Hydrogenase Active Site Research Articles

Introducing a dark reaction to photochemistry: photocatalytic hydrogen from [FeFe] hydrogenase active site model complexes

The light-driven splitting of water into its constituting elements gives access to a valuable fuel from an abundant substrate, using sunlight as the only energy source. Synthetic diiron complexes as functional models of the [FeFe] hydrogenase H2ase enzyme active site have moved into the centre of focus as potentially viable catalysts for the reductive side of this process, i.e. the reduction of protons to molecular hydrogen. The active site of the enzyme, as well as its mimics in an artificial system, are required to accumulate two electrons from single electron transfer events and to combine them with two protons to form hydrogen. Whereas in biology this reaction is not coupled to photosynthesis and thus proceeds in the dark, additional aspects need to be considered when designing a functional artificial system for the light-driven reduction of protons. Suitable photosensitizers have to be chosen that not only provide sufficient driving force for the reduction of the synthetic diiron catalyst, but also allow for selective excitation to minimize photodegradation. Electron transfer efficiencies have to be optimized for all steps and the sequential nature of the catalyst reduction requires a sufficient stability of potentially labile intermediates of the catalytic cycle. In this perspective, systems for the light-driven conversion of protons to molecular hydrogen are discussed where the catalyst is based on model complexes of the [FeFe] H2ase active site. Covalently linked dyads, supramolecular assemblies and multi-component systems will be examined with an emphasis on mechanistic electron transfer schemes, the properties of the individual components, their scope and their potential limitations.

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)].

Penta-carbonyl-1κC,2κC-(ferrocenyl-diphenyl-phosphine-1κP)[μ-2-(4-methyl-phen-yl)-2-aza-propane-1,3-dithiol-ato-1:2κS,S':S,S']diiron(I)(Fe-Fe).

The title compound, [Fe2(C9H11NS2){Fe(C5H5)(C17H14P)}(CO)5], was prepared as an aza­dithiol­ato–iron model for the iron-only hydrogenase active site. The Fe2S2 unit exhibits a butterfly conformation and the ferrocenyldiphenyl­phosphine ligand is trans to the Fe—Fe bond. The Fe—Fe distance of 2.5160 (8) Å is longer than found in related model structures. Intra­molecular C—H⋯S and inter­molecular C—H⋯O hydrogen bonds are observed.

Preliminary purification of an iron hydrogenase from marine green alga Platymonas subcordiformis

Three dinuclear iron complexes containing pendant nitrogen bases in phosphine ligands with general formular (μ-pdt) [Fe2(CO)5L] (where pdt is SCH2CH2CH2S, L = PPh2NH(CH2)2N(CH3)2 (5), PPh2NH(2-NH2C6H4) (6), PPh2[2-N(CH3)2CH2C6H4] (7)), were prepared as the models of the [Fe–Fe] hydrogenase active site. The molecular structures of 5–7 were characterized by X-ray crystallography. The secondary amine in 6 has weak intramolecular hydrogen bonding with both the terminal nitrogen and sulfur atom, which may suggest a proton transfer pathway from amine in phosphine ligand to the sulfur atom of active site. Protonation of complexes 5 and 6 only occurred at the terminal nitrogen atom. Electrochemical properties of the complexes were studied in the presence of triflic acid by cyclic voltammetry.

Nitrosyl derivatives of diiron(I) dithiolates mimic the structure and Lewis acidity of the [FeFe]-hydrogenase active site.

This study probes the impact of electronic asymmetry of diiron(I) dithiolato carbonyls. Treatment of Fe2(S2C(n)H(2n))(CO)(6-x)(PMe3)x compounds (n = 2, 3; x = 1, 2, 3) with NOBF4 gave the derivatives [Fe2(S2C(n)H(2n))(CO)(5-x)(PMe3)x(NO)]BF4, which are electronically unsymmetrical because of the presence of a single NO(+) ligand. Whereas the monophosphine derivative is largely undistorted, the bis(PMe3) derivatives are distorted such that the CO ligand on the Fe(CO)(PMe3)(NO)(+) subunit is semibridging. Two isomers of [Fe2(S2C3H6)(CO)3(PMe3)2(NO)]BF4 were characterized spectroscopically and crystallographically. Each isomer features electron-rich Fe(CO)2PMe3 and electrophilic Fe(CO)(PMe3)(NO)(+) subunits. These species are in equilibrium with an unobserved isomer that reversibly binds CO (DeltaH = -35 kJ/mol, DeltaS = -139 J mol(-1) K(-1)) to give the symmetrical adduct [Fe2(S2C3H6)(mu-NO)(CO)4(PMe3)2]BF4. In contrast to Fe2(S2C3H6)(CO)4(PMe3)2, the bis(PMe3) nitrosyl complexes readily undergo CO substitution to give the (PMe3)3 derivatives. The nitrosyl complexes reduce at potentials that are approximately 1 V milder than their carbonyl counterparts. Results of density functional theory calculations, specifically natural bond orbital analysis, reinforce the electronic resemblance of the nitrosyl complexes to the corresponding mixed-valence diiron complexes. Unlike other diiron dithiolato carbonyls, these species undergo reversible reductions at mild potentials. The results show that the novel structural and chemical features associated with mixed-valence diiron dithiolates (the so-called H(ox) models) can be replicated in the absence of mixed-valency by the introduction of electronic asymmetry.

X-ray Structure of the [FeFe]-Hydrogenase Maturase HydE from Thermotoga maritima

Maturation of the [FeFe]-hydrogenase active site depends on at least the expression of three gene products called HydE, HydF, and HydG. We have solved the high resolution structure of recombinant, reconstituted S-adenosine-L-methionine-dependent HydE from Thermotoga maritima. Besides the conserved [Fe(4)S(4)] cluster involved in the radical-based reaction, this HydE was reported to have a second [Fe(4)S(4)] cluster coordinated by three Cys residues. However, in our crystals, depending on the reconstitution and soaking conditions, this second cluster is either a [Fe(2)S(2)] center, with water occupying the fourth ligand site or is absent. We have carried out site-directed mutagenesis studies on the related HydE from Clostridium acetobutylicum, along with in silico docking and crystal soaking experiments, to define the active site region and three anion-binding sites inside a large, positive cavity, one of which binds SCN(-) with high affinity. Although the overall triose-phosphate isomerase-barrel structure of HydE is very similar to that of biotin synthase, the residues that line the internal cavity are significantly different in the two enzymes.

Preparation, characterization and electrochemistry of an iron-only hydrogenase active site model covalently linked to a ruthenium tris(bipyridine) photosensitizer

An NH2-functionlized [Fe2S2] model complex of the iron-only hydrogenase active site was covalently linked to the tris(bipyridine)ruthenium photosensitizer. The [RuFeFe] trinuclear complex 1 was characterized by MS, IR, UV-vis, 1H & 13C NMR spectra. A quasi-reversible reduction peak at −1.41 V versus Ag/Ag+ for the FeIFeI/FeIFe0 process is observed in the cyclic voltammogram of 1.

Pendant bases as proton transfer relays in diiron dithiolate complexes inspired by [Fe–Fe] hydrogenase active site

Three dinuclear iron complexes containing pendant nitrogen bases in phosphine ligands with general formular (μ-pdt) [Fe 2(CO) 5L] (where pdt is SCH 2CH 2CH 2S, L = PPh 2NH(CH 2) 2N(CH 3) 2 ( 5), PPh 2NH(2-NH 2C 6H 4) ( 6), PPh 2[2-N(CH 3) 2CH 2C 6H 4] ( 7)), were prepared as the models of the [Fe–Fe] hydrogenase active site. The molecular structures of 5– 7 were characterized by X-ray crystallography. The secondary amine in 6 has weak intramolecular hydrogen bonding with both the terminal nitrogen and sulfur atom, which may suggest a proton transfer pathway from amine in phosphine ligand to the sulfur atom of active site. Protonation of complexes 5 and 6 only occurred at the terminal nitrogen atom. Electrochemical properties of the complexes were studied in the presence of triflic acid by cyclic voltammetry.

A DFT investigation on structural and redox properties of a synthetic Fe6S6 assembly closely related to the [FeFe]-hydrogenases active site

In the present contribution, a density functional theory (DFT) investigation is described regarding a recently synthesized Fe6S6 complex – see C. Tard, X. Liu, S.K. Ibrahim, M. Bruschi, L. De Gioia, S.C. Davies, X. Yang, L.-S. Wang, G. Sawers, C.J. Pickett, Nature 433 (2005) 610 – that is structurally and functionally related to the [FeFe]-hydrogenases active site (the so-called H-cluster, which includes a binuclear subsite directly involved in catalysis and an Fe4S4 cubane). The analysis of relative stabilities and atomic charges of different isomers evidenced that the structural and redox properties of the synthetic assembly are significantly different from those of the enzyme active site. A comparison between the hexanuclear cluster and simpler synthetic diiron models is also described; the results of such a comparison indicated that the cubane moiety can favour the stabilization of the cluster in a structure closely resembling the H-cluster geometry when the synthetic Fe6S6 complex is in its dianionic state. However, the opposite effect is observed when the synthetic cluster is in its monoanionic form.

Thiolate-bridged dinuclear iron(tris-carbonyl)-nickel complexes relevant to the active site of [NiFe] hydrogenase.

The reaction of NiBr(2)(EtOH)(4) with a 1:2-3 mixture of FeBr(2)(CO)(4) and Na(SPh) generated a linear trinuclear Fe-Ni-Fe cluster (CO)(3)Fe(mu-SPh)(3)Ni(mu-SPh)(3)Fe(CO)(3), 1, whereas the analogous reaction system FeBr(2)(CO)(4)/Na(S(t)Bu)/NiBr(2)(EtOH)(4) (1:2-3:1) gave rise to a linear tetranuclear Fe-Ni-Ni-Fe cluster [(CO)(3)Fe(mu-S(t)Bu)(3)Ni(mu-Br)](2), 2. By using this tetranuclear cluster 2 as the precursor, we have developed a new synthetic route to a series of thiolate-bridged dinuclear Fe(CO)(3)-Ni complexes, the structures of which mimic [NiFe] hydrogenase active sites. The reactions of 2 with SC(NMe(2))(2) (tmtu), Na{S(CH(2))(2)SMe} and ortho-NaS(C(6)H(4))SR (R = Me, (t)Bu) led to isolation of (CO)(3)Fe(mu-S(t)Bu)(3)NiBr(tmtu), 3, (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{S(CH(2))(2)SMe}, 4, and (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{S(C(6)H(4))SR}, 5a (R = Me) and 5b (R = (t)Bu), respectively. On the other hand, treatment of 2 with 2-methylthio-phenolate (ortho-O(C(6)H(4))SMe) in methanol resulted in (CO)(3)Fe(mu-S(t)Bu)(3)Ni(MeOH){O(C(6)H(4))SMe}, 6a. The methanol molecule bound to Ni is labile and is readily released under reduced pressure to afford (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{O(C(6)H(4))SMe}, 6b, and the coordination geometry of nickel changes from octahedral to square planar. Likewise, the reaction of 2 with NaOAc in methanol followed by crystallization from THF gave (CO)(3)Fe(mu-S(t)Bu)(3)Ni(THF)(OAc), 7. The dinuclear complexes, 3-7, are thermally unstable, and a key to their successful isolation is to carry out the reactions and manipulations at -40 degrees C.

Tuning the electronic properties of Fe2(μ-arenedithiolate)(CO)6−n(PMe3)n (n = 0, 2) complexes related to the [Fe–Fe]-hydrogenase active site

Tuning the electronic properties of Fe2(-arenedithiolate)(CO)6-n(PMe3)n (n=0, 2) complexes related to the [Fe-Fe]-hydrogenase active site

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.

Mounting a hydrogenase analog on calixarenes—designing a nature‐inspired solid state catalyst for fuel cells by density functional theory

AbstractThe hydrogen‐fuel PEFC (polymer electrolyte fuel cell) is among the most promising alternative power generators of the future. But for it to become widely used, an alternative catalyst to platinum must be developed. While there have been previous attempts to synthesize hydrogenase mimics for this purpose, the current state of artificial hydrogenases can be summarized into two groups: sufficiently active but not in a stable solid state; and in the solid state but not sufficiently active.We model a modified active site of Fe‐only hydrogenases deposited on a variety of calixarenes for solid‐state support by density functional calculations. Among the calixarenes considered, our calculations have shown that calix[5]arene is so far the most viable macrocycle to hold the di‐iron site. Our calculations also show that the crucial hydrogenase active site characteristics are maintained in this material: having an FeFe bond and anti‐bond at the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) respectively; and the bonding orbitals on the Fe centers connected to the sulfur of the electron chain at the HOMO. Copyright © 2008 John Wiley & Sons, Ltd.

A proton–hydride diiron complex with a base-containing diphosphine ligand relevant to the [FeFe]-hydrogenase active site

A diiron dithiolate complex holding a mu-hydride on the iron atoms and a proton on the basic site of a chelating diphosphine ligand was prepared and crystallographically characterized as a structural model of the [FeFe]-hydrogenase active site, and its molecular structure shows the H(mu)(-)...H(N)(+) distance is 3.934 A.

Selenium-bridged diiron hexacarbonyl complexes as biomimetic models for the active site of Fe–Fe hydrogenases

Three N-substituted selenium-bridged diiron complexes [{(mu-SeCH2)2NC6H4R}Fe2(CO)6] (R = 4-NO2, 7; R = H, 8; R = 4-CH3, 9) were firstly prepared as biomimetic models for the Fe-Fe hydrogenases active site. Models could be generated by the convergent reaction of [(mu-HSe)2Fe2(CO)6] (6) with N,N-bis(hydroxymethyl)-4-nitroaniline (1), N,N-bis(hydroxymethyl)aniline (2), and N,N-bis(hydroxymethyl)-4-methylaniline (3) in 46-52% yields. All the new complexes were characterized by IR, 1H and 13C NMR and HRMS spectra and their molecular structures were determined by single-crystal X-ray analysis. The redox properties of and their dithiolate analogues [{(mu-SCH2)2NC6H4R}Fe2(CO)6] (R = 4-NO2, 7s; R = H, 8s; R = 4-CH3, 9s ) were evaluated by cyclic voltammograms. The electrochemical proton reduction by and were investigated in the presence of p-toluenesulfonic acid (HOTs) to evaluate the influence of changing the coordinating S atoms of the bridging ligands to Se atoms on the electrocatalytic activity for proton reduction.

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.

Diiron dithiolate complexes containing intra-ligand NH⋯S hydrogen bonds: [FeFe] hydrogenase active site models for the electrochemical proton reduction of HOAc with low overpotential

Four diiron dithiolate complexes containing ortho-acylamino-functionalized arenethiolato ligands, [(micro-S-2-RCONHC6H4)2Fe2(CO)6] (R = CH3, 1; CF3, 2; C6H5, 3; 4-FC6H4, 4), were synthesized and well characterized as biomimetic models of the Fe-Fe hydrogenase active site. The molecular structures of and 4 were determined by X-ray crystallography. The intra-ligand NHS hydrogen bonds were studied by the X-ray analysis and by the (1)H NMR spectroscopy. The contribution of the NHS hydrogen bonds to the reduction potentials of complexes was investigated by electrochemistry. The first reduction potentials of complexes exhibit large positive shifts, that is, 220-320 mV in comparison to that of the analogous complex [(micro-SPh)2Fe2(CO)6] and 370-470 mV to that of [(micro-pdt)2Fe2(CO)6] (pdt = propane-1,3-dithiolato). Complex is capable of electrocatalysing proton reduction of acetic acid at relatively low overpotential (ca. 0.2 V) in acetonitrile.

FeFe]-Hydrogenase active site models with relatively low reduction potentials: Diiron dithiolate complexes containing rigid bridges

Three diiron dithiolate complexes containing rigid and conjugated bridges, [μ-SC6H4-2-(CO)S-μ]Fe2(CO)6 (1), [2-μ-SC5H3N-3-(CO)S-μ]Fe2(CO)6 (2), and the PPh3-monosubstituted complex [μ-SC6H4-2-(CO)S-μ]Fe2(CO)5(PPh3) (1-P), were prepared as biomimetic models for the [FeFe]-hydrogenase active site. The structures of complexes 1 and 2 were determined by single crystal X-ray analysis, which shows that each complex features a rigid coplanar dithiolate bridge with a 2–3° deviation from the bisect plane of the molecule. The influence of the rigid bridge on the reduction potentials of complexes 1, 2 and 1-P was investigated by electrochemistry. The cyclic voltammograms of complexes 1 and 2 display large positive shifts for the primary reduction potentials, that is, 380–480mV in comparison to that of the pdt-bridged (pdt=propane-1,3-dithiolato) complex (μ-pdt)Fe2(CO)6 and 160–260mV to that of the bdt-bridged (bdt=benzene-1,2-dithiolato) analogue (μ-bdt)Fe2(CO)6.

Iron‐Only Hydrogenase Active Site Models Containing a Cysteinyl Group Coordinated through Its Sulfur Atom to One Iron Atom of the Diiron Subsite

AbstractWe have developed a simple and convenient method for the synthesis of the first H‐cluster models in which a L‐cysteinyl group is coordinated to one of the two iron atoms of the diiron subsite through its sulfur atom. This synthetic method includes (i) treatment of the Boc‐protected L‐cysteine ester Boc‐NHCH(CH2SH)CO2Et (1, Boc = tert‐butoxycarbonyl) with EtONa to give the L‐cysteinyl mercaptide NaSCH2CH(NH‐Boc)CO2Et (2); (ii) further treatment of 2 with [Cp(CO)2FeI] to produce the metallothioether ligand Cp(CO)2FeSCH2CH(NH‐Boc)CO2Et (3); and (iii) treatment of the parent diiron complex [Fe2(μ‐SCH2)2CH2(CO)6] (4), [Fe2(μ‐SCH2)2N(tBu)(CO)6] (5), or [Fe2(μ‐SCH2)2N(C6H4OMe‐p)(CO)6] (6) with Me3NO·2H2O followed by ligand 3 to afford the target model compounds [Fe2(μ‐SCH2)2CH2(CO)5(ligand 3)] (7), [Fe2(μ‐SCH2)2N(tBu)(CO)5(ligand 3)] (8), or [Fe2(μ‐SCH2)2N(C6H4OMe‐p)(CO)5(ligand 3)] (9), respectively. All the new compounds 2, 3, and 7–9 have been characterized by elemental analysis and various spectroscopic techniques. The X‐ray diffraction analysis of 8 has confirmed that these models contain a cysteinyl sulfur atom not only coordinated to one Fe atom of the diiron subsite, but also to the Fe atom of the Cp(CO)2Fe moiety to form the linkage [FeCp‐(μ‐cysteinyl‐S)‐Fesubsite], which is similar to [Fecubane‐(μ‐cysteinyl‐S)‐Fesubsite] found in natural enzymes. In addition, spectroscopic and electrochemical measurements have further demonstrated that the linkage [FeCp‐(μ‐cysteinyl‐S)‐Fesubsite] can provide substantial electronic communication between the diiron subsite and the Cp(CO)2Fe moiety. Under electrochemical conditions, 8 has been shown to be a catalyst for HOAc proton reduction to dihydrogen, and a new type of E*2E2C mechanism for this catalytic reaction is suggested.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Synthetic Hydrogenases: Incorporation of an Iron Carbonyl Thiolate into a Designed Peptide

[FeFe] hydrogenases catalyze reversible hydrogen oxidation at an unusual organometallic active site. Neither enzymatic studies nor synthesis of small molecule models has managed to elucidate the mechanisms of these enzymes. In this paper, we demonstrate the incorporation of an iron carbonyl thiolate mimic of the hydrogenase active site into a de novo artificial peptide, creating the first peptide-based model system for hydrogenases.