Hydrogenase H-cluster Research Articles

Fully Refined Semisynthesis of the [FeFe] Hydrogenase H-Cluster.

[FeFe] hydrogenases contain a 6-Fe cofactor that serves as the active site for efficient redox interconversion between H2 and protons. The biosynthesis of the so-called H-cluster involves unusual enzymatic reactions that synthesize organometallic Fe complexes containing azadithiolate, CO, and CN- ligands. We have previously demonstrated that specific synthetic [Fe(CO)x(CN)y] complexes can be used to functionally replace proposed Fe intermediates in the maturation reaction. Here, we report the results from performing such cluster semisynthesis in the context of a recent fully defined cluster maturation procedure, which eliminates unknown components previously employed from Escherichia coli cell lysate and demonstrate this provides a concise route to H-cluster synthesis. We show that formaldehyde can be used as a simple reagent as the carbon source of the bridging adt ligand of H-cluster in lieu of serine/serine hydroxymethyltransferase. In addition to the actual H-cluster, we observe the formation of several H-cluster-like species, the identities of which are probed by cryogenic photolysis combined with EPR/ENDOR spectroscopy.

FeFe]-hydrogenase H-cluster mimics mediated by ferrocenyl hetaryl thioketone derivatives

In this report, the influence of the substitution decoration in ferrocenyl selenophen-2-yl thioketone (1) and ferrocenyl furan-2-yl thioketone (2) utilized as a proligand in complexation reactions with Fe3(CO)12 was examined. As a result, three new complexes, considered [FeFe]-hydrogenase mimics, were prepared and characterized by IR, 1H, 13C{1H}, and 77Se{1H} NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray single-crystal diffraction methods. Moreover, the obtained complexes were compared with the analogous ferrocenyl α-thienyl thioketone as a starting material. Respectively, the reaction of compound 1 with Fe3(CO)12 is found to give two complexes that have similar structures to those obtained in analogous reactions with ferrocenyl α-thienyl thioketone. Conversely, compound 2 gives only an α,β-unsaturated aldehyde moiety as a result of the furan ring being opened. Furthermore, the electrochemical properties of the resulting complexes were investigated using cylic voltammerty in the absence and presence of acetic acid (AcOH) as a proton source and compared with their structurally related thiophene derivative. Accordingly, the nature of the linkage of these synthetic approaches has a significant effect in determining its redox properties.

Mono- and di-substituted [FeFe]-hydrogenase H-cluster mimics bearing the 3,4-dimercaptobenzaldehyde bridge moiety: Insight into synthesis, characterization and electrochemical investigations

This report describes the synthesis and characterization of [FeFe]-hydrogenase mimic, namely [Fe2(CO)6{μ-bdtf}] (2, bdtf = 3,4-dimercaptobenzaldehyde) obtained from the reaction of 3,4-dimercaptobenzaldehyde with Fe3(CO)12. Moreover, treatment of complex 2 with PPh3 ligand yielded both mono- and disubstituted complexes [Fe2(CO)5PPh3{μ-bdtf}] (3), [Fe2(CO)4(PPh3)2{μ-bdtf}] (4), while in the case of P(OMe)3 ligand afforded only the disubstituted complex [Fe2(CO)4(P(OMe)3)2{μ-bdtf}] (5). The molecular structures of complexes 2–5 were confirmed by X-ray diffraction analysis. Furthermore, the influence of the substituent on the benzene ring as well as at the iron center on the electrochemistry and catalysis of these mimics toward proton reduction have been evaluated by cyclic voltammetry. All complexes show their potentials toward hydrogen formation in the presence of weak acid, such as acetic acid (AcOH).

FeFe]-Hydrogenase In Vitro Maturation.

The [FeFe]-hydrogenase H-cluster is a complex organometallic cofactor whose assembly and installation requires three dedicated accessory proteins referred to as HydE, HydF, and HydG. The roles of these maturases and the precise mechanisms by which they synthesize and insert the H-cluster are not fully understood. This Minireview will focus on new insights into the [FeFe]-hydrogenase maturation process that have been provided by in vitro approaches in which the biosynthetic pathway has been partially or fully reconstructed using semisynthetic and enzyme-based approaches. Specifically, the application of these in vitro, semisynthetic, and fully defined approaches has shed light on the roles of individual maturation enzymes, the nature of H-cluster assembly intermediates, the molecular precursors of H-cluster ligands, and the sequence of steps involved in [FeFe]-hydrogenase maturation.

Accumulation and Pulse Electron Paramagnetic Resonance Spectroscopic Investigation of the 4-Oxidobenzyl Radical Generated in the Radical S-Adenosyl-l-methionine Enzyme HydG.

The radical S-adenosyl-l-methionine (SAM) enzyme HydG cleaves tyrosine to generate CO and CN- ligands of the [FeFe] hydrogenase H-cluster, accompanied by the formation of a 4-oxidobenzyl radical (4-OB•), which is the precursor to the HydG p-cresol byproduct. Native HydG only generates a small amount of 4-OB•, limiting detailed electron paramagnetic resonance (EPR) spectral characterization beyond our initial EPR lineshape study employing various tyrosine isotopologues. Here, we show that the concentration of trapped 4-OB• is significantly increased in reactions using HydG variants, in which the "dangler Fe" to which CO and CN- bind is missing or substituted by a redox-inert Zn2+ ion. This allows for the detailed characterization of 4-OB• using high-field EPR and electron nuclear double resonance spectroscopy to extract its g-values and 1H/13C hyperfine couplings. These results are compared to density functional theory-predicted values of several 4-OB• models with different sizes and protonation states, with a best fit to the deprotonated radical anion configuration of 4-OB•. Overall, our results depict a clearer electronic structure of the transient 4-OB• radical and provide new insights into the radical SAM chemistry of HydG.

Mono- and Di-Substituted [Fefe]-Hydrogenase H-Cluster Mimics Bearing the 3,4-Dimercaptobenzaldehyde Bridge Moiety: Insight into Synthesis, Characterization and Electrochemical Investigations

Mono- and Di-Substituted [Fefe]-Hydrogenase H-Cluster Mimics Bearing the 3,4-Dimercaptobenzaldehyde Bridge Moiety: Insight into Synthesis, Characterization and Electrochemical Investigations

Impact of the 2Fe2P core geometry on the reduction chemistry of phosphido-bridged diiron hexacarbonyl compounds

The effect of core geometry constraints of hydrogenase H-cluster analogues on reduction chemistry have been explored by a combination of structural, electrochemical and IR spectroelectrochemical (IR-SEC) studies. A series of phosphido-bridged diiron hexacarbonyl complexes, Fe2(µ2-PPh2(CH2)xPPh2)(CO)6, x = 2 (2P) and 4 (4P) and previously reported with x = 3 (3P) and the unlinked bis-diphenylphosphido (DP) analogues were investigated. The X-ray structures of the neutral complexes demonstrate the effect of the linking group on the Fe2P2 core geometry with P–Fe–Fe–P torsion angles of 95 (2P), 101 (3P), 108 (4P) and 109° (DP) and a twisting of the Fe(CO)3 fragments from an eclipsed geometry (2P, 3P and DP) for 4P. For all four compounds the primary reduction process involves two close-spaced one-electron reactions (E1 and E2) with a systematic trend to more negative reduction potentials with a shorter link between the bridging phosphorus atoms. This reflects the greater constraint that the bridging group places on the adoption of a planar 2Fe2P geometry. The sensitivity of the core geometry is greater for E2 than E1 and this impacts the stability of the monoanion with respect to disproportion (Kdisp(298 K) = 0.02 (2P), 2.4 (3P) and 3540 (4P and DP)). 4P has a stable dianion and gives reversible cyclic voltammetry at 298 K and is quasi-reversible at 253 K, whereas the response of 2P is irreversible at 298 K, with two distinct daughter products, but becomes quasi-reversible at 253 K. IR-SEC measurements enabled elucidation of the spectra and time evolution of the reduction products. These results are consistent with a bimolecular reaction giving a distinct reduced product modelled as a dimeric, 4Fe species. The sensitivity of the reduction chemistry of the bridged diiron compounds underpins their utility as catalytic proton reduction catalysts and the systematic trends delineated in this investigation provide the framework for charting the path of their redox-coupled chemical reactions.

Proposed Mechanism for the Biosynthesis of the [FeFe] Hydrogenase H-Cluster: Central Roles for the Radical SAM Enzymes HydG and HydE.

Radical S-adenosylmethionine (radical SAM or rSAM) enzymes use their S-adenosylmethionine cofactor bound to a unique Fe of a [4Fe–4S] cluster to generate the “hot” 5′-deoxyadenosyl radical, which drives highly selective radical reactions via specific interactions with a given rSAM enzyme’s substrate. This Perspective focuses on the two rSAM enzymes involved in the biosynthesis of the organometallic H-cluster of [FeFe] hydrogenases. We present here a detailed sequential model initiated by HydG, which lyses a tyrosine substrate via a 5′-deoxyadenosyl H atom abstraction from those amino acid’s amino group, initially producing dehydroglycine and an oxidobenzyl radical. In this model, two successive radical cascade reactions lead ultimately to the formation of HydG’s product, a mononuclear Fe organometallic complex: [Fe(II)(CN)(CO)2(cysteinate)]−, with the iron originating from a unique “dangler” Fe coordinated by a cysteine ligand providing a sulfur bridge to another [4Fe–4S] auxiliary cluster in the enzyme. In turn, in this model, [Fe(II)(CN)(CO)2(cysteinate)]− is the substrate for HydE, the second rSAM enzyme in the biosynthetic pathway, which activates this mononuclear organometallic unit for dimerization, forming a [Fe2S2(CO)4(CN)2] precursor to the [2Fe]H component of the H-cluster, requiring only the completion of the bridging azadithiolate (SCH2NHCH2S) ligand. This model is built upon a foundation of data that incorporates cell-free synthesis, isotope sensitive spectroscopies, and the selective use of synthetic complexes substituting for intermediates in the enzymatic “assembly line”. We discuss controversies pertaining to this model and some remaining open issues to be addressed by future work.

Structural studies and electrochemical catalysis investigation of [FeFe]-hydrogenase H-cluster mimics mediated by monophosphane ligands

In this research work we present the reaction of the precursor complexes [Fe2(CO)6{μ-(SeCH2)2CMeR}] (R = H (1) or Me (2)) with monophosphine/phosphane ligands, PPh3 and P(OMe)3, to afford the mono-substituted complexes [Fe2(CO)5PPh3{μ-(SeCH2)2CHMe}] (3), [Fe2(CO)5PPh3{μ-(SeCH2)2CMe2}] (4), [Fe2(CO)5P(OMe)3{μ-(SeCH2)2CHMe}] (5) and [Fe2(CO)5P(OMe)3{μ-(SeCH2)2CMe2}] (6). The structures of complexes 3–6 were confirmed by X-ray diffraction analysis. Moreover, we studied the influence of the steric interference of the μ-Se-to-Se linker with the nearby ligands (PPh3 and P(OMe)3) on the electronic properties, the solid-state structures and the mechanism of the cathodic process of complexes 3–6 compared to that of 1 and 2. In addition, a detailed electrochemical study of complexes 3–6 in the absence and presence of acetic acid AcOH, is also reported.

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 Hhyd state containing a terminal hydride at the distal Fe site. The 13C2H isotope effects were observed also in the Hhyd 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.

Biosynthesis of the [FeFe] hydrogenase H-cluster via a synthetic [Fe(II)(CN)(CO)2(cysteinate)]- complex.

The H-cluster of [Fe-Fe] hydrogenase consists of a [4Fe]H subcluster linked by the sulfur of a cysteine residue to an organometallic [2Fe]H subcluster that utilizes terminal CO and CN ligands to each Fe along with a bridging CO and a bridging SCH2NHCH2S azadithiolate (adt) to catalyze proton reduction or hydrogen oxidation. Three Fe-S "maturase" proteins, HydE, HydF, and HydG, are responsible for the biosynthesis of the [2Fe]H subcluster and its incorporation into the hydrogenase enzyme to form this catalytically active H-cluster. We have proposed that HydG is a bifunctional enzyme that uses S-adenosylmethione (SAM) bound to a [4Fe-4S] cluster to lyse tyrosine via a transient 5'-deoxyadenosyl radical to produce CO and CN ligands to a unique cysteine-chelated Fe(II) that is linked to a second [4Fe-4S] cluster via the cysteine sulfur. In this "synthon model", after two cycles of tyrosine lysis, the product of HydG is completed: a [Fe(CN)(CO)2(cysteinate)]- organometallic unit that is vectored directly into the synthesis of the [2Fe]H sub-cluster. However our HydG-centric synthon model is not universally accepted, so further validation is important. In this Frontiers article, we discuss recent results using a synthetic "Syn-B" complex that donates [Fe(CN)(CO)2(cysteinate)]- units that match our proposed HydG product. Can Syn-B activate hydrogenase in the absence of HydG and its tyrosine substrate? If so, since Syn-B can be synthesized with specific magnetic nuclear isotopes and with chemical substitutions, its use could allow its enzymatic conversions on the route to the H-cluster to be monitored and modeled in fresh detail.

Serine is the molecular source of the NH(CH2)2 bridgehead moiety of the in vitro assembled [FeFe] hydrogenase H-cluster.

The active site of [FeFe] hydrogenase, the H-cluster, consists of a canonical [4Fe–4S]H subcluster linked to a unique binuclear [2Fe]H subcluster containing three CO, two CN− and a bridging azadithiolate (adt, NH(CH2S−)2) ligand. While it is known that all five diatomic ligands are derived from tyrosine, there has been little knowledge as to the formation and installation of the adt ligand. Here, by using a combination of a cell-free in vitro maturation approach with pulse electronic paramagnetic resonance spectroscopy, we discover that serine donates the nitrogen atom and the CH2 group to the assembly of the adt ligand. More specifically, both CH2 groups in adt are sourced from the C3 methylene of serine.

Vibrational coherence transfer illuminates dark modes in models of the FeFe hydrogenase active site

Within the conceptual framework of Redfield theory, the optical response function arises from the dynamical evolution of the system’s density operator, where nonunitary relaxation is encoded in the Redfield relaxation superoperator. In the conventional approach, the so-called secular approximation neglects terms that induce transitions between distinct coherences and among coherences and populations. The rationale is that these nonsecular terms are small in comparison to the far more dominant population relaxation and coherence dephasing contributions. Since two-dimensional infrared (2D-IR) spectroscopy has significant contributions arising from population relaxation and transfer pathways, it can be challenging to isolate signatures of the nonsecular relaxation. We report here that in three diiron dithiolate hexacarbonyl complexes that serve as small-molecule models of the [FeFe] hydrogenase H-cluster subsite, a fortuitous vibrational energy structure enables direct and clear signatures of vibrational coherence transfer in alkane solution. This finding holds promise towards developing a molecularly detailed understanding of the mechanism of vibrational coherence transfer processes, thanks to the ease of synthesizing derivatives based on the chemical modularity of these well studied diiron compounds. In addition to the fundamental need to characterize coherence transfer in molecular spectroscopy, we find in this set of molecules a practical utility for the nonsecular dynamics: the ability to determine the frequency of an IR-inactive mode. A coherence generated during the waiting time of the 2D-IR measurement transfers to a coherence involving the single dark CO stretching mode, which modulates some peak amplitudes in the 2D spectrum, revealing its transient excitation.

Electrochemical and Computational Insights into the Reduction of [Fe2(CO)6{µ-(SCH2)2GeMe2}] Hydrogenase H-Cluster Mimic

The electrochemical reduction of the complex [Fe2(CO)6{µ-(SCH2)2GeMe2}] (1) under N2 and CO is reported applying cyclic voltammetry. Reduction of complex 1 in CO saturated solutions prevents the possible release of CO from the dianion 12−, while the latter reacts with additional CO forming a spectroscopically uncharacterized product P1. This product undergoes a reversible redox process at E1/2 = −0.70 V (0.2 V∙s−1). In this report, the structure of the neutral complex 1, isomers of dianionic form of 1, and P1 are described applying DFT computations. Furthermore, we propose reaction pathways for H2 production on the basis of the cyclic voltammetry of complex 1 in presence of the strong acid CF3SO3H.

Toward a Tunable Synthetic [FeFe]-Hydrogenase H-Cluster Mimic Mediated by Perylene Monoimide Model Complexes: Insight into Molecular Structures and Electrochemical Characteristics

The nature of the bridging dithiolate has an important role on tuning the physical and electrochemical properties of the synthetic H-cluster mimics of [FeFe]-hydrogenase and still of significant concern to scientists. In this report we describe the synthetic models of the active site of [FeFe]-hydrogenase containing perylene monoimide of peri-substituted disulfides as bridging linker. The resulting complexes were characterized by 1H and 13C{1H} NMR and IR spectroscopic techniques, mass spectrometry, and elemental analysis as well as X-ray analysis of complex 2a. The purpose of this work was to investigate the influence of the perylene-linker on the redox potentials of the complexes and their catalytic ability in the presence of acetic acid (AcOH) by applying cyclic voltammetry. Moreover, we compare these results with different diiron hexacarbonyl complexes previously reported in the literature. As a result, we have found that the presence of the rylene-linker provides further stability for the reduced spe...

Electronic Structure of Two Catalytic States of the [FeFe] Hydrogenase H-Cluster As Probed by Pulse Electron Paramagnetic Resonance Spectroscopy.

The active site of the [FeFe] hydrogenase (HydA1), the H-cluster, is a 6-Fe cofactor that contains CO and CN- ligands. It undergoes several different oxidation and protonation state changes in its catalytic cycle to metabolize H2. Among them, the well-known Hox state and the recently identified Hhyd state are thought to be directly involved in H2 activation and evolution, and they are both EPR active with net spin S = 1/2. Herein, we report the pulse electronic paramagnetic spectroscopic investigation of these two catalytic states in Chlamydomonas reinhardtii HydA1 ( CrHydA1). Using an in vitro biosynthetic maturation approach, we site-specifically installed 13C into the CO or CN- ligands and 57Fe into the [2Fe]H subcluster of the H-cluster in order to measure the hyperfine couplings to these magnetic nuclei. For Hox, we measured 13C hyperfine couplings (13CO aiso of 25.5, 5.8, and 4.5 MHz) corresponding to all three CO ligands in the H-cluster. We also observed two 57Fe hyperfine couplings (57Fe aiso of ∼17 and 5.7 MHz) arising from the two Fe atoms in the [2Fe]H subcluster. For Hhyd, we only observed two distinct 13CO hyperfine interactions (13CO aiso of 0.16 and 0.08 MHz) and only one for 13CN- (13CN aiso of 0.16 MHz); the couplings to the 13CO/13CN- on the distal Fe of [2Fe]H may be too small to detect. We also observed a very small (<0.3 MHz) 57Fe HFI from the labeled [2Fe]H subcluster and four 57Fe HFI from the labeled [4Fe-4S]H subcluster (57Fe aiso of 7.2, 16.6, 28.2, and 35.3 MHz). These hyperfine coupling constants are consistent with the previously proposed electronic structure of the H-cluster at both Hox and Hhyd states and provide a basis for more detailed analysis.

A [4Fe–4S]-Fe(CO)(CN)-l-cysteine intermediate is the first organometallic precursor in [FeFe] hydrogenase H-cluster bioassembly

Biosynthesis of the [FeFe] hydrogenase active site (the “H-cluster”) requires the interplay of multiple proteins and small molecules. Among them, the radical S-adenosyl-methionine enzyme HydG, a tyrosine lyase, has been proposed to generate an Fe(CO)2(CN) moiety-containing complex that is eventually incorporated into the H-cluster. Here we describe the characterization of an intermediate in the HydG reaction: a [4Fe-4S][(Cys)Fe(CO)(CN)] species, termed “Complex A”, in which a CO, a CN− and a cysteine (Cys) molecule bind to the unique “dangler” Fe site of the auxiliary [5Fe-4S] cluster of HydG. Identification of this intermediate—the first organometallic precursor to the H-cluster—validates the previously hypothesized HydG reaction cycle and provides a basis for elucidating the biosynthetic origin of other moieties of the H-cluster.

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.

Electrochemical Investigations of Hydrogenases and Other Enzymes That Produce and Use Solar Fuels.

Many enzymes that produce or transform small molecules such as O2, H2, and CO2 embed inorganic cofactors based on transition metals. Their active site, where the chemical reaction occurs, is buried in and protected by the protein matrix, and connected to the solvent in several ways: chains of redox cofactors mediate long-range electron transfer; static or dynamic tunnels guide the substrate, product and inhibitors; amino acids and water molecules transfer protons. The catalytic mechanism of these enzymes is therefore delocalized over the protein and involves many different steps, some of which determine the response of the enzyme under conditions of stress (extreme redox conditions, presence of inhibitors, light), the catalytic rates in the two directions of the reaction and their ratio (the "catalytic bias"). Understanding all the steps in the catalytic cycle, including those that occur on sites of the protein that are remote from the active site, requires a combination of biochemical, structural, spectroscopic, theoretical, and kinetic methods. Here we argue that kinetics should be used to the fullest extent, by extracting quantitative information from the comparison of data and kinetic models and by exploring the combination of experimental kinetics and theoretical chemistry. In studies of these catalytic mechanisms, direct electrochemistry, the technique which we use and contribute to develop, has become unescapable. It simply consists in monitoring the changes in activity of an enzyme that is wired to an electrode by recording an electric current. We have described kinetic models that can be used to make sense of these data and to learn about various aspects of the mechanism that are difficult to probe using more conventional methods: long-range electron transfer, diffusion along gas channels, redox-driven (in)activations, active site chemistry and photoreactivity under conditions of turnover. In this Account, we highlight a few results that illustrate our approach. We describe how electrochemistry can be used to monitor substrate and inhibitor diffusion along the gas channels of hydrogenases and we discuss how the kinetics of intramolecular diffusion relates to global properties such as resistance to oxygen and catalytic bias. The kinetics and/or thermodynamics of intramolecular electron transfer may also affect the catalytic bias, the catalytic potentials on either side of the equilibrium potential, and the overpotentials for catalysis (defined as the difference between the catalytic potentials and the open circuit potential). This is understood by modeling the shape of the steady-state catalytic response of the enzyme. Other determinants of the catalytic rate, such as domain motions, have been probed by examining the transient catalytic response recorded at fast scan rates. Last, we show that combining electrochemical investigations and MD, DFT, and TD-DFT calculations is an original way of probing the reactivity of the H-cluster of hydrogenase, in particular its reactions with CO, O2, and light. This approach contrasts with the usual strategy which aims at stabilizing species that are presumed to be catalytic intermediates, and determining their structure using spectroscopic or structural methods.

FeFe]-Hydrogenase H-Cluster Mimics with Various -S(CH2)nS- Linker Lengths (n = 2-8): A Systematic Study.

The effect of the nature of the dithiolato ligand on the physical and electrochemical properties of synthetic H-cluster mimics of the [FeFe]-hydrogenase is still of significant concern. In this report we describe the cyclization of various alkanedithiols to afford cyclic disulfide, tetrasulfide, and hexasulfide compounds. The latter compounds were used as proligands for the synthesis of a series of [FeFe]-hydrogenase H-cluster mimics having the general formulas [Fe2(CO)6{μ-S(CH2)nS}] (n = 4-8), [Fe2(CO)6{μ-S(CH2)nS}]2 (n = 6-8), and [Fe2(CO)6{(μ-S(CH2)nS)2}] (n = 6-8). The resulting complexes were characterized by 1H and 13C{1H} NMR and IR spectroscopic techniques, mass spectrometry, and elemental analysis as well as X-ray analysis. The purpose of this research was to study the influence of the systematic increase of n from 2 to 7 on the redox potentials of the models and the catalytic ability in the presence of acetic acid (AcOH) by applying cyclic voltammetry.