Hydrogenase Model Research Articles

Substitution Reactions at [FeFe] Hydrogenase Models Containing [2Fe3S] Assembly by Phosphine or Phosphite Ligands

In order to elucidate the role of the “on−off” coordination mode of the thioether group in the [2Fe3S] complex 1, which is related to the active site of [FeFe] hydrogenases, substitution studies of...

Investigation of amino acid containing [FeFe] hydrogenase models concerning pendant base effects

The present investigations deal with the modeling of the peptide surrounding of [FeFe] hydrogenase using amine containing disulphides to simulate possible influences of the amino acid lysine (K237) on the electrochemical and electrocatalytic properties of biomimetic compounds based on [Fe2S2] moieties. Fe(3)(CO)(12) was reacted with Boc-4-amino-1,2-dithiolane, Boc-Adt-OMe (Adt=4-amino-1,2-dithiolane-4-carboxylic acid, Boc=tert-butoxycarbonyl) and Boc-Adp tert-butyl ester (Adp=(S)-2-amino-3-(1,2-dithiolan-4-yl)propionic acid) to elongate the Fecdots, three dots, centeredN distance in comparison to the well known [Fe(2){(SCH(2))(2)NR}(CO)(6)] model complexes. Efforts to deprotect the complexes containing Boc-4-amino-1,2-dithiolane with trifluoroacetic acid result in the formation of [Fe(3)(mu(3)-O)(mu-O(2)C(2)F(3))(6)(OC(4)H(8))(2)(H(2)O)]. The novel [2Fe2S] complexes are characterized using spectroscopic, electrochemical techniques and X-ray diffraction studies.

Applications of X-ray absorption spectroscopy to biologically relevant metal-based chemistry

Recent developments in the understanding of the biosynthesis of the active site of the nitrogenase enzyme, the structure of the iron centre of [Fe]-hydrogenase and the structure and biomimetic chemistry of the [FeFe] hydrogenase H-cluster as deduced by application of X-ray spectroscopy are reviewed. The techniques central to this work include X-ray absorption spectroscopy either in the form of extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES) and nuclear resonant vibrational spectroscopy (NRVS). Examples of the advances in the understanding of the chemistry of the system through integration of a range of spectroscopic and computational techniques with X-ray spectroscopy are highlighted. The critical role played by ab initio calculation of structural and spectroscopic properties of transition-metal compounds using density functional theory (DFT) is illustrated both by the calculation of nuclear resonance vibrational spectroscopy (NRVS) spectra and the structures and spectra of intermediates through the catalytic reactions of hydrogenase model compounds.

An Iron Carbonyl Pyridonate Complex Related to the Active Site of the [Fe]-Hydrogenase (Hmd)

A mononuclear iron bis(carbonyl) pyridonate complex (1), which exhibits several common structural features with the active site of the iron-sulfur cluster-free [Fe]-hydrogenase, was synthesized and characterized. Spectroscopic data of 1 suggests a 2+ oxidation state for the Fe ion in the [Fe]-hydrogenase. Complex 1 serves as a precursor to other hydrogenase models.

Synthesis and Characterization of Diiron Diselenolato Complexes Including Iron Hydrogenase Models

Diiron diselenolato complexes have been prepared as models of the active site of [FeFe]-hydrogenases. Treatment of Fe3(CO)12 with 1 equiv of 1,3-diselenocyanatopropane (1) in THF at reflux afforded the model compound Fe2(μ-Se2C3H6)(CO)6 (2) in 68% yield. The analogous methyl-substituted complex, Fe2(μ-Se2C3H5CH3)(CO)6 (3), was obtained from the reaction of Fe3(CO)12 with the in situ generated compound 3-methyl-1,2-diselenolane (4). In contrast, the reaction of Fe3(CO)12 with 1,3,5-triselenacyclohexane (5) produced a mixture of Fe2(μ2,κ-Se,C-SeCH2SeCH2)(CO)6 (6), Fe2[(μ-SeCH2)2Se](CO)6 (7), and Fe2(μ-Se2CH2)(CO)6 (8). Compounds 2, 3, 6, and 7 were characterized by IR, 1H, 13C, and 77Se NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray single-crystal structure analysis. The He I and He II photoelectron spectra for 3 are reported, and the electronic structure is further analyzed with the aid of DFT computations. The calculated reorganization energy of the cation of 3 to the “rotated” structure, which has a semibridging carbonyl ligand, is less than that of the analogous complexes with sulfur instead of selenium. Complexes 2 and 3 have been proved to be catalysts for electrochemical reduction of protons from the weak acids pivalic and acetic acid, respectively, to give hydrogen.

Fluorophenyl-substituted Fe-only hydrogenases active site ADT models: different electrocatalytic process for proton reduction in HOAc and HBF4/Et2O

A set of fluorophenyl-substituted adt-bridged Fe2S2 active site models of Fe-only hydrogenase, [(micro-SCH2)2NR]Fe2(CO)6 (, R=C6F4CF3-p; , R=C6H4CF3-p) and [(micro-SCH2)2NR]Fe2(CO)5(PPh3) (, R=C6F4CF3-p; , R=C6H4CF3-p), have been synthesized and well characterized. Spectroscopic and electrochemical studies demonstrate that the aryl-substituted complexes are stable toward a strong acid HBF4/Et2O, and electrocatalytic process for the hydrogen production is mostly dependent on the strength of the available proton source. When CH3COOH is used as the proton source, the electrocatalytic process begins with successively two one-electron reduction processes to produce H2 at Fe(0)Fe(0) (E2pc); whereas in the presence of strong acid, HBF4/Et2O, the process is initiated by protonation of a N-bridged atom followed by reduction of the protonated N-bridged atom around -1.29 V, and then release of H2 at Fe(0)Fe(I) (E1pc). Varying the strength of acid leads to the initial electron-transfer step from the reduction of a protonated N-bridged atom to the active site of [Fe(I)Fe(I)].

FeFe]‐Hydrogenase Models and Hydrogen: Oxidative Addition of Dihydrogen and Silanes

Dieisendithiolate finden sich in der Natur in der [FeFe]-Hydrogenase und dienen der Synthese oder Oxidation von molekularem Wasserstoff. Einblicke in die Wechselwirkung von H2 mit Dieisenkomplexen lassen sich anhand von Carbonylmodellkomplexen (siehe Schema) gewinnen. Die dargestellten Reaktionspfade wurden rechnerisch und durch Synthesen aufgeklärt, wobei Silane anstelle von H2 verwendet wurden. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

FeFe]‐Hydrogenase Models and Hydrogen: Oxidative Addition of Dihydrogen and Silanes

Super model: Diiron dithiolates exist in nature in [FeFe]-hydrogenase, for the purpose of making or oxidizing molecular hydrogen. Evidence for how H2 interacts with diiron complexes has been realized using model diiron dithiolato carbonyl complexes. The pathway for this interaction has been elucidated computationally as well as synthetically, by using silanes in place of H2.

Aza- and Oxadithiolates Are Probable Proton Relays in Functional Models for the [FeFe]-Hydrogenases

The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH(2))(2)NH, and oxadithiolate (odt), (SCH(2))(2)O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solution and the terminal site on one Fe center. The results are consistent with the heteroatom of the dithiolate serving a gating role for both protonation and deprotonation. The pK(a) of the transiently formed ammonium (pK(CD(2))(Cl(2)) 5.7-8.2) or oxonium (pK(CD(2))(Cl(2)) -4.7-1.6) regulates the proton transfer. As a consequence, only the azadithiolate is capable of yielding the terminal hydride from weak acids. The aza- and oxadithiolates manifested the advantages of proton relays: the odt derivative proved to be a faster catalyst for hydrogen evolution than the pdt derivative as indicated from cyclic voltammetry plots of i(c)/i(p) vs [H(+)]. The adt derivative was capable of proton reduction from the weak acid [HPMe(2)Ph]BF(4) (pK(CD(2))(Cl(2)) = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe(2)(xdt)(mu-H)(CO)(2)(dppv)(2)](+), where the hydride is too distant and too basic to interact to be affected by the heteroatomic relay site. None of these mu-H species can be deprotonated.

Synthetic Support of De Novo Design: Sterically Bulky [FeFe]‐Hydrogenase Models

A twisted mimic: Upon oxidation of [(μ-SCH2C(CH3)2CH2S-){FeI(CO)2PMe3}2], rearrangement yields the mixed-valent FeIFeII cation in a square-pyramid/inverted square-pyramid geometry with a semibridging CO ligand, closely mimicking the [FeFe] hydrogenase enzyme active site. According to de novo design principles, the steric effect of bridgehead bulk in the S–S bridging ligand stabilizes this structure in the absence of the protein matrix.

Synthetic Support of De Novo Design: Sterically Bulky [FeFe]‐Hydrogenase Models

Bei der Oxidation lagert sich [(μ-SCH2C(CH3)2CH2S){FeI(CO)2PMe3}2] in ein gemischtvalentes FeIFeII-Kation mit quadratisch-pyramidaler und invertiert quadratisch-pyramidaler Koordinationsgeometrie und einem halbverbrückenden CO-Liganden um, das dem aktiven Zentrum der [FeFe]-Hydrogenase sehr ähnlich ist. Der sterische Einfluss des S,S-Brückenliganden stabilisiert diese Struktur auch außerhalb der Proteinmatrix.

Functionalized Sugars as Ligands towards Water‐Soluble [Fe‐only] Hydrogenase Models

AbstractOnly a small number of water‐soluble iron carbonyl type model compounds for the active centre in [Fe‐only] hydrogenase are known, and these models are mainly prepared by substitution of one of the CO ligands. In this publication we present new water‐soluble models for [Fe‐only] hydrogenase that contain a peripherally bound sugar residue. Compounds were prepared in which the 1,3‐disulfanyl‐2‐propyltetra‐O‐acetyl‐β‐D‐glucopyranoside unit is coordinated to the iron carbonyl core through either a sulfur or a selenium bridgehead atom. Electrochemical results for both types of compounds reveal hydrogen gas evolution from acetic acid and water. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A Ferrocene–Peptide Conjugate as a Hydrogenase Model System

AbstractThis work introduces a bis(cysteine) ligand to build a small peptidic model system of hydrogenase enzymes. Fe(C5H4CO–Cys–OMe)2 (LH2) has been employed as a chelate for an iron–carbonyl complex, which mimics two essential structural properties of the hydrogenase class of enzymes, namely the coordination of the iron–carbonyl core to peptide ligands and the presence of an electrochemical relay in spatial proximity. The treatment of LH2 with Fe3(CO)12 yields LFe2(CO)6 (3a), which is the first peptide‐coordinated iron hydrogenase active‐site model complex. Compound 3a was fully characterized spectroscopically (1H NMR, 13C NMR, IR and Mössbauer spectroscopy, mass spectrometry and elemental analysis). A single‐crystal X‐ray analysis confirms the proposed structure and reveals a staggered conformation of the Fe2(CO)6S2 core. Fourier transform infrared (FTIR) spectroelectrochemistry reveals an electronic interaction between the peptide backbone and the iron–carbonyl cluster, but not with the ferrocene subsite. The introduction of this peptidic cysteine‐based ligand into hydrogenase model chemistry helps to confirm the proposed cofactor biosynthesis and understand the electronic interplay between the metal–carbonyl active site and the protein environment in this important class of enzymes.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Precursors to [FeFe]-Hydrogenase Models: Syntheses of Fe2(SR)2(CO)6 from CO-Free Iron Sources

This report describes routes to iron dithiolato carbonyls that do not require preformed iron carbonyls. The reaction of FeCl 2, Zn, and Q 2S 2C n H 2 n (Q (+) = Na (+), Et 3NH (+)) under an atmosphere of CO affords Fe 2(S 2C n H 2 n )(CO) 6 ( n = 2, 3) in yields >70%. The method was employed to prepare Fe 2(S 2C 2H 4)( (13)CO) 6. Treatment of these carbonylated mixtures with tertiary phosphines, instead of Zn, gave the ferrous species Fe 3(S 2C 3H 6) 3(CO) 4(PR 3) 2, for R = Et, Bu, and Ph. Like the related complex Fe 3(SPh) 6(CO) 6, these compounds consist of a linear arrangement of three conjoined face-shared octahedral centers. Omitting the phosphine but with an excess of dithiolate, we obtained the related mixed-valence triiron species [Fe 3(S 2C n H 2 n ) 4(CO) 4] (-). The highly reducing all-ferrous species [Fe 3(S 2C n H 2 n ) 4(CO) 4] (2-) is implicated as an intermediate in this transformation. Reactive forms of iron, prepared by the method of Rieke, also combined with dithiols under a CO atmosphere to give Fe 2(S 2C n H 2 n )(CO) 6 in modest yields under mild conditions. Studies on the order of addition indicate that ferrous thiolates are formed prior to the onset of carbonylation. Crystallographic characterization demonstrated that the complexes Fe 3(S 2C 3H 6) 3(CO) 4(PEt 3) 2 and PBnPh 3[Fe 3(S 2C 3H 6) 4(CO) 4] feature high-spin ferrous and low-spin ferric as the central metal, respectively.

Noncovalent Assembly of a Metalloporphyrin and an Iron Hydrogenase Active-Site Model: Photo-Induced Electron Transfer and Hydrogen Generation

A noncovalent assembly of a pyridyl-functionalized hydrogenase active-site model complex and zinc tetraphenylporphyrin has been obtained and characterized. Upon light irradiation, fluorescence quenching by electron transfer was observed from the singlet excited state of the porphyrin to the diiron center, and the mechanism was verified by fluorescence lifetime and transient absorption spectroscopic measurements. In contrast to molecular dyads linked by covalent bonds, the assembled system was designed to avoid charge recombination via complex dissociation after photo-induced electron transfer. Visible light-driven hydrogen generation was observed from this self-assembled system. The assembling strategy employed in this study has the potential to be used for any other hydrogenase models in the future.

Models of the iron-only hydrogenase: Synthesis and protonation of bridge and chelate complexes [Fe2(CO)4{Ph2P(CH2)nPPh2}(μ-pdt)] (n = 2–4) – evidence for a terminal hydride intermediate

Abstract Reactions of [Fe 2 (CO) 6 (μ-pdt)] (pdt = SCH 2 CH 2 CH 2 S) and diphosphines, Ph 2 P(CH 2 ) n PPh 2 ( n = 2–4) and trans -Ph 2 PCH CHPPh 2 , have been carried out under different conditions. For all, at room temperature in MeCN with added Me 3 NO·2H 2 O the diphosphine-linked complexes [{Fe 2 (CO) 5 (μ-pdt)} 2 (μ,κ 1 ,κ 1 -diphosphine)] result. For trans -Ph 2 PCH CHPPh 2 this is the only product under all conditions. It has been crystallographically characterised revealing a C 2 symmetric structure with apical substitution at the diiron centres. In refluxing toluene, reactions with dppe and dppp lead to the formation of a mixture of diphosphine-bridged and chelate isomers [Fe 2 (CO) 4 (μ-diphosphine)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -diphosphine)(μ-pdt)], respectively, while with dppb the bridged complex [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)] is the only product. In MeCN at 60–70 °C (with added Me 3 NO·2H 2 O) similar products result although the ratios differ providing evidence for the conversion of chelate to bridge isomers. Three complexes, [Fe 2 (CO) 4 (μ-dppe)(μ-pdt)], [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] and [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)], have been crystallographically characterised and are compared to the previously reported dppm ( n = 1) complexes [Fe 2 (CO) 4 (μ-dppm)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -dppm)(μ-pdt)]. Diphosphine-bridged complexes are structurally superficially similar although significant differences are noted in some key bond lengths and angles, while chelate complexes [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -dppm)(μ-pdt)] differ in adopting basal–apical and dibasal coordination geometries, respectively, in the solid state. A number of protonation studies have been carried out. Addition of HBF 4 ·Et 2 O to [Fe 2 (CO) 4 (μ-dppe)(μ-pdt)] affords a bridging hydride complex with poor stability, while in contrast with [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)] the stable hydride [(μ-H)Fe 2 (CO) 4 (μ-dppb)(μ-pdt)][BF 4 ] results. This difference is partially ascribed to the greater flexibility of the diphosphine backbone in dppb. With [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] the bridging hydride complex [(μ-H)Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)][BF 4 ] is the final product, in which the diphosphine occupies two basal sites. Monitoring by NMR at low temperature shows the initial formation of a terminal hydride, which rapidly rearranges to a bridged isomer in which the diphosphine adopts a basal–apical geometry and this in turn rearranges in a slower process to the dibasal isomer. This behavior is similar to that recently communicated for [Fe 2 (CO) 4 (κ 2 -dppe)(μ-pdt)]. [S. Ezzaher, J.-F. Capon, F. Gloaguen, F.Y. Petillon, P. Schollhammer, J. Talarmin, R. Pichon, N. Kervarec, Inorg. Chem. 46 (2007) 3426–3428.]

Nanoscale ensembles using building blocks inspired by the [FeFe]-hydrogenase active site

Abstract The hydrogenase model [Fe2(S2C3H6)(CN)2(CO)4]2− was employed as a molecular tecton for the construction of supramolecular aggregates. IR spectroscopy indicated that cyanide bridged aggregates are formed when [Fe2(S2C3H6)(CN)2(CO)4]2− was treated with Lewis acids such as Zn(tetraphenylporphyrinate), [Cu(NCMe)(2,2′-bipyridine)]PF6 and [Cu(NCMe)4]PF6. Condensation of [Fe2(S2C3H6)(CN)2(CO)4]2− with the tritopic Lewis acid [Cp∗Rh]2+ afforded the novel expanded tetrahedron cage, {[Fe2(S2C3H6)(CN)2(CO)4]6[Cp∗Rh]4}4−. The tetrahedron cage was characterized crystallographically as the PPN salt.

On the electrochemistry of diiron dithiolate complexes related to the active site of the [FeFe]H2ase

Abstract A few recent electrochemical studies of diiron models of the iron-only hydrogenases' active site are summarized. Emphasis is put on the reduction mechanisms of hexacarbonyl complexes and on the different mechanisms of proton reduction that may operate depending on the nature of the complex and the strength of the acid. An attempt is made to discuss the thermodynamic and kinetic limitations of proton reduction processes supported by these compounds.

Photochemical studies of iron-only hydrogenase model compounds

FTIR studies of the photochemical reaction of Fe 2(SCH 2CH 2CH 2S)(CO) 6 ( 1) in both acetonitrile and toluene are described. Photochemical and thermal reactivity was studied under air, nitrogen and CO, and results are consistent with the loss of CO from 1 upon photolysis and the generation of a solvento species in both coordinating and non-coordinating solvents.

Influence of an electron-deficient bridging o-carborane on the electronic properties of an [FeFe] hydrogenase active site model

The IR carbonyl stretching frequencies of [Fe2(SRS)(CO)6] complexes correlate well with their first reduction potential; an [FeFe] hydrogenase model with a very mild reduction potential has been realized by using a strongly electron deficient carborane-dithiolate bridge.