Iron Hydride Research Articles

Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN{CH2CH2(PR2)}2 [R = isopropyl (((i)Pr)PNP) or cyclohexyl ((Cy)PNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes (((i)Pr)PNP)FeCl2(CO) (1a) and ((Cy)PNP)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex (((i)Pr)PNP)Fe(CO)2 (2a) was prepared through the reaction of Fe(CO)5 with ((i)Pr)PNP, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to 1a via the intermediate iron(II) complex [(((i)Pr)PNP)FeCl(CO)2]Cl (3a). The reaction of 2a with HCl generated the related complex [(((i)Pr)PNP)FeH(CO)2]Cl (4a), while the neutral iron hydrides (((i)Pr)PNP)FeHCl(CO) (5a) and ((Cy)PNP)FeHCl(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of (n)Bu4NBH4. The related reaction between 1a and excess NaBH4 generated the unusual η(1)-HBH3 complex (((i)Pr)PNP)FeH(η(1)-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η(1)-HBH3 ligand and the N-H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of 6a with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(((i)Pr)PNP)FeH(CO)}2(μ2,η(1):η(1)-H2BH2)][BPh4] (7a), which features a rare example of a μ2,η(1):η(1)-H2BH2 ligand. Unlike all previous examples of complexes with a μ2,η(1):η(1)-H2BH2 ligand, there is no metal-metal bond and additional bridging ligand supporting the borohydride ligand in 7a; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.

On the path to bulk FeH2: Synthesis and magnetic properties of amorphous iron (II) hydride

An amorphous Fe(II) hydride material approximating FeH2 in composition (FeH2−xRx(Et2O)y where R=mesityl) has been isolated as a bulk powder in the solid state. This was accomplished under moderate reaction conditions by the reaction of bis(mesityl) iron(II) in toluene and hydrogen gas at 100bar and 298K to give a 1:5 mixed phase amorphous material of Fe(0) and the iron (II) hydride. This represents an important advance because FeH2 has never been synthesised in bulk form. The material shows ferromagnetic behaviour with a magnetic susceptibility of 1.25Bohr magnetons per formula unit at 10K.

Photocatalytic Hydrogen Production Using Models of the Iron–Iron Hydrogenase Active Site Dispersed in Micellar Solution

Iron-thiolate complexes of the type [Fe2 (μ-bdt)(CO)6-x P(OMe3 )x ] (bdt=S2 C6 H4 =benzenedithiolate, x≤2) are simplified models of iron-iron hydrogenase enzymes. Recently, we have shown that these water-insoluble organometallic complexes, when included into micelles formed by sodium dodecyl sulfate (SDS), are good catalysts for the electrochemical production of hydrogen in aqueous solutions at pH<6. We herein report that the all-CO derivative [Fe2 (μ-bdt)(CO)6 ] (1), owing to its comparatively low reduction potential, is also a robust molecular catalyst for visible-light-driven production of H2 in aqueous SDS solutions at pH 10.5. Irradiation at λ=455 nm of a system consisting of complex 1, Eosin Y as a sensitizer, and triethylamine as an electron donor produced up to 0.86 mL of H2 in 4.5 h, corresponding to a turnover number of 117 mol of H2 per mol of catalyst. In the presence of a large excess of sensitizer, the production of H2 lasted for more than 30 h, stressing the relative stability of complex 1 under the photocatalytic conditions used herein. Thermodynamic considerations and UV/Vis spectroscopy experiments suggest that the catalytic cycle begins with the photo-driven reduction of complex 1. The reduced intermediate reacts with a proton source to yield iron hydride. Subsequent reduction and protonation steps produce H2 , regenerating the starting complex. As a result, the iron-thiolate complex 1 is a versatile proton reduction catalyst that can utilize either solar or electrical energy inputs, providing a starting point for the construction of noble metal-free molecular systems for renewable H2 production.

Nonstoichiometry and growth of some Fe carbides

Iron carbides containing from 31 to 17 atomic % carbon, with cohenite XRD structure and optical properties, were grown in experiments in Fe–Ni–S–C, Fe–Ni–C, and in Fe–C at 1, 6, and 7 GPa. X-ray cell volumes increase with C content. Compositions listed above vary considerably outside the nominal (Fe,Ni)3C stoichiometry of cohenite/cementite. Cohenites coexisting with Fe–C liquid are carbon poor. The Eckstrom-Adcock carbide, nominally Fe7C3, was found to show compositions from 29 to 36 atomic % C at 7 GPa in Fe–C. Both these materials are better regarded as solutions than as stoichiometric compounds, and their properties such as volume have compositional dependencies, as do the iron oxides, sulfides, silicides, and hydrides. The fraction of C dissolved in cohenite-saturated alloy is found to become smaller between 1 and 7 GPa. If this trend continues at higher pressures, the deep mantle should be easier to saturate with carbide than the shallow mantle, whether or not carbide is metastable as at ambient pressure. At temperatures below the cohenite-graphite peritectic, cohenite may grow as a compositionally zoned layer between Fe and graphite. The Eckstrom-Adcock carbide joins the assemblage at 7 GPa. Phases appear between Fe and C in an order consistent with metasomatic interface growth between chemically incompatible feed stocks. Diffusion across the carbide layer is not the growth rate limiting step. Carbon transport along the grain boundaries of solid Fe source stock at 1 GPa, to form C-saturated Fe alloy, is observed to be orders of magnitude faster than the cohenite layer growth. Growth stagnates too rapidly to be consistent with diffusion control. Furthermore, lateral variations in carbide layer thickness, convoluted inert marker horizons, and variable compositional profiles within the layers suggest that there are local transport complexities not covered by one-dimensional diffusive metasomatic growth. In contrast to many transport phenomena which slow with pressure, at 7 GPa and 1,162 °C, carbide growth without open grain boundaries is faster than at 1 GPa with fast grain boundary channels, again suggesting C transport is less of a constraint on growth than C supply. C supply at 7 GPa is enhanced by graphite metastability and the absence of fast grain boundary channels to divert C into the Fe instead of growing carbide. At both 1 and 7 GPa, the growth rate of carbide is found to systematically vary depending on which of two stock pieces of graphite are used to form the growth couple, suggesting that some property of each specific graphite, like C release rate, possibly from amorphous binder material, may influence the cohenite growth process. At temperatures near and above the cohenite-graphite peritectic at 1–1.5 GPa, complex intergrowths involving Fe–C liquids and extensive thermal migration transport were encountered, eroding the organized spatial resolution, and the range of cohenite compositions found grown below this peritectic from growth couples of crystalline Fe and graphite. The migration of graphite to a position in the metasomatic sequence between liquid and cohenite demonstrates that the solubility of graphite in liquid increases with temperature above the peritectic, whereas the solubility of graphite in cohenite below the peritectic decreases with temperature. The variable solubility of graphite in cohenite, shown by thermal migration, emphasizes that cohenite does have compositional variations.

Selective Reduction of Esters to Aldehydes under the Catalysis of Well‐Defined NHC–Iron Complexes

A simple methodol. for the chemoselective redn. of esters to aldehydes with a N-heterocyclic-carbene-iron complex, such as [(IMes)-Fe(CO)4], as the catalyst (1 mol%) in the presence of a secondary silane (diethylsilane or diphenylsilane) as the reducing agent. has been developed. This reaction occurs at room temp. under UV irradn. with both arom. and aliph. esters. Notably, this catalytic system also permitted the efficient and selective redn. of lactones to lactols. Exptl. evidence indicated that the hydrosilylation occurs by oxidative addn. of the hydrosilane to an unsatd. NHC-Fe species to yield a silyl iron hydride complex. [on SciFinder(R)]

Prospects for the development of uprated diesels

The development of an uprated engine are reviewed and determined. These power units are widely used for diesel locomotives in Ukraine. Thermodynamic improvement of diesel engines is to improve the air-supply systems, forming a mixture of combustion and exhaust. It is also improving the design and increasing longevity of pistons, rings, cylinder liners, connecting rods, crankshafts. Measures to improve the mechanisms for distributing gas and valve train are promising. Important environmental area is the development and implementation of measures to reduce the emissions of harmful substances into the environment. This includes providing excess air, application hybrid propulsion for shunting, frequency converters and corresponding exhaust fuel additives. The use of fuels that do not contain carbon, eliminates the emission of greenhouse gases. The development of diesel involves the use electric technology from a wide range of electronic and computer systems to control thermal barrier coatings. Improving the engine is also in the direction of creating a unified parametric and mixed series of similar sizes. It is also advisable to develop diesel gas engines and engines for heavy fuels. As an additive to diesel fuel are promising alcohols - methanol and ethanol. The most suitable type of fuel for future power plants and the energy of the future is hydrogen. However, there are problems with its storage. At present, the best system for hydrogen storage are hydrides of magnesium, iron, titanium and potassium

The influence of the milling time on the yield of Mg2FeH6 from a two-step synthesis conducted in a custom-made reactor

Dimagnesium iron hydride was synthesized by mechanical milling of a MgH2/Fe mixture followed by sintering under a high hydrogen pressure (120 bar). The influence of the milling time on the synthesis yield was observed. Properly chosen processing parameters led to a 94–97 % reaction yield (depending on the measurement method) for the formation of Mg2FeH6. Milling times that were too short or too long proved to be ineffective. A custom-made reactor for synthesis of a batch of up to 20 g is presented. Synthesized samples were characterized by XRD analysis and PCT measurements were performed.

高分解能X線非弾性散乱法とX線回折法による鉄水素化物の弾性波速度―密度関係の研究

In this article, my doctoral thesis, Behavior of hydrogen in the interior of the planets and satellites, was reviewed. I especially focus on the results of sound velocity measurements for iron hydride with the high resolution inelastic X-ray scattering here. The density evolution of the sound velocity of dhcp-FeH has been determined up to 70 GPa at room temperature from inelastic X-ray scattering and X-ray diffraction measurements. I found that the relationship between compressional wave velocity and density changes due to a magnetic transition from ferromagnetic to nonmagnetic FeH and that the nonmagnetic phase only follows the Birch's law. Assuming an ideal two-component mixing model, the hydrogen content of the Earth's inner core is obtained to be 0.30～0.37 wt% H, which corresponds to FeH0.17～FeH0.20.

Reactions of [FeFe]-hydrogenase models involving the formation of hydrides related to proton reduction and hydrogen oxidation

[FeFe]-hydrogenases are enzymes in nature that catalyze the reduction of protons and the oxidation of H2 at neutral pH with remarkably high activities and incredibly low overpotential. Structural and functional biomimicking of the active site of [FeFe]-hydrogenases can provide helpful hints for elucidating the mechanism of H2 evolution and uptake at the [FeFe]-hydrogenase active site and for designing bioinspired catalysts to replace the expensive noble metal catalysts for H2 generation and uptake. This perspective focuses on the recent progress in the formation and reactivity of iron hydrides closely related to the processes of proton reduction and hydrogen oxidation mediated by diiron dithiolate complexes. The second section surveys the bridging and terminal hydride species formed from various diiron complexes as well as the intramolecular proton transfer. The very recent progress in H2 activation by diiron dithiolate models are reviewed in the third section. In the concluding remarks and outlook, the differences in structure and catalytic mechanism between the synthetic models and the native [FeFe]-H2ase active site are compared and analyzed, which may cause the need for a significantly larger driving force and may lead to lower activities of synthetic models than the [FeFe]-H2ases for H2 generation and uptake.

Key Hydride Vibrational Modes in [NiFe] Hydrogenase Model Compounds Studied by Resonance Raman Spectroscopy and Density Functional Calculations

Hydrogenase proteins catalyze the reversible conversion of molecular hydrogen to protons and electrons. While many enzymatic states of the [NiFe] hydrogenase have been studied extensively, there are multiple catalytically relevant EPR-silent states that remain poorly characterized. Analysis of model compounds using new spectroscopic techniques can provide a framework for the study of these elusive states within the protein. We obtained optical absorption and resonance Raman (RR) spectra of (dppe)Ni(μ-pdt)Fe(CO)(3) and [(dppe)Ni(μ-pdt)(μ-H)Fe(CO)(3)][BF(4)], which are structural and functional model compounds for the EPR-silent Ni-SI and Ni-R states of the [NiFe] hydrogenase active site. The studies presented here use RR spectroscopy to probe vibrational modes of the active site, including metal-hydride stretching vibrations along with bridging ligand-metal and Fe-CO bending vibrations, with isotopic substitution used to identify key metal-hydride modes. The metal-hydride vibrations are essentially uncoupled and represent isolated, localized stretching modes; the iron-hydride vibration occurs at 1530 cm(-1), while the nickel-hydride vibration is observed at 945 cm(-1). The significant discrepancy between the metal-hydride vibrational frequencies reflects the slight asymmetry in the metal-hydride bond lengths. Additionally, time-dependent density functional theory (TD-DFT) calculations were carried out to obtain theoretical RR spectra of these compounds. On the basis of the detailed comparison of theory and experiment, the dominant electronic transitions and significant normal modes probed in the RR experiments were assigned; the primary transitions in the visible wavelengths represent metal-to-metal and metal-to-ligand charge transfer bands. Inherent properties of metal-hydride vibrational modes in resonance Raman spectra and DFT calculations are discussed together with the prospects of observing such vibrational modes in metal-hydride-containing proteins. Such a combined theoretical and experimental approach may be valuable for characterization of analogous redox states in the [NiFe] hydrogenases.

First-Principles Study on the Structural and Magnetic Properties of Iron Hydride

The magnetic and structural properties of iron hydride FeH with the double hexagonal close-packed (dhcp) and hexagonal close-packed (hcp) structures are investigated by first-principles density-functional theory calculations with a spin-polarized form of generalized gradient approximation. All the calculations are performed using all-electron full-potential linearized augmented plane wave method. Both dhcp and hcp FeH are ferromagnetic at ambient pressure. The ferromagnetic ordering of the dhcp structure collapses at a pressure of 48 GPa, while that of the hcp structure vanishes gradually from 48 GPa. The modification in the density of states (DOS) due to the applied pressure causes the collapse of the magnetization. The difference in magnetic moment reduction between dhcp and hcp FeH is attributed to their DOS around the Fermi level. The calculated magnetocrystalline anisotropy energies between in-plane and out-of-plane spin orientations are found to be 124 µeV/Fe for the dhcp structure, and 100 µeV/Fe for the hcp structure. The easy axis is in-plane direction for both structures.

Fe–C and Fe–H systems at pressures of the Earth's inner core

The solid inner core of Earth is predominantly composed of iron alloyed with several percent Ni and some lighter elements, Si, S, O, H, and C being the prime candidates. To establish the chemical composition of the inner core, it is necessary to find the range of compositions that can explain its observed characteristics. Recently, there have been a growing number of papers investigating C and H as possible light elements in the core, but the results were contradictory. Here, using ab initio simulations, we study the Fe–C and Fe–H systems at inner core pressures (330–364 GPa). Based on the evolutionary structure prediction algorithm USPEX, we have determined the lowest-enthalpy structures of all possible carbides (FeC, Fe2C, Fe3C, Fe4C, FeC2, FeC3, FeC4, Fe7C3) and hydrides (Fe4H, Fe3H, Fe2H, FeH, FeH2, FeH3, FeH4) and have found that Fe2C (space group Pnma) is the most stable iron carbide at pressures of the inner core, while FeH, FeH3, and FeH4 are the most stable iron hydrides at these conditions. For Fe3C, the cementite structure (space group Pnma) and the Cmcm structure recently found by random sampling are less stable than the I-4 and C2/m structures predicted here. We have found that FeH3 and FeH4 adopt chemically interesting thermodynamically stable crystal structures, containing trivalent iron in both compounds. We find that the density of the inner core can be matched with a reasonable concentration of carbon, 11–15 mol.% (2.6–3.7 wt.%) at relevant pressures and temperatures, yielding the upper bound to the C content in the inner core. This concentration matches that in CI carbonaceous chondrites and corresponds to the average atomic mass in the range 49.3–51.0, in close agreement with inferences from Birch's law for the inner core. Similarly made estimates for the maximum hydrogen content are unrealistically high: 17–22 mol.% (0.4–0.5 wt.%), which corresponds to the average atomic mass of the core in the range 43.8–46.5. We conclude that carbon is a better candidate light alloying element than hydrogen.

Characterization of the Fe-H bond in a three-coordinate terminal hydride complex of iron(I).

Three's company: Iron(I) hydride complexes are presented (see picture; N blue, O red) that are the first monomeric open-shell hydride complexes to be crystallographically verified as being three-coordinate at the metal. Backbonding into diketiminate π* orbitals stabilizes the low oxidation state. The FeH bonding has been analyzed using electron-nuclear double resonance (ENDOR).

Air‐Stable, Nitrile‐Ligated (Cyclopentadienone)iron Dicarbonyl Compounds as Transfer Reduction and Oxidation Catalysts

AbstractA series of air‐stable, nitrile‐ligated (cyclopentadienone)iron dicarbonyl compounds was synthesized and their activities as catalysts in the transfer reduction of acetophenone were explored. While all were active catalysts, the acetonitrile adduct was chosen for further study and was found to be active in the transfer reduction of aldehydes and ketones and in the Oppenauer‐type oxidation of secondary alcohols. The acetonitrile catalyst exhibited activities similar to those of an analogous air‐sensitive iron hydride, but unlike the iron hydride it was unreactive in carbonyl reductions using hydrogen gas.

Trimethylsilyl-Substituted Hydroxycyclopentadienyl Ruthenium Hydrides as Benchmarks to Probe Ligand and Metal Effects on the Reactivity of Shvo Type Complexes.

The bis(trimethylsilyl)-substituted hydroxycyclopentadienyl ruthenium hydride [2,5-(SiMe(3))(2)-3,4-(CH(2)OCH(2))(η(5)-C(4)COH)]Ru(CO)(2)H (10) is an efficient catalyst for hydrogenation of aldehydes and ketones. Because 10 transfers hydrogen rapidly to aldehydes and ketones and because it does not form an inactive bridging hydride during reaction, hydrogenation of aldehydes and ketones can be performed at room temperature under relatively low hydrogen pressure (3 atm); this is a significant improvement compared with previously developed Shvo type catalysts. Kinetic and (2)H NMR spectroscopic studies of the stoichiometric reduction of aldehydes and ketones by 10 established a two-step process for the hydrogen transfer: (1) rapid and reversible hydrogen bond formation between OH of 10 and the oxygen of the aldehyde or ketone, (2) followed by slow transfer of both proton and hydride from 10 to the aldehyde or ketone. The stoichiometric and catalytic activities of complex 10 are compared to those of other Shvo type ruthenium hydrides and related iron hydrides.

Experimental studies and molecular modelling of catalytic steam gasification of brown coal containing iron species

The paper presents experimental data of catalytic steam gasification of brown coal containing aqua-iron species, and the chemical mechanism(s) at a molecular level. Experimental techniques provided weight loss from catalysed reaction of char with steam of 17wt% at 800°C and 40wt% at 900°C, over 15min, on a dry ash free basis (daf). Inorganic and organic oxygen, identified using XPS in the char samples, was derived from reactions with steam. High yields of H2 resulted from catalysed reactions between char and steam. Semi-empirical (SE) quantum molecular modelling using MOPAC, of reaction routes for high temperature pyrolysis and steam gasification, provided results consistent with experimental data for weight loss, iron species, and the distribution of inorganic and organic oxygen in char samples after reaction with steam. The catalysis mechanism(s) that have been examined are considered to be a hybrid of organometallic and heterogeneous chemistry, involving iron hydride species that precede H2 formation; oxygen insertion into [Fe–C–] to form [Fe–O–C–] followed by elimination of CO, creating another [Fe–C] site to continue the catalytic cycle. SE modelling indicates concerted reactions were more energetically favoured. Initial results from molecular dynamics (MD) show a higher concentration of H2O molecules about the active site [Fe–C].

Structural, magnetic and electrical properties of iron hydride

Structural, magnetic and electrical properties of iron hydride

Iron Hydride Complexes Bearing Phosphinite-Based Pincer Ligands: Synthesis, Reactivity, and Catalytic Application in Hydrosilylation Reactions

Treatment of resorcinol-derived bis(phosphinite) ligands 1,3-(R2PO)2C6H4 (R = iPr and Ph) with Fe(PMe3)4 furnishes iron POCOP-pincer hydride complexes [2,6-(R2PO)2C6H3]Fe(H)(PMe3)2 (R = iPr, 1a; R = Ph, 1b) with two PMe3cis to each other. The isopropyl complex 1a undergoes ligand substitution upon mixing with CO to give [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)(CO). The kinetic product (2a) of this process contains a CO ligand trans to the hydride, whereas the thermodynamic product (2a′) has a CO ligand cis to the hydride. The displacement of PMe3 in 2a by CO takes place at an elevated temperature, resulting in the formation of [2,6-(iPr2PO)2C6H3]Fe(H)(CO)2 (3a). These new iron POCOP-pincer hydride complexes catalyze the hydrosilylation of aldehydes and ketones with different functional groups, and 1a is the most efficient catalyst for this process. Isotopic labeling experiments rule out the hydride ligand being directly involved in the reduction. The hydrosilylation reactions are more likely to proceed via the acti...

Light-Induced Enantioselective Hydrogenation Using Chiral Derivatives of Casey’s Iron–Cyclopentadienone Catalyst

We herein report the first example of an asymmetric ketone hydrogenation using chirally modified derivatives of the homogeneous iron(II)–cyclopentadienone–tricarbonyl system, known as Casey’s catalyst. For the synthesis of the chirally modified catalysts, one of three carbonyl ligands was exchanged for a chiral phosphoramidite. To this end, either oxidative decarbonylation using trimethylamine-N-oxide or photolysis was applied. Photolysis was also used to convert the tricarbonyl iron precatalyst (and, analogously, the dicarbonyl phosphoramidite complexes) to the coordinatively unsaturated dicarbonyl (monocarbonyl, respectively) complexes, which are intermediates in the catalytic cycle of ketone hydrogenation. Hydrogen uptake by the latter species affords the “loaded” hydride, as evidenced by 1H NMR spectroscopy. Thus, the preparation of sensitive iron hydrides by the typically low-yielding Hieber reaction could be avoided. Instead, the catalytic cycle is accessed from air-stable carbonyl precursors. In th...

X-ray diffraction and Mössbauer spectroscopy study of fcc iron hydride FeH at high pressures and implications for the composition of the Earth's core

The phase fcc FeH x (x ~ 1) was synthesized at pressures over 30 GPa and temperatures over 1600(50) K. At room temperature this iron hydride is stable at pressures from 19(1) GPa up to at least 68(2) GPa (the highest pressure achieved in this study). A fit of the PV data collected for fcc FeH x at room temperature gives the following parameters for the equation of state: V 0 = 53.8(3) Å 3, K 0 = 99(5) GPa, K′ = 11.7(5). Using this data the amount of H required to match the density of the Earth's core was estimated to be 0.5–1 wt.% hydrogen in the outer core and 0.08–0.16 wt.% hydrogen in the inner core. Our results also suggest that hydrogen and carbon do not occur together in the Earth's core.