Effort

The oxygen evolution reaction (OER) is a key bottleneck step of artificial photosynthesis and an essential topic in renewable energy research. Therefore, stable, efficient, and economical water oxidation catalysts (WOCs) are in high demand and cobalt-based nanomaterials are promising targets. Herein, we tackle two key open questions after decades of research into cobalt-assisted visible-light-driven water oxidation: What makes simple cobalt-based precipitates so highly active-and to what extent do we need Co-WOC design? Hence, we started from Co(NO₃)₂ to generate a precursor precipitate, which transforms into a highly active WOC during the photocatalytic process with a [Ru(bpy)₃]²⁺/S₂O₈^2-/borate buffer standard assay that outperforms state of the art cobalt catalysts. The structural transformations of these nanosized Co catalysts were monitored with a wide range of characterization techniques. The results reveal that the precipitated catalyst does not fully change into an amorphous CoO_x material but develops some crystalline features. The transition from the precipitate into a disordered Co₃O₄ material proceeds within ca. 1 min, followed by further transformation into highly active disordered CoOOH within the first 10 min. Furthermore, under noncatalytic conditions, the precursor directly transforms into CoOOH. Moreover, fast precipitation and isolation afford a highly active precatalyst with an exceptional O₂ yield of 91% for water oxidation with the visible-light-driven [Ru(bpy)₃]²⁺/S₂O₈^2- assay, which outperforms a wide range of carefully designed Co-containing WOCs. We thus demonstrate that high-performance cobalt-based OER catalysts indeed emerge effortlessly from a self-optimization process favoring the formation of Co(III) centers in all-octahedral environments. This paves the way to new low-maintenance flow chemistry OER processes.

Self-assembled nickel cubanes as oxygen evolution catalysts

Ni₄O₄ cubanes [(μ₃-L¹O)NiCl(MeOH)]₄ (1) and [(μ₃-L²O)NiCl(H₂O)]₄ (2) (L¹OH = 1-H-2-benzimidazolylmethanol, L²OH = 1-methyl-2-benzimidazolylmethanol) self-assemble from commercially available 1-H- and 1-methyl-2-benzimidazolylmethanol and NiCl₂·6H₂O in high yields under mild conditions. Both complexes were characterised spectroscopically and by X-ray crystallography. The cubanes oxidise water electrocatalytically to dioxygen at neutral pH in aqueous potassium phosphate buffer solutions.

Cobalt-Based Metal-Organic Cages for Visible-Light-Driven Water Oxidation

Water oxidation to molecular oxygen is indispensable but a challenge for splitting H₂O. In this work, a series of Co-based metal-organic cages (MOCs) for photoinduced water oxidation were prepared. MOC-1 with both bis(μ-oxo) bridged dicobalt and Co-O (O from H₂O) displays catalytic activity with an initial oxygen evolution rate of 80.4 mmol/g/h and a TOF of 7.49 × 10^-3 s^-1 in 10 min. In contrast, MOC-2 containing only Co-O (O from H₂O) in the structure results in a lower oxygen evolution rate (40.8 mmol/g/h, 4.78 × 10^-3 s^-1), while the amount of oxygen evolved from the solution of MOC-4 without both active sites is undetectable. Isotope experiments with or without H₂¹⁸O as the reactant successfully demonstrate that the molecular oxygen was produced from water oxidation. Photophysical and electrochemical studies reveal that photoinduced water oxidation initializes via electron transfer from the excited [Ru(bpy)₃]²⁺* to Na₂S₂O₈, and then, the cobalt active sites further donate electrons to the oxidized [Ru(bpy)₃]³⁺ to drive water oxidation. This proof-of-concept study indicates that MOCs can work as potential efficient catalysts for photoinduced water oxidation.

Do multinuclear 3d metal catalysts achieve O-O bond formation via radical coupling or via water nucleophilic attack? WNA leads the way in [Co4O4]n

Catalytic water oxidation is a required process for clean energy production based on the concept of artificial photosynthesis. Here, we provide in situ spectroscopic and computational analysis for the closest known photosystem II analog, [Co₄O₄]ⁿ⁺ ([Co₄O₄Py₄Ac₄]⁰, Py = pyridine and Ac = CH₃COO^-), which catalyzes electrochemical water oxidation. In situ extended X-ray absorption fine structure detects an ultrashort, Co^IV=O (~1.67 Å) moiety, a crucial intermediate for O-O bond formation. Density function theory analyses show that the intermediate has two Co^IV centers and a Co^IV=O unit of strong radicaloid character sufficient to support a Co^IV=O + H₂O = Co-OOH + H⁺ transition, where the carboxyl ligand accepts the proton and the bridging oxygen stabilizes the peroxide via hydrogen bonding. The proposed water nucleophilic attack mechanism accounts for all prior spectroscopic evidence on the Co₄O₄⁴⁺ core. Our results are important for the design and development of efficient water oxidation catalysts, which contribute to the ultimate goal of clean energy from artificial photosynthesis.

Ligand Controls the Activity of Light-Driven Water Oxidation Catalyzed by Nickel(II) Porphyrin Complexes in Neutral Homogeneous Aqueous Solutions

Finding photostable, first‐row transition metal‐based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)₃]²⁺ as photosensitizer and [S₂O₈]²⁻ as sacrificial electron acceptor. Electron‐poorer Ni‐porphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron‐poorer porphyrins afforded 6‐fold higher turnover frequencies (TOFs; ca. 0.65 min⁻¹) than electron‐richer analogues. However, the electron‐poorest Ni‐porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O₂ evolution starts was too high (+1.23 V vs. NHE) to be driven by the photochemically generated [Ru(bpy)₃]³⁺. Critically, these Ni‐porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1–3 % less O₂ than during the first photocatalytic run.

Soluble lanthanide-transition-metal clusters Ln36Co12 as effective molecular electrocatalysts for water oxidation

Here we report for the first time soluble lanthanide-transition-metal clusters Ln₃₆Co₁₂ (Ln = Eu, Gd and Dy) as effective homogeneous water oxidation electrocatalysts. The stable 48-metal Ln₃₆Co₁₂ clusters show an effective water oxidation activity under acidic conditions because of the synergistic effect between lanthanide and transition metals in O-O bond formation.

Co/Ni-polyoxotungstate photocatalysts as precursor materials for electrocatalytic water oxidation

An open-core cobalt polyoxometalate (POM) [(A-α-SiW9O34)Co4(OH)3(CH3COO)3]8-Co(1) and its isostructural Co/Ni-analogue [(A-α-SiW9O34)Co1.5Ni2.5(OH)3(CH3COO)3]8-CoNi(2) were synthesized and investigated for their photocatalytic and electrocatalytic performance. Co(1) shows high photocatalytic O2 yields, which are competitive with leading POM water oxidation catalysts (WOCs). Furthermore, Co(1) and CoNi(2) were employed as well-defined precursors for heterogeneous WOCs. Annealing at various temperatures afforded amorphous and crystalline CoWO4- and Co1.5Ni2.5WO4-related nanoparticles. CoWO4-related particles formed at 300 °C showed substantial electrocatalytic improvements and were superior to reference materials obtained from co-precipitation/annealing routes. Interestingly, no synergistic interactions between cobalt and nickel centers were observed for the mixed-metal POM precursor and the resulting tungstate catalysts. This stands in sharp contrast to a wide range of studies on various heterogeneous catalyst types which were notably improved through Co/Ni substitution. The results clearly demonstrate that readily accessible POMs are promising precursors for the convenient and low-temperature synthesis of amorphous heterogeneous water oxidation catalysts with enhanced performance compared to conventional approaches. This paves the way to tailoring polyoxometalates as molecular precursors with tuneable transition metal cores for high performance heterogeneous electrocatalysts. Our results furthermore illustrate the key influence of the synthetic history on the performance of oxide catalysts and highlight the dependence of synergistic metal interactions on the structural environment.

Photochemical Water Oxidation Using a Doubly N-Confused Hexaphyrin Dinuclear Cobalt Complex

A doubly N-confused hexaphyrin dinuclear cobalt complex (Co₂DNCH) is revealed as an efficient water oxidation catalyst, outperforming the mononuclear cobalt porphyrin with the same aryl group as those in Co₂DNCH. By photoirradiation of a water/acetone-d₆ (9:1) mixture containing Co₂DNCH, [Ru^II(bpy)₃]²⁺, and S₂O₈^2- as the water oxidation catalyst, photosensitizer, and sacrificial electron acceptor, respectively, with visible light, O₂ was obtained as the maximum with turnover number = 1200, turnover frequency = 3.9 s^-1, and quantum yield = 0.30.

An Unprecedented Bird Nest Molybdenum(V) Cobalto-Phosphate Nanosized Wheel Constructed from the [H55 (Mo24 O48 )(Co4 O)2 Co16 (PO4 )42 (py)6 (EtOH)2 (H2 O)11 ]3- Anion

An unprecedented bird-nest high-nuclear molybdenum(V) cobalto-phosphate nanosized wheel modified by imidazole (im) and pyridine (py), {[H₅₅ (Mo₂₄ O₄₈ )(Co₄ O)₂ Co₁₆ (PO₄ )₄₂ (py)₆ (EtOH)₂ (H₂ O)₁₁ ]- @[(Him)₂ (Hpy)]}(N-Et-py)(H₂ PO₄ )(py)₇ (EtOH)⋅12 H₂ O (1), has been successfully synthesized by self-assembly. The anionic huge wheel consists of two rare {Co₄ O} squares, four {Co₄ } tetramers, four {Mo₄ } tetramers and four {Mo₂ } dimers, linked by bridging oxygen atoms and [PO₄ ] groups and encloses two imidazolium cations and a protonated pyridium for charge balance. Surprisingly, 1 represents the first twisted wheel-shaped cluster with a record high-nuclear molybdenum(V) cobalto-phosphate. The delocalized electron effects of the cluster are enhanced with the help of coordinated py ligands, which endows 1 with an excellent third-order nonlinear optical (NLO) response. Additionally, 1 also shows a better photocatalytic water oxidation activity than Co(NO₃ )₂ with the O₂ production of 205 μmol during 6 h in the absence of the [Ru(bpy)₃ ]²⁺ photosensitizer.

Pentanuclear clusters resembling the cubane-dangler connectivity in the native oxygen-evolving center of photosystem II

A series of pentametallic "cubane-plus-dangler" complexes have been target synthesized. Among them, the [Fe3Ni2] aggregate strongly resembled the native oxygen-evolving center by mimicking the "cubane-plus-dangler" skeleton, the aqua binding site, and the connectivity between the pendent ion and the parent cubane. Our synthetic strategy that uses tri-substituted methanol as the "cubane-generator" and carboxylate as the pendant ligand provides a feasible approach for accessing model compounds of biological catalyst systems.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Bioinspired metal complexes for energy-related photocatalytic small molecule transformation

Bioinspired transformation of small-molecules to energy-related feedstocks is an attractive research area to overcome both the environmental issues and the depletion of fossil fuels. The highly effective metalloenzymes in nature provide blueprints for the utilization of bioinspired metal complexes for artificial photosynthesis. Through simpler structural and functional mimics, the representative herein is the pivotal development of several critical small molecule conversions catalyzed by metal complexes, e.g., water oxidation, proton and CO₂ reduction and organic chemical transformation of small molecules. Of great achievement is the establishment of bioinspired metal complexes as catalysts with high stability, specific selectivity and satisfactory efficiency to drive the multiple-electron and multiple-proton processes related to small molecule transformation. Also, potential opportunities and challenges for future development in these appealing areas are highlighted.

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir)₂(μ-κ³-O,N,O-H₄-EDTMP)] (Ir-H₄-EDTMP, H₄-EDTMP^4- = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)₂(μ-κ³-O,N,O-EDTA)] (Ir-EDTA, EDTA^4- = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR spectroscopy, and in the solid state, by single crystal X-Ray diffraction. They were supported onto rutile TiO₂ nanocrystals obtaining Ir-H₄-EDTMP@TiO₂ and Ir-EDTA@TiO₂ hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO₄, providing almost quantitative yields, and TON values only limited by the amount of NaIO₄ used. As for the molecular catalysts, Ir-H₄-EDTMP (TOF up to 184 min^-1) exhibited much higher activity than Ir-EDTA (TOF up to 19 min^-1), likely owing to the higher propensity of the former to generate a coordination vacancy through the dissociation of a Ir-OP bond (2.123 Å, significantly longer than Ir-OC, 2.0913 Å), which is a necessary step to activate these saturated complexes. Ir-H₄-EDTMP@TiO₂ (up to 33 min^-1) and Ir-EDTA@TiO₂ (up to 41 min^-1) hybrid materials showed similar activity that was only marginally reduced in the second and third catalytic runs carried out after having separated the supernatant, which did not show any sign of activity, instead. The observed TOF values for hybrid materials are higher than those reported for analogous systems deriving from heterogenized mononuclear complexes. This suggests that supporting dinuclear molecular precursors could be a successful strategy to obtain efficient heterogenized water oxidation catalysts.

Integration of bio-inspired lanthanide-transition metal cluster and P-doped carbon nitride for efficient photocatalytic overall water splitting

Photosynthesis in nature uses the Mn₄CaO₅ cluster as the oxygen-evolving center to catalyze the water oxidation efficiently in photosystem II. Herein, we demonstrate bio-inspired heterometallic LnCo₃ (Ln = Nd, Eu and Ce) clusters, which can be viewed as synthetic analogs of the CaMn₄O₅ cluster. Anchoring LnCo₃ on phosphorus-doped graphitic carbon nitrides (PCN) shows efficient overall water splitting without any sacrificial reagents. The NdCo₃/PCN-c photocatalyst exhibits excellent water splitting activity and a quantum efficiency of 2.0% at 350 nm. Ultrafast transient absorption spectroscopy revealed the transfer of a photoexcited electron and hole into the PCN and LnCo₃ for hydrogen and oxygen evolution reactions, respectively. A density functional theory (DFT) calculation showed the cooperative water activation on lanthanide and O−O bond formation on transition metal for water oxidation. This work not only prepares a synthetic model of a bio-inspired oxygen-evolving center but also provides an effective strategy to realize light-driven overall water splitting.

Grafting Molecular Cobalt-oxo Cubane Catalyst on Polymeric Carbon Nitride for Efficient Photocatalytic Water Oxidation

In this work, we have successfully constructed a cobalt-oxo (Co^III ₄ O₄ ) cubane complex on polymeric carbon nitride (PCN) through pyridine linkage. The covalently grafted Co^III ₄ O₄ cubane units were uniformly distributed on the PCN surface. The product exhibited greatly enhanced photocatalytic activities for water oxidation under visible-light irradiation. Further characterizations and spectroscopic analyses revealed that the grafted Co^III ₄ O₄ cubane units could effectively capture the photogenerated holes from excited PCN, lower the overpotential of oxygen evolution reaction (OER), and serve as efficient catalysts to promote the multi-electron water oxidation process. This work provides new insight into the future development of efficient photocatalysts by grafting molecular catalysts for artificial photosynthesis.

Cubes on a string: a series of linear coordination polymers with cubane-like nodes and dicarboxylate linkers

A series of semicrystalline and amorphous one-dimensional (1D) polymeric chains consisting of cubane-like CoII4L4 units (L = S-1,2-bis(benzimidazol-2-yl)ethanol) and dicarboxylates were synthesized and characterized by single crystal diffraction and X-ray total scattering. The polycationic chains are composed of [Co4L4(dicarboxylate)]2+ monomeric units, while one molecular dicarboxylate counterion is balancing the charge of each monomer. The linear compound series has five members, and the crystal structures were solved for [Co4L4(tph)](tph) and [Co4L4(ndc)](ndc), where tph = terephthalate and ndc = 2,6-naphthalenedicarboxylate. Partly crystalline compounds were produced by slow assembly at elevated temperature (over days), while the amorphous compounds were formed by fast precipitation (within minutes). Pair distribution function (PDF) analysis based on X-ray total scattering data reveals the presence of the cubane-like entity in both the amorphous and semicrystalline samples. While the powders are non-porous, precipitation is a fast and versatile method to produce compounds with cubane-like centres with moderate surface areas of 17-49 m2 g-1 allowing for surface chemical reactions. The powders have a high concentration of Lewis base sites as verified by their selective adsorption of CO2 over N2. The use of an amorphous cubane-like polymer for the electrocatalytic oxygen evolution reaction was demonstrated.

Catalysis of a Single Transition Metal Site for Water Oxidation: From Mononuclear Molecules to Single Atoms

Low-cost, nonprecious transition metal (TM) catalysts toward efficient water oxidation are of critical importance to future sustainable energy technologies. The advances in structure engineering of water oxidation catalysts (WOCs) with single TM centers as active sites, for example, single metallic molecular complexes (SMMCs), supported SMMCs, and single-atom catalysts (SACs) in recent reports are examined. The efforts made on these configurations in terms of design principle, advanced characterization, performances and theoretical studies, are critically reviewed. A clear roadmap with the correlations between the single-TM-site-based structures (coordination and geometric structure, TM species, support), and the catalytic performances in water oxidation is provided. The insights bridging SMMCs with SACs are also given. Finally, the challenges and opportunities in the single-TM-site catalysis are proposed.

Kinetic versus thermodynamic metalation enables synthesis of isostructural homo- and heterometallic trinuclear clusters

Temperature-dependent metalation of a new hexadentate enables the selective synthesis of both mononuclear (i.e. kinetic product) and trinuclear (i.e. thermodynamic product) complexes.

Ir- and Ru-doped layered double hydroxides as affordable heterogeneous catalysts for electrochemical water oxidation

Doping low-cost LDHs with noble metal atoms represents a promising approach to develop effective heterogeneous Water Oxidation Catalysts.

Dynamic Role of Cluster Cocatalysts on Molecular Photoanodes for Water Oxidation

While loading of cocatalysts is one of the most widely investigated strategies to promote the efficiency of photoelectrodes, the understanding of their functionality remains controversial. We established new hybrid molecular photoanodes with cobalt-based molecular cubane cocatalysts on hematite as a model system. Photoelectrochemical and rate law analyses revealed an interesting functionality transition of the {Co(II)₄O₄}-type cocatalysts. Their role changed from predominant hole reservoirs to catalytic centers upon modulation of the applied bias. Kinetic analysis of the photoelectrochemical processes indicated that this observed transition arises from the dynamic equilibria of photogenerated surface charge carriers. Most importantly, we confirmed this functional transition of the cocatalysts and the related kinetic properties for several cobalt-based molecular and heterogeneous catalysts, indicating wide applicability of the derived trends. Additionally, complementary analytical characterizations show that a transformation of the applied molecular species occurs at higher applied bias, pointing to a dynamic interplay connecting molecular and heterogeneous catalysis. Our insights promote the essential understanding of efficient (molecular) cocatalyzed photoelectrode systems to design tailor-made hybrid devices for a wide range of catalytic applications.