Role of the azadithiolate cofactor in models for [FeFe]-hydrogenase: novel structures and catalytic implications

Triiron Complexes Featuring Azadiphosphine Related to the Active Site of [FeFe]-Hydrogenases: Their Redox Behavior and Protonation

The design of iron clusters featuring a bimetallic core and several protonation sites in the second coordination sphere of the metal centers is important for modeling the activity of polymetallic active sites such as the H-cluster of [FeFe]-hydrogenases. For this purpose, the syntheses of complexes [Fe3(CO)5(κ2-PPh2NR2)(μ-pdt)2] (R = Ph (1), Bn (2)) and [Fe3(CO)5(κ2-PPh2NR2)(μ-adtBn)(μ-pdt)] (R = Ph (3), Bn (4)) were carried out by reacting hexacarbonyl precursors [Fe2(CO)6(µ-xdt)] (xdt = pdt (propanedithiolate), adtBn (azadithiolate) with mononuclear complexes [Fe(κ2-pdt)(CO)2(κ2-PPh2NR2)] (PPh2NR2 = (PPhCH2NRCH2)2, R = Ph, Bn) in order to introduce amine functions, through well-known PPh2NR2 diphosphine, into the vicinity of the triiron core. The investigation of the reactivity of these triiron species towards the proton (in the presence of CF3SO3H) and the influence of the pendant amines on the redox properties of these complexes were explored using spectroscopic and electrochemical methods. The protonation sites in such triiron clusters and their relationships were identified. The orientation of the first and second protonation processes depends on the arrangement of the second coordination sphere. The similarities and differences, due to the extended metal nuclearity, with their dinuclear counterparts [Fe2(CO)4(κ2-PPh2NR2)(μ-pdt)], were highlighted.

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as "P₂N₂" ligands, have profound effects on the reactivity of many catalysts. The resulting [Ni(P^R₂N^R'₂)₂]²⁺ complexes are electrocatalysts for both the oxidation and production of H₂. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with [Ni(P^R₂N^R'₂)₂]²⁺ complexes for the oxidation and production of H₂, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.

Inhibition of [FeFe]-hydrogenase by formaldehyde: proposed mechanism and reactivity of FeFe alkyl complexes

The mechanism for inhibition of [FeFe]-hydrogenases by formaldehyde is examined with model complexes. Key findings: (i) CH2 donated by formaldehyde covalently link Fe and the amine cofactor, blocking the active site and (ii) the resulting Fe-alkyl is a versatile electrophilic alkylating agent. Solutions of Fe2[(μ-SCH2)2NH](CO)4(PMe3)2 (1) react with a mixture of HBF4 and CH2O to give three isomers of [Fe2[(μ-SCH2)2NCH2](CO)4(PMe3)2]+ ([2]+). X-ray crystallography verified the NCH2Fe linkage to an octahedral Fe(ii) site. Although [2]+ is stereochemically rigid on the NMR timescale, spin-saturation transfer experiments implicate reversible dissociation of the Fe-CH2 bond, allowing interchange of all three diastereoisomers. Using 13CH2O, the methylenation begins with formation of [Fe2[(μ-SCH2)2N13CH2OH](CO)4(PMe3)2]+. Protonation converts this hydroxymethyl derivative to [2]+, concomitant with 13C-labelling of all three methylene groups. The Fe-CH2N bond in [2]+ is electrophilic: PPh3, hydroxide, and hydride give, respectively, the phosphonium [Fe2[(μ-SCH2)2NCH2PPh3](CO)4(PMe3)2]+, 1, and the methylamine Fe2[(μ-SCH2)2NCH3](CO)4(PMe3)2. The reaction of [Fe2[(μ-SCH2)2NH](CN)2(CO)4]2- with CH2O/HBF4 gave [Fe2[(μ-SCH2)2NCH2CN](CN)(CO)5]- ([4]-), the result of reductive elimination from [Fe2[(μ-SCH2)2NCH2](CN)2(CO)4]-. The phosphine derivative [Fe2[(μ-SCH2)2NCH2CN](CN)(CO)4(PPh3)]- ([5]-) was characterized crystallographically.

Mononuclear nickel(II) dithiolate complexes with chelating diphosphines: Insight into protonation and electrochemical proton reduction

Inspired by the metal active sites of [FeFe]- and [NiFe]‑hydrogenases, a series of mononuclear Ni(II) ethanedithiolate complexes [{(Ph₂PCH₂)₂×}Ni(SCH₂CH₂S)] (X = NCH₂C₅H₄N-p (2a), NCH₂C₆H₅ (2b), NCH₂CHMe₂ (2c), and CH₂ (2d)) with chelating diphosphines were readily synthesized through the room-temperature treatments of mononuclear Ni(II) dichlorides [{(Ph₂PCH₂)₂×}NiCl₂] (1a-1d) with ethanedithiol (HSCH₂CH₂SH) in the presence of triethylamine (Et₃N) as acid-binding agent. All the as-prepared complexes 1a-1d and 2a-2d are fully characterized through elemental analysis, nuclear magnetic resonance (NMR) spectrum, and by X-ray crystallography for 1b, 2a-2d. To further explore proton-trapping behaviors of this type of mononuclear Ni(II) complexes for catalytic hydrogen (H₂) evolution, the protonation and electrochemical proton reduction of 2a-2c with aminodiphosphines (labeled PCNCP = (Ph₂PCH₂)₂NR) and reference analogue 2d with nitrogen-free diphosphine (dppp = (Ph₂PCH₂)₂CH₂) are studied and compared under trifluoroacetic acid (TFA) as a proton source. Interestingly, the treatments of 2a-2d with excess TFA resulted in the unexpected formation of dinuclear Ni(II)-Ni(II) dication complexes [{(Ph₂PCH₂)₂×}₂Ni₂(μ-SCH₂CH₂S)](CF₃CO₂)₂ (3a-3d) and mononuclear Ni(II) N-protonated complexes [{(Ph₂PCH₂)₂N(H)R}Ni(SCH₂CH₂S)](CF₃CO₂) (4a-4c), which has been well supported by high-resolution electrospray ionization mass spectroscopy (HRESI-MS), NMR (³¹P, ¹H) as well as fourier transform infrared spectroscopy (FT-IR) techniques, and especially by X-ray crystallography for 3d. Additionally, the electrochemical properties of 2a-2d are investigated in the absence and presence of strong acid (TFA) by using cyclic voltammetry (CV), showing that the complete protonation of 2a-2d gave rise to dinuclear Ni₂S₂ species 3a-3d for electrocatalytic proton reduction to H₂.

Splitting of multiple hydrogen molecules by bioinspired diniobium metal complexes: a DFT study

Splitting of molecular hydrogen (H₂) into bridging and terminal hydrides is a common step in transition metal chemistry. Herein, we propose a novel organometallic platform for cleavage of multiple H₂ molecules, which combines metal centers capable of stabilizing multiple oxidation states, and ligands bearing positioned pendant basic groups. Using quantum chemical modeling, we show that low-valent, early transition metal diniobium(ii) complexes with diphosphine ligands featuring pendant amines can favorably uptake up to 8 hydrogen atoms, and that the energetics are favored by the formation of intramolecular dihydrogen bonds. This result suggests new possible strategies for the development of hydrogen scavenger molecules that are able to perform reversible splitting of multiple H₂ molecules.

[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.

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.

Unraveling the mechanism of biomimetic hydrogen fuel production – a first principles molecular dynamics study

The Fe₂(bdt)(CO)₆ [bdt = benzenedithiolato] complex, a synthetic mimic of the [FeFe] hydrogenase enzyme can electrochemically convert protons into molecular hydrogen. The free energy landscape reveals a different mechanism for the biomimetic cycle.

Anion control of tautomeric equilibria: Fe-H vs. N-H influenced by NH···F hydrogen bonding

Counterions can play an active role in chemical reactivity, modulating reaction pathways, energetics and selectivity. We investigated the tautomeric equilibrium resulting from protonation of Fe(PEtNMePEt)(CO)3 (PEtNMePEt = (Et2PCH2)2NMe) at Fe or N. Protonation of Fe(PEtNMePEt)(CO)3 by [(Et2O)2H]+[B(C6F5)4]- occurs at the metal to give the iron hydride [Fe(PEtNMePEt)(CO)3H]+[B(C6F5)4]-. In contrast, treatment with HBF4·OEt2 gives protonation at the iron and at the pendant amine. Both the FeH and NH tautomers were characterized by single crystal X-ray diffraction. Addition of excess BF4 - to the equilibrium mixture leads to the NH tautomer being exclusively observed, due to NH···F hydrogen bonding. A quantum chemical analysis of the bonding properties of these systems provided a quantification of hydrogen bonding of the NH to BF4 - and to OTf-. Treatment of Fe(PEtNMePEt)(CO)3 with excess HOTf gives a dicationic complex where both the iron and nitrogen are protonated. Isomerization of the dicationic complex was studied by NOESY NMR spectroscopy.

Models of the iron-only hydrogenase enzyme: structure, electrochemistry and catalytic activity of Fe2(CO)3(μ-dithiolate)(μ,κ1,κ2-triphos)

A series of Fe₂(triphos)(CO)₃(μ-dithiolate) complexes have been prepared and studied as models of the diiron centre in [FeFe]-hydrogenases.

Interaction of the H-Cluster of FeFe Hydrogenase with Halides

FeFe hydrogenases catalyze H₂ oxidation and production using an "H-cluster", where two Fe ions are bound by an aza-dithiolate (adt) ligand. Various hypotheses have been proposed (by us and others) to explain that the enzyme reversibly inactivates under oxidizing, anaerobic conditions: intramolecular binding of the N atom of adt, formation of the so-called "Hox/inact" state or nonproductive binding of H₂ to isomers of the H-cluster. Here, we show that none of the above explains the new finding that the anaerobic, oxidative, H₂-dependent reversible inactivation is strictly dependent on the presence of Cl^- or Br^-. We provide experimental evidence that chloride uncompetitively inhibits the enzyme: it reversibly binds to catalytic intermediates of H₂ oxidation (but not to the resting "Hox" state), after which oxidation locks the active site into a stable, saturated, inactive form, the structure of which is proposed here based on DFT calculations. The halides interact with the amine group of the H-cluster but do not directly bind to iron. It should be possible to stabilize the inhibited state in amounts compatible with spectroscopic investigations to explore further this unexpected reactivity of the H-cluster of hydrogenase.

Mechanistic Insight into Electrocatalytic H2 Production by [Fe2(CN){μ-CN(Me)2}(μ-CO)(CO)(Cp)2]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe₂(CN){μ-CN(Me)₂}(μ-CO)(CO)(Cp)₂] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN^- and amino-carbyne ligands have been elucidated. After the first reduction, CN^- works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN^- that forms a Fe(CNH) moiety: i.e., the acidic source for the H₂ generation. The hydride (formally 2e/H⁺), necessary to the heterocoupling with H⁺ is thus provided by the μ-CN(Me)₂ ligand and not by Fe centers, as occurs in typical L₆Fe₂S₂ derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN^- plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN^- to CN(Me)₂ is highlighted. The incompetence for the HER of the related species [Fe₂{μ-CN(Me)₂}(μ-CO)(CO)₂(Cp)₂]⁺ (2⁺) has been investigated and attributed to the loss of proton responsiveness caused by CN^- replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe₂ core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe₂ scaffold, could be exploited to develop more active and robust catalysts for the HER.

Proton Coupled Electronic Rearrangement within the H-Cluster as an Essential Step in the Catalytic Cycle of [FeFe] Hydrogenases

The active site of [FeFe] hydrogenases, the H-cluster, consists of a [4Fe-4S] cluster connected via a bridging cysteine to a [2Fe] complex carrying CO and CN^- ligands as well as a bridging aza-dithiolate ligand (ADT) of which the amine moiety serves as a proton shuttle between the protein and the H-cluster. During the catalytic cycle, the two subclusters change oxidation states: [4Fe-4S]_H²⁺ ⇔ [4Fe-4S]_H⁺ and [Fe(I)Fe(II)]_H ⇔ [Fe(I)Fe(I)]_H thereby enabling the storage of the two electrons needed for the catalyzed reaction 2H⁺ + 2e^- ⇄ H₂. Using FTIR spectro-electrochemistry on the [FeFe] hydrogenase from Chlamydomonas reinhardtii (CrHydA1) at different pH values, we resolve the redox and protonation events in the catalytic cycle and determine their intrinsic thermodynamic parameters. We show that the singly reduced state H_red of the H-cluster actually consists of two species: H_red = [4Fe-4S]_H⁺ - [Fe(I)Fe(II)]_H and H_redH⁺ = [4Fe-4S]_H²⁺ - [Fe(I)Fe(I)]_H (H⁺) related by proton coupled electronic rearrangement. The two redox events in the catalytic cycle occur on the [4Fe-4S]_H subcluster at similar midpoint-potentials (-375 vs -418 mV); the protonation event (H_red/H_redH⁺) has a pK_a ≈ 7.2.

Nickel-centred proton reduction catalysis in a model of [NiFe] hydrogenase

Hydrogen production through water splitting is one of the most promising solutions for the storage of renewable energy. [NiFe] hydrogenases are organometallic enzymes containing nickel and iron centres that catalyse hydrogen evolution with performances that rival those of platinum. These enzymes provide inspiration for the design of new molecular catalysts that do not require precious metals. However, all heterodinuclear NiFe models reported so far do not reproduce the Ni-centred reactivity found at the active site of [NiFe] hydrogenases. Here, we report a structural and functional NiFe mimic that displays reactivity at the Ni site. This is shown by the detection of two catalytic intermediates that reproduce structural and electronic features of the Ni-L and Ni-R states of the enzyme during catalytic turnover. Under electrocatalytic conditions, this mimic displays high rates for H₂ evolution (second-order rate constant of 2.5 × 10⁴ M^-1 s^-1; turnover frequency of 250 s^-1 at 10 mM H⁺ concentration) from mildly acidic solutions.

Effect of Bridgehead Steric Bulk on the Intramolecular C-H Heterolysis of [FeFe]-Hydrogenase Active Site Models Containing a P2N2 Pendant Amine Ligand

A series of pendant amine-containing [FeFe]-hydrogenase models, [X(CH2S-μ)2{Fe(CO)3}{Fe(CO)(P2(Ph)N2(Bn))}] (1H, X = CH2; 2Me, C(CH3)2; 3Et, C(CH2CH3)2; and P2(Ph)N2(Bn) = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) with different groups at the bridgehead carbon of the S-to-S linker were synthesized. The oxidations of these complexes as well as the reverse reduction reaction were studied by cyclic voltammetry and in situ IR spectroscopy. Regardless of the bridgehead steric bulk, all three complexes demonstrate intramolecular iron-mediated C(sp(3))-H bond heterolytic cleavage with the assistance of the pendant amine base within the chelating diphosphine ligand in the two-electron oxidation process. X-ray crystallographic analysis shows that the doubly oxidized products, [1'H](+), [2'Me](+), and [3'Et](+), all have a rigid FeSC three-membered ring at the open apical site of the rotated iron center. The most noticeable difference in structures of the oxidized complexes is that the single CO ligand of the rotated Fe(P2(Ph)N2(Bn))(CO) unit in [1'H](+) and [2'Me](+) is found below the Fe···Fe vector, while in [3'Et](+) an unusually rotated Fe(P2(Ph)N2(Bn))(CO) moiety positions one of the P donors within the bidentate ligand under the Fe···Fe vector. The starting Fe(I)Fe(I) complexes can be recovered from their corresponding doubly oxidized complexes by reduction in the presence of Brönsted acid.

Effect of the S-to-S bridge on the redox properties and H2activation performance of diiron complexes related to the [FeFe]-hydrogenase active site

Studies on diiron dithiolate complexes containing the same PNP ligand but different S-to-S bridges demonstrate that changing the S-to-S bridge can considerably alter the H₂activation performance of [FeFe]-hydrogenase models.

Diiron azadithiolates as models for the [FeFe]-hydrogenase active site and paradigm for the role of the second coordination sphere

The [FeFe] hydrogenases (H2ases) catalyze the redox reaction that interconverts protons and H2. This area of biocatalysis has attracted attention because the metal-based chemistry is unusual, and the reactions have practical implications. The active site consists of a [4Fe-4S] cluster bridged to a [Fe2(μ-dithiolate)(CN)2(CO)3](z) center (z = 1- and 2-). The dithiolate cofactor is [HN(CH2S)2](2-), called the azadithiolate ([adt(H)](2-)). Although many derivatives of Fe2(SR)2(CO)6-xLx are electrocatalysts for the hydrogen evolution reaction (HER), most operate by slow nonbiomimetic pathways. Biomimetic hydrogenogenesis is thought to involve intermediates, wherein the hydride substrate is adjacent to the amine of the adt(H), being bonded to only one Fe center. Formation of terminal hydride complexes is favored when the diiron carbonyl models contain azadithiolate. Although unstable in the free state, the adt cofactor is stable once it is affixed to the Fe2 center. It can be prepared by alkylation of Fe2(SH)2(CO)6 with formaldehyde in the presence of ammonia (to give adt(H) derivatives) or amines (to give adt(R) derivatives). Weak acids protonate Fe2(adt(R))(CO)2(PR3)4 to give terminal hydrido (term-H) complexes. In contrast, protonation of the related 1,3-propanedithiolate (pdt(2-)) complexes Fe2(pdt)(CO)2(PR3)4 requires strong acids. The amine in the azadithiolate is a kinetically fast base, relaying protons to and from the iron, which is a kinetically slow base. The crystal structure of the doubly protonated model [(term-H)Fe2(Hadt(H))(CO)2(dppv)2](2+) confirms the presence of both ammonium and terminal hydrido centers, which interact through a dihydrogen bond (dppv = cis-C2H2(PPh2)2). DFT calculations indicate that this H---H interaction is sensitive to the counterions and is strengthened upon reduction of the diiron center. For the monoprotonated models, the hydride [(term-H)Fe2(adt(H))(CO)2(dppv)2](+) exists in equilibrium with the ammonium tautomer [Fe2(Hadt(H))(CO)2(dppv)2](+). Both [(term-H)Fe2(Hadt(H))(CO)2(dppv)2](2+) and [(term-H)Fe2(adt(H))(CO)2(dppv)2](+) are highly active electrocatalysts for HER. Catalysis is initiated by reduction of the diferrous center, which induces coupling of the protic ammonium center and the hydride ligand. In contrast, the propanedithiolate [(term-H)Fe2(pdt)(CO)2(dppv)2](+) is a poor electrocatalyst for HER. Oxidation of H2 has been demonstrated, starting with models for the oxidized state ("Hox"), for example, [Fe2(adt(H))(CO)3(dppv)(PMe3)](+). Featuring a distorted Fe(II)Fe(I) center, this Hox model reacts slowly with high pressures of H2 to give [(μ-H)Fe2(adt(H))(CO)3(dppv)(PMe3)](+). Highlighting the role of the proton relay, the propanedithiolate [Fe2(pdt)(CO)3(dppv)(PMe3)](+) is unreactive toward H2. The Hox-model + H2 reaction is accelerated in the presence of ferrocenium salts, which simulate the role of the attached [4Fe-4S] cluster. The redox-complemented complex [Fe2(adt(Bn))(CO)3(dppv)(FcP*)](n+) catalyzes both proton reduction and hydrogen oxidation (FcP* = (C5Me5)Fe(C5Me4CH2PEt2)).

Formation, reactivity and redox properties of carbon- and sulfur-bridged diiron complexes derived from dibenzothienyl Schiff bases: effect of N,N- and N,P-chelating moieties

Redox properties of C,S-bridged diiron complexes were controlled by using dibenzothienyl Schiff base precursors with an N,N- or N,P-chelating moiety.

Diiron hexacarbonyl complexes bearing naphthalene-1,8-dithiolate bridge moiety as mimics of the sub-unit of [FeFe]-hydrogenase: synthesis, characterisation and electrochemical investigations

Reducing the formyl group into a hydroxyl group allows the further synthesis of novel diiron complexes with various functionalities.

Hydrogen generation: aromatic dithiolate-bridged metal carbonyl complexes as hydrogenase catalytic site models

The design, syntheses and characteristics of metal carbonyl complexes with aromatic dithiolate linkers reported as bioinspired hydrogenase catalytic site models are described and reviewed. Among these the complexes capable of hydrogen generation have been discussed in detail. Comparisons have been made with carbonyl complexes having alkyl dithiolates as linkers between metal centers.

Computational investigation of [FeFe]-hydrogenase models: characterization of singly and doubly protonated intermediates and mechanistic insights

The [FeFe]-hydrogenase enzymes catalyze hydrogen oxidation and production efficiently with binuclear Fe metal centers. Recently the bioinspired H₂-producing model system Fe₂(adt)(CO)₂(dppv)₂ (adt=azadithiolate and dppv=diphosphine) was synthesized and studied experimentally. In this system, the azadithiolate bridge facilitates the formation of a doubly protonated ammonium-hydride species through a proton relay. Herein computational methods are utilized to examine this system in the various oxidation states and protonation states along proposed mechanistic pathways for H₂ production. The calculated results agree well with the experimental data for the geometries, CO vibrational stretching frequencies, and reduction potentials. The calculations illustrate that the NH···HFe dihydrogen bonding distance in the doubly protonated species is highly sensitive to the effects of ion-pairing between the ammonium and BF₄^– counterions, which are present in the crystal structure, in that the inclusion of BF₄^– counterions leads to a significantly longer dihydrogen bond. The non-hydride Fe center was found to be the site of reduction for terminal hydride species and unsymmetric bridging hydride species, whereas the reduced symmetric bridging hydride species exhibited spin delocalization between the Fe centers. According to both experimental measurements and theoretical calculations of the relative pK_a values, the Fe_d center of the neutral species is more basic than the amine, and the bridging hydride species is more thermodynamically stable than the terminal hydride species. The calculations implicate a possible pathway for H₂ evolution that involves an intermediate with H₂ weakly bonded to one Fe, a short H₂ distance similar to the molecular bond length, the spin density delocalized over the two Fe centers, and a nearly symmetrically bridged CO ligand. Overall, this study illustrates the mechanistic roles of the ammonium-hydride interaction, flexibility of the bridging CO ligand, and intramolecular electron transfer between the Fe centers in the catalytic cycle. Such insights will assist in the design of more effective bioinspired catalysts for H₂ production.

Theoretical calculations in conjunction with supporting experimental data are used to analyze the mechanistic pathway for hydrogen evolution catalyzed by the bioinspired model Fe₂(adt)(CO)₂(dppv)₂. This study elucidates the site of reduction and the pK_a values associated with formation of the singly and doubly protonated species, as well as the roles of the ammonium-hydride interaction, flexibility of the bridging CO ligand, and intramolecular electron transfer between the Fe centers in the catalytic cycle for H₂ production.

Hydrogen Production Catalyzed by Bidirectional, Biomimetic Models of the [FeFe]-Hydrogenase Active Site

Active site mimics of [FeFe]-hydrogenase are shown to be bidirectional catalysts, producing H₂ upon treatment with protons and reducing equivalents. This reactivity complements the previously reported oxidation of H₂ by these same catalysts in the presence of oxidants. The complex Fe₂(adt^Bn)(CO)₃(dppv)(PFc*^Et₂ ) ([1]⁰; adt^Bn = (SCH₂)₂NBn, dppv = cis-1,2-bis(diphenylphosphino)ethylene, PFc*^Et₂ = Et₂PCH₂C₅Me₄FeCp*) reacts with excess [H(OEt₂)₂]BAr^F₄ (BAr^F₄^- = B(C₆H₃-3,5-(CF₃)₂)₄^-) to give ∼0.5 equiv of H₂ and [Fe₂(adt^BnH)(CO)₃(dppv)(PFc*^Et₂ )]²⁺ ([1H]²⁺). The species [1H]²⁺ consists of a ferrocenium ligand, an N-protonated amine, and an Fe^IFe^I core. In the presence of additional reducing equivalents in the form of decamethylferrocene (Fc*), hydrogen evolution is catalytic, albeit slow. The related catalyst Fe₂(adt^Bn)(CO)₃(dppv)(PMe₃) (3) behaves similarly in the presence of Fc*, except that in the absence of excess reducing agent it converts to the catalytically inactive μ-hydride derivative [μ-H3]⁺. Replacement of the adt in [1]⁰ with propanedithiolate (pdt) results in a catalytically inactive complex. In the course of synthesizing [FeFe]-hydrogenase mimics, new routes to ferrocenylphosphine ligands and nonamethylferrocene were developed.

The oxidative inactivation of FeFe hydrogenase reveals the flexibility of the H-cluster

Nature is a valuable source of inspiration in the design of catalysts, and various approaches are used to elucidate the mechanism of hydrogenases, the enzymes that oxidize or produce H2. In FeFe hydrogenases, H2 oxidation occurs at the H-cluster, and catalysis involves H2 binding on the vacant coordination site of an iron centre. Here, we show that the reversible oxidative inactivation of this enzyme results from the binding of H2 to coordination positions that are normally blocked by intrinsic CO ligands. This flexibility of the coordination sphere around the reactive iron centre confers on the enzyme the ability to avoid harmful reactions under oxidizing conditions, including exposure to O2. The versatile chemistry of the diiron cluster in the natural system might inspire the design of novel synthetic catalysts for H2 oxidation.

Heterolytic cleavage of H2 by bifunctional manganese(i) complexes: impact of ligand dynamics, electrophilicity, and base positioning

A balance of metal electrophilicity and ligand steric influences is required for facile, reversible H–H heterolytic cleavage in Mn complexes with pendant amines.

Production of hydrogen by electrocatalysis: making the H–H bond by combining protons and hydrides

Electrocatalytic production of hydrogen by nickel complexes is reviewed, with an emphasis on heterocoupling of protons and hydrides.

Mixed-valence [Fe(I)Fe(II)] hydrogenase active site model complexes stabilized by a bidentate carborane bis-phosphine ligand

A series of [FeFe]-hydrogenase active site analogues, with the general formula [Fe(2)(dt)(CO)(4)(BC)] 1-3 (dt = dithiolate, pdt = propyl-1,3-dt (1), bdt = benzene-1,2-dt (2), edt = ethyl-1,2-dt (3); BC = 1,2-bisdiphenylphosphine-1,2-o-carborane), has been prepared and structurally characterized. While the electrochemical reductions of 1-3 are largely invariant to the different nature of their dt bridges, the oxidations differ by more than 120 mV in between the series. Remarkably, all three compounds are reversibly oxidized, with complex 1 that contains the most electron-donating pdt ligand at the mildest potential of -0.09 V vs. Fc/Fc(+). The one-electron oxidized state 1(ox) is stable for several minutes and was spectroscopically characterized by FTIR and EPR. EPR spectroscopy provided evidence that in the mixed-valence [Fe(I)Fe(II)] state most of the spin density is located on the iron with the BC-ligand. This is monitored through the strong (31)P hyperfine coupling of the phenyl groups of the BC ligand, while further delocalization into the o-carborane unit is negligible.