Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

Bimetallic H2 Addition and Intramolecular Caryl-H Activation Mediated by an Iron-Zinc Hydride

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy2PC6H4)HZnEt (3) undergoes reversible intramolecular Caryl-H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy2PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D)2-Zn upon reaction with D2.

Two-Electron Mixed Valency in a Heterotrimetallic Nickel-Vanadium-Nickel Complex

Mixed-valence complexes represent an enticing class of coordination compounds to interrogate electron transfer confined within a molecular framework. The diamagnetic heterotrimetallic anion, [V(SNS)₂{Ni(dppe)}₂]^-, was prepared by reducing (dppe)NiCl₂ in the presence of the chelating metalloligand [V(SNS)₂]^- [dppe = bis(diphenylphosphino)ethane; (SNS)^3- = bis(2-thiolato-4-methylphenyl)amide]. Vanadium-nickel bonds span the heterotrimetallic core in the structure of [V(SNS)₂{Ni(dppe)}₂]^-, with V-Ni bond lengths of 2.78 and 2.79 Å. One-electron oxidation of monoanionic [V(SNS)₂{Ni(dppe)}₂]^- yielded neutral, paramagnetic V(SNS)₂{Ni(dppe)}₂. The solid-state structure of V(SNS)₂{Ni(dppe)}₂ revealed that the two nickel ions occupy unique coordination environments: one nickel is in a square-planar S₂P₂ coordination environment (τ₄ = 0.19), with a long Ni···V distance of 3.45 Å; the other nickel is in a tetrahedral S₂P₂ coordination environment (τ₄ = 0.84) with a short Ni-V distance of 2.60 Å, consistent with a formal metal-metal bond. Continuous-wave X-band electron paramagnetic resonance spectroscopy, electrochemical investigations, and density functional theory computations indicated that the unpaired electron in the neutral V(SNS)₂{Ni(dppe)}₂ cluster is localized on the bridging [V(SNS)₂] metalloligand, and as a result, V(SNS)₂{Ni(dppe)}₂ is best described as a two-electron mixed-valence complex. These results demonstrate the important role that metal-metal interactions and flexible coordination geometries play in enabling multiple, reversible electron transfer processes in small cluster complexes.

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.

Second and Outer Coordination Sphere Effects in Nitrogenase, Hydrogenase, Formate Dehydrogenase, and CO Dehydrogenase

Gases like H2, N2, CO2, and CO are increasingly recognized as critical feedstock in "green" energy conversion and as sources of nitrogen and carbon for the agricultural and chemical sectors. However, the industrial transformation of N2, CO2, and CO and the production of H2 require significant energy input, which renders processes like steam reforming and the Haber-Bosch reaction economically and environmentally unviable. Nature, on the other hand, performs similar tasks efficiently at ambient temperature and pressure, exploiting gas-processing metalloenzymes (GPMs) that bind low-valent metal cofactors based on iron, nickel, molybdenum, tungsten, and sulfur. Such systems are studied to understand the biocatalytic principles of gas conversion including N2 fixation by nitrogenase and H2 production by hydrogenase as well as CO2 and CO conversion by formate dehydrogenase, carbon monoxide dehydrogenase, and nitrogenase. In this review, we emphasize the importance of the cofactor/protein interface, discussing how second and outer coordination sphere effects determine, modulate, and optimize the catalytic activity of GPMs. These may comprise ionic interactions in the second coordination sphere that shape the electron density distribution across the cofactor, hydrogen bonding changes, and allosteric effects. In the outer coordination sphere, proton transfer and electron transfer are discussed, alongside the role of hydrophobic substrate channels and protein structural changes. Combining the information gained from structural biology, enzyme kinetics, and various spectroscopic techniques, we aim toward a comprehensive understanding of catalysis beyond the first coordination sphere.

Hydrogen evolution, electron-transfer, and hydride-transfer reactions in a nickel-iron hydrogenase model complex: a theoretical study of the distinctive reactivities for the conformational isomers of nickel-iron hydride

Hydrogen fuel is a promising alternative to fossil fuel. Therefore, efficient hydrogen production is crucial to elucidate the distinctive reactivities of metal hydride species, the intermediates formed during hydrogen activation/evolution in the presence of organometallic catalysts. This study uses density functional theory (DFT) to investigate the isomerizations and reactivities of three nickel-iron (NiFe) hydride isomers synthesized by mimicking the active center of NiFe hydrogenase. Hydride transfer within these complexes, rather than a chemical reaction between the complexes, converts the three hydrides internally. Their reactivities, including their electron-transfer, hydride-transfer and proton-transfer reactions, are investigated. The bridging hydride complex exhibits a higher energy level for the highest occupied molecular orbital (HOMO) than the terminal hydride during the electron-transfer reaction. This energy level indicates that the bridging hydride is more easily oxidized and is more susceptible to electron transfer than the terminal hydride. Regarding the hydride-transfer reaction between the NiFe hydride complex and methylene blue, the terminal hydrides exhibit larger hydricity and lower reaction barriers than the bridging hydride complexes. The results of energy decomposition analysis indicate that the structural deformation energy of the terminal hydride in the transition state is smaller than that of the bridging hydride complex, which lowers the reaction barrier of hydride transfer in the terminal hydride. To produce hydrogen, the rate-determining step is represented by the protonation of the hydride, and the terminal hydrides are thermodynamically and kinetically superior to the bridging ones. The differences in the reactivities of the hydride isomers ensure the precise control of hydrogen, and the theoretical calculations can be applied to design catalysts for hydrogen activation/production.

Introducing Water-Network-Assisted Proton Transfer for Boosted Electrocatalytic Hydrogen Evolution with Cobalt Corrole

Proton transfer is vital for many biological and chemical reactions. Hydrogen-bonded water-containing networks are often found in enzymes to assist proton transfer, but similar strategy has been rarely presented by synthetic catalysts. We herein report the Co corrole 1 with an appended crown ether unit and its boosted activity for the hydrogen evolution reaction (HER). Crystallographic and 1H NMR studies proved that the crown ether of 1 can grab water via hydrogen bonds. By using protic acids as proton sources, the HER activity of 1 was largely boosted with added water, while the activity of crown-ether-free analogues showed very small enhancement. Inhibition studies by adding (1) external 18-crown-6-ether to extract water molecules and (2) potassium ion or N-benzyl-n-butylamine to block the crown ether of 1 further confirmed its critical role in assisting proton transfer via grabbed water molecules. This work presents a synthetic example to boost HER through water-containing networks.

Modeling ligand electrochemical parameters by repulsion-corrected eigenvalues

Ligand electrochemical parameters, E_L , more commonly known as Lever parameters, have played a major research role in understanding redox processes involved in inorganic electrochemistry, enzymatic reactions, catalysis, solar cells, biochemistry, and materials science. Despite their broad usefulness, Lever parameters are not well understood at a first-principles level. Using density functional theory, we demonstrate in this contribution that a ligand's Lever parameter is fundamentally related to the ligand's ability to alter the eigenvalue of the electroactive spin-orbital in an octahedral transition metal complex. Our analysis furthers a first-principles understanding of the nature of Lever parameters.

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.

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.

Identifying a Real Catalyst of [NiFe]-Hydrogenase Mimic for Exceptional H2 Photogeneration

Inspired by the natural [NiFe]-H₂ ase, we designed mimic 1, (dppe)Ni(μ-pdt)(μ-Cl)Ru(CO)₂ Cl to realize effective H₂ evolution under photocatalytic conditions. However, a new species 2 was captured in the course of photo-, electro-, and chemo- one-electron reduction. Experimental studies of in situ IR spectroscopy, EPR, NMR, X-ray absorption spectroscopy, and DFT calculations corroborated a dimeric structure of 2 as a closed-shell, symmetric structure with a Ru^I center. The isolated dimer 2 showed the real catalytic role in photocatalysis with a benchmark turnover frequency (TOF) of 1936 h^-1 for H₂ evolution, while mimic 1 worked as a pre-catalyst and evolved H₂ only after being reduced to 2. The remarkably catalytic activity and unique dimer structure of 2 operated in photocatalysis unveiled a broad research prospect in hydrogenases mimics for advanced H₂ evolution.

Selective Oxidation of H2 and CO by NiIr Catalyst in Aqueous Solution: A DFT Mechanistic Study

One of the challenges in utilizing hydrogen gas (H₂) as a sustainable fossil fuel alternative is the inhibition of H₂ oxidation by carbon monoxide (CO), which is involved in the industrial production of H₂ sources. To solve this problem, a catalyst that selectively oxidizes either CO or H₂ or one that co-oxidizes H₂ and CO is needed. Recently, a NiIr catalyst [Ni^IICl(X)Ir^IIICl(η⁵-C₅Me₅)], (X = N,N'-dimethyl-3,7-diazanonane-1,9-dithiolate), which efficiently and selectively oxidizes either H₂ or CO depending on the pH, has been developed (Angew. Chem. Int. Ed. 2017, 56, 9723-9726). In the present work, density functional theory (DFT) calculations are employed to elucidate the pH-dependent reaction mechanisms of H₂ and CO oxidation catalyzed by this NiIr catalyst. During H₂ oxidation, our calculations suggest that dihydrogen binds to the Ir center and generates an Ir(III)-dihydrogen complex, followed by subsequent isomerization to an Ir(V)-dihydride species. Then, a proton is abstracted by a buffer base, CH₃COO^-, resulting in the formation of a hydride complex. The catalytic cycle completes with electron transfer from the hydride complex to a protonated 2,6-dichlorobenzeneindophenol (DCIP) and a proton transfer from the oxidized hydride complex to a buffer base. The CO oxidation mechanism involves three distinct steps, i.e., (1) formation of a metal carbonyl complex, (2) formation of a metallocarboxylic acid, and (3) conversion of the metallocarboxylic acid to a hydride complex. The formation of the metallocarboxylic acid involves nucleophilic attack of OH^- to the carbonyl-C followed by a large structural change with concomitant cleavage of the Ir-S bond and rotation of the COOH group along the NiIr axis. During the conversion of the metallocarboxylic acid to the hydride complex, intramolecular proton transfer followed by removal of CO₂ leads to the formation of the hydride complexes. In addition, the barrier heights for the binding of small molecules (H₂, OH^-, H₂O, and CO) to Ir were calculated, and the results indicated that dissociation from Ir is a faster process than the binding of H₂O and H₂. These calculations indicate that H₂ oxidation is inhibited by CO and OH^- and thus prefers acidic conditions. In contrast, the CO oxidation reactions occur more favorably under basic conditions, as the formation of the metallocarboxylic acid involves OH^- attack to a carbonyl-C and the binding of OH^- to Ni largely stabilizes the triplet spin state of the complex. Taken together, these calculations provide a rationale for the experimentally observed pH-dependent, selective oxidations of H₂ and CO.

Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates

Described are the syntheses of several Ni(μ-SR)₂Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS₃)^- and Ni(NS₃)^- (PS₃^3- = tris(phenyl-2-thiolato)phosphine, NS₃^3- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(C₆F₅)₂. The following new nickel precursor complexes were characterized: PPh₄[Ni(NS₃)] and the dimeric imino/amino-phosphine complexes [NiCl₂(PCH═N^An)]₂ and [NiCl₂(PCH₂NH^An)]₂ (P = Ph₂PC₆H₄-2-). The iron(II) reagents include [CpFe(CO)₂(thf)]BF₄, [Cp*Fe(CO)(MeCN)₂]BF₄, FeI₂(CO)₄, FeCl₂(diphos)(CO)₂, and Fe(pdt)(CO)₂(diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp*Fe(CO)Ni(NS₃), [Cp(CO)Fe(μ-pdt)Ni(dppbz)]BF₄, [( R,R-DIPAMP)Ni(μ-pdt)(H)Fe(CO)₃]BAr^F₄, [(PCH═N^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH₂NH^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH═N^An)Ni(μ-pdt)(H)Fe(dppbz)(CO)]BF₄, [(dppv)(CO)Fe(μ-pdt)]₂Ni, {H[(dppv)(CO)Fe(μ-pdt)]₂Ni]}BF₄, and (C₆F₅)₂Ni(μ-pdt)Fe(CO)₂(dppv) (DIPAMP = (CH₂P(C₆H₄-2-OMe)₂)₂; BAr^F₄^- = [B(C₆H₃-3,5-(CF₃)₂]₄^-)) Within the context of Ni-(SR)₂-Fe complexes, these new complexes feature new microenvironments for the nickel center: tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(μ-pdt)]₂Ni}⁺, four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR)₄ site, as supported by density functional theory calculations.

(Spectro)electrochemical and Electrocatalytic Investigation of 1,1'-Dithiolatoferrocene-Hexacarbonyldiiron

Hexacarbonyldiiron bridged by a 1,1'-dithiolatoferrocene, [Fe(C₅H₄S)₂{Fe(CO)₃}₂] (1), was synthesized, and the electrochemistry showed reversible oxidation at the Fe(C₅H₄S)₂ site and quasi-reversible reduction at the hexacarbonyldiiron site. Spectroelectrochemical techniques showed reduction-induced ligand isomerization, where the thiolate ligand went from bridging to terminal and one carbon monoxide ligand moved to a quasi-bridging position; this mechanism was further supported by cyclic voltammetry simulation and density functional theory calculations. Complex 1 showed electrocatalytic activity toward hydrogen-evolving reaction.

Bimetallic nickel-cobalt hydrides in H2 activation and catalytic proton reduction

The synergism of the electronic properties of nickel and cobalt enables bimetallic NiCo complexes to process H2. The nickel-cobalt hydride [(dppe)Ni(pdt)(H)CoCp*]+ ([1H]+ ) arising from protonation of the reduced state 1 was found to be an efficient electrocatalyst for H2 evolution with Cl2CHCOOH, and the oxidized [Ni(ii)Co(iii)]2+ form is capable of activating H2 to produce [1H]+ . The features of stereodynamics, acid-base properties, redox chemistry and reactivity of these bimetallic NiCo complexes in processing H2 are potentially related to the active site of [NiFe]-H2ases.

Nickel-Iron Dithiolato Hydrides Derived from H2 Activation by Their μ-Hydroxo Ligand-Containing Analogues

Here, we describe the synthesis and characterization of the two μ-hydrido models of [NiFe]H₂ases prepared from a new type of H₂ activation by the corresponding two μ-hydroxo models. In addition, another μ-hydrido model prepared by the anionic exchange between one of the two μ-hydrido models and KPF₆ is also reported. Interestingly, the synthesis of the two μ-hydrido models from H₂ activation represents the first functional modeling of the H₂ activation catalyzed by the [NiFe]H₂ase (Ni-SIr)_I state to give the Ni-R state.

Electrocatalytic hydrogen evolution with gallium hydride and ligand-centered reduction

Ga^III porphyrin is active for electrocatalytic hydrogen evolution with unusual features, including ligand-centered electron transfer and formation of post-transition metal hydride.

Ga^III chloride tetrakis(pentafluorophenyl)porphyrin (1) was synthesized and shown to be a highly active and stable post-transition metal-based electrocatalyst for the hydrogen evolution reaction (HER). Electrochemical and spectroscopic studies indicate that both the first and second reduction events of 1 are ligand-centered. The 2e-reduced form 1^2– reacts with a proton to give Ga^III–H species (1–H), which undergoes protonolysis with Brønsted acids to produce H₂. The identification of key intermediates 1^–, 1^2–, and 1–H leads to a catalytic cycle, which is valuable for the fundamental understanding of the HER. This study presents a rare but highly active HER electrocatalyst with unusual features, including ligand-centered electron transfer and formation of post-transition metal hydride.

Comprehensive reaction mechanisms at and near the Ni-Fe active sites of [NiFe] hydrogenases

[NiFe] hydrogenase (H2ase) catalyzes the oxidation of dihydrogen to two protons and two electrons and/or its reverse reaction. For this simple reaction, the enzyme has developed a sophisticated but intricate mechanism with heterolytic cleavage of dihydrogen (or a combination of a hydride and a proton), where its Ni-Fe active site exhibits various redox states. Recently, thermodynamic parameters of the acid-base equilibrium for activation-inactivation, a new intermediate in the catalytic reaction, and new crystal structures of [NiFe] H2ases have been reported, providing significant insights into the activation-inactivation and catalytic reaction mechanisms of [NiFe] H2ases. This Perspective provides an overview of the reaction mechanisms of [NiFe] H2ases based on these new findings.

Electron-Rich, Diiron Bis(monothiolato) Carbonyls: C-S Bond Homolysis in a Mixed Valence Diiron Dithiolate

The synthesis and redox properties are presented for the electron-rich bis(monothiolate)s Fe₂(SR)₂(CO)₂(dppv)₂ for R = Me ([1]⁰), Ph ([2]⁰), CH₂Ph ([3]⁰). Whereas related derivatives adopt C₂-symmetric Fe₂(CO)₂P₄ cores, [1]⁰-[3]⁰ have C_s symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [1]⁰-[3]⁰ undergo 1e^- oxidation upon treatment with ferrocenium salts to give the mixed valence cations [Fe₂(SR)₂(CO)₂(dppv)₂]⁺. As established crystallographically, [3]⁺ adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [1]⁺ and [2]⁺ and many other [Fe₂(SR)₂L₆]⁺ derivatives, [3]⁺ undergoes C-S bond homolysis, affording the diferrous sulfido-thiolate [Fe₂(SCH₂Ph)(S)(CO)₂(dppv)₂]⁺ ([4]⁺). According to X-ray crystallography, the first coordination spheres of [3]⁺ and [4]⁺ are similar, but the Fe-sulfido bonds are short in [4]⁺. The conversion of [3]⁺ to [4]⁺ follows first-order kinetics, with k = 2.3 × 10^-6 s^-1 (30 °C). When the conversion is conducted in THF, the organic products are toluene and dibenzyl. In the presence of TEMPO, the conversion of [3]⁺ to [4]⁺ is accelerated about 10×, the main organic product being TEMPO-CH₂Ph. DFT calculations predict that the homolysis of a C-S bond is exergonic for [Fe₂(SCH₂Ph)₂(CO)₂(PR₃)₄]⁺ but endergonic for the neutral complex as well as less substituted cations. The unsaturated character of [4]⁺ is indicated by its double carbonylation to give [Fe₂(SCH₂Ph)(S)(CO)₄(dppv)₂]⁺ ([5]⁺), which adopts a bioctahedral structure.

A mechanism study on the hydrogen evolution reaction catalyzed by molybdenum disulfide complexes

Water-mediated intermolecular H⁺/H⁻ coupling between two- or three-electron reduced sulfur hydride complexes with a hydrated proton is preferred to produce H₂ rather than intramolecular couplings between sulfur hydride and metal hydride complexes.

Interplay between Terminal and Bridging Diiron Hydrides in Neutral and Oxidized States

This study describes the structural, spectroscopic, and electrochemical properties of electronically unsymmetrical diiron hydrides. The terminal hydride Cp*Fe(pdt)Fe(dppe)(CO)H ([1(t-H)]⁰, Cp*^- = Me₅C₅^-, pdt^2- = CH₂(CH₂S^-)₂, dppe = Ph₂PC₂H₄PPh₂) was prepared by hydride reduction of [Cp*Fe(pdt)Fe(dppe)(CO)(NCMe)]⁺. As established by X-ray crystallography, [1(t-H)]⁰ features a terminal hydride ligand. Unlike previous examples of terminal diiron hydrides, [1(t-H)]⁰ does not isomerize to the bridging hydride [1(μ-H)]⁰. Oxidation of [1(t-H)]⁰ gives [1(t-H)]⁺, which was also characterized crystallographically as its BF₄^- salt. Density functional theory (DFT) calculations indicate that [1(t-H)]⁺ is best described as containing an Cp*Fe^III center. In solution, [1(t-H)]⁺ isomerizes to [1(μ-H)]⁺, as anticipated by DFT. Reduction of [1(μ-H)]⁺ by Cp₂Co afforded the diferrous bridging hydride [1(μ-H)]⁰. Electrochemical measurements and DFT calculations indicate that the couples [1(t-H)]^+/0 and [1(μ-H)]^+/0 differ by 210 mV. Qualitative measurements indicate that [1(t-H)]⁰ and [1(μ-H)]⁰ are close in free energy. Protonation of [1(t-H)]⁰ in MeCN solution affords H₂ even with weak acids via hydride transfer. In contrast, protonation of [1(μ-H)]⁰ yields 0.5 equiv of H₂ by a proposed protonation-induced electron transfer process. Isotopic labeling indicates that μ-H/D ligands are inert.

Synthetic Models for Nickel-Iron Hydrogenase Featuring Redox-Active Ligands

The nickel-iron hydrogenase enzymes efficiently and reversibly interconvert protons, electrons, and dihydrogen. These redox proteins feature iron-sulfur clusters that relay electrons to and from their active sites. Reported here are synthetic models for nickel-iron hydrogenase featuring redox-active auxiliaries that mimic the iron-sulfur cofactors. The complexes prepared are NiII(μ-H)FeIIFeII species of formula [(diphosphine)Ni(dithiolate)(μ-H)Fe(CO)2(ferrocenylphosphine)]+ or NiIIFeIFeII complexes [(diphosphine)Ni(dithiolate)Fe(CO)2(ferrocenylphosphine)]+ (diphosphine = Ph2P(CH2)2PPh2 or Cy2P(CH2)2PCy2; dithiolate = -S(CH2)3S-; ferrocenylphosphine = diphenylphosphinoferrocene, diphenylphosphinomethyl(nonamethylferrocene) or 1,1'-bis(diphenylphosphino)ferrocene). The hydride species is a catalyst for hydrogen evolution, while the latter hydride-free complexes can exist in four redox states - a feature made possible by the incorporation of the ferrocenyl groups. Mixed-valent complexes of 1,1'-bis(diphenylphosphino)ferrocene have one of the phosphine groups unbound, with these species representing advanced structural models with both a redox-active moiety (the ferrocene group) and a potential proton relay (the free phosphine) proximal to a nickel-iron dithiolate.

Tuning the overpotential of electrocatalytically active cyclopentadienylnickel complexes containing 1,4-diaza-1,3-butadienes (DAB) for proton reduction

The complexes [CpNi(DAB)]BF₄, Cp = cyclopentadienyl, DAB = substituted 1,4-diaza-1,3-butadienes, undergo two reduction processes. EPR spectroscopically active Ni(i) intermediates were observed despite the potential redox activity of the DAB ligands and these reduced species catalyse the conversion of electrons and protons (from acetic acid) into dihydrogen.

Dithiolato- and halogenido-bridged nickel–iron complexes related to the active site of [NiFe]-H2ases: preparation, structures, and electrocatalytic H2 production

A new series of [NiFe]-H₂ase mimics (5a,b–7a,b) has been prepared and structurally characterized; particularly, they have been found to be pre-catalysts for H₂ production from Cl₂CHCO₂H under CV conditions.

Heteronuclear assembly of Ni–Cu dithiolato complexes: synthesis, structures, and reactivity studies

A mild route was discovered to synthesize heterometallic [Ni^IICu^I] complexes featuring square-planar Ni(ii) and distorted tetrahedral Cu(i).

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.