Intramolecular hydrogen-bonding in a cobalt aqua complex and electrochemical water oxidation activity

Selective Cobalt-Mediated Formation of Hydrogen Peroxide from Water under Mild Conditions via Ligand Redox Non-Innocence

Despite the broad importance of hydrogen peroxide (H2O2) in oxidative transformations, there are comparatively few viable routes for its production. The majority of commercial H2O2 is currently produced by the stepwise reduction of dioxygen (O2) via the anthraquinone process, but direct electrochemical formation from water (H2O) would have several advantages─namely, avoiding flammable gases or stepwise separations. However, the selective oxidation of H2O to form H2O2 over the thermodynamically favored product of O2 is a difficult synthetic challenge. Here, we present a molecular H2O oxidation system with excellent selectivity for H2O2 that functions both stoichiometrically and catalytically. We observe high efficiency for electrocatalytic H2O2 production at low overpotential with no O2 observed under any conditions. Mechanistic studies with both calculations and kinetic analyses from isolated intermediates suggest that H2O2 formation occurs in a bimolecular fashion via a dinuclear H2O2-bridged intermediate with an important role for a redox non-innocent ligand. This system showcases the ability of metal-ligand cooperativity and strategic design of the secondary coordination sphere to promote kinetically and thermodynamically challenging selectivity in oxidative catalysis.

A homogeneous cobalt complex mediated electro and photocatalytic O2/H2O interconversion in neutral water

The O2/H2O redox couple is vital in various renewable energy conversion strategies. This work delves into the Co(L-histidine)2 complex, a functional mimic of oxygen-carrying metalloproteins, and its electrochemical behavior driving the bidirectional oxygen reduction (ORR) and oxygen evolution (OER) activity in neutral water. This complex electrocatalyzes O2 via two distinct pathways: a two-electron O2/H2O2 reduction (catalytic rate = 250 s-1) and a four-electron O2 to H2O production (catalytic rate = 66 s-1). The formation of the key trans-μ-1,2-Co(III)-peroxo intermediate expedites this process. Additionally, this complex effectively oxidizes water to O2 (catalytic rate = 15606 s-1) at anodic potentials via a Co(IV)-oxo species. Additionally, this complex executes the ORR and OER under photocatalytic conditions in neutral water in the presence of appropriate photosensitizer (Eosin-Y) and redox mediators (triethanolamine/ORR and Na2S2O8/OER) at an appreciable rate. These results highlight one of the early examples of both electro- and photoactive bidirectional ORR/OER catalysts operational in neutral water.

Boronated Cyanometallates

Thirteen boronated cyanometallates [M(CN-BR₃)₆]^3/4/5- [M = Cr, Mn, Fe, Ru, Os; BR₃ = BPh₃, B(2,4,6,-F₃C₆H₂)₃, B(C₆F₅)₃] and one metalloboratonitrile [Cr(NC-BPh₃)₆]^3- have been characterized by X-ray crystallography and spectroscopy [UV-vis-near-IR, NMR, IR, spectroelectrochemistry, and magnetic circular dichroism (MCD)]; CASSCF+NEVPT2 methods were employed in calculations of electronic structures. For (t_2g)⁵ electronic configurations, the lowest-energy ligand-to-metal charge-transfer (LMCT) absorptions and MCD C-terms in the spectra of boronated species have been assigned to transitions from cyanide π + B-C borane σ orbitals. CASSCF+NEVPT2 calculations including t_1u and t_2u orbitals reproduced t_1u/t_2u → t_2g excitation energies. Many [M(CN-BR₃)₆]^3/4- complexes exhibited highly electrochemically reversible redox couples. Notably, the reduction formal potentials of all five [M(CN-B(C₆F₅)₃)₆]^3- anions scale with the LMCT energies, and Mn(I) and Cr(II) compounds, [K(18-crown-6)]₅[Mn(CN-B(C₆F₅)₃)₆] and [K(18-crown-6)]₄[Cr(CN-B(C₆F₅)₃)₆], are surprisingly stable. Continuous-wave and pulsed electron paramagnetic resonance (EPR; hyperfine sublevel correlation) spectra were collected for all Cr(III) complexes; as expected, ¹⁴N hyperfine splittings are greater for (Ph₄As)₃[Cr(NC-BPh₃)₆] than for (Ph₄As)₃[Cr(CN-BPh₃)₆].

Characterization of Reaction Intermediates Involved in the Water Oxidation Reaction of a Molecular Cobalt Complex

Molecular cobalt(III) complexes of bis-amidate-bis-alkoxide ligands, (Me₄N)[Co^III(L₁)] (1) and (Me₄N)[Co^III(L₂)] (2), are synthesized and assessed through a range of characterization techniques. Electrocatalytic water oxidation activity of the Co complexes in a 0.1 M phosphate buffer solution revealed a ligand-centered 2e^-/1H⁺ transfer event at 0.99 V followed by catalytic water oxidation (WO) at an onset overpotential of 450 mV. By contrast, 2 reveals a ligand-based oxidation event at 0.9 V and a WO onset overpotential of 430 mV. Constant potential electrolysis study and rinse test experiments confirm the homogeneous nature of the Co complexes during WO. The mechanistic investigation further shows a pH-dependent change in the reaction pathway. On the one hand, below pH 7.5, two consecutive ligand-based oxidation events result in the formation of a Co^III(L^2-)(OH) species, which, followed by a proton-coupled electron transfer reaction, generates a Co^IV(L^2-)(O) species that undergoes water nucleophilic attack to form the O-O bond. On the other hand, at higher pH, two ligand-based oxidation processes merge together and result in the formation of a Co^III(L^2-)(OH) complex, which reacts with OH^- to yield the O-O bond. The ligand-coordinated reaction intermediates involved in the WO reaction are thoroughly studied through an array of spectroscopic techniques, including UV-vis absorption spectroscopy, electron paramagnetic resonance, and X-ray absorption spectroscopy. A mononuclear Co^III(OH) complex supported by the one-electron oxidized ligand, [Co^III(L^3-)(OH)]^-, a formal Co^IV(OH) complex, has been characterized, and the compound was shown to participate in the hydroxide rebound reaction, which is a functional mimic of Compound II of Cytochrome P450.

Dioxygen Activation and Mandelate Decarboxylation by Iron(II) Complexes of N4 Ligands: Evidence for Dioxygen-Derived Intermediates from Cobalt Analogues

The isolation, characterization, and dioxygen reactivity of monomeric [(TPA)M^II(mandelate)]⁺ (M = Fe, 1; Co, 3) and dimeric [(BPMEN)₂M^II₂(μ-mandelate)₂]²⁺ (M = Fe, 2; Co, 4) (TPA = tris(2-pyridylmethyl)amine and BPMEN = N¹,N²-dimethyl-N¹,N²-bis(pyridin-2-yl-methyl)ethane-1,2-diamine) complexes are reported. The iron(II)- and cobalt(II)-mandelate complexes react with dioxygen to afford benzaldehyde and benzoic acid in a 1:1 ratio. In the reactions, one oxygen atom from dioxygen is incorporated into benzoic acid, but benzaldehyde does not derive any oxygen atom from dioxygen. While no O₂-derived intermediate is observed with the iron(II)-mandelate complexes, the analogous cobalt(II) complexes react with dioxygen at a low temperature (-80 °C) to generate the corresponding cobalt(III)-superoxo species (S), a key intermediate implicated in the initiation of mandelate decarboxylation. At -20 °C, the cobalt(II)-mandelate complexes bind dioxygen reversibly leading to the formation of μ-1,2-peroxo-dicobalt(III)-mandelate species (P). The geometric and electronic structures of the O₂-derived intermediates (S and P) have been established by computational studies. The intermediates S and P upon treatment with a protic acid undergo decarboxylation to afford benzaldehyde (50%) with a concomitant formation of the corresponding μ-1,2-peroxo-μ-mandelate-dicobalt(III) (P1) species. The crystal structure of a peroxide species isolated from the cobalt(II)-carboxylate complex [(TPA)Co^II(MPA)]⁺ (5) (MPA = 2-methoxyphenylacetate) supports the composition of P1. The observations of the dioxygen-derived intermediates from cobalt complexes and their electronic structure analyses not only provide information about the nature of active species involved in the decarboxylation of mandelate but also shed light on the mechanistic pathway of two-electron versus four-electron reduction of dioxygen.

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.

Computational Scaling Relationships Predict Experimental Activity and Rate-Limiting Behavior in Homogeneous Water Oxidation

While computational screening with first-principles density functional theory (DFT) is essential for evaluating candidate catalysts, limitations in accuracy typically prevent the prediction of experimentally relevant activities. Exemplary of these challenges are homogeneous water oxidation catalysts (WOCs) where differences in experimental conditions or small changes in ligand structure can alter rate constants by over an order of magnitude. Here, we compute mechanistically relevant electronic and energetic properties for 19 mononuclear Ru transition-metal complexes (TMCs) from three experimental water oxidation catalysis studies. We discover that 15 of these TMCs have experimental activities that correlate with a single property, the ionization potential of the Ru(II)-O₂ catalytic intermediate. This scaling parameter allows the quantitative understanding of activity trends and provides insight into the rate-limiting behavior. We use this approach to rationalize differences in activity with different experimental conditions, and we qualitatively analyze the source of distinct behavior for different electronic states in the other four catalysts. Comparison to closely related single-atom catalysts and modified WOCs enables rationalization of the source of rate enhancement in these WOCs.

Homogeneous or heterogeneous electrocatalysis: reinvestigation of a cobalt coordination compound for water oxidation

A cobalt coordination compound with azo-ligand linkers combined with linked bisulfonate moieties has been argued to be an efficient catalyst for the oxygen-evolution reaction (OER) (H.-T. Shi, X.-X. Li, F.-H. Wu and W.-B. Yu, Dalton Trans., 2017, 46, 16321.). In the previously published report, this cobalt compound (compound 1) was believed to display a high turnover frequency (5 s^-1) at η = 720 mV at pH 9. Herein, the OER in the presence of compound 1 is reinvestigated. The nanosized oxide-based particles formed after the OER in the presence of compound 1 were tracked by electrochemical methods, scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), X-ray diffraction studies (XRD), (High-resolution) transmission electron microscopy ((HR)TEM), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). Based on these experiments, it is proposed that a candidate for the true catalyst of the OER in the presence of compound 1 is cobalt oxide. During the OER and using chronoamperometry, the oxidation state of Co ions for the formed Co oxide is (III), but after consecutive CVs the oxidation states of Co ions for the formed Co oxide are (II) and (III). The results shed new light on the role of Co oxide nanoparticles formed in the presence of this Co coordination compound during the OER. Our experimental data also show that for the OER in the presence of a homogeneous (pre)catalyst, careful analyses to find the role of metal oxides are necessary for informed progress. The present findings also might help to find the mechanism of the OER in the presence of coordination compounds.

Identifying Intermediates in Electrocatalytic Water Oxidation with a Manganese Corrole Complex

Water nucleophilic attack (WNA) on high-valent terminal Mn-oxo species is proposed for O-O bond formation in natural and artificial water oxidation. Herein, we report an electrocatalytic water oxidation reaction with Mn^III tris(pentafluorophenyl)corrole (1) in propylene carbonate (PC). O₂ was generated at the Mn^V/IV potential with hydroxide, but a more anodic potential was required to evolve O₂ with only water. With a synthetic Mn^V(O) complex of 1, a second-order rate constant, k₂(OH^-), of 7.4 × 10³ M^-1 s^-1 was determined in the reaction of the Mn^V(O) complex of 1 with hydroxide, whereas its reaction with water occurred much more slowly with a k₂(H₂O) value of 4.4 × 10^-3 M^-1 s^-1. This large reactivity difference of Mn^V(O) with hydroxide and water is consistent with different electrocatalytic behaviors of 1 with these two substrates. Significantly, during the electrolysis of 1 with water, a Mn^IV-peroxo species was identified with various spectroscopic methods, including UV-vis, electron paramagnetic resonance, and infrared spectroscopy. Isotope-labeling experiments confirmed that both O atoms of this peroxo species are derived from water, suggesting the involvement of the WNA mechanism in water oxidation by a Mn complex. Density functional theory calculations suggested that the nucleophilic attack of hydroxide on Mn^V(O) and also WNA to 1e^--oxidized Mn^V(O) are feasibly involved in the catalytic cycles but that direct WNA to Mn^V(O) is not likely to be the main O-O bond formation pathway in the electrocatalytic water oxidation by 1.

Electrochemical Polymerization Provides a Function-Integrated System for Water Oxidation

Water oxidation is a key reaction in natural and artificial photosynthesis. In nature, the reaction is efficiently catalyzed by a metal-complex-based catalyst surrounded by hole-transporting amino acid residues. However, in artificial systems, there is no example of a water oxidation system that has a catalytic center surrounded by hole transporters. Herein, we present a facile strategy to integrate catalytic centers and hole transporters in one system. Electrochemical polymerization of a metal-complex-based precursor afforded a polymer-based material (Poly-1). Poly-1 exhibited excellent hole-transporting ability and catalyzed water oxidation with high performance. It was also revealed that the catalytic activity was almost completely suppressed in the absence of the hole-transporting moieties. The present study provides a novel strategy for constructing efficient molecule-based systems for water oxidation.

Enhanced Electrocatalytic Activity of a Zinc Porphyrin for CO2 Reduction: Cooperative Effects of Triazole Units in the Second Coordination Sphere

The control of the second coordination sphere in a coordination complex plays an important role in improving catalytic efficiency. Herein, we report a zinc porphyrin complex ZnPor8T with multiple flexible triazole units comprising the second coordination sphere, as an electrocatalyst for the highly selective electrochemical reduction of carbon dioxide (CO₂ ) to carbon monoxide (CO). This electrocatalyst converted CO₂ to CO with a Faradaic efficiency of 99 % and a current density of -6.2 mA cm^-2 at -2.4 V vs. Fc/Fc⁺ in N,N-dimethylformamide using water as the proton source. Structure-function relationship studies were carried out on ZnPor8T analogs containing different numbers of triazole units and distinct triazole geometries; these unveiled that the triazole units function cooperatively to stabilize the CO₂ -catalyst adduct in order to facilitate intramolecular proton transfer. Our findings demonstrate that incorporating triazole units that function in a cooperative manner is a versatile strategy to enhance the activity of electrocatalytic CO₂ conversion.

Allan White and Solvento/Aqua Complexes: ScIII Solvation

Addition of various oxygen-donor ligands (L, all dipolar, aprotic solvents), to a solution of hydrated scandium picrate in weakly coordinating solvents (S), has yielded several crystalline adducts of the form Sc(pic)3(L)m(·nS) in which all ligands L are coordinated, subsequently characterised by a series of single crystal X-ray studies. For L=dmso, m=3 and the picrate anions are all bound as phenoxide-O donor units, while for nmp, dma, tmp, and hmpa, m=2 and one of the bound picrates becomes bidentate through phenoxide- and nitro-O donation. For L=ompa, m=2 and two picrate ligands are bound through phenoxide-O while one is displaced from the primary coordination sphere. All complexes contain six-coordinate ScIII, confirming that this coordination number is a consequence of ligand bulk.

Redox-active ligand assisted electrocatalytic water oxidation by a mononuclear cobalt complex

A cobalt complex bearing a redox-active monoanionic amidate ligand is shown to act as an efficient molecular electrocatalyst for water oxidation at a moderate overpotential (∼500 mV) in mildly alkaline medium.

Elucidating Proton-Coupled Electron Transfer Mechanisms of Metal Hydrides with Free Energy- and Pressure-Dependent Kinetics

Proton-coupled electron transfer (PCET) was studied in a series of tungsten hydride complexes with pendant pyridyl arms ([(PyCH₂Cp)WH(CO)₃], PyCH₂Cp = pyridylmethylcyclopentadienyl), triggered by laser flash-generated Ru^III-tris-bipyridine oxidants, in acetonitrile solution. The free energy dependence of the rate constant and the kinetic isotope effects (KIEs) showed that the PCET mechanism could be switched between concerted and the two stepwise PCET mechanisms (electron-first or proton-first) in a predictable fashion. Straightforward and general guidelines for how the relative rates of the different mechanisms depend on oxidant and base are presented. The rate of the concerted reaction should depend symmetrically on changes in oxidant and base strength, that is on the overall ΔG⁰_PCET, and we argue that an "asynchronous" behavior would not be consistent with a model where the electron and proton tunnel from a common transition state. The observed rate constants and KIEs were examined as a function of hydrostatic pressure (1-2000 bar) and were found to exhibit qualitatively different dependence on pressure for different PCET mechanisms. This is discussed in terms of different volume profiles of the PCET mechanisms as well as enhanced proton tunneling for the concerted mechanism. The results allowed for assignment of the main mechanism operating in the different cases, which is one of the critical questions in PCET research. They also show how the rate of a PCET reaction will be affected very differently by changes of oxidant and base strength, depending on which mechanism dominates. This is of fundamental interest as well as of practical importance for rational design of, for example, catalysts for fuel cells and solar fuel formation, which operate in steps of PCET reactions. The mechanistic richness shown by this system illustrates that the specific mechanism is not intrinsic to a specific synthetic catalyst or enzyme active site but depends on the reaction conditions.

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene-π Interactions: Ligand Binding and Ethylene Polymerization

Early metal complexes supported by hemilabile, monoanionic di(pyridyl) pyrrolide ligands substituted with mesityl and anthracenyl groups were synthesized to probe the possibility of second coordination sphere arene-π interactions with ligands with potential for allosteric control in coordination chemistry, substrate activation, and olefin polymerization. Yttrium alkyl, indolide, and amide complexes were prepared and structurally characterized; close contacts between the anthracenyl substituents and Y-bound ligands are observed in the solid state. Titanium, zirconium, and hafnium tris(dimethylamido) complexes were synthesized, and their ethylene polymerization activity was tested. In the solid state structure of one of the Ti tris(dimethylamido) complexes, coordination of Ti to only one of the pyridine donors is observed pointing to the hemilabile character of the di(pyridyl) pyrrolide ligands.

Low overpotential water oxidation at neutral pH catalyzed by a copper(ii) porphyrin

Low-overpotential water oxidation catalyzed by copper(ii) porphyrin to produce O₂ in neutral aqueous solution and H₂O₂ in acidic solution.

Low overpotential water oxidation under mild conditions is required for new energy conversion technologies with potential application prospects. Extensive studies on molecular catalysis have been performed to gain fundamental knowledge for the rational designing of cheap, efficient and robust catalysts. We herein report a water-soluble Cu^II complex of tetrakis(4-N-methylpyridyl)porphyrin (1), which catalyzes the oxygen evolution reaction (OER) in neutral aqueous solutions with small overpotentials: the onset potential of the catalytic water oxidation wave measured at current density j = 0.10 mA cm^–2 is 1.13 V versus a normal hydrogen electrode (NHE), which corresponds to an onset overpotential of 310 mV. Constant potential electrolysis of 1 at neutral pH and at 1.30 V versus NHE displayed a substantial and stable current for O₂ evolution with a faradaic efficiency of >93%. More importantly, in addition to the 4e water oxidation to O₂ at neutral pH, 1 can catalyze the 2e water oxidation to H₂O₂ in acidic solutions. The produced H₂O₂ is detected by rotating ring–disk electrode measurements and by the sodium iodide method after bulk electrolysis at pH 3.0. This work presents an efficient and robust Cu-based catalyst for water oxidation in both neutral and acidic solutions. The observation of H₂O₂ during water oxidation catalysis is rare and will provide new insights into the water oxidation mechanism.

Promoting proton coupled electron transfer in redox catalysts through molecular design

Mini-review on using the secondary coordination sphere to facilitate multi-electron, multi-proton catalysis.