Cobaloximes as Functional Models for Hydrogenases. 2. Proton Electroreduction Catalyzed by Difluoroborylbis(dimethylglyoximato)cobalt(II) Complexes in Organic Media

Effect of Ligand Backbone on the Electrochemical Hydrogen Evolution Reaction and Hydrogen-Atom-Transfer Reactivity Using a Nickel Polypyridine Quinoline Complex

Redox-active quinoline-containing [NiII(2PyN2Q) (H2O)]2+ complex (1) has been developed for the electrocatalytic (e) hydrogen evolution reaction (HER) in the presence of organic acids and water and for the hydrogen-atom-transfer (HAT) reaction with styrene in the presence of acids. Complex 1 shows promising e-HER performance in water up to pH 9. It exhibits a stepwise (E)ECEC mechanism with AcOH, while a potential-dependent bimolecular homolytic pathway and CEEC mechanism is operative with p-toluene sulfonic acid during the e-HER. The one- and two-electron-reduced species of 1 are characterized by spectro-electrochemistry, optical, and EPR studies. Moreover, the inverse kinetic isotope effect (KIE = 0.83) between AcOH and d4-AcOH during the e-HER and e-HAT with styrene for the hydro-functionalization reaction using catalyst 1 possibly suggests the involvement of nickel hydride species. The e-HER and e-HAT reactivity of 1 have been compared with redox-inactive redox-inactive [NiII(N4Py)(H2O)]2+ (2), demonstrating the prominent effect of quinoline in the e-HER and pyridine in the e-HAT. The proposed mechanism of the e-HER with AcOH is well supported by DFT studies.

Computational Study on the Proton Reduction Potential of Co, Rh, and Ir Molecular Electrocatalysts for the Hydrogen Evolution Reaction

In this study, comprehensive density functional theory calculations were conducted to investigate the molecular mechanism of electrocatalytic proton reduction using group 9 transition metal bpaqH (2-(bis(pyridin-2-ylmethyl)amino)-N-(quinolin-8-yl)acetamide) complexes. The goal was to explore how variations in the structural and electronic properties among the three metal centers might impact the catalytic activity. All three metal complexes were observed to share a similar mechanism, primarily characterized by three key steps: heterolytic cleavage of H2 (HEP), reduction protonation (RPP), and ligand-centered protonation (LCP). Among these steps, the heterolytic cleavage of H2 (HEP) displayed the highest activation barrier for cobalt, rhodium, and iridium catalysts compared to those of the RPP and LCP pathways. In the RPP pathway, hydrogen evolution occurred from the MII-H intermediate using acetic acid as a proton donor at the open site. Conversely, in the LCP pathway, H-H bond formation took place between the hydride and the protonated bpaqH ligand, while the open site acted as the spectator. The enhanced activity of the cobalt complex stemmed from its robust σ-bond donation and higher hydride donor ability within the metal hydride species. Additionally, the cobalt complex demonstrated a necessary negative potential in the first (MIII/II) and second (MII/I) reduction steps in both pathways. Notably, MIII/II-H exhibited a more crucial negative potential for the cobalt complex compared to those of the other two metal complexes. Through an examination of kinetics and thermodynamics in the RPP and LCP processes, it was established that cobalt and rhodium catalysts outperformed the iridium ligand scaffold in producing molecular hydrogen after substituting cobalt metal with rhodium and iridium centers. These findings distinctly highlight the lower-energy activation barrier associated with LCP compared to alternative pathways. Moreover, they offer insights into the potential energy landscape governing hydrogen evolution reactions involving group 9 transition metal-based molecular electrocatalysts.

Axial ligand-induced high electrocatalytic hydrogen evolution activity of molecular cobaloximes in homo- and heterogeneous medium

Three new molecular cobaloxime complexes with the general formula [ClCo(dpgH)2L] (1-3), where L1 = N-(4-pyridylmethyl)-1,8-naphthalimide, L2 = 4-bromo-N-(4-pyridylmethyl)-1,8-naphthalimide, L3 = 4-piperidin-N-(4-pyridylmethyl)-1,8-naphthalimide, have been synthesized and characterized by UV-Vis, multinuclear NMR, FT-IR and PXRD spectroscopic techniques. The crystal structures of all complexes have also been reported. The electrocatalytic activity of complexes is investigated under two catalysis conditions: (i) homogeneous conditions in acetonitrile using acetic acid (AcOH) as a proton source and (ii) heterogeneous conditions upon immobilization onto the surface of activated carbon cloth (CC). Complex 3 exhibited high electrocatalytic HER activity under both homogeneous and heterogeneous conditions. It catalyses proton reduction to molecular hydrogen in acetonitrile solution at a lower overpotential (640 mV) with a high turnover frequency (TOF) of 524.57 s-1 and demonstrates good stability in acidic conditions. Furthermore, catalytic (working) electrodes are prepared by immobilizing the complexes onto the surface of activated carbon cloth (CC) for electrocatalytic HER under heterogeneous conditions. An impressive HER performance was again obtained with catalytic electrode 3@CC in 1.0 M KOH, achieving a current density of -10 mA cm-2 at an overpotential of 262 mV. Chronoamperometric (CA) studies showed no significant decay of the initial current density for 10 h, indicating the excellent stability of 3@CC. Additionally, UV-Vis and NMR spectral studies of the recovered catalyst after electrocatalysis revealed no structural changes, demonstrating its robustness under reaction conditions.

Mononuclear Fe(III) Schiff base antipyrine complexes for catalytic hydrogen generation

Mononuclear Fe(III) complexes containing an antipyrine Schiff base ligand were prepared and fully characterized, demonstrating a planar tetradentate coordination geometry. These complexes were found to be active for the hydrogen evolution reaction. Catalysis occurs at -1.4 V vs. Fc+/Fc, with an overpotential of 700 mV. The complexes are active electrocatalysts with a turnover frequency of 700 s-1. Furthermore, when paired with a chromophore and sacrificial donor, the complexes are active photocatalysts demonstrating >1700 turnovers during 40 hours of irradiation with a quantum yield of up to 5.4%. The catalysts have also been found to operate in natural water samples of varying salinity.

Synergy of {Co(H2O)6}2+ with a Polyoxometalate Leads to Aqueous Homogeneous Hydrogen Evolution: Experiments and Computations

In this work, we have described a polyoxometalate (POM)-based inexpensive and easily synthesizable compound [Co(H2O)6]2[{K(H2O)}2V10O28]·2H2O (1), which exhibits electrocatalytic hydrogen evolution in its aqueous solution without its decomposition (or electrodeposition), acting as a rare homogeneous electrocatalyst. Even though the compound [Co(H2O)6]2[{K(H2O)}2V10O28]·2H2O (1) (soluble in water) shows electrocatalytic hydrogen evolution reaction (HER) activity because of the Coulombic attraction, including H-bonding interactions, between the [Co(H2O)6]2+ cationic species and [{K(H2O)}2V10O28]4-anionic species, the individual homogeneous solutions of [V10O28]6- (source: Na6[V10O28]·18H2O) and [Co(H2O)6]2+ (source: CoCl2·6H2O) do not show any electrocatalytic HER activity. We have thus established that the synergy of [V10O28]6- with [Co(H2O)6]2+ in crystal matrix as well as in the aqueous solution of 1 makes the compound 1 a stable and highly active electrocatalyst for homogeneous HER in an aqueous solution. In order to corroborate these homogeneous HER studies, we performed density functional theory (DFT) calculations to show that decavanadate cluster anion [V10O28]6- interacts with hexa-aqua complex cation [Co(H2O)6]2+ via strong H-bonding interactions, leading to a synergy effect that enables the cobalt center of [Co(H2O)6]2+ to be an active site of HER in the present work.

Mechanistic Insights into Electrocatalytic Hydrogen Evolution by an Exceptionally Stable Cobalt Complex

Co(aPPy) is one of the most stable and active molecular first-row transition-metal catalysts for proton reduction reported to date. Understanding the origin of its high performance via mechanistic studies could aid in developing even better catalysts. In this work, the catalytic mechanism of Co(aPPy) was electrochemically probed, in both organic solvents and water. We found that different mechanisms can occur depending on the solvent and the acidity of the medium. In organic solvent with a strong acid as the proton source, catalysis initiates directly after a single-electron reduction of CoII to CoI, whereas in the presence of a weaker acid, the cobalt center needs to be reduced twice before catalysis occurs. In the aqueous phase, we found drastically different electrochemical behavior, where the Co(aPPy) complex was found to be a precatalyst to a different electrocatalytic species. We propose that in this active catalyst, the pyridine ring has dissociated and acts as a proton relay at pH ≤ 5, which opens up a fast protonation pathway of the CoI intermediate and results in a high catalytic activity. Furthermore, we determined with constant potential bulk electrolysis that the catalyst is most stable at pH 3. The catalyst thus functions optimally at low pH in an aqueous environment, where the pyridine acts as a proton shuttle and where the high acidity also prevents catalyst deactivation.

Mechanisms of hydrogen evolution by six-coordinate cobalt complexes: a density functional study on the role of a redox-active pyridinyl-substituted diaminotriazine benzamidine ligand as a proton relay

The hydrogen evolution reaction is an important process for energy storage. The six-coordinate cobalt complex [CoIII(L1-)(LH)]2+ (LH = N-(4-amino-6-(pyridin-2-yl)-1,3,5-triazin-2-yl)benzamidine) was found to catalyze photocatalytic hydrogen evolution. In this work, we performed density functional calculations to obtain the reduction potentials and the proton-transfer free energy of possible intermediates to determine the preferred pathways for proton reduction. The mechanism involves the metal-based reduction of Co(III) to Co(II) before the protonation at the amidinate N on the pyridinyl-substituted diaminotriazine benzamidinate ligand L1- to form [CoII(LH)(LH)]2+. Essentially, the subsequent electron transfer is not metal-based reduction, but rather ligand-based reduction to form [CoII(LH)(LH˙1-)]1+. Through a proton-coupled electron transfer process, the cobalt hydride [CoIIH(LH)(LH2˙)]1+ is formed as the key intermediate for hydrogen evolution. As the cobalt hydride complex is coordinatively saturated, a structural change is required when the hydride on Co is coupled with the proton on pyridine. Notably, the redox-active nature of the ligand results in the low acidity of the protonated pyridine moiety of LH2˙, which impedes its function as a proton relay. Our findings suggest that separating the proton relay fragment from the electron reservoir fragment of the redox-active ligand is preferred for fully utilizing both features in catalytic H2 evolution.

Co-Catalyzed Hydrofluorination of Alkenes: Photocatalytic Method Development and Electroanalytical Mechanistic Investigation

The hydrofluorination of alkenes represents an attractive strategy for the synthesis of aliphatic fluorides. This approach provides a direct means to form C(sp3)-F bonds selectively from readily available alkenes. Nonetheless, conducting hydrofluorination using nucleophilic fluorine sources poses significant challenges due to the low acidity and high toxicity associated with HF and the poor nucleophilicity of fluoride. In this study, we present a new Co(salen)-catalyzed hydrofluorination of simple alkenes utilizing Et3N·3HF as the sole source of both hydrogen and fluorine. This process operates via a photoredox-mediated polar-radical-polar crossover mechanism. We also demonstrated the versatility of this method by effectively converting a diverse array of simple and activated alkenes with varying degrees of substitution into hydrofluorinated products. Furthermore, we successfully applied this methodology to 18F-hydrofluorination reactions, enabling the introduction of 18F into potential radiopharmaceuticals. Our mechanistic investigations, conducted using rotating disk electrode voltammetry and DFT calculations, unveiled the involvement of both carbocation and CoIV-alkyl species as viable intermediates during the fluorination step, and the contribution of each pathway depends on the structure of the starting alkene.

Hydrogen evolution catalysis by a cobalt porphyrin peptide: A proposed role for porphyrin propionic acid groups

Cobalt microperoxidase-11 (CoMP11-Ac) is a cobalt porphyrin-peptide catalyst for hydrogen (H2) evolution from water. Herein, we assess electrocatalytic activity of CoMP11-Ac from pH 1.0-10.0. This catalyst remains intact and active under highly acidic conditions (pH 1.0) that are desirable for maximizing H2 evolution activity. Analysis of electrochemical data indicate that H2 evolution takes place by two pH-dependent mechanisms. At pH < 4.3, a proton transfer mechanism involving the propionic acid groups of the porphyrin is proposed, decreasing the catalytic overpotential by 280 mV.

Electro- and photochemical H2 generation by Co(ii) polypyridyl-based catalysts bearing ortho-substituted pyridines

Cobalt(ii) complexes featuring hexadentate amino-pyridyl ligands have been recently discovered as highly active catalysts for the Hydrogen Evolution Reaction (HER), whose high performance arises from the possibility of assisting proton transfer processes via intramolecular routes involving detached pyridine units. With the aim of gaining insights into such catalytic routes, three new proton reduction catalysts based on amino-polypyridyl ligands are reported, focusing on substitution of the pyridine ortho-position. Specifically, a carboxylate (C2) and two hydroxyl substituted pyridyl moieties (C3, C4) are introduced with the aim of promoting intramolecular proton transfer which possibly enhances the efficiency of the catalysts. Foot-of-the-wave and catalytic Tafel plot analyses have been utilized to benchmark the catalytic performances under electrochemical conditions in acetonitrile using trifluoroacetic acid as the proton source. In this respect, the cobalt complex C3 turns out to be the fastest catalyst in the series, with a maximum turnover frequency (TOF) of 1.6 (±0.5) × 105 s-1, but at the expense of large overpotentials. Mechanistic investigations by means of Density Functional Theory (DFT) suggest a typical ECEC mechanism (i.e. a sequence of reduction - E - and protonation - C - events) for all the catalysts, as previously envisioned for the parent unsubstituted complex C1. Interestingly, in the case of complex C2, the catalytic route is triggered by initial protonation of the carboxylate group resulting in a less common (C)ECEC mechanism. The pivotal role of the hexadentate chelating ligand in providing internal proton relays to assist hydrogen elimination is further confirmed within this novel class of molecular catalysts, thus highlighting the relevance of a flexible polypyridine ligand in the design of efficient cobalt complexes for the HER. Photochemical studies in aqueous solution using [Ru(bpy)3]2+ (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbate as the sacrificial electron donor support the superior performance of C3.

Enzymatic and Bioinspired Systems for Hydrogen Production

The extraordinary potential of hydrogen as a clean and sustainable fuel has sparked the interest of the scientific community to find environmentally friendly methods for its production. Biological catalysts are the most attractive solution, as they usually operate under mild conditions and do not produce carbon-containing byproducts. Hydrogenases promote reversible proton reduction to hydrogen in a variety of anoxic bacteria and algae, displaying unparallel catalytic performances. Attempts to use these sophisticated enzymes in scalable hydrogen production have been hampered by limitations associated with their production and stability. Inspired by nature, significant efforts have been made in the development of artificial systems able to promote the hydrogen evolution reaction, via either electrochemical or light-driven catalysis. Starting from small-molecule coordination compounds, peptide- and protein-based architectures have been constructed around the catalytic center with the aim of reproducing hydrogenase function into robust, efficient, and cost-effective catalysts. In this review, we first provide an overview of the structural and functional properties of hydrogenases, along with their integration in devices for hydrogen and energy production. Then, we describe the most recent advances in the development of homogeneous hydrogen evolution catalysts envisioned to mimic hydrogenases.

Trends and progress in application of cobalt-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water splitting

There has been a growing interest in water oxidation in recent two decades. Along with that, remarkable discovery of formation of a mysterious catalyst layer upon application of an anodic potential of 1.13 V vs. standard hydrogen electrode (SHE) to an inert indium tin oxide electrode immersed in phosphate buffer containing Co^(II) ions by Nocera et.al, has greatly attracted researchers interest. These researches have oriented in two directions; one focuses on obtaining better understanding of the reported mysterious catalyst layer, further modification, and improved performance, and the second approach is about designing coordination complexes of cobalt and investigating their properties toward the application in water splitting. Although there have been critical debates on true catalysts that are responsible for water oxidation in homogeneous systems of coordination complexes of cobalt, and the case is not totally closed, in this short review, our focus will be mainly on recent major progress and developments in the design and the application of cobalt oxide-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water oxidation reaction, which have been reported since pioneering report of Nocera in 2008 (Kanan Matthew and Nocera Daniel in Science 321:1072-1075, 2008).

Use of a PCET Mediator Enables a Ni-HER Electrocatalyst to Act as a Hydride Delivery Agent

The generation of metal hydride intermediates during reductive electrocatalysis in the presence of acid most commonly leads to the hydrogen evolution reaction (HER). Redirecting the reactivity profile of such hydride intermediates toward the reduction of unsaturated substrates is an exciting opportunity in catalysis but presents a challenge in terms of catalyst selectivity. In this study, we demonstrate that a prototypical phosphine-supported Ni-HER catalyst can be repurposed toward the electrocatalytic reduction of a model substrate, methyl phenylpropiolate, via hydride transfer from a Ni^II-H when interfaced with a metallocene-derived proton-coupled electron transfer (PCET) mediator. Key to success is generation of the Ni^II-H at a potential pinned to that of the PCET mediator which is appreciably anodic of the onset of HER. Electrochemical, spectroscopic, and theoretical data point to a working mechanism where a PCET step from the metallocene-derived mediator to Ni^II generates Ni^III-H and is rate-determining; the latter Ni^III-H is then readily reduced to a Ni^II-H, which is competent for substrate reduction. Additional studies show that this tandem PCET-mediated hydride generation can afford high stereoselectivity (e.g., >20:1 Z/E using a phosphine-cobalt precatalyst with ethyl 2-heptynoate) and can also be used for the reduction of α,β-unsaturated ketones.

Intercepting Hydrogen Evolution with Hydrogen-Atom Transfer: Electron-Initiated Hydrofunctionalization of Alkenes

Hydrogen-atom transfer mediated by earth-abundant transition-metal hydrides (M-Hs) has emerged as a powerful tool in organic synthesis. Current methods to generate M-Hs most frequently rely on oxidatively initiated hydride transfer. Herein, we report a reductive approach to generate Co-H, which allows for canonical hydrogen evolution reactions to be intercepted by hydrogen-atom transfer to an alkene. Electroanalytical and spectroscopic studies provided mechanistic insights into the formation and reactivity of Co-H, which enabled the development of two new alkene hydrofunctionalization reactions.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Non-Covalent Functionalization of Graphene Oxide-Supported 2-Picolyamine-Based Zinc(II) Complexes as Novel Electrocatalysts for Hydrogen Production

Three mononuclear 2-picolylamine-containing zinc(III) complexes viz [(2-PA)2ZnCl]2(ZnCl4)] (Zn1), [(2-PA)2Zn(H2O)](NO3)2] (Zn2) and [Zn(2-PA)2(OH)]NO3] (Zn3) were synthesized and fully characterized. Spectral and X-ray structural characteristics showed that the Zn1 complex has a square-pyramidal coordination environment around a zinc(II) core. The hydroxide complex Zn3 was non-covalently functionalized with few layers of graphene oxide (GO) sheets, formed by exfoliation of GO in water. The resulting Zn3/GO hybrid material was characterized by FT-IR, TGA-DSC, SEM-EDX and X-ray powder diffraction. The way of interaction of Zn3 with GO has been established through density functional theory (DFT) calculations. Both experimental and theoretical findings indicate that, on the surface of GO, the complex Zn3 forms a complete double-sided adsorption layer. Zn3 and its hybrid form Zn3/GO have been individually investigated as electrocatalysts for the hydrogen evolution reaction. The hybrid heterogenized form Zn3/GO was supported on glassy carbon (GC) with variable loading densities of Zn3 (0.2, 0.4 and 0.8 mg cm−2) to form electrodes. These electrodes have been tested as molecular electrocatalysts for the hydrogen evolution reaction (HER) using linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) in 0.1 M KOH. Results showed that both GC-Zn3 and GC-Zn3/GO catalysts for the HER are highly active, and with increase of the catalyst’s loading density, this catalytic activity enhances. The high catalytic activity of HER with a low onset potential of −140 mV vs. RHE and a high exchange current density of 0.22 mA cm−2 is achieved with the highest loading density of Zn3 (0.8 mg cm−2). To achieve a current density of 10 mA cm−2, an overpotential of 240 mV was needed.

Elucidating Two Distinct Pathways for Electrocatalytic Hydrogen Production Using CoII Pincer Complexes

Hydrogen gas is a sustainable energy source with water as the sole combustion product. As a result, efforts to catalyze H₂ production are pertinent and widespread. The electrocatalytic H₂ generating capabilities of two Co^II complexes, [Co(κ³ -2,6-{Ph₂ PNR}₂ (NC₅ H₃ ))Br₂ ] with R=H (I) or R=Me (II), were presented for a variety of proton sources including trifluoroacetic acid (TFA), acetic acid (AA), and trifluoroethanol (TFE). Cyclic voltammetry and controlled potential coulometry demonstrated that electrocatalysis from I and II occurred at two different potentials and are associated with different reduction processes. Density functional theory analysis provided insight into the identities of the catalyst and supported two distinct reaction pathways for electrocatalytic proton reduction. Specifically, stronger acids (e. g., AA, TFA) proceeded at -1.31 to -1.45 V through a M^I /M^III pathway while sources with higher pK_a values (e. g., TFE, H₂ O) generated hydrogen at -2.4 V via M⁰ /M^II ligand-assisted metal-centered reduction.

Electrocatalytic reduction of protons to dihydrogen by the cobalt tetraazamacrocyclic complex [Co(N4H)Cl2]+: mechanism and benchmarking of performances

The cobalt tetraazamacrocyclic [Co(N₄H)Cl₂]⁺ complex is becoming a popular and versatile catalyst for the electrocatalytic evolution of hydrogen, because of its stability and superior activity in aqueous conditions. We present here a benchmarking of its performances based on the thorough analysis of cyclic voltammograms recorded under various catalytic regimes in non-aqueous conditions allowing control of the proton concentration. This allowed a detailed mechanism to be proposed with quantitative determination of the rate-constants for the various protonation steps, as well as identification of the amine function of the tetraazamacrocyclic ligand to act as a proton relay during H₂ evolution.

Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction

The efficiency of the hydrogen evolution reaction (HER) can be facilitated by the presence of proton-transfer groups in the vicinity of the catalyst. A systematic investigation of the nature of the proton-transfer groups present and their interplay with bulk proton sources is warranted. The HERs electrocatalyzed by a series of iron porphyrins that vary in the nature and number of pendant amine groups are investigated using proton sources whose pK_a values vary from ∼9 to 15 in acetonitrile. Electrochemical data indicate that a simple iron porphyrin (FeTPP) can catalyze the HER at this Fe^I state where the rate-determining step is the intermolecular protonation of a Fe^III-H^- species produced upon protonation of the iron(I) porphyrin and does not need to be reduced to its formal Fe⁰ state. A linear free-energy correlation of the observed rate with pK_a of the acid source used suggests that the rate of the HER becomes almost independent of pK_a of the external acid used in the presence of the protonated distal residues. Protonation to the Fe^III-H^- species during the HER changes from intermolecular in FeTPP to intramolecular in FeTPP derivatives with pendant basic groups. However, the inclusion of too many pendant groups leads to a decrease in HER activity because the higher proton binding affinity of these residues slows proton transfer for the HER. These results enrich the existing understanding of how second-sphere proton-transfer residues alter both the kinetics and thermodynamics of transition-metal-catalyzed HER.

Iron polypyridyl complex adsorbed on carbon surfaces for hydrogen generation

A series of homogeneous Fe(iii) complexes were recently reported that are active for electrocatalytic hydrogen generation. Herein we report a naphthalene-terminated Fe(iii) complex for use in the functionalization of glassy carbon surfaces for electrocatalytic hydrogen generation with retention of catalytic activity.

A blueprint for green chemists: lessons from nature for sustainable synthesis

The design of new chemical reactions that are convenient, sustainable, and innovative is a preeminent concern for modern synthetic chemistry. While the use of earth abundant element catalysts remains underdeveloped by chemists, nature has developed a cornucopia of powerful transformation using only base metals, demonstrating their viability for sustainable method development. Here we show how study of nature’s approach to disparate chemical problems, from alkene desaturation to photodetection in bacteria, can inspire and enable new approaches to difficult synthetic chemistry problems past, present, and future.

Reversible catalysis

We describe as 'reversible' a bidirectional catalyst that allows a reaction to proceed at a significant rate in response to even a small departure from equilibrium, resulting in fast and energy-efficient chemical transformation. Examining the relation between reaction rate and thermodynamic driving force is the basis of electrochemical investigations of redox reactions, which can be catalysed by metallic surfaces and biological or synthetic molecular catalysts. This relation has also been discussed in the context of biological energy transduction, regarding the function of biological molecular machines that harness chemical reactions to do mechanical work. This Perspective describes mean-field kinetic modelling of these three types of systems - surface catalysts, molecular catalysts of redox reactions and molecular machines - with the goal of unifying concepts in these different fields. We emphasize that reversibility should be distinguished from other figures of merit, such as rate or directionality, before its design principles can be identified and used to engineer synthetic catalysts.

Mononuclear Mn complexes featuring N,S-/N,N-donor and 1,3,5-triaza-7-phosphaadamantane ligands: synthesis and electrocatalytic properties

Mononuclear Mn(i) carbonyl complexes incorporating 2-mercaptobenzothiazole or 2-mercaptobenzimidazole and phosphaadamantane ligands were evaluated as electrocatalysts for the HER both in acetonitrile and acetonitrile/water.

Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation

Understanding the thermodynamics of paramagnetic transition metal hydride complexes, especially of the abundant 3d metals, is important in the design of electrocatalysts and organometallic catalysts. The pK_a^MeCN([MHL_n]⁺/[ML_n) of paramagnetic hydrides in MeCN are estimated for the first time using the ligand acidity constant (LAC) equation where contributions to the pK_a^MeCN from each ligand are simply added together, with the sum corrected for effects of charge and 5d metals. The pK_a^LAC-MeCN([MHL_n]⁺/ML_n) of over 200 hydride complexes MHL_n are used, along with their electrochemical potentials from the literature, in an uncommonly applied thermochemical cycle in order to reveal systematic trends in the redox couples M^III/II and M^V/IV (M = Cr, Mo, W), Mn^II/I, Re^VI/V and Re^IV/III, M^III/II and M^IV/III (M = Fe, Ru, Os), and M^III/II and M^II/I (M = Co, Rh, and Ir) and allow the estimation of the bond dissociation free energies BDFE(MH) of the unoxidized hydrides MHL_n and the prediction of the electrochemical potential for their oxidation. Density functional theory (DFT) calculations are used to validate the pK_a^LAC-MeCN values of hydrides of W^III, Mn^II, Fe^III, Ru^III, Co^II, and Ni^III. When a pK_a^LAC-MeCN is less than zero for a given complex [MHL_n]⁺, the oxidation of MHL_n is irreversible due to proton loss from the oxidized complex to the solvent. When pK_a^LAC-MeCN ≫ 0, the oxidation is reversible when there is no gross change in the coordination geometry upon a change in the redox state. Twenty paramagnetic hydrides prepared in bulk all have pK_a^LAC-MeCN > 8.

Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye

Mesoporous NiO photocathodes containing the push-pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye-NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H₂ evolution was not observed. Based on UV-vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

Precious-metal free photocatalytic production of an NADH analogue using cobalt diimine-dioxime catalysts under both aqueous and organic conditions

The photocatalytic generation of an NADH synthetic analogue, i.e. 1-benzyl-1,4-dihydronicotinamide (1,4-BNAH), has been studied using the cobalt diimino-dioxime complexes and the BF₂-bridged derivative as catalysts. 1,4-BNAH was produced in both aqueous and organic media at unprecedented turnover numbers with metal and organic photosensitizers, respectively.

Density Functional Theory Prediction of the Electrocatalytic Mechanism of Proton Reduction by a Dicobalt Tetrakis(Schiff Base) Macrocycle

A dicobalt tetrakis(Schiff base) macrocycle has recently been reported to electrochemically catalyze the reduction of H⁺ to H₂ in an acetonitrile solution. Density functional theory (DFT) calculations using the ωB97X-D functional are shown to produce structural and thermodynamic results in good agreement with the experimental data. A mechanistic model based on thermodynamics is developed that incorporates electrochemical and magnetic details of the complex, accounting for electron-spin reorganization of the metal center after redox steps. The model is validated through a comparison of the predicted electrochemical potentials with the irreversible cyclic voltammogram of [Co₂LAc]⁺, which shows redox-coupled spin-crossover (RCSCO) behavior for the Co^II/III transitions. Using our model, we predict the thermodynamically favored mechanism of H₂ evolution by [Co₂L]²⁺ to be one of heterolytic proton attack on a [Co^II₂L(μ-H)]⁺ species. Understanding the electronic details and thermodynamically preferred mechanism of this catalyst will aid in improving its efficiency and the future design of bimetallic Co-based H⁺ electrocatalysts. Also, this work will assist in the future DFT modeling of bimetallic RCSCO complexes.

New types of the hybrid functional materials based on cage metal complexes for (electro) catalytic hydrogen production

Successful using of cage metal complexes (clathrochelates) and the functional hybrid materials based on them as promising electro- and (pre)catalysts for hydrogen and syngas production is highlighted in this microreview. The designed polyaromatic-terminated iron, cobalt and ruthenium clathrochelates, adsorbed on carbon materials, were found to be the efficient electrocatalysts of the hydrogen evolution reaction (HER), including those in polymer electrolyte membrane (PEM) water electrolysers. The clathrochelate-electrocatalayzed performances of HER 2H+/H2 in these semi-industrial electrolysers are encouraging being similar to those for the best known to date molecular catalysts and for the promising non-platinum solid-state HER electrocatalysts as well. Electrocatalytic activity of the above clathrochelates was found to be affected by the number of the terminal polyaromatic group(s) per a clathrochelate molecule and the lowest Tafel slopes were obtained with hexaphenanthrene macrobicyclic complexes. The use of suitable carbon materials of a high surface area, as the substrates for their efficient immobilization, allowed to substantially increase an electrocatalytic activity of the corresponding clathrochelate-containing carbon paper-based cathodes. In the case of the reaction of dry reforming of methane (DRM) into syngas of a stoichiometry CO/H2 1:1, the designed metal(II) clathrochelates with terminal polar groups are only the precursors (precatalysts) of single atom catalysts, where each of their catalytically active single sites is included in a matrix of its former encapsulating ligand. Choice of their designed ligands allowed an efficient immobilization of the corresponding cage metal complexes on the surface of a given highly porous ceramic material as a substrate and caused increasing of a surface concentration of the catalytically active centers (and, therefore, that of the catalytic activity of hybrid materials modified with these clathrochelates). Thus designed cage metal complexes and hybrid materials based on them operate under the principals of “green chemistry” and can be considered as efficient alternatives to some classical inorganic and molecular (pre)catalysts of these industrial processes.

Analysis of multi-electron, multi-step homogeneous catalysis by rotating disc electrode voltammetry: theory, application, and obstacles

Rotating disc electrode (RDE) voltammetry has been widely adopted for the study of heterogenized molecular electrocatalysts for multi-step fuel-forming reactions but this tool has never been comprehensively applied to their homogeneous analogues. Here, the utility and limitations of RDE techniques for mechanistic and kinetic analysis of homogeneous molecular catalysts that mediate multi-electron, multi-substrate redox transformations are explored. Using the ECEC' reaction mechanism as a case study, two theoretical models are derived based on the Nernst diffusion layer model and the Hale transformation. Current-potential curves generated by these computational strategies are compared under a variety of limiting conditions to identify conditions under which the more minimalist Nernst Diffusion Layer approach can be applied. Based on this theoretical treatment, strategies for extracting kinetic information from the plateau current and the foot of the catalytic wave are derived. RDEV is applied to a cobaloxime hydrogen evolution reaction (HER) catalyst under non-aqueous conditions in order to experimentally validate this theoretical framework and explore the feasibility of RDE as a tool for studying homogeneous catalysts. Crucially, analysis of the foot-of-the-wave via this theoretical framework provides rate constants for elementary reaction steps that agree with those extracted from stationary voltammetric methods, supporting the application of RDE to study homogeneous fuel-forming catalysts. Finally, obstacles encountered during the kinetic analysis of cobaloxime, along with the voltammetric signatures used to diagnose this reactivity, are discussed with the goal of guiding groups working to improve RDE set-ups and help researchers avoid misinterpretation of RDE data.