Nuclear resonance vibrational spectroscopy reveals the FeS cluster composition and active site vibrational properties of an O2-tolerant NAD+-reducing [NiFe] hydrogenase

Hydrogenases are complex metalloenzymes that catalyze the reversible splitting of molecular hydrogen into protons and electrons essentially without overpotential. The NAD⁺-reducing soluble hydrogenase (SH) from Ralstonia eutropha is capable of H₂ conversion even in the presence of usually toxic dioxygen. The molecular details of the underlying reactions are largely unknown, mainly because of limited knowledge of the structure and function of the various metal cofactors present in the enzyme. Here, all iron-containing cofactors of the SH were investigated by ⁵⁷Fe specific nuclear resonance vibrational spectroscopy (NRVS). Our data provide experimental evidence for one [2Fe2S] center and four [4Fe4S] clusters, which is consistent with the amino acid sequence composition. Only the [2Fe2S] cluster and one of the four [4Fe4S] clusters were reduced upon incubation of the SH with NADH. This finding explains the discrepancy between the large number of FeS clusters and the small amount of FeS cluster-related signals as detected by electron paramagnetic resonance spectroscopic analysis of several NAD⁺-reducing hydrogenases. For the first time, Fe–CO and Fe–CN modes derived from the [NiFe] active site could be distinguished by NRVS through selective ¹³C labeling of the CO ligand. This strategy also revealed the molecular coordinates that dominate the individual Fe–CO modes. The present approach explores the complex vibrational signature of the Fe–S clusters and the hydrogenase active site, thereby showing that NRVS represents a powerful tool for the elucidation of complex biocatalysts containing multiple cofactors.     

Proton‐Coupled Reduction of the Catalytic [4Fe‐4S] Cluster in [FeFe]‐Hydrogenases

In nature, [FeFe]‐hydrogenases catalyze the uptake and release of molecular hydrogen (H₂) at a unique iron‐sulfur cofactor. The absence of an electrochemical overpotential in the H₂ release reaction makes [FeFe]‐hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one‐electron reduction of [FeFe]‐hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton‐coupled electron transport during the formation of the reduced state Hred′. Charge compensation stabilizes the excess electron at the [4Fe‐4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred′ in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes.     

Iron Oxide Supported on Al2O3 Catalyst for Methane Decomposition Reaction: Effect of MgO Additive and Calcination Temperature

Production of hydrogen is a challenging task and have significant impact in the recent scenario. The alumina supported iron oxide nanoparticle synthesized using non‐ionic surfactant Triton‐X was found very effective for steady production of hydrogen through methane decomposition reaction. The high surface area, easily reducible catalyst calcined at 500 °C and 800 °C temperature showed steady activity towards methane decomposition reaction. At a higher reaction temperature there was catalyst deactivation. The doping of MgO facilitated particle growth rendering the poor catalytic activity. The TPR study showed that reducibility of TPR was difficult in presence of MgO additive. The formation of Fe? Mg? Al solid solution confirmed by XRD study was found mainly responsible for the lower catalytic activity. The bamboo‐shaped carbon nanotube formed from 20 % Fe/Al₂O₃ catalyst which is mainly because of the poor wetting property of quasi‐liquid metal and carbon nanotube.     

Kinetic features of ethylene polymerization over supported catalysts [2,6‐bis(imino)pyridyl iron dichloride/magnesium dichloride] with AlR3 as an activator

The effects of polymerization temperature, polymerization time, ethylene and hydrogen concentration, and effect of comonomers (hexene‐1, propylene) on the activity of supported catalyst of composition LFeCl₂/MgCl₂‐Al(i‐Bu)₃ (L = 2,6‐bis[1‐(2,6‐dimethylphenylimino)ethyl] pyridyl) and polymer characteristics (molecular weight (MW), molecular‐weight distribution (MWD), molecular structure) have been studied. Effective activation energy of ethylene polymerization over LFeCl₂/MgCl₂‐Al(i‐Bu)₃ has a value typical of supported Ziegler–Natta catalysts (11.9 kcal/mol). The polymerization reaction is of the first order with respect to monomer at the ethylene concentration >0.2 mol/L. Addition of small amounts of hydrogen (9–17%) significantly increases the activity; however, further increase in hydrogen concentration decreases the activity. The IRS and DSC analysis of PE indicates that catalyst LFeCl₂/MgCl₂‐Al(i‐Bu)₃ has a very low copolymerizing ability toward propylene and hexene‐1. MW and MWD of PE produced over these catalysts depend on the polymerization time, ethylene and hexene‐1 concentration. The activation effect of hydrogen and other kinetic features of ethylene polymerization over supported catalysts based on the Fe (II) complexes are discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5057–5066, 2007     

Rapid Production of High‐Purity Hydrogen Fuel through Microwave‐Promoted Deep Catalytic Dehydrogenation of Liquid Alkanes with Abundant Metals

Hydrogen as an energy carrier promises a sustainable energy revolution. However, one of the greatest challenges for any future hydrogen economy is the necessity for large scale hydrogen production not involving concurrent CO₂ production. The high intrinsic hydrogen content of liquid‐range alkane hydrocarbons (including diesel) offers a potential route to CO₂‐free hydrogen production through their catalytic deep dehydrogenation. We report here a means of rapidly liberating high‐purity hydrogen by microwave‐promoted catalytic dehydrogenation of liquid alkanes using Fe and Ni particles supported on silicon carbide. A H₂ production selectivity from all evolved gases of some 98 %, is achieved with less than a fraction of a percent of adventitious CO and CO₂. The major co‐product is solid, elemental carbon.     

Biomimetic Hydrogenation Catalyzed by a Manganese Model of [Fe]‐Hydrogenase

[Fe]‐hydrogenase is an efficient biological hydrogenation catalyst. Despite intense research, Fe complexes mimicking the active site of [Fe]‐hydrogenase have not achieved turnovers in hydrogenation reactions. Herein, we describe the design and development of a manganese(I) mimic of [Fe]‐hydrogenase. This complex exhibits the highest activity and broadest scope in catalytic hydrogenation among known mimics. Thanks to its biomimetic nature, the complex exhibits unique activity in the hydrogenation of compounds analogous to methenyl‐H₄MPT⁺, the natural substrate of [Fe]‐hydrogenase. This activity enables asymmetric relay hydrogenation of benzoxazinones and benzoxazines, involving the hydrogenation of a chiral hydride transfer agent using our catalyst coupled to Lewis acid‐catalyzed hydride transfer from this agent to the substrates.     

Dinitrosyl iron complexes: From molecular electrocatalysts to electrodeposited‐film electrodes for hydrogen evolution reaction

Clean and large‐scale production of hydrogen via water splitting triggered by active, robust, and low‐cost electrocatalysts is a promising and sustainable strategy for energy conversion and storage. In this study, a series of four‐coordinated chelating amine‐bound {Fe(NO)₂}¹⁰ dinitrosyl iron complexes (DNICs) [(L)Fe(NO)₂] were synthesized to investigate how the electronic structure of [Fe(NO)₂] unit of DNICs was tailored to promote the electrocatalytic hydrogen evolution reaction (HER) triggered by the homogeneous DNICs' molecular catalysts and the heterogeneous DNIC‐derived electrodeposited‐film electrodes. The electrochemical studies demonstrate that HER onset potentials of those DNICs in neutral sodium sulfate aqueous solution are dependent on their IR ν_(NO) stretching frequencies, indicating that the electron‐rich [Fe(NO)₂] core modulated by the synergistic cooperation of the electron‐donating ability and steric effect of methyl‐/hydrogen‐substituted diamine‐coordinated ligands, presumably, benefits the formation of metal‐hydride intermediate to reduce the required onset potential. In contrast with homogeneous catalyst retaining its molecular integrity during the catalytic HER process, it is noticed that DNICs [(L)Fe(NO)₂] act as the precursor of the active heterogeneous HER catalyst during the electrocatalytic HER process. It is presumed that the intermolecular hydrogen‐bonding interactions among DNICs [(L)Fe(NO)₂] may control the particle sizes of DNIC‐derived electrodeposited film to modulate HER efficiency.     

N2O Reductase Activity of a [Cu4S] Cluster in the 4CuI Redox State Modulated by Hydrogen Bond Donors and Proton Relays in the Secondary Coordination Sphere

The model complex [Cu₄(μ₄‐S)(dppa)₄]²⁺ ( 1 , dppa=μ₂‐(Ph₂P)₂NH) has N₂O reductase activity in methanol solvent, mediating 2 H⁺/2 e^? reduction of N₂O to N₂+H₂O in the presence of an exogenous electron donor (CoCp₂). A stoichiometric product with two deprotonated dppa ligands was characterized, indicating a key role of second‐sphere N?H residues as proton donors during N₂O reduction. The activity of 1 towards N₂O was suppressed in solvents that are unable to provide hydrogen bonding to the second‐sphere N?H groups. Structural and computational data indicate that second‐sphere hydrogen bonding induces structural distortion of the [Cu₄S] active site, accessing a strained geometry with enhanced reactivity due to localization of electron density along a dicopper edge site. The behavior of 1 mimics aspects of the Cu_Z catalytic site of nitrous oxide reductase: activity in the 4Cu^I:1S redox state, use of a second‐sphere proton donor, and reactivity dependence on both primary and secondary sphere effects.     

An S‐Oxygenated [NiFe] Complex Modelling Sulfenate Intermediates of an O2‐Tolerant Hydrogenase

To understand the molecular details of O₂‐tolerant hydrogen cycling by a soluble NAD⁺‐reducing [NiFe] hydrogenase, we herein present the first bioinspired heterobimetallic S‐oxygenated [NiFe] complex as a structural and vibrational spectroscopic model for the oxygen‐inhibited [NiFe] active site. This compound and its non‐S‐oxygenated congener were fully characterized, and their electronic structures were elucidated in a combined experimental and theoretical study with emphasis on the bridging sulfenato moiety. Based on the vibrational spectroscopic properties of these complexes, we also propose novel strategies for exploring S‐oxygenated intermediates in hydrogenases and similar enzymes.     

Catalytic Systems for the H<Subscript>2</Subscript>S Wet Oxidation at room Temperature

The catalytic wet oxidation process is the most attractive process for small-scale hydrogen sulfide (H₂S) removal from natural gas. The catalytic wet oxidation process is anticipated to be cost effective and simple so that it can be used for treating sour gases containing small amounts of H₂S and can be easily operated even in isolated sites. The development of effective catalyst is the key technology in the wet catalytic oxidation of H₂S. The scale of operation for the process has to be flexible so its use will not be limited by the flow rates of the gas to be treated. The heterogeneous catalytic wet oxidation of H₂S has been attempted on activated carbons, but the H₂S removal capacity still shows the low removal efficiency. The catalytic wet oxidation of H₂S was studied over Fe/MgO for an effective removal of H₂S. In order to develop a sulfur removal technology, one has to know what surface species of catalyst are the most active. This article discusses the following systematic studies: (i) the catalytic preparation to disperse Fe metal well on MgO support for enhancing H₂S removal capacity, (ii) the effect of the catalytic morphology on the activity of Fe/MgO for the H₂S wet oxidation, (iii) the influence of precursor and support on the activity of Fe/MgO for catalytic wet oxidation of H₂S to sulfur.     

Two Fe3(μ3‐S)2(CO)8 clusters with terminal N‐heterocyclic carbenes

The title compounds with terminal N‐heterocyclic carbenes, namely octacarbonyl(imidazolidinylidene‐κC²)di‐μ₃‐sulfido‐triiron(II)(2 Fe—Fe), [Fe₃(C₃H₆N₂)(μ₃‐S)₂(CO)₈], (I), and octacarbonyl(1‐methylimidazo[1,5‐a]pyridin‐3‐ylidene‐κC³)di‐μ₃‐sulfido‐triiron(II)(2 Fe—Fe), [Fe₃(C₈H₈N₂)(μ₃‐S)₂(CO)₈], (II), have been synthesized. Each compound contains two Fe—Fe bonds and two S atoms above and below a triiron triangle. One of the eight carbonyl ligands deviates significantly from linearity. In (I), dimers generated by an N—H...S hydrogen bond are linked into [001] double chains by a second N—H...S hydrogen bond. These chains are packed by a C—H...O hydrogen bond to yield [101] sheets. In (II), dimers generated by an N—H...S hydrogen bond are linked by C—H...O hydrogen bonds to form [111] double chains.     

Silicon–Heteroaromatic [FeFe] Hydrogenase Model Complexes: Insight into Protonation,Electrochemical Properties,and Molecular Structures

To learn from Nature how to create an efficient hydrogen‐producing catalyst, much attention has been paid to the investigation of structural and functional biomimics of the active site of [FeFe]‐hydrogenase. To understand their catalytic activities, the μ‐S atoms of the dithiolate bridge have been considered as possible basic sites during the catalytic processes. For this reason, a series of [FeFe]‐H₂ase mimics have been synthesized and characterized. Different [FeFe]‐hydrogenase model complexes containing bulky Si–heteroaromatic systems or fluorene directly attached to the dithiolate moiety as well as their mono‐PPh₃‐substituted derivatives have been prepared and investigated in detail by spectroscopic, electrochemical, X‐ray diffraction, and computational methods. The assembly of the herein reported series of complexes shows that the μ‐S atoms can be a favored basic site in the catalytic process. Small changes in the (hetero)‐aromatic system of the dithiolate moiety are responsible for large differences in their structures. This was elucidated in detail by DFT calculations, which were consistent with the experimental results.     

Electrochemical Studies of CO2-Reducing Metalloenzymes

Only two enzymes are capable of directly reducing CO₂: CO dehydrogenase, which produces CO at a [NiFe₄S₄] active site, and formate dehydrogenase, which produces formate at a mononuclear W or Mo active site. Both metalloenzymes are very rapid, energy-efficient and specific in terms of product. They have been connected to electrodes with two different objectives. A series of studies used protein film electrochemistry to learn about different aspects of the mechanism of these enzymes (reactivity with substrates, inhibitors…). Another series focused on taking advantage of the catalytic performance of these enzymes to build biotechnological devices, from CO₂-reducing electrodes to full photochemical devices performing artificial photosynthesis. Here, we review all these works.     

Visible‐Light‐Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts

Linear poly(p‐phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H₂ evolution rate. The most active dibenzo[b,d]thiophene sulfone co‐polymer has a UV photocatalytic activity that rivals TiO₂, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co‐polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride.     

Mechanism of H2 production by the [FeFe]H subcluster of di-iron hydrogenases: implications for abiotic catalysts

To explore the possibility that the active center of the di-iron hydrogenases, the [FeFe] H subcluster, can serve by itself as an efficient hydrogen-producing catalyst, we perform comprehensive calculations of the catalytic properties of the subcluster in vacuo using first principles density functional theory. For completeness, we examine all nine possible geometrical isomers of the Fe(II)Fe(I) active-ready state and report in detail on the relevant ones that lead to the production of H 2. These calculations, carried out at the generalized gradient approximation level, indicate that the most efficient catalytic site in the isolated [FeFe] H subcluster is the Fe d center distal (d) to the [4Fe-4S] H cluster; the other iron center site, the proximal Fe p, also considered in this study, has much higher energy barriers. The pathways with the most favorable kinetics (lowest energy barrier to reaction) proceed along configurations with a CO ligand in a bridging position. The most favorable of these CO-bridging pathways start from isomers where the distal CN (-) ligand is in up position, the vacancy V in down position, and the remaining distal CO is either cis or trans with respect to the proximal CO. These isomers, not observed in the available enzyme X-ray structures, are only marginally less stable than the most stable nonbridging Fe d-CO-terminal isomer. Our calculations indicate that this CO-bridging CN-up isomer has a small barrier to production of H 2 that is compatible with the observed rate for the enzyme. These results suggest that catalysis of H 2 production could proceed on this stereochemically modified [FeFe] H subcluster alone, thus offering a promising target for functional bioinspired catalyst design.     

Isocyanide in Biochemistry? A Theoretical Investigation of the Electronic Effects and Energetics of Cyanide Ligand Protonation in [FeFe]‐Hydrogenases

The presence of Fe‐bound cyanide ligands in the active site of the proton‐reducing enzymes [FeFe]‐hydrogenases has led to the hypothesis that such Brønsted–Lowry bases could be protonated during the catalytic cycle, thus implying that hydrogen isocyanide (HNC) might have a relevant role in such crucial microbial metabolic paths. We present a hybrid quantum mechanical/molecular mechanical (QM/MM) study of the energetics of CN^? protonation in the enzyme, and of the effects that cyanide protonation can have on [FeFe]‐hydrogenase active sites. A detailed analysis of the electronic properties of the models and of the energy profile associated with H₂ evolution clearly shows that such protonation is dysfunctional for the catalytic process. However, the inclusion of the protein matrix surrounding the active site in our QM/MM models allowed us to demonstrate that the amino acid environment was finely selected through evolution, specifically to lower the Brønsted–Lowry basicity of the cyanide ligands. In fact, the conserved hydrogen‐bonding network formed by these ligands and the neighboring amino acid residues is able to impede CN^? protonation, as shown by the fact that the isocyanide forms of [FeFe]‐hydrogenases do not correspond to stationary points on the enzyme QM/MM potential‐energy surface.     

[FeFe]‐Hydrogenase with Chalcogenide Substitutions at the H‐Cluster Maintains Full H2 Evolution Activity

The [FeFe]‐hydrogenase HYDA1 from Chlamydomonas reinhardtii is particularly amenable to biochemical and biophysical characterization because the H‐cluster in the active site is the only inorganic cofactor present. Herein, we present the complete chemical incorporation of the H‐cluster into the HYDA1‐apoprotein scaffold and, furthermore, the successful replacement of sulfur in the native [4Fe_H] cluster with selenium. The crystal structure of the reconstituted pre‐mature HYDA1[4Fe4Se]_H protein was determined, and a catalytically intact artificial H‐cluster variant was generated upon in vitro maturation. Full hydrogen evolution activity as well as native‐like composition and behavior of the redesigned enzyme were verified through kinetic assays, FTIR spectroscopy, and X‐ray structure analysis. These findings reveal that even a bioinorganic active site with exceptional complexity can exhibit a surprising level of compositional plasticity.     

Organic solvent‐free catalytic hydrogenation of diene‐based polymer nanoparticles in latex form. Part II. Kinetic analysis and mechanistic study

The central challenge that has limited the development of catalytic hydrogenation of diene‐based polymer latex (i.e., latex hydrogenation) in large‐scale production pertains to how to accomplish the optimal interplay of accelerating the hydrogenation rate, decreasing the required quantity of catalyst, and eliminating the need for an organic solvent. Here, we attempt to overcome this dilemma through decreasing the dimensions of the polymer substrate (such as below 20 nm) used in the hydrogenation process. Very small diene‐based polymer nanoparticles were synthesized and then used as the substrates for the subsequent latex hydrogenation. The effects of particle size, temperature, and catalyst concentration on the hydrogenation rate were fully investigated. An apparent first‐order kinetic model was proposed to describe the rate of hydrogen uptake with respect to the concentration of the olefinic substrate (C?C). Mass transfer of both the hydrogen and catalyst involved in this solid (polymer)–liquid (water)–gas (hydrogen) three‐phase latex system is discussed. The competitive coordination of the catalyst between the C?C and acrylonitrile units within the copolymer was elucidated. It was found that (1) using very small diene‐based polymer nanoparticles as the substrate, the hydrogenation rate of polymer latex can be increased vastly to achieve a high conversion of 95% while a quite low level of catalyst loading is required; (2) this latex hydrogenation process was completely free of organic solvent and no cross‐linking was found; (3) the mass transfer of hydrogen is not a rate‐determining step in the present hydrogenation reactions; (4) the catalyst was dispersed homogeneously within the polymer nanoparticles; (5) for the reaction that has reached about 95 mol % conversion, the kinetic study shows that the reaction is chemically controlled with an apparent activation energy of 100–110 kJ/mol; (6) the strong coordination of C[tbond]N to the catalytically active species RhH₂Cl(PPh₃)₂ imposed a negative effect on the hydrogenation activity. The present research provides a comprehensive study to appreciate the underlying chemistry of latex hydrogenation of diene‐based polymer nanoparticles and more importantly shows great promise toward the commercialization of a “green” catalytic hydrogenation operation of a diene‐based polymer latex in industry. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Spectrofluorometric determination of hydrogen peroxide based on oxidative catalytic reactions of p-hydroxyphenyl derivatives with metal complexes of thiacalix[4]arenetetrasulfonate on a modified anion-exchanger.

The peroxidase-like catalytic activity of metal complexes of thiacalix[4]arenetetrasulfonate (TCAS[4]) on a modified anion-exchanger (Me(n+)-TCAS[4]A-500; Me(n+) = H2, Fe3+, Fe2+, Mn3+, Co3+, Co2+, Cu2+, Zn2+, Ni2+) for the oxidation of p-hydroxyphenyl derivatives to produce fluorescent substances in the presence of hydrogen peroxide has been investigated. Among the Me(n+)-TCAS[4]A-500 tested, Fe(3+)-TCAS[4]A-500 exhibited the highest level of catalytic activity for the oxidation of p-acetoamidophenol in a carbonate buffer solution of pH 10. The catalytic activity of Fe(3+)-TCAS[4]A-500 was then used for the spectrofluorometric determination of hydrogen peroxide. The calibration curve for the Fe(3+)-TCAS[4]A-500 method was linear over a range spanning from 0.1 to 5.0 microg of hydrogen peroxide in a 1.0 ml sample solution.