Molecular electrocatalysts for oxidation of hydrogen using earth-abundant metals: shoving protons around with proton relays

Dynamics and Coordination of a P2N2 Ligand - from Twisted Conformation to Chelation

Based on their general spacial flexibility, their Lewis and Brønsted basicity, and ability to mimic second sphere effects the 1,5-diaza-3,7-diphosphacyclooctane ligand family and their complexes have regained substantial scientific interest. It was now possible to structurally analyze a recently reported member of this family with p-tolyl and t-butyl substituents on P and N, respectively, (P2 p-tolN2 tBu). Notably, the ligand crystallizes with a 'twisted' backbone. This compound is the very first of its kind to have been unambiguously characterized with regard to its chemical and molecular structure as being in this conformation. A temperature-dependent NMR study provides insight into the molecular dynamics of two isomers in solution, which are most likely also both twisted, as judged by the observed limited reactivity. Despite the in principle unfavorable conformation of the free ligand, it was successfully chelated to tungsten and molybdenum centers in mononuclear carbonyl complexes. The ligand, a derivative thereof and four new complexes were comprehensively characterized and analyzed in comparison. This includes single crystal XRD molecular structures of P2 p-tolN2 tBu and all four complexes. P2 p-tolN2 tBu, regardless of its twisted conformation, is able to coordinate to metal centers given that enough energy (heat) for a conformational change is provided.

Molecular Dihydrogen Activation by (C5Me5)M/N (M=Rh, Ir) Transition Metal Frustrated Lewis Pairs: Reversible Proton Migration to, and Proton Abstraction from, the C5Me5 Ligand

The masked transition-metal frustrated Lewis pairs [Cp*M(κ3N,N',N''-L)][SbF6] (Cp*=η5-C5Me5; M=Ir, 1, Rh, 2; HL=pyridinyl-amidine ligand) reversibly activate H2 under mild conditions rendering the hydrido derivatives [Cp*MH(κ2N,N'-HL)][SbF6] observed as a mixture of the E and Z isomers at the amidine C=N bond (M=Ir, 3Z, 3E; M=Rh, 4Z, 4E). DFT calculations indicate that the formation of the E isomers follows a Grotthuss type mechanism in the presence of water. A mixture of Rh(I) isomers of formula [(Cp*H)Rh(κ2N,N'-HL)][SbF6] (5 a-d) is obtained by reductive elimination of Cp*H from 4. The formation of 5 a-d was elucidated by means of DFT calculations. Finally, when 2 reacts with D2, the Cp* and Cp*H ligands of the resulting rhodium complexes 4 and 5, respectively, are deuterated as a result of a reversible hydrogen abstraction from the Cp* ligand and D2 activation at rhodium.

Further Understanding the Roles of Solvent, Brønsted Acids, and Hydrogen Bonding in Iron Porphyrin-Mediated Carbon Dioxide Reduction

Improving our understanding of how molecules and materials mediate the electrochemical reduction of carbon dioxide (CO2) to upgraded products is of great interest as a means to address climate change. A leading class of molecules that can facilitate the electrochemical conversion of CO2 to carbon monoxide (CO) is iron porphyrins. These molecules can have high rate constants for CO2-to-CO conversion; they are robust, and they rely on abundant and inexpensive synthetic building blocks. Important foundational work has been conducted using chloroiron 5,10,15,20-tetraphenylporphyrin (FeTPPCl) in N,N-dimethylformamide (DMF) solvent. A related and recent report points out that the corresponding perchlorate complex, FeTPPClO4, can have superior function due to its solubility in other organic solvents. However, the importance of hydrogen bonding and solvent effects was not discussed. Herein, we present a detailed kinetic study of the triflate (CF3SO3-) complex of FeTPP in DMF and in MeCN using a range of phenol Brønsted acid additives. We also detected the formation of Fe(III)TPP-phenolate complexes using cyclic voltammetry experiments. Importantly, our new analysis of apparent rate constants with different added phenols allows for a modification to the established mechanistic model for CO2-to-CO conversion. Critically, our improved model accounts for hydrogen bonding and solvent effects by using simple hydrogen bond acidity and basicity descriptors. We use this augmented model to rationalize function in other reported porphyrin systems and to make predictions about operational conditions that can enhance the CO2 reduction chemistry.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions

Proton transfer reactions involving transition metal hydride complexes are prevalent in a number of catalytic fuel-forming reactions, where the proton transfer kinetics to or from the metal center can have significant impacts on the efficiency, selectivity, and stability associated with the catalytic cycle. This review correlates the often slow proton transfer rate constants of transition metal hydride complexes to their electronic and structural descriptors and provides perspective on how to exploit these parameters to control proton transfer kinetics to and from the metal center. A toolbox of techniques for experimental determination of proton transfer rate constants is discussed, and case studies where proton transfer rate constant determination informs fuel-forming reactions are highlighted. Opportunities for extending proton transfer kinetic measurements to additional systems are presented, and the importance of synergizing the thermodynamics and kinetics of proton transfer involving transition metal hydride complexes is emphasized.

Structural Determinants of the Catalytic Nia-L Intermediate of [NiFe]-Hydrogenase

[NiFe]-hydrogenases catalyze the reversible cleavage of H₂ into two protons and two electrons at the inorganic heterobimetallic NiFe center of the enzyme. Their catalytic cycle involves at least four intermediates, some of which are still under debate. While the core reaction, including H₂/H^- binding, takes place at the inorganic cofactor, a major challenge lies in identifying those amino acid residues that contribute to the reactivity and how they stabilize (short-lived) intermediate states. Using cryogenic infrared and electron paramagnetic resonance spectroscopy on the regulatory [NiFe]-hydrogenase from Cupriavidus necator, a model enzyme for the analysis of catalytic intermediates, we deciphered the structural basis of the hitherto elusive Ni_a-L intermediates. We unveiled the protonation states of a proton-accepting glutamate and a Ni-bound cysteine residue in the Ni_a-L1, Ni_a-L2, and the hydride-binding Ni_a-C intermediates as well as previously unknown conformational changes of amino acid residues in proximity of the bimetallic active site. As such, this study unravels the complexity of the Ni_a-L intermediate and reveals the importance of the protein scaffold in fine-tuning proton and electron dynamics in [NiFe]-hydrogenase.

Electrocatalytic metal hydride generation using CPET mediators

Transition metal hydrides (M-H) are ubiquitous intermediates in a wide range of enzymatic processes and catalytic reactions, playing a central role in H+/H2 interconversion1, the reduction of CO2 to formic acid (HCOOH)2 and in hydrogenation reactions. The facile formation of M-H is a critical challenge to address to further improve the energy efficiency of these reactions. Specifically, the easy electrochemical generation of M-H using mild proton sources is key to enable high selectivity versus competitive CO and H2 formation in the CO2 electroreduction to HCOOH, the highest value-added CO2 reduction product3. Here we introduce a strategy for electrocatalytic M-H generation using concerted proton-electron transfer (CPET) mediators. As a proof of principle, the combination of a series of CPET mediators with the CO2 electroreduction catalyst [MnI(bpy)(CO)3Br] (bpy = 2,2'-bipyridine) was investigated, probing the reversal of the product selectivity from CO to HCOOH to evaluate the efficiency of the manganese hydride (Mn-H) generation step. We demonstrate the formation of the Mn-H species by in situ spectroscopic techniques and determine the thermodynamic boundary conditions for this mechanism to occur. A synthetic iron-sulfur cluster is identified as the best CPET mediator for the system, enabling the preparation of a benchmark catalytic system for HCOOH generation.

Electrocatalysis with Molecular Transition-Metal Complexes for Reductive Organic Synthesis

Electrocatalysis enables the formation or cleavage of chemical bonds by a genuine use of electrons or holes from an electrical energy input. As such, electrocatalysis offers resource-economical alternative pathways that bypass sacrificial, waste-generating reagents often required in classical thermal redox reactions. In this Perspective, we showcase the exploitation of molecular electrocatalysts for electrosynthesis, in particular for reductive conversion of organic substrates. Selected case studies illustrate that efficient molecular electrocatalysts not only are appropriate redox shuttles but also embrace the features of organometallic catalysis to facilitate and control chemical steps. From these examples, guidelines are proposed for the design of molecular electrocatalysts suited to the reduction of organic substrates. We finally expose opportunities brought by catalyzed electrosynthesis to functionalize organic backbones, namely using sustainable building blocks.

Theoretical Modeling of Electrochemical Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) plays an essential role in a wide range of electrocatalytic processes. A vast array of theoretical and computational methods have been developed to study electrochemical PCET. These methods can be used to calculate redox potentials and pK_a values for molecular electrocatalysts, proton-coupled redox potentials and bond dissociation free energies for PCET at metal and semiconductor interfaces, and reorganization energies associated with electrochemical PCET. Periodic density functional theory can also be used to compute PCET activation energies and perform molecular dynamics simulations of electrochemical interfaces. Various approaches for maintaining a constant electrode potential in electronic structure calculations and modeling complex interactions in the electric double layer (EDL) have been developed. Theoretical formulations for both homogeneous and heterogeneous electrochemical PCET spanning the adiabatic, nonadiabatic, and solvent-controlled regimes have been developed and provide analytical expressions for the rate constants and current densities as functions of applied potential. The quantum mechanical treatment of the proton and inclusion of excited vibronic states have been shown to be critical for describing experimental data, such as Tafel slopes and potential-dependent kinetic isotope effects. The calculated rate constants can be used as input to microkinetic models and voltammogram simulations to elucidate complex electrocatalytic processes.

Proton-Coupled Electron Transfer: The Engine of Energy Conversion and Storage

Proton-coupled electron transfer (PCET) underpins energy conversion in chemistry and biology. Four energy systems are described whose discoveries are based on PCET: the water splitting chemistry of the Artificial Leaf, the carbon fixation chemistry of the Bionic Leaf-C, the nitrogen fixation chemistry of the Bionic Leaf-N and the Coordination Chemistry Flow Battery (CCFB). Whereas the Artificial Leaf, Bionic Leaf-C, and Bionic Leaf-N require strong coupling between electron and proton to reduce energetic barriers to enable high energy efficiencies, the CCFB requires complete decoupling of the electron and proton so as to avoid parasitic energy-wasting reactions. The proper design of PCET in these systems facilitates their implementation in the areas of (i) centralized large scale grid storage of electricity and (ii) decentralized energy storage/conversion using only sunlight, air and any water source to produce fuel and food within a sustainable cycle for the biogenic elements of C, N and P.

Outer-coordination sphere in multi-H+/multi-e-molecular electrocatalysis

Electrocatalysis is an indispensable technique for small-molecule transformations, which are essential for the sustainability of society. Electrocatalysis utilizes electricity as an energy source for chemical reactions. Hydrogen is considered the “fuel for the future,” and designing electrocatalysts for hydrogen production has thus become critical. Furthermore, fuel cells are promising energy solutions that require robust electrocatalysts for key fuel cell reactions such as the interconversion of oxygen to water. Concerns regarding the rising concentration of atmospheric carbon dioxide have prompted the search for CO₂ conversion methods. One promising approach is the electrochemical conversion of CO₂ into commodity chemicals and/or liquid fuels, but such chemistry is highly energy demanding because of the thermodynamic stability of CO₂. All of the above-mentioned electrocatalytic processes rely on the selective input of multiple protons (H⁺) and electrons (e^–) to yield the desired products. Biological enzymes evolved in nature to perform such redox catalysis and have inspired the design of catalysts at the molecular and atomic levels. While it is synthetically challenging to mimic the exact biological environment, incorporating functional outer coordination spheres into molecular catalysts has shown promise for advancing multi-H⁺ and multi-e^– electrocatalysis. From this Perspective, herein, catalysts with outer coordination sphere(s) are selected as the inspiration for developing new catalysts, particularly for the reductive conversion of H⁺, O₂, and CO₂, which are highly relevant to sustainability. The recent progress in electrocatalysis and opportunities to explore beyond the second coordination sphere are also emphasized.

A thiolate-bridged FeIVFeIV μ-nitrido complex and its hydrogenation reactivity toward ammonia formation

Iron nitrides are key intermediates in biological nitrogen fixation and the industrial Haber-Bosch process, used to form ammonia from dinitrogen. However, the proposed successive conversion of nitride to ammonia remains elusive. In this regard, the search for well-described multi-iron nitrido model complexes and investigations on controlling their reactivity towards ammonia formation have long been of great challenge and importance. Here we report a well-defined thiolate-bridged Fe^IVFe^IV μ-nitrido complex featuring an uncommon bent Fe-N-Fe moiety. Remarkably, this complex shows excellent reactivity toward hydrogenation with H₂ at ambient conditions, forming ammonia in high yield. Combined experimental and computational studies demonstrate that a thiolate-bridged Fe^IIIFe^III μ-amido complex is a key intermediate, which is generated through an unusual two-electron oxidation of H₂. Moreover, ammonia production was also realized by treating this diiron μ-nitride with electrons and water as a proton source.

Spin-state crossover in photo-catalyzed nitrile dihydroboration via Mn-thiolate cooperation

The role of a phosphine-free SNS-pincer ligand in metal–ligand cooperative hydroboration catalysis was investigated. The bifunctional thiolate donor and spin-state change to high-spin Mn are crucial to accessing low-energy activation barriers.

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen

The articulation of the notion of "frustrated Lewis pairs" (FLPs) emerged from the discovery that H₂ can be reversibly activated by combinations of sterically encumbered main group Lewis acids and bases. This has prompted numerous studies focused on various perturbations of the Lewis acid/base combinations and the applications to organic reductions. This Perspective focuses on the new directions and developments that are emerging from this FLP chemistry involving hydrogen. Three areas are discussed including new applications and approaches to FLP reductions, the reductions of small molecules, and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chemistry, this simple concept of FLPs is being applied across the discipline.

Reductive C(sp3)-C(sp3) homo-coupling of benzyl or allyl halides with H2 using a water-soluble electron storage catalyst

This paper reports the first example of a reductive C(sp³)-C(sp³) homo-coupling of benzyl/allyl halides in aqueous solution by using H₂ as an electron source {turnover numbers (TONs) = 0.5-2.3 for 12 h}. This homo-coupling reaction, promoted by visible light, is catalysed by a water-soluble electron storage catalyst (ESC). The reaction mechanism, and four requirements to make it possible, are also described.

Modulation of H+/H- exchange in iridium-hydride 2-hydroxypyridine complexes by remote Lewis acids

A series of iridium hydride complexes featuring dihydrogen bonding are presented and shown to undergo rapid H⁺/H^- exchange (1240 s^-1 at 25 °C). We demonstrate that the H⁺/H^- exchange rate can be modified by post-synthetic modification at a remote site using BH₃, Zn(C₆F₅)₂, and [Me₃O][BF₄]. This route provides a complementary strategy to traditional methods that rely on pre-metalation modifications to a metal's primary sphere.

Metal Effect Meets Volcano Plots: A DFT Study on Tris(phosphino)borane-Transition Metal Complexes Catalyzed H2 Activation

Bifunctional transition metal complexes are of particular interest in metal-ligand cooperative activation of small molecules. As a novel type of bifunctional catalyst, Lewis acid transition metal (LA-TM) complexes have attracted increasing interest in hydrogen activation and storage. To advance the catalyst design, herein the metal effect of LA-TM complexes on the hydrogen activation has been systematically studied with a series of tris(phosphino)borane (TPB) complexes with V, Cr, Mn, Fe, Co, and Ni as metal centers. The metal effect not only influences the mechanism of hydrogen activation, but also notably casts a volcano plot for the activity. TPB complexes of V, Cr, Mn, Fe, and Co tend to activate H₂ through a stepwise mechanism, while TPB-Ni prefers a synergetic mechanism for H₂ activation. More importantly, the metal effect significantly influences the activity of H₂ activation and the formation of the LA-H-TM bridging hydride. The trend of changes in the LA-H-TM structures, the second-order perturbation stabilization energies, and the Laplacian bond orders, along with different metals (from V to Ni), are all interestingly constitute volcano plots for the performance of TPB-TM complexes catalyzed H₂ activation. TPB-Mn and TPB-Fe are found to be the optimal catalysts among the discussed TPB-TM complexes. The volcano plots disclosed for the metal effects should be informative and instructive for homogeneous and heterogeneous LA-TM catalysts development.

Same ligand, three first-row metals: comparing M-amido bifunctional reactivity (Mn, Fe, Co)

The bifunctional reactivity of three metal SNS (bis)amido complexes was computationally assessed by comparing the nucleophilicity of the M-N_amido donor (Mn, Fe, Co). Hirshfeld charges identified the Mn-N_amido donor as most nucleophilic and Fe as most electrophilic metal. Reaction energy profiles of a model bifunctional H₂ activation showed Mn with the lowest reaction barrier (17 kcal mol^-1), followed by Fe and Co (21 and 29 kcal mol^-1).

Catalytic Aldehyde and Alcohol Arylation Reactions Facilitated by a 1,5-Diaza-3,7-diphosphacyclooctane Ligand

We report a catalytic method to access secondary alcohols by the coupling of aryl iodides. Either aldehydes or alcohols can be used as reaction partners, making the transformation reductive or redox-neutral, respectively. The reaction is mediated by a Ni catalyst and a 1,5-diaza-3,7-diphosphacyclooctane. This P₂N₂ ligand, which has previously been unrecognized in cross-coupling and related reactions, was found to avoid deleterious aryl halide reduction pathways that dominate with more traditional phosphines and NHCs. An interrupted carbonyl-Heck type mechanism is proposed to be operative, with a key 1,2-insertion step forging the new C-C bond and forming a nickel alkoxide that may be turned over by an alcohol reductant. The same catalyst was also found to enable synthesis of ketone products from either aldehydes or alcohols, demonstrating control over the oxidation state of both the starting materials and products.

Vibrational Perturbation of the [FeFe] Hydrogenase H-Cluster Revealed by 13C2H-ADT Labeling

[FeFe] hydrogenases are highly active catalysts for the interconversion of molecular hydrogen with protons and electrons. Here, we use a combination of isotopic labeling, ⁵⁷Fe nuclear resonance vibrational spectroscopy (NRVS), and density functional theory (DFT) calculations to observe and characterize the vibrational modes involving motion of the 2-azapropane-1,3-dithiolate (ADT) ligand bridging the two iron sites in the [2Fe]_H subcluster. A -¹³C²H₂- ADT labeling in the synthetic diiron precursor of [2Fe]_H produced isotope effects observed throughout the NRVS spectrum. The two precursor isotopologues were then used to reconstitute the H-cluster of [FeFe] hydrogenase from Chlamydomonas reinhardtii (CrHydA1), and NRVS was measured on samples poised in the catalytically crucial H_hyd state containing a terminal hydride at the distal Fe site. The ¹³C²H isotope effects were observed also in the H_hyd spectrum. DFT simulations of the spectra allowed identification of the ⁵⁷Fe normal modes coupled to the ADT ligand motions. Particularly, a variety of normal modes involve shortening of the distance between the distal Fe-H hydride and ADT N-H bridgehead hydrogen, which may be relevant to the formation of a transition state on the way to H₂ formation.

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

Copper catalysts for photo- and electro-catalytic hydrogen production

Square planar 1, square pyramidal 2 and trigonal bipyramidal 3 copper complexes are poor catalysts for hydrogen evolution (HER) under photocatalytic conditions, whereas 1 is, or forms, a good and enduring electrocatalyst for HER, but 2 and 3 do not.

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.

Electrocatalytic Proton Reduction by a Cobalt(III) Hydride Complex with Phosphinopyridine PN Ligands

Cobalt complexes with 2-(diisopropylphosphinomethyl)pyridine (PN) ligands have been synthesized with the aim of demonstrating electrocatalytic proton reduction to dihydrogen with a well-defined hydride complex of an Earth-abundant metal. Reactions of simple cobalt precursors with 2-(diisopropylphosphino-methyl)pyridine (PN) yield [Co^II(PN)₂(MeCN)][BF₄]₂ 1, [Co^III(PN)₂(H)(MeCN)][PF₆]₂ 2, and [Co^III(PN)₂(H)(Cl)][PF₆] 3. Complexes 1 and 3 have been characterized crystallographically. Unusually for a bidentate PN ligand, all three exhibit geometries with mutually trans phosphorus and nitrogen ligands. Complex 1 exhibits a distorted square-pyramidal geometry with an axial MeCN ligand in a low-spin electronic state. In complexes 2 and 3, the PN ligands lie in a plane leaving the hydride trans to MeCN or chloride, respectively. The redox behavior of the three complexes has been studied by cyclic voltammetry at variable scan rates and by spectroelectrochemistry. A catalytic wave is observed in the presence of trifluoroacetic acid (TFA) at an applied potential close to the Co(II/I) couple of 1. Bulk electrolysis of 1, 2, or 3 at a potential of ca. -1.4 V vs E(Fc⁺/Fc) in the presence of TFA yields H₂ with Faradaic yields close to 100%. A catalytic mechanism is proposed in which the pyridine moiety of a PN ligand acts as a pendant proton donor following opening of the chelate ring. Additional mechanisms may also operate, especially in the presence of high acid concentration where speciation changes.

A NiRhS fuel cell catalyst - lessons from hydrogenase

We present a novel fuel cell heterogeneous catalyst based on rhodium, nickel and sulfur with power densities 5-28% that of platinum. The NiRhS heterogeneous catalyst was developed via a homogeneous model complex of the [NiFe]hydrogenases (H₂ases) and can act as both the cathode and anode of a fuel cell.

Synthesis and Structure of Iron (II) Complexes of Functionalized 1,5-Diaza-3,7-Diphosphacyclooctanes

In order to synthesize new iron (II) complexes of 1,5-diaza-3,7-diphosphacyclooctanes with a wider variety of the substituents on ligands heteroatoms (including functionalized ones, namely, pyridyl groups) and co-ligands, it was found that these ligands with relatively small phenyl, benzyl, and pyridin-2-yl substituents on phosphorus atoms in acetonitrile formed bis-P,P-chelate cis-complexes [L₂Fe(CH₃CN)₂]²⁺ (BF₄)₂⁻, whereas P-mesityl-substituted ligand formed only monoligand P,P-complex [LFe(CH₃CN)₄]²⁺ (BF₄)₂⁻. 3,7-dibenzyl-1,5-di(1′-(R)-phenylethyl)-1,5-diaza-3,7-diphosphacyclooctane reacted with hexahydrate of iron (II) tetrafluoroborate in acetone to give an unusual bis-ligand cationic complex of the composition [L₂Fe(BF₄)]⁺ BF₄⁻, where two fluorine atoms of the tetrafluoroborate unit occupied two pseudo-equatorial positions at roughly octahedral iron ion, according to X-ray diffraction data. 1,5-diaza-3,7-diphosphacyclooctanes replaced tetrahydrofurane and one of the carbonyl ligands of cyclopentadienyldicarbonyl(tetrahydrofuran)iron (II) tetrafluoroborate to form heteroligand complexes [CpFeL(CO)]⁺BF₄⁻. The structural studies in the solid phase and in solutions showed that diazadiphosphacyclooctane ligands of all complexes adopted chair-boat conformations so that their nitrogen atoms were in close proximity to the central iron ion. The redox properties of the obtained complexes were performed by the cyclic voltammetry method.

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.

[NiFe], [FeFe], and [Fe] hydrogenase models from isomers

The study of hydrogenase enzymes (H₂ases) is necessary because of their importance to a future hydrogen energy economy. These enzymes come in three distinct classes: [NiFe] H₂ases, which have a propensity toward H₂ oxidation; [FeFe] H₂ases, which have a propensity toward H₂ evolution; and [Fe] H₂ases, which catalyze H^- transfer. Modeling these enzymes has so far treated them as different species, which is understandable given the different cores and ligand sets of the natural molecules. Here, we demonstrate, using x-ray analysis and nuclear magnetic resonance, infrared, Mössbauer spectroscopies, and electrochemical measurement, that the catalytic properties of all three enzymes can be mimicked with only three isomers of the same NiFe complex.

Heterogeneous aqueous CO2 reduction by rhenium(i) tricarbonyl diimine complexes with a non-chelating pendant pyridyl group

A graphite-adsorbed tricarbonylrhenium(i) terpyridine complex supports CO₂ reduction electrocatalysis over a wide range of pH values.

Catalytic H2 evolution/oxidation in [FeFe]-hydrogenase biomimetics: account from DFT on the interplay of related issues and proposed solutions

A DFT overview on selected issues regarding diiron catalysts related to [FeFe]-hydrogenase biomimetic research, with implications for both energy conversion and storage strategies.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

Thermodynamic and kinetic studies of H2 and N2 binding to bimetallic nickel-group 13 complexes and neutron structure of a Ni(η2-H2) adduct

Understanding H2 binding and activation is important in the context of designing transition metal catalysts for many processes, including hydrogenation and the interconversion of H2 with protons and electrons. This work reports the first thermodynamic and kinetic H2 binding studies for an isostructural series of first-row metal complexes: NiML, where M = Al (1), Ga (2), and In (3), and L = [N(o-(NCH2PiPr2)C6H4)3]3-. Thermodynamic free energies (ΔG°) and free energies of activation (ΔG ‡) for binding equilibria were obtained via variable-temperature 31P NMR studies and lineshape analysis. The supporting metal exerts a large influence on the thermodynamic favorability of both H2 and N2 binding to Ni, with ΔG° values for H2 binding found to span nearly the entire range of previous reports. The non-classical H2 adduct, (η2-H2)NiInL (3-H2), was structurally characterized by single-crystal neutron diffraction-the first such study for a Ni(η2-H2) complex or any d10 M(η2-H2) complex. UV-Vis studies and TD-DFT calculations identified specific electronic structure perturbations of the supporting metal which poise NiML complexes for small-molecule binding. ETS-NOCV calculations indicate that H2 binding primarily occurs via H-H σ-donation to the Ni 4p z -based LUMO, which is proposed to become energetically accessible as the Ni(0)→M(iii) dative interaction increases for the larger M(iii) ions. Linear free-energy relationships are discussed, with the activation barrier for H2 binding (ΔG ‡) found to decrease proportionally for more thermodynamically favorable equilibria. The ΔG° values for H2 and N2 binding to NiML complexes were also found to be more exergonic for the larger M(iii) ions.