Homogeneous Water Oxidation Catalyzed by First-Row Transition Metal Complexes: Unveiling the Relationship between Turnover Frequency and Reaction Overpotential

The utilization of earth-abundant low-toxicity metal ions in the construction of highly active and efficient molecular catalysts promoting the water oxidation reaction is important for developing a sustainable artificial energy cycle. However, the kinetic and thermodynamic properties of the currently available molecular water oxidation catalysts (MWOCs) have not been comprehensively investigated. This Review summarizes the current status of MWOCs based on first-row transition metals in terms of their turnover frequency (TOF, a kinetic property) and overpotential (η, a thermodynamic property) and uses the relationship between log(TOF) and η to assess catalytic performance. Furthermore, the effects of the same ligand classes on these MWOCs are discussed in terms of TOF and η, and vice versa. The collective analysis of these relationships provides a metric for the direct comparison of catalyst systems and identifying factors crucial for catalyst design.

Development of a panchromatic photosensitizer and its application to photocatalytic CO2 reduction

We designed and synthesized a heteroleptic osmium(ii) complex with two different tridentate ligands, Os. Os can absorb the full wavelength range of visible light owing to S–T transitions, and this was supported by TD-DFT calculations. Excitation of Os using visible light of any wavelength generates the same lowest triplet metal-to-ligand charge-transfer excited state, the lifetime of which is relatively long (τ_em = 40 ns). Since excited Os could be reductively quenched by 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole, Os displays high potential as a panchromatic photosensitizer. Using a combination of Os and a ruthenium(ii) catalyst, CO₂ was photocatalytically reduced to HCOOH via irradiation with 725 nm light, and the turnover number reached 81; irradiation with light at λ_ex > 770 nm also photocatalytically induced HCOOH formation. These results clearly indicate that Os can function as a panchromatic redox photosensitizer.

The osmium(ii) complex functioned as a panchromatic photosensitizer and drove CO₂ reduction.

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.

Redox Activity of Noninnocent 2,2'-Bipyridine in Zinc Complexes: An Experimental and Theoretical Study

We report on a systematical reactivity study of β-diketiminate zinc complexes with redox-active 2,2'-bipyridine (bpy). The reaction of LZnI (L = HC[C(Me)N(2,6-iPr₂C₆H₃)]₂) with NaB(C₆F₅)₄ in the presence of bpy yielded [LZn(bpy)][B(C₆F₅)₄] (1), with bpy serving as a neutral ligand, whereas reduction reactions of LZnI with 1 or 2 equiv of KC₈ in the presence of bpy gave the radical complex LZn(bpy) (2) and [2.2.2-Cryptand-K][LZn(bpy)] (3), in which bpy either acts as a π-radical anion or a diamagnetic dianion, respectively. The paramagnetic nature of 2 was confirmed via solution magnetic susceptibility measurements, and UV-vis spectroscopy shows that 2 exhibits absorption bands typical for bpy radical species. The EPR spectra of 2 and its deuterated analog 2-d ₈ demonstrate that the spin density is localized to the bpy ligand. Density functional theoretical calculations and natural bond orbital analysis were employed to elucidate the electronic structure of complexes 1-3 and accurately reproduced the structural experimental data. It is shown that reduction of the bpy moiety results in a decrease in the β-diketiminate co-ligand bite angle and elongation of the Zn-N(β-diketiminate) bonds, which act cooperatively and in synergy with the bpy ligand by decreasing Zn-N(bpy) bond lengths to stabilize the energy of the LUMO.

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.

Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes

This review provides a comprehensive overview on characterisation techniques for light-driven redox-catalysts highlighting spectroscopic, microscopic, electrochemical and spectroelectrochemical approaches.

A highly active and robust iron quinquepyridine complex for photocatalytic CO2 reduction in aqueous acetonitrile solution

[Fe(qnpy)(H₂O)₂]²⁺ is a highly efficient and robust catalyst for visible-light-driven reduction of CO₂ to CO, with a TON for CO of up to 14 095 and selectivity of 98% using Ru(phen)₃Cl₂ as photosensitizer and BIH as sacrificial reductant.

Preparative History vs Driving Force in Water Oxidation Catalysis: Parameter Space Studies of Cobalt Spinels

The development of efficient, stable, and economic water oxidation catalysts (WOCs) is a forefront topic of sustainable energy research. We newly present a comprehensive three-step approach to systematically investigate challenging relationships among preparative history, properties, and performance in heterogeneous WOCs. To this end, we studied (1) the influence of the preparative method on the material properties and (2) their correlation with the performance as (3) a function of the catalytic test method. Spinel-type Co3O4 was selected as a clear-cut model WOC and synthesized via nine different preparative routes. In search of the key material properties for high catalytic performance, these cobalt oxide samples were characterized with a wide range of analytical methods, including X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, Raman spectroscopy, BET surface area analysis, and transmission electron microscopy. Next, the corresponding catalytic water oxidation activities were assessed with the three most widely applied protocols to date, namely, photocatalytic, electrocatalytic, and chemical oxidation. The activity of the Co3O4 samples was found to clearly depend on the applied test method. Increasing surface area and disorder as well as a decrease in oxidation states arising from low synthesis temperatures were identified as key parameters for high chemical oxidation activity. Surprisingly, no obvious property-performance correlations were found for photocatalytic water oxidation. In sharp contrast, all samples showed similar activity in electrochemical water oxidation. The substantial performance differences between the applied protocols demonstrate that control and comprehensive understanding of the preparative history are crucial for establishing reliable structure-performance relationships in WOC design.

Photoinduced hole transfer from tris(bipyridine)ruthenium dye to a high-valent iron-based water oxidation catalyst

An efficient water oxidation system is a prerequisite for developing solar energy conversion devices. Using advanced time-resolved spectroscopy, we study the initial catalytic relevant electron transfer events in the light-driven water oxidation system utilizing [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as a light harvester, persulfate as a sacrificial electron acceptor, and a high-valent iron clathrochelate complex as a catalyst. Upon irradiation by visible light, the excited state of the ruthenium dye is quenched by persulfate to afford a [Ru(bpy)3]3+/SO4˙- pair, showing a cage escape yield up to 75%. This is followed by the subsequent fast hole transfer from [Ru(bpy)3]3+ to the FeIV catalyst to give the long-lived FeV intermediate in aqueous solution. In the presence of excess photosensitizer, this process exhibits pseudo-first order kinetics with respect to the catalyst with a rate constant of 3.2(1) × 1010 s-1. Consequently, efficient hole scavenging activity of the high-valent iron complex is proposed to explain its high catalytic performance for water oxidation.

Ruthenium Picolinate Complex as a Redox Photosensitizer With Wide-Band Absorption

Ruthenium(II) picolinate complex, [Ru(dmb)₂(pic)]⁺ (Ru(pic); dmb = 4,4′-dimethyl-2,2′-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N^∧N)₃]²⁺ (N^∧N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, Ru(pic) was found to display certain advantageous characteristics of wide-band absorption of visible light (λ_abs < 670 nm) and stronger reduction ability in a one-electron reduced state (E1/2red = −1.86 V vs. Ag/AgNO₃), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using Ru(pic) as a redox photosensitizer combined with a Re(I) catalyst reduced CO₂ to CO under red-light irradiation (λ_ex > 600 nm). TON_CO reached 235 and Φ_CO was 8.0%. Under these conditions, [Ru(dmb)₃]²⁺ (Ru(dmb)) is not capable of working as a redox photosensitizer because it does not absorb light at λ > 560 nm. Even in irradiation conditions where both Ru(pic) and Ru(dmb) absorb light (λ_ex > 500 nm), using Ru(pic) demonstrated faster CO formation (TOF_CO = 6.7 min⁻¹) and larger TON_CO (2347) than Ru(dmb) (TOF_CO = 3.6 min⁻¹; TON_CO = 2100). These results indicate that Ru(pic) is a superior redox photosensitizer over a wider wavelength range of visible-light absorption.

Efficient visible light-driven water oxidation catalysed by an iron(iv) clathrochelate complex

A water-stable FeIV clathrochelate complex catalyses fast and homogeneous photochemical oxidation of water to dioxygen with a turnover frequency of 2.27 s-1 and a maximum turnover number of 365. An FeV intermediate generated under catalytic conditions is trapped and characterised using EPR and Mössbauer spectroscopy.

Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation

Stoichiometric electron-transfer (ET) oxidation of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O₂ evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O₂-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O₂ evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. In order to clarify the O₂-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O₂-evolving rate constants, indicating that the O₂-releasing step is the rate-determining step in the O₂ evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O₂ evolution for the two isomers should be derived from the thermodynamic stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.

Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites

The nature of the oxidizing species in water oxidation reactions with chemical oxidants catalyzed by α-[Fe(OTf)₂(mcp)] (1α; mcp = N, N'-dimethyl- N, N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion) and β-[Fe(OTf)₂(mcp)] (1β) has been investigated. Mössbauer spectroscopy provides definitive evidence that 1α and 1β generate oxoiron(IV) species as the resting state. Decomposition paths of the catalysts have been investigated by identifying and quantifying ligand fragments that form upon degradation. This analysis correlates the water oxidation activity of 1α and 1β with stability against oxidative damage of the ligand via aliphatic C-H oxidation. The site of degradation and the relative stability against oxidative degradation are shown to be dependent on the topology of the catalyst. Furthermore, the mechanisms of catalyst degradation have been rationalized by computational analyses, which also explain why the topology of the catalyst enforces different oxidation-sensitive sites. This information has served in creating catalysts where sensitive C-H bonds have been replaced by C-D bonds. The deuterated analogues D₄-α-[Fe(OTf)₂(mcp)] (D₄-1α), D₄-β-[Fe(OTf)₂(mcp)] (D₄-1β), and D₆-β-[Fe(OTf)₂(mcp)] (D₆-1β) were prepared, and their catalytic activity has been studied. D₄-1α proves to be an extraordinarily active and efficient catalyst (up to 91% of O₂ yield); it exhibits initial reaction rates identical with those of its protio analogue, but it is substantially more robust toward oxidative degradation and yields more than 3400 TON ( n(O₂)/ n(Fe)). Altogether this evidences that the water oxidation catalytic activity is performed by a well-defined coordination complex and not by iron oxides formed after oxidative degradation of the ligands.

Mimicry and functions of photosynthetic reaction centers

The structure and function of photosynthetic reaction centers (PRCs) have been modeled by designing and synthesizing electron donor–acceptor ensembles including electron mediators, which can mimic multi-step photoinduced charge separation occurring in PRCs to obtain long-lived charge-separated states. PRCs in photosystem I (PSI) or/and photosystem II (PSII) have been utilized as components of solar cells to convert solar energy to electric energy. Biohybrid photoelectrochemical cells composed of PSII have also been developed for solar-driven water splitting into H2 and O2. Such a strategy to bridge natural photosynthesis with artificial photosynthesis is discussed in this minireview.

2,2'-Bipyridin-1'-ium 1-oxide bromide monohydrate

The title compound 2,2'-bipyridin-1'-ium 1-oxide bromide crystallizes as a monohydrate, C10H9N2+·Br-·H2O. Structural disorder is observed due to the fact that protonation, as well as oxidation, of the N atoms of 2,2'-bi-pyridine occurs at either of the N atoms. The disorder extends to the remainder of the cation, with a refined occupancy rate of 0.717 (4) for the major moiety. An intra-molecular N-H⋯O hydrogen bond forces the bi-pyridine unit into an s-cis conformation. Each pair of neighbouring 2,2'-bipyridin-1'-ium ions forms a dimeric aggregate by hydrogen bonds between their respective N-O and the N-H functions. These dimers and hydrogen-bonding inter-actions with bromide ions and the water mol-ecule give rise to a complex supra-molecular arrangement.

An octanuclear Cu(ii) cluster with a bio-inspired Cu4O4 cubic fragment for efficient photocatalytic water oxidation

An octanuclear Cu(ii) cluster [Cu₈(dpk·OH)₈(OAc)₄](ClO₄)₄ was found to be an efficient homogeneous catalyst for photocatalytic water oxidation with an oxygen yield, TON and TOF of 35.6%, 178 and 3.6 s⁻¹, respectively.

Catalytic Conversion of Dinitrogen into Ammonia under Ambient Reaction Conditions by Using Proton Source from Water

Molybdenum-catalyzed conversion of molecular dinitrogen into ammonia under ambient reaction conditions has been achieved by using a proton source generated in situ from the ruthenium-catalyzed oxidation of water in combination with visible light and a photosensitizer. The preset reaction system is considered as a new model for the nitrogen fixation by photosynthetic bacteria.

Photocatalytic water oxidation by persulphate with a Ca2+ ion-incorporated polymeric cobalt cyanide complex affording O2 with 200% quantum efficiency

Incorporation of a small amount of Ca²⁺ ions into a polymeric cobalt cyanide complex enhanced the activity for photocatalytic water oxidation by persulphate with [Ru(bpy)₃]²⁺ at pH 7.0 to achieve a maximum quantum efficiency of 200%.

Frontiers of water oxidation: the quest for true catalysts

Development of advanced analytical techniques is essential for the identification of water oxidation catalysts together with mechanistic studies.

Flower-like Cobalt Hydroxide/Oxide on Graphitic Carbon Nitride for Visible-Light-Driven Water Oxidation

Direct water oxidation via photocatalysis is a four-electron and multiple-proton process which requires high extra energy input to produce free dioxygen gas, making it exacting, especially under visible light irradiation. To improve the oxygen evolution reaction rates (OERs) and utilize more visible light, flower-like cobalt hydroxide/oxide (Fw-Co(OH)₂/Fw-Co₃O₄) photocatalysts were prepared and loaded onto graphitic carbon nitride (g-C₃N₄) by a facile coating method in this work. Influenced by the unique three-dimensional morphologies, the synthesized Fw-Co(OH)₂ or Fw-Co₃O₄/g-C₃N₄ hybrids reveal favorable combination and synergism reflected by the modified photoelectric activities and the improved OER performances. Attributed to its prominent hydrotalcite structure, Fw-Co(OH)₂ shows better cocatalytic activity for g-C₃N₄ modification compared with that of Fw-Co₃O₄. Specifically, 7 wt % Fw-Co(OH)₂/g-C₃N₄ photocatalyst exhibits photocurrent density 4 times higher and OER performance 5 times better than pristine g-C₃N₄. This work unambiguously promotes the application of sustainable g-C₃N₄ in water oxidation.

Water oxidation using earth-abundant transition metal catalysts: opportunities and challenges

Catalysts for the oxidation of water are a vital component of solar energy to fuel conversion technologies. This Perspective summarizes recent advances in the field of designing homogeneous water oxidation catalysts (WOCs) based on Mn, Fe, Co and Cu.

A carbon-free polyoxometalate molecular catalyst with a cobalt–arsenic core for visible light-driven water oxidation

Na₁₂[{Co^II₇As^III₆O₉(OH)₆}(A-α-SiW₉O₃₄)₂]·8H₂O with the first reported cobalt–arsenic core was synthesized, thoroughly characterized and employed to catalyze water oxidation under visible-light-driven conditions.