Hydrogen Evolution from Aqueous Solutions Mediated by a Heterogenized [NiFe]‐Hydrogenase Model: Low pH Enables Catalysis through an Enzyme‐Relevant Mechanism

Investigating the interplay between charge transfer and CO2 insertion in the adsorption of a NiFe catalyst for CO2 electroreduction on a graphite support through DFT computational approaches

This article describes a density functional theory (DFT) study to explore a bio-inspired NiFe complex known for its experimental activity in electro-reducing CO2 to CH4 when adsorbed on graphite. The coordination properties of the complex are investigated in isolated form and when physisorbed on a graphene surface. A comparative analysis of DFT approaches for surface modeling is conducted, utilizing either a finite graphene flake or a periodic carbon surface. Results reveal that the finite model effectively preserves all crucial properties. By examining predicted structures arising from CO2 insertion within the mono-reduced NiFe species, whether isolated or adsorbed on the graphene flake, a potential species for subsequent electro-reduction steps is proposed. Notably, the DFT study highlights two positive effects of complex adsorption: facile electron transfers between graphene and the complex, finely regulated by the complex state, and a lowering of the thermodynamic demand for CO2 insertion.

Connecting Biological and Synthetic Approaches for Electrocatalytic CO2 Reduction

Electrocatalytic CO2 reduction has developed into a broad field, spanning fundamental studies of enzymatic 'model' catalysts to synthetic molecular catalysts and heterogeneous gas diffusion electrodes producing commercially relevant quantities of product. This diversification has resulted in apparent differences and a disconnect between seemingly related approaches when using different types of catalysts. Enzymes possess discrete and well understood active sites that can perform reactions with high selectivity and activities at their thermodynamic limit. Synthetic small molecule catalysts can be designed with desired active site composition but do not yet display enzyme-like performance. These properties of the biological and small molecule catalysts contrast with heterogeneous materials, which can contain multiple, often poorly understood active sites with distinct reactivity and therefore introducing significant complexity in understanding their activities. As these systems are being better understood and the continuously improving performance of their heterogeneous active sites closes the gap with enzymatic activity, this performance difference between heterogeneous and enzymatic systems begins to close. This convergence removes the barriers between using different types of catalysts and future challenges can be addressed without multiple efforts as a unified picture for the biological-synthetic catalyst spectrum emerges.

Bimetallic Molecular Catalyst Design for Carbon Dioxide Reduction

The core challenge in developing cost-efficient catalysts for carbon dioxide (CO2 ) conversion mainly lies in controlling its complex reaction pathways. One such strategy exploits bimetallic cooperativity, which relies on the synergistic interaction between two metal centers to activate and convert the CO2 substrate. While this approach has seen an important trend in heterogeneous catalysis as a handle to control stabilities of surface intermediates, it has not often been utilized in molecular and heterogenized molecular catalytic systems. In this review, we gather general principles on how natural CO2 activating enzymes take advantage of bimetallic strategy and how phosphines, cyclams, polypyridyls, porphyrins, and cryptates-based homo- and hetero-bimetallic molecular catalysts can help understand the synergistic effect of two metal centers.

Silver nanostructure-modified graphite electrode for in-operando SERRS investigation of iron porphyrins during high-potential electrocatalysis

In-operando spectroscopic observation of the intermediates formed during various electrocatalytic oxidation and reduction reactions is crucial to propose the mechanism of the corresponding reaction. Surface-enhanced resonance Raman spectroscopy coupled to rotating disk electrochemistry (SERRS-RDE), developed about a decade ago, proved to be an excellent spectroscopic tool to investigate the mechanism of heterogeneous oxygen reduction reaction (ORR) catalyzed by synthetic iron porphyrin complexes under steady-state conditions in water. The information about the formation of the intermediates accumulated during the course of the reaction at the electrode interface helped to develop better ORR catalysts with second sphere residues in the porphyrin rings. To date, the application of this SERRS-RDE setup is limited to ORR only because the thiol self-assembled monolayer (SAM)-modified Ag electrode, used as the working electrode in these experiments, suffers from stability issues at more cathodic and anodic potential, where H₂O oxidation, CO₂ reduction, and H⁺ reduction reactions occur. The current investigation shows the development of a second-generation SERRS-RDE setup consisting of an Ag nanostructure (AgNS)-modified graphite electrode as the working electrode. These electrodes show higher stability (compared to the conventional thiol SAM-modified Ag electrode) upon exposure to very high cathodic and anodic potential with a good signal-to-noise ratio in the Raman spectra. The behavior of this modified electrode toward ORR is found to be the same as the SAM-modified Ag electrode, and the same ORR intermediates are observed during electrochemical ORR. At higher cathodic potential, the signatures of Fe(0) porphyrin, an important intermediate in H⁺ and CO₂ reduction reactions, was observed at the electrode-water interface.

Bioinspired and biomolecular catalysts for energy conversion and storage

Metalloenzymes are remarkable for facilitating challenging redox transformations with high efficiency and selectivity. In the area of alternative energy, scientists aim to capture these properties in bioinspired and engineered biomolecular catalysts for the efficient and fast production of fuels from low-energy feedstocks such as water and carbon dioxide. In this short review, efforts to mimic biological catalysts for proton reduction and carbon dioxide reduction are highlighted. Two important recurring themes are the importance of the microenvironment of the catalyst active site and the key role of proton delivery to the active site in achieving desired reactivity. Perspectives on ongoing and future challenges are also provided.

Through-Space Electrostatic Effects of Positively Charged Substituents on the Hydrogen Evolution Reaction

Elucidating the effects of various structural components on energy-related small molecule activation is of fundamental and practical significance. Herein the inhibition effect of positively charged substituents on the hydrogen evolution reaction (HER) was reported. With the use of Cu porphyrins 1-5 containing different numbers and locations of positively charged substituents, it was demonstrated that their electrocatalytic HER activities significantly decreased when more cationic units were located close to the Cu ion: the i_cat /i_p (i_cat is the catalytic peak current, i_p is the one-electron reduction peak current) value decreased from 38 with zero cationic unit to 15 with four closely located cationic units. Inspired by this result, Cu porphyrin 6, with four meso-phenyl groups each bearing a negatively charged para-sulfonic substituent, was designed. With these anionic units, 6 outperformed the other Cu porphyrins for electrocatalytic HER under the same conditions.

Bioinorganic Chemistry on Electrodes: Methods to Functional Modeling

One of the major goals of bioinorganic chemistry has been to mimic the function of elegant metalloenzymes. Such functional modeling has been difficult to attain in solution, in particular, for reactions that require multiple protons and multiple electrons (nH⁺/ne^-). Using a combination of heterogeneous electrochemistry, electrode and molecule design one may control both electron transfer (ET) and proton transfer (PT) of these nH⁺/ne^- reactions. Such control can allow functional modeling of hydrogenases (H⁺ + e^- → 1/2 H₂), cytochrome c oxidase (O₂ + 4 e^- + 4 H⁺ → 2 H₂O), monooxygenases (RR'CH₂ + O₂ + 2 e^- + 2 H⁺ → RR'CHOH + H₂O) and dioxygenases (S + O₂ → SO₂; S = organic substrate) in aqueous medium and at room temperatures. In addition, these heterogeneous constructs allow probing unnatural bioinspired reactions and estimation of the inner- and outer-sphere reorganization energy of small molecules and proteins.

6,6′-Di-(2″-thiophenol)-2,2′-bipyridine

This short note describes the synthesis of compound 6,6′-di-(2″-thiophenol)-2,2′-bipyridine from its methyl phenyl sulfane precursor via deprotection of the methyl groups. The product as well as the intermediate in the synthetic route have been characterized by UV-Vis spectroscopy, 1H- and 13C-NMR spectroscopy, FT-IR spectroscopy, and HR-MS analysis. This work presents a rare example of tetradentate chelators that bears pyridyl backbones and thiophenol donors for the coordination with 3d-transition metal cations.

A bio-inspired heterodinuclear hydrogenase CoFe complex

Herein, a new heterobimetallic CoFe complex is reported with the aim of comparing its performance in terms of H₂ production within a series of related MFe complexes (M = Ni, Fe). The fully oxidized [(L^N2S2)Co^II(CO)Fe^IICp]⁺ complex (CoIIFeII, L^{N2S2 2-} = 2,2'-(2,2'-bipyridine-6,6'-diyl)bis(1,1'-diphenylethanethiolate), Cp^- = cyclopentadienyl anion) can be (electro)chemically reduced to its CoIFeII form, and both complexes have been isolated and fully characterized by means of classic spectroscopic techniques and theoretical calculations. The redox properties of CoIIFeII have been investigated in DMF, revealing that this complex is the easiest to reduce by one-electron among the analogous MFe complexes (M = Ni, Fe, Co). Nevertheless, it displays no electrocatalytic activity for H₂ production, contrary to the FeFe and NiFe analogs, which have proven remarkable performance.

Carbon Dioxide Reduction: A Bioinspired Catalysis Approach

While developed in a number of directions, bioinspired catalysis has been explored only very recently for CO₂ reduction, a challenging reaction of prime importance in the context of the energetic transition to be built up. This approach is particularly relevant because nature teaches us that CO₂ reduction is possible, with low overpotentials, high rates, and large selectivity, and gives us unique clues to design and discover new interesting molecular catalysts. Indeed, on the basis of our relatively advanced understanding of the structures and mechanisms of the active sites of fascinating metalloenzymes such as formate dehydrogenases (FDHs) and CO dehydrogenases (CODHs), it is possible to design original, active, selective, and stable molecular catalysts using the bioinspired approach. These metalloenzymes use fascinating metal centers: in FDHs, a Mo(W) mononuclear ion is coordinated by four sulfur atoms provided by a specific organic ligand, molybdopterin (MPT), containing a pyranopterin heterocycle (composed of a pyran ring fused with a pterin unit) and two sulfhydryl groups for metal chelation; in CODHs, catalytic activity depends on either a unique nickel-iron-sulfur cluster or a dinuclear Mo-Cu complex in which the Mo ion is chelated by an MPT ligand. As a consequence, the novel class of catalysts, designed by bioinspiration, consists of mononuclear Mo, W, and Ni and as well as dinuclear Mo-Cu and Ni-Fe complexes in which the metal ions are coordinated by sulfur ligands, more specifically, dithiolene chelates mimicking the natural MPT cofactor. In general, their activity is evaluated in electrochemical systems (cyclic voltammetry and bulk electrolysis) or in photochemical systems (in the presence of a photosensitizer and a sacrificial electron donor) in solution. This research is multidisciplinary because it implies detailed biochemical, functional, and structural characterization of the inspiring enzymes together with synthetic organic and organometallic chemistry and molecular catalysis studies. The most important achievements in this direction, starting from the first report of a catalytically active biomimetic bis-dithiolene-Mo complex in 2015, are discussed in this Account, highlighting the challenging issues associated with synthesis of such sophisticated ligands and molecular catalysts as well as the complexity of reaction mechanisms. While the very first active biomimetic catalysts require further improvement, in terms of performance, they set the stage in which molecular chemistry and enzymology can synergistically cooperate for a better understanding of why nature has selected these sites and for developing highly active catalysts.

Time-Resolved Spectroscopy and High-Efficiency Light-Driven Hydrogen Evolution of a {Mo3 S4 }-Containing Polyoxometalate-Based System

Polyoxothiometalate ions (ThioPOM) are active hydrogen-evolution reaction (HER) catalysts based on modular assembly built from electrophilic clusters {MoS_x } and vacant polyoxotungstates. Herein, the dumbbell-like anion [{(PW₁₁ O₃₉ )Mo₃ S₄ (H₂ O)₃ (OH)}₂ ]^8- exhibits very high light-driven HER activity, while the active cores {Mo₃ S₄ } do not contain any exposed disulfido ligands, which were suspected to be the origin of the HER activity. Moreover, in the catalyst architecture, the two central {Mo₃ S₄ } cores are sandwiched by two {PW₁₁ O₃₉ }^7- subunits that act as oxidant-resistant protecting groups and behave as electron-collecting units. A detailed photophysical study was carried out confirming the reductive quenching mechanism of the photosensitizer [Ir(ppy)₂ (dtbbpy)]⁺ by the sacrificial donor triethanolamine (TEOA) and highlighting the very high rate constant of the electron transfer from the reduced photosensitizer to the ThioPOM catalyst. Such results provide new insights into the field of molecular catalytic systems able to promote high HER activity.

H2 activation by hydrogenase-inspired NiFe catalyst using frustrated Lewis pair: effect of buffer and halide ion in the heterolytic H-H bond cleavage

Hydrogen is a clean fuel alternative to fossil fuels, and it is vital to develop catalysts for its efficient activation and production. We investigate the reaction mechanism of H₂ activation in an aqueous solution by the recently developed NiFe complex (Ogo et al. Sci. Adv. 2020, 6, eaaz8181) using density functional theory (DFT) calculation. Our computational results showed that H₂ is activated using frustrated Lewis pair. That is, H₂ binds to the Fe site of the NiFe complex, acting as a Lewis acid, while the added buffer, acting as Lewis base, abstracts protons to form a hydride complex. Furthermore, the higher basicity in the proton abstraction reaction characterises reaction more exergonic and lowers the reaction barrier. In addition, in the proton abstraction by the water molecule, the reaction barrier was lowered when anion such as Cl⁻ is in the vicinity of the water. Understanding the chemical species that contribute to the catalytic process in cooperation with the metal catalyst at the atomic level should help to maximise the function of the catalyst.

Hydrogen is a clean fuel alternative to fossil fuels, and it is vital to develop catalysts for its efficient activation and production.

Structural tuning of heterogeneous molecular catalysts for electrochemical energy conversion

Heterogeneous molecular catalysts based on transition metal complexes have received increasing attention for their potential application in electrochemical energy conversion. The structural tuning of first and second coordination spheres of complexes provides versatile strategies for optimizing the activities of heterogeneous molecular catalysts and appropriate model systems for investigating the mechanism of structural variations on the activity. In this review, we first discuss the variation of first spheres by tuning ligated atoms; afterward, the structural tuning of second spheres by appending adjacent metal centers, pendant groups, electron withdrawing/donating, and conjugating moieties on the ligands is elaborated. Overall, these structural tuning resulted in different impacts on the geometric and electronic configurations of complexes, and the improved activity is achieved through tuning the stability of chemisorbed reactants and the redox behaviors of immobilized complexes.

Biochemical and artificial pathways for the reduction of carbon dioxide, nitrite and the competing proton reduction: effect of 2nd sphere interactions in catalysis

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment.

Bio-inspired, Multifunctional Metal-Thiolate Motif: From Electron Transfer to Sulfur Reactivity and Small-Molecule Activation

Sulfur-rich metalloproteins and metalloenzymes, containing strongly covalent metal-thiolate (cysteinate) or metal-sulfide bonds in their active site, are ubiquitous in nature. The metal-sulfur motif is a highly versatile tool involved in various biological processes: (i) metal storage, transport, and detoxification; (ii) electron transfer; (iii) activation of the sulfur atom to promote different types of S-based reactions including S-alkylation, S-oxygenation, S-nitrosylation, or disulfide or thiyl radicals formation; (iv) activation of small earth-abundant molecules (such as water, dioxygen, superoxide radical anion, carbon oxides, nitrous oxide, and dinitrogen).This Account describes our investigations carried out during the past 10 years on bio-inspired and biomimetic low-nuclearity complexes containing metal-thiolate bonds. The general objective of these structural, spectroscopic, electrochemical, and catalytic studies was to determine structure-properties-function correlations useful to (i) understanding the peculiar features or the mechanism of the mimicked natural systems and/or (ii) reproducing enzymatic reactivities for specific catalytic applications.By employing a unique highly preorganized N₂S₂-donor ligand with two thiolate functions, in combination with different first-row transition metals (Mn, Fe, Co, Ni, Cu, Zn, or V), we got access to a series of bio-inspired sulfur-rich complexes displaying a widespread spectrum of structures, properties, and functions. We isolated a dicopper(I) complex that, for the first time, mimicked concomitantly the key structural, spectroscopic, and redox features of the biological Cu_A center, a highly efficient electron transfer agent involved in the respiratory enzyme cytochrome c oxidase. In the field of sulfur activation, we explored (i) sulfur methylation promoted by a Zn-dithiolate complex that mimics Zn-dependent thiolate alkylation proteins and shows different selectivity compared to the Ni and Co congeners and (ii) a series of Co, Fe, and Mn complexes as the first copper-free systems able to promote thiolate/disulfide interconversion mediated by (de)coordination of halides. Concerning metal-centered reactivity, we investigated two families of metal-thiolate catalysts for small-molecule activation, especially relevant in the fields of sustainable fuel production and energy conversion: (i) two isostructural Mn and Fe dinuclear complexes that activate and reduce dioxygen selectively, either to hydrogen peroxide or water as a function of the experimental conditions; (ii) a family of dinuclear MFe (M = Ni or Fe) hydrogenase mimics active for catalytic H₂ evolution both in organic solution and on modified electrodes in water.This Account thus illustrates how the versatility of thiolate ligation can support selected functions for transition metal complexes, depending on the nature of the metal, the nuclearity of the complex, the presence and type of co-ligands, the second coordination sphere effects, and the experimental conditions.

First-row transition metal porphyrins for electrocatalytic hydrogen evolution — a SPP/JPP Young Investigator Award paper

A series of first-row transition metal complexes of tetrakis(pentafluorophenyl)porphyrin (1), denoted as 1-M (M [Formula: see text] Mn, Fe, Co, Ni, Cu, and Zn), were synthesized and examined as electrocatalysts for the hydrogen evolution reaction (HER). All these transition metal porphyrins were shown to be active for HER in acetonitrile using trifluoroacetic acid (TFA) as the proton source. The molecular nature and the stability of these metal porphyrins when functioning as HER catalysts were confirmed, and all catalysts gave Faradaic efficiency of >97% for H2 generation during bulk electrolysis. Importantly, by using 1-Cu, a remarkably high turnover frequency (TOF) of 48500 s[Formula: see text] 1-Cu the most efficient among this series of metal porphyrin catalysts. This TOF value also represents one of the highest values reported in the literature. In addition, electrochemical analysis demonstrated that catalytic HER mechanisms with these 1-M complexes are different. These results show that with the same porphyrin ligand, the change of metal ions will have significant impact on both catalytic efficiency and mechanism. This work for the first time provides direct comparison of electrocatalytic HER features of transition metal complexes of tetrakis(pentafluorophenyl)porphyrin under identical conditions, and will be valuable for future design and development of more efficient HER electrocatalysts of this series.

Synthetic models of hydrogenases based on framework structures containing coordinating P, N-atoms as hydrogen energy electrocatalysts – from molecules to materials

Nowadays, hydrogen has become not only an extremely important chemical product but also a promising clean energy carrier for replacing fossil fuels. Production of molecular H2 through electrochemical hydrogen evolution reactions is crucial for the development of clean-energy technologies. The development of economically viable and efficient H2 production/oxidation catalysts is a key step in the creation of H2-based renewable energy infrastructure. Intrinsic limitations of both natural enzymes and synthetic materials have led researchers to explore enzyme-induced catalysts to realize a high current density at a low overpotential. In recent times, highly active widespread numerous electrocatalysts, both homogeneous or heterogeneous (immobilized on the electrode), such as transition metal complexes, heteroatom- or metal-doped nanocarbons, metal-organic frameworks, and other metal derivatives (calix [4] resorcinols, pectates, etc.), which are, to one extent or another, structural or functional analogs of hydrogenases, have been extensively studied as alternatives for Pt-based catalysts, demonstrating prospects for the development of a “hydrogen economy”. This mini-review generalizes some achievements in the field of development of new electrocatalysts for H2 production/oxidation and their application for fuel cells, mainly focuses on the consideration of the catalytic activity of M[P2N2]2 2+ (M = Ni, Fe) complexes and other nickel structures which have been recently obtained.

The large subunit of the regulatory [NiFe]-hydrogenase from Ralstonia eutropha - a minimal hydrogenase?

Chemically synthesized compounds that are capable of facilitating the reversible splitting of dihydrogen into protons and electrons are rare in chemists' portfolio. The corresponding biocatalysts – hydrogenases – are, however, abundant in the microbial world. [NiFe]-hydrogenases represent a major subclass and display a bipartite architecture, composed of a large subunit, hosting the catalytic NiFe(CO)(CN)₂ cofactor, and a small subunit whose iron–sulfur clusters are responsible for electron transfer. To analyze in detail the catalytic competence of the large subunit without its smaller counterpart, we purified the large subunit HoxC of the regulatory [NiFe]-hydrogenase of the model H₂ oxidizer Ralstonia eutropha to homogeneity. Metal determination and infrared spectroscopy revealed a stoichiometric loading of the metal cofactor. This enabled for the first time the determination of the UV-visible extinction coefficient of the NiFe(CO)(CN)₂ cofactor. Moreover, the absence of disturbing iron–sulfur clusters allowed an unbiased look into the low-spin Fe²⁺ of the active site by Mössbauer spectroscopy. Isolated HoxC was active in catalytic hydrogen–deuterium exchange, demonstrating its capacity to activate H₂. Its catalytic activity was drastically lower than that of the bipartite holoenzyme. This was consistent with infrared and electron paramagnetic resonance spectroscopic observations, suggesting that the bridging position between the active site nickel and iron ions is predominantly occupied by water-derived ligands, even under reducing conditions. In fact, the presence of water-derived ligands bound to low-spin Ni²⁺ was reflected by the absorption bands occurring in the corresponding UV-vis spectra, as revealed by time-dependent density functional theory calculations conducted on appropriate in silico models. Thus, the isolated large subunits indeed represent simple [NiFe]-hydrogenase models, which could serve as blueprints for chemically synthesized mimics. Furthermore, our data point to a fundamental role of the small subunit in preventing water access to the catalytic center, which significantly increases the H₂ splitting capacity of the enzyme.

Spectroscopic investigation of an isolated [NiFe]-hydrogenase large subunit enables a unique view of the NiFe(CO)(CN)₂ cofactor.

Diazaphosphinanes as hydride, hydrogen atom, proton or electron donors under transition-metal-free conditions: thermodynamics, kinetics, and synthetic applications

Exploration of new hydrogen donors is in large demand in hydrogenation chemistry. Herein, we developed a new 1,3,2-diazaphosphinane 1a, which can serve as a hydride, hydrogen atom or proton donor without transition-metal mediation. The thermodynamics and kinetics of these three pathways of 1a, together with those of its analog 1b, were investigated in acetonitrile. It is noteworthy that, the reduction potentials (E_red) of the phosphenium cations 1a-[P]+ and 1b-[P]+ are extremely low, being −1.94 and −2.39 V (vs. Fc^+/0), respectively, enabling corresponding phosphinyl radicals to function as neutral super-electron-donors. Kinetic studies revealed an extraordinarily large kinetic isotope effect KIE(1a) of 31.3 for the hydrogen atom transfer from 1a to the 2,4,6-tri-(tert-butyl)-phenoxyl radical, implying a tunneling effect. Furthermore, successful applications of these diverse P–H bond energetic parameters in organic syntheses were exemplified, shedding light on more exploitations of these versatile and powerful diazaphosphinane reagents in organic chemistry.

A new 1,3,2-diazaphosphinane, serving as a formal hydride, hydrogen-atom or proton donor without transition-metal mediation was exploited thermodynamically and kinetically. And, its promising potentials in versatile syntheses have been demonstrated.

Influence of nitro substituents on the redox, electronic, and proton reduction catalytic behavior of phenolate-based [N2O3]-type cobalt(iii) complexes

We report on the synthesis, redox, electronic, and catalytic behavior of two new cobalt(iii) complexes, namely [CoIII(L1)MeOH] (1) and [CoIII(L2)MeOH] (2). These species contain nitro-rich, phenolate-based pentadentate ligands and present dramatically distinct properties associated with the position in which the -NO2 substituents are installed. Species 1 displays nitro-substituted phenolates, and exhibits irreversible redox response and negligible catalytic activity, whereas 2 has fuctionalized phenylene moieties, shows much improved redox reversibility and catalytic proton reduction activity at low overpotentials. A concerted experimental and theoretical approach sheds some light on these drastic differences.