Proton-coupled Electron Transfer Process Research Articles

A Ru-bda Complex with a Dangling Carboxylate Group: Synthesis and Electrochemical Properties.

Ruthenium complexes containing the tetradentate 2,2'-bipyridine-6,6'-dicarboxylato (bda2-) equatorial ligand and ortho-subsituted pyridines in the axial position have been prepared and characterized using spectroscopic, crystallographic and electrochemical techniques. Complexes [Ru(Hbda)(DMSO)(pyC)] (1) and [Ru(bda)(DMSO)(pyA)] (2) (where pyC is 2-pyridinecarboxylate, pyA is pyridine-2-ylmethanol and DMSO is dimethyl sulfoxide) have been isolated in moderate to high yields. The solid state structures of (1-H)- and 2 reveal the strong chelate effect of the axial pyridine ligand that coordinates in a bidentate fashion leaving the bda2- equatorial ligand coordinating in a tridentate mode. In solution, compound 2 shows a dynamic equilibrium between different coordination modes of the bda2- and pyA ligands. This phenomenon does not occur for 1 because the carboxylate binds stronger than the labile alcohol in 2. Cyclic voltammetry analysis of 1 reveals a complex behavior with a pH-independent wave at E1/2 = 1.12 V that is tentatively associated with the two-electron RuIV/II couple. In sharp contrast, complex 2 shows a pH-dependent one-electron wave at E1/2 = 0.83 V (pH 1), assigned to the proton-coupled electron transfer process of the RuIII/II couple and a pH-independent wave at E1/2 = 1.06 V assigned to the RuIV/III couple. Compound 2 is used to prepare complex [Ru(bda)(pic)(pyA)] (4). This complex is air sensitive and converts to complex [Ru(bda)(pic)(pyE)] (5) (where pyE is methyl 2-pyridine carboxylate) in the presence of methanol. This oxidation also occurs by applying a positive potential to an aqueous solution of 4, producing the derivative [Ru(bda)(pic)(pyC)] (3). Cyclic voltammetry of 3 shows two pH-independent one-electron oxidation waves at E1/2 = 0.64 V and E1/2 = 1.0 V, corresponding to the RuIII/II and RuIV/III couples, respectively. In addition, a water oxidation catalytic wave appears at Eonset ≈ 1.4 V. Foot-of-the-wave analysis of this catalytic wave based on a water nucleophilic attack accounts for a TOFmax = 0.63-0.74 s-1.

Impact of Local Electrostatics on the Redox Properties of Tryptophan Radicals in Azurin: Implications for Redox-Active Tryptophans in Proton-Coupled Electron Transfer.

Tyrosine and tryptophan play critical roles in facilitating proton-coupled electron transfer (PCET) processes essential to life. The local protein environment is anticipated to modulate the thermodynamics of amino acid radicals to achieve controlled, unidirectional PCET. Herein, square-wave voltammetry was employed to investigate the electrostatic effects on the redox properties of tryptophan in two variants of the protein azurin. Each variant contains a single redox-active tryptophan, W48 or W108, in a unique and buried protein environment. These tryptophan residues exhibit reversible square-wave voltammograms. A Pourbaix plot, representing the reduction potentials versus pH, is presented for the non-H-bonded W48, which has potentials comparable to those of tryptophan in solution. The reduction potentials of W108 are seen to be increased by more than 100 mV across the same pH range. Molecular dynamics shows that, despite its buried indole ring, the N-H of W108 hydrogen bonds with a water cluster, while W48 is completely excluded from interactions with water or polar groups. These redox properties provide insight into the role of the protein in tuning the reactivity of tryptophan radicals, a requirement for controlled biological PCET.

Boosting water oxidation activity by tuning the proton transfer process of cobalt phosphonates in neutral solution.

Water oxidation is a vital step in both natural and artificial photosynthetic processes. However, the effect of second coordination sphere for efficient oxygen evolution electrocatalysts has rarely been studied, becoming a bottleneck in many energy-related issues. In this article, the cobalt phosphonate (NH3C6H4NH3)Co2(hedpH)2·H2O (Co-PDA) displayed decent electrocatalytic water oxidation activity in 50 mM PBS solution (pH 7.0), comparable to the activity of state-of-the-art IrO2. Moreover, it exhibited a 160 mV lower onset potential and 6 times higher TOF than those of the counterpart, (NH4)2Co2(hedpH)2 (Co-NH4+), which existed with the same Co active center, while surrounded by different ligands. The related mechanistic studydemonstrates that the ligand in Co-PDA would benefit the proton-coupled electron transfer (PCET) processes and the formation of high valence state Co(iv).

Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires.

Designing molecular platforms for controlling proton and electron movement in artificial photosynthetic systems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons and electrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature in myriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as to generate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies of a series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol (BI2P) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups. The set of TPAs spans more than 6 pKa units. These compounds have been designed to explore the role of the bridging benzimidazole(s) in a one-electron oxidation process coupled to intramolecular proton translocation across either two (the BIP series) or three (the BI2P series) acid/base sites. These molecular constructs feature an electrochemically active phenol connected to the TPA group through a benzimidazole-based bridge, which together with the phenol and TPA group form a covalent framework supporting a Grotthuss-type hydrogen-bonded network. Infrared spectroelectrochemistry demonstrates that upon oxidation of the phenol, protons translocate across this well-defined hydrogen-bonded network to a TPA group. The experimental data show the benzimidazole bridges are non-innocent participants in the PCET process in that the addition of each benzimidazole unit lowers the redox potential of the phenoxyl radical/phenol couple by 60 mV, regardless of the nature of the TPA group. Using a series of hypothetical thermodynamic steps, density functional theory calculations correctly predicted the dependence of the redox potential of the phenoxyl radical/phenol couple on the nature of the final protonated species and provided insight into the thermodynamic role of dibenzimidazole units in the PCET process. This information is crucial for developing molecular “dry proton wires” with these moieties, which can transfer protons via a Grotthuss-type mechanism over long distances without the intervention of water molecules.

Design and synthesis of benzimidazole phenol-porphyrin dyads for the study of bioinspired photoinduced proton-coupled electron transfer

Benzimidazole phenol-porphyrin dyads have been synthesized to study proton-coupled electron transfer (PCET) reactions induced by photoexcitation. High-potential porphyrins have been chosen to model P680, the photoactive chlorophyll cluster of photosynthetic photosystem II (PSII). They have either two or three pentafluorophenyl groups at the meso positions to impart the high redox potential. The benzimidazole phenol (BIP) moiety models the Tyr[Formula: see text]-His190 pair of PSII, which is a redox mediator that shuttles electrons from the water oxidation catalyst to P680[Formula: see text]. The dyads consisting of a porphyrin and an unsubstituted BIP are designed to study one-electron one-proton transfer (E1PT) processes upon excitation of the porphyrin. When the BIP moiety is substituted with proton-accepting groups such as imines, one-electron two-proton transfer (E2PT) processes are expected to take place upon oxidation of the phenol by the excited state of the porphyrin. The bis-pentafluorophenyl porphyrins linked to BIPs provide platforms for introducing a variety of electron-accepting moieties and/or anchoring groups to attach semiconductor nanoparticles to the macrocycle. The triads thus formed will serve to study the PCET process involving the BIPs when the oxidation of the phenol is achieved by the photochemically produced radical cation of the porphyrin.

A bio-inspired coordination polymer as outstanding water oxidation catalyst via second coordination sphere engineering

First-row transition metal-based catalysts have been developed for the oxygen evolution reaction (OER) during the past years, however, such catalysts typically operate at overpotentials (η) significantly above thermodynamic requirements. Here, we report an iron/nickel terephthalate coordination polymer on nickel form (NiFeCP/NF) as catalyst for OER, in which both coordinated and uncoordinated carboxylates were maintained after electrolysis. NiFeCP/NF exhibits outstanding electro-catalytic OER activity with a low overpotential of 188 mV at 10 mA cm−2 in 1.0 KOH, with a small Tafel slope and excellent stability. The pH-independent OER activity of NiFeCP/NF on the reversible hydrogen electrode scale suggests that a concerted proton-coupled electron transfer (c-PET) process is the rate-determining step (RDS) during water oxidation. Deuterium kinetic isotope effects, proton inventory studies and atom-proton-transfer measurements indicate that the uncoordinated carboxylates are serving as the proton transfer relays, with a similar function as amino acid residues in photosystem II (PSII), accelerating the proton-transfer rate.

Redox-neutral photocatalytic strategy for selective C–C bond cleavage of lignin and lignin models via PCET process

It remains challenging to achieve the selective cleavage of C–C bonds in lignin or lignin model compounds to produce aromatic products in high yield and selectivity. We have developed a redox-neutral photocatalytic strategy to accomplish this goal in both β-O-4 and β-1 lignin models at room temperature (RT) via proton-coupled electron transfer (PCET) process without any pretreatments of substrate, by adjusting the alkalinity of base to obtain a lignin models/base PCET pair with a bond dissociation free energy close to 102 kcal/mol. Without breaking down Cβ–Cγ bond and any C–O bonds, this PCET method is 100% atom economy and produces exclusive Cα–Cβ bond cleavage products, such as benzaldehydes (up to 97%) and phenyl ethers (up to 96%), in high to excellent yields and selectivities. Preliminary studies indicated that the PCET strategy is also effective for the depolymerization of native lignin at RT, thus providing significantly important foundation to the depolymerization of lignin.

Ultrasmall Silver Clusters Stabilized on MgO for Robust Oxygen-Promoted Hydrogen Production from Formaldehyde Reforming.

Efficient molecular hydrogen generation from renewable biomass-derived resources and water is of great importance to the sustainable development of the future society. Herein, ultrasmall Ag nanoclusters supported on a defect-rich MgO matrix (AgUCs/MgO) are synthesized by a facile impregnation/calcination method and are applied to robust oxygen-promoted formaldehyde reforming into H2 at room temperature. Density functional theory calculations and experimental observations show that the catalyst spatially builds up a channel for directional electron transfer from electron-rich Ag sites to the anti-bonding π orbital of chemisorbed bridged O2 molecules, leading to the implementation of low-temperature O2 adsorption and activation. The catalytically active species, •OOH, is thus selectively generated via a preferential two-electron reduction of O2 with a low energy barrier on Ag sites, involving an unusual long-range proton-coupled electron transfer process. The •OOH-AgUCs/MgO active center is efficient for the subsequent C-H activation and H2 generation, leading to a 3-fold improvement of the turnover frequency as compared with its analogous AgNPs/MgO catalyst. Our atomic-level design and synthetic strategy provide a platform that facilitates the construction of an electron-proton transfer channel for catalysis, altered adsorption configurations of activated reactants, and enhancement of catalytic hydrogen generation activity, extending a promising direction for the development of next-generation energy catalysts.

Proton-Coupled Electron Transfer Drives Long-Range Proton Translocation in Bioinspired Systems.

Proton-coupled electron transfer (PCET) combines the movement of fundamental charged species to form an essential link between electron- and proton-transport reactions in bioenergetics and catalysis in general. The length scale over which proton transport may occur within PCET processes and the thermodynamic consequences of the resulting proton chemical potential to the oxidation reaction driving these PCET processes have not been generally established. Here we report the design of bioinspired molecules that employ oxidation-reduction processes to move reversibly two, three, and four protons via a Grotthuss-type mechanism along hydrogen-bonded networks up to ∼16 Å in length. These molecules are composed of benzimidazole moieties linking a phenol to the final proton acceptor, a cyclohexylimine. Following electrochemical oxidation of the phenol, the appearance of an infrared band at 1660 cm-1 signals proton arrival at the terminal basic site. Switching the electrode potential to reducing conditions reverses the proton translocation and resets the structure to the initial species. In addition to mimicking the first step of the iconic PCET process used by the Tyrz-His190 redox relay in photosystem II to oxidize water, this work specifically addresses theoretically and experimentally the length scale over which PCET processes may occur. The thermodynamic findings from these redox-driven, bioinspired "proton wires" have implications for understanding and rationally designing pumps for the generation of proton-motive force in artificial and reengineered photosynthesis, as well as for management of proton activity around catalytic sites, including those for water oxidation and oxygen reduction.

Photocatalytic Water Splitting Cycle in a Dye-Catalyst Supramolecular Complex: Ab Initio Molecular Dynamics Simulations

A dye-sensitized photoelectrochemical cell (DS-PEC) is a promising device for direct conversion of solar energy into fuel. The basic idea, inspired by natural photosynthesis, is to couple the photoinduced charge separation process to catalytic water splitting. The photo-oxidized dye coupled to a water oxidation catalyst (WOC) should exert a thermodynamic driving force for the catalytic cycle, while water provides the electrons for regenerating the oxidized dye. These conditions impose specific energetic constraints on the molecular components of the photoanode in the DS-PEC. Here, we consider a supramolecular complex integrating a mononuclear Ru-based WOC with a fully organic naphthalene-diimide (NDI) dye that is able to perform fast photoinduced electron injection into the conduction band of the titanium-dioxide semiconductor anode. By means of constrained ab initio molecular dynamics simulations in explicit water solvent, it is shown that the oxidized NDI provides enough driving force for the whole photocatalytic water splitting cycle. The results provide strong evidence for the significant role of spin alignment and solvent rearrangement in facilitating the proton-coupled electron transfer processes. The predicted activation free energy barriers confirm that the O–O bond formation is the rate-limiting step. Our results expand the current understanding of the photocatalytic water oxidation mechanism and provide guidelines for the optimization of high-performance DS-PEC devices.

On the Mechanism of Carbon Dioxide Reduction on Sn-Based Electrodes: Insights into the Role of Oxide Surfaces

The electrochemical reduction of carbon dioxide into carbon monoxide, hydrocarbons and formic acid has offered an interesting alternative for a sustainable energy scenario. In this context, Sn-based electrodes have attracted a great deal of attention because they present low price and toxicity, as well as high faradaic efficiency (FE) for formic acid (or formate) production at relatively low overpotentials. In this work, we investigate the role of tin oxide surfaces on Sn-based electrodes for carbon dioxide reduction into formate by means of experimental and theoretical methods. Cyclic voltammetry measurements of Sn-based electrodes, with different initial degree of oxidation, result in similar onset potentials for the CO2 reduction to formate, ca. −0.8 to −0.9 V vs. reversible hydrogen electrode (RHE), with faradaic efficiencies of about 90–92% at −1.25 V (vs. RHE). These results indicate that under in-situ conditions, the electrode surfaces might converge to very similar structures, with partially reduced or metastable Sn oxides, which serve as active sites for the CO2 reduction. The high faradaic efficiencies of the Sn electrodes brought by the etching/air exposition procedure is ascribed to the formation of a Sn oxide layer with optimized thickness, which is persistent under in situ conditions. Such oxide layer enables the CO2 “activation”, also favoring the electron transfer during the CO2 reduction reaction due to its better electric conductivity. In order to elucidate the reaction mechanism, we have performed density functional theory calculations on different slab models starting from the bulk SnO and Sn6O4(OH)4 compounds with focus on the formation of -OH groups at the water-oxide interface. We have found that the insertion of CO2 into the Sn-OH bond is thermodynamically favorable, leading to the stabilization of the tin-carbonate species, which is subsequently reduced to produce formic acid through a proton-coupled electron transfer process. The calculated potential for CO2 reduction (E = −1.09 V vs. RHE) displays good agreement with the experimental findings and, therefore, support the CO2 insertion onto Sn-oxide as a plausible mechanism for the CO2 reduction in the potential domain where metastable oxides are still present on the Sn surface. These results not only rationalize a number of literature divergent reports but also provide a guideline for the design of efficient CO2 reduction electrocatalysts.

CO2 Reduction on an Iron-Porphyrin Center: A Computational Study.

Carbon dioxide is a greenhouse gas and is available in abundance in the atmosphere. Because of this, many researchers have searched for opportunities to utilize CO2 and convert it into valuable materials. In this work, we study the electrochemical reduction of CO2 to CO on an iron-porphyrin center using computational modeling. We tested two types of iron-porphyrins, namely, the tetraphenylporphyrin (TPP) and meso-(ortho-2-amide-phenyl)(triphenyl)porphyrin ligands. Density functional theory calculations investigated a catalytic cycle that involves a reduction, CO2 binding, two protonations, and another reduction step. We tested several density functional theory methods, basis sets, and model structures. There is a certain degree of variation between the results obtained with different density functional methods, but the same general trends are found. The calculations show that during the reduction processes, a ligand rather than metal reduction takes place, which enables stable binding of CO2 as an [FeIII(CO22-)(TPP-•)]2- complex. The subsequent proton transfer from phenol has a small barrier and is identified as a proton-coupled electron-transfer process, while the second proton transfer does not change the electronic configuration of the metal complex. Overall, the studies show that iron porphyrins are efficient CO2 reducing systems that should be able to turnover CO2 into CO efficiently. Second-coordination sphere perturbations influence CO2 positioning but are not seen to have electronic or thermochemical effects on the overall reaction mechanism.

Visible-Light-Mediated Organocatalyzed Thiol–Ene Reaction Initiated by a Proton-Coupled Electron Transfer

A convenient method for performing a thiol-ene reaction is described. The reaction is performed under blue-light irradiation and catalyzed by photoactive Lewis basic molecules such as acridine orange or naphthalene-fused N-acylbenzimidazole. It is believed that the process is initiated by a proton-coupled electron transfer process within the complex between the thiol and the Lewis basic catalyst.

Perylene-3,4,9,10-tetracarboxylic acid accelerated light-driven water oxidation on ultrathin indium oxide porous sheets

Engineering the organic/inorganic nanocomposites by taking advantage of the high electrical conductivity of the inorganic semiconductors and the structure flexibility of the organic semiconductors (OSCs) provides chances to understand the structural and electronic contributions that give rise to increased activities, which is the imperious demand for methodical design of next-generation of oxygen evolution photocatalysts. Herein, perylene-3,4,9,10-tetracarboxylic acid (PTCAD) bonded porous In2O3 nanosheets, PTCAD/In2O3 NSs, were fabricated to elucidate the performance of OSCs in light harvesting and charge separation toward photo-driven oxygen evolution reaction (OER). PTCAD coupled strongly to the conduction band (CB) of In2O3 NS and type-II energy alignment was constructed between these two organic-inorganic components. Consequently, upon excitation, electrons injected smoothly from PTCAD to In2O3 NS, leaving holes behind to perform water oxidation, which results in accelerated charge separation and transportation. Associated with the large specific area, high-speed electron transmission channels, abundant oxygen vacancies and active sites provided by the In2O3 NS, the PTCAD/In2O3 composites demonstrated highly reduced band gap (BG), boosted and stable photocurrent, as well as largely reduced overpotential, suggesting promises for broadband-light-triggered OER. In addition to the synergistic effects in promoting charge separation and inhibiting electron-hole recombination, the π-conjugated molecule, PTCAD, played a role of manipulating the lattice plane of the In2O3 nanostructure without disturbing its porous 2D structure, providing another potential approach for enhancement of catalytic activity by regulating the crystal faces. Importantly, water oxidation over PTCAD/In2O3 NSs is indeed a proton-coupled electron transfer (PCET) processes, a key process for charge separation and water oxidation, which is significantly promoted by PTCAD. Ultimately, the photo-catalytic oxygen evolution yield enhanced greatly at the presence of PTCAD, indicating that our findings may provide clues for development of organic/inorganic hybrid semiconducting photocatalysts for solar fuels generation.

Activity and Durability of Organic Ligands Modified Platinum Catalyst for Oxygen Reduction Reaction

Last decade witnessed significant advances of PEMFC technology, which has led to the commercialization of several fuel cell electric vehicles (FCEVs) such as Toyota Mirai and others. However, various technical hurdles remained to be solved for future development toward large scale of deployment. One major issue involves the high cost of the cathode catalyst, which has been identified as the most expensive among stack materials for high production volume. Current PEMFCs use platinum-based catalysts to accelerate oxygen reduction reaction (ORR) on the cathode. ORR has a sluggish kinetics, as it involves multiple sequential steps of proton coupled electron transfer processes and a “scaling” issue makes it difficult to optimize every elemental reaction step simultaneously. While many research efforts have focused on ORR catalyst material design, for example, alloy, core-shell, shape control and so on., engineering catalyst-electrolyte interface properties may offer another way to overcoming the problem. It has been reported that the modification of Pt catalyst surface with organic ligands could enhance catalyst activity. The improvement was mostly derived from either the change of surface electronic structure of the catalyst, or the mitigation of ionomer poisoning by strong anion adsorption. In this study, we systematically examined a series of organic ligands with different structures. Combining material characterization with electrochemical analysis allowed us to evaluate not only the catalyst activity, but also the ligand stability. The modified Pt/C catalyst was further examined in MEA for the validation of the possible effect of ligand modification to catalyst layer properties and performance.

Switching between CO2 Electroreduction Pathways

Electrochemical carbon dioxide reduction (CO2R) is a promising technology to use renewable electricity to convert CO2 into valuable carbon-based products (1-4). Of metal-based electrocatalysts, silver is one of the best known materials to produce CO in aqueous media, via two proton-coupled electron-transfer processes, with a high selectivity of near 100% Faradaic efficiency (FE) (5). Here, in this study, using density functional theory (DFT), we showed that hydronium (H3O+) is a key intermediate in the first oxygen hydrogenation step to form adsorbed carboxyl (*COOH), and lowers the activation energy barrier for CO formation. Removing the hydronium influence, we found that the activation energy barrier for oxygen hydrogenation and therefore adsorbed carboxyl formation increases significantly, while the activation energy barrier for carbon hydrogenation and consequently adsorbed formate (*HCOO) formation reduces. This mechanism suggests that formate formation pathway at lower concentration of hydronium is more favorable than CO formation pathway. Inspired by these DFT results, we designed experiments at highly concentrated KOH solution, to limit the hydronium availability in the aqueous electrolyte. Using a gas diffusion electrode in flow cell configuration enabled us to utilize a very basic electrolyte by separating it from the CO2 gas stream, and also to run experiments at a high current density of 300 mA/cm2. We found that, the CO2R pathways switches from entirely CO formation under neutral condition, to almost 60% FE for formate formation in 11 M KOH. Different in situ and ex situ materials characterization such as XAS, XPS, SEM and XRD, demonstrated that the electrocatalysts before and after the reactions were identical. In addition, control experiments excluded the applied potential effect, confirming that formate formation was a direct effect of alkaline media and lack of hydronium. Observing high selectivity for formate formation on silver in aqueous media and at high current density has never been reported before. We believe that selectivity shift provides new insights into the role of hydronium on CO2 electroreduction processes and the ability for electrolyte manipulation to directly influence transition state kinetics, altering favored CO2 reaction pathways. C.-T. Dinh et al., CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface. Science 360, 783 (2018). T.-T. Zhuang et al., Steering post-C–C coupling selectivity enables high efficiency electroreduction of carbon dioxide to multi-carbon alcohols. Nature Catalysis 1, 421-428 (2018). Z.-Q. Liang et al., Copper-on-nitride enhances the stable electrosynthesis of multi-carbon products from CO2. Nat Commun 9, 3828 (2018). M. G. Kibria et al., A Surface Reconstruction Route to High Productivity and Selectivity in CO2 Electroreduction toward C2+ Hydrocarbons. Advanced Materials 30, 1804867 (2018). C. M. Gabardo et al., Combined high alkalinity and pressurization enable efficient CO2 electroreduction to CO. Energy & Environmental Science, (2018). Figure 1

Decoding Proton-Coupled Electron Transfer with Potential-p Ka Diagrams: Applications to Catalysis.

The applied potential at which [NiII(P2PhN2Bn)2]2+ (P2PhN2Bn = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) catalyzes hydrogen production is reported to vary as a function of proton source p Ka in acetonitrile. By contrast, most molecular catalysts exhibit catalytic onsets at p Ka-independent potentials. Using experimentally determined thermochemical parameters associated with reduction and protonation, a coupled Pourbaix diagram is constructed for [NiII(P2PhN2Bn)2]2+. One layer describes proton-coupled electron transfer reactivity involving ligand-based protonation, and the second describes metal-based protonation. An overlay of this diagram with experimentally determined E cat/2 values spanning 15 p Ka units, along with complementary stopped-flow rapid mixing experiments to detect reaction intermediates, supports a mechanism in which the proton-coupled electron transfer processes underpinning the p Ka-dependent catalytic processes involve protonation of the ligand, not the metal center. For proton sources with p Ka values in the range 6-10.6, the initial species formed is the doubly reduced, doubly protonated species [Ni0(P2PhN2BnH)2]2+, despite a higher overpotential for this proton-coupled electron transfer reaction in comparison to forming the metal-protonated isomer. In this complex, each ligand is protonated in the exo position with the two amine moieties on each ligand binding a single proton and positioning it away from the metal center. This species undergoes very slow isomerization to form an endo-protonated hydride species [HNiII(P2PhN2Bn)(P2PhN2BnH)]2+ that can release hydrogen to close the catalytic cycle. Importantly, this slow isomerization does not perturb the initially established proton-coupled electron transfer equilibrium, placing catalysis under thermodynamic control. New details revealed about the reaction mechanism from the coupled Pourbaix diagram and the complementary stopped-flow studies lead to predictions as to how this p Ka-dependent activity might be engendered in other molecular catalysts for multi-electron, multi-proton transformations.

Synergistic Mn-Co catalyst outperforms Pt on high-rate oxygen reduction for alkaline polymer electrolyte fuel cells

Alkaline polymer electrolyte fuel cells are a class of fuel cells that enable the use of non-precious metal catalysts, particularly for the oxygen reduction reaction at the cathode. While there have been alternative materials exhibiting Pt-comparable activity in alkaline solutions, to the best of our knowledge none have outperformed Pt in fuel-cell tests. Here we report a Mn-Co spinel cathode that can deliver greater power, at high current densities, than a Pt cathode. The power density of the cell employing the Mn-Co cathode reaches 1.1 W cm−2 at 2.5 A cm−2 at 60 oC. Moreover, this catalyst outperforms Pt at low humidity. In-depth characterization reveals that the remarkable performance originates from synergistic effects where the Mn sites bind O2 and the Co sites activate H2O, so as to facilitate the proton-coupled electron transfer processes. Such an electrocatalytic synergy is pivotal to the high-rate oxygen reduction, particularly under water depletion/low humidity conditions.

Molybdenum-catalysed ammonia production with samarium diiodide and alcohols or water.

The production of ammonia from nitrogen gas is one of the most important industrial processes, owing to the use of ammonia as a raw material for nitrogen fertilizers. Currently, the main method of ammonia production is the Haber-Bosch process, which operates under very high temperatures and pressures and is therefore very energy-intensive1. The transition-metal-catalysed reduction of nitrogen gas2-6 is an alternative method for the formation of ammonia. In these reaction systems, metallocenes or potassium graphite are typically used as the reducing reagent, and conjugate acids ofpyridines or related compounds are used as a proton source. To develop a next-generation nitrogen-fixation system, these reagents should be low cost, readily available and environmentally friendly. Here we show that the combination of samarium(II) diiodide (SmI2) with alcohols or water enables the fixation of nitrogen to be catalysed by molybdenum complexes under ambient conditions. Up to 4,350 equivalents of ammonia can be produced (based on the molybdenum catalyst), with a turnover frequency of around 117 per minute. The amount of ammonia produced and its rate of formation are one and two orders of magnitude larger, respectively, than those achieved in artificial reaction systems reported so far, and the formation rate approaches that observed with nitrogenase enzymes. The high reactivity is achieved by a proton-coupled electron-transfer process that is enabled by weakening of the O-H bonds of alcohols and water coordinated to SmI2. Although the current reaction is not suitable for use on an industrial scale, this work demonstrates an opportunity for further research into catalytic nitrogen fixation.

Activation of C-H, N-H, and O-H Bonds via Proton-Coupled Electron Transfer to a Mn(III) Complex of Redox-Noninnocent Octaazacyclotetradecadiene, a Catenated-Nitrogen Macrocyclic Ligand.

Reaction of 1,3-diazidopropane with an electron-rich Mn(II) precursor results in oxidation of the metal center to a Mn complex with concomitant assembly of the macrocyclic ligand into the 1,2,3,4,8,9,10,11-octaazacyclotetradeca-2,9-diene-1,4,8,11-tetraido (OIM) ligand. Although describable as a Werner Mn(V) complex, analysis by X-ray diffraction, magnetic measurements, X-ray photoelectron spectroscopy, cyclic voltammetry, and density functional theory calculations suggest an electronic structure consisting of a Mn(III) metal center with a noninnocent OIM diradical ligand. The resulting complex, (OIM)Mn(NH tBu), reacts via proton-coupled electron transfer (PCET) with phenols to form phenoxyl radicals, with dihydroanthracene to form anthracene, and with (2,4-di tert-butyltetrazolium-5-yl)amide to extrude a tetrazyl radical. PCET from the latter generates the isolable corresponding one-electron reduced compound with a neutral, zwitterionic axial 2,4-di tert-butyltetrazolium-5-yl)amido ligand. Electron paramagnetic resonance and density functional theoretical analyses suggest an electronic structure wherein the manganese atom remains Mn(III) and the OIM ligand has been reduced by one electron to a monoradical noninnocent ligand. The result indicates PCET processes whereby the proton is transferred to the axial ligand to extrude tBuNH2, the electron is transferred to the equatorial ligand, and the central metal remains relatively unperturbed.