The Third Hydrogenase: More Natural Organometallics

Synthesis, Structures and Chemical Reactivity of Dithiolato-Bridged Ni-Fe Complexes as Biomimetics for the Active Site of [NiFe]-Hydrogenases

To develop the structural and functional modeling chemistry of [NiFe]-H2ases, we have carried out a study regarding the synthesis, structural characterization and reactivity of a new series of [NiFe]-H2ase model complexes. Thus, treatment of diphosphine dppb-chelated Ni complex (dppb)NiCl2 (dppb = 1,2-(Ph2P)2C6H4) with (dppv)Fe(CO)2(pdt) (dppv = 1,2-(Ph2P)2C2H2, pdt = 1,3-propanedithiolate) and NaBF4 gave dicarbonyl complex [(dppb)Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([A](BF4)2). Further treatment of [A](BF4)2 with Me3NO and Bu4NCN or KSCN afforded t-cyanido and t-isothiocyanato complexes [(dppb)Ni(pdt)Fe(CO)(t-R)(dppv)]BF4 ([1]BF4, R = CN; [2]BF4, R = NCS), respectively. While azadiphosphine MeN(CH2PPh2)2-chelated t-hydride complex [MeN(CH2PPh2)2Ni(pdt)Fe(CO)(t-H)(dppv)]BF4 ([3]BF4) was prepared by treatment of dicarbonyl complex [MeN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([B](BF4)2) with Me3NO and 1.5 MPa of H2, treatment of dicarbonyl complex [B](BF4)2 with Me3NO (without H2) in pyridine resulted in formation of a novel monocarbonyl complex [MeN(CH2PPh2)2Ni(SCHCH2CH2S)Fe(CO)(dppv)]BF4 ([4]BF4) via the unexpected sp3 C-H bond activation reaction. Furthermore, azadiphosphine PhN(CH2PPh2)2-chelated µ-mercapto complex [PhN(CH2PPh2)2Ni(pdt)Fe(CO)(µ-SH)(dppv)]BF4 ([5]BF4) was prepared by treatment of dicarbonyl complex [PhN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([C](BF4)2) with Me3NO and H2S gas, whereas treatment of azadiphosphine Ph2CHN(CH2PPh2)2-chelated dicarbonyl complex [Ph2CHN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppe)](BF4)2 ([D](BF4)2, dppe = 1,2-(Ph2P)2C2H4) with Me3NO⋅2H2O gave rise to µ-hydroxo complex [Ph2CHN(CH2PPh2)2Ni(pdt)Fe(CO)(µ-OH)(dppe)]BF4 ([6]BF4). All the possible pathways for formation of the new model complexes are briefly discussed, and their structures were fully characterized by various spectroscopic techniques and for six of them by X-ray crystallography.

Biomimetic models of [Fe]-hydrogenase featuring a 2-acylphenylthiomethyl-6-R-pyridine (R = H or OMe) ligand

A new flexible pyridine ligand (FPL)-based method is developed, by which two novel biomimetic models of [Fe]-H2ase are prepared and their enzyme-like H2/D2 activation functions are studied.

Transport of nano zerovalent iron (nZVI) coupling with Alcaligenes sp. strain in porous media

Coupling nano zerovalent iron (nZVI) particles with anaerobic bacteria is a potentially powerful approach for remediating polluted groundwater. However, little is known about the transport of these mixed systems in porous media, which could potentially affect the system's activity and half-life in aqueous environments. This study assessed the transport and stability of nZVI coupled with Alcaligenes sp. TB by column experiments and sedimentation tests. The results showed that combined bio-nZVI systems experienced significantly higher transport and lower sedimentation rates than stand-alone nZVI. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images showed that Alcaligenes sp. TB reduced aggregation of nZVI to some extent, though slight toxicity to bacteria was observed. The results of ζ-potential measurements demonstrated that the presence of bacteria increased the electrostatic force between the particles. Voltammetry, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analysis confirmed that the bio-nZVI system undergoes different redox processes. The presence of bacteria favored the formation of FeOOH not Fe₂O₃ or Fe₃O₄, resulting in weaker surface magnetic properties.

Coupling nano zerovalent iron (nZVI) particles with anaerobic bacteria experienced significantly higher transport and lower sedimentation rates than stand-alone nZVI.

Synthesis, characterization, and some properties of two types of new [Fe]-H2ase models containing a 4-phosphatopyridine or a 4-phosphatoguanosinepyridine moiety

The novel [Fe]-H₂ase active site framework-containing model 6 was first prepared and structurally characterized.

Embedding a Ruthenium-Based Structural Mimic of the [Fe]-Hydrogenase Cofactor into Papain

We describe the synthesis of the ruthenacyclic carbamoyl complexes [Ru(2-NHC(O)C₅H₃NMe)(CO)₂( o,o-Me₂-C₆H₃S)(L)] (L = H₂O or MeCN), which have a labile water or acetonitrile ligand at their sixth coordination sites. Steric bulk around the ruthenium center is essential in preventing isomerization and dimerization, and embedding within papain can be achieved via coordination of its sole free cysteine residue. The observed chemistry parallels that of the natural [Fe]-hydrogenase.

Structural Mimics of the [Fe]-Hydrogenase: A Complete Set for Group VIII Metals

A set of structural mimics of the [Fe]-hydrogenase active site comprising all the group VIII metals, viz., [M(2-NHC(O)C₅H₄N)(CO)₂(2-S-C₅H₄N)], has been synthesized. They exist as a mixture of isomers in solution, and the relative stability of the isomers depends on the nature of the metal and the substituent at the 6-position of the pyridine ligand.

Reversible Hydride Transfer to N,N'-Diarylimidazolinium Cations from Hydrogen Catalyzed by Transition Metal Complexes Mimicking the Reaction of [Fe]-Hydrogenase

[Fe]-hydrogenase is a key enzyme involved in methanogenesis and facilitates reversible hydride transfer from H₂ to N⁵,N¹⁰-methenyltetrahydromethanopterin (CH-H₄MPT⁺). In this study, a reaction system was developed to model the enzymatic function of [Fe]-hydrogenase by using N,N'-diphenylimidazolinium cation (1⁺) as a structurally related alternative to CH-H₄MPT⁺. In connection with the enzymatic mechanism via heterolytic cleavage of H₂ at the single metal active site, several transition metal complex catalysts capable of such activation were utilized in the model system. Reduction of 1[BF₄] to N,N'-diphenylimidazolidine (2) was achieved under 1 atm H₂ at ambient temperature in the presence of an equimolar amount of NEt₃ as a proton acceptor. The proposed catalytic pathways involved the generation of active hydride complexes and subsequent intermolecular hydride transfer to 1⁺. The reverse reaction was accomplished by treatment of 2 with HNMe₂Ph⁺ as the proton source, where [(η⁵-C₅Me₅)Ir{(p-MeC₆H₄SO₂)NCHPhCHPhNH}] was found to catalyze the formation of 1⁺ and H₂ with high efficiency. These results are consistent with the fact that use of 2,6-lutidine in the forward reaction or 2,6-lutidinium in the reverse reaction resulted in incomplete conversion. By combining these reactions using the above Ir amido catalyst, the reversible hydride transfer interconverting 1⁺/H₂ and 2/H⁺ was performed successfully. This system demonstrated the hydride-accepting and hydride-donating modes of biologically relevant N-heterocycles coupled with proton concentration. The influence of substituents on the forward and reverse reactivities was examined for the derivatives of 1⁺ and 2 bearing one para-substituted N-phenyl group.

Substitution reactions of iron(ii) carbamoyl-thioether complexes related to mono-iron hydrogenase

A C,N,S pincer complex has been synthesized for structural modeling of the organometallic active site of mono-[Fe] hydrogenase (HMD).

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

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

A Functional Model of [Fe]-Hydrogenase

[Fe]-Hydrogenase catalyzes the hydrogenation of a biological substrate via the heterolytic splitting of molecular hydrogen. While many synthetic models of [Fe]-hydrogenase have been prepared, none yet are capable of activating H2 on their own. Here, we report the first Fe-based functional mimic of the active site of [Fe]-hydrogenase, which was developed based on a mechanistic understanding. The activity of this iron model complex is enabled by its unique ligand environment, consisting of biomimetic pyridinylacyl and carbonyl ligands, as well as a bioinspired diphosphine ligand with a pendant amine moiety. The model complex activates H2 and mediates hydrogenation of an aldehyde.

Mononuclear, Dinuclear, and Trinuclear Iron Complexes Featuring a New Monoanionic SNS Thiolate Ligand

The new tridentate ligand, S(Me)N(H)S = 2-(2-methylthiophenyl)benzothiazolidine, prepared in a single step from commercial precursors in excellent yield, undergoes ring-opening on treatment with Fe(OTf)2 in the presence of base affording a trinuclear iron complex, [Fe3(μ2-S(Me)NS(-))4](OTf)2 (1) which is fully characterized by structural and spectroscopic methods. X-ray structural data reveal that 1 contains four S(Me)NS(-) ligands meridionally bound to two pseudooctahedral iron centers each bridged by two thiolates to a distorted tetrahedral central iron. The combined spectroscopic (UV-vis, Mössbauer, NMR), magnetic (solution and solid state), and computational (DFT) studies indicate that 1 includes a central, high-spin Fe(II) (S = 2) with two low-spin (S = 0) peripheral Fe(II) centers. Complex 1 reacts with excess PMePh2, CNxylyl (2,6-dimethylphenyl isocyanide), and P(OMe)3 in CH3CN to form diamagnetic, thiolate-bridged, dinuclear Fe(II) complexes {[Fe(μ-S(Me)NS(-))L2]2}(OTf)2 (2-4). These complexes are characterized by elemental analysis; (1)H NMR, IR, UV-vis, and Mössbauer spectroscopy; and single crystal X-ray diffraction. Interestingly, addition of excess P(OMe)3 to complex 1 in CH2Cl2 produces primarily the diamagnetic, mononuclear Fe(II) complex, {Fe(S(Me)NS(-))[P(OMe)3]3}(OTf) (5).

Synthesis, structural characterization, and chemical properties of pentacoordinate model complexes for the active site of [Fe]-hydrogenase

Four pentacoordinate iron dicarbonyl with bulky NHC ligands were synthesised as model of [Fe]-hydrogenase active site, which exhibited different protonation reactivity due to the variable electronic and steric effects of introduced ligands.

Toward functional type III [Fe]-hydrogenase biomimics for H2 activation: insights from computation

The chemistry of [Fe]-hydrogenase has attracted significant interest due to its ability to activate molecular hydrogen. The intriguing properties of this enzyme have prompted the synthesis of numerous small molecule mimics aimed at activating H2. Despite considerable effort, a majority of these compounds remain nonfunctional for hydrogenation reactions. By using a recently synthesized model as an entry point, seven biomimetic complexes have been examined through DFT computations to probe the influence of ligand environment on the ability of a mimic to bind and split H2. One mimic, featuring a bidentate diphosphine group incorporating an internal nitrogen base, was found to have particularly attractive energetics, prompting a study of the role played by the proton/hydride acceptor necessary to complete the catalytic cycle. Computations revealed an experimentally accessible energetic pathway involving a benzaldehyde proton/hydride acceptor and the most promising catalyst.

[FeFe] hydrogenase: protonation of {2Fe3S} systems and formation of super-reduced hydride states

The synthesis and crystallographic characterization of a complex possessing a well-defined {2Fe3S(μ-H)} core gives access to a paramagnetic bridging hydride with retention of the core geometry. Chemistry of this 35-electron species within the confines of a thin-layer FTIR spectro-electrochemistry cell provides evidence for a unprecedented super-reduced Fe(I)(μ-H)Fe(I) intermediate.

Bioinhibitory effect of hydrogenotrophic bacteria on nitrate reduction by nanoscale zero-valent iron

Hydrogenotrophic bacteria (HTB) were introduced into a nitrate removal system, which used nanoscale zero-valent iron (nZVI) as reductant, to investigate its bioinhibitory effect. Based on the results, it was noted that addition of HTB culture (10-50 mL) led to 58.9-91.4% decrease in the first observed rate constant (kobs1), which represented the nitrate removal rate by nZVI, and a reduction in the generated poisonous by-products from 94.9% to 38.5%. In other words, HTB had a significant inhibitory effect on nitrate reduction by nZVI. However, the pathway of this bioinhibition only prevented the occurrence of chemical reduction, but not competition for nitrate. Furthermore, FeOOH coating was observed on the surface of nZVI, instead of Fe3O4 or Fe2O3, which could prevent electron transmission from nZVI to nitrate. Considering that FeOOH was the product of iron corrosion, the result indicated that HTB could inhibit chemical reduction by enhancing the reaction between nZVI and water.

Synthesis, structural characterization, and some properties of 2-acylmethyl-6-ester group-difunctionalized pyridine-containing iron complexes related to the active site of [Fe]-hydrogenase

The first four acylmethyl/ester group-disubstituted pyridine-containing models for [Fe]-hydrogenase have been synthesized and crystallographically characterized.

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.

Towards [NiFe]-hydrogenase biomimetic models that couple H2 binding with functionally relevant intramolecular electron transfers: a quantum chemical study

[FeFe]- and [NiFe]-hydrogenases are dihydrogen-evolving metalloenzymes that share striking structural and functional similarities, despite being phylogenetically unrelated. Most notably, they are able to combine substrate binding and redox functionalities, which has important bearings on their efficiency. Model complexes of [FeFe]-hydrogenases that are able to couple H2 binding with a substrate-dependent intramolecular electron transfer promoting dihydrogen activation were recently shown to reproduce the complex redox chemistry of the all-iron enzyme. Notably, coupling of H2 binding and intramolecular redox events was proposed to have a key role also in [NiFe]-hydrogenases, but this feature is not reproduced in currently available nickel-iron biomimetic compounds. In the present study, we exploit dedicated density functional theory approaches to show that H2 binding and activation on a NiFe core can be favored by the installment of conveniently substituted isocyanoferrocenes, thanks to their ability to undergo intramolecular reduction upon substrate binding. Our results support the concept that a unified view on hydrogenase chemistry is a key element to direct future efforts in the modeling of microbial H2 metabolism.

H2 binding and splitting on a new-generation [FeFe]-hydrogenase model featuring a redox-active decamethylferrocenyl phosphine ligand: a theoretical investigation

[FeFe]-hydrogenases are dihydrogen-evolving metalloenzymes that are able to combine substrate binding and redox functionalities, a feature that has important bearing on their efficiency. New-generation bioinspired systems such as Fe(2)[(SCH(2))(2)NBn](CO)(3)(Cp*Fe(C(5)Me(4)CH(2)PEt(2)))(dppv) were shown to mimic H(2) oxidation and splitting processes performed by the [FeFe]-hydrogenase/ferredoxin system, and key mechanistic aspects of such reaction are theoretically investigated in the present contribution. We found that H(2) binding and heterolytic cleavage take place concomitantly on DFT models of the synthetic catalyst, due to a substrate-dependent intramolecular redox process that promotes dihydrogen activation. Therefore, formation of an iron-dihydrogen complex as a reaction intermediate is excluded in the biomimetic system, at variance with the case of the enzyme. H(2) uptake at the synthetic system also requires an energetically disfavored isomerization of the amine group acting as a base during splitting. A possible strategy to stabilize the conformation competent for H(2) binding is proposed, along with an analysis of the reactivity of a triiron complex in which di(thiomethyl)amine--the chelating group naturally occurring in [FeFe]-hydrogenases--substitutes the benzyl-containing dithiolate ligand.